14.2. 14.2 The need for communications network modelling and simulation

In a world of more and more data, computers, storage systems, and networks, the design and management of systems are becoming an increasingly challenging task. As networks become faster, larger, and more complex, traditional static calculations are no longer reasonable approaches for validating the implementation of a new network design and multimillion dollar investments in new network technologies. Complex static calculations and spreadsheets are not appropriate tools any more due to the stochastic nature of network traffic and the complexity of the overall system.

Organisations depend more and more on new network technologies and network applications to support their critical business needs. As a result, poor network performance may have serious impacts on the successful operation of their businesses. In order to evaluate the various alternative solutions for a certain design goal, network designers increasingly rely on methods that help them evaluate several design proposals before the final decision is made and the actual systems is built. A widely accepted method is performance prediction through simulation. A simulation model can be used by a network designer to analyse design alternatives and study the behaviour of a new system or the modifications to an existing system without physically building it. A simulation model can also represent the network topology and tasks performed in a network in order to obtain statistical results about the network's performance.

It is important to understand the difference between simulation and emulation. The purpose of emulation is to mimic the original network and reproduce every event that happens in every network element and application. In simulation, the goal is to generate statistical results that represent the behaviour of certain network elements and their functions. In discrete event simulation, we want to observe events as they happen over time, and collect performance measures to draw conclusions on the performance of the network, such as link utilisation, response times, routers' buffer sizes, etc.

Simulation of large networks with many network elements can result in a large model that is difficult to analyse due to the large amount of statistics generated during simulation. Therefore, it is recommended to model only those parts of the network which are significant regarding the statistics we are going to obtain from the simulation. It is crucial to incorporate only those details that are significant for the objectives of the simulation. Network designers typically set the following objectives:

Depending on the objectives, the same network might need different simulation models. For instance, if the modeller wants to determine the overhead of a new service of a protocol on the communication links, the model's links need to represent only the traffic generated by the new service. In another case, when the modeller wants to analyse the response time of an application under maximum offered traffic load, the model can ignore the traffic corresponding to the new service of the protocol analysed in the previous model.

Another important question is the granularity of the model, i.e., the level of details at which a network element is modelled. For instance, we need to decide whether we want to model the internal architecture of a router or we want to model an entire packet switched network. In the former case, we need to specify the internal components of a router, the number and speed of processors, types of buses, number of ports, amount of port buffers, and the interactions between the router's components. But if the objective is to analyse the application level end-to-end response time in the entire packet switched network, we would specify the types of applications and protocols, the topology of the network and link capacities, rather then the internal details of the routers. Although the low level operations of the routers affect the overall end-to-end response time, modelling the detailed operations do not significantly contribute to the simulation results when looking at an entire network. Modelling the details of the routers' internal operations in the order of magnitude of nanoseconds does not contribute significantly to the end-to-end delay analysis in the higher order of magnitude of microseconds or seconds. The additional accuracy gained from higher model granularity is far outweighed by the model's complexity and the time and effort required by the inclusion of the routers' details.

Simplification can also be made by applying statistical functions. For instance, modelling cell errors in an ATM network does not have to be explicitly modelled by a communication link by changing a bit in the cell's header, generating a wrong CRC at the receiver. Rather, a statistical function can be used to decide when a cell has been damaged or lost. The details of a cell do not have to be specified in order to model cell errors.

These examples demonstrate that the goal of network simulation is to reproduce the functionality of a network pertinent to a certain analysis, not to emulate it.