=[ x25-box.com ]=

• What is the style of addresses-numeric, alphabetic and so on.
How are addresses assigned in the network, and how do you
ensure there are no duplicates?

• What happens if the packet is corrupted in transit?

• What happens if the packet is completely lost in transit?

• What happens if there is more than one conversation going on?

Even if these issues are resolved in a particular network then how are the
issues resolved in equipment from different manufacturers and how is
connection between dissimilar networks achieved?

Fortunately, international standards bodies, such as the International
Consultative Committee for Telephony and Telegraphy (CCITT), the
Institute of Electrical and Electronic Engineers (IEEE), and the International
Organization for Standardization (ISO) have addressed these issues and
produced recommendations and standards. ISO have developed a model
of a communication network which divides the problems into seven groups,
and this is referred to as the Reference Model for Open Systems
Interconnection. This is defined in the ISO 7498 standard. More concisely
it is referred to as the ISO OSI Seven Layer Model.

The model is discussed in more detail in Chapter 6; however, for the
moment we need only be concerned with the bottom three layers:

• Layer 1
Covers the problems of getting a data bit from one component,
via a transmission medium, to another component.

• Layer 2
Covers the problems of ensuring that the bits are transferred
with error notification.

• Layer 3
Covers the problems of synchronizing the two users of the
network.

The model only relates the functions of the layers and does not attempt
to solve the problems. A network must therefore choose from a range of
standards which are practical implementations of the narrow layers. Since
the model defines what tasks are performed in each layer, any choice of
standards is satisfactory as long as each standard conforms to the service
required from the layer of the model.

In order that equipment from various manufacturers can interwork,
and so that different networks can successfully be connected together, it
is necessary that everyone chooses the same standards. There are therefore
a limited number of network technologies, the most popular being X.25
and Ethernet. The international standards for these networks that conform
to the OSI model will be discussed later.

In order to illustrate the functions of the layers we will examine X.25.
Before doing so, a simple analogy. Consider a normal postal letter being
carried in a postal van along the road between two postal depots. The

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