3Com PathBuilder S200 Series, Owner'S Manual

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36 Bridging
Configuring the PathBuilder S200 Series Switch for Bridging Operation
Increasing the frame size also causes the reduction in frame overhead. If a 1000 byte
data packet required a 50 byte header (frame + IP + TCP), then if 2000 bytes were
placed in the frame with the same frame, the difference in overhead is 50/1000 = 5%
versus 2.5%. As the size of the data increases, the overhead becomes even less.
However, at these levels, the gain is marginal. Other factors may reduce this method
of gain considerably. For example, intermediate systems have a limit on how large a
frame they can handle.
As the size of the frame becomes larger, there is a corresponding increase in the time
the frame spends in transmission media. The error rate of transmission media is finite
and becomes a problem when the time for transmitting a frame becomes long enough
that the probability of an error occurring during the transmission time is likely. An
error on a large frame with its subsequent retransmission means the media are used
with unproductive transmissions and reduced efficiency.
Standard Frame
Sizes
In general, these industry standards can be used as a guideline for selecting the
maximum frame size.
Bridge Transit Time The transit time for bridged traffic within a PathBuilder S200 series switch is fixed to
an upper bound of approximately one second. If the time is exceeded, the frame is
discarded. This avoids extra traffic being sent (especially due to LLC2 recovery
procedures).
Duplicate frames will frustrate normal recovery procedures and cause extra traffic to
be generated. When a frame is discarded in this manner, the port statistic in the
Detailed Port Stat screen displays “Frames Discarded: Congestion.”
Max Frame Size Line Speed Range (kbps)
512 9.6 to 38.4
1500 38.4 to 56
2052 56 to 1544
4472 1544