Subject: RE: [Tsvwg] draft-baker-diffserv-basic-classes-02

From: "Jozef Babiarz" <[email-masked]>
Date: Tue, 1 Jun 2004 21:31:20 -0400

In a message dated May 27, 2004 [email-masked] <mailto:[email-masked]>
writes:

[Mike]

I believe this remains as one of the major open questions for the
application of SIP or H.323 in a large, managed network. Some comments: 

I've heard of speech clipping (on answer), but I don't know what
"ring-clipping" is. I presume you are referring to call setup delay (or
delay to ring). The issue is signaling delay in general (call setup and
release), but speech clipping has always been the most serious impact of
delay. 

It seems to me that this issue is really not related to "interfacing to a
TDM telephony switch", but occurs equally within a fully IP environment. 

[Joe]

Ring-clipping can occur if SS7 signaling path from the softswitch to the TDM
CO is much faster than the signaling path from softswitch to IP gateway that
interfaces to the TDM CO were the ringing tone is generated. Speech-clipping
can occur if the signaling path from the call answered phone to the
softswitch and back to the IP gateway that enables the audio path takes
longer then for the person answering the phone to say hello. 

[Mike]

No matter what service class is used for signaling (above, below, or same as
speech bearer) there must be guaranteed bandwidth for the expected signaling
traffic to provide sufficient low latency and almost zero discard rate
independent from the amount of speech bearer traffic. In effect, if
signaling is in a separate class and is guaranteed, say 5% of the bandwidth
on an outgoing link and speech bearer (EF) is guaranteeed 50%, then it isn't
really necessary to think of one being above or below or the same as the
other. The forwarding algorithm guarantees a minimum bandwidth to both in
what is effectively "time-division multiplexing", but we don't call it that
since it is not based on strict timing. The result is that the signaling
does not experience "higher latency", but probably has "higher delay
variation", mostly due to the varying packet sizes. 

[Joe]

Agree. Putting signaling traffic into a distinct class and engineering that
class so that queuing dose not occur does work. In next draft (03) I will
total separate telephony signaling from telephony bearer traffic.

Regards,

Joe Babiarz 

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