Modelling represents a powerful method of real objects studying. At use of modelling the investigated object is replaced with more simple object named by model. It is possible to consider as the basic versions of modelling two its kinds - mathematical and physical modelling. At physical (natural) modelling the investigated system is replaced with other material system corresponding it which reproduces properties of studied system with preservation of their physical nature. As an example of this kind of modelling the test-network by means of which the basic opportunity of a network construction on the basis of computers is studied, communication devices, operating systems and applications can serve.

Opportunities of physical modelling are limited enough. It allows to solve separate problems at the task of a small amount of combinations of system investigated parameters. At natural modelling the computer network it is practically impossible to check up its work for variants with use of various types of communication devices (routers, switches). Check in practice even ten different types of routers is connected with huge time and material inputs.

But even when by optimization of a network types of devices and operational systems but only their parameters, carrying out of experiments in real scale is practically impossible change not. Even simple change of the maximal size of a package in any protocol demands reconfiguration operating system in hundreds computers of a network. It demands from the network administrator of carrying out very much great volume of work.

By optimization of networks in many cases preferable there is use of mathematical modelling. The mathematical model represents set of parities (formulas, equations, inequalities, logic conditions), defining process of a condition change of system depending on its parameters.

Special class of mathematical models are imitating models. Such models represent the computer program which step by step reproduces the events occuring in real system.

Imitating models of computing systems reproduce processes of messages generation by applications, splitting of messages into packages and the staff of the certain protocols, the delays connected with processing of messages, packages and the staff inside of operating system. As process of reception of access by a computer to the divided network environment and process of acting packages processing by a router is modelled.

At imitating modelling a network it is not required to get the expensive equipment. Work of the network equipment is simulated by the programs precisely enough reproducing all its basic features and parameters.

Advantage of imitating models is the opportunity of process substitution of events change in investigated system in real time on the accelerated process of change of events in rate of work of the program. As a result in some minutes it is possible to reproduce work of a network within several days. It enables to estimate work of a network in a wide range of varied parameters.

Result of imitating model work are collected during supervision over proceeding events statistical data about the most important characteristics of a network. To such characteristics concern:

• Time of reaction;
• Operating ratios of throughput of channels;
• Congestion of units;
• Probability of loss of packages.

There are special languages of imitating modelling which facilitate process of program model creation in comparison with use of universal programming languages. As examples of imitating modelling languages - SIMULA, GPSS, SIMDIS can serve.

There are also systems of imitating modelling which are guided by a narrow class of studied systems and allow to build models without programming.