H.320 gateways a MEGACO / ITU
Ami Amir
amir at RADVISION.RAD.CO.IL
Wed Mar 31 04:14:57 EST 1999
Hi
During the last MEGACO conference call - Tom Taylor asked what could hinder
ITU acceptance of the MEGACO work. It is obviously to everybody's interest
that IETF and ITU standards merge. As an example - it would be great if the
MEGACO work could become an ITU SG 16 contribution.
An obvious item is efficient multimedia support (as reflected also in John's
mail).
However, I think that one of the even more major difficulties facing us will
be the encoding scheme (ASN.1 vs Text).
There are many who feel that ASN.1 is too heavy and complex for simple
devices, and should be avoided. This was one of the major reasons for the
emergence of SIP.
On the other hand, experience in the ITU H.323 work has shown that since
ASN.1 is the encoding scheme on the PSTN side, the use of ASN cleared the
way for PSTN to IP interoperability. This feature will be extremely
important in hybrid networks that need to provide Intelligent Network (IN)
services (e.g. "800"), while retaining the investment in existing IN,
billing and directory services (411).
Another problem is that if ASN.1 is not chosen, every device that will have
to connect between a MEGACO component and H.323 will need to dis-assemble
and re-assemble (transcode) messages, and hence network performance will
suffer, and those devices will be more complex. A prime example - MGC to GK
communications.
I am not promoting a specific approach. I just think that this complex issue
needs to be addressed if we want to be able to be able accept a universal
protocol.
Do you think this is really a problem?
If so - any ideas on how to bridge the gap?
Ami
-----Original Message-----
From: John Segers [SMTP:jsegers at lucent.com]
Sent: Tuesday, March 30, 1999 6:27 PM
To: ITU-SG16 at mailbag.cps.intel.com
Subject: H.320 gateways
People,
In yesterday's conference call, the subject of H.320 GWs was raised
briefly. In my opinion, the connection model and protocol should be
able
to deal with H.320. I would like to continue discussion on it on
the
mailing list.
H.320 allows a user to have a session with both audio and video on a
single 64 kbit/s channel such as an ISDN B-channel. The same
channel
carries some signalling information (frame alignment, bitrate
allocation). To a MG supporting H.320, this means that on a single
endpoint, three streams can come in, carrying different types of
media.
The current connection model of megaco/H.gcp does not cater to this.
I
see two possible solutions:
The first is to allow multiple media in one context and to describe
for
terminations the logical streams they carry. In a picture:
+----------+
| |
| +--------- signalling (FAS, BAS)
| |
B-channel ==========+ +--------- audio (16 kbit/s)
| |
| +--------- video (46.4 kbit/s)
| |
+----------+
The second solution is to have separate terminations for the
different
streams. They would all "connect to" the same physical endpoint.
In
order to properly identify the terminations, it is necessary to have
logical names for them. The physical endpoint they connect may have
the
hierarchical name proposed in the megaco document.
Another example of a H.320 session is the case of two B-channels
being
used for an audiovisual call. The following frame structure is then
possible.
+--------------------------++-----------------------+
| Channel 1 || Channel 2 |
+-----+--+--+--+--+--+--+--++--+--+--+--+--+--+--+--+
|Bit 1|B2|B3|B4|B5|B6|B7|B8||B1|B2|B3|B4|B5|B6|B7|B8|
+-----+--+--+--+--+--+--+--++--+--+--+--+--+--+--+--+
1 | a1 |a2|a3|a4|a5|a6|v1|F ||v2|v3|v4|v5|v6|v7|v8|F |
2 | a7 |a8|a9|a |a |a |v9|F ||v |v |v |v |v |v |v |F |
3 | a |a |a |a |a |a |v |F ||v |v |v |v |v |v |v |F |
4 | a | | | | |a |v |F ||v | |v |F |
5 | a | | | | |a |v |F ||v | |v |F |
6 | a | | | | |a |v |F ||v | |v |F |
7 | a | | | | |a |v |F ||v | |v |F |
8 | a | | | | |a |v |F ||v | |v |F |
+---------------------------------------------------+
9 | a | | | | |a |v |B ||v | |v |B |
10 | a | | | | |a |v |B ||v | |v |B |
11 | a | | | | |a |v |B ||v | |v |B |
12 | a | | | | |a |v |B ||v | |v |B |
13 | a | | | | |a |v |B ||v | |v |B |
14 | a | | | | |a |v |B ||v | |v |B |
15 | a | | | | |a |v |B ||v | |v |B |
16 | a | | | | |a |v |B ||v | |v |B |
+---------------------------------------------------+
17 | a | | | | |a |v |v ||v | |v |v |
.
.
.
80 | a | | | | |a |v |v ||v | |v |v |
+---------------------------------------------------+
(a=audio, v=video, F=FAS, B=BAS).
We see that the video stream is split up over two channels. In
order to
cater to this, it seems we have to allow terminations to receive
media
from and send it to multiple physical endpoints. The two approaches
outlined above can both be extended to allow this. Both extensions
will
lead to the introduction of logical names for terminations. In the
first approach there will be one termination "containing" two
B-channels
on one side and three logical streams on the other. In the second
approach there will be three terminations, the one for the video
stream
referencing both B-channels, the ones for signalling and audio
referencing only channel 1.
The second approach allows us to keep separate contexts for
different
media types. It is then easy to delete, for instance, the video
part of
a session (session used loosely to desribe the contexts for the
audio
and video).
The first approach groups the streams coming from/going to one user,
making it possible to remove a user from a context more easily.
Personally, I can't decide which approach I would prefer. How do
others
feel about these ideas?
Regards,
John Segers
--
John Segers email:
jsegers at lucent.com
Lucent Technologies Room HE
344
Dept. Forward Looking Work phone: +31 35 687
4724
P.O. Box 18, 1270 AA Huizen fax: +31 35 687
5954
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