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WAN for Public Use
The most powerful two-way wireless network is the WAN, or Wide Area Network. A cellular tower signal can range up to 10 miles, although most city towers reach a 2500-meter radius, usually connecting no more than 400 devices at a time. The WAN is heavily regulated and licensed to specific territories.
1G, 2G, 3G
Cellular WAN technology has evolved over three generations since 1979, when the first national cellular network was born in Japan. Each generation uses spectrum more efficiently, thereby adding more subscribers who can generate more revenue for a carrier. The first generation (1G) voice-only analog network was used entirely for voice calls. The current second generation (2G) is a digital network permitting some data. The emerging third generation (3G) allows high-speed data and voice. One generation doesn't wipe out the previous generation; rather, a 2G tower operates next to a 1G tower operating at a different part of the spectrum. Since it takes a while to install new hardware, cellular devices are made to fall back to use the previous generation network.
All networks have service features, air interface standards, and spectrum allocated. 3G networks feature packet-switched data, transparent roaming service, broadcast-quality sound and video, a target 2 Mbps data rate, and, if you're in the U.S., automatic location identification (ALI). The International Telecommunications Union (ITU) 3G standards provide five official air interfaces, with CDMA technology leading the 3G air interface parade.
The most novel ITU 3G air interface is Time Division Duplex (TDD). It has the highest theoretical efficiency for mixing voice and data. TDD doesn't rely on the FDD division of paired spectrum bands, wasteful of spectrum in 2G and proposed 3G networks. Let's explore this a bit.
The Science of Two-Way Wireless Communication
What makes cellular radio different from broadcast radio is being able to receive as well as transmit signals. Two general techniques to coordinate sending and receiving signals are the entrenched FDD and the promising TDD.
Frequency Division Duplex (FDD) is a transmission protocol that uses two frequency bands. All licensed cellular WANs use FDD. In cellular calls, a paired band transmits conversations with one band to transmit (uplink) to a tower and the other to receive (downlink from) a tower. A 2G network uses a set of paired bands, the uplink near 1850 MHz and the downlink near 1990 MHz. The lower band is always for the handset transmitter uplink, since lower frequencies take less power to transmit.
Time Division Duplex (TDD) is a transmission protocol that uses one entire band for sending and receiving. Most wireless LANs uses TDD. TDD has a great advantage over FDD when it comes to data. TDD can dynamically allocate the amount of spectrum needed to send or receive data. FDD bands are symmetric, dedicating fixed amounts of spectrum to send as well as receive. Since far more data is sent from the tower than from the field, the uplink spectrum allocated for FDD is largely wasted.
3G TDD air interfaces can dynamically adjust the transmission in the given spectrum to send and receive data or voice traffic. The spectral efficiency of TDD bandwidth allows it to better adapt to bursty data traffic and higher bandwidth demands of IP-based packet data networks. As WAN systems evolve to carry more data, they may turn to more spectrally efficient TDD. This is the conclusion of the Chinese, who are busy developing with Siemans a new TD-SCDMA (Time Division-Synchronous Code Division Multiple Access) standard. Their web site is http://www.tdscdma-forum.org. 12 other companies formed the TDD Coalition to press their standard.
WAN Data Networks
Bits, not bytes, are how telecommunication network rates are measured. In data networks, when you read that a T1 line is 1.54 Mbps, that's 1.54 million bits per second—not bytes per second.
In the U.S., three distinct data networks have been operating since the mid 1990s: CDPD (19.2 Kbps), Mobitex (8 kbs), and Motient (19.2 Kbps). Cellular Digital Packet Data (CDPD) is IP-based and is overlaid on a TDMA voice circuit. Overlays work like a gopher, popping up its head to find an unused channel. When no one is talking, it grabs the channel and nibbles on the data overlay. When a voice call appears in the air, it ducks its head, waiting for the next free space. The overlay protocol can never interrupt a voice call.
As data moves forward from the commonly deployed 2G networks of the world, there is an interim 2.5G standard and a 3G standard. These provide faster data and are entirely packet-based.
2.5G data—As a data upgrade to 2G European GSM voice networks, General Packet Radio Service (GPRS) implements a packet-switching protocol that combines neighboring 19.2 Kbps time slots up to a potential 115 Kbps. To use all the GPRS slots takes more power—not to mention denying subscriber access. GPRS typically uses one uplink slot and two downlink slots. Enhanced Data for GSM Evolution (EDGE) is another packet-switched technology that peaks at 384 Kbps with typical 64 to 100 Kbps rates. EDGE is an emerging standard favored by AT&T Wireless and is discussed at http://www.uwcc.org.
3G data—The battle to build a third-generation, always-on, 2 Mbps, transparent roaming wireless network using all IP protocols is very intense. While the maturity of European and Asian wireless markets are perhaps two years ahead of the U.S. in 2001, U.S. wireless technology is headed for an order-of-magnitude jump over them all. The ITU targets a 2 Mbps data rate target for 3G, but EDGE and Japanese W-CDMA tops off at 384 Kbps. Although the theoretical limit for W-CDMA appears to be 5 Mbps, there are only papers written about high-speed downlink packet access (HSDPA). Meanwhile, the CDMA company QUALCOMM stamps out High Data Rate (HDR) silicon that can reach a 2.4 Mbps peak wireless connection (HDR is also called 1xEV-DO). Consider that the fastest personal connection generally found at an office terminal—a dedicated T1 line—is 1.5 Mbps.
To be fair, these are all peak rates. Peak rates are based on one device using the maximum channel capacity. In realistic use, an average cellular connection "on a good day" is likely to run 40 Kbps using GPRS, 100 Kbps using EDGE, 400 Kbps for DSL or T1, and 800 Kbps using HDR.
Future WAN
The time it takes to go from one air interface generation to the next is about 10 years. If this is true, we can expect to see 4G implemented around 2010. By then, NTT DoCoMo research expects a 4G network based on a TDD packet-based IPv6, next-generation Internet network. Like all radio transmission technology, 4G will need new spectrum allocation, a technical definition for the world industries to build towers to cover territory, handset radios for operation, and standards for testing and certification.
Other emerging air interfaces include Local Multipoint Distribution Service (LMDS), which can deliver up to 155 Mbps data. Multipoint Multichannel Distribution Service (MMDS), popular with Sprint and WorldCom (MCI), delivers 750 Kbps to 11 Mbps of data. How about 1 gigabit per second wireless broadband data? 1 Gps Ultra Wideband (UWB) doesn't use a frequency band to modulate a carrier. It works by firing pulses of low energy over a wide spectral band. Military and intelligence establishments have been using UWB radio applications to image objects buried under the ground or behind walls. In 2000, the FCC granted permission for wide-range testing of UWB technology on an unlicensed basis. For more on UWB, visit http://www.uwb.org/.
Software Defined Radio (SDR) is also in the near future. All cellular phones produced today are at least dual-mode, meaning that they can operate on a 2G network like TDMA at 1900 MHz, and, when the 2G towers are not available, fall back to a 1G analog protocol at 800 MHz. But international travelers using GSM cannot cross to CDMA. When my friends from Paris visit with their GSM world phone, even if they're two blocks away I have to dial France to talk to them. SDR may finally harmonize diverse standards and provide the technical basis for an open handset market, although billing is another matter. The FCC legally cleared use for SDR chip sets and transmissions in 2001. A true SDR can download and upgrade the entire phone operating system, air interface, and frequency use for any carrier network on any continent. The SDR Forum web site is http://www.sdrforum.org.
But the enduring future of the wireless Internet WAN may be the wireless LAN.