Establishing a SIP Connection

June 1, 2008


For two codecs to successfully communicate via SIP through a SIP server, information must be exchanged between the two devices. Codecs manufactured by members of the Audio-via-IP Experts Group are SIP compliant. These manufacturers include AETA, Mayah, Oban/CRL and Tieline.

When one codec attempts to communicate with another via SIP, the two units begin the negotiation process shown in the figure.

Codec 1 first suggests an audio encoding algorithm, sampling rate and coding rate, such as MPEG Layer 2 at 48kHz and 384kb/s. Codec 1 can also suggest an alternate encoding method, such as MPEG layer 3 at 44.1kHz and 256kb/s. Codec 2 is unable to use Layer 2 audio, so it selects the alternative (Layer 3) by responding with those settings. Codec 1 acknowledges the chosen algorithm by transmitting an ACK signal. The two devices are then connected via the Layer 3 option.

It is common for a device to offer several options of encoding to ensure that a connection can be established. The SDP portion of the SIP message carries this information, and the choices are offered in an order of priority for the receiving codec to choose.



Want to read more stories like this?
Get our Free Newsletter Here!

Comments