RE: Inter-frame transmission length
The IPG was and still is created by the MAC as specified in clause 4. In
clause 4 a minimum length is specified. The average IPG cannot be specified
because it is dependent on network load, and that is also true of the RS
specification in clause 46.
The start control character alignment function of the RS (see 46.3.1.4)
either adds or deletes IPG. The specification through a Deficit Idle Count
(DIC) is such that if a running total of MAC IPG were maintained, the
aligned data at the XGMII would produce a total IPG that is +0, -3 compared
to the MAC IPG total. Only at maximum frame rate can this be restated as
maintaining a 12 byte average IPG.
In an integrated MAC/RS design, the implementer may choose to either always
add idle to produce alignment, or may produce a design that mimics the
behavior of the clause 4 bit serial MAC and 32-bit wide RS alignment
function.
The answers to your direct questions:
The table was removed because of comments from 802.3ae participants. The
table was felt by some to be too restrictive and by others as difficult to
read and even misleading. The clause still specifies the handling of the
interframe, but the clause organization was changed significantly since
D1.1.
Yes, one of the important characteristics of accepted comments on the start
alignment function is to allow implementer flexability. This also includes
a modification in TF ballot (i.e., a DIC does not have to be implemented,
the implementation only has to behave as if it was implemented).
Another algorithm can be used as long as it behaves like either 1) or 2) of
46.3.1.4.
Conformance is not easily and precisely specified using the average of IPG,
even when only discussing the average at maximum frame rate. The algorithm
specifies the allowd difference on the total IPG. An implementation that
transmitted three back to back frames with the sum of the frame1 to frame2
IPG and frame2 to frame3 IPG being 21 bytes or more (average >= 10.5) at the
XGMII would is in conformance. With minimum frame spacing, after a million
frames, the transmitted IPG at the XGMII could be an average of 11.999997 in
a conformant implementation, if at the end of the test, DIC = 3.
--Bob Grow
-----Original Message-----
From: Lior Reem [mailto:LReem@xxxxxxxxx]
Sent: Sunday, April 29, 2001 3:12 AM
To: '802.3ae'
Subject: Inter-frame transmission length
Hi,
In D1.1 clause 46.2.5.1, a Table (46-4) was attached, which described how to
implement an IPG of 12 bytes in average.
Since then, the following draft excluded this table, and almost nothing is
said about how IPG should be transmitted.
This clause doesn't even talks about the average IPG length anymore, which
looks like a real "hole" in the text.
Why is the inter-frame length table excluded?
Is it to enable more flexibility?
Is it aloud to transmit other IPG length algorithms (any exist)?
Is it aloud to transmit shorter average IPG?
Thanks,
Lior.
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Lior Re'em
Avaya Inc.
Email : lreem@xxxxxxxxx
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