H.323 Overhead

[Ami Amir]

Hi Steve and All

Video does not gain from silence suppression. The equivalent to "silence" is
no motion. H.261 was really designed for constant GOBS per second (Group of
Blocks). The number of GOBs varies by the level of compression and quality
(CIF vs QCIF). No motion = a  GOB which is practically empty - which creates
a packet that is only overhead => not very efficient. Since this the 'best
case" situation as far as total bandwidth taken up by the video stream, I
would not worry about it.

When there is lots of motion - the GOBS are very big, and the frame size is
really the maximum  RTP frame allowed by the network. When this occurs, the
overhad is negligible.

To summarize - for H.323 (it is really for RTP - since all protocols i.e.
SIP and MEGACO/MGCP/.... use RTP) the more data you send - the lower the
overhead.

H.323 signaling overhead is practically nill (again like SIP and MGCP).
Signaling information is sent only when there are changes - e.g. call set up
or tear down.

As Steve correctly states - the only real issue is at the edge (say PPP
connections via RAS - remote access modems). This is the place where the
attention needs to be placed, i.e. header compression etc.

H.320 (H.221 really) was designed for maximum efficiency in TDM circuits.
H.221 sacrifices simplicity for bandwidth. The data is NOT organized in
bytes and requires a lot of bit manipulation. Overhead is really neglible.
As far as I remember - for 128kbps - it is less than 5%.

Ami

-----Original Message-----
From: Stephen Casner [mailto:casner at CISCO.COM]
Sent: Friday, February 11, 2000 6:34 PM
To: ITU-SG16 at MAILBAG.INTEL.COM
Subject: Re: H.323 Overhead


On Fri, 11 Feb 2000, Francois Menard List Account wrote:

> In terms of the overhead related to
> packets transmission, it is RTP encapsulation which is causing a lot of
> overhead.

Actually, the 12 bytes of RTP aren't the biggest part.  There's also 8
for UDP, 20 for IP, and some more for a link-level header.  But the
IP/UDP/RTP part can be compressed down to a few bytes across links
where that matters.

This compares to the H.221 overhead for H.323 -- I don't know that number.

But in the IP case, you also gain from silence suppression, which I
believe is only done on specialized links in the circuit case.

In my opinion, the focus on transmission overhead is misguided.  If
you believe predictions that voice bandwidth will be a small fraction
of the converged IP network, then that overhead in the backbone should
not be an issue.  Header ompression can be used on narrow links at the
edges.
                                                        -- Steve