Nortel Networks NN43001-314 SIP Offer/Answer model

60 Bandwidth Management

Algorithm details The H.323 Master/Slave algorithm operates in the

following manner:

•The Master node uses its own codec list as the preferred one and ﬁnds

a common codec in the far end’s list. In other words, the Master gets the

ﬁrst codec in its list (for example,C1), checks in the far end’s list if it is a

common codec; if it is, C1 is the selected codec. Otherwise, it gets the

second codec in its list and veriﬁes it against the far end, and so on.

•The Slave node uses the farend’s list as the preferred one and ﬁnds

in its own list the common codec.

Issues caused by the H.323 Master/Slave algorithm The issues

caused by the Master/Slave algorithm are due to the random nature of the

Master/Slave information. In other words, one cannot predetermine the

codec that is used during a Virtual Trunk call.

The following are the issues associated with the H.323 Master/Slave

algorithm:

•After an on-hold and off-hold scenario (which triggers Master/Slave

negotiation), the codec used for the restored call mightbe different than

the one used before on-hold, because the Master/Slave information

could have been changed.

•When using "Fast Start" codec selection, a call from Telephone 1

(node1) to Telephone2 (node2) can use a different codec than a

call from Telephone2 (node2) to Telephone 1 (node1), because the

terminating end is always Master.

•For tandem calls, the Master/Slave information is not relevant. The

Master/Slave information is designed for use between two nodes only,

not between three or more nodes. It makes the codec selection for

tandem calls more complex and inefﬁcient.

Tosolve the issues, another codec selection algorithm, not based on the

unpredictable Master/Slaveinformation, is needed. Since any change to the

Master/Slave algorithm implies a change to the H.323 standard, the new

codec algorithm is used for Virtual Trunkcalls between Nor tel equipment.

SIP Offer/Answer model

The SIP codec negotiation is based on the Offer/Answer model with Session

Description Protocol (SDP).

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.

Nortel Communication Server 1000

Branch Ofﬁce Installation and Commissioning

NN43001-314 01.02 Standard

Release 5.0 20June 2007