by Seth Prandini and David Sims
07/28/2000
If you have heard terms like GSM, 3G, and GPRS and not known what they've meant, our handy chart and quick reference should give you an idea of what's coming down the pipe in wireless carrier technologies and the bandwidths they promise.
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Figure 1. Today's GSM, TDMA and CDMA digital wireless systems are expected to be upgraded over the next two years to broadband, packet-based wireless systems that can deliver smooth media over the ether. |
TDMA |
Time Division Multiple Access (also known as IS-136) is the second most popular digital wireless standard, the most popular in the U.S. TDMA uses time division technology, which divides each cellular channel into three time slots, to increase the bandwidth of the connection. TDMA and GSM are so similar that the two standards may someday be brought back together (via EDGE technology -- see below). |
CDMA |
With its roots in military application, Code Division Multiple Access is the least popular digital wireless technology. Unlike its competitors (GSM and TDMA), CDMA is a "spread spectrum" technology, which means that after it digitizes a signal, it spreads it out over the entire bandwidth that is available to it. This concept was originally developed to prevent enemy "signal-jamming" and to make the communication harder to detect overall. Qualcomm helped develop the technology, and it continues to champion its use. |
GSM |
The Global System for Mobile communications is the most popular digital wireless carrier technology for mobile phones in the world, although it's the least popular in the United States. It digitizes and compresses data (whether voice or other), then sends it out at one of three frequencies, depending on the system: 900 MHz, 1800 MHz, or 1900 MHz. GSM appears to be the best bet for a technology that lets a cell phone user move gracefully from one continent to another. The recent news that Deutsche Telekom would buy U.S. carrier VoiceStream highlights this fact: VoiceStream is one of the few North American carriers that relies on GSM, and that fact was part of the attraction to Deutsche Telekom. Sources for more information: |
GPRS |
General Packet Radio Service, in tests now, will use a packet-based rather than a circuit-based system. This means that instead of getting your own direct line (as with a phone call), your message travels in packets through an open channel (which is how e-mail and web pages travel across the Internet). When you transmit a file through a packet-switched (as opposed to a circuit-switched) connection, the file is initially broken up into many equally sized "chunks." Each packet contains both the IP address of the file's ultimate destination and a unique number that allows the file to be properly reconstructed when all the packets have reached the other end. Because each packet is labeled and has directions, it does not necessarily need to follow the route of the packet that was sent before it. Each packet can decide which is the fastest route at the second it is being sent, resulting in a decrease in the time it takes to transfer the entire file. The current radio channel used for voice and data is a 200-kHz wide band that is divided up into eight separate data streams. Because GPRS is packet-based, it can send packets through all eight streams concurrently. Each stream can handle about 13 Kbps of throughput, so when you add up all the channels, you get a rate that is over 100 Kbps. This increases the bandwidth available for your message, and that's why GPRS is considered the next step toward third-generation (3G) high-speed access. Promoters originally hyped GPRS as being able to deliver over 100 Kbps, but some now suspect those expectations are unrealistic. Charles Davies, chief technology officer of Psion (U.K. maker of handheld computers), speaking at the WWW9 conference last May, said real speeds are somewhere between "old modem" and a still-life painting. "It's still going to be better than we have today," said Davies, "but it's not going to live up to the hype." The new selling feature: constant connection, similar to DSL. GPRS is in tests in some parts of Europe and Asia. North America will not be seeing it for at least another six months. Ericsson, one of GPRS' biggest supporters, plans to roll out GPRS-enabled devices by the end of this year. |
EDGE |
Once GPRS implementation has been completed, the next step on the road to 3G will be a technology known as Enhanced Data rates for GSM Evolution. EDGE encompasses a series of hardware and software enhancements and has the potential to deliver speeds as high as 384 Kbps. Again, as with other technologies, some analysts are skeptical. A recent article on ZDNet predicted EDGE speeds would come in around 56 Kbps, so there's some debate about whether it should be considered a 2G, 2.5G, or 3G technology. EDGE also has the potential to bring back together forking technologies, as an EDGE-compliant phone may be able to work on GSM and TDMA networks, greatly improving one's ability to roam. But it will, reportedly, require carriers to overhaul their infrastructure. It introduces a new physical layer in the network architecture, including new ways of encoding data to protect against errors. Trial deployments are set to begin as soon as 2001; EDGE-enabled phones and services will not be available until at least 2002 or 2003. Sources for more information: |
UMTS |
The Universal Mobile Telecommunications System is the promise of fat pipes over the ether, constant connection at speeds that can deliver entertainment content. UMTS is broadband, packet-based digital wireless technology, based on GSM and GPRS systems. It has been approved by international bodies, including the International Telecommunications Union, which includes it as part of its "IMT-2000" vision of a global network of 3G mobile communications systems. The promise is to begin delivering its 2-megabits-per-second speeds to commercial users as early as 2001 -- though that seems optimistic. Sources for more information: |
4G |
As expectations for the speeds of 3G networks decrease, interest rises in fourth-generation (4G) wireless networks. 4G networks are expected to be based on orthogonal frequency division multiplexing (OFDM), which sends data over hundreds of parallel streams, increasing the available bandwidth. Fourth-generation systems will also use technologies like adaptive processing (which helps clear up interference in transmission) and so-called smart antennas, which include a signal-processing capability to optimize their reception and radiation patterns.
Sources for more information: |
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Figure 2. What is 3G? It means different things, depending on how fast you're moving. Loosely, mobile services are considered third generation if they can deliver 128-Kbps throughput to devices moving in cars, 384 Kbps to devices people are carrying, or 2 Mbps to stationary devices. |
Seth Prandini is a student at Pepperdine University in Malibu, Calif.
David Sims was the editorial director of the O'Reilly Network.
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