Wireless Cell Phones

We have seen the future of wireless, and the future belongs to your phone. Phones take pictures, phones connect to the Internet and get your e-mail, phones now come with keyboards and productivity suites—there are phones and more phones. Did I forget to mention that you can even hold a conversation on your phone?

The cell phone is the single most successful wireless device, and in this age of digital convergence, your cell phone is asked to do more and more. Dick Tracy had his wrist watch, you have your phone. Tracy would be amazed.

Many of the things you can do with cell phones are specific to your cellular network carrier, but we’ll look at some of the more popular things people do to make their cell phones easier, more customized, and of course, more fun.

The cell phone has changed the way people interact with each other and has created a culture all its own. As phone networks get better, faster, and have nearly universal coverage, other wireless services are climbing on board.

So please turn off your cell phone ringer while you read this (so as not to disturb the audience) and let’s have a look at the wonderful world of cell phones.

Cell Phone Networks

Even if your interest in wireless technology extends no further than your laptop, you can’t ignore what’s happening in the cell phone marketplace. The cell phone networks of the future will most likely be the mobile network of choice for any device that must maintain its connection during travel.

And the cell phone of tomorrow may be the mobile PC of choice. The features that cell phone networks offer that make them attractive include large coverage areas, significant bandwidth, established service groups, and reasonable, although not worldwide, transmission protocol agreements.

Cell phone networking technology developed from the earlier radio telephone networks. Radio telephone networks used powerful, centrally located base stations and large antenna towers to provide around 25 channels per station; each station covered a small city with a 50-mile radius.

The problem with this technology was that the number of channels limited the number of connected users. You’ll find that a walkie-talkie works over a single channel (inconvenient on that crowded ski slope), a CB radio can use up to 40 channels, whereas a cell phone might have as many as a couple thousand channels to work with.

Both CB radios and walkie-talkies use what is called a half duplex mode of operation. Both devices use the same single frequency, so that when one person is transmitting, the other is listening.

Cellular technology uses a very different system with very different results. On a cell network a system of distributed small transceivers called cells are placed in the coverage area. Because cells can overlap, you get many more channels than radio telephones could offer.

Also, because your cell phone is much closer to the system transceiver, it can operate at powers as low as 0.5 watts, with smaller batteries, whereas radio telephones use as much as 4 watts and much more powerful batteries.

The base station of a cellular network can also be much lower power than a radio telephone network, thus greatly diminishing the cost of the system.

Analog or Digital?

When you go into a cellular network store to purchase a cell phone you might be offered a choice between an analog and a digital phone and wonder what the difference is. Mostly the difference between analog phones and digital phones is how the network processes the signal.

Over time cellular networks are being converted to digital technology, so this distinction will be significantly less important in the next year or two. An analog cellular network assigns a frequency range to a service provider that can be sliced up into 832 different frequencies using what is called the Advanced Mobile Phone System (AMPS) protocol.

To make communications in an analog phone system work, the send and receive signals in a duplex connection must be at least 45 kHz apart, each signal being 30 kHz wide in the 824 to 894 MHz range.

When you connect to an analog cellular network, you use two frequencies to send and receive—a duplex channel mode. Of 832 possible frequencies, 42 channels are reserved as control channels, with 790 channels remaining.

Thus the system can connect up to 395 users, or up to 56 channels and users per cell. Digital phone networks offer features that can’t be obtained from an analog network, including Internet access, e-mail, and more.

In operation they aren’t much different than analog networks, but they use the connection between your phone and the cellular network to compress the transmission signal in order to pass more data between the two devices.

Thus many more digital phones can share a single cell in the network—up to ten times as many connections. Your cell phone is a small personal computer in its own right.

It has its own processor, memory, operating system, and a number of special processors that do digital-to-audio conversion and audioto- digital conversion (DAC and ADC), as well as specialized Digital Signal Processors (DSP).

That means that it’s relatively easy to convert a computer with a little extra circuitry into a cell phone, the first generation of smart phones is based on the operating systems of personal data assistants, or PDAs.

That isn’t a coincidence, as many of the PDA OSes were based on projects involving cell phone manufacturers and network providers. When you buy a cell phone, the number one thing you want to get is a reliable, strong network connection to your native cellular network.

However, for people who travel with their cell phones outside of their network area, it is also important to buy a phone that is capable of communicating with as many different types of cellular networks as possible. The way you connect to networks is called an access technology.

You’ll run across three common types: Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Time Division Multiple Access (TDMA). If you have to choose one of these access technologies, in the United States that would be CDMA.

CDMA and TDMA can be and often are used over the same network. When you buy a phone that is dual mode or tri-mode, these are the transmission standards that they are talking about.

It isn’t important that you know all that much about these access technologies because you can’t really choose which one is used in a connection. That intelligence is built into your phone.

However, if you are traveling you want as many of these access methods as possible. Unlike the United States, the rest of the world uses a cell phone standard called GSM, or Global System for Mobile communications. GSM uses a TDMA access technology, but one that is slightly different than TDMA in use in non-GSM networks.

You’ll find GSM in use in 900 MHz and 1800 MHz bands in Europe, and in the 1.9 GHz range in the U.S. where it is available, but not in the most popular networks. Among the digital cellular networks using GSM in the U.S. are PCS systems and IDN (Integrated Digital Network) systems used by Nextel with Motorola phones.

The PCS, or Personal Communications System, is meant to differentiate that system from cellular phone networks, and offers additional digital services such as paging and e-mail.

The problem with traveling with a cellular phone across country is that you may need to use different bands or modes in different locations. A band is the frequency spread that the cellular network operates on. A dual band phone can operate at either 800 MHz or 1.9 GHz frequency bands.

A mode is the type of transmission technology used, as we just described. A tri-mode phone might use AMPS for analog transmission, TDMA, and CDMA thus providing not only analog connections, but two types of digital connections.

It’s not uncommon to find dual band, tri-mode phones, and these phones provide the widest connection coverage you can get at the moment for cellular phone systems.

Making a Connection

Setup a wireless cell phone is similar to what you do when you setup a wireless network or network node.

When you purchase a cell phone, your cell phone provider registers the identification code of the phone as set by the phone manufacturer (also called the Electronic Serial Number, or ESN) into its database and assigns that ID code to a Mobile Identification Number (MIN), which is most often your phone number.

Essentially the ESN is the MAC address of the phone, and the MIN is equivalent to the device name. If you ever have to replace a cell phone, your provider may ask you to activate your phone by matching your ESN, which is fixed in each phone, to your MIN either through an automated system or by contacting its customer support line.

When you turn on your cell phone, it searches for available networks using its special control channel frequency, which is similar to the discovery process that a laptop goes through.

Depending on the System Identification code (SID) of any network—a unique five-digit number assigned by the FCC—your phone finds it and knows whether it is in your service area (if the SID matches your provider).

Whether it’s roaming outside of your area (if the SID doesn’t match), or whether it’s outside of any coverage area and thus cannot provide service. An SID of a network other than your own may be a provider who has a reciprocating agreement with your provider and part of your network service.

Otherwise, you are in an area where you are “roaming,” and additional charges (often excessive) may apply. With many phones you will see a different colored flashing light, get an “R” icon on your screen, hear a different ring or dial tone, or some other feature that lets you know that you are roaming and out of your service plan area.

The quality of your service provider’s network is one of the most important features you should look for when purchasing a service plan. As we all find out, calls in a roaming area of a cellular network without a reciprocating agreement can be quite expensive.

Different levels of service agreements may include areas of the country that other levels do not. When you make a phone call, you create a registration request at the MTSO, and the system validates your phone and account, determines your current cell, and then assigns you a frequency pair that it knows isn’t in use at the moment.

The process is similar to a directory service search and authentication. You are connected and your conversation begins. Cellular networks monitor your signal strength in your current cell and determine when its strength has dropped in one cell and is higher in another adjacent cell.

Then over the control channel your phone is sent a switching signal to switch to a different set of frequencies in that second cell, essentially handing you off seamlessly to that cell. Of course, if there is no adjacent cell and you are in an area of no service, you’ve lost your cell, meaning you’ve been dropped, and your connection terminates.