Subject: [AVT] VBD vs. RFC 2833 vs. V.150.1 for V.21

From: Tom Taylor <[email-masked]>
Date: Thu, 20 Nov 2003 09:33:35 -0500

I don't know if this is the right place to ask this question, or whether 
I should go to Study Group 16.  During my rewrite of RFC 2833 I'm trying 
to add a few words of advice about the encoding of V.21 used to carry 
the conversation, as opposed to control signalling.  This comes up with 
respect to text telephones.

V.21 is a frequency shift modem operating at up to 300 bits/s, one pure 
tone for "0", another for "1".  By my rough calculations, using G.711 
for carrying the tones takes more bandwidth than RFC 2833, assuming 
similar packetization intervals.  I see it as:

G.711 = 64000 bits/s.

RFC 2833 events = 300 bit events/s x 16 bits/event report x 4 repetition 
factor = 19200 bits/s.  There's more for redundancy headers, but this is 
the right order of magnitude.

V.150.1 modem relay would be more compact still, since it reports octets 
rather than bits per sample.

So the advice would be to carry V.21 as RFC 2833 events or use V.150.1 
modem relay in preference to G.711.  Do I have it right, or was I too 
careless with my calculations?

Tom Taylor

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

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From: "Gunnar Hellstrom" <[email-masked]>
Date: Thu, 20 Nov 2003 23:48:48 +0100

Tom,
If you know that the V.21 is for text telephony, you would be even better
off using RFC2793 for its transport, because it is textphone type
independant, and can be converted to IP text telephony or any other kind of
text telephony.

But if you are in a big general borderline gateway between PSTN and IP, it
may be hard to tell if V.21 signals are for text telephony or some kind of
data communication. If the origin or destination is a SIP terminal UA, you
may assume text telephony, and use RFC2793.

But for gateway to gateway traffic it may be impossible to distinguish
between the applications.
For that case, the digital transports you mention may be used, and it seems
that your conclusions are right.

If there is a slightest risk ( >0.5%) of loosing packets when transmitting
audio coded modem signals in G.711, redundancy must be used on G.711, and
that is even more bandwidth consuming.

There are textphones based on V.23 as well. They transmit at 1200 bits/s. I
think that including V.23 bit events in RFC2833 would not be good
bit-economy.

Issues like this are studied in ITU-T Q.11/16 and Q.H/16. So, I think the
question should be treated there, to find out if there is any benefit of
RFC2833-code text telephony. The idea was touched last spring, but abandoned
because of bad bit-economy and too tight coupling between the ends.

If any such coding would be of interest, there could be a reason to study
the carrierless modes separately: Baudot(1400/1800Hz FSK), EDT (=V.21 110
bits/s half duplex ) and DTMF.
The carrier based V.21 (full duplex 300 bits/s), V.23, Bell 103, and Native
V.18( based on V.21 full duplex  ) may better be transported by some byte
coded mechanism, like RFC2793 or some other relay coding.

It is the intention of RFC2793bis to offer a transport for all these modes.
So, I think that, after considering the alternatives, we would like to stick
with that code for text transport.

Agree?

Regards

Gunnar
-------------------------------------------
Gunnar Hellstrom
Omnitor AB
Renathvagen 2
SE 121 37 Johanneshov
SWEDEN
+46 8 556 002 03
Mob: +46 708 204 288
www.omnitor.se
[email-masked]
--------------------------------------------

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Tom
Taylor
Sent: Thursday, November 20, 2003 3:34 PM
To: [email-masked]
Subject: [AVT] VBD vs. RFC 2833 vs. V.150.1 for V.21

G.711 = 64000 bits/s.

Tom Taylor

---

From: Tom Taylor <[email-masked]>
Date: Thu, 20 Nov 2003 19:06:03 -0500

I forgot about RFC2793.  I'll have a look, then add it to my words of 
advice.

I'll be sending my draft out shortly to selected readers, once Henning 
gives the basic OK.  I'll make sure you're on the list -- there seemed 
to be a few oversights in the definition of V.18-related events, and 
perhaps with my new words you'll spot the gaps or else suggest changes.