Digital Network 9789350433096, 9789350243725

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DIGITAL NETWORK

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DIGITAL NETWORK VINIT THAKUR M.Sc. Sr. Consul/am, Indo-German Training Centre. Mumbai - 20.

Hal GJiimalaya GJlublishing'flouse MUMBAI • DELHI • NAGPUR • BANGALORE • HYDERABAD

©

AUTHOR

ISBN

: 978-93-5024-372-5

Revised Edition :2010

Published by

Branch Offices: Delhi

Nagpur

Bangalore

Hyderabad

Printed by

Mrs. Meena Pandey for HIMALAYA PUBLISHING HOUSE, "Ramdoot", Dr. Bhalerao Marg, Girgaon, Mumbai - 400 004. Phones: 886 01 70/386 38 63, Fax: 022-387 71 78 Email: [email protected] Website: www.himpub.com "Pooja Apartments", 4-B, Murari Lal Street, Ansari Road, Darya Ganj, New Delhi - 110002. Phone: 327 03 92, Fax: 011-325 62 86 Email: [email protected] Kundanlal Chandak Industrial Estate, Ghat Road, Nagpur - 440018. Phone: 72 12 16, Telefax: 0712-72 12 15 No. 12, 6th Cross, Opp. Hotel Annapoorna, Gandhinagar, Bangalore - 560009. Phone: 228 1541, Fax: 080-228 66 11 No. 2-2-1 167/2H, 1st Floor, Near Railway Bridge, Tilak Nagar, Main Road, Hyderabad - 500 044. Phone: 650 1745, Fax: 040-756 00 41 Printline

New DeThi.

CONTENTS

1.

Basics of Networking

2.

Wide Area Network -

The Internet

3.· Local Area Networks 4.

Man (Metropolitan Area Network)

5.

Commercial Lan System -

6.

Integrated Services Digital Network Introduction to ISDN

Ethernet

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Basics 01 N

rllil

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Chapter 1

BASICS OF NETWORKING

•

Introduction

•

Applications of a network

•

Analog & Digital Signals

•

Serial & Parallel Transmission

•

Asynchronous & Synchronous Transmission

•

Bandwidth

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5

Basics of Networking

WHAT IS A COMPUTER

NETWORK~

A computer network is a collection of devices that can store and manipulate electronic data, interconnected in such a way that network users can store, retrieve and share information. A network may be vast, encompassing hundreds of computers and spread across continents; it may link together mainframes, minicomputers and micros, printers, fax machines, and pagers; its users may be a host of individual enthusiasts or firms; or the network may consist of no more than two machines, joined with the sole purpose of sharing a printer or hard disk. In the near future, numerous other types of devices will be network connectable, including interactive TVs, videophones, navigational and environmental control systems. 0

The larger systems are generally referred to as Wide Area Networks or WAN's. Some are run by single organisations, with perhaps the biggest being the world-wide network run by IBM for its own use, linking its many research establishments and sales organisations, so that the scientists and managers in America, Europe, Australia and elsewhere can pool ideas and draw upon each others expertise. Within the UK many of the leading chain stores and supermarkets have networks that span the whole country, with every store feeding data back to the central organisation. Some wide area networks like SEAQ, the Stock Exchange System, are not restricted to a single firm but are run as a service for interested individuals and businesses. In today's business world, a computer network is a resource, which enables the businesses to gather, analyse, organise and disseminate the information that is essential to their profitability. Businesses are currently implementing intranets at a breakne~k pace as they enable a business to collect, manage and disseminate

Digital Network

6

information more quickly than ever before. The rise of intranets and extranets is an indication of the crucial importance of computer networking to businesses. Intranets and extranets are private business networks that are based on Internet technology. The Importance of Computer Networks

Information and communication are two of the most important strategic issues for the success of every enterprise. Today nearly every organization uses a substantial number of computers and communication tools. To enable managers to communicate with other departments and participate in information retrieval programs, effective usage of information technology, computer networks are necessary. These networks are a kind (one might call it paradigm) of organization of computer systel'!ls produced by the need to merge computers and communications. At the same time they are the means to converge the two areas; the unnecessary distinction between tools to process and store information and tools to collect and transport information can disappear. Computer networks can manage to put down the barriers between information held on several (not only computer) systems. Only with the help of computer networks can a borderless communication and information environment be built. Computer networks allow the user to access remote programs and remote databases either of the same organization or from other enterprises or public sources. Comp.uter networks provide communication possibilities faster than other facilities. Because of these optimal information and communication possibilities, computer networks may increase the organizational learning rate, which many authors declare as the only fundamental advantage in competition. Besides this major reason why any organization should not fail to have a c0!'l1puter network, there are other reasons as well: • •

cost reduction by sharing hard- and software resources high reliability by having multiple sources of supply

Basics of Networking • •

7

cost reduction by downsizing to microcomputer-based networks iristead of using mainframes greater flexibility because of the possibility to connect devices from various vendors .

What makes up a networkJ The most important components are, obviously, the computers. In a small business, these will normally be IBM. Larger firms, where greater speed or capacity may be needed, might include a minicomputer in the network. A design or engineering office may well have ~ network composed largely of high-resolution graphic terminals to run their CAD software, with a smattering of PC's for routine word processing and accounting. That brings up an important, point about a network; you can link different types of machines together. Next come the peripherals: hard disk drives and tape streamers, printers and plotters, modems and mice. With a network, usually fewer peripherals are needed than with the same number of separate computers, for each user will have access to every peripheral that is attached to the network. Instead of having, say, a 20 Megabyte hard dis,k drive on each of half a doze~ machines, two 60 Megabyte drives could service them all, for each computer will be able to store its files on either of the networked drives. One high quality printer may be enough to satisfy the needs of a 4-computer office, which without the network would have found it difficult to function without one per machine. Cables are then needed to create the physical links between the computers. Special networking software, or netware is also essential. This provides a means of identifying and addressing each component, and controls the flow of data around the system. When a file is sent to be printed, it is the netware that ensures that it reaches the right printer; when one computer user wants to access a file on another machine, it is the netware that makes this possible or that prevents it. Security - the control of access to data is an important function of the software.

Digital Network

8

APPLICATIONS OF COMPUTER NETWORKS Sharing of applications It allows all network users to share the same application, saving disk space, because the application only needs to be installed on one of the computers.

Sharing of databases Allowing multi-user access and modify to the same database at the same time is definitely better than having the same database in all the computers, and periodically combine all the modifications together. It is particularly useful for companies like Banks and Travel Agencies.

Sharing ()f resources Allows each user to have access to the same peripheral device, • like printers and scanners. It is certainly cheaper than each terminal having its own peripheral device.

Personal Communications Allowing users to communicate with each other, sending computer files to another user, just clicking a button improves companies efficiency. Users can send messages quickly without having to move. .

Effective Collaboration A well-designed computer network enables users' to collaborate effectively. For example, 'a managing editor, associate editors, writers, and to work together on a publication. With a computer artists may network, they can share the same electronic files, each from his or hflr own computer, without copying or transferring files. If the applications they are using feature even basic integration with the network operating system, they can perform such tasks as opening, viewing, and printing the same file simultaneously.

need

To be able to collaborate electronically from widely separate physical locations has significant advantages. It enables people to

Basics of Networking

9

avoid the considerable time investments and costs connected with traveling. It enables people to communicate instantaneously, regardless of the distance. It frees people from having to reconcile the differences in multiple information files. Electronic collaboration enables people to minimize the amount of work required to complete projects-it frees them from redoing werk they would do correctly in the first place if they had instantaneous access to upto-date information and instructions.

Freedom to Choose the Right Tool The design of any particular computer can make it well suited for some tasks and not as well suited for others. In an open network environment, you can combine many kinds of computers to take advantage of the special strengths of each type of machine. For example, Novell network users can use IBM pes running any version of Windows or DOS, Macintosh computers running a version of the Macintosh operating system, Sun workstations running the UNIX operating system, and many other types of computers, all on the same network. Scientists, secretaries, doctors, lawyers, writers, editors, artists, engineers-everyone can use the type of computer equipment best suited to the type of work he or she does, yet each can still easily share information with everyone· else.

Cost-Effective Resource Sharing A very important reason for having a computer network is that it enables users to share an expensive equipment. /'

By selecting the right mix of printers and allowing each network user an appropriate access to them, one could have enough printing power to take care of the needs of all users; you could ensure that the expensive equipment was not standing idle; and you could provide users with the latest, most powerful productivity features, freeing them from many.tasks they would otherwise have to do manually-all for a significantly lower cost than if you were to buy an inexpensive printer for each of the computers connected to your network.

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Digital Network

A network enables you to share any networkable equipmen or software and realize the same benefits that you would enjoy from sharing printers. On a network, users can share modems; data storage devices, suc~ as hard disks and CD-ROM drives; data backup devices, such as tape drives; E-mail systems; fax machines; and all networkable software. When you compare sharing these resources to purchasing them for each computer, the cost savings can be enormous. Secure Management of Sensitive Information There is another advantage to computer networking that may be even more important than instantaneous, coordinated information and resource sharing. The best networks have extremely powerful security features,that enable you to exercise flexible control of who will have access to sensitive data, equipment and other resources. Easy, Immediate Information Dissemination With networking in a business environment, one can create or update information and easily and immediately make it accessible to all company employees. This information will then be available to anyone who has rights to access it, anywhere in the world. Integrated, flexible information sharing; instantaneous information updating and access; lower equipment costs; flexible use of computing power; secure management of sensitive information-these are the benefits of computer networking. And these benefits help produce the results to all looking for increased efficiency, productivity, and profitability.

ANALOG & DIGITAL SIGNALS Continuously variable as opposed to discretely variable. Physical quantities such as temperatures are continuously variable and so are described as Analog. Analog signals vary in accordance. with the physical quantities they represent. The public telephone network was designed to transmit voice in analog form.

Basics of Networking

11

Analog Data: Data that is in the form of continuously variable physical quantities.

Analog Signaling: An analog signal is one that varies in a continuous manner, such as voice or music.

Analog Transmission: Transmission of a continuously variable signal as opposed to a discretely variable signal. Physical quantities such as temperature are continuously variable and so are described as "analo~."

Digital: Discretely variable as opposed to continuously variable. Data characters are coded in discrete, separate pulses or signal levels.

Digital Data: Information represented by a code consisting of a sequence of discrete elements.

Digital Data Network: A network specially designed for transmission of data, wherever possible in digital form, as distinct from analog networks such as telephone systems, on which data transmission is an exception. The purpose of computer networks is to enable users to manipulate data so it can be stored, retrieved and shared. To understand how available technology enables us to do this, we need to define a few terms and understand some basic concepts. Computers in a network must 'communicate' with each other to have the desired benefits of the network. This 'communication' is by the way of signals. These signals can be either 'Analog' .or 'Digital'. We will try to see in detail what these terms mean. The term "analog" comes from the word "analogous" meaning something is similar to something else. It is used to describe devices that turn the movement or condition of a natural event into similar

12

Digital Network

electronic or mechanical signals. There are numerous examples,but let's look at a couple. A non-digital watch contains a movement that is constantly active in order to display time, which is also constantly active. Our time is measured in ranges of hours, minutes, seconds, months, years, etc. The display of a watch constantly tracks time within these ranges. In effect, the data represented on a watch may have any number of values within a fairly large range. The watch's movement is 'analogous to the movement of time. In this respect the data produced is analog data. Another prime example of an analog device is a non-digital thermometer measuring a constantly changing temperature. The action is continuous and the range is not very limited, though sortJetimes we wish it were. The data produced by a thermometer is analogous to the change in temperature. Therefore, it is an analog signal. Digital signals, on the other hand, are distinctively different. Digital signals don't have large ranges, nor do they reflect constant activity. Digital signals have very few values. Each signal is unique from a previous digital value and unique from the one to come. In effect, a drgital signal is a snapshot of a condition and does not represent continual movement. Of course, the most-obvious example of digital data is that communicated on-board a computer. Since a computer's memQl'¥ is simply a series of switches that can either be on or off, digital data directly represents one of these two conditions. We typically represent this.on and off status with 1s and Os where 1 represents an "on" bit and 0 represents "off". Analog data, by its nature, more closely captures the essence of natural'phenomenon, with its action and subtlety. Digital data can only attempt to capture natural phenomenon by "sampling'" it at distinct intervals, creating a digital representation composed of 1s and Os. Obviously, if the interval between samples is too large, the digital representation less accurately represents the phenoa:nenon. If the sampling occurs at too short of an interval,

Basics of Networking

13

then an inordinate amoun' of digital resources may be utilized to capture the phenomenon. The changes involved may not be significant enough to warrant so frequent a sampling for accuracy's sake. To digitally represent sound authentically, a sample must be taken over 44,500 times per second. A reference to digital resources would certainly include digital storage media. In terms of storage, digital samples of natural phenomenon, or encoding of analog signals from such phenomenon, generally requires a significant amount of recording media (i.e., disk space). To record a second of authentic sound, 1.5 million bits of storage is required. Analog signafs don't require such great storage capaCity, but they do suffer in the area of duplication. When copying an analog signal from one generation to another, deterioration of the original signal occurs. A prime example is when we copy a videotape. Since video recorders' are analog machines, copying a tape several times results in the accumulation of Uilwanted analog values called "noise". Eventually these signals become so evident, that the original analog signal is compromised and the video "dub" suffers from intense graininess and poor audio sound. Our technology is limited in the transmission ~nd duplication of analog signals because of the infinite number of values that are allowable. Digital signals, however, have basically two values. It is much easier to work with two values rather than an infinite number. Consequently our current level of technology allows us to maintain the original quality of a digital signal. With a value of "on" or "off", it's pretty hard to miss. Both analog signals and digital signals have found a home in the networking world. Analog signals are used in. a certain type of network known as broadband networks. Digital data is typically used in what is known as a baseband .network. Broadband networks incorporate technology similar to that of cable television. Data, whether it is video, audio, or digital, is transmitted on the wire at certain frequencies. The typical medium

Digital Network

14

is coaxial cable. Just like you can have cable TV at home bringing you several channels at once, so broadband systems can bring you several channels of data. More on this type of network will be presented later. Digital technology is generally utilized exclusively for baseband networks. These networks devote the entire cable (and subsequently its bandwidth) to network transmissions. The baseband network will be more fully discussed later in this chapter. In comparing analog and digital signals, advantages lie on either end of the spectrum. Analog signals. suffer far less from attenuation over long distances. This rather makes-sense. Since digital data can only be a 1 or 0, what happe'ns when a signal becomes so weak that it is hard to distinguish between each state? Sometimes we just can't. Analog devices, on the other hand, are equipped to handle the infinite values between 1 and O. Digital devices are a lot less sophisticate9, meaning that they are fairly easy to manufacture and cost~ffective. In'addition, digital devices are more resilient to EMI and make more efficient use of the cabling bandwidths than analog systems do. Parallel Transmission: Parallel Transmission i~ the technique that sends each bit simulta-neously over a separate line. Normally Parallel Transmission t~h-nique is used to send data a byte (8 bits over eight lines) at a time to a high-speed printer or other locally attached peripheral. I

Serial Transmission: The standard method of ASCII transmission where bits are sent, one at a time, in sequence. Each 7-bit ASCII character is preceded by a start bit and ended with a parity bit and stop bit. A group of SNA networks connected in series by gateways is called as Serial Network. A technique in which each bit of information is sent sequentially on a single channel, rather than simultaneously as in parallel transmission. Serial Transmission is the normal mode of data communications. Parallel Transmission is often used between

15

- Basics of Networking computers and local peripheral devices. Block Diagram of Data Communications Model: I

n

p u t

M

Inpu data

e

Transmitted Signal 5 (t)

D(t)

s s

ReceiSignal

Output data

) (t)

d

ved

Output Message m

(t)

a g e M

.-

A (~

Input

s

device

)

--+

Trans- --;;.. mitter

Circuit --;.. Recei- --+

ver

Out-

..;.

put

A(r)

device

In the diagram above, the block .label circuit indicates some media that is carrying the electromagnetic signal. line Transmission Mode involves the number of paths being used to convey the data. If only a single path is available, then the transmission of data must occur bit-by-bit in a sequence. This requires that the input data D (t) must be converted into a sequential emission of single bits that travel one after the other across the circuit. This is known as Serial Transmission Mode. A simple telephone circuit on the loql loop is a common circuit used for serial transmission. If there are many paths available, then several bits may travel, each along its own path, at the same time. Multiple paths may be provided by separate physical circuits or by sub-circuits within a .frequency-division multiplexed [FDM] signal. In either case, bits may tt2':~1 simultaneously in parallel, all departing and arriving at

Digital Network

16

the same instant. This is known as Parallel Transmission Mode. Parallel transmission is normally used for communication to locally attached peripheral equipment. Data Transmission: One major difficulty in data transmission is that of synchronising the receiver with the sender. Clearly, this is • particularly true in seridl data transfer, where the receiver must be able to detect the beginning of each new character in the bit stream being presented to it. Obviously, if it is unable to achieve this, it will not be able to interpret the incoming bit stream correctly. Two approaches exist to solve the problem of synchronisation: these are asynchronous and synchronous transmission.

Asynchronous Transmission Using this approach, synchronisation is implemented at character level and each individual character is transmitted along with the necessary control information to allow this to take place. The control information consists of additional bits added to each character, START BITS - which indicate that transmission is about to commence - and STOP BITS - which indicate that it is about to cease. Usually, the stop bits are of the same polarity as the Channel idle state (for example zero) and consequently the start bits are of opposite polarity. The initial change in the state of polarity, from the idle state to the first bit, is known as the START PULSE. Clearly, this enables the receiver's clock to be synchronised with the transmitter's clock. The di~gram below shows one common system, using two start bits and one stop bit, to transmit II ABC": Associated with the approach of asynchronous transmission are various advantages: •

One principal advantage is that each individual character is complete in itself - therefore if a character is corrupted during transmission, its successor and predecessor will be unaffected.

Basics of Networking

•

11

Particularly s'uit~ for applications where the characters are generated at irregular intervals e.g. data entry from the keyboard.

On the other hand, an asynchronous transmission system has extensive disadvantages: •

Successful transmission inevitably depends on the recognition of the start bits - clearly these can be easily missed, or occasionally spurious start bits can be generated by line interference.

•

A high proportion of the transmitted bits (usually 3 out of 11 if using ASCII code) are uniquely for control purposes and thus carry no useful information.

•

As a result of the effects of distortion the speed of transmission is limited.

Thus, in conclusion, asynchronous serial transmission is only normally used for speeds of upto 3000 bits per second, with only simple, single-character error detection.

Synchronous Transmissiun Thus, second approach again transmits the message via a single channel. However, in this instance it is imperative to note that there is no control information (tssociated with INDIVIDUAL characters. Instead the characters are grouped together in blocks of some fixed size and each block transmitted is preceded by one or more special synchronisation characters, which can be recognised by the receiver. ASCII provides a control character, SYN (ASCII code 22) for this unique purpose. On this ocCasion, the diagram below shows how IIABC· would be transmitted on a synchronous system, preceded by two SYN characters: As with asynchronous transmission it is apparent that the synchronous approach offers advantages-

18

Digital Network

•

The amount of. central information which requires to be transmitted is restricted to only a few characters at the start of each block.

•

The system is not so prone to distortion as asynchronous communication and can thus be used at higher speeds.

Yet the synchronous approach also suffers from a few detrimental attributes•

If an error does occur rather than just a single character. the whole block of data is lost (usually containing 100 or more characters).

•

The sender cannot transmit characters simply as they occur and consequently has to store them until it has built up a block, thus the system is unsuitable for applications where characters are generated at irregula~ intervals.

Thus, in conclusion, serial synchronous transmission is principally used for high-speed communication between computers.

BANDWIDTH In the simplest sense, bandwidth refers to the amount of information that can be transferred between computers. Think of it as the main water pipe in your home-the larger the pipe, the more water. By increasing the pressure you may get more water faster, but the amount you can pump through the pipe is ultimately limited by its size. likewise, you can send and receive more information over phone lines by compressing data. However, there comes a point at which the lines reach maximum capacity. This is where the need for more bandwidth-bigger pipes-comes in. Right now, telephone and cable companies are racing to be the first to deliver higher speed, higher capacity Internet access. It may take a few years, but when it hits, expect the extra bandwidth to make today's Internet look like black-and-white newsprint. The bandwidth is the speed at which the physical connection can move· data, and it actually constrains Web access or access

Basics of Networking

19

across the network more than the speed of your computer. For example a 14.4 kbps modem can receive only about 1.2 - 1.4 kilobytes of data per second, even if there is no other-traffic on the network. IIBandwidth" describes the amount of data a network can transport in a certain period of time. In other words, bandwidth is a capacity for rate of transfer, usually expressed in bits per second. Think of it this way: Your corporate network is like a highway. In a given moment, only one car per lane can pass a given point on that highway. During rush hour, bottlenecks cause traffic to slow to a crawl, and you arrive home late for dinner. Too many cars, not ehough lanes. It's the same when your network has too much information flowing between computers and not enough bandwidth. Everything slows down, and in extreme cases, your data may never get to where it's going. So what does bandwidth mean for an organisation ? Every time you communicate with another person or another computer (like the server or mainframe) via your computer, you are moving bits of information over the network. The more people using your corporate network, the more congested the network becomes. At first, that may simply slow down the system and force you to sit idle while your computer retrieves the information you've requested. If the overcrowding gets too bad, data may get lost in transit because it crashes into other stuff. Many networks today are based on a technology called Ethernet, which has a standard bandwidth of 10Mbps (10 million bits per second); in one second, 10 megabits of data can move through any given spot on the network. And the new Fast Ethernet has transmission speeds of 100Mbps. As technology continues .to evolve, even more advanced networks have been developed that offer transmission rates greater than 1Gbps (that's gigabits, or 1 billion bits, per second). The bandwidth you require should be decided by the speed at which information has to be transferred across the network and

Digital Network

20

the cost of increasing your bandwidth versus the perceived benefits that your organisation will derive from the added bandwidth. REFERENCES 1.

Data & COl1lputer Communications - Terms, Definitions & Abbreviations:

Gilbert Held. 2.

Data Communications & Distributed Networks: Uyless D. Black.

3.

Using Computer Networks & Communications Software in Business.

4.

Unpublished Reference Material by Vinit Thakur.

5.

Internet

Wide Area Network -Theloleroet

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Chapter

2

WIDE AREA NETWORK - THE INTERNET •

Internet Basics

•

Internet in India

•

TCP/IP

•

IP Addresses and URl's

•

Domain Name Servers

•

File Transfer Protocols

•

Telnet

•

World Wide Web •

Hypetext Transfer Protocol

•

Web Servers

•

Browsers

•

Search Engines

•

Web Services

•

Hyper Text Markup language (HTMl)

•

Usenet

•

Shell Account PPP/SlIP Account

•

Firewalls

•

Intranets

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Wide Area Network - The Internet

INTERNET BASICS Internet is not a program, it's not a piece of hardware, it's not software, and it's not even a system. Instead, it's a place where you can get information, make information available and where you can meet people. Essentially, the Internet is a network of computers that offer access to people and information. Over 60 million people use the Internet, and that number is expected to increase over 120 million within a few years. To use Internet, you may run different programs depending upon the type of information you wish to have. The kind of information freely available includes-govemment documents, scientific data, hobbyist lists, business and personal advertising, databases and much more. Internet is a combination of network, on which all the programs are used to get information. People from all over the world can access the Internet, and more than 10 million people do so daily. Most of these people only use one feature of the Internetmail-but many also use additional information. The kinds of communication that happen on the Internet daily include the following:•

Exchanging short social notes.

•

Getting the latest news from around the world.

•

Conducting business negotiations.

•

Collaborating on scientific research.

•

Exchanging information with others who have similar hobbies or ·interestS.

•

Transferring computer files.

INTERNET IN INDIA In India Internet was started to serve the Educational Institute to help in the research work. In 1986 the liT (Indian Institute of

26

Digital Network

Technology) were liked up with Indian Institute of Science by ERNET (Educational Research Network) which later connected with foreign universities. On 15th August 1996 government body called VSNl (Videsh Sanchar Nigam limited) started it's dial-up services as first Internet Service Provider. In the present situation there are more than 100 ISP given license to provide internet servi~e some of them are MTNl, Satyam Computers ltd., Wintech etc. TCP/IP Transfer Control Protocol and Internet Protocol are two sets of rules that allow computers and networks to communicate effectively. They regulate the flow of data and make sure that it reaches its destination safe and sound. TCP and IP goes hand-in-hand to enable the safe delivery of data over a Networ~. In order to speed up transmission of data over a network, the data is split into a number of smaller packets. This is where TCP/IP plays a vital role. TCP attaches a header to the data packet, which contains information like the address, its origin, length of the packet and so on. IP, on the other hand, works like a postal department and ensures that once the data packets reach the receiver's end, they are reassembled in the same sequence they were broken up and are ready for the application they are meant for. IP work as routing agent falls under the network layer which has function of making decision for transmitting data across device not connected to each other.

Layers of Travel The two protocols TCP/IP are stacked over each other and occupy the network layer and the transport layer of the network. These layers are a part of virtual model of networking called OPEN SYSTEM INTERCONNECTION (OS I) model, which defines how network devices interact with each other. The OSI models consists of physical, data link, network, transport, sessions, presentation and application layers.

Wide Area Network - The Internet

27

The Physical layer transmits data from one location to the other and is made up of physical aspects of the network like cables and connectors. The Data Link layer ensures error-free transmission of data and consists of networking cards, modems, etc. The function of the Network layer is to make routing decisions for transmitting data across devices not connected to each other. As IP is a routing agent, it falls under this layer. The Transport layer comes next and its primary function is to ensure error-free transmission of data. Transfer Control Protocol or TCP falls under this layer. The remaining layers such as the Session, Presentation and Application layers form the Application Group, which synchronises links across programs and converts network data to user-readable formats. As the data from the sender's side transcends the 051 layers, an identifier header is attached to the data payload at each layer. The data block, with all the attachments, then embarks on its jolJrney through the Internet. When the data packet reaches its destination, it works its way· up the layers, detaching headers at their respective layers. Once the data packet reaches the Application layer, the last header is detached and when you open the file, you see the original data. TCP forms the virtual circuit of the network. A virtual circuit refers to the temporary path where data and messages follow between two parties when a connection is established between them. TCP is thus a connection-oriented service . . Internet Protocol or IP on the other hand provides datagram services. Datagram refers to data packets that have destination addresses on them and are floated on the Net. It is a connection less service and the packet travel using the shortest available path traversing several networks on the way. Transfer Control Protocol As transport layer protocol, TCP accepts message information from the applications, and divides it into multiple segments, and encapsulates each segment into a datagram. Each datagram is passed over to the Network layer protocol UP) for further transmission and routing. As the receiver's end, TCP reassembles the data and distributes it to the concerned application program.

Digital Network

28

.TCP transmits data in the form ~f packets that comprise a header and a data block. The header consists of information like the address of the packet, its origin, the length of the packet and more. The 'data block carries the payload, which is t~e text or pictures th~t ~u dQwnloador browse off the Net. Th~

header indudes a destination port address to identify the packet's ultimate destination. The Source and Destination Port fields are both 16-bit in length and identify user processes and applications. Since data is. split into a number of packets, the Sequence Number in the second layer provides a mechanism to maintain the sequentiality of the data stream on the network. The Hlen field denotes the length of the segment header in 32bit mUltiples and the Code Bits field contains six flag bits. These bits indicate which header is significant and which field value shoula be interpreted, while the other bits are used to control the connection and data transfer operations. The Checksum Field provides reliability for the Tep header and the data packet, thereby detecting errors in the segment. The Urgent Pointer field completes the heaaer and identifies the position of data in the Tep segment.

Jntemet Protpcol Internet Protocol or IP works like the postal department. It routes data packets to the address mentioned in the header and fragmentS -them. These are then marked so that the fragmentation sequence is maintained and are reassembled upon reaching their destination. The routing of datagrams over a network can occur over different paths and the possibility of some datagrams arriving out of sequence is not ruled out. In addition, as datagrams flow between various networkS, they also f.3ce physical li'mitations in terms of the amount of data that can be transferred over a particular network. Sometimes, these fragmented datagrams can be too big for smaller networks and thus are further fragmented to overcome this hurdle. IP also attached a small header on the data packet, which provides information about the handling of the datagram,

Wide Area Network - The Internet identification of fragmet\ted datagrams and the like. The Version field contains a 4,bit code that identifies the IP protocol used to create the datagram. While the Hlen field defines the length of the header, the Service field denotes the importance level that has been assigned to the packet by the sender. The Identification field proves the identity of a datagram. In case the datagram has been further fragmented, the Fragment Offset field specifies the other offset of the datagram. These two fields keep track of the origin of the datagram and how it will be merged with the original data or datagram upon reaching the receiver end. The Flag field contains information about the nature of fragmentation. It provides information about the current fragment and also gives the total number of the fragments. The header also has a field called 'Time To live' or TTL that defines the number of routers a data packet can encounter en route to its destination computer, thereby avoiding choked networks. Each packet is forwarded based on a four-byte destination address (called IP address). Internet authorities assign a range of numbers to different organisations. These organisations further assign groups of their numbers to departments. IP operates on gateway machines that move data from the department to the organisation, then to the region and finally across the world.

IP ADDRESS & URl To connect to another machine on the Internet, you need to know its IP address, which is an ide~tifier for a particular machine on a particular network. These are also referred to as IP numbers or Internet addresses. The IP address is represented by four decimal numbers separated by dots and is basically divided into the host computer section (which is assigned by the Internet Information Network Centre) and the host computer section (which is defined by the network administrator of the particular network).

Digital Network

30

Depending on their size, networks are classified into:

Class A This comprises of very lar.ge networks with millions of nodes. They have their IP addresses ranging from 1.0.0.0 to 127.0.0.0. The first number before the dot defines the network, with the remaining three sections assigned to hosts. Class 8 These are smaller networks and can have only about 65,000 nodes. Their IP addresses range from 128.0.0.0 to 191.0.0.0. The first two numbers are allocated to the network and the remaining two numbers for the hosts. Class C These are much smaller networks, whicH support a maximum of 254 nodes. The IP addresses range from 192.0.0.0 to 223.0.0.0. In this case, the first three numbers denote the network and the last one denotes the host. To make life simpler, ordinary names are assigned to each address using the Domain Name System (DNS). Each Domain Name corresponds to a numeric IP address. The Internet uses the IP address to identify the network and the node and send data to the same. As you type in the address www.yahoo.com from your browser, the Internet actually connects you to the IP number 204.71.200.74 (which is the domain name for the Web site). UNIFORM RESOURCE LOCATORS URLs (Uniform Resource Locators) is the way to represent site name on the World Wide Web. URLs are similar to postal addresses or telephone numbers which are used to represent the destinations. Most URLs consist of 3 parts: 1. Service Name 2. Host Name 3. Request

e.g.

iiI

www.mrsoft.com!iniemetlP~mv Request Host name Service

Wide Area Network - The Internet

31

It is common to have URL's where the request is just a single

"/" The most common service names you use in URLs are "http", "gopher", "ftp:" , and "news:". These refer to Web servers, Gopher servers, FTP servers and Usenet news servers, respedively. A few URLs do not have a host name.

DOMAIN NAME SERVERS Internet works on the numbering system. These numbers are called IP. When we connect to the net you must have seen a set of 4 numbers being dialed i.e. for each address on Internet there is a unique set of these 32 bit numbers. Domain Name Servers are the servers, which maintain a distributed list of all domains against Internet Protocol addresses UP addresses}. Earlier to Domain Name Servers there was a system of having a host table maintained by SRI-NIC. It was updated twice a week to include new sites. System would download the copy of this table through FTP. Currently there are two types of servers as below :

1.

Resolver

2.

DNS

There are a number of servers, which maintain the addresses of sites. When browser needs the address of any site, resolver queries the nearest name server. This server becomes name server, replies immediately if it knows the answer or it asks another server. When the answer is found out it travels backwards. Thus, every server has two roles to play : 1.

As a server for name server

2.

Super server to extend functionality.

All web sites are arranged in 7 branches namely arpa, com, edu, net, gOY, mil, org. Following this are 236 country name abbreviations like ".in" for India. This helps to locate the site easily. The IP addresses of name servers at each of the domain name tags are lTIaintained by 10 root servers.

32

Digital Network When a DN5 fields a query that it cannot answer

1.

It sends a query to. root server

2.

Root server says it does not know but a mach1ne at say 195.95.251.10 might know

3.

DN~ sends a query to the above machine

4.

Server at 195.95.251.10 knows the answer (because it is primary name server)

5.

DN5 returns answer to your Pc.

Special ·features of DNS Cache: Name server caches alliP addresses for domain names that were requested recently. 50 that if requested again it responds imme~Uately.

Load Balancing: large sites like www.msn.com can have multiple addresses for same domain name. Name servers currently returns alliP addresses leaving PC to choose at random. But some name servers will now evaluate all addresses to find out the one with least load.

FILE TRANSFER PROTOCOL FTP stands for File Transfer Protocol, which is the standard system for moving files on the Internet. With it, it is possible to send or receive files to and from a machine on the Internet. The machine to which you are connecting must have FTP server and its address generally starts with 1ft!)' code. FTP requires that you be directly linked to the machine in question. 50 you are unlikely to use FTP to transfer your files, unless you or your client has dedicated FTP serv~r permanently connected to the Net.

As FTP computer knows the name of the Internet service you are calling from, you only need to type 'username@' followed by return. Once logged on, we can get access to public accessible software. Here one can get all sorts of files inside them. Thus, numerous FTP servers all over the world allow the people anywhere

Wide Area Network - The Internet

33

on the internet to log in and download whatever files have been placed on the FTP server. The web does not change this; it just makes obtaining of files easier, as FTP has somewhat arcane interface. In due course, FTP will probably vanish, as there is no particular advantage for a site to run an FTP server instead of HTIP server, who can do everything that the FTP server can do, and more in 1993, the Internet had already achieved a substantial size. Searching on the net comprised the enormous and exhaustive task of connecting to each server and finding the requisite information on it. Two doctorate students, David Filo and Jerry Lang, studying electronics at Stanford University decided to do something' about it. They decided to develop a universal database using which one could find information 'in a quick and ~imple way. Consequently the worlds first search engine Yahoo I was born in April 1994. Today there exist a variety of search engines, each with their own individual characteristics. Betw~n all of them they would possibly cQver almost a major part of the information on the web at any given time. However not a single search engine can claim

Wide Area Network - The Internet

37

to cover all of the information on the Internet. With hundreds of Web-sites springing up eV'eryday search engines simply cannot be expected to keep up with it. T~is is because search engines are programmed to seek out for new and updated information at periodic time slots, so they invariably lag behind by a few days to a few weeks or so depending upon the search engine in question. One of the biggest problems while searching the Web is that the information vastly differs in type and origin. Data is spread across numerous servers across the world, which are not subject to any central administration, in other words there is no central directory that can boast of covering the entire Internet at any given time. Search engines are, composed of databases that comprise indexing schemes, a query processor and 'spiders'. Spiders are programs that are designed to look up Web pages which are listed in every database, follow up on each and every link and update their databases to reflect the updated information, if any, as and when' they are programmed to. The records in these databases consist of the uniform resource locator (URl) or more simply, the dares where the Web site or page is located, the title of the page and the keywords for that page along with a short summary of the site in a few lines. To search, you have to enter keywords. The query engine picks up your request and tries to find the desired information from the millions of pages recorded in the index. How do search engines worki A search engine continuously sends out so-called 'spiders', a special kind of program, which starts in a homepage of a Server and pursues all links stepwise. Word indices are created from individual pages and the database i·s updated. (1)

In some search engines, the operators make entries using forms. Depending on the system, the data is released only after editorial processing.

(2)

To search for data, the search criteria are entered in the form provided by the search engine. The query is forwarded to the database.

Digital Network

38

(3)

The result displays a list with all pages that correspond to the search criteria. At the same time, the entries are displayed as links. You can reach the corresponding pages with a click.'

Search site types Search sites are basically of two types, search directories and search engines. Search directories are lists of Web sites organized into categories and sub-categories. Search directories are created manually rather than being automated. Their coverage is far less than that of search engines but comprise recommendations and reviews of sites. Yahoo! and Magellana are typical search directories. Search engines are huge computer generated databases containing information on millions of Web sites. They have programs called spiders that automatically look up Web sites and update their databases. Altavista, Hotbot, lycos*, Infoseek*, Excite* and Webcrawler* are search engines. (* these are hybrid sites i.e. they are search engines as wen as offering search directories.) There is also a third type that does not fit into any of the above two types, they are called metasearch sites.

Ranking of web pages There are various methods of ranking web pages and there are different approaches to ranking by different search engines. Confidence ranking is a method by which the various search result pages are ranked according to the number of times the specified keyword(s) appear in the document. Relevancy ranking is a slightly different approach to ranking the_ search results and depends upon the particular search engine you are employing. Some search engines have an algorithm to find words or phrases that are closest to or related significantly to the keyword(s) specified by you. Your search is then processed using these related words or phrases to get the number of matches in the search result documents, and a ranking is generated.

Wide Area Network - The Internet

39

Other factors like number of links pointing to the site, whether the search engine reviews a site or not are also very important in the ranking mechanism. Example, Webcrawler uses the number of links to the site very significantly in it's ranking, whereas Magellan will boost the ranking of a page for the simple reason that it has reviewed the site. The logic is that if the site was good enough to earn a review, the chances of it being relevant are more than an unreviewed site.

Web services The web service is facilities which is provided to the user, with or without charge with some limitations. Hotmail was one of the first success stories on the Net. It promises you free web-based e-mail account that you can access from anywhere in the world. You can send and receive e-mail, though there is a strong limit of 2 MB. The source of revenue generation for the service provider is advertising banner and other attracting offers. The popular services are : Example E-mail service Faxing Pager service Electronic card Free web pages

www.Hotmail.com www.tcp.int or www.tacstems.comlfax.htm www.icq.com www.123greetings.com www.geocities.com or or www.xoom.com

HYPER TEXT MARKUP LANGUAGE (HTML) It is a language for describing how documents are to be formatted. The term "Markup" comes from the old days when copy editors actually marked up documents to tell the printer-in those days, a human being-which fonts to use, and so on. Markup languages thus contain explicit commands for formatting. For example, in H'TMl, < B > means start boldface < IB > means leave boldface mode. The advantage of a markup language over one with no explicit markup is that writing a browser for it is

40

Digital Network

straight forward: the browser simply has to understand the markup commands. Documents written in a markup language can be contrasted to documents produced with WYSIWYG (What You See Is What You Get) word processor, such as MS-Worda or Word Perfecta. These systems may store their files with hidden embedded markup so they can reproduce them later, but not all of them work this way. Word Processors for the Macintosh, for example, keep the formatting information in separate data structures, not as commands embedded in the user files. By embedding the markup commands within each HTML file and standardizing them, it becomes possible for any Web browser Jo read and reformat any Web page. Being able to reformat Web pages after receiving them is crucial because a page may have been produced full screen on a 1024 x 768 display with 24-bit color but may have to be displayed in a small window on a 640. X 480 screen with 8-bit color. Proprietary WYSIWYG word processors cannot be used on the Web because their internal markup languages (if any) are not standardized across vendors, machines and operating systems. Also, they do not handle reformatting for different-sized windows and different resolution displays. Commonly used HTML Tags:

Beginning and end of web page.

< TITAL > < rrlTAL >

The text between these tags does not appear on the page but becomes the title of the page that appears in the browser title bar.

< BODY>

Makes the beginning and· the end of a text in web page.

< P> < IP >

Beginning and end of the paragraph.

Makes the begir,ning and end of a script section. The script itself is not displayed on the page only the result are dictated.

Wide Area Network - The Internet

41

USENET A newsgroup is a worldwide discussion forum on some specific topic. People interested in the subject can "subscribe" to the newsgroup. Subscribers can use a special kind of user agent, a newsreader, to read all the articles (messages) posted to the newsgroup. People can also post articles to the newsgroup. Each article 'posted to a newsgroup is automatically delivered to all the subscribers, wherever they may be in the world. Delivery typicai'ly takes between a few seconds and a few hours, depending how far off the beaten path the sender and receiver are. In effect, a newsgroup is somewhat like a mailing list, but internally it is implemented differe~tly. It can also be thought of as a kind of high-level multicast. The number of newsgroups is so large (probably over 10,000) that they are arranged in a hierarchy to make them manageable.

Alt is to the official groups as a flea market is to a department store. It is a chaotic, unregulated mishmash of news groups on all topics, some of which are very popular, and most of which are worldwide. The eomp groups were the original USENET groups. Computer scientists, computer professionals, and computer hobbyists populate these groups. Each one features technical discussions on a topic related to computer hardware or software. The sei and humanities groups are populated by scientists, scholars, and amateurs with an interest in physics, chemistry, biology, Shakespeare, and so on. The news hierarchy is used to discuss and manage the news system itself. System administrators can get help here, and a discussion about whether to create new newsgroups occurs here. The hierarchies covered so far have a professional, somewhat academic tone. That changes with Tee which is about recreational activities and hobbies.

42

Digital Network

soc, which has many newsgroups concerning, politics, gender, religion. Various national cultures, and genealogy. Talk covers controversial topics and is populated by people who are strong on opinions, weak on facts. Air is a complete alternative tree that operates under'its own rules.

Numerous newsreaders exist. like email readers, some are' keyboard based; others are mice based. In nearly all cases, when the newsreader is started, it checks a file to see which newsgroups the user subscribes to. It then typically displays a one-line summary of each as-yet-unread article in the first newsgroup and waits for the user to select one or more for reading. The selected articles are then displayed one at a time. After being read, they can be' discarded, saved, printed, and so on. Newsreaders also allow users to' subscribe and unsubscribe to newsgroups. Changing a subscription simply means editing the local file listing which newsgroups the user is subscribed to. To make an analogy, subscribing to a newsgroup is. like watching a television program. If you want to watch some program every week, you just do it. You do not have to register with some central authority first. Newsreaders also handle posting. The user composes an article and then gives a command or clicks on a icon to send the article on its way. Within a day, it will reach almost everyone in the world subscribing to the newsgroup to which it was posted. It is possible to crosspost an article, that is, to send it to multiple newsgroups with a single command. It is also possible to restrict the geographic distribution of a posting. The sociology of USENET is unique, to put it mild1¥. Never before has it been possible for thousands of people who do not know each other to have worldwide discussions on a vast variety of topics. If enough subscribers to a group get annoyed with newsgroup pollution (irresponsible messages) , they can propose having the newsgroup be moderated. .

Wide Area Network - The Internet

43

A moderated newsgtoup is one in which only one person, the moderator, can post articles to the newsgroup. All postings to a moderated newsgroup are automatically sent to the moderator, who posts the good ones and discards the bad ones. Some topics have both a moderated newsgroup and an unmoderated one. SHELL ACCOUNT PPP/SLIP ACCOUNT Shell Account describes the authorisation to access another computer at the operating system level. Shell accounts are useful to the user who needs data in textual format. In shell account, user has no direct IP-link via SLIP/PPP. He rather logs into the Unix computer of the provider using a terminal program. PPP/SLIP In order to conn,ect a computer using the TCP/IP communication protocol to another TCP/)P computer over a modem or a serial line, both computers must be running on an additional protocol. This can either be PPP (Point To Point) or SLIP ( Serial line IPl. Both protocols perform the same task but they are not interoperable (i.e. both ends of the connection must be running on either PPP or SLIP) PPP was derived in 1991 by IETF (Internet Engineering Task Force). FIREWAlLS As private networks integrate with the Internet, there is a need to ensure your network. A study revealed that out of the 250,000 attacks on the Department's computer systems, about 65 per cent succeed. To minimize such problems, the companies need to add a firewall between the network and the interest. The firewall consist of Hardware such as routers and host systems software both hard and software. In terms of functionality a firewall is basically a data packets between ,trusted and untrusted networks. Any kind of network (Internet, Intranet or extranet) that uses TCP/IP for data transmission depends on source address, and the port number. A firewall uses these addresses and port numbers

Digital Network

44

to control the flow of data packets between the trusted and untrusted network. Depending on how they deal with ports and addresses, the firewalls are classified as packet filter, application proxy or application gateway and packet inspection firewall. 1. Application filter firewall : The fastest and the simplest of the three, is also one of the earliest. Usually the Router (hardware)based-, in this system a packet filter compares the' header information-source and destination add~ess, and port number-of each incoming or outgoing packet against a table of access control rules. The packet having the right address and port is allowed to pass through, or else is dropped. 2. Application proxy firewall: Proxy firewalls are built on the principle that security can be reliable only if there is no direct connection between the trusted and untrusted networks. An application firewall works by examining what application or service (such as e-mail or file transfer) a data packet is directed to. If no service is avail~ble, the packet is discarded at the firewall. If the service is avpilable to that packet, then it is allowed to pass through. Perhaps the only problem with application proxy firewalls is that they are slow. 3. In the packet inspection firewall the content of the packets is also considered. This inspection of packet can be either based on its 'state' or 'session'. In case of.state filtering, the firewall only allows the incoming packet if it can be mat~hed with an outbound request (or 'invitation') for that packet. In case of session filtering, the network station is tracked. Once the trusted user terminates the session, all incoming packets with identity pertaining to that session are rejected. Real life firewalls There are two types in which a firewall can be set up:

Wide Area Network -The Internet

45

1. Dual homed gateway : Here there -is only one firewall with two connections, one for trusted network another for untrusted network. 2. Demilitarized zone: Here two firewalls are used. The first firewall has one connection leading to untrusted network and second leading to host systems that can be accessed through untrusted network. The host system i.e. usually e-mail or web server is connected to another firewall, which links them to trusted system. The area between the firewalls is called demilitarized zone.

INTRANET It is a network connecting an affiliated set of clients using standard internet protocols, especially Tep/IP and HITP. It is also defined as an IP-based network of nodes behind a firewall, or behind several firewalls connected by secure, possibly virtual, networks. Advantages

Streamlining business processes Intranets are phenomenally powerful tools to streamline business processes. From decision support, customer service and product engineering to distributed channel operations, and from sales force automation and executive information systems, business applications based on intranets can substantially improve the efficiency of complex operations. This is possible because intranet applications are typically much less expensive to develop and deploy, and much easier to use than applications based on older proprietary platforms. Benefiting from a universal client interface the Web browser-intranet business applications can be deployed and managed from a central location. At the same time, standardsbased protocols and development technologies enable separate departments across a company to create intranet solutions that remain compatible and compliant with companywide systems and processes.

46

Digital Network

Facilitating Information dissemination A key benefit of the intranet technology is its ability to provide up-to-date information quickly and cost-effectively to the entire user community. An intranet puts vital information at the fingertips of employees, regardless of their location or the location of the information. Information disseminated on an intranet enables a high degree of coherence for the entire company because communications are consistent. A'news' section of an intranet, for example, can include recent company press releases regarding management strategies, partnerships, and new products. A finance section can keep employees informed of vital financial reports and forecasts. A customer section can allow customers to check the status of an order or repair. Vendors can, submit invoices online and check procurement status. The possibilities are virtually limitless. By giving people the ability to access time-critical information, intranets improve the decision-making process by empowering individuals with the knowledge necessary for faster and better informed business decisions. Intranets allow the centralisation of information, which makes it easier to maintain and keep data up to date. The benefit to the end-user is the simplicity and speed of information access. For example, the interactive capability enabled by hyper-text links makes it ~asy for users to gather all the information they need from Web pages quickly, just by clicking on a related icon or button. They do Rot have to spend long hours searching through arcane directories and servers. Providing instant and secure access to business-critical information saves time and increases productivity, and publishing information online eliminates the production, duplication and distribution costs associated with paper. The result-significantly reduced costs, and increased profitability. .

Enriching communications and collaboration Intranet technologies enable teams to share knowledge and information regardless of their locations or time zones. Engineering groups can share research data, design concepts, schedules and

Wide Area Network - The Internet

47

other project materials for comments and reviews during a developm~nt pro.cess. Training groups can distribute training schedules and multimedia computer-assist~d training courses using ~ Web-based technologies. Project teams can take advantage of intranet newsgroups and threaded discussions to communicate issues and solutions, and can use online chat technology when real-time interaction is required. With intranet teleconferencing, participants can share conference materials in a variety of formats, including text, graphics, audio and video. As E-maiJ is already a standard business tool in most companies, its integration with other intranet technologies can be seen as a natural extension of this accepted communications medium.

"This page is Intentionally Left Blank"

locllArea Networlls

"This page is Intentionally Left Blank"

Chapter 3

LOCAL AREA NETWORKS

•

Local Area Network (LAN) Metropolitan Area Networking (MAN) & Wide Area Networking (WAN)

•

International Organisation for Standardization (ISO)

•

150-051 Reference Model

•

TCP/IP Reference Model

-. Characteristics & -Uses of LAN

•

LAN PROTOCOLS

•

LAN STAN DARDS

"This page is Intentionally Left Blank"

Local Area Networks

53

LOCAL AREA NETWORK (LAN), METROPOLITAN AREA NETWORKING (MAN) & WIDE AREA NETWORK (WAN).

An interconnection of computers and peripheral devices contained within a limited geographical area utilizing a communication link and operating under some form of standard control is called as local Area Network. The term-localized area could mean a small room, twenty feet by ten feet, or it could mean a factory spanning several acres. Another definition, widely accepted, states that a LAN is a computer network that is confined to a building or a cluster of buildings. A LAN is a network that is typically personal to an Organisation and is installed for the exclusive use of an office or factory of a given Organisation. It is not often that you will come across two or more organisations in an office complex sharing a LAN. The traditional definition of wide area networking has been "connecting two or more networks existing at widely separate geographic sites." ' Suppose you connect two networks in two different buildings 100 meters apart by means of asynchronous modems and common telephone lines. Is that wide area networking? Most knowledgeable computer networking people would say no-this would be "onesite" or "campus" networking. What if the networks were two miles apart and separated by a major highway? Or, what if they were 15 kms apart, on opposite sides of a major city? There are many computer networking people who would still not call this Wide area networking; they might use a re~ently coined term"Metropolitan Area Networking. (MAN)". Others consider metropolitan area networking a part of wide area networking. Of course, everyone would agree that two networks connected on opposite sides of a continent by means of a satellite microwave link rented from a common carrier is an example of a wide area network.

54

Digital Network

Wide Area Network

Metropolitan Area Network

Local Area Network

INTERNATIONAL ORGANIZATION FOR STANDARDIZATION (ISO) The ISO was founded in 1946 and is currently headquartered in Geneva, Switzerland. Its mission at its inception was to create international standards regarding the threads of screws used for manufacturing worldwide. As needs for other standards arose, the ISO oth a server and a workstation. In a peer-to-peer network, the operating system is installed on every networked computer; this enables any networked computer to provide resources and services to all other networked computers. For example, each networked computer can allow other computers to access its files and use connected printers while it is in use as a workstation. Peer-to-peer operating systems have both advantages and disadvantages when compared to client-server operating systems. They provide many of the same resources and services as do clientserver operating systems, and under the right circumstances, can provide good performance. They are also easy to install and are usually inexpensive.

PEER TO

PEER

70

Digital Network

However, peer-to-peer networks provide fewer services than client-server operating systems. Also, the services they provide are a great deal less robust than those provided by mature, full-featured client-server- operating systems, and the performance of peer-topeer networks commonly decreases significantly under a heavy load. Advantages of a peer-ta-peer network: •

Less initial expense - No need for a dedicated server.

•

Setup - An operating system (such as Windows 95) already in place may only need to be reconfigured for peer-topeer operations.

Disadvantages of a peer-ta-peer network: •

Decentralized - No central repository for files and appl ications.

•

Security - Does not provide the security available on a client/server network.

Classification of LAN: Network Topologies : A network topology is the way the cabling is laid out. This doesn't mean the physical layout (how it loops through walls and floors), but ~ow the logical layout looks when viewed in a simplified diagram. You may hear many different names for the type of network you have: ring, bus, star, and so on. They all refer to the shape of the network schematic. Bus Topology: All devices in this topology share a common wire to transmit and receive data through in the bus topology (Figure). This approach is very economical, as a single cable is cheaper to purchase than several individual cables for each device. Additionally, a,single cable is easier to install than several cables. These apparent advantages of the bus topology are offset, however, by the difficulty in troubleshooting a problem in this layout scheme. Since all devices use the common wire, how do you

Local Area Networks

71

track down one that has gone bad? The worst-case scenario in this situation may involve a trouble-shooter visiting and detaching every unit on the LAN until the offending machine is located. In short, troubleshooting bus topologies may require a good pair of sneakers. On the ends of the common cable or bus, a device called a terminator is utilized to absorb signals that have traversed the entire length of the bus. Since everyone shares the same cable, no two machines can transmit at once or the bits of data from each will collide destroying both pieces of information. This event is called a collision and obviously too many of them can be disastrous to traffic flow on a network. The terminator's vital role in absorbing data that has traveled the whole network, ensures that bits of information do not reflect back across the bus. A data reflection can occur any time an electronic signal encounters a short (where the wires inside a ca,ble get connected accidentally) or an open (where the wires are left unconnected as in when a terminator is removed). The end result is the same reflected data collides with the "good" data on the LAN and traffic flow is impacted. Advantages of a Linear Bus Topology e

Easy to connect a computer or peripheral to a linear bus.

•

Requires less cable length than a star topology.

i?isadvantages of a Linear Bus Topology •

Entire network shuts down if there is a break in the main cable.

•

Terminators are required at both ends of the backbone cable.

•

Difficult to identify the problem if the entire network shuts down.

•

Not meant to be used as a stand-alone solution in a large building.

72

Digital Network T-COnnector

Coaxial Cable

PC Network Interface Card

PC Network Interface Card

+

t

Terminator

Terminator

I

f

STA R TOPOLOGY Today if you decide to install a lAN, your local LAN dealer will probablY suggest you look seriously at star topology networks. Star topology networks are nothing new, they just offer some benefits that are hard to overlook. Star topology derives its name from the arrangement of devices so that they radiate from a central point. At the central point we usually see a device generically called a hub (Figure).

Local Area Networks

73

Key to the benefits of the star topology is the hub unit which may vary in function from a simple signal splitter (called a passive hub) to one that amplifies and keeps statistics on data traveling through them (termed as an active and intelligent hub). -In fact, hubs may be sophisticated enough to selectively disconnect any machine connected to them that is misbehaving, as well as allow network operators to dial to them and monitor the performance of a single workstation. It's these advantages that make the star topology a popular choice in the networking market place. Hubs that amplify signals coming through are called active hubs or multiport repeaters. Star topologies do require more cable than a simple bus topology, but most use a relatively inexpensive type of cable called twisted pair cabling which helps control costs of wiring. The hubs themselves require expense and the level of that expense is directly attributasle to how complex. a hub is needed. Troubleshooting a star topology network is a bit easier than bus topology. At the very least, one may disconnect devices from a' central hub to isolate a problem as opposed to visiting each individual machine. Above this physical level of troubleshooting, there is hub management software that can report problems back to you. It's obvious how the central hub device offers advantages, but there is one 9rawback. The hub itself represents a single point of failure. If you lose a hub, you effectively lose all workstations attached to it. Quality and reliability of hub products you purchase cannot be over-stressed.

Digital Network

74

Advantages of a Star Topology •

Easy to install and wire.

•

No disruptions to the network when connecting or removing devices.

•

Easy to detect faults and to remove parts.

Disadvantages of a Star Topology •

Requires more cable length than a linear topology.

•

If the hub or concentrator fails, nodes attached are disabled.

•

More expensive than linear bus topologies because of the cost of the concentrators.

Ring Tapology Ring Topology describes the logical layout of token ring and FOOl networks. In this scheme, a ring is created to which each device (workstation, server, etc.) is attached (Figure 2-4). A special signal called a token travel around this ring visiting each machine, letting it know that it is that machine's turn to transmit. Since the token visits every node, every one gets the chance to transmit, creating a very "fair" LAN. This simplistic explanation belies the true complexity of ring topology systems available today. Token ring LANs, and their FOOl cousins, are the most sophisticated, fault-tolerant, and consequently, the most expensive systems available in the current market place. The logical creation of a ring allows information on such a LAN to travel in one direction. Since only one device is allowed to transmit at a time, collisions are not a problem on ring systems. Of course there are always problems that can occur like bad network cards or hub units that will bring a ring topology LAN to a grinding halt, but they are often very resilient. Typical ring system Network Interface Cards (NICs) contain the ability to perform what is known as signal regeneration. This means information received by them is copied and retransmitted at a higher amplification.

75

Local Area Networks

Since every piece of data traveling around a ring must visit each device, the signal gets regenerated numerous times: This feature allows for greater distances between nodes and increased chances that good data will completely traverse the ring.

Tree A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 4). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs. Cable

Terminator

I Advantages of a Tree Topology •

PoinHo-point wiring for individual segments.

•

Supported by several hardware and software vendors.

76

Digital Network

Disadvantages of a Tree T,?pology • Overall the type of cabling used limits length of each segment. • If the backbone line breaks, the entire segment go.es down. • More difficult to configure and wire than other topologies. Star-Wired Ring A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the (multistation access unit) of a star-wired ring contains wiring that allows information to pass from one device to another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring topology.

~Q(f~

Considerations when choosing a Topology: Money. A linear bus network may be the least expensive • way to install a network; you do not have to purchase concentrators. • Length of cable needed. The linear bus network uses shorter lengths of cable. • Future growth. With a star topology, adding another concentrator easily does expanding a network. • Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies. LAN Access Control Collision sense multiple access I collision detection In bus topology systems like this, all devices are attached to a common wire. As mentioned in a previous section, this means that only one device may use the common wire at a time. Since several devices may need to use the wire at once, machines are

Local Area Networks

77

said to be contending for the media. If the system is operating within tolerable limits, every machine will eventually get the opportunity to transfer data. Ethernet systems use a channel access method known as CSMA/CD, short for Carrier Sense Multiple Access I Collision Detection. Though this seems a lot of words, the meaning is quite simple. Carrier Sense means that each device checks the LAN before it starts transmitting to see if some other device is using the media then. If another signal (containing a "carrier") was present, then the device attempting to send would wait until the LAN is clear. Then it transmits its data. The collision detection part means that each workstation listens to make sure that only one signal is present on the LAN. In the event there are two then obviously the data from one device has collided with that of another. Once a workstation detects a collision, it sends out a series of 1 bits alerting the rest of the network. At that point everyone stops transmitting and each workstation waits a random amount of time before attempting to transmit again. A random number generator on-board each Ethernet card regulates the delay time. Local Talk LANs used by Macintosh PCs also use CSMA contention schemes, but these machines incorporate a technology called time- division multiplexing to allow avoidance of collisions. In fact, Local Talk Systems are said to be CSMAlCA systems, with CA standing for Collision Avoidance. The major advantage of contention systems is that devices may transmit when~ver they like just as long as the LAN is free. Consequently the overhead of devices waiting on the opportunity is generally low. Since any device can participate at almost any time, no attempt is made to prioritize LAN access in any way. However, as traffic increases in a contention system, collisions can become excessive, severely impacting the overall performance of the network. The capacity of the LAN may be far underutilized in this event. The other major disadvantage is that contention systems do not follow an easily predictable pattern of performance

78

Digital Network

degradation as traffic increases. The true loss in performance can only be guessed at statistically. Token Passing Scheme This technology is used for token ring systems.. Its incorporation along with complimentary fault-tolerance capabilities yield a LAN with a fair amount of sophistication, manageability and reliability. In this channel access method, a small signal called a token regularly visits each device. The token gives permission for the device to transmit if it needs to. If a transfer of data is needed, the device receives a set amount of time to broadcast its data. When it is done, the machine then retransmits the token to another machine giving that recipient permission to transmit, and so the system continues. This mechanism ensures opportunity for all devices to gain access to the, LAN. Because of its predictable behavior, token scheme LANs offer the advantage of priorities, where a certain group of devices may have enhanced access to the LAN if warranted. As traffic demand increases on a token LAN, the overall throughput of data rises as well, until a point is reached where the networks simply cannot accommodate anymore. The function in this case is somewhat like a waterwheel. The wheel itself receives water from a sluice. You may increase the capacity 'of the wheel, but the sluice can only hold so much water, so there is a finite limit to the throughput of 'the system.

Because the throughput characteristics of token lANs are so predictable, and because of the ch,aracteristics of traffic d~mand vs. throughput, these systems ,are ideal for heavy traffic situations. However, the complexity of such a LAN does come at some cost. Token systems require overhead to carry out their many functions including fault-tolerance. Plus, toke~,ring systems are considerably more expensive than Ethernet systems. Factors weighing in deciding which system to choose should include traffic. demand and budgetary restraints.

Local Area Networks

79

LAN Cabling Cable is the medium through which information usually moves from one network device to another. There are several types of cable, which are commonly used with LANs. In some' cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network.The various types of cables are as follows :

Twisted Pair Cabling Twisted pair cabling is the current popular favourite for new LAN installations. The marketplace popularity is primarily due to twisted pair's (TP's) low cost in proportion to its functionality. Phone companies have justified its usage through years of implementation, as it is the medium used by them to connect our world together. In many cases, the phone company during telephone installation removing the need to put in any new cabling for a local area network has already installed TP cabling in a site. The construction of TP is simple. Two insulated wires are twisted around one another a set number of times within one foot of distance. If properly manufactured, the twists themselves fall 'in no' consistent pattern. This is to help offset electrical disturbances, which can affect TP cable such as radio frequency interference (RFI) and electromagnetic interference (EM!). These "pairs" of wires are then bundled together and coated to form a cable.

> .'. . .....--

80

Digital Network

Twisted pair comes in two different varieties - shielded and unshielded. Shielded twisted pair (STP) is often implemented with Local Talk by Apple and by IBM's token ring systems. STP is simply TP cabling with a foil or mesh wrap inside the outer coating. This special layer is designed to help offset interference problems. The shielding has to be properly grounded, however, or it may cause serious problems for the LAN. Twisted pair cabling with no shielding is simply called unshielded twisted pair (UTP). TP cabling has been around a while and IS a tried and true medium. It hasn't been able to support high-speed data transmissions until relatively recently however. New development is focusing on achieving 100 Mbps throughput on UTP without costing the user an arm and a leg. A copper version of fiber optic's FOOl, called COOl, will continue to mature while standardization is worked out for 100 Mbps Ethernet systems by the mid 90s. Copper cable will not allow the speeds attainable with fiber optic cable. However, the standard for fiber stipulates LAN speeds of only 100 Mbps, far below the fiber optic cable's actual capacity. Twisted pair is grouped into certain classifications based on quality and transmission characteristics. IBM calls the classifications "types". UTP by itself is often grouped by "grades". Unshielded Twisted Pair Grades Grade 1 Suitable for voice transmission and data transfer upto 1 Mbps Grade 2 Capable of carrying data at 4 Mbps Grade 3 Carries data at upto 10 Mbps Grade 4 Rated at 20 Mbps Grade 5 Supports speeds at upto 100 Mbps Twisted Pair Cable Types (18M Standards) Type 1 Type 2 Type 3 Type 5

STP, two pair, 22 gauge, solid conductors, and braidedshield Type 1 cable with additional four pairs of UTP UTP, 22' or 24 gauge, 2 twists per foot, four pairs Fiber optic cable used to link -MAUs

Local Area Networks Type 6 Type 8

81

Two pair, stranded (not solid) 26 gauge, patch cables Two pair, 26 gauge, and untwisted but shielded cable

Twisted Pair Cable Advantages 1.

2. 3.

Disadvantages

Inexpensive Often available in existing phone system Well tested and easy to get

1. Susceptible to RFI and EMI 2. Not as durable as coax

3.

Doesn't support as high a speed as other media

Coaxial Cable Coaxial cable or just ·coax" enjoys a huge installed base among LAN sites in the US. It has fit the bill perfectly for applicati9ns requiring stable transmission characteristics over fairly long distances. It has been used in ARC net systems, Ethernet systems and is fometimes used to connect one hub device to another in other systems. This is due to coax's superior distance allowances. Construction-wise coax is a little more complex then TP. It is typically composed of a copper conductor that serves as the ·core" of the cable. Thts conductor is covered by a piece of insulating plastic, which is covered by a wire mesh serving as both a shield and second conductor. PVC or other coating then coats this second conductor. The conductor within a conductor sharing a single axis is how the name of the cable is derived. Shielding

Outer Jacket

Insulator

Coaxial cable's construction and components make it superior to twisted pair for carrying data. It can carry data farther and faster thSl.n TP can. These characteristics improve as the size of the coax

82

Digital Network

increases. There are several different types of coax used in the network world. Each has its own RG specification that governs size and impedance, the measure of a cable's resistance to an alternating current. One must be cautious in acquiring coax to make sure the right kind has been obtained. Different cable can differ widely in many important areas.

Common Coaxial Cable Types Used In Networking Type Common Usage Impedance RG-8 Thick Ethernet 50 Ohms RG-ll Broadband LANs 75 Ohms RG-58 Thin Ethernet 50 Ohms RG-59 Television 75 Ohms RG-62 ARC net 93 Ohms

Coaxial Cable

Advantages

Disadvantages

1. Fairly resistant to RFI and EMil. Can be effected by strong interference 2. Supports faster data rates 2. More c~stly than TP than twisted pair 3. More durable than TP 3. Bulkier and more rigid than TP

Fiber Optics Carrying data at dizzying speeds, fiber has come into its own as the premier bounded media for high-speed LAN use. Because of fiber's formidable expense, however, you're not likely to see it at the local workstation any time real soon. Instead, fiber is used to·link vital components (like file servers) in a LAN or multi-LAN environment together. Consequently we often hear-terms like ufiber backbone" thrown around. Fiber optic is unsophisticated in its structure, but expensive in its man'ufacture. The crucial element for fiber is glass that makes up the core of the cabling. The glass fibers may be only a few microns thick or bundled to produce something more sizable. It is worth noting that there are two kinds of fiber optic cable

Local Area Networks

83

commercially available - single mode and multimode. Companies like AT use single mode in'the telecommunications industry and T or US Sprint to carry huge volumes of voice data. Multimode is what we use in the LAN world. The glass core of a fiber optic cable is surrounded by and bound to a glass tube called "cladding". Cladding adds strength to the cable while disallowing any stray light wave from leaving the central core. A plastic then surrounds this cladding or PVC outer jacket with provides additional strength and protection for the inwards. Some fiber optic cables incorporate Kevlar fibers for added strength and durability. Kevlar is the stuff of which bulletproof vests are made" so it's tough. Outer Jacket CladdIng

FE:---l::==::J Optic Core Kevlar Fibres

Fiber optic is lightweight and is utilized often with LEOs (LightEmitting Diodes) and ILDs (Injection Laser Diodes). Since it contains no metal, it is not susceptible to problems that copper wiring encounters like RFI and EM!. Plus, fiber optic is extremely difficult to tap, so security is not a real issue. The biggest hindrance to fiber is the cost. Special tools and skills are needed to work with fiber. These tools are expensive and hired skills are expensive too. The cable itself is pricey, but demand will ease that burden as more people invest in this medium. Attempts have been made to ease the cost of fiber. One solution was to create synthetic cables from plastic as opposed to glass. While this cable worked, it didn't possess the near capabilities of glass fiber optic, so its acceptance has been somewhat limited. The plastic fiber cables are constructed like glass fiber only with a plastic core and cladding.

84

Digital Network

The, bandwidth or capacity of fiber is enormous in comparison with copper cabling. Multimode fiber can carry data in excess of 5 gigabits per second (that's million megabits). Single mode fiber used in telecommunications has a theoretical top speed in excess of 25,000 Gbps. That much data is the equivalent of all the catalogued knowledge of man transmitted through a single small glass tube in less than 20 seconds. That's impressive. The standard governing implementation of fiber optic in the marketplace is called the Fiber Oistributed Oata Interface standard or FDOL FOOl specifies the speed of the LAN, the construction of the cable, and distance of transmission guidelines. FOOl behaves very much like token ring, only much faster. An added feature for FOOl is a backup ring in case the main ring fails. This fault tolerance along with the fault tolerance already incorporated in token ring technology makes FOOl LANs pretty resilient. One minor drawback for fiber optic LANs is that they can be difficult to layout. Fiber Optic Cable

Advanta es

Disadvantages

1. Highly secure

1. Extremely costly in product and service

2. Not affected by RFI and EMI

2. Sophisticated tools and methods for installation

3. Highest bandwidth available 4. Very durable

3. Complex to layout and design

LAN PROTOCOLS A protocol is a set of rules that governs the·communications between computers ona network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer.

Local Area Networks

85

The most common protocols are: Ethernet The Ethemet protocol is by far the most widely used. Ethemet uses an access method called CSMNCD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is dear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this acces.s method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not nOrTllally effect the speed of transmission on the network. The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over twisted pair, coaxial or fiber optic cable at a speed of 10 Mbps.

.

Fast Ethernet

To allow for an increased speed of transmission, the Ethernet protocol has developed a new standard that supports 100 Mbps'. This is commonly called Fast Ethernet. Fast Ethernet requires the use of different, more expensive network concentratorslhubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. local Talk Local Talk is a network protocol that was developed by Apple Computer, Inc. for Macintosh Computers. The method used by Local Talk is called CSMNCA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMNCD e.xcept that a computer signals its intent to transmit before it actually does so. Local Talk ad~pters and special twisted pair cable can be used to connect a series of computers through the

86

Digital Network

serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of Apple Share software, a client/server network can be established. The local Talk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of local Talk is speed. Its speed of transmission is only 230 Kbps.

Token Ring IBM developed the Token Ring protocol in the mid-1980s. The access method used inv.olves token passing. rn Token Ring, the computers are connected so that the signal travels around the network from one computer to another in ~ logical ring. A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the receiving computer captures the data. The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased.

a

FDOI Fiber Distributed Data Interface (FOOl) is a network protocol that is used primarily to interconnect two or more local area networks, often over large distances. The access method used by FOOl involves token passing. FOOl uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new. complete ring. A major advantage of FOOl is speed. It operates over fiber optic cable at 100 Mbps.

87

Local Area Networks

Protocol Summary

Protocol

Cable

Speed

Topology

Ethernet

Twisted Pair Coaxial, Fiber

10 Mbps

Linear Bus, Star, Tree

Fast Ethernet

Twisted Pair, Fiber

100 Mbps

Star

Local Talk

Twisted Pair

23 Mbps

Linear Bus or Star

Token Ring

Twisted' Pair

4 Mbps16 Mbps

Star - Wired Ring

FOOl

Fiber

100 Mbps

Dual Ring

LAN STANDARDS Institute of Electrical and Electronic Engineers (IEEE) The IEEE has done notable work in the standards area of networking. This organization is huge with over 300,000 members made up of engineers, technicians, scientists, and students in related areas. The Computer Society of IEEE alone has over 100,000 members. IEEE is credited with having provided definitive standards in local area networking. These standards fall under a group of standards known as the 802 Project executed by the Computer Society's 802 subcommittee. The 802 standards were the culmination of work performed by the subcommittee starting in 1980. The first published work was 802.1, which specified a framework for LANs and intemetworking. This was followed in 1985 with specific LANoriented standards titled 802.2 - 802.5. Since that time there have been other references set up as well. Most of the work performed by the 802 Project committee revolves around the first two layers of the 051 model initiated by the ISO. These layers involve the physical medium on which we move data (cable type) and the way that we interact with it. It addresses such crucial issues of how data is placed on the network and how we insure its accura«!:y

88

Digital Network

and flow. In order to better define these functions, the IEEE split the Data ~ink layer of the OSI model up into two separate components. Here is a summary of what committees there are and what standards areas are being defined within IEEE: 802 IEEE committee responsible for setting standards concerning cabling, physical topologies, logical topologies and physical access methods for networking products. The Compute~ Society of IEEE's 802 Project Committee is divided into several subcommittees that deal with specific standards in these general areas. Specifically the Physical layer and the Data link layer of the ISO's OSI model are addressed. 802.1 This work defines an overall picture of LANs and connectivity. 802.1 B This set of standards specifically address the network management. 802.1 D Standards for bridges used to connect various types of LANs together were set up with 802.1 D. 802.2 Called the LogiCal link Control (LLq standards, this specification governs the communication of packets of information from one device to another on a network. Specifically it deals with communication, not access to the network itself. 802.3 Defines the way data has access to a network for multiple topology systems using Carrier Sense Multiple Accessl Collision Detection (CSMNCD). A prime example is Ethernet and Star LAN systems. These LAN types operate at 10 Mb/sec. 802.4 Standards developed for a token-passing scheme on a bus topology. The primary utilizer of this specification was the Manufacturing Autpmation Protocol LANs developed by General Motors. Operates at 1 0 Mb/sec. 802.5 This standard defines token ring systems. It involves the token-passing concept on a ring topology with twisted pair cabling.

Local Area Networks

89

IBM's token ring system uses this specification. The speed is either 4 Mb/sec. or 16 Mb/sec. 802.6 ty\etropolitan Area Networks are defined by this group. MANs are networks that are larger than lANs typically falling within 50 kilometers. They operate at speeds ranging from 1 Mbl sec. up to about 200 Mb/sec. 802.7 These are standards concerning broadband lANs. 802.8 This group sets up standards for lANs using fiber optic cabling and access methods. 802.9 This specification covers voice and digital data integration. 802.10 These members set standards for interoperable security. 802.11 Wireless lANs are the subject of this particular subcommittee's works. Both infrared and radio lANs are covered. Advantages of LAN over Mini and Mainframe Computers Mainframe Computers or Mini Computers have a huge processing power. Many users are attached to the CPU with the help of 'dumb terminals'. Though, the processing power and number of people interacting with the computer is great, economically, such computing power would be very expensive. The advantage of the Mainframe and Mini systems can be had at a fraction of the cost with networking with PCs. Flexibility is another advantage of the networked PCs. The setup and operations of Mainframe and Mini systems are rather rigid giving very little room for the flexibility in design and approach. LAN on the contrary is modular which can be altered as per the user requirement. Scalability, (changing capacity of computing) is very difficult and time and money consuming for Mainframes and Minis while LAN is ideally suitable for this. Skilled and highly qualified engineers are required for the operations of Mainframe and Minis whiles users themselves can

90 '

Digital Network

manage LANs without any problem. Even the installation and commissioning is extremely easy for LANs. In short, when the user applications does not require processing power very high LAN offers tremendous benefits over Mainframes and Minis. BIBLIOGRAPHY

1.

Local Area Network Design - Andrew Hopper, Steven Temple & Robin Williamson

2.

Local Area Network Architectures - David Hutchison

3.

Telecommunication Switching Systems & Networks - Thiagarajan Vishwanathan

4.

How Networks Work (4th Edition) - Frank

5.

NUT Course Material

6.

Information from the Internet

J.

Derfler, Jr. & Les Freed

MAN [Metropolitan Area Network]

"This page is Intentionally Left Blank"

Chapter

4

MAN (Metropolitan Area Network) •

Metropolitan Area Network (MAN) •

DQDS (Distributed Queue Dual Bus) Structure

•

•

Data Transfer in DQDB

Asynchronous Transfer Mode (ATM) •

PSTN (Public Switched Telephone Network) Structure

•

Circuit & Packet Switching

•

Asynchronous & Synchronous Transfer Mode

•

Introduction to ATM

•

Benefits of ATM

•

ATM Technology

•

Classes of Services in ATM

"This page is Intentionally Left Blank"

MAN [Metropolitan Area Network1

95

MAN (METROPOLITAN AREA NETWORK) Introduction to MAN A Metropolitan area network is basically a bigger version of LAN and uses similar technology. It might cover a group of near by corporate offices or a city and might be either private or public. It can support both data and voice and might even be related to the local cable television network. The key aspect of MAN is that there is a broadcast medium, to which all the computers are attached. For networks covering an entire city, IEEE (Institute of Electrical & Electronics Engineers) defined one MAN called DQDB (Distributed Queue Dual Bus), as standard 802.6. The basic geometry of 802.6 is shown in the diagram on the left side. Two parallel uni-directio.nal busses make through the city with station5 attached to both busses in parallel. Each bus has a head end, which generates a steady stream of 53 byte cells. Each cell travels down stream from the head end. When it reaches the end it falls off the bus. Traffic that is destined for a computer to the right of the sender uses the upper bus. Traffic to the left use~ the lower one. Each cell carries a 44 byte payload field, and it also holds two protocol bits, busy set to indicate that a cell is occupied, and request, which can be set when a station wants to make a request. Unlike all the other 802 LAN Protocols 802.6 is not greedy. In all the others, if a station gets the chance to send, it will. Here stations queue up in the order till they become ready to send and transmit in FIFO orde·r. The basic rule is that stations are polite: they defer to stations downstream from them. This politeness is needed to prevent a situation in which the station nearest to the head end simply graps all the empty cells as they come by and fills them up, starving every down stream.

96

Digital Network Bus A

Direction of flow on bus A _

Computer

Bus B

-

Direction of floW on bus B

To stimulate the F1FO queue, each station maintains two counters, RC and CD. RC (Request counter) counts the number of downstream request pending until the statjon itself has a frame to send at that point RC is copied to CD, RC is reset to 0, and now counts the number of request made after the station became ready. For example i.f CD = 3 and RC = 2 for station k, the next three empty cells that pass by station k are reserved for downstream stations, then station k may send, then two more cells are reserved for down stream stations. For simplicity in the discussion below it is assumed that a station can have only one cell ready for transmission at a time.

MAN [Metropolitan Area Network]

97

5.1.2 Data Transfer in DQDB: To send the cell, a station must first make a reservation by setting the request bit in some cell on the reverse bus (i.e. on bus B for a transmission that will later take place on bus A). As this cell propogates down the reverse bus, every station along the way notes it and increments its RC. Initially all the RC counters are 0 and no cells are queued up as shown in the figure. Then station 0 makes a request, which causes station C, B, and A, to increment their RC counters, After that 0 makes a request copying its current RC value in CD. At this point the head end on bus A generates an empty cell. As it passes by B, that station sees that its CD > 0, so it may not use the empty cell (when a station has a cell queue, CD represents its position in the queue, with 0 being front of the queue). Instead it decrements CD. When the still empty cell gets to B, that station sees that CD=O, meaning that no one is ahead of it on the queue, so it inserts its data into the cell and sets the busy bit. When the next empty cell is generated, station D sees that it is now at the head of the queue, and seizes the cell (by setting 10 Regional offices (fu\y

~~~~~§~~~§~~~~~Interconnected)

19,000 End offices t

234 5

200 Million telephones

19,00('

98

Digital Network

one bit). In this way s~tions queue up to take turns without a centralized queue manager. This process is illustrated in the diagram shown on the left page. Many carriers throughout the entire cities are now installing OQOB systems. Typically they run for upto 160km at speeds of 44.736 Mbps.

ATM (ASYNCHRONOUS TRANSFER MODE) The PSTN (Public Switched Telephone Network): In AT&T system which can be looked at as a general model the telephone system has five classes of switching offices as shown in the diagram on the left page. Calls _are generally connected at the lowest possible level. Thus if a subscriber connected to end office 1 calls another subscriber connected to end office 1 the call will be completed to that office. However calls from customer attached to end office 1 to a customer attached to end office 2 will have to go to toll office 1. However a call from end office 1 to end office 4 will have to go to primary office 1, and so on. With a pure tree, there is only one minimal route, that could normally be taken. During years of operation, the telephone companies noticed that some routes were busier than others e.g. there were many calls from New York to los Angeles. Rather than go all the way up the hierarchy, they simply installed direct trunks for the busy routes. Few such lines are shown in the- figure as dashed lines. As a consequence many calls can now be routed along many paths. The actual route choosed is generally the most direct one, but if the necessary trunks along it are full, the alternative is chosen.

"

Switching office

99

MAN [Metropolitan Area Network) Circuit Switching and Packet Switching:

Two different switching techniques are used in the telecommunication systems namely Circuit Switching & Packet Switching. Circuit

S~itching

When a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from the senders telephone to the receivers telephone, this technique is called Circuit Switching and is shown schematically in the diagram on the left page. In the earlier days of telephone the connection was met by having the operator plug a jumper cable into the input and output sockets. Once the call has been s~tup, a dedicated path between both ends exists and will continue to exist until the call is finished.

r~{·~ni

........ -:,.

,Pkt 2~ ....• '. ::.:...:::.::.: ~Pkt 1.: . . . . . . . . .4'

'Pkt 3,

!:':;~.;:~ ;:::k. ."" ". 8~f:1') ~n~M~ .~nm :lutgoin!

AB

BC

CD

trunk

trunk

trunk

B

~i............ _! : ........... '~

iPkt2.

·1-.. •.. ; - ."",

CoO accep1

>

A

r ••••

-

for an

Time

J.

: p~. ~.I ....... '_' ". .. Pkt 2:: "_ " " .-... : .~PKt 1~ ,,_ ... ...~ .....,;. ;:.~

Time -" .... spent hunting

C

D

A

B

D

An important property of circuit switching is the need to setup an end-to-end path before any data can be sent. The elapsed time. between the end of dialing and ,the start of ringing can easily be 10 seconds, more, on long'distant or international calls_ During this I time 'interval the telephone system is hunting for a copper path,

100

Digital Network

Before data transmission can even begin the call request signal must propogate all the way to the destination and be acknowledged. For many computer applications (e.g. point of sale credit verification) long set up time are undesirable. Once the set up is completed the only delay for data is the propogation time for the electro magnetic signal about 5 msec. per thousand kms. As a result of the established path there is no danger of congestion i.e. once_ the call is put through, you never get busy signals, although you might get one before the connection has been established due to lack of switching or trunk capacity.

Packet Switching : In packet switching fixed length blocks or packets of information are sent over the transmission line. By making sure that no user can monopolize any transmission line for very long (milliseconds) packet switching networks are well suited for handling interactive traffic. The further advantage of packet switching is that the first packet of a Multi-packet message can be forwarded before the second one has fully arrived, reducing delay and improving throughput.

Parameter

CircuitSwitched

PacketSwitched

Dedicated Ncopper" path

Yes

No

Bandwidth available

Fixed

Dynamic

Potentially wasted Bandwidth

Yes

No

Store-and-Forward transmission

No

Yes

Each packet follows the same route

Yes

No

Call setup

Required

Not required

When can congestion occur

At setup time

Every packet

Charging

Per minute

Per packet.

101

MAN [Metropolitan Area Network) M~_-----T1 frame(125M5eC.)

'-Channel 1 gets exactly 1 t:

-----601

~e at the start of each frame-.l

(a)

There is no requirement about cell ordering (b)

5.2.3

H 1 cell (53 bytes)

Synchronous & Asynchronous Transfer mode:

Analog signals at digitized in the end office by a device called Codec (Coder Decoder), reducing a 8bitnumber. The Codec makes 8000 samples per second (125 micro second per sample) because the Nyquist Theorem says that this is sufficient to capture all the information from the 4-kHz telephone channel bandwidth. At a lower sampling rate, information will be lost; at a higher one, no extra information would be gained. This technique is called PCM (Pulse Code Modulation). One method that is in wide spread use in North America and Japan is the TI carrier. The T1 carrier consists of 24-voice channel mUltiplex together. Usually, the analog signals are sampled on a round robin basis with resulting analog screen being fade to the Codec rather than having 24 separate codecs and then merging the digital output. One T1 frame is generated precisely every 125 micro second. This rate is govern by a master clock. ATM in contrast has no requirement that cells rigidly alternate ~ among the various sources. Cells arrive randomly 'from different

sources with no particular pattern. Benefits of ATM

The benefits of ATM are the following: • High performance via hardware watching • Dynamic bandwidth for bursty traffic

102

Digital Network

• • • • •

Class-of-service support for multimedia Scalability in speed and network size Common LANIWAN architecture Opportunities for simplification via VC architecture International standards compliance.

The high-level benefits delivered through ,ATM services deployed on ATM technology using International ATM standards can be summarized as follows: High performance via hardware switching with terabit switches on the horizon.