What vexes VoIP users?
mike at mtcc.com
Wed Mar 2 10:36:08 CST 2011
On 03/02/2011 06:23 AM, Jay Ashworth wrote:
> ----- Original Message -----
>> From: "Michael Thomas"<mike at mtcc.com>
>> Yes, really. The only difference was which L2 channels the RTP
>> packets were flowed onto, which was determined by the MGCP/SIP
>> signalling and interaction with the telephony gateway. There
>> is a **very** complicated state machine that deals with this
>> using some bastardized IETF protocols (COPS IIRC).
> Ok, see, now I (like, I suspect, Frank Bulk) am confused again:
> when you say "which L2 channels the RTP packets were flowed onto", that
> sounds to me a *whole* lot like "which VLAN on the end-user drop carried
> the RTP packets from the terminal adapter in their cable box to our
> concentrator"... which is pretty much the point I was originally trying
> to make, if perhaps in slightly different terms.
> Am I still misunderstanding you?
They're kind of like VLAN's, but not exactly. It's been a long
time since I worked on this... The RTP is flowed over what is
called unsolicited grants (UGS) which give slots on the upstream
for transmission. There are several other types of qos treatment
between the CM and CMTS... I think that packetcable flows the
MGCP and SIP over nrt-PS, but I might be misremembering.
The signalling between the CM (MTA) and CMS (eg MGCP)
is what fields the requests for better qos treatment for the
RTP stream, and the CMS talks to the CMTS via COPS to
set up the UGS flows to the cable modem/voice box (ie,
an embedded MTA).
In any case, this is 100% IP end to end, with all kinds of
goodies for LEA and privacy to boot, which make the entire
problem of faithfully reproducing the PSTN over IP a giant
Mike, I should know about the LEA aspect since I was the
first one at Cisco to find and then dutifully step on that mine
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