INTERNETWORKING

The challenge of enterprise networks is to achieve invisible interoperability, the ability to allow users from one part of the network to access others on the network despite the differences in underlying technologies and hardware deployed. Interoperability is to do with protocols, and as part of these tutorials, we will be taking a closer look at these protocols to see how large-scale networks achieve this.

The Internet is best example of this interoperability, with millions of users every day traversing a whole array of networks to obtain information which may be stored anywhere in the world. This is the sheer power and incredible achievement of standardisation and international agreements on what protocols to use to enable this to happen. The user does n’t know, indeed does n’t need to know the underlying technologies, the different LANs/WANs traversed and protocols used, as long as it works, the wires and boxes do not matter. But for our purposes, we want to learn how do we connect things up together to make this happen, we are very much interested in the devices/boxes that allow us to achieve this.

To start this investigation, we need to consider these individual components and what do they mean.

LOCAL AREA NETWORKS (LANS) & WIDE AREA NETWORKS (WANS)

IEEE definition of a LAN is as follows:

A Local Area Network is distinguished from other types of data networks in that the communications are usually confined to a moderately sized geographical area, such as a single office building, warehouse, or a campus area, and can depend on a physical communications channel of moderate to high data rate which has consistently low error rate.

In contrast, WANs will cover large geographical areas, spanning several cities, a whole country or even several countries.  LANs are broadcast networks, shared media (cable) environments and are described as connectionless data is sent onto the media without first establishing a ‘connection’ with the device to which it is being sent (Note: SMDS is strictly connectionless, but we will cover this somewhere else). WANs generally speaking are connection-oriented environments.

I have an interesting side note here, which concerns the remarkable exponential rise of the internet to become an ubiquitous network which we all enjoy today.  When I first started my career in telecommunications in the UK, we were busy working with the CCITT standards committees in defining and implementing a connection-oriented protocol called X25.  I was employed to write software in Z80 assembler language using this protocol, and we developed multiplexors called X25 switches and other devices called Packet Assemblers and Disassemblers.  We would write our software, convert it into machine code and download it into Eraseable Programmable Read Only Memories (EPROMS) so it could be tested on our development platforms.  The X25 protocol relied on the concept of establishing a connection by generating a Call Request packet to the host (computer) you wished to send data to, and once the host accepted the request with a Call Accept packet, the connection (a switched virtual circuit, SVC) was established and you could send your data freely.  Once the data had been sent, the connection was closed down with a Clear Request packet which the host confirmed with a Clear Confirmation packet.

The reason for going into it in some detail, is to demonstrate how robust the system was designed to be so that it could be deployed by the banking services community with trust, and indeed I was involved in the development of the software that allowed the clearing backs to have a common timing source for the CHAPS clearing house system.  Meanwhile, in the US, the department of defence were busy developing TCP/IP protocol stack, the important part of which, IP, was the Internet Protocol, a connectionless protocol, in which the ordering of data packets was left to the higher layers of the OSI stack.  And we all know, which protocol survived to this day to become king of the network protocols!!!

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