Evaluation of CRTP performance over cellular radio links

IP telephony gaining momentum and cellular telephony having several hundred million users, it seems inevitable that some future wireless telephony systems will be based on IP technology. What we know today as cellular phones may, in addition to telephony and video, have IP stacks, Web browsers, e-mail clients, net-worked games, and so on. If based on IP, telephony service will be much more flexible than today. This document concentrates on the problem of providing a good IP solution for speech, but it is understood that applications for video, games, and so forth will also have to be supported. It is vital for cellular phone systems to use radio resources efficiently in order to support a sufficient number of users per cell. Only then can deployment costs be kept low enough. It will also be important to provide sufficiently high-quality voice and video. In particular, voice service should be as good as users expect from the cellular phone systems of today. A lower quality may only be accepted if costs are significantly reduced. The radio channels used in cellular systems have very high bit error rates (BER) due to shadow fading, multipath fading, and continuous mobility. The radio signals of one user will interfere with those of other users, so with the desired number of users per cell BERs will be high. Even after error correcting channel coding, the remaining BER can be as high as 10 –3 (one in 1000) or even 10 –2 (one in 100) for some users some of the time. The only cost efficient way to achieve sufficient voice quality over such channels appears to be to use clever speech coders and decoders that can tolerate some damage to the encoded sound data. It is not feasible to use a link layer that delivers all data reliably through an automatic repeat request (ARQ) scheme with link-local retransmission. Intolerably high delays would be the result. If the long maximum delays caused by an ARQ scheme were acceptable, it would be better to spread the signal over time in order to reduce the BER rather than using an ARQ protocol. Nor is it feasible to have the link layer discard all damaged frames. The large fraction of discarded frames would result in insufficient speech quality. Unless explicitly stated otherwise, the numbers and figures presented in this document are for IPv4, not IPv6. Speech data for IP telephony will …