IMG 1010 - Early Media

Early media is the ability of two SIP User Agents to communicate before a SIP call is actually established. Typically, this scenario occurs when the called party is a PSTN gateway. Before the call is set up, the gateway might provide in-band tones or announcements that inform the caller of the call progress.

Early media can involve the transfer of media from caller to callee. Within the PSTN, forward channels can be established to convey DTMF signaling to select a final destination to call. This feature can be used to access Interactive Voice Response (IVR) systems “behind” 800 numbers.

Implementation

• connects the media path prior to the 200 OK message.

• supports pre-answer DTMF and announcements.

• converts the SS7 Call Progress (CPG) message to a 183 response message with SDP.

• When the called party wishes to send early media to the caller, the called party sends a 183 response to the caller. That response contains the SDP. When the caller receives the 183 response, it suppresses any local alerting of the user (for example, audible ring tones or pop-up window) and plays out media that it receives.

• If the call is ultimately rejected, that called party generates a non-2xx final response. When the caller receives this response, the caller stops playing out or sending media. If the call is accepted, the called party generates a 2xx response and sends that to the caller. Media transmission continue as before.