Nortel Networks NN43001-307 Best Bandwidth codec selection algorithm

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 sendrecvattribute.

For more information on this algorithm, refer to RFC 3264 – An

Offer/AnswerModel with the SessionDescription 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. Every 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 Table8 "Codec

types" (page 100).

Tabl e 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. 729AB G. 723.1

G.729 A G.729A G.729 A G.729 A G. 729 AB G.729 A

Nortel Communication Server 1000

Main Ofﬁce Conﬁguration Guide forSRG 50

NN43001-307 02.02 Standard

Release 5.0 3 December 2007