Section A.8. CDPD, 1xRTT, and GPRS: Cellular Data Networks

A 8 CDPD, 1xRTT, and GPRS Cellular Data Networks

When it comes to data rates, most people are in agreement that faster is better. But current communications technology always involves a trade-off between speed, power, and range. 54 Mbps may be great if you can get it, but can be difficult to maintain on a large scale. The 802.11 protocols compensate for increased range by scaling back the data rate, but these devices simply aren't designed to serve hundreds of people scattered over many miles.

There are times when any data to the Internet is better than none at all, no matter how slow it might be. For example, you might need to log in to a remote machine or send a quick email while traveling, when Wi-Fi or even wired network access just isn't available. Or maybe you want to have an alternate communications channel into a wireless node in a remote place (say, on a mountaintop or deep in the woods) where telephone lines aren't even available. For these situations, you might consider exploiting the biggest advantage of the commercial mobile data networks: their ubiquity.

Mobile networks maybe be slow and relatively expensive, but you can't beat their coverage compared to current Wi-Fi networks. They can give you an IP address just about anywhere, but be warned that most mobile data services are not cheap. Most charge by the byte, and all charge for airtime while you are using it.

The type of data service you can use depends on the underlying wireless technology. Obviously, before choosing a technology, determine the coverage area of the mobile network in the place you intend to use it. The three leading mobile data services are described next, in decreasing order of availability in the United States.

A.8.1. CDPD on TDMA

CDPD stands for Cellular Digital Packet Data. It works over the enormously popular Time Division Multiple Access (TDMA) mobile network, which is easily the most widely deployed mobile network in the United States. CDPD "modems" typically use a serial port or PCMCIA slot and offer speeds of up to 19.2 Kbps (real world is typically closer to 9, 600 bps).

All of the TDMA operators are generally migrating to GSM, so it is probably unlikely that TDMA data services will ever be upgraded. In some areas, TDMA is being phased out altogether, making it difficult to obtain a CDPD account. But despite the relatively slow speed of CDPD, you can't beat its coverage. Virtually all of the populated regions of the United States are covered by TDMA.

A.8.2. 1xRTT on CDMA

CDMA stands for Code Division Multiple Access: it is the second most popular mobile technology in the United States. The original CDMA data services offered speeds of 9600 bps to 14.4 Kbps. 1xRTT boasts speeds of up to 144 Kbps, but by many reports, real-world throughput is somewhere between 60 and 80 Kbps, occasionally bursting to 144 Kbps if you get lucky. If you think the 802.11 protocol names aren't confusing enough, you should really try following mobile phone technology. 1xRTT is also known in various circles as CDMA2000 Phase 1, or simply 95-C.

1xRTT is just the first phase of the CDMA2000 plan. A few communities are lucky to have the newer 1xEV-DO technology, which can theoretically achieve 2 Mbps from fixed locations over CDMA. This technology hasn't yet been widely deployed.

A.8.3. GPRS on GSM

GPRS stands for General Packet Radio Service, and is the data service available on Global System for Mobile communications (GSM) networks. The original GSM data services offered only 9, 600 bps throughput, but GPRS allows real-world speeds of 20 to 30 Kbps. GPRS is a packet-based protocol, meaning that the GPRS radio transmits only when it actually has data to send. This can save on battery usage, and theoretically makes more efficient use of the network. A number of nifty gadgets such as the HipTop by Danger (http://www.danger.com/) use GPRS for connectivity.

Eventually, GPRS will be replaced by technologies like Enhanced Data for Global Evolution (EDGEyou have to ask yourself how they can use these acronyms with a straight face), which offers theoretical speeds of up to 384 Kbps over GSM. EDGE is still experimental, and hasn't yet been widely deployed. GSM coverage is increasing rapidly in the United States, but still isn't as ubiquitous as CDMA or TDMA. Much of the rest of the world has a more thoroughly deployed GSM network.

If you find that you need simple wireless connectivity beyond what you can hope to provide with 802.11 technologies, commercial data networks are a viable alternative. They don't come cheap, but can be perfect for many low-bandwidth applications.