Cellular Networks Detailed

by EchoMirage  (webmaster@echomirage.com)

Not so long ago there was only one basic type of cellular network: the analog network.

In the last few years there has been a great divergence in the technology that cellular phones communicate with.  Digital is only the tip of the iceberg, as there are a handful of different digital technologies and even more radio frequency bands within those digital spectrums.  We will look at each of the currently available cellular networks and the basic differences between them.

The Phones

First, let's look at the small side of the system.

A cellular phone is not all that different from a regular cordless phone or a similar radio frequency device.  It sends voice signals out over the airwaves to a base station, which then connects into the POTS network and completes the call.

"Mobile" phones, or car phones, and "transportable" phones, or bag phones, usually output three watts of RF power, whereas a handheld cellular phone outputs 0.6 watts of RF power.

Analog phones work by sending your voice signal more or less directly out over the airwaves.  Digital phones use a device called a "vocoder" to compress the analog sound waves of your voice into binary data that it can send digitally.  Analog phones are, therefore, much less secure than digital phones, but analog has the benefit of being much more widely used.  Analog networks cover 95 percent of the United States.  Digital networks cover only 65-70 percent.

Now, let's look at the different types of networks.

AMPS

AMPS stands for "Advanced Mobile Phone System."  Basically, the AMPS network is the analog network.  These phones operate in the 800 MHz band.  Each phone requires its own frequency to operate on, henceforth, a great deal of individual frequencies are required to operate an AMPS network, and the phone has decreased battery life, because it is constantly "talking" on the network.

AMPS phones do have the benefit of being able to achieve up to 19.2 kbps data transfer rates.  AMPS phones use ESNs (Electronic Serial Numbers) for tracking information.  ESNs are usually 11-digits in decimal form or 8-digits in hexadecimal form, and are found on the back of the phone (as with handheld phones) or on the transmitter (as with mobile and transportable phones).

TDMA

TDMA, and all the networks mentioned from here on out, are D-AMPS (Digital Advanced Mobile Phone System) networks.  TDMA, stands for "Time-Division Multiple Access."  These phones operate in either the 800 MHz ("digital") band or the 1900 MHz ("PCS") band.  TDMA, is the most ubiquitous digital network in the United States, used by companies such as AT&T and BellSouth Wireless.

Since digital phones transmit much less frequently than analog phones because the binary information can be relayed faster, digital phones "share" radio frequencies.  TDMA, works by assigning each phone a talk time on the frequency.  Thus, a cellular phone will transmit on the frequency only when its assigned time frame comes.  Since this time is measured in nanoseconds, it is transparent to the user.

TDMA, provides roughly three to four times the capacity of AMPS.  Data transmissions are possible on straight TDMA, networks but are strangely rare.  Many TDMA, companies prefer to use their legacy analog systems to perform data transmission than the TDMA, system.

TDMA, phones use ESNs for tracking.

CDMA

CDMA is a digital technology designed and pioneered by Qualcomm.  CDMA stands for "Code-Division Multiple Access."  These phones operate in either the 800 MHz ("digital") band or the 1900 MHz ("PCS") band.  CDMA is based on military technology, and is the most efficient cellular technology publicly available.  CDMA technology is used by companies such as Sprint PCS and Airtouch.

Rather than assigning each phone a time to talk, CDMA basically allows an open-channel.  CDMA binary transmissions are "tagged" to be unique to the phone from which they originated, so they are never mixed up.  Although several cellular phones may be "talking" at the same time, they are all kept separate because each binary packet has a unique tag on it, which identifies it as coming from or belonging to a specific phone.  CDMA technology allows for approximately ten times the capacity of AMPS and roughly three times the capacity of TDMA,.

CDMA has additional benefits.  Since there are no "time slots" to worry about, data transmission is more feasible on a CDMA network and is less subject to interference or noise than an AMPS network.  CDMA phones, like TDMA, and AMPS phones, use ESN numbers for tracking purposes.

A great deal of information on CDMA network technology can be found on the Qualcomm and Ericsson websites, at www.qualcomm.com and www.ericsson.com, respectively.

GSM

GSM is more or less the worldwide standard for digital cellular communications.  GSM stands for "Global System for Mobile communications."  GSM technology is used by companies such as Omnipoint, Pacific Bell, and Western Wireless (i.e., Voicestream).

These phones operate in the 800 or 900 MHz ("digital") bands or the 1800 or 1900 MHz ("PCS") bands.  The frequency on which the phone operates depends on where in the world it is being used.  GSM is a derivative of TDMA, technology, operating on the same "time sharing" principle as TDMA,.  GSM technology is the declared European standard, and is the most widely used technology everywhere else (except North America).

In North America, GSM phones operate in the 800 and 1900 MHz bands, while in the rest of the world they operate in the 900 and 1800 MHz bands (the same is true for TDMA, and CDMA technology when they are used elsewhere in the world).

GSM phones use smart cards or Subscriber Identity Modules (SIM) as part of their functionality.  SIM cards come in two types: regular credit card-shaped card and smaller cards approximately the size of a third of a stick of gum.  In addition to storing information on the account and the user, the SIM card usually also holds the contents of the address book or phone directory, unique phones settings, etc.

Additionally, GSM phones use "A5/1" encryption to encode the network traffic.  The algorithms and authentication keys are held in the SIM card.  While this was originally hailed as a fail-safe method for communication, it has since been cracked sever times and has been shown to be a flawed: encryption technology, on the whole.

GSM's strong point, however, is data transmission.  GSM is ideally suited to be used to transmit both data and voice signals very rapidly.  GSM phones use International Mobile Equipment Identity (IMEI) numbers for tracking the phone, though certain other types of tracking are done using the SIM card number.

An excellent source of information on GSM technology and GSM providers worldwide can be found at the GSM Alliance homepage at www.gsm.org.

iDEN

The iDEN network is the brainchild of Motorola and was designed to accommodate both cellular transmissions and two-way radio-like transmissions into one network.

iDEN supposedly stands for "Integrated Digital Electronics Network."  iDEN is A yet another implementation of TDMA, network technology, but operates solely in the 800 MHz band (Motorola is currently designing a 1.5 GHz version of iDEN for use in Japan).

In the United States, the only current iDEN provider is Nextel Communications.

The unique feature about the iDEN network is that users have the option of placing a traditional cellular call or using the "Direct Connect" feature to turn the phone into a two-way radio that can communicate with one or hundreds of other iDEN phones that are "tuned" to that channel.  This is primarily being marketed as a business solution, and rightfully so, as Nextel and other iDEN companies have priced the technology out of the range of most consumers.

iDEN phones, though operating on TDMA, technology, are more capable of supporting data transmissions, and it appears that Motorola is attempting to develop this into iDEN's second "killer app" just in case the "Direct Connect" feature falls flat.  iDEN phones use IMEI numbers for tracking purposes.

More information about the iDEN network can be found on the Motorola website at www.motorola.com/iden.

The cellular world is constantly being changed and transformed, and it doesn't look like the battle for standards will end anytime soon.

Hackers and phreakers can have no end of fun exploring the cellular networks.  What I have provided here is just an overview.  If you are further intrigued, there are thousands of web pages, books, and technical documentations on cellular phone technology.

Go out and explore and learn.

Shout outs to Nightbanshee, Zombie, Ri'Hahn, Voyager, and TNo.