Subject: RE: Packet loss draft

From: "Bob Barrett" <[email-masked]>
Date: Thu, 19 Dec 2002 21:05:47 -0000

As a general comment:

I have been off doing other things for a month, just trawling 200+ pwe3
emails ....

I haven't seen any discussion / papers on the affect of packet loss on
signalling (CAS and CCS). If there have been any that I have missed could I
ask Yaakov to point me at them please?

It makes sense for the implementation of a PE to have the option to look for
frame alignment (and align frame boundaries to the PW payload, as they would
be for structured, if alignment is found) even when configured for
unstructured.

The function has to be optional (and able to be turned off or on), as
non-voice data can spoof frame alignment (BERT on E1 for sure).

A subscriber may well run a framed circuit (most likely voice) over an
unstructured service, and it would make sense to take advantage of the
framing (if there) to manage substitution on packet loss (by whatever
means). However, the full 256 / 193 bits per frame would still be carried
without extraction of signalling, as in unstructured mode, but with frame
alignment and not arbitrary / no alignment. This would aid re-play /
substitution for the odd packet loss, or burst of packet losses, if the
subscriber was running a voice circuit over the unstructured PW.

However, the one packet lost in 100 (1%) test, or 1 lost in 25 (4%) would
likely mess up any CCS. CAS is not so susceptible (sic).

Best regards

Bob

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Shahram
Davari
Sent: 25 November 2002 22:36
To: 'Sasha Vainshtein'; [email-masked]'
Cc: 'Yaakov Stein'; [email-masked]
Subject: RE: [PWE3] Packet loss draft

Sasha,

> [Sasha] To the best of my knowledge, the tolerance to CRC errors
> is much higher than tolerance to loss of Frame Alignment signal.
> Please see ITU-T G.706 for the details. E.g., E1 requires
> just two valid
> MF alignment signals within 8 milliseconds to retain MF
> alignment. Hence
> replay of, say, the last received packet that carried 256
> bytes (equivalent
> of 8 frames) instead of the lost one will preserve FAS and will NOT
> result in loss of multiframe alighnment. T1 requirements are
> even more relaxed.

In your example you are assuming that the 8 frames are aligned to
MF. But the unstructured draft assumes no such alignment. In the
absence of no such alignment, any packet loss could span over
2 MFs. Only 2 non-consecutive packet loss could affect 4 MFs ( = 8 msec),
so you would get zero MF alignment in 8 msec.

Yours,
-Shahram
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