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Essays of

"MISSING SCRIPT" by Dr. Kunle Bello
 
kunbel@runbox.com

 

MS011
GSM CALL ARCHITECTURE


Contingent on the any-to-any-principle whereof, any user of any network anywhere is capable at anytime to communicate (call & receive) with any other user of the same or any other network anywhere at anytime, we are now poised and ready to make and receive all types of calls and even transfer & receive data.


OUTGOING GSM VOICE CALLS
Armed with a working MS, the user who wishes to make a call dials the mobile telephone number of the person to be called i.e. MSISDN of the interested party, presses the call initiation key i.e. “SEND” or “TALK” key, and the MS sends an access request message to the GSM network via the nearest functioning GSM mast or tower i.e. BTS.


The access request message is administered next by the associated MSC, which verifies the subscriber’s record held in the VLR to know if the outgoing call is allowed. If yes, the MSC then routes the call in the same way that a telephone exchange does in a fixed network.


If the subscriber is on a ''Pay As You Go'' tariff also known as “Prepaid” then an additional verification is made to know if the subscriber has enough credit to make the call. If not, the call is rejected. Where the call is allowed to continue, then it is continually monitored and the appropriate amount is deducted from the subscriber's account based on the duration of the call in relation to the concomitant tariff(s). If and when the credit is exhausted i.e. reaches zero, the call is dropped or cutoff by the network. It should be noted here that the system that monitors and provides the prepaid service is not part of the GSM standard services rather, the relevant service provider incorporates it. That monitoring system is an example of Intelligent Network (IN) services that operators provide to guarantee their Return on Investment (ROR).


ROUTING INCOMING GSM VOICE CALLS

For an incoming call to a Mobile Station (MS) for instance, from a fixed network (PSTN), the landline caller dials the relevant MSISDN and the call is immediately routed to the nearest Gateway Mobile Switching Centre (GMSC). As the entry point to the mobile network, the GMSC checks on the current location of the called mobile phone through the HLR which is aware of the particular VLR that the mobile phone is associated with, if any.
In the event the owner of the mobile phone has requested that all calls be diverted a number known as the Call Forward Unconditional (CFU) number then this number is stored in the HLR. As soon as GMSC asks HLR for routing of an incoming call, the HLR gives out the particular CFU to GMSC that in turn routes the call to the CFU.


If however, the called mobile phone is not currently associated with any VLR as it off-line, the HLR then sends a number called Call Forward Not Reachable (CFNRc) number to the GMSC that subsequently passes the call to that number. Most operators attach CFNRc to their voicemail system that enables callers to drop messages.


RINGING GSM PHONE & RECEIVING CALLS

Where the called Mobile Station (MS) is roaming, the HLR will request the attendant VLR for the temporary Mobile Subscriber Roaming Number (MSRN) which is sent back to the GMSC that in turn directs the call to the appropriate MSC where the called MS is roaming. The Visited MSC then pages the BTSs in the area to inform the MS of an incoming call. If the subscriber answers, then the Visited MSC with the GMSC establishes a speech path between the called MS and the calling landline.


If however, the MS is not answered as the phone is busy on another call and the call-waiting feature of the MS is not activated, the Visited MSC routes the call to a predetermined Call Forward Busy (CFB) number or to a Call Forward No Reply (CFNRy) number if after typically, 30seconds the call remains unanswered. Here again, the service provider may designate that number to a voicemail system to enable callers to drop messages.


Similarly, the call can also be routed to a predetermined Call Forward Not Reachable (CFNRc) number if unanswered for other reasons like, out of coverage, switched off or battery energy constraints. And of course, messages can be also dropped in a similar fashion as before.


SIGNALLING & CONTROL


We have just observed part of the wherewithal and techniques of making and receiving calls with a GSM mobile phone i.e. a Mobile Station (MS). Incidentally, we only saw the paths or routes, the network elements and subsystems that are involved in the speech or voice communication from the calling MS or landline to the called MS. The manner, method, action and how the signals are actually sent, received and controlled are not yet unfolded. This is the aspect of signalling.


Signalling is the action of communication between the users & networks and between various network elements for the control and supervision, setting up and clearing down of telephone calls. Additional functions include the support of advanced features like supplementary & special services, network operation & maintenance (O&M), billing information and other housekeeping activities related to the calls and the maintenance of the signalling communication channel. Signalling types, also referred to as signalling protocols are determined by network architecture, switching system and the transmission (transport) system slated to carry the user data and the signalling information. As switching and transmission systems have evolved so also have signalling systems kept appropriate pace to adequately support the multiplicity of emerging telecoms services.


Up until the early 1980s,the signalling for a telephone call used voice circuit the telephone call travelled on. Referred to as in-band signalling, this method of signalling used the same physical path for both the call-control signalling and the actual connected call. Please note that any signal inside the allowed speech band is “in-band” otherwise it is “out-of-band”. This inefficient “in-band” signalling method has since been replaced by “out-of-band” or “Common Channel Signalling” (CCS) techniques such as Signalling System no 7 (SS7) that is efficient and security-oriented. SS7 is an internationally standardised protocol used by service provider’s inter-exchange messaging e.g. MSC-to-MSC or MSC to GMSC, call establishment, billing, routing and exchange of information across the network. In PSTN and today’s wireless networks, SS7 is the de facto system that puts the information required to set up and manage telephone calls (with greater security) in a separate, dedicated overlay network rather than within the same network used to make call.


SS7 signalling provides a highly reliable and robust packet-switched network over which the network control information as SS7 messages can be transferred. Thus, SS7 signalling supports call control, supplementary services, Intelligent networks (INs) in fixed networks whilst mobile networks also get support for the additional features like mobility management, Short Message Service (SMS), Multimedia Messaging Service (MMS) and other advanced services that are evolving.



MS010
JUST BEFORE THE GSM CALL


Now that we have beamed our searchlight on the organisation of the GSM network, touched on the attendant subsystems, described network elements in their singular & distributed forms and expounded on the features & functions of all these, it is apposite to now put them into action(s). The simplest of such actions is to make, trace, sustain and terminate a call or send data across a GSM network. If anything, this will bring home that seeming behemoth of a cellular mobile system the effects of which are actively and rapidly turning around & enhancing the fortunes of small and big economies.


This impending step shall precede our foray into other areas that will consummate our effort in the current expose of the GSM specification. These areas include Transmission Systems & Techniques, Signalling & Control, Platforms, Intelligent Networks (INs), Mediation & Billing, Services & Applications, Operations & Maintenance (O&M), Interoperability, Standardisation, Regulation, Developments and Evolutionary Trends, to mention a few.


As a quick reminder, the main features of the GSM network are:
• Mobile Station Subsystem (MSS) with Mobile Equipment (ME) and Subscriber Identity Module (SIM) card as component parts.
MSS or MS equals ME plus SIM
• Base Station Subsystem (BSS) with Base Transceiver Station (BTS) and Base Station Controller (BSC) as constituent parts also forming the Radio Access Network (RAN)
BSS equals (BTS plus BSC) which is same as Radio Access Network (RAN)
• Network & Switching Subsystem also called Core Network (CN) having Mobile Switching Centre (MSC) and Network Databases (ND) as its fragments.
NSS equals (MSC plus ND) which is the same as Core Network (CN)
At this point too, it is relevant to describe the possible switching methods and techniques with which calls and data could be sent and received through a GSM phone (MS). Also, it is important to note that, since the phones under consideration are continually mobile, it is advisable to understand the roaming features, properties and possible situations of the Mobile Station (MS).


CIRCUIT SWITCHING (CS)

Traditionally, circuit switching is ideal for voice communications and any other real-time service as it establishes or sets-up and maintains or dedicates connection (path) between parties until the end of conversation. As the paths are exclusive to parties even when idle, inefficient use is therefore made of network resources. Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN) are examples of standards & services that use circuit switching.


Where data is similarly circuit-switched i.e. Circuit-Switched Data (CSD), accompanying gaps in data transfer while connection is still retained, also leads to network resource wastages. This is experienced in Public Switched Data Networks (PSDNs). It should be noted here that for both voice & data, circuit-switched networks are charged on occupation or time duration basis.

PACKET SWITCHING (PS)

This is a method of transferring data by breaking it up into chunks called packets, cells or frames before transmission (transportation). Packet data is how most data travel over the Internet - a global system of interconnected computer networks. Superior to the traditional Circuit-Switched Data (CSD) transfer method where an open data connection which uses network resources, must be maintained even when idle (like standard voice connections), with Packet Switched Data Network (PSDN), each user consumes network resources only when data is actually being transferred. It is thus a more modern way of data transfer and also cheaper & faster than its circuit-switched counterpart as resources e.g. transmission links can be shared by many users. With PSDN, charges are based on throughput i.e. average rate of successful message delivery in data packets per second or data packets per time slot.

ROAMING
When a Mobile Station (MS) travels outside of its calling area, (i.e. outside subscriber’s home service area) it is said to roam or it is roaming. As it roams, an MS continues to send a signal out that tells any BTS it can reach (regardless of the service provider) that “I am here”. If it can communicate with a BTS then the roam indicator on the MS is displayed and the MS can then place and/or receive calls provided there is a roaming service or arrangement in place. There are roaming arrangements and agreements amongst and between operators within and across borders to enable subscribers make and receive calls when they are outside of their calling areas.






MS009
THE GSM CORE NETWORK


Just as the core of anything depicts the heart, kernel and nucleus of that thing, the Core Network (CN) is the chief & principal constituent of a generic GSM architecture & specification.


Core Network is a convoluted system of switches, transmission systems, interfaces, platforms, databases, billing systems, network management centres and customer service centres - all aimed at providing virtually any type service that may be required by any prospective user.


Core Network can conveniently be grouped into two categories viz: Network & Switching Subsystem (NSS) and Operations & Support Subsystem (OSS). For a gradual understanding and imprint on the basics, we shall defer the discussion on OSS till we are through with the main functional issues.


NSS comprises of the Mobile Switching Centre (MSC) and Network Databases (ND). Its main role is to carry out switching functions with which it manages communications between mobile users and other users such as mobile users, Integrated Services Digital Network (ISDN) users and Public Switched Telephone Network (PSTN) users etc.


For completeness, please observe that Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, data, video and other network services over the traditional circuits of the legacy, circuit-switched, Public Switched Telephone Network (PSTN) often referred to as landline or fixed network i.e. the Plain Old Telephone System (POTS).


Network Switching Subsystem (NSS) is also responsible for mobility management of subscribers whose compendium of information is stored in databases inside the NSS.


Mobile Switching Centre (MSC) is the primary delivery service node for GSM specification as it provides relevant voice, data & video path connections between a mobile phone and a landline telephone, between two mobile phones, or between a mobile phone and the Internet. From the customers’ perspective, the MSC is functionally supposed to appear as a seamless extension of the PSTN. Any MSC that directly interfaces with other networks e.g. PSTN is called a Gateway MSC or GMSC.

In summary, the Mobile Switching Centre is responsible for the following:
Manages the location of mobiles
• Switches calls – call control, routing & registration
• Manages security features
• Controls handovers between BSCs
• Resource management
• Interworks with and manages network databases
• Collects call billing data and sends to billing system
• Collects traffic statistics for monitoring performance
• Interfaces with external service centres and other networks: PSTN, ISDN, PSPDN (Packet Switched Public Data Networks)


The other component of NSS that interworks with the MSC is the Network Databases. In order to manage subscriptions data for network users and to support mobility, the Core Network (CN) contains a series of Network Databases known as Location Registers, and specifically Home Location Registers (HLRs) and Visitor Location Registers (Velars). In conjunction with MSC, the Location Registers provide call routing and roaming capabilities of GSM. As part of Network Databases are also Authentication Centres (Auks) and Equipment Identification Registers (Ears).


A Home Location Register (HLR) is a database that contains information on all subscribers to that particular network for the purpose of call routing and position determination. Considered as a single network element, there is logically one HLR per GSM network albeit; it may be implemented as a distributed database around a network. Stored in the HLR are the following:

• International Mobile Subscriber Identity (IMSI), a number that is unique to each GSM subscriber.
• Mobile Subscriber ISDN (MSISDN) number – the mobile subscriber directory number, the number a calling party dials to reach the called party.
• Information on the multiplicity of possible services subscribed to, e.g. teleservices, bearer and supplementary services.
• Mobile Service Roaming Number (MSRN) – the number that is used to route any mobile terminated call to the appropriate VLR.
• Any service restriction.
A Visitor Location Register (VLR) is a temporary database of any subscriber while that particular subscriber is located and registered within a geographical area controlled by the associated MSC. Thus, a VLR contains information on home subscribers and visiting subscribers from other networks. Generally, there is one VLR for every MSC albeit, not mandatory as it is possible to associate a VLR with many MSCs but not vice-versa. In summary, the information contained within the VLR includes:
• IMSI & MSISDN.
• TMISI – a temporary version of an IMSI that is used for paging purposes. As paging messages are not encrypted in GSM the use of an IMSI for this purpose may allow unauthorised eavesdroppers to associate a subscriber to a given geographical location.
• The Mobile Subscriber Roaming Number (MSRN), used to direct mobile terminated calls to the appropriate MSC.
• The Location Area Code (LAC), the consequence of mobile registration and it is used to identify the general geographic location of a mobile for terminating calls.
• Mobile status (IMSI attached/detached). The mobile will generally inform the network when it is switching on (attached) and off (detached). This arrangement avoids wastage of resources while attempting to contact a detached subscriber. In this scenario, an instantaneous voicemail option can be offered to the caller.
• Authentication parameters – parameters used in the authentication of subscribers are generated by the Authentication Centre (AuC) and downloaded to the serving VLR. The VLR administers the process of authentication.
• A copy of all other subscriber information from the HLR, for example information relating to subscriber services.
The Authentication Centre (AuC) is a protected database that stores the security information about each subscriber (a copy of the individual subscriber secret key stored in each SIM card). Parameters generated by the AuC are passed to the serving VLR. The VLR manages the process of authentication of subscribers and in turn provides parameters to the BSS that provides the foundation for confidentiality by encryption of both user traffic and signalling over the air interface. The HLR also directly communicates with the AuC and provides the relevant information to Velars in visited networks in the roaming scenario.
Equipment Identity Register (EIR) contains information on the identity of Mobile Equipment (ME) to prevent calls from stolen, unauthorised and defective mobile stations (MSs). Please remember that MS is equal to ME plus SIM card.
GSM network operators maintain three lists of International Mobile Equipment Identities (IMEIs) in their Equipment Identity Register (EIR):
• GREY - GSM mobile phones to be tracked.
• BLACK – Barred GSM mobile phones.
• WHITE – Valid GSM mobile phones.

 

MS008
RADIO ACCESS NETWORK (RAN)

The Radio Access Network (RAN) does the management of the radio link between the Mobile Station Subsystem (MSS) and the Core Network (CN). In GSM networks, RAN is called GsmRAN or GRAN consisting of Base Transceiver Stations (BTS) and Base Station Controllers (BSC) both of which form the Base Station Subsystem (BSS).

The BSS is that portion of a GSM network which handles traffic and the control data routing (activity) i.e. signalling, between Mobile Station System (MSS) and the Core Network (CN). The BSS does the direct digital-to-digital conversion of one encoding to another i.e. converting an incompatible & obsolete data to a more suitable format for further processing. This is called transcoding of speech channels or data streams. Amongst other radio-related network tasks, BSS allocates radio channels to mobile phones, does paging and quality management of transmission & reception over the air interface.

BASE TRANSCEIVER STATION (BTS)

A Base Transceiver Station (BTS) also called Radio Base Station (RBS) or just, Base Station (BS) is the cellular mobile phone equipment you often see on rooftops and farmers’ fields, that basically provides radio coverage for mobile phones to communicate within the assigned GSM radio spectrum. A BTS has multiple transceivers (TRXs) typically between 1 and 16 with antennas, each supporting eight physical channels inclusive of the control channel. Ordinarily, a BTS defines and serves a single cell but has the capability to transmit to several cells. This singular characteristic enables a BTS to serve different frequencies and different sectors of the cell (in case of sectorised base stations). The radius or coverage area of a BTS varies dramatically from a few metres (for indoor deployments) to over 30kms usually in rural areas, and a radius of up to 70kms (and even greater) is possible using modified BTSs. It should however be noted that the spectral efficiency of a BTS is degraded as the radius of coverage increases and thus a very wide radius is applicable only in areas where there is a low traffic volume. For completeness, please observe that Spectral efficiency quantifies the amount of traffic a network can carry for a given spectrum or frequency band. It is a measure of radio performance and thus, for a given traffic load, a higher spectral efficiency depicts a higher quality of service.

At this point we shall cursorily observe that other collective functions of BTS include broadcast & control channel arrangement (scheduling), channel ciphering (coding) including error protection & encryption / decryption (decoding), detection of messages (bursts) from mobile stations (MSs) and frequency reuse activity (hopping).

A Base Station Controller (BSC) is the most robust element in the BSS and the control computer that manages multiple BTSs in their tens and even hundreds examining which radio channels are being used by which BTS. It provides, classically, the intelligence behind the BTSs. Usually housed in the Core Network (CN), a BSC is connected to a number of BTSs through an interface called Abis interface. It manages the call handoff (handover) process between BTSs. Handover is the process of transferring an ongoing call or data session from one channel connected to the core network to another. The BSC also regulates the transmit power levels of the towers and the handsets. For instance, if a handset travels away from the tower the BSC raises the transmitter power levels but reduces the transmitter power levels if the handset gets closer to the tower. In summary, BSC handles much of the overhead and administrative burdens associated with frequency management that is used to allocate frequencies to BTSs, call setup, inter-cell handovers, power management, creates the operation & maintenance (O&M) interface, radio resource management for BTSs and frequency reuse (hopping) that improves voice quality and enhances data throughput. In effect, the BSC functions like a front-end processor to the core network thus encouraging it  (core network) to concentrate on processing and switching mobile calls.

 

MS007
GSM NETWORK ORGANISATION

Starting with the basic structure of a GSM network comprising of the Mobile Station (MS), Radio Access Network (RAN) and the Core Network (CN), we are now ready to elicit or launch our curiosity into the workings of the cellular mobile phone system, as promised, hitherto.

MOBILE STATION (MS)

The Mobile Station (MS) sometimes called Mobile Station Subsystem (MSS) consists of the Mobile Equipment (ME) and the Subscriber Identity Module (SIM).

ME consists of the typical hand portable phone and the vehicle mounted unit both of whose types, sizes, features & functions have revolutionised in direct synchronism with the generational developments of the cellular mobile phone system. Indeed, the manufacturers in close liaison with cellular mobile network providers or operators have pervaded the global market with a flurry of handsets that are tailored to meeting the demands of all strata of customers. Armed with information on service requirements, class distinctions, image and even aesthetics, handsets & automobile units are now produced in their relevant quantities and attendant quality worldwide. It is also relevant to add that some mobile handsets are manufactured to support more than one standard, contextually called modes e.g. dual-mode (2), tri-mode (3) and quad-mode (4) possible operations.                                                   

Every GSM handset manufactured has a unique identification number known as the International Mobile Equipment Identity (IMEI). The IMEI number of each handset is stored in the Equipment Identity Register (EIR), which is one of the databases used for security and anti-fraud purposes. For instance, if a handset is reported lost or stolen, the IMEI number is placed on the blacklist of the EIR thereby making it not work again on the network. IMEI is not only a serial number, it also indicates the manufacturer, the country in which it was produced and the type approval. IMEI is assigned at the factory.

The main function of the handset is to code (cipher), transmit, receive and decode (decipher) voice transmission according to the GSM standard.

The other component of MSS is the Subscriber Identity Module (SIM). The SIM usually called SIM card or detachable smart card is a microcontroller (chip) embedded into a small piece of plastic, which holds the GSM operating program and the entire customer related information. A chip or an Integrated Circuit (IC) is a miniaturised electronic circuit consisting mainly of semiconductor devices manufactured in the surface of a thin substrate of semiconductor material. When a customer purchases service from a GSM network operator, the sales office will program the SIM card with the user’s identification information and plug the card into the mobile phone.

Stored in the SIM card is a unique number called the International Mobile Subscriber Identity (IMSI). It is the only absolute identity of a subscriber in a GSM network. IMSI is usually a 15-digit number, the first 3digits being the Mobile Country Code (MCC) followed by the Mobile Network Code (MNC) which is either 2digits-European standard or 3digits –North American standard. The remaining digits represent the Mobile Station Identification Number (MSIN) that is within the network customer base. Thus, IMSI is MCC plus MNC plus MSIN.

Different and distinct from IMSI is the Mobile Services ISDN number (MSISDN) standing for the mobile subscriber directory number, the number the calling party dials to reach the called party. Meanwhile, Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, data, video and other network services over the traditional circuits of the Public Switched Telephone Network (PSTN). Each IMSI may have more than one MSISDN related to it. Different services to a particular subscriber e.g. voice, data or facsimile (fax) are identified by different MSISDNs. 

The SIM card provides authentication, information storage, subscriber account information and data encryption. Such subscriber information can be retained with seamless functionality when a user changes his/her handset but would need another SIM card if the service provider is changed.

However, some operators block either the SIM (SIM locking) or even their peculiar mobile phones (phone locking) both essentially for revenue generation reasons. Please observe that this practice is not universal as some countries are opposed to the policy. It is usual for a subscriber to contact the operator to remove the lock for a fee, use private services or take advantage of available software & Internet websites to remove the lock on the phone but certainly not the lock on the SIM card.

MS 006
THE ONLINE COURSE

You have earlier seen the tip of the iceberg in MS000-MS005 as they are only starters to wet or stimulate your appetite in preparation for the main meal, the MOBILE TELECOMS course that is designed to unravel or demystify the seeming wonders of the mobile phone. It should provide adequate, simplified & satisfactory information, more than a stop-gap or superficial treatment of the relevant issues for readers including especially, people who are still astounded by the sprawling inventiveness, ingenuity & resourcefulness of the ordinary mobile telephone.

THE MOBILE NETWORK

As we have presumably agreed, we shall start the series of courses with the cellular mobile aspect of land-based mobile systems. This choice is engendered by the advantages cellular networks offer over alternative solutions in terrestrial mobile networks. These advantages are essentially: increased capacity, lower power usage, larger coverage area and reduced interference from other signals. Besides, it is the much-sought-after mobile telephone technology and its global deployment, development, standardisation & regulation have been phenomenal. Also, the investment & funding strategies and business potentials exemplified by the kaleidoscope of continual cross-border acquisitions, mergers & joint ventures blazing its trail have equally been awesome.

Please note from the outset that alternate terminologies are given here to indicate that various regional standards use different nomenclatures or names to define or describe an entity but functionally meaning the same thing. However, any such name that is appropriate to GSM networks i.e. Global System for Mobile communication networks (our interest) shall be highlighted.

The concept of cellular mobile is hinged on the dexterous use & control of ElectroMagnetic (EM) waves transmission generically called Radio, instead of wire lines to provide telephone & related services comparable to the Plain Old Telephone Service (POTS). A Radio wave depicts an intricate combination of electrostatic & magnetic energy necessary and needed for the transportation of information in form of signals (waves) from one point or multipoint to another or others. 

The cellular system is similar in functional design to the legacy, circuit-switched & Public-Switched Telephone Network (PSTN) or landline network. A cellular network is a radio network that comprises of radio cells with pre-assigned frequencies, each served by at least one fixed-location transceiver referred to as a cell site or Base Transceiver Station (BTS) or Radio Base Station (RBS). A transceiver is a device that contains both a transmitter and a receiver, which are combined & share a common circuitry. These cells cover different geographic areas to provide radio coverage over a wider area than the area of one cell. A variable number of transceivers can therefore be used in any one cell and moved through more than one cell during transmission. 

Please note that whereas a cell hence the name cellular, is a geographic (land) area, a cell site is simply a single location or point where an RBS or BTS is situated. Also, in order to ascertain maximum or effective radio coverage for each cell, a cell is visually depicted as an approximate hexagon rather than a circle, triangle, rhombus or any other shape whatsoever. This is because, apart from a hexagonal shape all other shapes will leave gaps in any contiguous cluster or pack of cells or layout, hence the hexagon choice for maximum benefit (coverage).

In a GSM network there are five different sizes of cells – macro, micro, pico, femto and umbrella cells. The coverage of each cell varies according to the environment in which it is implemented. Whereas, macro cells are often seen as cells where the BTS antenna is installed on a mast or a building above average roof top level, their micro cells counterpart usually found in urban areas have BTS antennas below average roof top height. Often used indoors, pico cells are small cells whose coverage diameter is a few dozen metres as distinct from femto cells that are smaller cells intended for residential or small business areas thereby attracting broadband Internet connections to the operators network. However, umbrella cells as the name indicates, are used to cover shadowed regions of smaller cells and fill gaps in coverage between those cells.

For better efficiency and improved capacity to allow more calls, a cell may be divided into sectors or individual areas typically three. Sectorisation reduces signal interference problems thereby enhancing frequency reuse capability of a cell. Usually occurring between two transmitting radio signals whose frequencies are close or even identical, interference refers to the interaction of radio signals resulting in noise & possible cancellation of signals. And Frequency reuse or frequency agility is ability of a cell to operate on any given number of radio frequencies pre-assigned to that cell.

For the same reason of effective coverage, an RBS or BTS with its antennas (transceiver towers) are situated at the edges or corners of converging or cluster of cells. Antennas transmit inward to each cell and only cover a portion or sector of each cell and not the whole cell thereby leaving antennas from other cell sites to cover the other portions. The cell site equipment provides each sector with its own channels operating with pre-assigned frequencies.

In both fixed and mobile networks it is possible to divide the network into various parts according to their functions. Some parts of the network will deal with connection to the user and others may deal with the management of the users and the services they wish to subscribe to and continually enjoy.

Essentially, a cellular mobile network consists of a mobile terminal device often called cell phone, user equipment or Mobile Station (MS); an air interface or Radio Access Network (RAN) and the Core Network (CN)- all depending on the adopted standard. Any user has the liberty to move anywhere within the system radio coverage and expect to communicate with any other user elsewhere through his/her own RAN & CN.  This proven possibility or prerogative is referred to as the any-to-any-principle whereof any user of any network anywhere is capable at anytime to communicate with any other user of the same or any other network anywhere at anytime.

Radio Access Network is the air interface between MS & CN and it enables users to make connections to the core network anywhere in the coverage area of the network. It does this by assigning radio base stations around the area where service is required & the mobile station then uses radio signals to relay the users’ information to the core network via the RBS. The link from an MS to the CN is referred to as an uplink while that from CN to an MS is called a downlink. Downlink frequencies are by rule-of-the-thumb, usually higher than uplink ones because of a higher attenuation experienced by downlink signals compared to uplink signals. Attenuation is the reduction in signal characteristics especially strength, due to atmospheric vagaries e.g. extant weather conditions.

Core Network is a convoluted system of switches, transmission systems, interfaces, platforms, databases, billing systems, network management centres and customer service centres all aimed at providing virtually any type service that may be required by users. Today, core networks are split into two segments to differentiate voice-based from data-based services but not without shared service platforms and interfaces. Examples of voice-oriented services include teleservices – voice & facsimile and supplementary services like call transfer, conference call, call forwarding, call identification, malicious call etc. However, connections to data terminals, business systems, the Internet and the like, all fall under data-oriented services.  

It is apposite to mention here that the combination of MS, RAN & CN make up the basic structure of a GSM network i.e. Global System for Mobile communication network, our primary interest.

 

MS 005
THIRD GENERATION 3G (GSM)

As expected, the cauldrons in the minds of scientists, operators & customers alike, continue to bubble in search of networks that would resolve the backdrops of 2G GSM including those unmet services even by its latest edition-Enhanced EDGE (2.75G), hence the concept & evolution of 3G standard. As a reminder, no development can be introduced into the marketplace without the approval of relevant world standards’ bodies.

Implicit in the demand or clamour for 3G networks was & still is the necessity & yearning to have a single network that would have the capacity & capability to link-up, bunch or converge all possible traffic including voice, data, video & the Internet, whilst simultaneously supporting mobility, using very high speed switching & transmission resources, embracing packet-oriented techniques in a spectrally efficient environment & as well, delivering flexible value-added services of all sorts.

The International Mobile Telecommunications-2000 (IMT-2000) popularly referred to as 3G represents a family of standards for mobile telecoms defined by the International Telecommunications Union (ITU) since year 2000, aimed at enhancing growth, increase spectral efficiency & also have greater capacity for more diverse applications. This kindred comprises of EDGE-Enhanced Data for GSM Evolution (Environment); UMTS-Universal Mobile Telephone Service in Europe but elsewhere the service is called WCDMA- Wideband Code Division Multiple Access; CDMA2000 service; DECT-Digital European Cordless Telephone service and Wimax- Worldwide (Wireless) Interoperability for Microwave Access service (that was added in 2007).

The Partnership Project

Within the scope of the IMT-2000 project of the International Telecommunications Union (ITU), there is a collaboration called 3rd Generation Partnership Project (3GPP) between groups of worldwide telecommunications associations designed to develop a specification for 3rd Generation (3G) mobile phone system that is globally applicable & acceptable. The groups are: ETSI- European Telecommunications Standards Institute; ARIB/TTC (Japan)- Association of Radio Industries & Businesses/Telecommunication Technology Committee; CCSA- China Communications Standards Association; ATIS (North America)- Alliance for Telecommunications Industry Solutions and TTA (South Korea)- Telecommunications Technology Association.

3GPP should not be confused with 3rd Generation Partnership Project 2 (3GPP2) which specifies standards for a competing 3G technology based on International Standard, IS-95 (CDMA) popularly referred to as CDMA2000, predominantly a North American service. Both 3GPP & 3GPP2 are already working on further extensions to 3G standards, named Long Term Evolution (LTE) & Ultra Mobile Broadband (UMB) respectively.

Being based on an All-IP network infrastructure and using advanced wireless technologies such as MIMO-Multiple-Input & Multiple-Output, being the use of multiple antennas at both the transmitter & receiver to improve communication performance like significant increases in data throughput & long range without additional bandwidth or transmit power, these specifications already display features characteristic of IMT-Advanced (4G), the successor of 3G. It however falls short of the bandwidth requirements for 4G, which is 1Gbits/sec for stationary & 100Mbits/sec for mobile operation. These standards are classified as 3.9G or Pre-4G.

Established in 1998, the project groups examined aspects of Radio, Core Network and service architecture for standardisation based on evolved GSM specifications.

As 3GPP standards are structured as “RELEASES”, discussions by the groups essentially revolve around the functionality in one Release or another.  

Each Release incorporates hundreds of individual standard documents each of which may have been through many revisions.

In the 4th quarter of 2007, a very important series of advancements through Release 7 came into focus that defined a turning point, as subsequent Releases from then fell under developments known as “Long Term Evolution” (LTE). Release 7 focused on improvements to Quality of Service (QoS) & real-time applications like VoIP (Voice over Internet Protocol- a general term for a family of transmission technologies for delivery of voice communication over IP networks e.g. the Internet & other packet-switched networks).

Also included in the list is High Speed Packet Access Evolution (HSPA+) & amongst others, is also SIM (Subscriber Identity Module) high-speed protocol & contactless front-end interface that allows operators to deliver contactless service like Mobile Payments.

3GPP Release 8 (LTE) of 2008 is an All-IP Network Infrastructure (SAE-Service Architecture Evolution) constituting a refactoring of UMTS as an entirely IP-based Fourth Generation (4G) network. 

GSM & 3G- THE DIFFERENCE

In spite of other competing & fecund but complementary mobile phone formats, GSM has not compromised its global predominance while 3G being a new technology in mobile phones is still at its infancy as far as service is concerned.

With the gamut of progressively expanding & improved services attending the phased developments of GSM from the first tranche of 2G to the most developed 2G i.e. Enhanced EDGE (2.75G) that allows the multiplicity of simultaneous state-of-the-art diverse services in a converged mobile channel, 3G is an entirely new technology that is here to replace the supposedly ageing GSM (2G) as a non-competitive transition from an older technology to a new one. It therefore smacks of substantial improvements over its predecessor in every conceptualised possibility.

All these services include wide-area wireless telephony, video calls, wireless data, high-speed Internet & interactive multimedia-all in a mobile environment.

A main wrench in the works of 3G however, is its backward incompatibility with its forerunners. Thus, in practice, 3G phones cannot communicate with 2G radio towers & correspondingly, 3G towers will not handshake with 2G phones.

 

MS004
SECOND GENERATION (2G/2.5G/2.75G) GSM:


By its very nature, speech (audio) is analogue & thus analogue mobile phones could only provide limited service that is basically voice, as it did not naturally support data transmission. Modems could however be used to send faxes & data at extremely slow speed but generally, not cost-effective. A modem is a device that adjusts (modulates) an analogue carrier signal to encode digital information and later readjusts (demodulates) the carrier signal to decode the transmitted information.

These limitations were resolved at the instance of 2nd Generation (2G) digital mobile phone systems that generally improved the quality, variety & number of services.

2G (GSM) digital mobile phones by design, enable voice & data traffic, starting with text messages i.e. Short Message Service (SMS) and electronic mail (e-mail) in its later versions of 2G & beyond.

As 2G (GSM) is digital, more information (calls) can be compressed & arranged together (multiplexed) into the same amount of radio frequency (bandwidth) than in its analogue predecessor. Thus, more calls can be handled thereby improving system capacity, a result of enhanced spectral efficiency.
Being digital too, less radio power is emitted by comparatively miniature handsets, thus stimulating the use of smaller cells to provide better radio coverage. Subsequently, more cells could be packed in the same amount of space. In effect, system capacity is enhanced, again.

Other generic advantages of 2G (GSM) over its analogue predecessor include but not limited to,
• Digital radio signals with less power require less battery power resulting in longer periods between charges & smaller batteries
• Lower power emissions resolve health issues (i.e. less apprehensiveness or anxiety)
• The case of handset cloning that leads to fraud is also addressed
• With security algorithms (finite sequence of problem-solving instructions), privacy is improved
 

Of necessity, operators of 2G networks have subsequently re-engineered their networks to support the transmission of data in a more efficient manner called packet data. This is a method of transferring data by breaking it up into chunks called packets. Packet data is how most data travel over the internet- a global system of interconnected computer networks. Superior to the usual Circuit-Switched Data (CSD) transfer method where an open data connection which uses network resources, must be maintained even when idle (like standard voice connections), with packet-switched data network (PSDN), each user consumes network resources only when data is actually being transferred. It is thus a more modern way of data transfer and also cheaper & faster than its circuit-switched data transfer method counterpart.

The term “second & a half generation” (2.5G) is used to describe 2G-digital mobile phone networks that use packet-switched mode in addition to circuit-switched domain. Representing a bridge between 2G & 3G (3rd Generation), 2.5G is not a GSM standard but a marketing fallout to highlight enhancements on 2G (GSM) especially, in relation to data services.

General Packet Radio Service (GPRS), a packet-based wireless technology is used for various data applications on mobile phones including wireless internet (Wireless Application Protocol-WAP), Multimedia Messaging Service (MMS) & software that connects to the internet.

Basically, any network connection that is not voice or text messaging uses a connection like GPRS. It is designed to offer a tenfold increase in data speed over previous circuit-switched technologies. It also complements Bluetooth, a standard for replacing wired connection between devices with wireless radio connections. GPRS is an evolutionary step toward Enhanced Data for GSM Environment (EDGE- 2.75G) or Enhanced Data rates for Global Environment (Evolution) and Universal Mobile Telephone Service (UMTS)- a service based on 3rd Generation GSM standard endorsed by major international standards’ (world) bodies.

EDGE or Enhanced GPRS (EGPRS) also called 2.75G is a superset or bolt-on enhancement on GPRS as it can handle four times the traffic of a standard GPRS network. EDGE meets International Telecommunication Union (ITU)’s requirement for a 3rd Generation (3G) network.

 

 

MS003
THE DIGITAL EPOCH


Given the inherent & generic limitations that analogue systems in general, and analogue Cellular Mobile Telephone System (CMTS) in particular are wont of, digital mobile phone systems evolved across the globe. In particular, the Global System for Mobile communication (GSM) era took off with 2nd Generation (2G) version in Europe in 1991. Correspondingly, the digital editions of Advanced Mobile Phone System-AMPS (analogue), D-AMPS e.g. 2G (IS-54 & IS-136) were developed in USA. IS stands for International Standard. In the US, 2G services are frequently referred to as Personal Communication Service (PCS). And exclusively in Japan, the equivalent digital mobile phone format called Public Digital Cellular (PDC) system was consequential.


As distinguished from the GSM format, the other leading & competing but complementary standard in digital mobile phone system is conveniently referred to as code division multiplex access (cdma). However, this extraction should not be confused with the channel access method used by various radio communication technologies and fundamentally named Code Division Multiple Access (CDMA).


The marriage of convenience between cdma & CDMA resides in the fact that this digital mobile phone standard, e.g. (IS-95) marketed as cdmaOne or (IS-2000) marketed as cdma2000 (both simply referred to as cdma or CDMA) uses CDMA as its underlying channel access method whereby transmitters with assigned codes and of different frequencies simultaneously send information over the same physical communication channel. Thus, several users are obliged to share a range of different frequencies culminating in bandwidth effectiveness i.e. spectral efficiency.


This kind of channel access method (CDMA) contrasts with that used in the GSM standard, whereby many signals staggered in time (into sub-channels) are simultaneously sent over the same physical communication channel. Thus, the time domain is split into recurrent but fixed timeslots, one timeslot for each sub-channel prior to bunching together (multiplexing). This channel access method is referred to as TDMA-Time Division Multiple Access.


For completeness, the other main method of channel access used by many radio technologies is called FDMA-Frequency Division Multiple Access, whereby many signals of different frequencies are bunched together (multiplexing) & simultaneously sent over the same physical communication channel. Here, various users are distinguished by frequency (FDMA) rather than time (TDMA) or code (CDMA).

 
From this point henceforth, we shall focus on the GSM extraction & its concomitant generations to-date, being the global choice (80%), as the cdma format commands only 17% patronage worldwide, whilst the other digital mobile phone protocols share the balance of only 3%. In Nigeria, the current percentages are: GSM-87.65% & CDMA-10.25%.


It is relevant to mention here, that the rationale for the evolution of GSM in particular, & its seemingly ceaseless developmental phenomena to-date, is a result of an allegory between customers’ ferocious palate & humongous appetite for the ever-expanding services & applications and the continual quest for technological advancements by researchers. Which precedes the other, customer-driven improvements or technology-driven advancements?




MS002
STANDARDS & SYSTEMS


At every stage in the design of engineering systems, efforts are made to align, situate & ensure that designs take strict cognisance of internationally established standards beyond proprietary & factory benchmarks. If anything, this international best practice is to ascertain compatibility, interoperability and consequently, seamless global deployment & effective utilisation of systems or products are guaranteed. In effect, global services, markets & business opportunities are enhanced. And even global developmental initiatives are also engendered.


ANALOGUE (1G):
The following is a catalogue of the various standards and systems (analogue) that culminated as a result of concurrent initiatives & efforts in the various countries, worldwide.


AMPS (Advanced Mobile Phone Systems):
Developed by Bell Laboratories for the American market, AMPS uses 800MHz band allocated to mobile services in ITU Region 2 (the Americas) with 30KHz channel spacing in common with established PMR practice.


AMPS underwent a long development period, and an extended trial (technical & commercial), which not only fixed the system parameters but also contributed to the basic planning rules, which hold true for all cellular systems.


AMPS is in operation extensively across the North American continent (USA & Canada). So also is its presence in a number of Central & South American countries, in Australia and some East Asian countries.


TACS (Total Access Communications Systems): Adapted from the AMPS standard by the UK when cellular radio was licensed for operation from 1985, the adaptation was necessary to suit European frequency allocations, which were at 900MHz, with 25KHz channel spacing.


TACS was originally specified to use the full 1000 channels (2x25MHz) allocated to mobile services in Europe. However, in the UK only 600channels (2x15MHz) were released by the licensing authority and the reminder was reserved for Global System for Mobile communications (GSM).


Subsequently, an additional allocation of channels below the existing TACS channels were made, namely the Extended TACS (ETACS) channels, and the standard was modified accordingly.


TACS was adopted by several European countries -(UK, Eire, Spain, Italy, Austria, & Malta), in the Middle East (Kuwait, UAE, & Bahrain) and the Far East (Hong Kong, Singapore, Malaysia, & China). A variant of TACS (called J-TACS) was also adopted in Japan.


NMT (Nordic Mobile Telephone) Systems:
Jointly developed by the Posts, Telegraphs & Telephones (PTTs) of Sweden, Norway, Denmark and Finland in the late 1970s & early 1980s, the system was designed to operate in the 450MHz band & was later adapted in 1987 to also use the 900MHz band as an overlay system, to cater for capacity requirements.


Although NMT was developed after AMPS, it saw commercial service before it, opening in late 1981.
NETZ-C (C-450) System:
SIEMENS developed this system during the early 1980s under the direction of the German PTT, DEUTSCHE BUNDESPOST.
Commercially deployed in 1987, C-450 with an analogue speech transmission is a hybrid technology having advanced features & characteristics such as time slotted signalling channels & continuous signalling during call & these properties have been carried through into the GSM design.
Whilst C-450 has mostly served the German market, it also showed some appearances in Europe, Portugal & South Africa.


MS 001
PREAMBLE

Broadly, telecommunications is the science & business of transferring & receiving any kind of information from one point or multi-point to another or others. It primarily manifests in wired (fixed) e.g. the Plain Old Telephone System (POTS) generically referred to landline, or wireless (full mobile) e.g. Cellular Mobile Telephone System (CMTS), or fixed-wireless (mixed).

In this discourse, the mobile (full wireless) aspect will be expounded on first, leaving the balance of fixed & fixed-wireless to other series in this venture.


Even in the mobile domain we have terrestrial (land-based) and satellite options, the latter of which operates through radio relay systems that are situated, either 36,000kms; or 24,000kms & 16,000kms above the earth in Geostationary & Non-Geostationary orbits, respectively. Incidentally, the land-based option is the darling or chosen one in the ensuing exposition.


Here again, in land-based mobile systems, there are public and private types. The simplest of the private land-based mobile systems is the Cordless Telephone (CT1, CT2 etc), which is designed to support local mobility, & usually attached to the fixed house telephone for convenience whilst talking & moving around the premises of a building. Predominantly, the cordless telephone system is analogue until the development of the digital & more versatile version called DECT (Digital European Cordless Telephone). The other private type of land-based mobile systems i.e. Private Mobile Radio (PMR) available are the dedicated & independent mobile radio kind, popularly referred to as “walkie-talkie” type radios that find applications in cabs & company radio systems called resource-sharing or trunked radio networks. These, among other applications, enhance information flow or dissemination of information within & between offices and precincts of the same organisation.


For the more cognate & targeted public land-based mobile system counterpart especially the “cellular radio system”, there is a sprawling development & deployment of this sector that is much-sought-after, considering the spate of technological advancement, customer & technology-driven services, ever-increasing set of applications, global standardisation, regulation and competition coupled with its enormous & expanding investment capability.


Unlike the old generation radio systems that operate on high-powered fixed radio stations, the cellular network as the name indicates provides radio coverage to mobile stations (hand-held equipment or sets) in large geographical areas by seamless, overlapping & re-usable cells with each cell having its own radio base station.



BACKGROUND


The cellular radio systems are by far the most common of all public land-based mobile networks and are now the dominant technology in all parts of the world.


Bell Laboratories established the basic principles of cellular systems in 1949, but it was not until the early 1980s that technology allowed real commercial networks to be built and services offered to the public.


First generation (analogue, 1G) systems were developed at different times in different countries and were subject to a variety of constraints such as frequency band, channel spacing etc. As a result, a number of incompatible cellular standards were developed throughout the world and a summary of the most important standards is explained in MS 002.


Second generation (digital, 2G) cellular systems, developed during the 1980s & deployed in the 1990s have now superseded & have finally replaced their analogue counterparts (1G). Given this development, common standards & systems have been created across the world.


Incidentally, GSM, (Global System for Mobile comunication) originally coined from Groupe Speciale Mobile (a French group), which was developed & launched in 1991 in Europe, has been the most widely used extraction or protocol across the globe and is therefore the attraction and focus in the first series of this undertaking.


At this point, we shall defer a mention of subsequent generations of GSM e.g. 2G, 2.5G, 3G & 3rd Generation Partnership Project (3GPP) Release 8, Long Time Evolution (LTE) that is currently touted as 4th Generation (4G) GSM to the appropriate MS.

All of these are precursors to the next generation of systems & networks that would also impart on the next generation of market opportunities et cetera.

 

KUNLE BELLO writes for CyberschuulNews
as “MISSING SCRIPT” debuts

 

Telecommunications expert, Dr. Kunle Bello, has accepted to write a regular column for CyberschuulNews. The column to be named THE MISSING SCRIPT, is designed to be a compendium of facts about telecoms, be it wired (fixed), wireless (full mobile) or fixed-mobile (mixed) that would be equivalent to sitting in front of a college professor & receiving lectures on a telecoms course but without banging any diploma or degree at the end of the long semi-online course. All you need do is, subscribe to CyberschuulNews.com and you are on to the telecoms-made-easy column.  

It will start running in the New Year. 

The author, a doctorate holder in telecommunications who had record of researched works in digital technology, is a telecommunications consultant of repute whose last public post was Managing Director/CEO of Mobile Telecommunications Ltd, Mtel. He lives in Abuja, Nigeria. 

At the outset of this premier venture, it would be strictly like reading a book albeit queries could be posted to the author’s personal e-mail box - kunbel@runbox.com   

By design, the column would take-off with the wireless (mobile) aspect as this is the currently most-sought-after technology beyond its complementary but competing predecessor- the wired (fixed) genre and of course, the hybrid, i.e. the fixed-wireless counterpart. Each MS shall to a large extent, be an easily readable standalone to assist readers who may inadvertently miss any MISSING SCRIPT and because of its very compelling nature, the reader might want to do a catch-up. However, a continuous & seamless reading is recommended to enhance a thorough understanding of the subject matter & its attendant throughput. 

The stuff will not be exclusive to telecoms engineers or practitioners as MS derives its simplicity & comprehension from first principles’ presentations that calibrate or graduate therefrom to today’s seemingly complex & topical issues as Worldwide (Wireless) Interoperability for Microwave Access (Wimax) and (its complement) 3rd Generation Partnership Project (3GPP) Release 8, Long Time Evolution (LTE) that is currently touted as 4th Generation (4G) mobile (radio) communications technology, designed to increase the capacity & speed of mobile telephone networks. 

Meet MS come 2010. 


 

 
 
 
 
 
 
 
 

 

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