Μελέτη προβλημάτων ομαδοποίησης δικτυακών σταθμών και χρηστών

Abstract

Nowadays, the spread of computer networks and Internet increases dramati- cally the number of end users who, at the same time, become more demand- ing in terms of the provided capacity as well as the provided Internet services and applications. Optical networking using Wavelength Division Multiplex- ing (WDM) o ers an excellent way to exploit the huge bandwidth of optical bers and, thus, it is the most promising technology for coping with the aforementioned demands and meeting both present and future needs. Using the star topology, a WDM network can be con gured as a broadcast-and- select network in which all of the inputs from the various network nodes will be combined by a passive star coupler via two-way bers. An important issue in such WDM networks is to specify the way that the nodes transmit on the available channels. Thus, a media access control (MAC) protocol is necessary in order to unleash the network's capabilities by introducing a scheduling algorithm which will allocate the network resources in an e#cient way. The rst part of the thesis presents two novel MAC protocols for broadcast- and-select WDM star networks with tunable transmitters and xed re- ceivers. The proposed protocols fall in a special category of pre-transmission coordination-based protocols, since they do not use any control channel and the transmission coordination is accomplished by transmitting control pack- ets over the data channels. Their core idea is based on the observation that the network load is asymmetric and, thus, there is a need of a scheduling scheme which would take into account the speci c demands of each node. The new protocols are based on the use of clustering techniques and, more speci cally, they organize the network's nodes into groups, i.e. clusters, ac- cording to the number of their requests per channel. Based on the obtained clusters, they de ne their transmission priority beginning from the nodes belonging to the cluster with greater demands and ending to the nodes of cluster with fewer requests. In this way, they manage to decrease both the unused timeslots as well as the schedule length and as a result the net- work performance is signi cantly upgraded without aggravating the time complexity of the scheduling algorithm. The protocols presented in the second part of the thesis are for broadcast- and-select WDM star networks with tunable transmitters and tunable re- ceivers. Channel assignment and message sequencing are two main key issues that have to be addressed when designing MAC protocols for these networks. Up to now, popular scheduling techniques consider either channel assignment or message sequencing issue but not both of them, and, thus, they su er from low performance, especially when operating under heavy tra#c. The proposed protocols of this part are also based on clustering techniques, as the ones of the rst part, but in this case both the criterion and aim of clustering is di erent. More speci cally, these protocols cre- ate groups of similar source nodes on the basis of the destination nodes of their messages and their goal is to decrease the probability of scheduling consecutive messages to the same destination. Extensive simulation results are presented which indicate that the proposed schemes lead to a signi - cantly higher throughput-delay performance, in comparison to conventional scheduling algorithms. It is important to be noted that this is the rst time that clustering techniques are applied to the MAC layer and to the optical networks generally.