Cellular Phone and Roaming Service


Table of Contents


How do cell phones work?. 2

Some basics about cell phones. 2

Why is it called "Cellular"?. 3

Analog Cellular Systems in the US AMPS Variants. 3

An Overview of the Cell Phone System.. 3

Analog Cellular Systems outside the US TACS Variants. 4

Cellular System Specifications. 4

Cell Phone Identity. 5

Analog v. Digital Cell Phones. 5

What Happens When Your Cell Phone Places A Call?. 6

Cellular Radiotelephone Service Fact Sheet 7

Cellular Frequently Asked Questions. 9

How Does Roaming Work?. 13


How do cell phones work?

You've seen them, perhaps you've used one. Each day something like 100,000 people around the world sign up for and start using a cellular phone. Therefore it is likely that you or someone you know has a cell phone and uses it on a regular basis. They are such great gadgets - with a cell phone you can talk to anyone on the planet from just about anywhere (80% of the U.S. has coverage).

But have you ever wondered how a cell phone works? Or what happens when you talk on one? Or how a call gets routed to a regular phone? we will discuss the technology behind cell phones so that you can see how amazing they really are.

One of the most interesting things about a cell phone is that it is really a radio - an extremely sophisticated radio, but a radio nonetheless. A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie. A CB radio is a simplex device. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a duplex device, so it uses one frequency for talking and a second, separate frequency for listening. A CB radio has 40 channels. A cell phone can communicate on 1,664 channels. Cell phones also operate within cells and they can switch cells as they move around. Cells give cell phones incredible range. A walkie-talkie can transmit perhaps a mile. A CB radio, because it has much higher power, can transmit perhaps 5 miles. Someone using a cell phone, on the other hand, can drive clear across a city and maintain a conversation the entire time. Cells are what give a cell phone its incredible range.

Some basics about cell phones

Always remember: a cell phone is a radio !!

A cellular telephone is basically a two-way walkie-talkie that acts like a telephone. With a walkie-talkie, you either talk or you listen; with a cell phone, you can talk and listen at the same time. You can dial a number to place a call. You can receive calls. You can do fancy things like three-way calls, conference calls, call hold, and voice mail.

All conversations on cell phones are unprotected and can be intercepted. Don't think of a cell phone as a telephone, think of it as a radio. Sure, there are laws to protect you against illegal eavesdroppers, but obviously these laws are hard to enforce since it's hard to catch someone listening in on your call. Be careful of what you say (or transmit with a modem) on a cell phone!

Cellular phone systems can be "analog" or "digital". Older systems are analog and newer systems are digital.

Each cell phone identifies itself to the cellular system each time it places or receives a call so that the cellular system can verify it is a valid paying customer. The cell phone's identity includes the phone number that is assigned by the service provider.


Why is it called "Cellular"?

Many different types of systems have provided city-wide two-way communications, such as radio systems for taxis or the police. In these systems, a single antenna is located near the center of the city. Each two-way conversation occupied one channel so if there were 100 channels in the city, only 100 simultaneous conversations could be held. To cover the entire city, the antenna is placed on a tall building and emits a very strong signal.

In the cellular phone system, the city is divided into smaller sections, or cells. Each cell contains its own antenna and uses only a subset of all the channels. Each antenna is lower in height and emits a much weaker signal so that the same subset of channels can be used in a cell somewhere else within the city. For example, within a typical cellular system, every seventh cell uses the same set of channels.



Analog Cellular Systems in the US
AMPS Variants

There are three variations of the analog cellular system in operation in the US:

AMPS - Advanced Mobile Phone System

Original system that had 666 channels (42 control, 624 voice)

EAMPS - Extended AMPS

Current system that has 832 channels (42 control, 790 voice) and has replaced AMPS as the US standard

NAMPS - Narrowband AMPS

New system that has three times as many voice channels as EAMPS with no loss of signal quality

All three systems have 42 control channels that are for setting up calls; the remaining channels are for voice conversations. All three systems are also analog systems. The systems are listed in chronological order and are backward compatible; that is, the infrastructure is designed so that older phones work on the newer systems.

The UK TACS system was based on the US AMPS system.

The NAMPS system was designed to increase the number of calls that can be handled by the system. This is intended as an interim solution until a digital cellular system is agreed upon.




An Overview of the Cell Phone System

The predominant cellular system operating in the US is the Advanced Mobile Phone System, AMPS. There are two minor variations of AMPS: EAMPS and NAMPS. They are all basically the same and were the basis for the European analog cellular system, TACS (and its variants).



Analog Cellular Systems outside the US
TACS Variants

There are several variations of analog cellular systems in operation outside the US that were originally based on the US AMPS cellular system:

TACS - Total Access Communications System

Original UK system that has either 600 or 1000 channels (42 control, 558 or 958 voice)

ITACS - International TACS

Minor variation of TACS to allow operation outside of the UK by allowing flexibility in assigning the control channels

ETACS - Extended TACS

Current UK system that has 1320 channels (42 control, 1,278 voice) and has replaced TACS as the UK standard


A version of TACS designed for operation in Japan

IETACS - International ETACS

Again, a minor variation of ETACS to allow operation outside of the UK by allowing flexibility in assigning the control channels

NTACS - Narrowband TACS

New system that has three times as many voice channels as ETACS with no loss of signal quality

All systems have 42 control channels that are for setting up calls; the remaining channels are for voice conversations. All these systems are also analog systems. The systems are listed in chronological order and are roughly backward compatible; that is, the infrastructure is designed so that older phones work on the newer systems.

The European cellular systems were originally based on the US AMPS system and are being replaced by GSM systems which is a new digital cellular system.




Cellular System Specifications

Full specifications of the over-the-air interfaces for the cellular systems are available from the appropriate standards body.



Approved: April 19, 1989

"Mobile Station - Land Station Compatibility Specification"


ANSI/EIA/TIA-PN-2925 DD>October 5, 1992

"Mobile Station - Land Station Compatibility Standard for Dual-Mode Narrowband Analog Cellular Technology"


"Total Access Communication System Mobile Station - Land Station Compatibility Specification"

Issue 3

Ammendment 5

September, 1992

Note: The versions listed might not be the most recent.



Cell Phone Identity

Each cell phone identifies itself to the cell system with two unique values:

MIN - Mobile Identification Number

A ten-digit number that is similar to a landline phone number in that it has a three-digit area code and a seven-digit phone number. The MIN is assigned by the cellular service provider and can be changed, such as when changing service providers.

ESN - Electronic Serial Number

A 32-bit binary number. The ESN is assigned by the manufacturer and can never be changed.

When a cell phone is signed up for service, the service provider assigns a new MIN to the phone and updates its customer database to associate that MIN with the phone's ESN. Then, every time the cell phone places or receives a call, the customer database is searched and the call is allowed if the transmitted MIN-ESN pair is found (and you've paid your bill).



Analog v. Digital Cell Phones

Cellular telephone systems can be "analog" or "digital". Older systems (AMPS, TACS, NMT) are "analog" and newer systems (GSM, PCS) are "digital".

The major difference is in how the audio signals, e.g. your voice, is transmitted between the phone and base station. "Analog" and "digital" refer to this transmission mechanism. Think of it as audio casettes versus CDs - audio casettes are analog and CDs are digital.

In either system, the audio at the microphone always starts out as a voltage level that varies continuously over time. High frequencies (high pitch) cause rapid changes and low freqs cause slow changes.

With analog systems, the audio is modulated directly onto a carrier. This is very much like (if not identical to) FM radio where the audio signal (in that case music) is translated to the RF signal.

With digital systems, the audio is converted to digitized samples at about 8000 samples per second or so. The digital samples are numbers that represent the time-varying voltage level at specific points in time. These samples (numbers) are now transmitted as 1s and 0s. At the other end, the samples are converted back to voltage levels and "smoothed out" so that you get about the same audio signal. (There is some loss, but it may be unnoticable - depending on how it's done.)

With analog transmissions, interference (RF noise or some other anomaly that affects the transmitted signal) gets translated directly into the recovered signal - there is no "check" that the signal makes sense. The neat thing about digital is that the 1s and 0s can not be easily confused or distorted during transmission, plus extra data is typically included in the transmission to help detect and correct any errors.



What Happens When Your Cell Phone Places A Call?

Let's say you've just entered a phone number into your cell phone and pressed the [SND] key. Here's what happens next:

  1. Scan Control Channels: Your cell phone needs to use the "closest" base station because that's the one with the strongest signal and the one that will give the best connection. To find the closest base station, your phone checks all 21 control channels and determines which has the strongest signal.
  2. Choose Strongest: Your cell phone chooses the strongest signal and decides to use that one for placing the call.
  3. Send Origination Message: Your cell phone now transmits a very short message (about 1/4 second) that contains the MIN (Mobile Identification Number, aka your cell phone number), its ESN (Electronic Serial Number), and the number you just dialed.
  4. Get Channel Assignment: After the cellular service provider verifies that you are a valid, paying customer (based on the MIN and ESN your phone sent), the base station sends a Channel Assignment message to your phone (also a short 1/4-second burst). This message tells your phone where (that is, on which channel) the conversation will take place.
  5. Begin Conversation: Your cell phone tunes to the assigned channel and begins the call. The spurts you hear that indicate the phone is ringing at the other end (which is called "ring back") or the busy signal that you hear do not begin until you get to this step. Both of these are transmitted by the base station as an audio signal just like the voice of the person you are calling.



Cellular Radiotelephone Service Fact Sheet

In 1981, the FCC adopted rules creating a commercial cellular radio telephone service. The FCC set aside 50 MHz of spectrum in the 800 MHz frequency band for two competing cellular systems in each market (25 MHz for each system). From the beginning, the FCC has encouraged competition in the cellular radio market by dividing the available spectrum into two channel blocks, one for the local wireline telephone companies and the other for the nonwireline companies, e.g., Radio Common Carriers (RCC).

The FCC established rules and procedures for licensing cellular systems in the United States and its Possessions and Territories. These rules designated 305 Metropolitan Statistical Areas (MSAs) defined by counties according to the 1980 census. The FCC revised the MSAs in some of the top 30 markets. The Gulf of Mexico Service Area was added as Market 306. From the remaining counties that were not included in the MSAs the Commission created 428 Rural Service Areas (RSAs) for a total of 734 cellular markets. The Commission used comparative hearings to select the licensees in the top 30 markets in cases where there was more than one applicant. In the remaining markets (31 through 734), where there was more than one applicant, lotteries were used to select applications to process where there was more than one applicant.

A cellular system operates by dividing a large geographical service area into cells and assigning the same channels to multiple, nonadjacent cells. This allows channels to be reused, increasing spectrum efficiency. As a subscriber travels across the service area the call is transferred (handed-off) from one cell to another without noticeable interruption. All the cells in a cellular system are connected to a Mobile Telephone Switching Office (MTSO) by landline or microwave links. The MTSO controls the switching between the Public Switched Telephone Network (PSTN) and the cell site for all wireline-to-mobile and mobile-to-wireline calls. The MTSO also processes mobile unit status data received from the cell-site controllers, switches calls to other cells, processes diagnostic information, and compiles billing statistics.

Each cell is served by its own radio telephone and control equipment. Each cell is allocated a set of voice channels and a control channel with adjacent cells assigned different channels to avoid interference. The control channel transmits data to and from the mobile/portable units. This control data tell the mobile/portable unit that a call is coming from the MTSO or, conversely, tells the controller that the mobile/portable unit wishes to place a call. The MTSO also uses the control channel to tell the mobile/portable unit which voice channel has been assigned to the call. The 25 MHz assigned to each cellular system presently consists of 395 voice channels and 21 control channels.

Low powered transmitters are an inherent characteristic of cellular radio systems. As a cellular system matures, the effective radiated power of the cell site transmitters is reduced so channels can be reused at closer intervals, thereby increasing subscriber capacity. There are over 30 million mobile/portable cellular units and more than 20 thousand cell sites operating in the United States and its Possessions and Territories. In order to insure nationwide compatible service, all cellular systems must operate in accordance with the technical specifications in the Commission's Rules. However, present analog FM cellular systems are fast approaching system capacity in many of the larger cellular markets.

In order to meet the future demands for service, the cellular industry is developing new standards using various digital modulation techniques that will increase system capacity. Licensees using new digital technology will continue to also use analog equipment to serve those customers who have not purchased new digital phones.

Each initial cellular licensee in the MSAs and RSAs were given 5 years from the date of their initial authorization to build and expand their cellular system within their market. During this 5-year fill-in period no applications from anyone other than the licensee were accepted within the market on their channel block without their consent. Amy remaining area within the market not covered by the licensee is considered an "unserved area." The Commission recently completed rules for the acceptance and processing of applications for unserved areas within a channel block after the 5-year build-out period expires. Cellular station files, maps, pending applications and related documents are available from the Commercial Wireless Division Reference Room located in Room 5608, at 2025 M Street, N.W., Washington, D.C. The room is open to the public, Monday through Thursday, 9 AM to 4 PM and Friday 9 AM to 2 PM. Copy machines are available for public use for 8-10 cents per page.

The rules governing cellular radio are found in the Code of Federal Regulations Volume, 47 CFR, Parts 20 to 39. They can be ordered from: E-mail at mspann@fcc.gov


Cellular Frequently Asked Questions

1.   Does the FCC regulate or approve tower locations for cellular companies? How do I get a list of all cellular tower sites? Does the FCC have this information ?

No. This is a local matter that is handled by state and local zoning boards. The new Telecommunications Act instructs the Commission to insure that the state and local zoning boards do not unreasonably delay the construction of towers or the approval of sites. Tower siting and antenna registration is also contained on this web site.

The FCC does not maintain a data base with all cellular cell sites. The FCC only maintains a data base of the external cell sites of each cellular system. These are the cell sites whose contours make up the outer boundary of the cellular system. These external cell sites are listed on the authorization for each cellular system.

You may review or make copies of cellular authorizations in the Public Reference Room of the Wireless Telecommunications Bureau's Commercial Wireless Division which is located on the fifth floor of 025 M Street, NW, Washington DC 20554, telephone (202) 418-1350. On-line dababase searches can also be accomplished in the Public Reference Room.

The FCC does not duplicate these records, but has contracted with International Transcription Service, Inc. to provide this service. Requests for copies of information should be addressed to International Transcription Service, Inc. (ITS, Inc.) 2100 M St., NW, Suite 140, Washington, DC 20037, Telephone (202) 857-3800.

2.   Can a subscriber have multiple phones with the same telephone number?

Yes, but there are restrictions. Each individual phone must have a unique Electronic Serial Number (ESN). The ESN is a unique number programmed into each cellular telephone at the time it is manufactured and is the means by which a cellular carrier identifies a telephone to determine whether the user of that phone is entitled to obtain service and to insure that the proper accounting is made of all activity. Most cellular phone emulators or extension services simply "clone" cellular phones, duplicating not only the telephone number but also the ESN. This activity is in violation of current Commission rules.

The Code of Federal Regulations Title 47, Section 22.915, entitled Cellular System Compatibility Specifications, generally sets forth the standards of cellular operation as reflected in the Cellular System Mobile Station-Land Station Compatibility Specification (April 1981 ed.), Appendix D to the Report and Order in CC Docket No. 79-318, 86 FCC 2d 469, 567 (1981). It is a violation of Section 22.915 of the Commission's rules for an individual orcompany to alter or copy the ESN of a cellular telephone so that the telephone emulates the ESN of any other cellular telephone. Moreover, it is a violation of the Commission's rules to operate a cellular telephone that contains an altered or copied ESN.

Part 22 of the Commission's rules was recently revised to add a new rule Section 22.919, to further clarify the issue of ESNs. Pursuant to subpart (c) of the referenced section, it is a violation to remove, tamper with, or change the ESN chip, its logic system, or firmware originally programmed by the manufacturer.

It currently is possible to obtain two cellular phones with the same telephone number if the cellular carrier in the market has the software in place to handle the billing and its fraud detection system has been notified not to be triggered by the use of two phones with the same phone number in suspicious circumstances.

3.  How do I get a cellular license?

The Commission divided the United States up into 734 different markets where it licensed two entities for each market. The largest 306 markets are Metropolitan Statistical Areas (MSAs) and the remaining 428 markets are smaller Rural Service Areas (RSAs). The majority of these markets have been licensed and all that remains to possibly be licensed is what the Commission has defined as "unserved area". Each of the MSA and RSA licensees was provided five years to build out their systems within their designated market area. At the end of the five year buildout period, licensees are required to notify the Commission of what area they actually cover within the market area. This covered market area is called their Cellular Geographic Service Area or CGSA. Any area within their MSA or RSA that is not their CGSA or covered service area after five years is available for unserved area licensing. The unserved area licensing process is two-fold.

Phase I is a one-time process that provides an opportunity for eligible parties to file competing applications for authority to operate a new cellular system in, or to expand an existing cellular system into, unserved areas as soon as these new areas become available. Phase I initial applications must be filed on the 31st day after the expiration of the five year build-out period of the authorized system(s) on the channel block requested in the market containing the unserved area. Each Phase I application must request authorization for one and only one CGSA in one and only one cellular market. Additionally, each licensee whose Phase I initial application is granted is afforded one opportunity during the Phase I process to file an application proposing major modifications to the cellular system authorized by that grant, without being subject to competing applications.

Phase II is an ongoing filing process that allows eligible parties to apply for any unserved areas that may remain in a market after the Phase I process is complete. If a Phase I initial application is granted for a market and channel block, Phase II applications (applications for authority to operate a cellular system in any remaining unserved area) for that market and channel block may be filed on or after the 121st day after the Phase I application was granted. If no Phase I initial applications are granted for a market and channel block, Phase II applications for that market and channel block may be filed on or after the 31st day after the FCC dismissed the last pending Phase I application. If no Phase I initial applications are received for a market and channel block, Phase II applications for that market and channel block may be filed on or after the 32nd day after the expiration of the relevant five year build-out period. There is no limit to the number of Phase II applications that may be granted on each channel block in each market. Consequently, Phase II applications are mutually exclusive only if the proposed CGSAs would overlap. Mutually exclusive applications are processed using the general procedures in  22.131. Phase II applications may propose a CGSA covering more than one cellular market. Each Phase II application must request authorization for one and only one CGSA.

Competing applications that are mutually exclusive (i.e. both cannot be granted) are resolved by auction. Prospective applicants must research the Commission's files to determine whether or not there is any unserved area in markets that they might be interested. Applicants may review or make copies of cellular authorizations or coverage maps in the Public Reference Room of the Wireless Telecommunications Bureau's Commercial Wireless Division which is located on the fifth floor of 025 M Street, NW, Washington DC 20554, telephone (202) 418-1350. On-line dababase searches can also be accomplished in the Public Reference Room.

The FCC does not duplicate these records, but has contracted with International Transcription Service, Inc. to provide this service. Requests for copies of information should be addressed to International Transcription Service, Inc. (ITS, Inc.) 2100 M St., NW, Suite 140, Washington, DC 20037, Telephone (202) 857-3800.

4.   How can I determine the cellular licensees in certain markets or for all the cellular markets?

A list of all cellular providers is provided on the WTB web site . As mentioned in the answer to question 3 above, other licensing information may be obtained from our Public Reference Room or from ITS.

5.   Where can I find the rules and regulations that pertain to cellular providers?

47 Code of Federal Regulations (CFR) Part 22 contains the specific technical rules and requirements governing cellular providers. The CFR can be obtained from the Government Printing Office or from any local library.

6.  What sort of regulations exist concerning the radiation hazards associated with cellular?

The Office of Engineering and Technology (OET), an office within the Federal Communications Commission, works closely with the scientific industry on this issue and releases periodic reports on human exposure to radiation. The latest is from December 1994, entitled Information on Human Exposure to Radiofrequency fields from Cellular Radio Transmitters and can be obtained through OET's radiation information line at 202-418-2464. Additionally, other technical questions concerning radiation can be answered at this information number or at the OET web site.

7.  What kind of regulation is there concerning reselling cellular service? Do I need a license to become a reseller?


The Commission does not actively regulate resale of cellular service, only mandates that it must occur. Section 20.12(b) of the Commission's Rules, 47 CFR Part 20, provides that each cellular system licensee must permit unrestricted resale of its service and that this requirement shall cease to be effective five years after the last group of initial licenses for broadband PCS spectrum in the 1850-1910 and 1930-1990 MHz bands is awarded. The Commission does not regulate resellers rates or any other obligations; these are all considered private contracts and negotiations.

8.  Where can I file a complaint about cellular service, coverage or billing.

You may contact our Enforcement Division at 202-418-0569 or if you wish to file an informal or formal complaint in writing these should be filed to:

 Federal Communications Commission 
 Wireless Telecommunications Bureau 
 Washington, DC 20554 

9.  Can I use my cellular phone in aircraft (to include hot air balloons, etc.)

Section 22.925 of the Commission's rules, 47 CFR Part 22, provides that cellular telephones installed in or carried aboard airplanes, balloons or anyother type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off. The following notice must be posted on or near each cellular telephone installed in any aircraft:

"The use of cellular telephones while this aircraft is airborne is prohibited by FCC rules, and the violation of this rule could result in suspension of service and/or a fine. The use of cellular telephones while this aircraft is on the ground is subject to FAA regulations."

10.  How do I obtain data on cellular systems, such as number of subscribers, number of calls on the system, number of cell sites, etc.

The Commission does not collect data of this type, with much of the information considered proprietary by cellular carriers. Cellular data is collected by two associations in the Washington, DC, area. The two associations are: CTIA, its phone number is 202-785-0081 and the second is PCIA, its phone number is 703-739-0300. These associations can provide some but probably not all information of this nature.


How Does Roaming Work?

Last Updated: 12/22/98

The NACN is a network that interconnects wireless systems (switches, home location registers, visitor location registers, service control points, and other intelligent peripherals).

A wireless roaming network has five components that make it work:

These five components interact to validate a cellular phone when it is powered on and to deliver calls to and from the subscriber anywhere in the network.


A registration cycle keeps track of a phone as it travels around the network. It begins when a wireless user powers on their phone. The general steps for this process are:

1. When the phone is powered on, it sends a data message to the cellsite. This data message contains the Mobile Identification Number (MIN or phone number) and the Electronic Serial Number (ESN). The cellsite forwards this information to the switch.

2. The switch compares the MIN with a table of all MINs in the network. It will determine if the MIN belongs to a home customer, or to a visiting customer. In either case, the switch will request the subscriber's feature profile from the Home Location Register (HLR). The HLR for home customers may be integrated into the same switch or stored on a separate platform.

3. If the HLR is a separate platform, or if the customer is visiting from another system, the switch then sends a data message to the HLR across the signaling network. Routing specifications stored at Signaling Transfer Points (STPs) provide the necessary information to direct the message to the home location register.

4. When the Home Location Register (HLR) receives the message, it checks the MIN & the ESN. If the numbers are valid, the HLR records the location of the phone and returns a message containing the subscriber's feature list and calling restrictions to the visited switch.

5. Once the visited switch receives the return message, it creates a Visitor Location Register (VLR) to store information about the roamer, including the MIN, ESN, features, etc... This register will be used by the roamer as long as they are registered in the visited system.


Once the subscriber is properly registered in the visited system, they can place outgoing calls to whatever numbers they are authorized to dial. This may include local numbers, long-distance numbers, and international numbers. Be aware that many wireless service providers restrict the dialing of international numbers for visiting roamers. For some outgoing calls, the switch may re-validate the subscriber with another message to the HLR.

Feature code dialing strings are usually sent back to the HLR for processing. For example, if a subscriber dials *71-555-555-1211 to initiate call-forwarding, a message will go to the HLR. The HLR will record the forward-to number string and send an updated feature list back to the visited switch.


Call Delivery occurs when someone calls to a cellular phone. This example assumes that the person is roaming in a system away from their home system. The general steps for this process are:

1. Someone dials the phone number of the cellular phone.

2. The call is routed to the subscriber's home switch. The home switch sends a query to the Home Location Register (HLR) to determine the location of the phone. The HLR recorded the location of the phone when the phone registered in the visited system.

3. The HLR sends a data message across the signaling network to the visited system requesting a temporary local directory number (TLDN) for routing the call.

4. The visited switch returns a data message with a TLDN to the HLR. The HLR sends the TLDN to the home switch for routing across the public telephony network.

5. When the visited switch receives a call to that TLDN, it associates it to the phone, sends out a regular cellular page and delivers the call to the phone.

The TLDN is a regular telephone number that is routable through the public network. The visited switch usually assigns TLDNs on a per-call basis, when it receives the routing request from the home system. The visited switch uses a timer to determine how long it keeps the TLDN associated with that subscriber. The timer lasts long enough for the home switch to route a call to that number across the public network.

How Long Does This Process Take?

These procedures take only 2 seconds. When the visited switch becomes aware that the customer has turned off his phone, the home switch is notified and the customer is "de-registered" in the visited market. The next time the customer turns on his/her phone, the registration procedure is repeated.

If a customer moves to another service area, the new system will go through the registration process with the HLR. When the HLR sees that the phone has moved into a new system, it sends a "registration cancellation" to the previous system and records the new location. This way, the HLR can always keep track of a phone anywhere in the network.


The network operates on a combination of network hardware and software to operate.

Mobile Base Stations - otherwise known as the cell site. The radio signal of each cell site covers a portion of a service area. A service area can have hundreds of cell sites.

Mobile Switching Centers (MSC) - The MSC connects the voice path for a call. The MSC and cell site work together to determine which radios the mobile phone will use when delivering calls. It interprets the digits entered by the mobile phone user and delivers the call accordingly. As the mobile subscriber moves to new cell site areas, the MSC reconnects the call to a new radio in that cell. It provides the path to the public switched telephony network (PSTN), for delivering calls to land-line phones or to long-distance numbers. There can be many MSCs in a single service area.

Home Location Register (HLR) - The HLR is a database where the subscriber's feature profile is stored. It will keep information on the subscribers phone number, the electronic serial number of the mobile phone, and the features the customer has (call forwarding, call waiting, 3-way calling, voice mail, etc.). The HLR also knows of any dialing restrictions. The HLR is a software element. It can be an integral part of the MSC, or stored on a separate platform.

Service Control Point (SCP) - The SCP holds databases that control customer features and services. The HLR is a primary example of a database on the SCP. Some other examples include 800-number lookup services, calling card services, calling number identification, short message service, message waiting indicator, and debit card services.

Signal Transfer Point (STP) - The STP is a network routing element. It takes a message in, checks the routing information, and sends the message toward its destination. STPs use the signaling system seven (SS7) protocol to transport messages.

Protocols - Data messages are formatted in a way that allows switches and network elements to understand them. The formatting follows certain rules and these rules are called a protocol. The predominant protocols used in roaming networks are signaling system seven (SS7), and interim standard-41 (IS-41). We also use X.25 packet switched data, global system for mobile communications (GSM), Ethernet, and the transmission control protocol/internet protocol (TCP/IP) for messaging :

SS7 - SS7 is an out-of-band signalling mechanism used in telephony. More information on SS7 can be found on this page.

IS-41 - IS stands for "interim standard." Standards such as IS-41 are created by the Electronics Industry Association (EIA) and the Telecommunications Industry Association (TIA). IS-41 defines a set of messages to manage wireless mobility between cell sites and switching systems. The registration cycle and call delivery mechanisms described above use IS-41 messaging.

X.25 - A packet switched public data network protocol.

GSM - Global System for Mobile communications. This is an entire set of standards governing a particular type of mobile phone system used throughout the world. It also defines how mobility is managed in GSM systems.

TCP/IP - If you're viewing this page over the world wide web, you are using the TCP/IP protocol. It is the primary protocol used between internet elements. We use it to get information from one intelligent peripheral to another.