Overview of Computer Networks

Basically, TCP/IP is ubiquitous today for general-purpose usage. But there are other protocols for specialpurpose

usage, such as one designed for fast communication in parallel processing applications.

Since the protocol ID (e.g. 0x0800 for TCP/IP, 0x8137 for Netware, etc.) will be placed into the bit frames

going out onto the Ethernet, each computer on the network will be able to determine what protocol an

incoming message was sent under. This is especially useful if the same computer is running two or more

protocols. An incoming message from the Ethernet at the computer causes a hardware interrupt, and the

interrupt service routine can then pass on the message to the proper protocol software, according to the

protocol ID specified in the Ethernet frame.

The jagged line between citroen.abc.com and angus.moo.edu is a phone line, dedicated to communication

between the two machines, operating 24 hours a day.

So you can see that there are two ways that we can connect two LANs together:

• We can install a computer and connect it to both LANs.

• We can put in a phone line (or microwave link, etc.) between a computer on one LAN and a computer

on another LAN.

3 The Famous, Overrated But Useful 7-Layer Model

Every network textbook includes a picture of the famous "seven-layer" model. Actually, this model is vague,

and it does not always correspond to specific portions of specific networks. Nevertheless, it serves as a useful

overview of the field. Here is how some of the layers relate to our sample network above.

3.1 Overview of the Layers

The layers collectively are often referred to as the protocol stack.

3.1.1 Physical Layer

This is concerned with the nature of the physical media (metal or optical cable, free-space microwave, etc.)

used to send signals, the nature of the signals themselves, and so on.

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There is also the question of signal form; the signals themselves may be in the form of pure 0-1 bits, or may

be in the form of certain frequencies. In addition there are questions concerning how a receiver distinguishes

two bits which are adjacent in time.

A major issue is the form of the medium, both in terms of the materials it uses and its topology. A basic

wired Ethernet, for example, consists of cable conducting electrical signals; the connections could also be

wireless. More complicated networks, including Ethernets, may consist of more than one cable, with all of

them connected via a hub. The latter has become common even at the household level.

3.1.2 Data Link Layer

For example, in an Ethernet, this layer is concerned with ensuring that two network stations connected to

the same cable do not try to access the line at the same time.4 For this reason the Ethernet operation is an

example of what is called a Medium Access Control (MAC) Protocol.5