There has been a tremendous rush to acquire and roll out 3G broadband wireless services in key markets. While both 3G standards bodies--3GPP (3rd Generation Partnership Project) for UMTS networks and 3GPP2 for CDMA2000 networks--envision 3G as running entirely over a pure IP-based communications network, real network behavior facts tells a different story.

In this article we will discuss why IP is not ready to support real-time 3G multimedia, and explore a new protocol that has gained in popularity lately as a way to enable the delivery of real-time high-bandwidth content, such as video telephony and video-on-demand (VOD), to 3G handsets using traditional circuit-switched channels rather than IP packets.

3G: A growing worldwide market for wireless broadband applications

Today more than 30 operators serving more than 32 million 2.5G-3G users in over a dozen countries, with numbers steadily increasing, operate 2.5G/3G networks. The broadband wireless revolution, namely 2.5G/3G, has begun! Nonetheless, consumers will not change service suites for a concept. In order to drive market adoption of broadband wireless and 2.5/3G, there has to emerge appealing differentiated services that will leverage the broadband functionality of 3G.

Real-time conversational multimedia communication services and applications, capitalizing on the broadband nature of 3G, provides these extremely high value, differentiated services including:

• Videoconferencing and video telephony for business or private use

• Surveillance services

• Doctors on the move

• Adult entertainment

• Multimedia dating

• Live video broadcasting

• Video-on-demand

While delay-tolerant applications such as multimedia messaging (MMS), presence, Web browsing and e-mail can tolerate the narrow bandwidth and latency delivered by 2G/2.5G networks for real-time multimedia communication services, such as those listed above, the cellular network needs a format that supports the call and service quality found on the public network and circuit-switched communications, fixed delay, not the best effort of the public Internet.

Today public mobile networks cannot operate real-time multimedia communication using IP

While the vision of an all IP network is shared by all, the current state of today's IP network and infrastructure presents several severe drawbacks to operate delayed sensitive applications running IP over 3G public mobile networks.

The following describes the main drawbacks of conversational multimedia using IP over pubic mobile network:

Lack of guaranteed QoS--A fixed low delay is required for the entire conversional multimedia session. The reality of IP is that end-to-end QoS cannot be provisioned in most IP networks today, on a per-call basis, using existing IP infrastructure. While MPLS-based networks do provide that QoS as long as both peers are on the same network, we do not see the MPLS peering that is required to truly support an all-IP broadband wireless solution for real- time multimedia communications.

High overhead and bit error rate sensitivity--For carrying conversational multimedia communication, codec streams between the communicating peers should be provisioned in real time, fixed low delay manner and with minimal codecs transportation overhead. IP multimedia communication incorporates a high overhead as it contains the codec data carried in the data portion of an IP PDU. The public mobile network naturally has a high bit error rate (BER). Each bit error that cannot be recovered by the receiving side will cause a re-transmission request, and the entire IP packet retransmission will be required. That makes IP an extremely delay-sensitive transport protocol for high BER environments and therefore conversational multimedia communication over 3G networks is not feasible over IP today.

Signaling address space--In order to use IP-based signaling, in mass-market mobile devices, each device should be assigned with a permanent IP address. Having more than 1 billion mobile devices today, the address space required for paramagnet IP address allocation can not be carried with the limited 32 bits address space of IP version 4, with its subnet schemes that limit the valid address space even more. The 3GPP/3GPP2 working groups defined IPv6 to be used for allocating a permanent IP address to each Mobile device. However, IPv4 is everywhere today while IPv6 is far from being deployed, and this situation might take at least few more years to overcome. Until there is a networkwide deployment of IPv6, with its additional signaling address space, IP will not support real-time broadband applications over 3G.  

To enable 3G conversational multimedia communications over IP, a full IPv6 deployment is required with end-to-end IPv6 interoperable network equipment, configured for required QoS.

Saying that, is there any feasible solution today to enable the conversational multimedia for the Mobile 3G? Yes, a workable solution is, instead of embracing "next generation" IP for conversational multimedia, simply leverage the high QoS and mature signaling of "old generation" time division multiplexing (TDM), based on a circuit-switched communication channel between mobile peers. For other IP delay-tolerant applications, such as Web browsing, MMS and e-mail, IP can be used in additional packet channels.

The 3GPP standard body responsible for UMTS/WCDMA specifications defined very specifically the structure and implementation requirements of a circuit switch-based channel, the 3G-324M standard. The 3GPP2 standard body, responsible for CDMA2000 specifications in August 2002 approved a similar technical spec for 3G-324M operation requirements over CDMA2000 networks--named "3GPP2 C.S0042 for Circuit-Switched Video Conferencing Services." Currently several CDMA2000 vendors are planning to add 3G-324M capabilities into their mobile and central products based on 3-324M, most notably NTT/DoCoMo, which is already offering 3G videoconferencing service using the 3G-324M standard. Figure 1 shows a 3G network using the 3G-324M protocol for real-time multimedia delivery, while using IP packets for provisioning other common Internet services.

Figure 1: 3G-324M within the 3G network

3G-324M is currently in operation in a live UMTS/WCDMA network by J-Phone and NTT DoCoMo in Japan. In addition there are other operators in Europe that are planning to have 3G-324M technology services this year. Currently J-Phone and DoCoMo are offering 3G-324M-based cell phones from several large mobile vendors such as Toshiba, SHARP, Sanyo and Panasonic. More operators such as Vodafone, AT&T Wireless and Orange are planning to deploy such services in the near future. An interesting trend of thought is to enable two channels of communication operating at the same time--IP and circuit switched--so that non delay-sensitive applications such as SIP presence will operate using the IP channel. The circuit-switched channel is used to support a broadband multimedia session, such as videoconferencing, between parties, and IP is used for less delay-sensitive applications. 

3G-324M basics

3G-324M is a derivative of H.324, which was developed for an analog public network connection and V.34 modems. H.324 is a long-in-the-tooth protocol for the setup and tear-down of videoconferencing sessions over analog phone lines. As a baseline of the terminal specification, H.324M was adopted as a mobile extension, with bit error handling procedures to support the delivery of delay-sensitive applications (video streaming, videoconferencing) over 3G networks. The protocol does not handle addressing but comes into play only after a call setup using ISUP logical procedure and E.164 addressing methods (very mature standard) over underlying 3G wireless protocol (e.g., WCDMA). In that stage the called peer is located and the call is set up between the two call peers, establishing a circuit-switched channel between them. When that circuit-switched channel is operating, the 3G-324M is initiated to synchronize the mux-demux between parties and to start the H.245 call control operation.

Figure 2: The 3G-324M protocol architecture

Integrating 3G-324M with other multimedia over IP protocols

While 3G-324M is a straightforward protocol to implement in end devices and media servers, primarily through the use of a protocol toolkit such as that provided by Radvision, those developers of gateways must by necessity interface 3G-324M end points with tradition H.323 and SIP-based protocols. The interworking function resides in a gateway network element; refer to Figure 3 for the architecture of the interworking function.

H.323 and 3G-324M: H.323 is based on Q.931 for call setup and H.245 for call control. 3GPP defines TS.26.112 for call setup procedure in UMTS. The interworking device shall map theTS-26.112 call setup into Q.931 H.323 calls and vise versa. For Call control mapping, since both protocols uses H.245 the mapping is trivial, however the H.245 in 3G-324M is address-less. For codecs mapping needs, to assure operation with various H.323 devices supporting Codecs such as H.261 and H.263, while 3G-324M uses MPEG-4 as standard de-facto, a transcoding function may also be required.  

SIP and 3G-324M: SIP is based on Session Description Protocol (SDP) for both call setup and call control. Hence both TS 26.112 and 3G-324M H.245 call control should be mapped into SDP messages and vise versa. For codecs mapping needs, SIP terminals are similar to H.323 and hence similar transcoding function may also be required.

Figure 3: 3G-324M protocol interworking

Conclusion

The crux of the problem is that today's IP network (the Internet) is not sufficiently robust for delay-sensitive applications, such as conversational multimedia, especially in wireless environment with high BER such as public mobile networks and, in fact, will not be so at least until service providers move to a fully deployed IPv6 networks and SIP-based IP communications. IP, with its variant transmission delays (many hops of routing processing and congestion delays) and packet overheads, is not ready for "prime time." The fact that IP is BER sensitive and has a high overhead for codec transportation may delay its use for conversational multimedia even more.

While the vision of a true 3G IP-based network has been pushed out a bit, the promise of a feature-rich, multimedia wireless experience has not. This is due to the emergence of a standard, called 3G-324M, which addresses and supports the real-time conversational multimedia over 3G wireless network by routing traffic as a fixed delay one, over the circuit-switched network. Being circuit-switched based, the standard has all the hallmarks of a protocol ideal for provisioning bi-directional real-time multimedia, including a fixed delay, low overhead of codecs, and no IP/UDP/RTP header overheads.

3G-324M, based on ITU H.324M and specified in detail by 3GPP (3GPP TS 26.112 and 3GPP TS 26.111 Working Groups), enables the development, deployment and support of a wide variety of delay-sensitive applications over UMTS and CDMA2000 networks. These include multimedia conferencing with other 3G mobile end points, and wire lined H.323 or SIP terminals, cell phone TV, video-on-demand (news, sports, etc.), and multimedia, multi-participant gaming, to name a few applications.

Combing 3G-324M services with non-delay sensitive SIP based services such as presence can enable new type of differentiated mobile 3G services and truly represents the architecture of real time multimedia over 3G for years to come.

Eli Orr is Product Marketing Manager, Technology Business Unit, for Radvision.

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