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RE: (Another) Question Regarding Open-Loop Control Mechanism


Part of the purpose of the non-extended IPG (that is, the traditional 96
bits) is to allow physical
layer devices to drop some of the idle in order to move between clock
domains that have the 
ppm clock difference. The difference in maximum size packet duration between
slowest and
fastest clocks is less than 3 bits. The MAC does not need to lengthen the
gap to compensate
for this.

Because our physical layer devices will only be able to drop or add idles in
increments of 4
bytes, dropping an idle from a 12 byte IPG will reduce it to 8 bytes. We
will need to consider
the limits on shortening idle in order to specify the limit of IPG
shrinkage. For instance, if
a device gets a 9 byte IPG, can it shorten that IPG or should it wait for a
longer idle? For
some alignments, the 10GBASE-R PCS encoding requires 8 bytes of idle
(counting the /T/
as part of the idle) to produce valid codes so at least for that case, it
will need to look for
a long enough IPG before shortening. Since short gaps can not happen
constantly, within
a packet or two a long enough one will arrive.

By the way, if you do the calculation for the open-loop control mechanism,
you will see that
when extending the IPG to adapt to the WIS data rate, it adds less than a
byte per a minimum
size packet. Therefore, a 10GBASE-R PCS doing the adaptation for WIS also
receives IPGs
as short as 9 bytes and will need to wait for a long enough IPG to do the
idle deletion.


-----Original Message-----
From: Boaz Shahar [mailto:boazs@xxxxxxxxxxxx]
Sent: Tuesday, August 29, 2000 11:53 PM
To: HSSG (E-mail)
Subject: (Another) Question Regarding Open-Loop Control Mechanism

When the MAC device and the PHY device are driven by a different clock
source, How does  Clock Tolerance Compensation between the two done? 
(That is, since the max difference between the two is 200ppm, then the MAC
may always adjust the rate to the nominal rate - 200ppm? And if so: Is it
acceptable ?)

Sorry if this was asked before.