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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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