3G-324M Helps 3G Live Up To Its Potential

Broadband wireless is now a reality. Towers are converting to 3G throughout Europe and Asia. 3G-enabled phones are flying off the shelf and service providers are making a substantial commitment to the new format. At the same time, consumers are happily embracing all of the new features and functionality that broadband wireless delivers.

Everyone wants to know what will drive adoption; give service providers and equipment vendors new revenue streams; and increase usage in both subscriptions and minutes used. Obviously, it will be something that leverages the broadband capabilities of the 3G network architecture—namely, mobile real-time multimedia communication.

At lower speeds, SMS and MMS work fine. Imagine if users were comfortable with non-real-time multimedia. 3G would be unnecessary, as 2G and 2.5G provide sufficient bandwidth—as long as the streams can be cached and reconfigured at the end point.

Obviously, the real applications for 3G are the real-time ones. They include video telephony (videoconferencing), video streaming, remote wireless surveillance, multimedia real-time gaming, video on demand, and more (FIG. 1). These "killer apps" will drive usage and increase service-provider revenue. Correspondingly, they also will raise equipment sales.

Originally, 3G was conceived as an "all-IP" solution by both the Third Generation Partnership Project (3GPP) and Third Generation Partnership Project 2 (3GPP2). In reality, however, 3G multimedia is not enabled by the IP protocol (SIP). The problem is that IP communications are sensitive to high bit error rates (BERs). Such high rates are found throughout the public cellular network.

Using IP as the underlying transport results in poor quality for conversational real-time communication. As an alternative, 3G real-time multimedia is now delivered over a circuit-switched protocol. Called 3G-324M, this protocol provides adequate quality for any sort of latency-sensitive applications.

3G-324M enables conversational real-time multimedia over third-generation (3G) technology. The 3G real-time multimedia services based on 3G-324M started in Japan. They have now expanded to the U.K., Italy, Australia, and Spain. They're currently finding footholds in more and more countries. Correspondingly, the subscriber growth rate continues to grow per month in each of these markets. In addition, more and more 3G-324M-enabled mobile terminals (3G PDAs, smart phones, and feature phones) are becoming available.

Instead of operating over IP protocol, as most communication protocols do today, 3G-324M operates over a Time Division Multiplexing (TDM) circuit-switched (CS) channel (FIG. 2). That channel is opened by the baseband protocol between communicating peers. TDM has the benefit of a fixed-low-delay service, which saves the need for routing on every hop of the IP's communication path. For low-fixed-delay services in a high-bit-error-rate environment, such as conversational voice and video, it has been found to operate well in public cellular networks.

3G-324M may be a throwback to the circuit-switched world and not "next-generation IP." Unlike IP, however, it does work for conversation video calls in a cellular network. It also allows service providers and equipment developers to enable the broadband killer applications that were previously mentioned. Those applications are delivered as a hybrid of communication technologies based on IP and 3G-324M CS.

To put it simply: IP isn't ready to support real-time multimedia over wireless. Take video telephony, for example. It requires medium to high bandwidth, low delay (two-way), medium to high quality, and a continuous connection. To provide video that's acceptable to the mobile user, the wireless network must provide a certain quality of service (QoS). Frame-delay variation, bit errors, and frame loss can have severe effects on the video quality.

Even in the fastest CDMA2000 EV-DO network, 3G video telephony for conversational multimedia communications over IP was found to be unsuitable. By "packaging" many bits into an IP packet that is carried over a public mobile network, this approach incurred a large rate of packet loss. Because IP needs to be processed for addressing in each hop on the network, each packet loss and the request to retransmit caused additional delays. In a real operational scenario of public cellular networks, the overall delay that's caused is unacceptable for conversational multimedia services.

Often, the IP packet has too many bits that cannot be recovered. The entire packet then needs to be retransmitted. As a result, the multimedia experience becomes unacceptable. Yet when a circuit-switch-based time-division-multiplexing session was opened between two communicating peers, it was found to operate well in the same bit-error-rate conditions. Of course, this session boasted suitable error detection and correction for the bits (H.223 Annexes A and B) and concealment for the codecs (MPEG-4, SP-L0, and GSM-AMR).

3G-324M's role as the video-telephony enabler to all 3G technologies is now becoming clear. Consider a transmission link with a BER of 10⁻⁵. It might be acceptable for non-real-time data transmission with some form of error correction. In a video stream, however, this error rate would cause a serious degradation in the quality of the received video. Frame-delay, frame-loss, and rate-control issues also have a significant impact on the quality of the video that's received. Simulation is needed to assess the picture quality under different propagation channels along with error-correction and/or concealment schemes.

To understand the role of 3G-324M today, it's important to know the technology's background. For conversational multimedia services in a 3G network, 3G-324M operates on an established circuit-switched channel between source and destination parties. Multipoint communication between more than two 3G-324M terminals is possible. It requires both a gateway-to-IP network and an H.323 multipoint control unit (MCU).