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Graphics and more info on this page...
http://www.ee.washington.edu/class/498/sp98/final/marsha/final.html

How Cell Phones Work

An Overview

It is common knowledge that Cellular Phones (referred to as "cell" phones from here on) are wireless phones; however, many are confused about how a cell phone actually works. Essentially, cell phones use high-frequency radio signals to communicate with "cell towers" located throughout the calling area. Cell phones communicate in the frequency range of 806-890 MHz and 1850-1990 MHz for the newly allocated "PCS" frequency range.

When the user wants to make a call, the cell phone sends a message to the tower, asking to be connected to a given telephone number. If the tower has sufficient resources to grant the request, a device called a "switch" patches the cell phone?s signal throughout to a channel on the "public switched telephone network" (otherwise known as the PSTN). This call now takes up a wireless channel as well as a PSTN channel that will be held open until the call is completed. The following figure illustrates this process.

This channel cannot be used for anyone else?s call until the cell phone call is discontinued.

Given this simple description of how cell phones work, we will add technical details about various facets of cell phone systems throughout the remainder of this section.

Cells

As the name implies, cell phone systems are made up of many small "cells." Each cell in a cell phone system represents the area served by one cell phone tower. The concept of cells is key behind the success of cell phones because by spacing many cells fairly close to each other, the cell phones may broadcast at very low power levels (typically 200mW ? 1W, depending on system). Since the cell phones may broadcast at low power levels, they use small transmitters and small batteries, and thus are able to fit in a shirt pocket, unlike amateur radios can occupy a tabletop.

Cells are typically spaced around 1-2 miles apart but can be spaced up to 20 miles apart in rural areas. In loaded areas or areas with many obstacles (such as tall buildings), the cell sites may be spaced closer together. Some technologies, like PCS, require closer cell spacing due to their higher frequency and lower power operation. Additionally, buildings interfere with cell signals coming from outside, so many buildings have their own "microcell." The Kingdome and New York subway are two examples of where microcells are used. Microcells may also be used to increase overall capacity within a heavily populated area such as a city?s core downtown area. In fact, homes may have "picocells" connected to the home?s PSTN connection to allow the cell phone to be used as a cordless phone. An example of typical microcell and picocell environments is pictured in the following figure.

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Switching

Overview

When a user places a call on a cell phone, the system must figure out how to route the call to the PSTN. Additionally, when someone calls the cell phone, the system must figure what cell the user is in. This section describes how this is done.

Finding the user

Whenever you turn your cell phone on, the phone sends its identification to the cell phone tower. This includes the "MIN" (mobile identification number, usually the phone number) and the "ESN" (electronic serial number). The cell tower forwards this information to a centrally located switch via special leased phone lines that connect a switch to many cell sites (T-1 lines are often used). When the switch gets this information, it forwards it to any higher level switches.

Connecting the call

Whenever a call comes in, it will come to the switch that serves the exchange (the exchange is the 555 in (206) 555-1212). This top-level switch will pass that call onto any lower level switches, if there is one, although there usually is not. When the call is passed to the lowest level switch, it checks to make sure the phone is still registered (it is turned on and in range). If it is registered, the phone is notified via the signaling channel and the phone begins ringing. When the user chooses to accept the call, the switch establishes the voice channel and the call begins.

Roaming

Roaming was one of the most challenging issues the cell phone industry faced. The goal was simple: a phone could be used anywhere in the US or the world where compatible technology is used. The difficult part is getting various systems to communicate and pass routing and billing information to each other.

When a user turns his or her cell phone on in a roaming area, the cell phone identifies itself to the switch. When the switch looks up the information and discovers it is not a local phone, it will attempt to find the "home" switch based on the exchange. When it locates the home switch, it will determine if roaming is possible. If roaming is possible, the switch (referred to here as the "roaming switch") sets up a "Visitor Location Register" (VLR) registering the phone in the locality. The home switch will also be notified about the change so that it can route calls to the switch in the roaming location. Outbound calls are handled through the roaming switch as they would be handled if the user were at home. Incoming calls are routed from the home switch to the roaming switch after sending a message to the roaming switch requesting a "temporary local directory number" (TLDN). This TLDN will be used to make a connection from the home switch to the roaming switch across the PSTN. Finally, whenever the roaming phone is turned off, the phone is unregistered with the roaming switch and the home switch is notified. The process of registering the phone and notifying the home switch takes 2 seconds.

Wireless Data

Overview

In modern times, we depend on e-mail and on the World Wide Web. We have access to these resources at home and at the offices, and we would like to use these resources while traveling. This is the goal that "wireless data" hopes to accomplish. In the following section, we will discuss various types of wireless data systems that run on top of cellular networks. Systems that run on other systems, such as ricochet, which runs on the pager network, will not be discussed.