cycle time properties of the fddi token ring protocol (extended abstract)  communication technology now makes it possible to support high data transmission rates at relatively low cost. in particular, optical fiber can be used as the medium in local area networks with data rates in the range of 100 megabits per second. unfortunately, local area network topologies and communication protocols that work well with lower speed media are not necessarily appropriate when the data transmission rate is scaled up by approximately an order of magnitude. recognizing this fact, an ansi sub- committee (ansix3t9) has been working for the past two years on a proposed standard for a token ring protocol tailored to a transmission medium with transmission rate in the 100 megabits per second range. the protocol is referred to as the fddi (fiber distributed data interface) token ring protocol. the proposal for the standard is now quite mature and nearly stable.  while numerous analyses of the performance of token ring protocols have been carried out and described in the literature, these have for the most part dealt with protocol variations of less complexity than fddi. the major feature that distinguishes fddi from token ring protocols that have been analyzed previously is the concept of a "timed token", which selectively allocates the right to transmit data among the stations depending in part on how rapidly the token progressed around the ring on the previous cycle. a station is allowed to transmit certain types of data only if the token's last cycle has been shorter than a "target" token rotation time. this feature makes it possible to give guaranteed response to time-critical messages. the "timed token" creates some dependencies among transmissions at various stations, however, and these dependencies complicate the analysis of the protocol's performance.  the basic ideas of the timed token protocol on which the fddi protocol is based were first presented by grow ["a timed-token protocol for local area networks", electro '82, 1982]. he distinguished two types of traffic. synchronous traffic is a type of traffic that has delivery time constraints. examples include voice and video transmissions, where delays in transmission can result in disruptions of the sound or picture signal. asynchronous traffic has no such time constraints, or at least the time constraints are measured in units that are large relative to the token cycle time.  here is a brief overview of how the "timed token" protocol works. the stations on the local area network choose, in a distributed fashion, a target token rotation time (ttrt). basically, the ttrt is chosen to be sufficiently small that requirements for responsiveness at every station will be met. the right to use network bandwidth for transmission of synchronous traffic is allocated among the stations in a manner that guarantees that network capacity is not exceeded. the token is then forced by the protocol to circulate with sufficient speed that all stations receive their allocated fractions of capacity for synchronous traffic. this is done by conditioning the right to transmit asynchronous messages on the fact that the token has rotated sufficiently fast that it is "ahead of schedule" in delivering synchronous allocations to the stations. in essence, the ttrt value dictates a departure schedule for the token to pass from station to station, and asynchronous traffic can be transmitted only when doing so does not cause that schedule to broken. subsequently, ulm ["a timed token ring local area network and its performance characteristics", proc. of conf. on local area networks, ieee, 1982] analyzed the protocol described by grow and determined its sensitivity to various parameters. he considered the effect of overheads and provided a number of graphs indicating the impact of various parameters on maximum transmission capacity.  as well as describing the timed token protocol, grow and ulm included intuitive arguments supporting two fundamental properties of (a somewhat idealized version of) the protocol. these two properties are:  the average token cycle time in the absence of failures is at most the ttrt.  the maximum token cycle time in the absence of failures is at most twice the ttrt.  both these properties are important to the successful operation of the protocol. the first one guarantees that the average long run bandwidth provided to each station is at least its allocated fraction of the network's capacity. the second property guarantees that, in the absence of component failures, the time between a station's successive opportunities to transmit synchronous traffic will never exceed twice the target token rotation time. while grow and ulm assert that these properties hold for the timed-token protocol, neither formal proofs nor references are provided. because the fddi protocol is based on the same timed- token protocol, subsequent publications specifically describing the fddi protocol have also claimed that the two properties hold.  in this paper, we prove both properties using a common notational framework. we first treat an idealized situation in which several types of overhead are ignored. we actually study a protocol that is slightly more liberal that the fddi proposed standard in that it allows asynchronous transmission more often because "lateness" is not carried forward from cycle to cycle. the protocol variation, which still guarantees properties (1) and (2), is at least as easily implemented as the original version. also, it guarantees sufficient responsiveness and capacity for the transmission of synchronous traffic, while providing improved responsiveness to asynchronous transmissions.  when overheads are considered, it is found that the proposed standard fddi protocol satisfies the constraint on average token rotation time (relying on the retention of "lateness" from cycle to cycle), but not the one on maximum cycle time.  we analyze a variation of the protocol that ignores accumulated lateness, but accounts for the various overhead sources. the advantages of the new rule include:  it guarantees both desired properties without having to retain "lateness" from one cycle to the next.  it provides better service to asynchronous requests in the case where the amount of overhead is small relative the token rotation time. (when the amount of overhead is large, the original proposed protocol may have token rotation times significantly in excess of twice the ttrt.)  it is easier to implement.  work is underway on the task of quantifying the performance of the fddi protocol by determining estimates of, or tighter bounds on, the average token rotation time and on the average delivery time of a submitted message. the properties established in this paper are required to form the basis of the quantitative analysis.  kenneth sevcik marjory j. johnson