Sun Microsystems 1.0 - Manual (Page 52)

26 SunFDDI/P 1.0 Adapter User’s Guide—May 1997
4
fragmented so that they do not exceed the MTU of the Ethernet network. Some
low-cost bridges that do not support fragmentation will reject the 4352-byte
packets.
Fragmenting and reassembling the packets introduces a considerable overhead
that affects performance. It also increases the risk of out-of-sequence or
dropped packets. Therefore, if there is heavy network traffic across the bridge,
it may be more efficient to force the MTU in the FDDI network to be 1500 to
match the MTU on the Ethernet side of the bridge. This argument can also be
applied to a mixed FDDI/NetWare® network. In this case, the MTU in the
NetWare LAN is 4178 octets.
Target Token Rotation Time (TTRT)
Caution – Modifying the target token rotation time (TTRT) can affect network
performance adversely. In the worst case, it can reduce network throughput to
zero. In general, the TTRT should not be set to less than
8000 microseconds (ms).
The target token rotation time (TTRT) is the key FDDI parameter used for
network performance tuning. In general, increasing the TTRT increases
throughput and increases access delay.
For SunFDDI/P, the TTRT must be between 4000 and 165000 microseconds,
and is set to 8000 ms by default. The optimum value for the TTRT is dependent
on the application and the type of traffic on the network:
•
If the network load is very irregular (bursty traffic), the TTRT should be set
as high as possible to avoid lengthy queueing at any one station.
•
If the network is used for the bulk transfer of large data files, the TTRT
should be set relatively high to obtain maximum throughput, without
allowing any one station to monopolize the network resources.
• If the network is used for voice, video, or real-time control applications, the
TTRT should be set low to decrease access delay.
The TTRT is established during the claim process. Each station on the ring bids
a value (
T_req ) for the operating value of the TTRT ( T_opr ) and the station
with the lowest bid wins the claim. Setting the value of T_req on a single
station does not guarantee that this bid will win the claim process.
!