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Posted by James Knott on July 22, 2005, 7:42 am
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James Knott wrote:
> Andy Ball wrote:
>
>>
>> Hello,
>>
>> I have a hypothetical question with practical implications: Two
>> computers, 'A' and 'B', are plugged into a 100baseTX switch. Computer
>> 'A' has a 100baseTX interface and computer 'B' has a 10baseT card.
>> When 'A' is talking to 'B' through the switch, does it send its frames
>> at the slower 10 Mbit/sec rate, or does it send to the switch at the
>> full 100 Mbit/sec and the switch shift it out slower to 'B'?
>
> "A" will send at the full rate. If the switch doesn't use flow control,
> the
> buffers will eventually fill and frames will be lost. However, the TCP
> protocol also has it's own flow control mechanisms, that cause it to back
> off, when switch congestion causes delays or lost frames. This will
> reduce the amount of data that "A" tries to send through the switch.
Forgot to mention: look up IEEE 802.3x flow control. This flow control
mechanism uses a "pause" MAC control frame to tell another device to stop
sending for a specified period of time. However, flow control is optional,
so not all switches use it.
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