Implementation Details

To the Access SBC (A-SBC), the Transcoding SBC (T-SBC) is a transcoding agent that offers services that the A-SBC can invoke on-demand when an offer-answer exchange requires transcoding. The A-SBC uses its public SIP interface and a corresponding realm reserved for communication with a T-SBC, which you configure as a transcoding agent on the A-SBC. You can configure multiple transcoding agents, which can be IP addresses, session agents, and session agent groups (SAG).

In a deployment with multiple transcoding agents, the A-SBC initiates communication in the order in which the transcoding agents were entered on the list. The A-SBC tries to communicate with each transcoding agent listed one-by-one until it:

• Receives a 2xx response from a transcoding agent
• The list is exhausted
• The original transaction times out

When a transcoding agent is a session agent with hostnames, the A-SBC uses DNS to resolve the hostname and tries the hosts in order. When transcoding agents are session agent groups (SAG), the A-SBC selects the session agent according to the selection strategy configured for the SAG. When you enable recursing, the A-SBC recurses through all members of the SAG. When session agents and SAGs do not have ports or transport protocols specified, the defaults are 5060 and UDP respectively.

When the A-SBC identifies a transcoding agent, the A-SBC sends an INVITE to which the T-SBC responds with a 2xx message. Configuring the A-SBC as a session agent and disabling dialog transparency (in global SIP configuration) on the T-SBC allows the T-SBC to accept SIP messages from the A-SBC. Then the T-SBC acts as a B2BUA, using the information received in the INVITE from the A-SBC to invoke transcoding and to route SIP signaling back to the A-SBC on egress.