An alternative to FDDI: DPMA and the Pretzel ring

This paper proposes an efficient, fair, and simple capacity-1 protocol replacing FDDI. In the recent years, two network protocols gained predominance in very fast local and in metropolitan area networks: FDDI and DQDB. The FDDI ring-protocol combination is well-suited for large local ("campus") area networks. However, for larger networks, FDDI users suffer excessive access delays, even under very light traffic. Moreover, FDDI achieves full capacity only in special circumstances, and heavy load is not a sufficient condition for achieving it. We propose a network topology that is very similar to the dual ring and a token-passing protocol that yield a true capacity-1 network even for modest values of the TTRT. Its additional properties include total fairness and very small access delays under light load. This makes the proposed protocol a reasonable option to use in combination with the dual counter-rotating ring. The proposed network should not be difficult to implement using standard FDDI components.