Making Sense of Cellular

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Wireless technology begins and ends with the radio spectrum. Comprising a wide-band array of low-frequency electromagnetic radiation, radio waves operate in the 3 KHz to 300 GHz range. To give you some idea of scale, these frequencies correspond to wavelengths between 3 centimeters (the length of a common paper clip) and 300 meters (three football fields). Compare this to the frequencies common in broadcasting and satellite transmission: 1 MHz for the AM radio band, 100 MHz for the FM radio band, and 1.5 GHz for the GPS (Global Positioning Satellite) band, the microwave band used by location-based technologies.

Communication and Control Channels

The radio spectrum, which includes microwave frequencies, is divided into channels, each occupying a 30 kHz band. These may be either control or communications channels. Communication channels carry both voice and data content. Control channels, operating at slightly higher frequencies than the voice and data channels they command, control signaling inputs and outputs, and manage other network transmission chores.

Why Radio

So, what's the big advantage of radio frequencies? Why not use the higher end of the electromagnetic spectrum to transmit data? Several reasons: Unlike short wavelengths, which are easily absorbed by objects and materials, long wavelengths penetrate obstructions — forests, hills, towns, cities — without significant distortion. They can travel long distances, bounce off satellites, and reflect halfway across a continent. Moreover, because low-frequency waves are characterized by low energy, there is little health hazard to human populations. Finally, low energy means cheap transmission.

AMPS

AT&T pioneered American mobile wireless communications when it launched the Advanced Mobile Phone Service (AMPS) in Chicago in the early 1980s. AT&T later divested itself of AMPS when the US government forced the company's breakup. The service originally operated in the 800 MHz band, but later expanded to include transmissions in the 1900 MHz range, putting AMPS in the VHF (Very High Frequency) range of radio transmissions, the band in which most wireless carriers operate.

AMPS is a first-generation analog, circuit-switched network. Because it uses analog signals, AMPS is better suited for voice than data. Because it is circuit-switched, each transmission starts and ends with a dedicated connection. AMPS consequently fails to meet several key needs of daily life in the computer age: it cannot transmit digital signals and cannot transport data packets without assistance from newer technologies such as TDMA and CDMA.

TDMA and CDMA

TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access) are the two most prevalent second-generation attempts to digitize AMPS in order to facilitate wireless data transmission. Bandwidth for TDMA and CDMA is allocated in the 800 MHz, 900 MHz, and 1900MHz ranges.

AMPS is a frequency-division multiple-access (FDMA) system. This means that the frequency band has been chopped up into a number of smaller bands, providing access to more users at the same time. AMPS, however, didn't make efficient use of available bandwidth. Remember the discussion we had earlier regarding channels? It turns out that FDMA channels can be further subdivided into time slots so that multiple users can actually share the same channel. This method of increasing the carrying capacity of the network is the concept underlying TDMA. TDMA makes more efficient use of channels.

In some sense, TDMA is similar to client-server time-sharing systems of the 80s. Each voice channel is divided into time slots, enabling up to three simultaneous conversations modulated on the same channel. TDMA consequently enables multiple users to occupy the same channel. The following diagram provides a quick visual comparison of FDMA and TDMA.

As you can see, TDMA effectively triples network capacity. CDMA, which came later, took an entirely different tack. Created by Qualcomm in the mid 1990s, CDMA is a so-called spread-spectrum technology. Rather than split the frequency bands into smaller and smaller subunits, CDMA allows all users to share the same frequency at the same time. The entire frequency band is fair game for all users. The secret to managing this wireless free-for-all lies in the packaging.

Signals from different users are distinguished by unique code sequences. Each voice (or data) communication is broken up into little pieces, and each piece is given an identifying code. At the receiving end, knowledge of the code sequence being sent allows the signal to be extracted and reconstructed. This process is closely analogous to the way TCP/IP packets are sent across the Internet; it is much more complicated, however, because all the transmissions may occur at the same time. Even so, the code sequence enables the receiving software to isolate and reassemble the pieces of each message.

Modulation

At the heart of TDMA and CDMA is a trick of physics called modulation. A high-frequency digital signal is grafted onto a lower-frequency analog wave, so that digital packets are able to ride piggyback on the analog airwave.

This hybrid, modulated approach to signal processing has helped bridge the evolution of analog to digital services, but at its core it's an imperfect technology. Still, until full-scale digital services arrive, with packet-switched delivery of built-from-the-ground-up digital voice and data services, it will have to do. And, at least in Europe, it hasn't done half bad.

GSM

The close proximity of diverse nations in Europe made it evident early on that a transnational, standards-based approach would be crucial to the success of wireless, particularly in regard to roaming. What was needed was a kind of European Union for the airwaves. The agreed-upon solution implemented in the late 80s was GSM (originally a French phrase, now generally understood to stand for Global System for Mobile Communications). More specifically, GSM is a set of ETSI (European Telecommunications Standards Institute) standards that ensure network interoperability across national boundaries.

GSM, initially set to operate in the 900 MHz range, now also operates in the 1800 MHz range in Europe and (since the early 1990s) in the 1900 MHz range in North America. As an aside — and as an insight into some of the interoperability concerns that haunt the wireless market: even though GSM sets standards for a common frequency range, Americans can't call business associates or relatives in Europe on their cell phones, in part because American mobile phones operate in the 800 and 1900 MHz ranges while their European counterparts operate in the 900 and 1800 MHz range. This incompatibility is analogous to the differences in standard household current: 220 volts, 50 Hz in Europe; 110 volts, 60 Hz in the US.

GSM is a TDMA-based technology. However, unlike TDMA and CDMA, GSM was designed from scratch as a system for analog voice with modulated digital data capabilities built in.

GSM handsets in Europe are unique in that they require a Subscriber Identity Module (SIM). SIMs are stripped-down smart cards — essentially a chip containing information about the identity of the subscriber and subscriber authentication and service information. Because the SIM uniquely identifies the subscriber, international roaming is not a problem. SIMs are even portable among handsets. You can remove the SIM from a GSM phone you bought in Spain, and plug it into a GSM phone bought in France. The network recognizes you as the same unique caller on either instrument.

PCS, GPRS, i-mode, WAP

To round out the discussion of wireless protocols, a few words should be said about some emerging technologies that may soon affect developments in the wireless world:

• PCS — Personal Communications Service is a suite of second-generation, digitally modulated mobile-communications interfaces that includes TDMA, CDMA, and GSM. It's easiest to think of PCS as just an umbrella term for second-generation wireless technologies operating in the 1900MHz range.
• GPRS — General Packet Radio Service is an always-on connection. As a result, phones can deliver services like instant messaging and instant notification of email. GPRS is built on top of GSM and is widely deployed in Europe. GPRS provides packet-mode data on a packet-overlay network that runs parallel to the existing wireless network.
• i-mode — Japanese wireless devices use i-mode technology to access cHTML (compact HTML) Web sites and display animated GIFs and other multimedia content. Introduced by NTT DoCoMo in 1999, i-mode simply wiped out WAP in Japan, and now boasts roughly 40 million users. One Web developer estimated he could produce a cHTML Web site for i-mode in one day, whereas creating the same Web site using WAP would have taken him ten days.
• WAP — Wireless Application Protocol is a heavy-duty but somewhat heavy-handed technology to deliver wireless Web services. WAP is a technology pushed from the top down by wireless providers, while cHTML bubbled up from the bottom as an easier way to deliver content to handsets.

Beyond these technologies lies the world of 3G, or third-generation wireless, a world where broadband, always-on, packet-switched networks allow for full-scale multimedia and high-speed data communications.

Air Interface

Having seen the various protocols that drive — and sometimes fragment — the wireless industry, we turn now to a discussion of the cellular infrastructure that supports mobile communications. How, in other words, are voice and digital data actually transmitted across the cell network?

Central to cell networks is the concept of the cell. Unlike traditional radio broadcast networks, in which a single, centrally-located base station with a tall antenna serves a sprawling geographical region, cell networks are characterized by a small number of tightly integrated and computer-controlled cells, each covering an area no larger than several city blocks. As mobile devices roam, automated switching centers hand them off from one cell to the next according to the signal strength of a cell relative to the position of the mobile device.

Base Station

A base station is the cellular relay station (or cell tower) that a cell phone talks to when initiating or receiving a wireless call. A base station transmits calls to devices over the Forward Control Channel (FOCC). Mobile devices transmit calls to the base station over the Reverse Control Channel (RECC).

Radio transmissions use a bi-directional (full duplex) configuration, transmitting and receiving on separate frequencies. A mobile device transmits on the radio frequency the base station is tuned to, and the base station transmits on the frequency the mobile device is tuned to.

All transmissions are managed by the base station, which acts as a kind of clearinghouse for wireless communications. The base station's primary responsibility is to transmit voice and data traffic between mobile devices and an MSC (Mobile Switching Center), which is a computer-controlled switch for managing automated network operations.

Mobile Switching Center

A Mobile Switching Center is the electronic field office of a cellular carrier, which automatically coordinates and switches calls between mobile phones in a given service area. Each cell in a cellular network is controlled by an MSC, which constantly monitors each caller's signal strength and arranges cellular handoffs. When a signal begins to fade, the MSC locates another MSC, better positioned to manage the call, and re-routes it to maintain the communications link.

MSCs are connected to base stations by T1 landlines or microwave channels, and by landlines to the Public Service Telephone Network (PSTN).

MSCs maintain individual subscriber records, current status of subscribers, and information on call routing and billing in two subscriber databases: the Home Location Register (HLR) and the Visitor Location Register (VLR).

The Home Location Register (HLR) is a database for permanent storage of subscriber data and service profiles. The Visitor Location Register (VLR) is a database that contains temporary information about subscribers. The MSC uses this information to keep track of and serve visiting subscribers — in other words, roamers.

When a cellular phone roams outside its home MSC, the local carrier communicates with the home provider to obtain device and subscriber data, which it loads into its VLR. This information is maintained by the local provider as long as the mobile device roams within the local paging area.

The following diagram shows the various components that make up the cellular network. Notice that gateways provide mobile devices access to the Internet across TCP/IP lines.

Cellular Application Areas

Over and above traditional mobile voice communications, the cellular network plays host to a variety of new applications and services that promise to revolutionize the way mobile devices transport and use data. A brief overview of some of the more promising of these technologies follows.

Short Message Service (SMS)

One of the hottest areas of cellular technology today is SMS, the Short Message Service. SMS is a protocol for sending short (up to 160 characters) alphanumeric messages to and from wireless devices across networks such as GSM, which offers SMS as a built-in subscriber service. In fact, SMS originally appeared on the wireless scene with the advent of GSM in Europe in 1991. Because SMS is characterized by out-of-band packet delivery and low-bandwidth message transfer, it was thought to be a mechanism for carriers to squeeze a little more capacity out of their underused cellular networks. No one, least of all the carriers themselves, imagined the tremendous popularity of SMS as a means of "mini- chat" among youthful subscribers. much less as a conduit for the transmission of "ring-tones,"- short, custom melodies that play when a phone is addressed — as pioneered by Nokia in Finland.

In North America, Bell South and Nextel were among the pioneers who first made SMS available on their wireless networks. By 1998, with the build-out of PCS networks based on GSM, CDMA, and TDMA, SMS started to take root in the United States.

SMS is a cost-effective way of sending short bursts of information across a cellular network. All SMS messages must pass through an SMS operations center, similar to an MSC, which operates in conjunction with a network carrier.

Aside from the youth-oriented applications mentioned, SMS supports such potential value-added enterprise services as stock quotes, currency quotes, airline scheduling, reminder services, email notifications, bank services, and over- the-air downloads of firmware to upgrade device hardware.

Location-based Services

Location-based services are available on many local cell networks, where base towers and signal triangulation are used to locate handsets. Location-based services also rely on Global Positioning Satellite (GPS) transmissions to pinpoint subscribers and offer location-specific services such as traffic reports, weather conditions, and restaurant recommendations.

GPS-enabled devices transmit latitude and longitude, which are mapped by device firmware to city and street addresses and used to track mobile devices and monitor subscriber location and movement.

Location-based communication is made possible by device hardware components that receive microwave signals from government-maintained satellites, which hover above the earth in geostationary orbits. A total of 24 such satellites ensure that the location of any device can be determined within a few meters. GPS data is transmitted to subscribers across cellular networks.

Telematics

One emerging industry that provides a highly specialized location-based service is called telematics. It routes event notification and control data over wireless networks to and from mobile devices installed in automobiles. Telematics makes use of GPS technology to track vehicle latitude and longitude, which are transformed into maps displayed in LED consoles mounted in dashboards. In effect, telematics transforms vehicles into mobile, thin-client Web appliances that connect to remote processing centers, which in turn provide server-side Internet and voice services, as well as access to database resources.

The Future

Cell phones, PDAs, pagers, handsets, GPS tracking devices, and so on all seem destined to converge into one multi-purpose device, the so-called pico pad.

Not as futuristic as it might sound, the pico pad, driven by voice and data content transmitted over next-generation 3G packet-switched cellular networks, promises to move computing off the desktop and supplant the micro in much the same way that the micro supplanted the mini and the mainframe. The evolution of computing has always been toward the more compact, the more modular, and the more portable. The progress made in cellular technology, and the nascent development of hybrid, multi-purpose handsets seems a harbinger of fully mobile personal computing in the 21st century.

References

Newton's Telecom Dictionary
16th Edition
Harry Newton
Telecom Books, 2000

3G Wireless Networks
Clint Smith, Daniel Collings
McGraw-Hill, 2002

Wireless Internet Enterprise Applications
Chetan Sharma
Wiley, 2001

How Wireless Works
Preston Gralla
Que, 2002

¹ As used in this document, the terms "Java virtual machine" or "JVM" mean a virtual machine for the Java platform.