[Asterisk-video] H320

[John Martin]

Hi Olle,
  I see there's been some follow up posts while I was writing this, but
here's my input...
  Like the IP standards H.320 is a layered protocol. So you can think of
ISDN being like IP and H.320 being like SIP/H.323. In fact for every
protocol you find in H.323 (H.245, H.225, Q.931) there's an equivalent
in H.320 (H.241, H.221, Q.931 plus others).
The H.320 is encapsulated in the B-channels, whereas with SIP and H.323
the protocols are encapsulated in IP packets. So the call control is
basic ISDN with different bearer types in the Q.931 setup etc. Once the
calls are up the H.320 protocols worry about how to align the channels
and negotiate capabilities etc. To take the comparison a bit further:
the call being spread across multiple B-channels is like trying to take
the RTP in a SIP call and de-mulitplex every RTP packet into n packets
(where n=8: RTP byte 0 goes to packet0[0], RTP byte 1 goes to
packet1[0]... RTP byte 8 goes to packet0[1] etc), and then send the n
packets in parallel - with the commensurate differing delays between the
n parallel streams that have to be re-synchronised when they reach their
destination. This scheme is very similar to the way that GSM does stuff
like HSCSD.

 One way to think of H.320 would be as the video protocol for ISDN, so
in that sense it could be included into libpri/zaptel. But then you've
got to add it to mISDN and they'd have to share the same code, but, as
you say, with different API's :-(


John Martin
http://www.AuPix.com


-----Original Message-----
From: asterisk-video-bounces at lists.digium.com
[mailto:asterisk-video-bounces at lists.digium.com] On Behalf Of Olle E
Johansson
Sent: 09 June 2006 09:50
To: Development discussion of video media support in Asterisk
Subject: Re: [Asterisk-video] H320


9 jun 2006 kl. 10.43 skrev John Martin:

> Good point. I was wondering this on the way into work this morning.  
> I'm
> not sure how a lump of code that provided H.320 would be able to be  
> used
> in both PRI and BRI channels, or if it's an app, func, channel or res?
>
> It brings up an interesting point about things like SIP, IAX and H.323
> they all use IP as the transport but have their own channel drivers
> rather than a common IP channel driver and an app/func/res... just an
> observation. So things that are IP based can have their own channel
> drivers but ISDN based things can't :-)

Well, I was just a bit curious. It's not obviously a new channel seen
with eyes from someone who does not know the details. Is the signalling
very much different on an ISDN video call than an audio call?

Unfortunately the API for MISDN and ZAP channels are very different.

For IAX2, SIP and H.323 the signalling is *very* different. Do remember
though that SIP, H.323 and MGCP all share the RTP driver :-)

/O
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