Krister Svanbro

Evaluation of CRTP performance over cellular radio links

To make mobile IP telephony over cellular radio systems an economically viable alternative to circuit-switched voice, it is an absolute requirement that the 40-octet IP/UDP/RTP headers on IP telephony packets be reduced in size to conserve bandwidth and radio spectrum. We evaluate the performance of the default header compression scheme for IP/UDP/RTP, CRTP (RFC-2508), over links built on cellular radio access technology. We find that CRTP does not perform adequately over such links, and suggest that a more robust header compression scheme must be developed to make IP telephony over cellular economically viable.

Wireless real-time IP services enabled by header compression

The world of telecommunications is currently going through a shift of paradigm from circuit switched, connection oriented information transfer towards packet switched, connection-less transfer. For application independence and to decrease costs for transport and switching it is attractive to go IP all the way over the air interface to the end user equipment, i.e., to not terminate the IP protocols before the air interface. A major reason to avoid using voice over IP over the air interface has, up to now, been the relatively large overhead imposed by the IP/UDP/RTP headers of voice packets. This paper presents a novel header compression scheme, ROCCO, that can compress the large headers to a minimum of one octet and is robust against the errors imposed by cellular links. Its performance is excellent both in terms of robustness and compression efficiency. Moreover, system capacity simulations are presented, which show that ROCCO gives significantly higher system capacity than the other current header compression candidate, CRTP. Finally it is argued that with ROCCO voice over IP over wireless becomes feasible from a spectrum-efficiency point of view.

Efficient transport of voice over IP over cellular links

Internet and cellular technologies are merging to decrease transport costs and enable IP-based applications to hundreds of millions of cellular users. Real time voice services will continue to be dominant in future cellular systems and to reduce system costs it is desirable to base all services, including voice, on IP. However, the radio spectrum must be efficiently used to provide mass-market services at reasonable prices, but Internet protocol headers are large in size. Moreover, data discarding policies are poorly suited to the error-proneness of cellular links. Protocol overhead for IP-based voice services can be reduced by compressing headers, e.g. with the compressed real time protocol (CRTP). CRTP, however, does not perform well over cellular links since it is not robust in the face of errors. ROCCO, a header compression scheme suitable for cellular systems, delivers more packets to the user than CRTP at realistic error rates with a better average compression ratio. Voice services in cellular systems should also be able to use damaged speech data. This can be done using the UDP Lite protocol which provides a more flexible checksumming policy that enables delivery of data possibly usable to the application. This paper shows that CRTP/UDP fails to deliver sufficient service quality at the error rates of a cost-efficient cellular system and that ROCCO combined with UDP Lite can provide an IP-based voice service over such cellular systems.

Capacity and speech quality aspects using adaptive multi-rate (AMR)

The AMR (adaptive multi-rate) is an emerging speech codec cellular standard in the ETSI. This standard should be ready during as a speech GSM evolution. It is a new concept for achieving a high speech quality maintaining an efficient spectrum usage. According to the channel quality and the traffic load, the radio resource algorithm allocates a half-rate or a full-rate channel in order to obtain the best balance between quality and capacity. Within this channel, the codec is quickly adapted to track changes in the radio link. An AMR system model has been developed to show the impact on speech quality by varying the capacity from only full-rate channels to only half-rate channels. The aim is also to show the gain provided by an AMR system compared with an existing GSM system using second generation EFR (enhanced full rate) and HR (half rate) coders. The results show that there is a trade-off between capacity increase and speech quality degradation. It is also very clear that there is a potential gain in quality by using AMR compared to existing speech codecs in GSM systems.

Voice-over-IP-over-wireless

The IP-technology is gaining momentum and it seems inevitable that future wireless telephony services will be based on IP-technology. However, in order for this to become reality a number of issues must be solved. In this paper we present solutions-header compression and traffic classification-and figures indicating that for WCDMA, voice over IP can be competitive with traditional circuit switched speech services.