Analysis of IEEE 802.5 Token Ring Performance

This analysis assumes "Early Release" of the token. Let:

µ

be mean frame service rate (1/DT)

lambda

be mean aggregate frame arrival rate at all stations

U

be channel efficiency; at equilibrium, U = rho = lambda

q

be mean frame queue length at a station when the token arrives

N

be number of stations on the ring

DP

be propagation delay around cable

R

be data rate

TRT

be Token Rotation Time, the period between appearances of the token at a point in the ring

Token Rotation Time, TRT = cable propagation delay (DP) + (at least) 1-bit delay (1/R) at each of N stations + Station Service Time (q/µ) at each of N stations:

TRT = DP + N/R + qN

assuming that the station service time, q/µ, is less than the Token Holding Time (THT) the Lightly Loaded case. The queue of frames which accumulates at a station in a period TRT is 1 N'th of the total number of frames which arrive at all stations:

q = TRT × lambda / N

Thus, substituting for q in the expression for TRT:

TRT = DP + N/R + TRT × lambda × N/(µ×N) = DP + N/R + rho × TRT

which can be re-arranged to give an expression for Token Rotation Time, TRT:

TRT =

When a frame arrives at a station, it can expect to be about half way through a period of Token Rotation Time, and thus delay is on average, about ½TRT. As efficiency --> 1, TRT --> infinity.

In case of a system at equilibrium, q/µ<THT because each station clears its entire backlog of accumulated frames before the Token Holding Timer expires. Then:

U = = rho    Lightly Loaded Case

In case of a temporarily overloaded (i.e. non-equilibrium) system, where, q/µ>THT, channel efficiency, U < rho, because each station can not clear its entire backlog of accumulated frames. Then: