RTP payload multiplexing between IP telephony gateways

We describe a new method to multiplex a number of low bit rate audio streams into a single RTP stream between IP telephony gateways. Audio frames from different users are assembled into a multiplexed RTP payload thus reducing the overhead of RTP/UDP/IP headers. To maintain distinction between the multiplexed audio streams sharing a single RTP stream, a 2-byte mini-header is added to each frame of an audio stream. A unique channel identifier is associated between the peer gateway entities by means of signaling procedures. Major applications of this method will be in the areas of IP telephony gateways that interconnect PSTN/PBX systems and cellular access networks. Results have indicated that the overhead is reduced by 50% to 80% depending on the size of the audio frame. Other advantages of the proposed scheme include fewer UDP/IP connections between gateways, a reduced possibility of congestion at intermediate IP routers, and a reduction of processing delays in multiprotocol label switching (MPLS), differentiated service (diff-serv) and IP security (IPsec) environments.