Avaya Application Solutions Deployment Manual page 172

Traffic engineering
The TDM usage rates of 130.0 Erlangs for Site 1 and 46.7 Erlangs for Site 2 can both be easily
handled by the TDM facilities of a single PN, which is capable of carrying up to 200 Erlangs of
TDM traffic at a P001 GOS. Therefore, the assumption that all calls between two
circuit-switched endpoints is intra-Port Network is valid. If one PN was insufficient to support the
TDM usage in one of the sites, the calculations would have been repeated under the
assumption of two PNs. If a pair of PNs was still insufficient, the number would continually be
incremented until there were enough port networks to handle the TDM usage in that particular
site. Finally, the TDM resources on the two G350 Media Gateways are easily sufficient for
supporting the 17.9 Erlangs of TDM traffic in Site 3.
Note:
The more PNs, the more inter-Port Network calls there are, and hence more TDM
Note:
usage, since each interport network call requires resources in each PN that is
involved in each call.
Each TN2302 Media Processing circuit pack (or Media Gateway VoIP Media Module) can
support only a finite number of simultaneous calls. However, the exact number that can be
supported varies according to the codecs of the calls to be supported. In general, compressed
calls (for example, G.729 codec) require twice as many media processing resources as
uncompressed calls (for example, G.711 codec). Also, calls utilizing AES media encryption
require approximately 25% more media processing resources than unencrypted calls.
A TN2302 Media Processing circuit pack (or a MG VoIP Media Module) can support both
compressed and uncompressed calls, as well as both encrypted and unencrypted calls, all
simultaneously. Therefore, the general model for sizing the media processing resources is very
complex. The model is a "batch arrival and service" model, and the details are beyond the
scope of this document.
In practice, a fairly common strategy is to use an uncompressed codec for intrasite calls, and a
compressed codec for intersite calls. This is due to the trade-off between bandwidth savings,
increased media processing costs, and voice quality for compressed calls. If a private LAN is
used for intrasite calls, bandwidth usage is of less concern than media processing cost and
voice quality. However, for intersite calls, especially over a public WAN, the bandwidth savings
offered by the use of compression outweighs the extra processing costs and slight degradation
of voice quality.
Recall that any usage that is expressed in Erlangs represents the average number of busy
servers at any given time. For the total media processing usages provided at the bottom of
Table 30, a "server" can be thought of as the set of media processing resources that is
necessary to support a single bidirectional media stream through a media processing circuit
pack. Consider the total of 89.4 Erlangs of media processing usage in Site 1. This usage
consists of 76.8 Erlangs of intrasite usage, and 12.6 Erlangs of intersite usage. Assume that an
uncompressed codec is used for the intrasite calls, and a compressed codec is used for the
intersite calls. Since each compressed call requires twice as many media processing resources
as each uncompressed call, the 12.6 Erlangs must be counted twice. Therefore, the media
processing load is actually 76.8 + (2 x 12.6) = 102.0 Erlangs. Similarly, the total media
processing loads in Sites 2 and 3 are 43.9 Erlangs and 17.6 Erlangs, respectively. Those
numbers are also based on the assumption that media encryption was not used.
172 Avaya Application Solutions IP Telephony Deployment Guide