PDQRAP - Prioritized Distributed Queueing Random Access Protocol

A new MAC layer protocol, DQRAP (Distributed Queueing Random Access Protocol), provides performance with respect to throughput and delay that approaches that of an ideal protocol. This paper introduces PDQRAP, a prioritized version of DQRAP. Two formats, the Extra-bit and the Extra-slot, each a variation of the basic DQRAP are described. Simulation results are presented that show that under a total traffic load of 90%, a high priority segment of 20%-30% in either format has delay characteristics ranging from one-third to one-half of the delay characteristics of the normal priority traffic. The relative advantages and disadvantages of each format are discussed. It is suggested that PDQRAP could be the mechanism that allowed a shared medium to economically transport traffic associated with multimedia services utilizing LANs, MANs, and WANs. DQRAP, developed by Xu and Campbell at the Illi-nois Institute of Technology, is an extended model multiple access protocol that utilizes control minislots and two global queues to provide performance superior to other protocols with respect to throughput and delay. DQRAP provides immediate access at light offered loads but moves seamlessly to a reservation system at heavier traffic. DQRAP is effective in networks regardless of the value of "a", the ratio of propagation delay to frame transmission time. All previous protocols based upon the extended channel model, i.e., use of control minislots, provided a throughput of one only by use of an infinite number of minislots [6]. DQRAP provides a throughput of one with use of three control minislots and thus can be considered a practical protocol. DQRAP is amenable to a variety of network topologies and is scaleable to any speed and distance. DQRAP is described in detail by Xu and Campbell [1] and the presentation of PDQRAP in this paper effectively provides details of how DQRAP operates. However we provide a short narrative of how and why DQRAP works. DQRAP utilizes three control minislots and two global queues: a transmission queue (TQ), and a collision resolution queue (RQ). The states of these queues, which are actually counters maintained by each station, provide the basis for a four-state machine that governs the operation of DQRAP. There are three sets of rules governing the operation of DQRAP: DTR-data transmission rules, RTR-request transmission rules, and, QDR-queueing discipline rules. The equivalent rules in PDQRAP are presented later. When a station has a frame ready for transmission it transmits what is effectively a reservation request …