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Wireless data has always been next year’s big success. While it has achieved some penetration of the market, it is only a tiny fraction of the dominantly voice market. While, a few years ago, CDPD was all the rage, now talk is centering around wireless internet access. This is, for example, one of the major aims of the ITU’s IMT-2000 3rd generation wireless initiative, which requires 144 kbps for mobile users (and 2,000 kbps for stationary users).
The internet is closely associated with steadily increasing access speeds. Today, individual users are generally connecting with 28.8 kbps modems from homes and offices, and migrating to 56 kbps. While ISDN has not been a roaring success, one can expect the next big leap to be to xDSL services at multi-megabit speeds over existing phone lines, which will become widely installed as availability increases and prices decline. This technology is already available in some areas, offering high speed access to the internet at a price that, even today, most small information-economy businesses can afford, although it is still expensive for the average consumer. The speeds offered by xDSL will likely eliminate the access technology as a constraining factor, leaving internet routing and server capacity as limiting factors for some time.
Wireless, by contrast, can only offer data speeds in the 10-20 kbps range, although future developments in both TDMA and CDMA will make speeds of 30 kbps (TDMA) and 64 kbps (CDMA) a reality. The problem with wireless internet access is not, however, technology limitations, but that increasing wireless speeds come at a direct cost in terms of capacity available for voice users. Digital cellular and PCS carriers are in the business of selling bits, whether they like it or not. If data users require more bits than voice users, they will have to pay more, or wireless carriers just won’t see data as a profitable business.
Currently, voice users require raw bit rates of 9.6-14.4 kbps for TDMA and CDMA voice coders – much less than normally used for internet access, even today. Not only that, but the bit rates required for voice coders will decline over time, not increase. At the same time, the bit rates required to avoid the World Wide Wait on the internet will be increasing, probably at a rapid rate. You can call this divergence in speeds “Crowe’s Capacity Conundrum” if you like.
When digital cellular was born, data rates were of the same magnitude as the bit rate required for voice coders. The TDMA IS-54 voice coder (VSELP) ran at approximately a 9.6 kbps rate, while data rates on high speed modems of the late '80s were 9.6kbps, rising to 14.4 kbps. Since then, common modem data rates for landline internet access have risen to 28.8 kbps, 33 kbps and now 56 kbps, while voice coder bit rates have grudgingly been allowed to rise to 14.4 kbps. I say grudgingly because, although the 14.4 kbps CDMA voice coder is generally agreed to have better voice quality than the 9.6 kbps alternatives, it also reduces the total capacity of a CDMA system by about one-third. There is little question that if a 9.6 kbps voice coder could be designed that would provide the same voice quality as the current 14.4 kbps voice coder, it would be adopted immediately. Similarly, if the original TDMA plans for a 4.8 kbps voice coder could provide the same voice quality as the current 9.6 kbps ACELP coder, it would also be adopted.
A useful guide to the cost of wireless data services is to look at the ratio of bit rates between data and voice users. Currently, desirable internet bit rates are about 2-6 times the current voice coder requirements. This ratio will rise dramatically in the future. If we assume that xDSL will become widely available at speeds of 1,000 kbps, the ratio will become 100:1. If a practical 4 kbps voice coder is ever invented, and xDSL speeds rise to 10,000 kbps (not a completely unrealistic scenario), the ratio will be a spectacular 2500:1. The most optimistic scenario is to assume, as IMT-2000 does, that a 144 kbps data rate would be a satisfactory internet access speed, and that more efficient voice coders will not be invented. Even this most optimistic case provides a 10:1 data:voice ratio for CDMA and 15:1 for TDMA.
If we assume an airtime rate of 10 cents per minute, and a data:voice bit-rate ratio of 10:1, the data user will have to be charged 1 dollar per minute. Worse yet, the cost ratio will continue to climb over time. I cannot see users lining up for wireless internet service at these prices, when they can get unlimited access from a wireline modem for around 10 dollars per month!
Internet capacity requirements can theoretically be reduced by taking advantage of the generally unidirectional nature of sessions (i.e. data is usually being transmitted or received, not both), and of periods when the user is looking at the retrieved data, and not causing data to be transmitted. If radio capacity is allocated only when needed, internet requirements could be halved if transmissions are always unidirectional, and halved again if the user spends equal amounts of time transmitting as looking at their screen. However, the unidirectional nature of internet access is also asymmetrical, and could result in capacity being exhausted in one direction before the other, while voice users need capacity in both directions to make calls. Voice users are, in fact, more likely to be able to benefit from this approach, because conversations are generally symmetrical. Even if a four times capacity reduction is achieved in this way, the advantage will soon be lost as internet users continue to need increasing bit rates to access services that incorporate more graphics, sound and video.
Wireless data usage will continue to expand slowly, but data applications will most likely be those that run at speeds not much faster than a voice coder. This includes the CDPD IS-732 standard (19.2 kbps shared among multiple users), as well as the competing RAM and ARDIS data systems (that run at similar speeds, also shared among multiple users). Furthermore, data uses that can take advantage of lower priced off-peak hours will be more desirable to carriers. These will most likely be providing computer-to-computer communications (e.g. meter reading) rather than human-to-computer. While wireline internet access does have different peaks from voice usage, there is no guarantee that wireless internet access will follow the same trend. If people were freed from accessing the internet from the comfort of their own homes, they may well start accessing the internet more during the day and less in the evenings.
The fundamental problem of consumer-oriented wireless data is that the airwaves are a shared resource, and a phone line is not. If clever engineers can squeeze more capacity out of an existing phone line (which has been done through more sophisticated modems, and now through xDSL technology), the only user who can take advantage of the extra capacity is the person with equipment connected to that line. The problem with cellular and PCS wireless systems is that even when the capacity of a cellsite is increased many times over through the application of more advanced technology (e.g. TDMA, CDMA), carriers can sell the additional capacity to new customers. There is no advantage in providing the extra bits to their current customers. Even if wireless carriers were left with significant extra capacity, there would be no motivation to sell access to data users at a lower cost, because that would simply drive users toward internet telephony and an erosion of their current income base.
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