Keith Cambridge

COMT 391

Wireless Networking

Winter 2000

All rights reserved to Nokia

International Roaming Using PCS Technology

 

 

As you travel around the world, you simply can't afford to be out of touch with your associates and colleagues. Would it be interesting to traverse the world with one phone number? However, to accomplished this you need to have an idea as to what the term PCS (personal communications system) entails and how it allows you to be in-touch with others when you are on your vacation in Paris or in the lovely island of the Bahamas.

 

 

What is PCS?

 

Personal Communications Services or PCS refers to a whole family of next-generation wireless communications services that include new categories of wireless voice, two-way messaging and high-speed data transmission using lightweight, multi-function, digital pocket phones and mobile computers.

Licenses to provide these new services in the 1850-1990 MHz band of the radio spectrum were auctioned by the Federal Communications Commission in 1995.

The higher frequencies at which PCS networks operate accommodate more information, more efficiently than lower frequencies. (Cellular networks operate

in the lower 800 MHz band).

 

How does it work?

PCS networks operate similarly to other radio technologies in that signals are sent using a series of transmitters (also called base stations) mounted on buildings, ding towers of buildings and other structures. These specialized phones send and receive digital signals via these base stations and a system of computerized switches. However, when a call is made on a PCS phone, the base receives the signal from call station, which in turns connects the signal to the serving PCS switch. The switch similar to a LEC CO houses a variety of computers, software and control systems that processes, routes setup, tear down, and completes voice and data transmissions. The switching unit then decides under what condition transmitted signal from the mobile unit should be delivered. Typically in most conditions, the unit will decipher whether it should be transmitted to another wireless phone or connect to the local PSTN and routed via wire medium. Having this concept in mind there will be issues such as to where the destination of the call would be handed over. For example the customer may be in his hometown or in another state. However my paper would be focusing on the nature that subscriber of the mobile handset is located in an international zone. In trying to grab with the concept of international concept we would embark on the term called roaming.

 

What is roaming?

 

Roaming allows you to use your mobile phone when traveling abroad in countries with compatible networks. You can stay in touch with friends, family and associates without them having to learn a new number as it stays the same. Simply switch your phone on when you reach your destination and select a local network through your mobile phone menu options. Roaming can be described as the ability for one network to permit the subscribers of another network to make and receive calls on to the network of your destination. Roaming is also possible even between different types of networks where suitable interface infrastructure is deployed.   Examples of circumstances under which roaming have been permitted include the following: -“ International roaming between GSM networks worldwide; At the inception of the MTN and Vodacom businesses, national roaming between these two networks was an obligation placed on the operators in order to cater for the network roll out differences between them. Consequently, subscribers of one operator could make calls on the network of the other in areas where the other operator had not yet rolled out. Prior to the issue of the licenses to the incumbent MCTS's operators, Telkom, a partner to Vodacom, had already deployed certain infrastructure in anticipation of a license for Vodacom. MTN was then considered disadvantaged in the roll out process, hence the roaming arrangements was permitted by the then Government.  It is common in Europe that where a subscriber (customer) to a network of one kind (eg. DCS1800 network), who has access to another kind of a network (eg. GSM 900 network), and where such a subscriber has a dual mode hand set, then such a subscriber is able to roam on the other network in areas where the other network is not available.” (Techno Surf Adwav)

Now that we have an idea as to what is PCS and roaming we will move to the main focus of my paper. As I said earlier if we need to use one PCS number around the world we need to migrate to the concept of International Roaming.  The technology that is widely used to achieve this international roaming feature is the GSM (Global Systems For Mobile Computing)

 

What is GSM?

 

GSM, which stands for Global System for Mobile Communications, is a digital cellular radio network operating in over 200 countries world-wide. Today we see that this standard is the prevalent standard that provides almost complete coverage in western Europe, and growing coverage in the Americas, Asia and elsewhere. Today we have seen that this standard is able to achieve the capability of providing data computing.  The vast number of the consumer population of mobile handheld user think of voice calls when they refer to that of their cellular phones. In the GSM environment the signal in use is of digital nature. This makes provision for you initiate a connection on  your GSM-enabled phone to your laptop computer which in turns allows the subscriber to  send or receive e-mail, faxes, browse the Internet, securely access your company's LAN/intranet, and use other digital data features including Short Messaging Service.

  Global System for Mobile Communication still have need for radio frequencies, GSM is a wireless platform, in contrast to technologies, which require connecting one's laptop modem to a telephone outlet to use the land-based telephone network.  Today we see a vast amount the people using the benefits of GSM which provides a complete  virtual environment that allows users to accomplish  and establish  wireless data computing from any given location given consideration for the terrain and the type of medium that the signal is being used to transmit the signal. This is turn relieves the mobile user the headache of worrying about adapters, telephone jacks, cables, etc. The unique roaming features of GSM allow cellular subscribers to use their services in any GSM service area in the world in which their provider has a roaming agreement. In essence the mobile phone that you utilized in your home will be able to work on your dream vacation in Paris, this will also be somehow dependant on the type of roaming agreement you have established with your local provider.. GSM-enabled phones have a "smart card" inside called the Subscriber Identity Module (SIM). The SIM card is personalized to you and you alone. It identifies your account to the network and provides authentication, which allows appropriate billing.  No wires! This means connecting with the office: anytime, anywhere. Turn waiting time into email or fax time -- at the airport, train station, the dentist's waiting room. Turn any place into a productive work space: a hotel room, a client's office or home, the factory floor, a loading dock, a cafe table -- anywhere.

 

GSM Frequencies Usage

 

 Global System for Mobile Computing today presently operates in three different frequency ranges.  The frequencies that are popular are as follows:

·        GSM 900 (also called GSM) - operates in the 900 MHz frequency range and is the most common in Europe and the world.

·        GSM 1800 (also called PCN (Personal Communication Network), and DCS 1800) - operates in the 1800 MHz frequency range and is found in a rapidly- increasing number of countries including France, Germany, Switzerland, the UK, and Russia. A European Commission mandate requires   European Union members to license at least one DCS 1800 operator before 1998.

·        GSM 1900 (also called PCS (Personal Communication Services), PCS 1900, and DCS 1900) - the only frequency used in the United States and Canada for GSM. Note that the terms PCS is commonly used to refer to any digital cellular network operating in the1900 MHz frequency range, not just GSM (http://www.gsmdata.com)

 

 

Benefits of International Roaming

 

The benefits of using the International roaming are numerous. However, these benefits will vary among the subscribers. As consumers the very concept of international access will be directly tied into cost, billing arrangements and in some cases the value of the currency compare to that of the U.S dollar.  Needless to say, since we are dealing with roaming with reference to our cellular unit, they would be some vital benefits that one should into account such as:

 

 

 

 

How International Roaming is Made Possible?

 

All rights reserved to Globecom Technologies

 

  On many occasion while walking or sitting at major airports, I would observe passengers who are traveling with gadgets of communication devices. Sometimes with curiosity I would form a conversation and find out what is the main idea of traveling with all these devices. Some of them may be scientist researchers, or on numerous occasion executives from wealthy firms traveling overseas to either sell contracts or invest. However, one occasion there was a chief engineer from Lufthansa Air fleet who was in the U.S on business.

    However, his flight was cancelled because of bad weather.  As he heard the announcement he went into his traveling pack whipped out his IBM laptop with a cellular phone and began to communicate with his international branch. Which went on for a while. When he was finished he smiled and said “ it is amazing that I could use my company phone anywhere in the world with no confusion.”

After a long flight from New York to London Heathrow you have decided to call your wife or CEO of your organization.   It is important to note that upon arrival to an international country you need to switch on your GSM mobile phone. Your mobile unit goes through a phase seeking and search for a GSM signal within the international country.

   The visited network detects that a foreign GSM phone is present, and if there is a roaming agreement with your provider and that of the visiting country it will validate your status with the home network in that of the visited country. During this time the name or the code of the visited network will then be displayed on your phone indicating that you have successfully been connected to the network, when this connection and validation process is complete your mobile unit is ready to act as though it is functioning within its local coverage area. Needless to say the said phone subscriber will have to make some changes and adjust to his new environment.

   Now you are ready to use your mobile unit. To make a call while in the visited country, you should dial in the same way as local users in that country would do. All codes for long distance and international calls should be applied, including calls to your country of origin. If you want to make an international call but do not know the International Access Code of the country you are visiting, you may enter '+' which all GSM networks will translate into the relevant International Access Code. Your phone user's manual will explain where the '+' key is on your GSM mobile phone.

Example:  To call Dubai from London, UK, you would dial in the following way:

                                   1.International Access Code           00 or '+'

                                   2.Country Code                                971

                                   3.Area Code                                       4

                                   4.Phone No.                                  123456

 

It is very important to note to consumers that will be some billing issues associated with international roaming. Whilst traveling abroad and using your SIM card, you will be liable for call charges in the following cases:

Outgoing calls:  The visited GSM network operator will charge you their standard tariff, be it local, long distance or international. The charges worked out by the visited operator are then sent to your home operator. These charges are converted into your telephone bill, which you receive in your country or origin.              

 Incoming calls: When receiving incoming calls whilst roaming you will be charged for the international re-routing of the call from the visited country to the country where you receive the call, regardless of where the call was originated. The calling party will be charged for the call from the country where he/she is making the call.

 
Technical Overview

 

 

 

 

                                                                                  

GSM uses a series of radio transmitters called Base Stations of BS to link the subscriber and their cell phone to the cellular network. Base stations are also called a cell, because it covers a specific distance within a given area. Each station are connected thus leading to subscribers having the capability of moving from one cell to another, this is done via the process of hand over.

  There are also controllers. The controllers are called base stations these controllers connects the base stations to a particular base station. The MSC or mobile switching unit is another major part of the system. The cell phone and the SIM creates a digital signature that includes the subscriber number, that is then sent from your handset to the nearest base station which in turns ask the desired network to allow the subscriber to use the network. The request is then passed on the network of the base station to the center of the cellular network in this case it’s the MSC. The MSC routes all incoming and outgoing traffic to and from the fixed line network. 

   The HLR or the home locator register provides the administrative information that is necessary to authenticate, register and locate the user as a subscriber. When the HLR receives the information, the HLR immediately checks the special signature contained in the request against its special subscriber database. In the event of inactive subscription the subscriber will be rejected from the network. If you area valid subscriber, then the MSC sends a message back to the phone via the network base station that indicates that you have been granted permission to access the network. The code or name of the network will appear on the LCD of the handset. When a call is need to be routed to your handset it goes through a process of polling i.e. the HLR registers which base station that the subscriber is connected to. This is mainly for the network advantage, where the MSC have an interest in routing the call it will then first check the HLR to see where you are. By this the handset will transmit a message to the network indicating its location.

   Multiplexing is also a part of the system, whereby each base station utilize digital methods to enable a number of phone to be simultaneously connected to it, as well as simultaneously allowing a number of subscribers to make and receive calls. There is the notion of signals being handed over to other cells in the network aka as handing over. The HLR automatically updates and continue to monitor where exactly the call should be routed as you begin to move between another base stations. The VLR or visitor locator register paves the way for out going calls to be accomplished. The VLR verifies to see if you are permitted to make such calls. There is also a mailbox in the system. The mailbox is the automated response that a caller may get when the subscriber is unavailable.  The SMSC or SMS center is a special facility that handles short messages. The SMSC generates the special SMS message that notifies you that you have mail waiting in your mailbox. The SMS is capable of receiving messages even when the subscriber is engaged in a voice call. This is possible because SMS are sent on a different radio frequency. There is a data channel and a voice channel and the never mingled with each other. As a result of this we have seen that GSM is the de facto global cellular standard.

 

System architecture

 

The functional architecture of a GSM system can be broadly divided into the mobile station, the base station subsystem, and the network subsystem. Each subsystem is comprised of functional entities, which communicate through the various interfaces using specified protocols.

 

Mobile Station

 

The mobile station in GSM is really two distinct entities. The actual hardware is the mobile equipment, which is anonymous. The subscriber information, which includes a unique identifier called the International Mobile Subscriber Identity (IMSI), is stored in the Subscriber Identity Module (SIM), implemented as a smart card. By inserting the SIM card in any GSM mobile equipment, the user is able to make and receive calls at that terminal and receive other subscribed services. By decoupling subscriber information from a specific terminal, personal mobility is provided to GSM users.

 

Base Station Subsystem

 

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BCS). The BTS houses the radio

transceivers that define a cell and handles the radio (Um) interface protocols with the mobile station. Due to the potentially large number of BTSs, the requirements for a BTS are ruggedness, reliability, portability, and minimum cost. The Base Station Controller (BSC) manages the radio resources for one or more BTSs, across the Abis interface. It manages the radio interface channels (setup, teardown, frequency hopping, etc.) as well as handovers.

 

Network Subsystem

 

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). The MSC emulates a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, including registration, authentication, location updating, inter-MSC handovers, and call routing to a roaming subscriber. These services are provided in conjunction with four intelligent databases, which together with the MSC form the Network Subsystem. The MSC also provides the connection to the public fixed networks.

 

The Home Location Register (HLR) contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the subscriber. The location assists in routing incoming calls to the mobile, and is typically the SS7 address of the visited MSC. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

 

The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although the VLR can be implemented as an independent unit, to date all manufacturers of switching equipment implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR. The proximity of the VLR information to the MSC speeds up access to information that the MSC requires during a call.

 

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile

equipment on the network, where each mobile equipment is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, used for authentication and ciphering on the radio channel.

 

 

 GSM Security/ Fraud For International Roaming

 

  To be truly useful, mobile phones should allow roaming between networks and across countries. However, fraudsters can exploit the time lag in                     sharing account activation and billing data between these operators. For example, a user can evade billing limits set by his home network by making high value calls whilst roaming across other networks. It is even possible to close the account in the home country, but still use the phone abroad for a certain period. Faster data transfer between operators - preferably by EDI - is the most effective solution to this problem. At certain times, such as during a call set up, the network's VLR (Visitor Location Register) decides to authenticate a subscriber. It requests authentication parameters from the mobiles HLR (Home Location Register). The HLR in turn requests these parameters from the AuC (Authentication Centre). The AuC passes a 128 bit random number, RAND, and the mobiles Key, Ki, through the algorithms A3 and A8 which generate the desired 32 bit SRES (Signed Response) and Kc (Cipher Key) respectively. This Authentication Triplet (RAND+SRES+Kc) is passed to the BTS, which the mobile is operating through. The BTS issues an authentication challenge;

 

 

By storing all the Keys and Algorithms within a single entity of the Home network it is possible to enhance security- the Keys and Algorithms are never transmitted outside the AuC hence cannot be intercepted on the fixed part of the GSM network. Additionally it allows different networks to have different algorithms since roaming mobile only needs to get the same RAND and Kc as the current BTS has. At all time these authentication parameters are calculated by its home network, not the network it has roamed onto. The only common algorithm is the encryption algorithm A5 used to encrypt traffic on the air interface between the MS and BTS. Without knowledge of Ki (which is never transmitted over the air) you cannot form Kc. Ki is only stored in two places, the authentication center within the network and the SIM. It cannot be read out of the SIM and networks will have security systems to stop all but the most essential employees getting access to Kc at the AuC. Attacking using a code book response - you get all possible combinations of RAND and respond with a pre-recorded SRES- would mean that you need to record the SRES for the 3.4*10^38 combinations of RAND- for the mobile under attack. Clearly this method appears difficult to say the least  (http://www.digicomglobal.com)

 

Authentication and security

 

Since the radio medium can be accessed by anyone, authentication of users to prove their identity. It is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the AuC. During authentication, the AuC generates a random number that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated [16].

 

The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key, together with the

TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is XORed with the 114 bits of a burst (the two 57 bit blocks). Enciphering is an option for the fairly paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers. Another level of security is performed on the mobile equipment itself, as opposed to the mobile subscriber. As mentioned earlier, a unique International Mobile Equipment Identity (IMEI) number identifies each GSM terminal. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR). The status returned in response to an IMEI query to the EIR is one of the following:

 

White-listed : The terminal is allowed to connect to the network.

Grey-listed:  The terminal is under observation from the network for possible problems.

Black-listed:  The terminal has either been reported stolen, or is not type approved (the correct type of terminal for a GSM network).

(GSM Engineering Handbook)

 

Tips you need to do before leaving on an international trip in order to achieve international roaming with your cellular handset.

 

1.Ask my Service Provider to lift any international bars on my handset.

2.If I don't want to receive incoming calls, set Divert Immediate to automatically divert incoming calls to recall in the country of origination

3.Check I have my Call barring code to hand and a full list of Call Barring options.

4.Make sure I have a multi-band GSM/PCS phone or have a hired PCS phone for travel in the USA or Canada.

5.Make a note of any security PIN and PUK (PIN Unblocking Key) numbers I may need as well as the number of my Service Provider.

6.Switch off my phone before I board the plane.

 

Call Barring

One useful option while you're overseas is Call Barring. You can bar certain types of calls, such as   international, premium rate services and/or incoming calls. Ask your Service Provider for your barring code and a full list of Call Barring options.

 

Security

With a communication tool as vital as your mobile, the last thing you want is someone else using it to stay in touch - at your expense. In case your phone is lost or stolen, some companies have several built-in security features that can be turned on and off as necessary. You'll find additional instructions on the various security features in your handset manual.   To give yourself extra protection, it is recommend you activate the Personal Identification Number   (PIN) security code. For details one should read his or her handset user manual or contact your Service Provider.  To report a lost or stolen SIM/handset:  Contact your Service Provider during office hours.

 

 

 

Some Main Features of GSM

 

GSM today allows SMS (Short Message Service), which allows subscribers to send and receive text messages with a length of 126 characters. More importantly the number portability issues of roaming from country to country is not a issue of concern cause with this technology subscribers are able to use the same phone number in a number of network related countries.

  The data transmission today is at 9,600 bps the rate is similar for reception across GSM networks too. This rate is also similar for facsimile transmission. When dealing with the issues of interference especially static; GSM provides No-static connections.  Emergency number dialing is another hands up for GSM, in the majority of countries; the global 112 emergency numbers can be dialed for free. Billing convenience is one of the benefits enjoyed by subscribers, they could have a real-time call costs on the handset’s display.

    Encryption for security measures is constructed in such a way that the system cannot be tapped. For the subscriber that wishes to protect his handset i.e. regarding types of calls he or she could implement blockage on calls for locations, and at the same time blocks incoming calls. Capacity is another great advantage it allows more capacity and provides a rapid call set – up procedure. Today we se that call conferencing i.e. talking to several parties simultaneously is supported on the GSM handset. Subscriber’s handsets are capable of being recognized as a member of PBX class extensions. In addition to having your personal callers ID or your LCD, subscribers have to the advantage to bar anyone for seeing their number via CLIR

 

 

 

GSM Advantages and Disadvantages.

 

     Advantages

The GSM technology for international roaming has several advantages when it comes to roaming at both local and internationally. Its capacity i.e. 2 to 3 times that of the conventional AMPS systems. When it comes to data rate, GSM allows high- speed (full duplex at 9.6Kbits/s) facsimile. GSM also has the both paging and messaging services capability. Power consumption is also a major advantage when compare to AMPS and other technologies. GSM technology provides better security than other technologies when it compare to other technological formats. The system performance is better in low signal and interference situations

 

     Disadvantages

There are several disadvantages of the GSM protocol for International Roaming. Today a few of the main disadvantages are:

 

Some Latest Development with GSM

 

A number of competing Mobile Satellite Services (MSS) plan to blanket the globe with satellite telephone coverage from   a constellation of over 1,000 satellites by

 2004. These systems include Iridium, Odyssey, Globalstar,  Teledesic, ICO, Thuyra, ACes, Agrani, and EAST.

     According to a report from Frost & Sullivan "World Mobile Satellite Telephony Service and Terminal Equipment,’’ the world global mobile personal communication service  (GMPCS) service market will reach $47.2 billion by 2006. Only the Iridium satellite phone system is currently commercial (as from November 1 1998), Globalstar successfully launched many of it's satellites during 1999 and went live in October 1999. It will be commercial in 3/2000.

Inmarsat has operated a portable phone service since the early 1990's. It's smallest is the briefcase sized Inmarsat-M(ini) phone system. It will be superceeded by the ICO system. ICO filed for bankruptcy protection in August 1999. It was bailed out in November 1999 by a $1.2 billion reprieve from investors led by industry veteran Craig McCaw.

  Almost all these satellite services offer a combination of all-digital transparent voice, data, fax and paging services to and from hand-held telephone devices, some no larger than current GSM cellular handsets. Agrani, AceS (Asian regional systems built by Lockheed Martin), and EAST (a regional system that will focus on

Africa and the Middle East) aim to provide interoperability between mobile satellite and cellular networks. As a result, any GSM subscriber can continue to use his existing GSM SIM cards with the new dual-mode (GSM/satellite) handsets.

    The three systems will share an air interface standard named GMSS (Geostationary Mobile Satellite Standard) that is similar to GSM. This means that satphone

customers will be able to use mobile phones that are  compatible with satellite systems in any country where GMSS is offered, in effect creating roaming capabilities

between the three systems' regional footprints (http://www.cellular.co.za/)

 

 

Some Important GSM Terminology

 

TCAP: Transaction Capabilities Application Part

VLR:   Visitor Location Register

AIC:   Advanced Intelligent Networks

Control

Channel: The channel the phone and cell base station first communicate on.

 Reverse

Control

Channel: The opposite frequency, 45 MHz lower than the control channel. Used by the mobile.

Voice channel:  The channel you are assigned by the switch to commence the call on after the exchange of subscriber data.

Base Transceiver Station: The network base station that talks to the mobile.

TDMA Time Division Multiple Access

FDMA Frequency Division Multiple Access

A-TDMA   Advanced Time Division Multiple Access

CDMA Code Division Multiple Access

AuC Authentication Center

LAI Location Area Identity

Ki Individual subscriber authentication key

OMS Operation and Maintenance Subsystem of the GSM network, IMEI The IMEI (International Mobile Equipment Identity) is a 15 digit Code used to identify the GSM/DCS/PCS phone to the network.

CLIP Caller Line Identity Presentation

CLIR Caller Line Identity Restriction

SMS Short Message Service

HLR Home Location Register

PIN Personal Identity Number

PUK PIN Unblocking Code

BSC Base Station Controller

BS   Base Station

 MSC Mobile Switching Centre - The computer that places the calls, and takes and   receives data from the subscriber or   from PSTN (Public switched Telephone  Network)

SMSC   Short Message Service Centre

SMD-PP   Short Message Delivery Point-to-Point

VLR Visitor Location Register

SMS-IWMSC SMS Interworking Mobile Switching   Center

TAP    Telocator Alphanumeric Protocol

SS7   Signaling System 7

GMSC Gateway Mobile Switching Center

MO-SMS Mobile-Originated Short Message

ESN Electronic Serial Number

IWF  Interworking Function (Modem)

IEMI   International Equipment Mobile Identity

GSM Global System For Mobile Communication

EIR Equipment Identity Register

PE   Primary Exchange

 

 

Sources :

 

(a)IEEE Communications Magazine April  1996.

 

(b) Robert G.Winch . Telecommunication Transmission Systems. Mc Graw-Hill. New York, 1993

 

© http://www.gpsdata.com

 

(d) Michel Mouly and Marie- Bernadette Pautet. The GSM System for Mobile  Communications

 

(e) http://vodafone.sksl.com/

 

(f) http://www.digicomglobal.com

 

(g) http://www.cellular.co.za/