SIP and AMR

Another post on SIP, yes I am doing a lot of experimenting around it these days. While we don't have AMR-Wideband yet on Nokia phones, one experiment has at least confirmed that for Nokia SIP to SIP device calls, the bandwidth efficient AMR (Adaptive Multi Rate) codec is used instead of the default G.711 codec. The figure on the left shows how this looks like in Wireshark. While the bandwidth requirement of G.711 is 2 x 85.6 kbit/s (uplink + downlink), AMR brings the transmission bandwidth, including Ethernet, IP, UDP and RDP header down to 2x 34.4 kbit/s. The codec rate itself is only 12.2 kbit/s, so 2/3 of the required bandwidth goes into packetization header overhead. While not much can be done about that in most parts of the network, Robust Header Compression (ROHC) in future wireless radio networks can reduce the overhead from 54 bytes down to just a few, thus making SIP calls almost as efficient as current wireless circuit switched calls.