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Monday, April 16, 2012

Getting Multimedia Telephony to replace CS (Circuit Switched) Telephony: Realizing the Radio Bearer


The Multimedia Telephony communication service is an IMS service that can be realized over any access technology. However, since Multimedia Telephony aims to replace CS telephony, the realization of the Multimedia Telephony must meet strict performance requirements such as low end-to-end delay and low packet loss rates. And at the same time, Multimedia Telephony should be a flexible communication tool that allows the end-users to choose between a set of different communication methods, including voice, video and text communication. This flexibility also puts requirements on the underlying access. As a result of the different requirements, certain access methods are more suitable for Multimedia Telephony than others.


The illustration below indicates one possible radio realization suitable for Multimedia Telephony is shown;

Multimedia Telephony use combinations of RABs (1 – in the illustration above) to provide differentiated QoS for different types of media. In the simplest case, Multimedia Telephony basic voice call, two RABs are used. One RAB (Radio Access Bearers) is used for IMS related SIP signaling, and that RAB uses the Interactive traffic class. The speech data is sent over a Conversational RAB. In the case of more complex media combinations, more advanced RAB combinations may be used. For instance, in the case of Multimedia Telephony video calls, two Conversational RABs can be used (one RAB is used for speech transfer and another RAB for video transfer). The use of RABs for Multimedia Telephony is depicted in the illustration below;


IP technology has a number of benefits, but one apparent drawback is quite large overhead of real-time media due to generous IP header sizes. To be able to compete with CS telephony in terms of capacity, these headers need to be compressed. A number of header compression algorithms have been included in the 3GPP specifications but the preferred technology is called ROHC (2 – in the first illustration above). ROHC can compress the IP, UDP and RTP headers from 60 bytes (IPv6) or 40 bytes (IPv4) down to 3 bytes.

Real-time media, like speech and video, are quite tolerant to packet losses but are intolerant to delay variations. Therefore, to avoid time-consuming retransmissions, UDP is used rather than TCP as the IP transport protocol. A radio realization suitable for realtime media should also avoid slow retransmissions on the radio link layer. Therefore, the proposed radio realization uses RLC UM (3) as the radio link control protocol mode for speech and video. For non-real-time data such as the SIP signaling, the radio bearer uses RLC AM. Optimized RLC PDU size is beneficial for capacity. When Multimedia Telephony is implemented on a 3GPP compliant mobile terminal, the speech will be encoded by AMRNB or AMR-WB. This knowledge, plus the knowledge of how ROHC works, is used to optimize the RLC PDU size so that the padding on the RLC layer is minimized.

The media flexibility aspects of Multimedia Telephony (i.e. the instant adding and dropping of media) and the variation of packet sizes due to ROHC and other ‘bursty’ IMS service enablers like Presence that may be used in parallel with Multimedia Telephony make it beneficial to use radio bearers that can transfer packets of varying sizes during e.g. speech transfer without causing significant delay variations in the media transfer. In the uplink, the preferred radio technology is the E-DCH


E-DCH can be realized using 2 ms or 10 ms TTIs. Both options perform well for Multimedia Telephony. The addition of a Hybrid ARQ enables fast retransmissions and soft combination of multiple attempts, which is beneficial for capacity as well as media quality. The E-DCH can be scheduled to increase control over the interference level. However, for low bandwidth media like speech, the uplink scheduling of E-DCH (5) may not increase capacity significantly; rather the overhead associated with the uplink scheduling may reduce capacity. Therefore, in the uplink voice capacity evaluations, non-scheduled mode is used

In the downlink, HSDPA is the preferred radio technology. HS-DSCH (6) is the shared channel
IP technology has a number of benefits, but one apparent drawback is quite large overhead of real-time media due to generous IP header sizes. To be able to compete with CS telephony in terms of capacity, these headers need to be compressed. A number of header compression algorithms have been included in the 3GPP specifications but the preferred technology is called ROHC (2 – in the first illustration above). ROHC can compress the IP, UDP and RTP headers from 60 bytes (IPv6) or 40 bytes (IPv4) down to 3 bytes.

Real-time media, like speech and video, are quite tolerant to packet losses but are intolerant to delay variations. Therefore, to avoid time-consuming retransmissions, UDP is used rather than TCP as the IP transport protocol. A radio realization suitable for realtime media should also avoid slow retransmissions on the radio link layer. Therefore, the proposed radio realization uses RLC UM (3) as the radio link control protocol mode for speech and video. For non-real-time data such as the SIP signaling, the radio bearer uses RLC AM. Optimized RLC PDU size is beneficial for capacity. When Multimedia Telephony is implemented on a 3GPP compliant mobile terminal, the speech will be encoded by AMRNB or AMR-WB. This knowledge, plus the knowledge of how ROHC works, is used to optimize the RLC PDU size so that the padding on the RLC layer is minimized.

The media flexibility aspects of Multimedia Telephony (i.e. the instant adding and dropping of media) and the variation of packet sizes due to ROHC and other ‘bursty’ IMS service enablers like Presence that may be used in parallel with Multimedia Telephony make it beneficial to use radio bearers that can transfer packets of varying sizes during e.g. speech transfer without causing significant delay variations in the media transfer. In the uplink, the preferred radio technology is the E-DCH


E-DCH can be realized using 2 ms or 10 ms TTIs. Both options perform well for Multimedia Telephony. The addition of a Hybrid ARQ enables fast retransmissions and soft combination of multiple attempts, which is beneficial for capacity as well as media quality. The E-DCH can be scheduled to increase control over the interference level. However, for low bandwidth media like speech, the uplink scheduling of E-DCH (5) may not increase capacity significantly; rather the overhead associated with the uplink scheduling may reduce capacity. Therefore, in the uplink voice capacity evaluations, non-scheduled mode is used

In the downlink, HSDPA is the preferred radio technology. HS-DSCH (6) is the shared channel



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