Performance evaluation of the DQDB layer in IEEE 802.6 metropolitan area networks

The authors analyse the medium access control service offered to the logical link control (LLC) sublayer by the DQDB layer. The MCF block is responsible of a segmentation process that makes the segment sources of the queue arbitrated (QA) block much more bursty than classic mathematical models for bursty data sources. The considered scenario is that of a LAN interconnected to a MAN and using a transport/network protocol suite like TCP/IP, XTP or similar protocols. Measurements on local area networks have shown that traffic distributions are far from theoretical models for packet length (such as the uniform or the exponential distribution). Their models are conformant to the IEEE 802.6 standard with 3 priority levels and bandwidth balancing. The evaluation of the DQDB layer was carried out using different traffic profile and discrete distribution that are much more similar to realistic traffic scenarios. Simulation analysis shown that not only the Access delay is deeply sensitive to the burstiness of the sources, but also the MAC delay both in underload and in slightly overload condition. The unfairness of the network in terms of access delay is also analyzed among the three active priorities