Chapter 2 - Communications Models

Modeling

Before explaining the models used in data communications, it might be useful to explain
what models are and what purpose they serve. A model is a simulation of a real object. It
may be a mathematical description, it may be an analogue of the original (a model of the
object’s physical characteristics), a set of logical constructions, or even a set of functions. A
model can contain all of these properties. We use models to simplify explanation, when the
real object is too elusive to use in human terms and to represent objects that we cannot
physically capture.

In communications we use models of different types to explain functional or circuit operation
and to design communications. In this chapter we look at six models. We will first
consider the International Organization for Standardization (ISO) Open Systems
Interconnection (OSI) model, which is a model of the functionality required to communicate
from end user to end user. The second model we look at is the Institute of Electrical and
Electronic Engineers’ (IEEE) 802 LAN model. We then close the chapter by discussing a set
of four modern applications models. To understand data communications, it is important
that you grasp what these models represent, as almost all discussions [both in this book
and generally] of protocols and standards are based on these models.

ISO OSI Model

How are the groups of ones and zeros, the data’s organization, transmitted from one end
user to another? It is not enough that data be organized; there must be a system for
moving this data from one location to another. This system is comprised of rules, rules
about who has access to the media and when (media access), who does packet (frame)
error detection, who counts packets, who performs routing, who is responsible for enduser-
to-end-user communication, who keeps track of the variety of traffic, who ensures that
the traffic is compatible with the host or remote stations, and who interfaces with the user
program. All of these functions have to be performed when communicating from one end
user to another. The OSI model describes these functions, and generally specifies the order
in which they take place in transmission. The OSI model is complex, and understanding its
functions is crucial to understanding data communications.

Yet the actual information needed to gain this understanding is actually quite simple.
Though most readers have probably been exposed to the OSI model, it is usually explained
only in brief statements or in the implementing standards themselves. One reason for this is
that this is a model of function, not a hardware or a software specification. If a system is
OSI compliant, then particular OSI standards are implemented in that layer. Other standards
may perform a particular layer’s functions, but that does not make the implementation OSI
compliant. The best way to begin an explanation of the OSI model is with an analogy.

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