Computer Networking

Introduction to Computer Networking:

      Computer networks allow people and machines to communicate, using a number of services. A computer network is made of two distinct subsets of components

- distributed applications are programs running on interconnected computers; a

web server, a remote log in server, an e-mail exchanger are examples. This is

the visible part of what people call “the Internet”. In this lecture we will

study the simplest aspects of distributed applications. More sophisticated

aspects are the object of lectures called “Distributed Systems” and

“Information Systems”.

- the network infrastructure is the collection of systems which are required

for the interconnection of computers running the distributed applications. It

is the main focus of this lecture.

The network infrastructure problem has itself two aspects:

- distance: interconnect remote systems that are too far apart for a direct

cable connection

- meshing: interconnect systems together; even in the case of systems close to

each other, it is not possible in non-trivial cases to put cables from all

systems to all systems (combinatorial explosion, cable salad management

problems etc.).

The distance problem is solved by using a network, such as the telephone

network with modems (see later). The meshing problem was originally solved

easily because the terminals were not able to communicate with each other, but

always has to go through a main computer. The mesh in such cases is reduced to

a star network. Today this is solved by a complex set of bridges and routers.

Like any complex computer system, a network is made of components.

This decomposition is, to a large extent, stable: computer networking people

have agreed on a reasonable way to divide the set of functions into what is

called “layers”.

We use the term layer because the decomposition always assumes that different

components can be ordered such that one component interfaces only with two

adjacent components. We call “layers” the components.

We start with the simplest, and the oldest network example: it is a mainframe

connected to terminals. In this case, there are mainly two functions

• physical layer: translates bits into electromagnetic waves;

• data link layer: translates frames into bits.

These two functions are implemented on cables or on radio links. The physical

layer has to do with signal processing and coding; it is the object of the

lecture called “Telecommunications”. The data link layer has to do with bits

and bytes; we will study the data link layer in this lecture.