Introduction 

Opnet (Optimised Network Engineering Tools) is a tool for modelling, simulation and performance analysing of communication networks and communications protocols. Opnet is developed by MIL3, Inc. and runs on both Unix and Windows NT machines. The tool can be used by developers who are: 

  • Developing new protocols.
  • Optimising existing protocols.
  • Studying the performance of existing protocols in different network topologies during varying traffic loads.
  • Evaluating competing protocols.
 Opnet combines a graphical interface and a programmable interface: 
  • The graphical interface gives the designer a good overview of the model and lets him specify parts of the model in an easy to use and intuitive graphical manner.
  • The programmable interface allows the user to define parts of the model in raw c-code. This feature makes it possible build very detailed models and makes the tool very flexible since almost any communication system can be modelled.
Together with Opnet comes a library with ready-made models. This library includes models of the most widely used communications protocols such as: Ethernet, IP, UDP, TCP, ATM, FDDI, Frame Relay and CDPD. These models are very detailed and include all features of the mentioned protocols. Opnet also contains some models of the most commonly used routers and switches (for example a number of Cisco routers). When developing an Opnet model the user can either: 
  • Use a ready-made model. If there already exists a ready-made model for the protocol or network that the he wants to model.
  • Use ready-made models as a part of his model. For example the ready-made Ethernet, IP and TCP models could be used as parts of an FTP model.
  • Develop a new model from scratch.