Understanding Wireless Networking Standards

First there was copper. Then there was fiber optics. And last of all, wireless. This, of course, is a gross oversimplification. Many voice/data/video (VDV) networks today actually use a combination of copper, fiber, and wireless technologies to achieve seamless connectivity among a wide variety of wired and wireless terminal devices: desktop computers, peripherals, laptops, smart telephones, personal digital assistants. Cabled systems will continue to be the basis of VDV networking within buildings and campuses for the foreseeable future, for reasons of both bandwidth and speed. But wireless is an increasingly important part of the VDV networking mix. Nor is this bad news for installing companies, as long as they keep the forward-looking attitude that got them into this business in the first place. Although wireless networks obviously use less copper and fiber optic cable and fewer outlets, backbone portions of systems up to the telecommunications closet are approximately the same. Wireless networks also have special components of their own such as wireless modems, infrared repeaters, and air bridges for linking from one building to another in campus environments. Many companies that perceived themselves strictly as electrical contractors only three to five years ago now have separate data cabling divisions. Some are also installing active networking components and performing final system programming and commissioning. Those electrical-electronic installation firms that prosper in the future will be those that add wireless networking expertise to their services. The many different wireless networking standards go by a bewildering alphabet soup of names, but these are easier to sort out when you understand the basic principles behind the different wireless communications standards and technologies. Early days Back in the 70s, Bell Telephone Laboratories (today's Lucent Technologies) developed the first wireless system to provide telephone services to mobile subscribers. It consisted of three major components: cellular telephones, base stations, and a Mobile Telephone Switching Office (MTSO). Service areas were divided into geographic zones called "cells," with wireless calls automatically being transferred from cell to cell as the mobile user driving in a car, for example, moved from place to place. Although major technological developments have occurred since then, the basic concepts and components of a cellular system have remained the same. AMPS The original analog cellular telephone system invented by Bell Labs was known as Advanced Mobile Phone Service (AMPS). It operates in the frequency spectrum ranging from 800 to 900 MHz and divides this band into 25 kHz channels using a technique called Frequency Division Multiple Access (FDMA). Subdividing the frequency band into channels allows multiple callers to operate in a cell at the same time. However, restrictions on using the same frequencies in adjacent cells limits the number of simultaneous calls a cell can support under actual operating conditions to fewer than the theoretical 4,000 [100 MHz / 25 kHz = 4,000]. As cellular telephones became more popular, users began to encounter blocking, in which calls couldn't be placed, or they were "dropped" when leaving one cell and moving to another. Seeing that fixed-cell analog cellular systems couldn't serve the growing base of subscribers, cellular developers came up with two new and more flexible systems, TDMA and CDMA. TDMA Time Division Multiple Access (TDMA) divides radio channels into time slots, each consisting of a small fraction of a second. These time slots are then assigned among eight subscribers, which substantially increases the capacity of a cell. Figure 1 shows how two separate conversations can share a single channel using TDMA. TDMA-based cellular systems operate at 800 or 1900 MHz in North America and are referred to as digital cellular or personal communication services (PCS). Because wavelength is proportional to the reciprocal of frequency, higher wavelength results in a smaller cell diameter. This means that 1900 MHz PCS systems requires more cells in the same size geographic area than 800 MHz systems. TDMA can co-exist with analog channels on the same wireless network. This allows subscribers with dual-mode mobile telephones to enjoy the broader coverage of existing analog networks while digital TDMA systems (with their extra features) continue to grow. The name game. TDMA is sometimes also referred to as Digital-AMPS (D-AMPS), North America TDMA (NA-TDMA), IS-54, and IS-136. These last two are the designations of so-called "interim standards" published by the Telecommunications Industry Association (TIA) in Arlington, Va. IS-54 is the original implementation of TDMA, and a proposed next generation version of cellular TDMA is referred to as IS-136. This technology will provide wireless data transmissions of up to 43.3 kbps (kilobits per second), nearly as fast as conventional wired telephone modems. CDMA Code Division Multiple Access (CDMA) is a digital cellular technology that works on a completely different principle than AMPS or TDMA. Rather than dividing the radio frequency spectrum into separate channels by frequency, or creating time slots, it assigns digital codes to different pieces of a conversation, allowing them to be spread out within a channel by frequency and time as shown in Figure 1. A major advantage of CDMA is its ability to extend system capacity, using spread-spectrum technology to create wider radio channels. On the same network, it can provide approximately 10 to 20 times the capacity of analog AMPS and four to six times the capacity of digital TDMA . TIA first issued its IS-95 CDMA standard in 1992. It is now used in 60 countries, and is also the basic technique behind wireless Ethernet as defined in IEEE standard 802.11. CDMA also permits a more graceful "handoff" than either of the other technologies because a subscriber's mobile telephone can monitor and communicate with several adjacent cells simultaneously. This translates into fewer accidental disconnections when moving from one cell to another. GSM In an ideal world, all communications standards would be global, providing seamless connectivity (i.e., just pick up your mobile telephone and dial) anywhere around the globe. But this isn't an ideal world yet, and differences exist in the standards adopted in different sections of it. The European version of TDMA is known as Global System for Mobile communications (GSM). More than 90 countries have adopted GSM and approximately half of all wireless telephones worldwide employ GSM technology, with the highest coverage in Europe. Originally it operated at 900 MHz, but a newer version works on the 1800 MHz band. GSM supports a feature called Short Message Service, which enables compatible telephones to receive short text messages on their liquid crystal display (LCD) screens. In addition, GSM phones can be connected to a personal computer to transmit data and fax messages at data rates up to 9.6 kbps (about the same speed as a five-year-old conventional modem). In North America, GSM is presently something of an also-ran system with an estimated 6 million users. However, it operates in this hemisphere at 1900 MHz, which is not compatible with global GSM. There are dual-band GSM (900/1800 and 900/1900) telephones, and a few "universal" tri-band 900/1800/1900 telephones that enable roaming. Without a universal telephone, GSM telephones used on North American systems can't operate when traveling abroad (and vice versa). CDPD Cellular Digital Packet Data (CDPD) more commonly referred to as wireless Internet Protocol (IP) is implemented as an "overlay" or additional communications method on an existing digital cellular network. CDPD provides Internet access to mobile users at a transmission rate of up to 19.2 kbps, with sophisticated error correction and encryption (security) capabilities. CDPD is currently available in major metropolitan areas covering approximately 75 percent of U.S. business users, and its usage is growing rapidly as sales of Internet-compatible phones, personal digital assistants (PDAs), and wireless notebook computers increase. The Future Many high-tech buffs find it fashionable to refer to evolving wireless communications technologies as "generations." Analog AMPS represents the first generation, while digital TDMA and CDMA represent the second-generation. The next evolution of digital wireless technologies that will greatly enhance the ability to communicate anything from anywhere at anytime are collectively referred to as third-generation. Research to define 3G wireless systems dates to 1986, when the International Telecommunications Union (ITU), a European-based global standards organization, launched its IMT-2000 project. IMT-2000 is a family of systems that provide wireless communications access through a combination of satellite and land-based station, for both stationary and mobile users. The ITU's ambitious plan calls for 3G networking to be compatible with all of the existing wireless services described above, and to operate at much faster data rates than today's cellular systems. As presently outlined, IMT-2000 will support voice, high-speed data, and video communications at data rates ranging between 144 kbps and 2 Mbps (Megabits per second). It will also support police-fire-emergency dispatch, and emergency locating of mobile telephones and other wireless devices based on Global Positioning System (GPS) satellites. Stauffer is director of codes and standards for the National Electrical Contractors Association (NECA) in Bethesda, Md. He writes and lectures widely about voice/data/video networking technologies.

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