Re: Pause Frame transmission

In article <e4rsfa$602$1@xxxxxxxxxxxxx>,
Sven-Arne Reinemo <svenar@xxxxxxxxx> wrote:


Was there ever a specific reason for including a configurable pause time
instead of a pure on/off scheme? To me it seems hard to make any good
use of the pause timer.

In *theory*, one can make a reasonable estimate of the required
pause_time, and optimize operation at both ends of the link. In
*practice*, the benefit doesn't make much difference; in fact, in
*practice*, PAUSE flow control is turned off (at least at speeds up to
1000 Mb/s).

In the standards committee, we discussed a pure "on-off" scheme at great
length (commonly called XON/XOFF, the flow control method of RS-232).
The problem is that there is no guarantee of receiving the "XON" message
after receiving an "XOFF" which would cause a link to hang indefinitely.
That's the reason we put in the timer, which automatically expires after
a pause_time. Thus, if a PAUSE with pause_time = 0x0000 is send but not
received, the system at least clears the hang after a time. That time
can be quite long, however; at 10 Mb/s, 0xFFFF is a pause of over 3
seconds. That's another reason to use a better estimate for pause_time
than just 0xFFFF (although both schemes "work").


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Rich Seifert Networks and Communications Consulting
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