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100 Bandwidth Management
The following three cases of codec negotiation are supported:
•
The calling user agent sends an SDP offer with its codec list in the
INVITE message with a sendrecv attribute. In this case, the called user
agent selects one codec and sends the selected codec in an SDP
answer. The SDP answer is included in the 200 OK message (which is
the response to the INVITE) with the sendrecv attribute.
This is the preferred method of operation.
•
The calling user agent sends an SDP offer with its codec list in the
INVITE message with a sendrecv attribute. The called user agent
returns more than one codec in the SDP answer. In the case that many
codecs are included in the response, the calling user agent picks the
first compatible codec from the called user agent’s list, and sends a new
SDP offer with a single codec to lock it in.
• If the SDP of the calling user agent is not present in the INVITE
message, then the called user agent sends its codec list in an SDP
offer in the 200 OK message, with the sendrecv attribute. The calling
user agent selects one codec and sends the selected codec in an SDP
answer inside the ACK message, with sendrecv attribute.
For more information on this algorithm, refer to RFC 3264 – An
Offer/Answer Model with the Session Description Protocol (SDP).
Best Bandwidth codec selection algorithm
The “Best Bandwidth” codec selection algorithm solves the issues caused
by the H.323 Master/Slave algorithm. The “Best Bandwidth” algorithm
selects one common codec based on two codec lists. Ever y time the
selection is done with the same two lists, the selected codec is the same.
The “Best Bandwidth” codec decision is based on the codec type only,
it does not take into account the fact that some codecs, while generally
using less bandwidth, can consume more bandwidth than others at certain
payload sizes.
Algorithm details The selected codec is the type considered as the
best bandwidth codec type. To know whether one codec type has better
bandwidth than another, see the rule as summarized in Table 8 "Codec
types" (page 100).
Table 8
Codec types
G.711 A-law G.711 mu-law G.729 A G. 729 AB G. 723.1
G.711 A-law
G.711 A-law G.711 mu-law G.729 A G. 729 AB G. 723.1
G.711 mu-law
G.711 mu-law G.711 mu-law G.729 A G. 729 AB G. 723.1
G.729 A
G.729 A G.729 A G.729 A G. 729 AB G.729 A
Nortel Communication Server 1000
Main Office Configuration Guide for SRG 50
NN43001-307 02.02 Standard
Release 5.0 3 December 2007
Copyright © 2005-2007, Nortel Networks
.