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Packet switching right choice for wireless

Posted: 01 Feb 2001 ?? ?Print Version ?Bookmark and Share

Keywords:packet switching? ip? wap? voip? acm?

Internet protocol (IP) has become remarkably popular in the last few years, largely due to reduced costs and the integration of voice and data. The Internet switching model can cost two orders of magnitude less than conventional circuit-switched architectures, making the economics of an IP-based network very attractive.

A telecom circuit switch costs around $5 million to $20 million, whereas packet-switch routers with the same bandwidth cost about $1 million or less. Additionally, the ease of integration of packet-switched IP-based systems is attractive for developing and installing new services, compared to a circuit-switched model in which a wide variety of open and proprietary interfaces may be needed to implement even a single service. Finally, for the service operator supporting both voice and data, a single IP network can be simpler to maintain and provision than a hybrid network of circuit and packet lines.

Today, wireless networks are almost completely circuit-switched and voice-oriented. Yet, in Japan, the i-mode is receiving phenomenal acceptance. WAP has also debuted in Europe, and is making inroads in the United States. Consequently, there has been a strong push toward wireless IP architectures, paralleling the trend in wired networks.

To address this trend, the two worldwide wireless standards organizations?Third Generation Partnership Project (3GPP) and Third Generation Partnership Project #2 (3GPP2)?have initiated programs to create an IP-based architecture for their core networks to support both voice and data on a single architecture.

Data is inherently IP-based. To date, however, voice-over-IP (VoIP) has not been used in the wireless domain. Currently, participants from all areas of the industry are actively pursuing ways of transporting VoIP in the wireless arena and are trying to define and resolve the key issues surrounding wireless VoIP.

Among the major issues facing wireless-system designers is the fact that there are a large variety of VoIP protocols and related algorithms. In a system, some level of support is needed for all of these algorithms, to preserve backward compatibility with existing and previous standards. In addition, new voice codecs are continually being developed to increase compression ratios and reduce latencies.

A second issue is the close coupling of existing vocoder technology with the air-interface protocol. The air-interface protocol and the vocoder in a CDMA, TDMA, GSM or wideband CDMA (W-CDMA) handset have been optimized together for maximum efficiency.

A third issue designers face is packet loss, a phenomenon common in all packet-switching networks, including IP networks. Unlike circuit-switched networking in public telephone systems, no end-to-end physical circuits are established in IP networks. IP packets from many sources are queued for transmission over an outgoing link in a router. Packets are transmitted one-by-one from the head of the queue. If there is no space in the queue, an arriving packet is lost in the network. As traffic grows, routers often become congested, resulting in packet loss.

Packet loss can cause severe damage to voice quality for IP telephony. Each IP packet contains 20ms to 80ms of speech data, matching the duration of critical units of meaning, which in speech are called phonemes. When a packet is lost, a phoneme of continuous speech is lost. While the human brain is capable of reconstructing a few lost phonemes in speech, the loss of too many packets creates unintelligible messages.

Beyond these initial barriers, system designers are confronted with the use of conventional microprocessor, DSP and ASIC technologies, which have processing power limitations, given the computationally intensive demands of VoIP. Computational requirements for VoIP algorithms range from 8Mips to 35Mips. Adaptive computing machine (ACM) circuitry is an emerging class of IC technology that gives designers higher performance and lower power to resolve these VoIP processing limitations.

ACM is a specialized computing circuitry consisting of adaptive function blocks married to programmable interconnections, all of which can be changed in real-time. In short, it allows software to create hardware on demand, providing a solution with the flexibility of software and the performance of hardware. Consequently, an ACM architecture is significantly more efficient than a rigid and inflexible DSP- or ASIC-based VoIP implementation.

Resources on an ACM chip can be allocated in parallel to the limit of availability, since these resources are not dedicated to particular functions as they normally would be in a microprocessor, DSP, or ASIC. This allows an ACM-based device to act as a parallel processing machine, as opposed to the single-threaded and/or limited parallelism in a typical microprocessor or DSP. While a large fraction of a microprocessor's or DSP's fixed resources may sit idle at any particular moment, most of the time they are idle while awaiting an assignment for which they are suited. Also, DSPs generally operate on fixed data sizes that are typically multiples of a byte. However, the dynamic or adaptive logic of an ACM operates on any data width, and the width can vary with time to suit the needs of a VoIP design problem.

The necessary logic for each portion of VoIP algorithm is thus optimally implemented on the adaptive silicon at any given point in time. As a result, the need for DSPs, ASICs and microprocessors or microcontrollers is eliminated.

The challenges are whether to carry existing VoIP over the air-interface protocols and rely on existing VoIP vocoders; introduce new vocoders that are better suited to the air interface; or transcode between existing air interface and VoIP vocoders so that these two technologies will operate efficiently together. All of these changes must be made without discarding many of the existing efficiencies of the air-interface protocols.

Any IP-based application carries with it a certain amount of overhead, meaning it contains header information. In a wireless environment, this header information is very significant because it is correlated with a loss of capacity. In a 10Mbps or 100Mbps ethernet-based system, a 20 percent or 30 percent overhead for header information is insignificant. However, that same amount of overhead is highly unacceptable in a wireless environment and to a wireless carrier, which is expected to include VoIP.

The industry must therefore find a way to efficiently transport or extend VoIP protocols to a terminal. It is not yet clear how best to do this.

The 3GPP2 organization has wrestled with this issue for some time. Part of the discussion involves further design phases as a way to resolve this problem. Also, the group has discussed extensively whether the IP stops at the cell site or continues to the mobile station. As far as transferring data, most agree that some compression techniques should be developed to shrink the header overhead. However, the jury is still out on whether this is adequate for voice.

Considering these perplexing and problematic areas, these questions arise: How can the industry design cellphones that can support VoIP? What VoIP algorithm can handle the highly computationally intensive VoIP codecs? And the system designer must keep in mind that there are two sets of vocoder standards?one set from the wired domain, the other from wireless. Although their function is similar, they are not the same.

Further questions include: How can a VoIP wireless handset be designed to deal with all of these codecs? Should system designers rely on conventional wireless codecs on the air-interface portion, and then reconvert data from these codecs to VoIP codecs at some point over the network? Or, do they use existing VoIP codecs and transport them to mobile stations? Or, will wireless OEMs have to encourage VoIP codec vendors to support wireless codecs in addition to the current VoIP standards?

From the view of a wireless handset OEM, these crucial questions pose significant engineering and profit/loss concerns as the demand for VoIP continues. Presently, there is no single right answer; instead, there are numerous right answers, and this number will only continue to grow. Thus, trying to design a VoIP wireless handset is a major challenge.

? Bob Plunkett

Director, Product Management

QuickSilver Technology Inc.

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