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Info about H.323

Posted: Thu Dec 16, 2010 11:55 am
by peyus
I Have testing scenarios with H.323.Please could anyone provide me with information about:

Early/Late media
Slow/Fast start

Re: Info about H.323

Posted: Thu Dec 16, 2010 8:33 pm
by paulej
Early media / late media is a term usually associated with SIP, not H.323.

In any case, some devices support Fast Connect (aka, fastStart) and some do not. Some support H.245 tunneling and some do not. One does not have to support either of them, though, which is why we see devices doing different things. All devices must support H.245 signaled as a separate connection.

Over the years, I've encouraged implementation of H.245 tunneling. It avoids the need to have a separate TCP connection just for H.245 control traffic. Lots of folks implemented Fast Connect, because it significantly reduced the time required to exchange messages to get a call established. In so doing, this helped to avoid the media clipping issues we sometimes see with use of a separate H.245 channel.

There is away to avoid media clipping entirely in H.323, which is to use H.460.11. This works fine for most point-to-point calls, but is not useful in call center scenarios where any one of 20 agents might answer the phone, for example: media can only be established after the phone is answered in that case. (Same is true for SIP, too.)

Re: Info about H.323

Posted: Tue Dec 21, 2010 1:11 pm
by peyus
Hi Paulej,
Thanks for your reply.I was wondering if this particular scenario occurs:if one device enables slow start and another device supports fast start..what would happen?would there be media exchange betwen them.Please advise on this scenario...

Re: Info about H.323

Posted: Tue Dec 21, 2010 3:16 pm
by tsahil
H.323 works in a way that it knows at each time to "fall back" to the previous method.
This means that an endpoint that dials out using fast start is going to notice in the reply that he will receive from the remote endpoint that it doesn't support fast start and both will simply end up using slow start.
This holds true for other features such as Q.931 multiplexing of multiple calls on a single TCP connection, running H.245 in parallel to fast start, etc.

I hope this helps.

Tsahi Levent-Levi

Re: Info about H.323

Posted: Wed Dec 22, 2010 12:47 pm
by peyus
Hi Tsahil,
Thanks so much for your reply.Can You share documents or any kind of material which would give me a deep insight into Fast/Slow start feature of H.323.We have testing scenarios and we are completely new to H.323.Your help is much appreciated..

Re: Info about H.323

Posted: Wed Dec 22, 2010 1:45 pm
by willamowius

there are some very good books on H.323 that are a lot easier to read than the standards document itself (which is available for free from the ITU).

See this page and start with the first two, they are my favorites.


Re: Info about H.323

Posted: Thu Dec 23, 2010 1:13 am
by paulej
You might also want to look at these two presentations I prepared some time ago:

Overview of H.323
H.323 Protocol Overview

I would read them in that order. The first one is a little higher-level, but has flows and content not in the second deck. The second deck is what I use when I provide training to people. I think it has useful content, though I'm sure it helps having a person explain things on a whiteboard.