Expect standards to drive killer apps in mobile video telephony

Expect standards to drive killer apps in mobile video telephony
As mobile operators migrate to 3G networks and services, they see video telephony services as the killer app that will differentiate new 3G service from existing 2G and 2.5G ones.

Although it is possible to have limited video-based services such as delivery of pictures and video clips as well as streaming video with 2.5G, these are delay-insensitive applications that run over a packet-based wireless network. But with 3G, two approaches for delay-sensitive applications like video telephony have been defined by the 3GPP consortium: known as Release 99 and Release 5.

In Release 99 architectures, services that require guaranteed delivery with real-time constraints and no repudiation of billing are presented over the more traditional circuit-switched network. Release 5 provides packet-based delivery for such services, but requires features to enable quality-of-service guarantees and reliable billing.

All 3G mobile networks are likely to eventually migrate to the Release 5 architecture, but most early deployments are based on Release 99. In some cases, specifications for the required protocols or functionality are not mature and in other cases such features are not yet available in network infrastructure.

Networks conforming to Release 99 are based on 3G-324M, a derivative of the existing ITU-T H.324 standard for low-bit-rate media communication, which was initially intended for video telephony over wired nets. Through a series of annexes it has been extended to make it suitable for use on mobile devices and networks. These annexes (A, B and C, also known as Mobile Levels 1, 2 and 3) provide increasing levels of error resilience to cope with data loss over radio interfaces.

H.324 together with Annexes A, B and C is usually referred to as H.324M, an umbrella standard referencing other standards that specify mandatory and optional video (H.263 mandatory) and audio (G.723.1 mandatory) codecs; the messages to be used for call setup, control and tear-down (H.245); and the way that audio, video, control and other data are multiplexed into a bit stream to be sent over the network and demultiplexed when received (H.223). The main differences between H.324 and H.324M are in the H.223 multiplexer, which provides different levels of protection of the transmitted data and control information depending on the mobile level that is set.

The 3GPP (http://www.3gpp.org) and 3GPP2 (http://www.3gpp2.org) standards bodies have taken H.324M as a starting point and modified it to create 3G-324M. The 3GPP body produces specifications for 3G systems based on the Universal Mobile Telecommunications System standard and the evolving Global System for Mobile Communication (GSM) standard core networks that prevail in Europe while 3GPP2 performs a similar function for 3G systems based on CDMA2000 and the evolving ANSI-41 core network.

The principal differences between 3G-324M and H.324M are in the codecs supported and in the limiting of mandatory support to Mobile Levels 1 and 2. The overheads associated with Mobile Level 3, in terms of computational requirements and impact on available payload, are considered to be too great.

The change to mandatory codecs is due in part to the limited resources and battery life available in handsets. 3G phones already have built-in audio codecs to support basic voice telephony. GSM-AMR is used for 3G GSM networks and EVRC for CDMA2000. Allowing GSM-AMR or EVRC to be used in the place of G.723.1 means that an additional codec does not have to be provided on the handset. Also, unlike G.723.1, the GSM-AMR and EVRC algorithms have been specifically developed to give good voice quality on mobile networks.

For Release 5 networks, handsets will be based on the Session Initiation and Session Description Protocols (SIP/SDP). Work in this area is much less advanced than for 3G-324M, where many handset manufacturers are currently readying their 3G-324M products for the market.

Many factors will ultimately decide whether circuit-switched mobile video telephony is a success. 3G-324M is considerably more complex and sophisticated than standard voice telephony. Just ensuring that handsets from different manufacturers will operate with each other and with network equipment is a significant task. Good tools for analyzing and debugging will be essential in the early stages of deployment.

People using video telephony will require access to the same basket of services that they get with their voice service. This includes roaming, call forwarding and mailbox services, which will have to be enabled or enhanced to take account of the video aspect of the call.

In the early stages of 3G rollout there are likely to be islands of 3G within a largely 2G and 2.5G network; the service will have to handle this by building video-awareness into the network, allowing video-enabled and voice-only terminals to operate with one another. Therefore, enabling 3G video telephony services is as much about building 3G-324M awareness into the network as it is about providing 3G-324M handsets.

Video telephony has had a glorious future that has failed to materialize when launched on other networks, often because of a lack of a critical mass of users to bootstrap the service. Mobile video telephony services must overcome this barrier by providing access to as wide a variety of video-enabled end users as possible. This means allowing 3G video telephony customers to call people on Internet-based H.323 and SIP video and voice over IP terminals and to call ISDN based H.320 video telephone users.

This interoperability with wired phones will make the emerging video services much more attractive initially to corporate customers. Allowing mobile video phones to access streamed content available on the Internet, such as sport and music clips, will appeal to younger users. Providing multipoint calling, which appeals to many markets, will drive the uptake toward and then well beyond critical mass.

Offering this rich set of services to achieve the vision of mass-market mobile video telephony requires sophisticated media gateways at the edge of the mobile network. These gateways convert control signaling and transcode media between 3G-324M and a range of other popular protocols including H.323, SIP, H.320 and RTSP. As Release 5 services become available, such gateways will also allow seamless interconnection of circuit-switched Release 99 mobile video telephony users to communicate with users of packet-based Release 5 mobile video telephony.