The second-generation air interface technologies introduced digital techniques to enhance the efficiency of transmission and therefore increase network capacity. Many of these methods increased the capacity several times over analog systems by digitizing the voice signal before it is sent over the air. Voice packets from individual conversations could then be packed more tightly into channels to boost system capacity. They first began being deployed in the early 1990s, often times alongside analog systems still in operation.

To increase capacity, many of these air interface protocols employ the concept of multiple access (or multiplexing). Individual calls can be multiplexed in time (time division), in frequency (frequency division), or by codes (code division) to “stack” multiple callers in a frequency band that, using analog technology, could handle far fewer callers. Here’s a brief description of each technology:

• FDMA (Frequency Division Multiple Access): A spectrum band is divided up into paired frequency channels, one dedicated to each caller. Total system capacity is determined simply by the number of channels. FDMA is predominantly used in early analog cellular systems.

• TDMA (Time Division Multiple Access): An individual channel is divided up into time slots, then each slot is dedicated to a single caller. By digitizing speech, three or more individual conversations can be packed into a single paired channel. TDMA technology has been combined with FDMA in many of today’s wireless networks.

• CDMA (Code Division Multiple Access): Digitized speech from each caller is broken up and assigned a code, then transmitted on any available channel in a spectrum band. The receiver can then use the codes to piece together individual conversations or ignore those that may not apply (in the case of the mobile unit). This technology boosts capacity, since codes can be selected so they don’t interfere with each other. This technology is also the basis for most wireless networks of the future.

In addition to their digital techniques for transmitting voice, most of these air interface protocols also have provisions for sending limited amounts of data. This enables features such as paging, messaging, and Internet browsing. The digital nature of transmission also makes these protocols more secure and much harder to tap into illegally.

Below are several advanced 2G wireless standards that exist now or will be deployed soon. Keep in mind that these standards are each based on one of the three technologies above. Confusion arises because many of the standards are simply referred to by the technology they use, as in the case of TDMA and CDMA. It’s actually more accurate to refer to them as IS-136 and IS-95 respectively.

• IS-136 (TDMA): A digital wireless technology that uses time division to split a channel into multiple time slots. Largely deployed in the Americas, IS-136 is one of the most popular TDMA protocols. Even though there are several protocols based upon TDMA, the term is most often used to refer to the IS-136 standard.

• IS-95 (CDMA): A technology that uses digital spread spectrum techniques in communication between wireless devices, this was developed originally by the U.S. military. Qualcomm Inc. pioneered this air interface technology for commercial use in cellular and other wireless networks and holds a significant amount of intellectual property in the standards. Also called cdmaOne.

• GSM (Global System for Mobile communication): A digital wireless protocol based on TDMA technology, GSM was developed largely through the efforts of European Union and Asian countries, it is noted for being highly secure and reliable. GSM also employs a removable smart card for the handheld unit, so subscriber information can be easily transferred. It is now by far the most popular protocol in the world.

• iDEN (Integrated Digital Enhanced Network): A proprietary standard developed by Motorola Inc. that is also based upon TDMA, iDEN combines voice telephony, two-way radio, and short messaging features in one network. Nextel Communications Inc. is the largest carrier supporting this technology in the U.S.

• CDPD (Cellular Digital Packet Data): This overlay protocol uses open voice channels to transmit low levels of data over wireless networks. AT&T Corp. and other carriers have set up CDPD networks in the main populated areas of the U.S. for data-centric services.

• PDC (Personal Digital Communications): Established in 1991, this digital system is based on TDMA technology and operates in both the 800MHz and 1.5GHz bands. This protocol is employed predominantly in Japan.

• PHS (Personal Handyphone System): A digital cordless phone system used primarily in Japan, with some use in other Asian countries as well, the range of the system is very small and is incorporated with the traditional home phone system to offer better economy.

• GPRS (General Packet Radio System): A packet-based technology that can be overlaid on TDMA and GSM architectures for data services. When not used for voice conversations, individual time slots can be allocated for passing data back and forth.

• EDGE (Enhanced Data Rates for Global Evolution): Similar to and based upon GPRS, EDGE is seen as the next step towards 3G for TDMA and GSM networks. It uses an enhanced modulation technique to further boost data transmission capabilities.

• 1xRTT (Single Carrier Radio Transmission Technology): An evolution of IS-95 CDMA protocol, some consider this the first phase of 3G for CDMA networks. The first iteration of the standard offers data rates of up to 144 kbit/s with backward compatibility to existing IS-95 systems. Also abbreviated to 1x.

• i-Mode: This packet-based data service is available in Japan. It has a relatively slow data rate of 9.6 kbit/s but is very popular due to its ease of use and appealing services. While NTT DoCoMo brought this to the masses initially in Japan, the company has recently entered into partnerships to spread this service to Europe and the Americas.

• HSCSD (High Speed Circuit Switched Data): A circuit-switched data transmission technology that can support speeds up to 38.4 kbit/s, this technology fits into TDMA and GSM systems, using voice channels to pass data.