The Importance of Sliding Window Protocol Generalisation in a Communication Protocols Course

⎯ All modern communication systems rely on the correct operation of communication protocols that are running in different layers of protocol stacks. Hence, a deep knowledge and understanding of them must be possessed by all communications engineers. Therefore, a basic communication protocols course must be an essential constituent of any higher education communications engineering programme. Automatic repeat request (or ARQ for short) protocols that provide for reliable data transfer over noisy and lossy channels and are usually used in data-link and transport layers of a protocol stack often form the core of such courses. One of the reasons for their high pedagogic importance within the frame of a protocols course is that they are typical but not trivial distributed algorithms and therefore not very easy to grasp; the other reason is that they must be both logically correct and efficient. Pedagogically, the general notions of correctness and efficiency can easily be introduced and emphasised through the correctness and efficiency of ARQ protocols. Automatic repeat request protocols are usually implemented in the form of sliding window protocols. Traditionally, three basic types of sliding window protocols are distinguished, namely, stop-and-wait, go-back-N, and selective-repeat protocols. Although all the three types were often claimed to be members of a common sliding window protocols family, this was not quite clear and evident until recently, which was quite confusing for both students and teachers. However, several years ago, the generalised sliding window protocol was defined and formally specified so that the three above mentioned basic types can easily be shown as its special cases. The protocol behaviour depends on two essential parameters, namely transmit window width and receive window width which are directly related to the amount of memory used by transmitter and receiver, respectively. In the generalised protocol, these parameters can have arbitrary values, while in the three special cases their values are differently restricted. When the three basic types of sliding window protocols are explained as special cases of the generalised protocol, the relation of their behaviours to memory usage at both transmitting and receiving sides is much easier to be seen. A simulator of the generalised sliding window protocol was implemented and is used for both pedagogic and research purposes, where any of the three basic protocol types can be chosen to be simulated simply by choosing appropriate values of transmit and receive window widths. The generalised protocol was shown to be potentially equally efficient as the basic types while using less resources. From the pedagogic viewpoint, it is extremely important to be aware that a systematic approach to this complex topic can be both much easier and more understandable for students. Index Terms ⎯ Communication Protocol, Communication Protocol Course, Generalisation, .Sliding Window Protocols.