Performance of priorities on an 802.5 token ring

ABSTRACT

The IEEE 802.5 token ring allows the user to define multiple message priorities. The priority mechanism is implemented using a reservation scheme whereby packets attempt to reserve the token. We present an analytic model that predicts mean delay for each priority level. We show that operation of the priority mechanism is not free; that is, there is a well-defined cost (measured in terms of increased average message delay) associated with its implementation, and that in general there is minimal discrimination made between the various priority levels. We also show that the cost of the priority mechanism is an easily computable function of various network parameters. One of the circumstances where the priority mechanism results in a large discrimination between priorities is when service is limited to a single packet per received token. We show that single-packet-per-token service is necessary (but not sufficient) to gain maximum benefit from priority operation. Our main conclusion is that the network user will not, in general, receive much benefit from the priority mechanism since the priority mechanism can in fact increase slightly the delay for all priorities. The exception to this statement occurs when the network is operating under fairly extreme conditions which are well defined, and which we believe are unlikely in most installations.

REFERENCES

• 1: D, P. Heyman, "Data-transport Performance Analysis of Fasnet," Bell System Technical Journal 62, 8, Oct. 1983, pp. 2547-2560.
• 2: Alan G. Konheim , Bernd Meister, Waiting Lines and Times in a System with Polling, Journal of the ACM (JACM), v.21 n.3, p.470-490, July 1974 [doi>10.1145/321832.321845]
• 3: J.H. Peden, "Performance Analysis of the IEEE 802.5 Token Ring," Master's Thesis, Department of Computer Science, University of Virginia, Jan. 1987.
• 4: Mischa Schwartz, Telecommunication networks: protocols, modeling and analysis, Addison-Wesley Longman Publishing Co., Inc., Boston, MA, 1986
• 5: "Token Ring Access Method and Physical Layer Specifications," The Institute of Electrical and Electronics Engineers, Inc., 1985.

INDEX TERMS

Primary Classification:
  C. Computer Systems Organization
  C.2 COMPUTER-COMMUNICATION NETWORKS
      C.2.5 Local and Wide-Area Networks
          Subjects: Token rings

Additional Classification:
  C. Computer Systems Organization
  C.4 PERFORMANCE OF SYSTEMS
      Subjects: Performance attributes; Modeling techniques

  K. Computing Milieux
  K.1 THE COMPUTER INDUSTRY
      Subjects: Standards

General Terms:
Design, Measurement, Performance, Standardization