Voice Signal Level in H323 Terminal/Gateway

Shan Lu slu at nextone.com
Fri Jun 30 14:14:16 EDT 2000


I'd like to start a discussion on how to set speech signal level in an
H323 environment.

Speech signal level is an important part of QoS. It directly impacts end
user's perception of how good the system works. It is also an integral
part of interoperability testing. After the interoperability issues at
the protocol level have been ironed out, the signal levels from
different vendors have to "match" in order to provide true
interoperability for end users.

Some vendors make speech signal level a configurable parameter in their
equipment and leave it to end user to decide. However, not all end users
are capable of making intelligent decisions on this issue. When signal
levels are set improperly, end users will complain about low volume,
high volume, and/or echo. If the system consists of equipment from
multiple vendors or involves interworking with PSTN/PBX, things can
become complicated quickly.

So a set of guidelines on how to determine speech level can benefit
equipment manufacturers as well as their customers, including end users
and service providers.

I envision the guidelines should cover a number of topics:

1. desired end-end signal attenuation and acceptable range;
2. desired place of attenuation (tx or rx?) and how much;
3. desired level for DTMF and tones
4. desired level of echo cancellation;
5. connection scenarios: terminal-terminal, terminal-gateway, and

Issues that we have to contend with include: end user acceptance, voice
coder and echo cancellor optimal operation ranges, idle noise levels.

For many of these issues, we can draw upon the wealth body of knowledge
from the telephony world. Some of them may only take a reference to an
existing standard. Others may require some serious thinking and, more
importantly, agreement among multiple vendors.

The discussion certainly is not restricted to voice only applications.
But I am not sure if video has an interoperability issue as serious as
voice. I am open to any suggestions. We may continue the discussion
online or we can discuss face to face at SuperOp next week.

Now here is my question #1: Is SourceInfo/DestinationInfo structure in
H225 call setup messages a reliable way to distinguish end point type
(terminal, gateway, gk)? What if in a GK-routed call? Does the GK modify
that field?


Shan Lu

NexTone Communications, Inc
9700 Great Seneca Highway
Rockville, MD 20850
+1 240-453-6315

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