K.2 Procedure for computing the bandwidth

26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS

The bandwidth is calculated using the following procedure when no extra bandwidth is allocated for redundancy:

1) Calculate the size of the RTP payload, see below.

2) Calculate the size of the IP packets by taking the RTP payload size (in bytes) and adding the IP/UDP/RTP overhead: 20 bytes for IPv4; 40 bytes for IPv6; 8 bytes for UDP; 12 bytes for RTP.

3) Convert the IP packet size to bits.

4) Calculate the required bit rate (bps) given the packet size and the packet rate: 50 packets per second for 1 frame per packet; 25 packets per second for 2 frames per packet.

5) The b=AS bandwidth is then calculated by converting the required bit rate to kbps and rounding to the nearest higher integer value.

When redundancy is used then the RTP payload contains several frames, both non-redundant and redundant. For example, for 100% redundancy each RTP payload contains one non-redundant frame and one redundant frame, giving 2 frames per packet. The packet rate is however still 50 frames per packet.

If the SDP includes multiple codecs and/or configurations then the bandwidth is calculated for each configuration and the b=AS bandwidth is set to the highest of the bandwidths.