Nortel ISDN Signaling Link, Описание, установка и эксплуатация

Страница: 136 / 380

Page 136 of 378 ITG Engineering Guidelines
553-3001-202 Standard 1.00 April 2000
The DSP coding algorithm parameter sets the preferred codec of each ITG
card. The recommendation is to use Profile 1, and to set the preferred codec
to G.729A with Voice Activity Detection/Silence Suppression with a payload
setting of 30 ms. With this codec-payload combination the ITG can deliver a
good QoS but loads less than 10 kbit/s per port on the intranet.
It is recommended that all the nodes in the ITG Trunk network have a
common preferred codec. From a network planning perspective this provides
a predictable load on the intranet since all calls will negotiated on one codec.
If multiple preferred codecs are configured in the network, some calls will
negotiate a G.723 5.3K call successfully, while other calls will default to the
G.711A/G.711U codec when the originating and destination codecs do not
match, since this codec is available in all three images.
Consider if the ITG network results in tandem encoding for some of the users.
Too much consecutive coding and encoding by G.729 Annex AB, G.723
6.3K, G.723 5.3K, or G.729A codecs can lower the end-to-end quality of
service.
To maintain an acceptable QoS on speech, silence suppression can be
disabled under some conditions (e.g., in tandem networking conditions when
some trunk facilities have excessively low audio levels).
Fall back threshold
There are two parameters, the receive fall back threshold, and the transmit
fall back threshold, which can be set on a per site pair basis.
“Set QoS” on page 108 and “Measure intranet QoS” on page 114 sections
describe the process of determining the appropriate QoS level for operating
the ITG network. Site pairs can have very different QoS measurements if
some traffic flows are local, while other traffic flows are inter-continental.
You can consider setting a higher QoS level for the local sites compared to
the international sites, keeping costs of international WAN links down.
Normally you must set the fall back threshold in both directions to the same
QoS level. In site pairs where one direction of flow is more important, you
can set up asymmetric QoS levels.
Payload size
The ITG default payload sizes are as follows: