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Cell Phones

Cell Phones

Among all consumer electronic devices ever invented, cellular telephones (cell phones for short) have been the most popular in terms of the number of devices sold as a function of time. Today, cell phones are almost as common as the wired telephone. In certain European countries, they exceed the total number of wired telephones, and in several Scandinavian countries, cell phones have become a way of life.

The concept behind the working of a cell phone is based on what is known as frequency reuse. There is only so much of available electromagnetic spectrum, or frequency band, available for transmitting a voice call with a cell phone and this has to be shared by many users. Each voice conversation requires a certain amount of bandwidth , which is a chunk of the available frequency band. In order to support the large number of subscribers, the service providers split the frequency band into smaller sub-bands and spread them over a given area. They install radio transmitters, called base stations, that serve an area called a cell. Each cell utilizes a sub-band of the spectrum. Two cells that are sufficiently far apart can use the same sub-bands because radio signals lose their strength with distance and do not interfere. Thus, frequency sub-bands are spatially reused by the creation of several cells supported by base stations. The name "cellular telephony" comes from this architecture.

Each base station is connected to a mobile switching center that handles cell phone calls and also performs a very important function called mobility management. If you make a phone call to a wired telephone, a telephone switch will know how to route your call because the telephone device is fixed. If you make a phone call to a cell phone, there must be a mechanism that keeps track of where the cell phone is and accordingly routes the call. This mechanism is called mobility management. It is performed by the mobile switching center, with the assistance of two databases called the home location register and the visitor location register.

Every cell phone is registered with a home location register in its home calling area. When you make a call to a cell phone, the telephone switch contacts the mobile switching center associated with the home location register. The home location register contains information about the visitor location register with which the mobile phone is currently registered while roaming. From this point, the call is routed to the mobile switching center associated with the visitor location register and handled from there. As the person with the cell phone moves, the cell phone constantly updates its location with the visitor location register, which in turn updates the home location register upon a change in location.

Another important aspect of managing mobility is what is known as the process of handoff. As users move with their cell phones, they cross the coverage of one base station and enter the coverage area of anotherthat is they cross cell boundaries. At this point, the cell phone should switch its connection from the old base station to the new base station. This procedure is called handoff and it involves measuring and comparing the received signal strengths from multiple base stations.

Within a cell, multiple users may simultaneously conduct phone conversations using the available frequency sub-band. They may do this in several different ways. The earliest technique was to further divide the sub-band into smaller chunks of frequency and use one chunk for one conversation. This is called frequency division multiplexing and was used in the analog cellular service first implemented in the early 1980s by AT&T. This was also adopted in Europe by several countries, each using its own standard. These systems were called the first generation cellular telephone systems. The frequency division multiplexing technique is inefficient and the resulting capacity to support subscribers is smaller than desired.

In the 1990s, there was a trend to move to digital standards to enhance the capacity of cellular systems. In Europe, all the countries agreed upon a common digital standard called the global system for mobile communications (GSM). In the United States, two major standards emerged called the IS-136 and IS-95 standards. These are the second-generation cellular telephone systems.

The GSM and IS-136 standards are based on time-division multiplexing where users appear to talk continuously, but their signals are transmitted one after another. That is, their signals are separated in time and these are commonly called the TDMA (Time Division Multiple Access) standards. The IS-95 standard employs code-division multiplexing where the signals of all users are simultaneously transmitted using the same frequency band. Each user is assigned a unique code that is designed to enable easy separation of signals with minimum interference. This is called the CDMA (Code Division Multiple Access) standard and it has proved to provide a significant increase in capacity and other additional benefits. Today, there is a global evolutionary trend toward third generation (3G) systems where cell phones in Europe and in the United States will all use some variation of CDMA.

Today, cell phones are becoming much more than simply tools used for voice conversations. The most modern phones are equipped with a microbrowser that can display whittled down versions of web pages using a wireless application protocol (WAP). In Japan, a different system called I-mode provides access to web pages on the cell phone display. In either case, a subscriber can get stock quotes, news, traffic, weather information, sports scores, and other personalized and customized information on a cell phone. In Europe, a service called short-messaging service (SMS) is extremely popular. This allows almost instantaneous exchange of very short messages (up to 140 characters long) between two cell phones. This is very similar to computer-based instant messaging and is growing in popularity, especially with young adults. In summary, the cell phone is becoming a pivotal part of popular culture worldwide. There are also increasing trends toward integrating the cell phone with personal digital assistants (PDAs) , whether they run the PalmOS or WindowsCE operating systems.

Even as cell phones have become the single-most popular consumer electronic device, questions remain as to how safe electromagnetic signals are to the health of human beings. Studies relating cell phone usage to cancer are inconclusive, but manufacturers are gradually including transmit power and radiation information as part of cell phone specifications and documentation to alert consumers of potential hazards associated with long-term usage of cell phones.

see also Internet; Telecommunications; Wireless Technology.

Prashant Krishnamurthy

Bibliography

Oliphant, M. W. "The Mobile Phone Meets the Internet." IEEE Spectrum 36, no. 8 (1999).

Sweet, W. "Cell Phones Answer Internet's Call." IEEE Spectrum 37, no. 8 (2000).

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