Introduction
Some Terminology
LAN, MAN and WAN Characteristics
WAN Definition
Communication between two nearby computers
Communication between two distant computers
Making Communication Modular
Communications Architectures
History of OSI vs TCP/IP
Conclusion
Reading References
Web References
Example Networks

Introduction

To communicate between two (or more) separate computers we need to make use of communication channels such as cables or radio or light techniques. By signaling via the communication channels, messages can be sent between separate entities. This course will explore the techniques that are used when these entities are far apart and have to use the facilities of a carrier such as the telephone network to enable the communication.

Some Terminology

Before embarking upon this course, it will be instructive to define a few networking terms.

Network - A computer network is a series of points or nodes interconnected by communication paths. Here, the nodes are computers and the communication paths are cables or free space. Networks can interconnect with other networks and contain subnetworks.

Subnetwork - A small autonomous network. End systems communicate directly with each other, e.g. Ethernet, Token Ring, FDDI

Internetwork - A collection of subnetworks linked by internetworking devices such as bridges, routers, gateways

Defining LAN, MAN and WAN

A loose definition is that they are all networks. However, we ned to be more precise if we are going to study WANs in detail. There are many differences between a local area network LAN, a metropolitan area network MAN and a wide area network WAN.

Some such as size and coverage are listed in the table below. Can you think of any more?

WAN - a definition

A WAN  is a geographically dispersed telecommunications network and the term distinguishes a broader telecommunication structure from a LAN. A WAN may cover a few miles to the entire globe. A WAN may be privately owned or rented, but the term usually connotes the inclusion of public (shared user) networks. An intermediate form of network in terms of geography is a metropolitan area network (MAN).

An example of a WAN is the Internet.

Communication between two nearby computers

When we wish to establish communications between two computers that are linked with a cable (or other communication medium), there are several steps that we need to undertake in order that the communication can take place.

• Source system must activate the transmission medium or identify its intended destination
• Source system must ascertain that destination system is prepared to accept data
• Source system's file transfer application must communicate with destination system's file management system
• If data formats are different, one system must perform format translation
• Destination system must control data arrival rate to avoid buffer overflow
• Errors in arriving data must be identified and then corrected

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When the communication is to be achieved between two distant computers that are separated by multiple transparent networks, the steps that are involved above must also include

• Source system must not flood the intervening network(s)
• MTU (maximum transmission unit) of networks to be negotiated or fragmentation must take place
• Addressing and routing to be performed transparently
• & many more

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To be able to overcome these different problems that are posed by communications, a generalised set of procedures for a communications model has been defined. This is known as the ISO OSI model. It has seven stages that encompass all of these problems. For a more complete discussion of the OSI model, refer to previous notes.

In these stages or layers are protocols that must be adhered to during communications.

A protocol is a set of rules that are implemented in software that govern the exchange of data between two entities. In the sphere of computing, a protocol has three key elements:-

Syntax - this covers data format and signal levels

Semantics - control information for co-ordination and error handling

Timing - speed matching and sequencing

Exercise

Think of an algorithm that could be used to implement the stop-and-wait protocol that is used for flow control. Describe this algorithm using a flow diagram.

Communications Architectures

OSI is fine in theory, but in practice protocol stacks such as the TCP/ IP model are used. Remember that OSI is only a reference model, a yardstick by which we can measure other communications architectures.
 

Some examples of practical communications architectures are TCP/ IP, WAP, SNA.

Below is a comparison between OSI and TCP/ IP.

Figure 1-1 Comparing TCP/ IP to OSI model

History of OSI vs TCP/IP

During the latter part of the 1970s there was a universal realisation that interoperability of different manufacturers' equipment was going to be necessary to encourage and promote the use of computers for communications. To help bring this about, the ISO developed the OSI model for communications. Unfortunately, they were too late because the TCP/ IP model had already been developed and was fully functional. Thus TCP/ IP was almost universally adopted. The main use of OSI model is in teaching and comparisons with other communications models.

Overview of the operation of  TCP/IP architecture

Refer to figure 1-2. The process (application) is associated with port 1 on host A and wishes to communicate with a process associated with port 2 on host B. The process on host A sends the message to TCP with instructions to send to host B, port 2. TCP hands the message down to IP with instructions to send to host B. IP does not need to know the destination port as this has been taken care of by TCP. IP then passes the message down to the network access layer, Ethernet in this case with instructions to send the message to a router R which is the first hop on the route to host B.

Figure 1-2 Using TCP/ IP to deliver a message

For data to be transferred between two similar applications on remote computers to take place, extra information other than the data generated by the application has to be transmitted too. For each piece of data passed to TCP, there is some control information too that is added by TCP in the form of a header, a few bytes of information that is prepended to the data. This forms a TCP segment. This information will be used when the TCP segment is examined by host B to pass the data to the correct port. It is not used or seen at any other point between host A and B.

Some of the information included in the TCP header are:

• Destination port number for the remote host's application
• Sequence number. TCP numbers the segments in case they arrive at the destination out of order so that TCP at host B can re-order the segments correctly
• Checksum. This is in the form of a cyclic redundancy check (CRC) and is used to check that the data have arrived correctly and not been corrupted during the transmission process.

This IP datagram is presented to the network access layer for transmission across the first subnetwork in its journey towards its destination. The network access layer then appends its own header containing information that is required to transfer data across this first subnetwork.

Information contained in the network header includes:

• Subnetwork destination address. Here this will be a router (in our case R).
• Facilities request. The network access protocol may request use of subnetwork facilities such as priority.

The data will then be delivered to host B where firstly the network header is stripped off. The IP header is removed too, revealing the TCP data (port address). The TCP layer in host B then is able to remove the TCP control information and pass the (original) data to the correct port for the residing application to decode and present to the user.

Conclusion

A WAN is a geographically separated network of computers that often use a bearer network to make the long haul connections between the end nodes. Because of the complexity of the task of arbitrarily connecting any two computers across any number of different intervening networks, a set of tasks was drawn up by the ISO to help achieve this communication.

These tasks are implemented in the TCP/IP model that is used widely in today's communications via the Internet, although other schemes such as WAP exist to overcome different problems.

Reading References

Standards bodies:

Tannenbaum, Computer Networks, Prentice Hall, 3rd Edition,  pp 28-38

Comparison of standards, Tannenbaum p38

Problems with OSI, why it never happened, Tannenbaum pp40 -41

Problems with TCP/ IP, Tannenbaum p43

Web References

http://www.atmforum.com/atmforum/specs/specwatch.htm

http://www.atmforum.com/atmforum/specs/approved.htm

 http://www.frforum/5000/5000index.htm

http://whatis.techtarget.com/

Example Networks
 

Arpanet, Tannenbaum p47

NSFNET, Tannenbaum p50,

Internet, Tannenbaum p52 uses, email, news, remote login, file transfer

Transport Schemes, Tannenbaum p66

SMDS, X25, Frame Relay, BISDN & ATM

(C) M M Clements 25/01/01