66 SunFDDI/P1.0 Adapter User’s Guide—May 1997
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The maximum time for which a station can hold the permission token, and
therefore the time for which a station can transmit on the ring, is determined
by these two station timers that work together to maintain the target token
rotation time (TTRT), irrespectiveof the load on the network:
•TokenRotation Timer (TRT)
The TRT measures the time between successive arrivals of the token or,the
time taken for the token to rotate once around the ring. The TRTis reset to
the operating value of TTRT (T_opr) each time the token is received. If the
TRT expires beforethe token arrives, the token is considered late and the
flag Late_Ct is set. Only synchronous (high priority) trafficcan be
transmitted when Late_Ct is set.
•TokenHold Timer (THT)
The THT determines how long a station can hold a token or,how long a
station can transmit asynchronous traffic. It is loaded with the value
remaining in the token rotation timer (TRT)each time the token arrives.
Thus, the faster the token rotates, the more transmission time is allocatedto
each station. If the token is delayed because the network is heavily loaded,
the amount of transmission time is reduced. If THT expires during
transmission, the current transmission is completed before the token is
released.
The interaction of these two timers causes token rotation to increase (reduced
transmission time per station) in a heavily loaded network, and to decrease
(increased transmission time per station) in a lightly loaded network.
FDDI Performance
Although the FDDI standards define a medium that supports data transfer
rates of up to 100 Mbps, this does not translate directlyto an increase in overall
system performance. If fact, it frequently transfers the bottleneck elsewhere,so
that the FDDI connection is not used to full capacity.
An FDDI ring has the potential to carry more information rather than to be a
faster connection. If the applications running over the network do not use the
available bandwidth efficiently,you will not see much improvement in the
performance of your network above that of traditional Ethernet connections.
Among the factors that affect FDDI network performance are:
•Network topology