Desktop Videoconferencing over a switched LAN/MAN interconnected system: modeling, designing, dimensioning and tuning issues

This paper mainly concerns with the modeling, the designing, the dimensioning and the tun- ing of a network system supporting a high quality desktop videoconference service for a large community of users scattered over a metropolitan area. The network topology of reference consists in different groups of Ethernet LANs interconnected by a DQDB backbone through high performances multiport switches, trying to exploit the bandwidth saving introduced by best efforts techniques and to maintain the investment of a large community of users. Three main subnetwork systems have been investigated: the single LAN segment, each group of locally switched LAN segments, the DQDB backbone. A simulation model has been designed for each subsystem and has been run separately. The obtained results have been of great aid in dimensioning and tuning a simulation model of the whole system, by which the tested packet switched network protocols and technologies have been shown to effectively support high quality multimedia real time services, at the price of a careful planning and tuning activity. More in details, our simulations show that up to a 70% occupation of the available bandwidth is compatible with a satisfactory quality of user perception, and that Ethernet is the system bottleneck, due to sudden and unpredictable peaks in the LAN access delay. Finally, the adoption of the Real Time Transport Protocol, especially conceived to improve the steadfastness of the delivered frames inter-arrival time, revealed itself to be a crucial choice in this context in order to improve the quality of the user perception.