GSM Summary
Source of information: http://www.privateline.com
1. History of GSM
During the early 1980s, analog cellular telephone systems were experiencing
rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but
also in France and Germany. Each country developed its own system, which was
incompatible with everyone else's in equipment and operation. This was an
undesirable situation, because not only was the mobile equipment limited to
operation within national boundaries, which in a unified Europe were increasingly
unimportant, but there was also a very limited market for each type of equipment,
so economies of scale and the subsequent savings could not be realized.
The Europeans realized this early on, and in 1982 the Conference of European
Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial
Mobile (GSM) to study and develop a pan-European public land mobile system.
The proposed system had to meet certain criteria:
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Good subjective speech quality
Low terminal and service cost
Support for international roaming
Ability to support handheld terminals
Support for range of new services and facilities
Spectral efficiency
ISDN compatibility
Pan-European means European-wide. ISDN throughput at 64Kbs was never
envisioned, indeed, the highest rate a normal GSM network can achieve is
9.6kbs.
Europe saw cellular service introduced in 1981, when the Nordic Mobile Telephone
System or NMT450 began operating in Denmark, Sweden, Finland, and Norway in the
450 MHz range. It was the first multinational cellular system. In 1985 Great Britain
started using the Total Access Communications System or TACS at 900 MHz. Later, the
West German C-Netz, the French Radiocom 2000, and the Italian RTMI/RTMS helped
make up Europe's nine analog incompatible radio telephone systems. Plans were afoot
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during the early 1980s, however, to create a single European wide digital mobile service
with advanced features and easy roaming. While North American groups concentrated on
building out their robust but increasingly fraud plagued and featureless analog network,
Europe planned for a digital future. Link to my mobile telephone history series
In 1989, GSM responsibility was transferred to the European Telecommunication
Standards Institute (ETSI), and phase I of the GSM specifications were published
in 1990. Commercial service was started in mid-1991, and by 1993 there were 36
GSM networks in 22 countries [6]. Although standardized in Europe, GSM is not
only a European standard. Over 200 GSM networks (including DCS1800 and
PCS1900) are operational in 110 countries around the world. In the beginning of
1994, there were 1.3 million subscribers worldwide [18], which had grown to
more than 55 million by October 1997. With North America making a delayed
entry into the GSM field with a derivative of GSM called PCS1900, GSM systems
exist on every continent, and the acronym GSM now aptly stands for Global
System for Mobile communications.
According to the GSM Association (external link) , here are the current GSM
statistics:
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No. of Countries/Areas with GSM System (October 2001) - 172
GSM Total Subscribers - 590.3 million (to end of September 2001)
World Subscriber Growth - 800.4 million (to end of July 2001)
SMS messages sent per month - 23 Billion (to end of September 2001)
SMS forecast to end December 2001 - 30 Billion per month
GSM accounts for 70.7% of the World's digital market and 64.6% of the World's
wireless market
http://www.gsmworld.com/membership/mem_stats.html (external link)
The developers of GSM chose an unproven (at the time) digital system, as
opposed to the then-standard analog cellular systems like AMPS in the United
States and TACS in the United Kingdom. They had faith that advancements in
compression algorithms and digital signal processors would allow the fulfillment
of the original criteria and the continual improvement of the system in terms of
quality and cost. The over 8000 pages of GSM recommendations try to allow
flexibility and competitive innovation among suppliers, but provide enough
standardization to guarantee proper interworking between the components of the
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system. This is done by providing functional and interface descriptions for each
of the functional entities defined in the system.
The United States suffered no variety of incompatible systems as in the different
countries of Europe. Roaming from one city or state to another wasn't difficult . Your
mobile usually worked as long as there was coverage. Little desire existed to design an all
digital system when the present one was working well and proving popular. To illustrate
that point, the American cellular phone industry grew from less than 204,000 subscribers
in 1985 to 1,600,000 in 1988. And with each analog based phone sold, chances dimmed
for an all digital future. To keep those phones working (and producing money for the
carriers) any technological system advance would have to accommodate them.
GSM was an all digital system that started new from the beginning. It did not
have to accommodate older analog mobile telephones or their limitations.
American digital cellular, first called IS-54 and then IS-136, still accepts the
earliest analog phones. American cellular networks evolved, dragging a legacy of
underperforming equipment with it. Advanced fraud prevention, for example, was
designed in later for AMPS, whereas GSM had such measures built in from the
start. GSM was a revolutionary system because it developed fully digital from the
beginning.
2. Services provided by GSM
From the beginning, the planners of GSM wanted ISDN compatibility in terms of
the services offered and the control signalling used. However, radio transmission
limitations, in terms of bandwidth and cost, do not allow the standard ISDN Bchannel bit rate of 64 kbps to be practically achieved.
Isn't this a shame? What many wireless customers need most is a high speed
data connection and this is what GSM provides least. Only 9.6kbs if everything
works right. It is possible the GSM designers in the early 1980s never envisioned
the need for such bandwidth. It may be true, too, that in most countries the radio
spectrum needed to give every caller a 64kbs channel was never available. The
add on technology EDGE (external link) promises higher data speed rates in the
near to mid-term for GSM. Highest data rates will come in the long term when
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GSM changes into a radio service based on wide band code division multiple
access, and not TDMA.
Using the ITU-T definitions (external link), telecommunication services can be
divided into bearer services, teleservices, and supplementary services. The most
basic teleservice supported by GSM is telephony. As with all other
communications, speech is digitally encoded and transmitted through the GSM
network as a digital stream. There is also an emergency service, where the
nearest emergency-service provider is notified by dialing three digits (similar to
911).
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Bearer services: Typically data transmission instead of voice. Fax and SMS are
examples.
Teleservices: Voice oriented traffic.
Supplementary services: Call forwarding, caller ID, call waiting and the like.
A variety of data services is offered. GSM users can send and receive data, at
rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN,
Packet Switched Public Data Networks, and Circuit Switched Public Data
Networks using a variety of access methods and protocols, such as X.25 or X.32.
Since GSM is a digital network, a modem is not required between the user and
GSM network, although an audio modem is required inside the GSM network to
interwork with POTS.
GSM is an all digital network but many machines are still analog, as is most of
the local loop. Thus, we need a modem, even though we are dealing with digital.
A FAX machine's digital signal processor converts an analog image into an
instantaneous digital representation; a series of bits, all 0s and 1s. A modulator
then turns these bits into audio tones representing the digital values. An analog
FAX machine at the other end converts the tones received back into digital bits
and then into an image.
This tedious process was required initially because local loops were and are
primarily analog. In addition, digital services such as T1, fractional T1, or ISDN,
where available, was and is extremely expensive. All digital equipment, such as
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Group 4 Fax machines, are far higher priced than their analog counterparts. The
local loop will remain primarily analog for some time.
Other data services include Group 3 facsimile, as described in ITU-T
recommendation T.30, which is supported by use of an appropriate fax adaptor.
A unique feature of GSM, not found in older analog systems, is the Short
Message Service (SMS). SMS is a bidirectional service for short alphanumeric
(up to 160 bytes) messages. Messages are transported in a store-and-forward
fashion. For point-to-point SMS, a message can be sent to another subscriber to
the service, and an acknowledgement of receipt is provided to the sender. SMS
can also be used in a cell-broadcast mode, for sending messages such as traffic
updates or news updates. Messages can also be stored in the SIM card for later
retrieval [2].
Supplementary services are provided on top of teleservices or bearer services. In
the current (Phase I) specifications, they include several forms of call forward
(such as call forwarding when the mobile subscriber is unreachable by the
network), and call barring of outgoing or incoming calls, for example when
roaming in another country. Many additional supplementary services will be
provided in the Phase 2 specifications, such as caller identification, call waiting,
multi-party conversations.
Excellent IEC tutorial on SMS is here:
http://www.iec.org/online/tutorials/wire_sms/ (external link)
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3.Architecture of the GSM network
A GSM network is composed of several functional entities, whose functions and
interfaces are specified. Figure 1 shows the layout of a generic GSM network.
The GSM network can be divided into three broad parts. The Mobile Station is
carried by the subscriber. The Base Station Subsystem controls the radio link
with the Mobile Station. The Network Subsystem, the main part of which is the
Mobile services Switching Center (MSC), performs the switching of calls between
the mobile users, and between mobile and fixed network users. The MSC also
handles the mobility management operations. Not shown is the Operations and
Maintenance Center, which oversees the proper operation and setup of the
network. The Mobile Station and the Base Station Subsystem communicate
across the Um interface, also known as the air interface or radio link. The Base
Station Subsystem communicates with the Mobile services Switching Center
across the A interface.
As John states, he presents a generic GSM architecture. Lucent, Ericsson,
Nokia, and others feature their own vision in their own diagrams. But they all
share the same main elements and parts from different vendors should all work
together. The links below show how these vendors picture the GSM architecture.
You can remember the different terms much better by looking at all these
diagrams.
Figure 1. General architecture of a GSM network
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SIM: Subscriber identify module.
ME: Mobile
equipment.
BTS: Base transceiver
station.
BSC: Base station controller.
HLR: Home
location register.
VLR: Visitor location
register.
MSC: Mobile services switching center.
EIR: Equipment
identity register.
AuC: Authentication
Center.
UM: Represents the radio link.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data
network.
3.1. Mobile Station
The mobile station (MS) consists of the mobile equipment (the terminal) and a
smart card called the Subscriber Identity Module (SIM). The SIM provides
personal mobility, so that the user can have access to subscribed services
irrespective of a specific terminal. By inserting the SIM card into another GSM
terminal, the user is able to receive calls at that terminal, make calls from that
terminal, and receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile
Equipment Identity (IMEI). The SIM card contains the International Mobile
Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret
key for authentication, and other information. The IMEI and the IMSI are
independent, thereby allowing personal mobility. The SIM card may be protected
against unauthorized use by a password or personal identity number.
GSM phones use SIM cards, or Subscriber information or identity modules.
Memory modules. They're the biggest difference a user sees between a GSM
phone or handset and a conventional cellular telephone. With the SIM card and
its memory the GSM handset is a smart phone, doing many things a
conventional cellular telephone cannot. Like keeping a built in phone book or
allowing different ringtones to be downloaded and then stored. Conventional
cellular telephones either lack the features GSM phones have built in, or they
must rely on resources from the cellular system itself to provide them. Let me
make another, important point.
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With a SIM card your account can be shared from mobile to mobile, at least in
theory. Want to try out your neighbor's brand new mobile? You should be able to
put your SIM card into that GSM handset and have it work. The GSM network
cares only that a valid account exists, not that you are using a different device.
You get billed, not the neighbor who loaned you the phone.
This flexibility is completely different than AMPS technology, which enables one
device per account. No swtiching around. Conventional cellular telephones have
their electronic serial number burned into a chipset which is permanently
attached to the phone. No way to change out that chipset or trade with another
phone. SIM card technology, by comparison, is meant to make sharing phones
and other GSM devices quick and easy.
On the left above: Front of a Pacific Bell GSM phone. In the middle above: Same phone, showing
the back. The SIM card is the white plastic square. It fits into the grey colored holder next to it. On
the right above. A new and different idea, a holder for two SIM cards, allowing one phone to
access either of two wireless carriers. Provided you have an account with both. :-) The Sim card
is to the left of the body.
3.2 Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base Transceiver
Station (BTS) and the Base Station Controller (BSC). These communicate across
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the standardized Abis interface, allowing (as in the rest of the system) operation
between components made by different suppliers.
An explanation of the Abis interface is here
The Base Transceiver Station houses the radio tranceivers
that define a cell and handles the radio-link protocols with
the Mobile Station. In a large urban area, there will
potentially be a large number of BTSs deployed, thus the
requirements for a BTS are ruggedness, reliability,
portability, and minimum cost.
The BTS or Base Transceiver Station is also called an RBS or
Remote Base station. Whatever the name, this is the radio gear
that passes all calls coming in and going out of a cell site.
The base station is under direction of a base station controller so
traffic gets sent there first. The base station controller, described below, gathers the calls
from many base stations and passes them on to a mobile telephone switch. From that
switch come and go the calls from the regular telephone network.
Some base stations are quite small, the one pictured here is a large outdoor unit. The large
number of base stations and their attendant controllers, are a big difference between GSM
and IS-136.
Want to read more about a base station? Download this product brochure from Siemens. It's
about 228K in .pdf
The Base Station Controller
The Base Station Controller manages the radio resources for
one or more BTSs. It handles radio-channel setup, frequency
hopping, and handovers, as described below. The BSC is the
connection between the mobile station and the Mobile service
Switching Center (MSC).
Another difference between conventional cellular and GSM is the
base station controller. It's an intermediate step between the base
station transceiver and the mobile switch. GSM designers thought
this a better approach for high density cellular networks. As one
anonymous writer penned, "If every base station talked directly to the
MSC, traffic would become too congested. To ensure quality communications via traffic
management, the wireless infrastructure network uses Base Station Controllers as a way
to segment the network and control congestion. The result is that MSCs route their
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circuits to BSCs which in turn are responsible for connectivity and routing of calls for 50
to 100 wireless base stations."
Want to read more about a base station controller? Download this product brochure from
Siemens. It's about 363K in .pdf
Two page .pdf file on the network subsystem by Nokia. It's a glossy product brochure but it
does mention all the important elements. (363k in .pdf)
Many GSM descriptions picture equipment called a TRAU,
which stands for Transcoding Rate and Adaptation Unit. Of
course. Also known as a TransCoding Unit or TCU, the TRAU is
a compressor and converter. It first compresses traffic coming
from the mobiles through the base station controllers. That's
quite an achievement because voice and data have already
been compressed by the voice coders in the handset. Anyway,
it crunches that data down even further. It then puts the traffic
into a format the Mobile Switch can understand. This is the
transcoding part of its name, where code in one format is
converted to another. The TRAU is not required but apparently it saves quite a bit
of money to install one. Here's how Nortel Networks sells their unit:
"Reduce transmission resources and realize up to 75% transmission cost savings
with the TCU."
"The TransCoding Unit (TCU), inserted between the BSC and MSC, enables
speech compression and data rate adaptation within the radio cellular network.
The TCU is designed to reduce transmission costs by minimizing transmission
resources between the BSC and MSC. This is achieved by reducing the number
of PCM links going to the BSC, since four traffic channels (data or speech) can
be handled by one PCM time slot. Additionally, the modular architecture of the
TCU supports all three GSM vocoders (Full Rate, Enhanced Full Rate, and Half
Rate) in the same cabinet, providing you with a complete range of deployment
options."
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(PCM? To read more about that click here.)
The voice coders or vocoders are built into the
handsets a cellular carrier distributes. They're
the circuitry that turns speech into digital. The
carrier specifies which rate they want traffic
compressed, either a great deal or just a little. The cellular system is designed
this way, with handset vocoders working in league with the equipment of the
base station subsystem.
3.3 Network Subsystem
The Mobile Switch
The central component of the Network Subsystem is the Mobile services
Switching Center (MSC). It acts like a normal switching node of the PSTN or
ISDN, and additionally provides all the functionality needed to handle a mobile
subscriber, such as registration, authentication, location updating, handovers,
and call routing to a roaming subscriber. These services are provided in
conjunction with several functional entities, which together form the Network
Subsystem. The MSC provides the connection to the fixed networks (such as the
PSTN or ISDN). Signalling between functional entities in the Network Subsystem
uses Signalling System Number 7 (SS7), used for trunk signalling in ISDN and
widely used in current public networks.
.pdf file on SS7 and mobile networking -- Good reading!
Mobile switches go by many names: mobile switch (MS), mobile switching center
(MSC), or mobile telecommunications switching office (MTSO). They all do the
same thing, however, and that is to process mobile telephone calls. This switch
can be a normal landline switch like a 5ESS (external link), a Nokia, an Alcatel,
or an Ericsson AXE (Automatic Exchange Electric) or a dedicated switch, built
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just to handle mobile calls. Each mobile switch manages dozens to scores of cell
sites. In GSM the mobile switch handles cell sites by first directing the base
station controllers. Large systems may have two or more MSCs. It's easy
understand what a switch does. What is harder to understand is the role the
switch has to do with other network resources.
Two page .pdf file on the network subsystem by Nokia. It's a glossy product brochure but it
does mention all the important elements. (363k in .pdf)
Home Location Register and the Visitor/ed Location Register
The Home Location Register (HLR) and Visitor Location Register (VLR), together
with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR
contains all the administrative information of each subscriber registered in the
corresponding GSM network, along with the current location of the mobile. The
location of the mobile is typically in the form of the signalling address of the VLR
associated with the mobile station. The actual routing procedure will be described
later. There is logically one HLR per GSM network, although it may be
implemented as a distributed database.
The Visitor Location Register (VLR) contains selected administrative information
from the HLR, necessary for call control and provision of the subscribed services,
for each mobile currently located in the geographical area controlled by the VLR.
Although each functional entity can be implemented as an independent unit, all
manufacturers of switching equipment to date implement the VLR together with
the MSC, so that the geographical area controlled by the MSC corresponds to
that controlled by the VLR, thus simplifying the signalling required. Note that the
MSC contains no information about particular mobile stations --- this information
is stored in the location registers.
The Home Location Register and the Visitor or Visited Location Register work
together -- they permit both local operation and roaming outside the local service
area. You couldn't use your mobile in San Francisco and then Los Angeles
without these two electronic directories sharing information. Most often these
these two directories are located in the same place, often on the same computer.
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The HLR and VLR are big databases maintained on
computers called servers, often UNIX workstations.
Companies like Tandem, now folded into Compaq (external
link), make the servers, which they call HLRs when used for
cellular. These servers maintain more than the home location register, but that's
what they call the machine. Many mobile switches use the same HLR. So, you'll
have many Home Location Registers. To operate its nationwide cellular system,
iDEN, Motorola uses over 60 HLRs nationwide.
The HLR stores complete local customer information. It's the main database.
Signed up for cellular service in Topeka? Your carrier puts your information on its
nearest HRL, or the one assigned to your area. That info includes your
international mobile equipment identity number or IMEI, your directory number,
and the class of service you have. It also includes your current city and your last
known "location area," the place you last used your mobile.
The VLR or visitor location registry contains roamer information. Passing through
another carrier's system? Once the visited system detects your mobile, its VLR
queries your assigned home location register. The VLR makes sure you are a
valid subscriber, then retrieves just enough information from the now distant HLR
to manage your call. It temporarily stores your last known location area, the
power your mobile uses, special services you subscribe to and so on. Though
traveling, the cellular network now knows where you are and can direct calls to
you.
The equipment Identity Register and the Authentication Center
The other two registers are used for authentication and security purposes. The
Equipment Identity Register (EIR) is a database that contains a list of all valid
mobile equipment on the network, where each mobile station is identified by its
International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it
has been reported stolen or is not type approved. The Authentication Center
(AuC) is a protected database that stores a copy of the secret key stored in each
subscriber's SIM card, which is used for authentication and encryption over the
radio channel.
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"The Equipment Identity Register (EIR) is a standard GSM network element that
allows a mobile network to check the type and serial number of a mobile device
and determine whether or not to offer any service." The EIR or equipment identity
register is yet another database. It's first purpose is to deny stolen or defective
mobiles service. Good mobiles are allowed on the network, of course, as is faulty
but still serviceable equipment. In the latter case such mobiles are flagged for the
cellular carrier to monitor.
The AC or AuC is the Authentication Center, a secured database handling
authentication and encryption keys. Authentication verifies a mobile customer
with a complex challenge and reply routine. The network sends a randomly
generated number to the mobile. The mobile then performs a calculation against
it with a number it has stored in its SIM and sends the result back. Only if the
switch gets the number it expects does the call proceed. The AC stores all data
needed to authenticate a call and to then encrypt both voice traffic and signaling
messages.
The Interfaces
Cellular radio's most cryptic terms belong to these names: A, Um, Abis, and Ater.
A telecom interface means many things. It can be a mechanical or electrical link
connecting equipment together. Or a boundary between systems, such as
between the base station system and the network subsystem. GSM calls that
one Interface "A", remember? To be more specific, Smith says "A" is the
signaling link between the two subsystems. Which brings us to the point I want to
make.
Interfaces are standardized methods for passing information back and forth. The
transmission media isn't important. Whether copper or fiber optic cable or
microwave radio, an interface insists that signals go back and forth in the same
way, in the same format. With this approach different equipment from any
manufacturer will work together. See my page on standards.
Let's consider the the A-bis interface as an example. Tektronix says the A-bis "is
a French term meaning 'the second A Interface.' " Good grief! In most cases the
actual span or physical connection is made on a T1 line or in Europe its
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equivalent, the E1.But regardless of the material used, the transmission media, it
is the signaling protocol that is most important.
Although the interface is unlabeled, the mobile switch communicates with the
telephone network using Signaling System Seven, an internationally agreed upon
standard. More specifically, it uses ISUP (external link) over SS7. As the
Performance Technologies people tersely put in in their tutorial on SS7, "ISUP
defines the protocol and procedures used to set-up, manage, and release trunk
circuits that carry voice and data calls over the public switched telephone
network (PSTN). ISUP is used for both ISDN and non-ISDN calls."
Using SS7 throughout is a big difference between conventional cellular and
GSM. IS-136 and IS-95 also uses SS7 but to communicate between the HLR
and VLR it uses a standard called IS-41.
What about the mysterious UM? That's the radio link between a mobile and a
base station. Um are the actual radio frequencies that calls are put on. Possibly
the letters stand for User Mobile. R.C. Levine clears up this matter nicely,
"Interface names (A, Abis, B, C, etc.) were arbitrarily assigned in
alphabetical order. The Um label is taken from the customernetwork U interface label used in ISDN. Although mnemonics have
been proposed for these letters, they are after-the-fact."
.pdf file on SS7 and mobile networking -- Good reading!
SIM: Subscriber identify module.
ME: Mobile
BTS: Base transceiver
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equipment.
station.
BSC: Base station controller.
HLR: Home
location register.
VLR: Visitor location
register.
MSC: Mobile services switching center.
EIR: Equipment
identity register.
AuC: Authentication
Center.
UM: Represents the radio link.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data
network.
Figure 1. General architecture of a GSM network
4. Radio link aspects
The International Telecommunication Union (ITU), which manages the
international allocation of radio spectrum (among many other functions),
allocated the bands 890-915 MHz for the uplink (mobile station to base station)
and 935-960 MHz for the downlink (base station to mobile station) for mobile
networks in Europe. Since this range was already being used in the early 1980s
by the analog systems of the day, the CEPT had the foresight to reserve the top
10 MHz of each band for the GSM network that was still being developed.
Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.
Cellular Radio frequencies around the world
American Cellular
AMPS, N-AMPS, D-AMPS (IS-136) CDMA
824-849 MHz
869-894 MHz
Mobile to base
Base to mobile
901-941 MHz
1850-1910MHz
1930-1990 MHz
Mobile to base
Base to mobile
872-905 MHz
917-950 MHz
Mobile to base
Base to mobile
American PCS/GSM
Narrowband
Broadband
E-TACS
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GSM
935-960MHz
GSM has three main frequency bands around the world: 890-915MHz
900 MHz, 1800 MHz, and 1900 MHz. It all depends on 1800MHz
the country. Other bands may be used in the future or
1900 MHz.
may be in trial right now.
JDC
810-826 MHz
940-956 MHz
1429-1441 MHz
1477-1489 MHz
Mobile to base
Base to mobile
Base to mobile
Mobile to base
GSM frequency spacing is 200Khz, AMPS is 30 Khz
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4.1 Multiple access and channel structure
Since radio spectrum is a limited resource shared by all users, a method must
be devised to divide up the bandwidth among as many users as possible. The
method chosen by GSM is a combination of Time- and Frequency-Division
Multiple Access (TDMA/FDMA). The FDMA part involves the division by
frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies
spaced 200 kHz apart. One or more carrier frequencies are assigned to each
base station. Each of these carrier frequencies is then divided in time, using a
TDMA scheme. The fundamental unit of time in this TDMA scheme is called a
burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods
are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which
forms the basic unit for the definition of logical channels. One physical channel
is one burst period per TDMA frame.
This is the correct, complete view of GSM. It's not enough to say, as I have too
many times, that GSM and conventional cellular (IS-136) are TDMA based.
While that it is true, it is more true to say such systems are TDMA and FDM
based. First, we have a number of radio frequencies, each separated by
200khz. This is the frequency division multiplexing part. (Or the FDMA part, a
minor semantic difference.) Secondly, we have the transmission technology,
TDMA, by which we put several calls on a single frequency. These calls are
broken into many pieces, each piece of each call sent one after another. Each
call separated by slight differences in time. GSM is a TDMA/FDMA system.
Weick calls a burst "a sequence of signals counted as a unit in accordance
with some specific criterion or measure." Bits are single pulses of electrical
energy. Much like the single dash of a Morse Code key. With Morse code we
use long and short pulses of energy to stand for letters. Although of uniform
length, the pulses we use in digital radio do the same thing. Bits grouped in
patterns represent voice and data. We also use bits, as shown in the diagram
below, for signaling. In the channel depicted a burst of bits is a marker, an
indicator, a signal within a signal. It's what the mobile first looks for in the
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digital stream flowing from the base station. More on this on the next page.
Channels are defined by the number and position of their corresponding burst
periods. All these definitions are cyclic, and the entire pattern repeats
approximately every 3 hours. Channels can be divided into dedicated
channels, which are allocated to a mobile station, and common channels,
which are used by mobile stations in idle mode.
Terminology alert! Cellular radio uses the word channel in many ways. It is a
pair of radio frequencies. And channels are part of the digital stream that flows
back and forth from the mobile to the base station. Channels, therefore, can be
carried on a channel. Confusing, isn't it? The discussion below focuses on
data channels, not radio channels.
4.1.1. Traffic channels
A traffic channel (TCH) is used to carry speech and data traffic. Traffic
channels are defined using a 26-frame multiframe, or group of 26 TDMA
frames. The length of a 26-frame multiframe is 120 ms, which is how the
length of a burst period is defined (120 ms divided by 26 frames divided by 8
burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is
used for the Slow Associated Control Channel (SACCH) and 1 is currently
unused (see Figure 2). TCHs for the uplink and downlink are separated in time
by 3 burst periods, so that the mobile station does not have to transmit and
receive simultaneously, thus simplifying the electronics.
We've seen these characters before. Reading the Channels page might help
you understand what follows. We'll discuss them individually as they come up
later in the article.
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In addition to these full-rate TCHs, there are also half-rate TCHs defined,
although they are not yet implemented. Half-rate TCHs will effectively double
the capacity of a system once half-rate speech coders are specified (i.e.,
speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are
also specified, and are used for signalling. In the recommendations, they are
called Stand-alone Dedicated Control Channels (SDCCH).
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Figure 2. Organization of multiframes, individual TDMA frames, and bursts for
speech and data
(NB: Quarter bits don't exist. The quarter bit shown as shown in guard bit is an
effective rate. More on this on the next page. Keep reading!)
5. Network aspects
Ensuring the transmission of voice or data of a given quality over the radio link is
only part of the function of a cellular mobile network. A GSM mobile can
seamlessly roam nationally and internationally, which requires that registration,
authentication, call routing and location updating functions exist and are
standardized in GSM networks. In addition, the fact that the geographical area
covered by the network is divided into cells necessitates the implementation of a
handover mechanism. These functions are performed by the Network
Subsystem, mainly using the Mobile Application Part (MAP) built on top of the
Signalling System No. 7 protocol.
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Mobiles can in fact only roam seamlessly if they are multi-band units. Most
international phones have two bands, one for the Americas at 1900Mhz, and one
for Europe at 900Mhz. Others such as the Ericsson R380 show below, cover the
1800Mhz band as well. This lets the phone roam on Asian and African networks.
The mobile switch communicates with the telephone network using Signaling
System Seven, an internationally agreed upon standard. IS-136 and IS-95 also
uses SS7. But it uses a standard called IS-41 when communicating between the
Home Location Register and the Visitor Location register. (Source for this IS-41
information is http://www.mobilein.com/mobile_basics.htm)
.pdf file on SS7 and mobile networking -- Good reading!
The signalling protocol in GSM is structured into three general layers [1], [19],
depending on the interface, as shown in Figure 3. Layer 1 is the physical layer,
which uses the channel structures discussed above over the air interface. Layer
2 is the data link layer. Across the Um interface, the data link layer is a modified
version of the LAPD protocol used in ISDN (external link), called LAPDm. Across
the A interface, the Message Transfer Part layer 2 of Signalling System Number
7 is used. Layer 3 of the GSM signalling protocol is itself divided into 3 sublayers.





Radio Resources Management
Controls the setup, maintenance, and termination of radio and fixed channels,
including handovers.
Mobility Management
Manages the location updating and registration procedures, as well as security and
authentication.
Connection Management
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
Handles general call control, similar to CCITT Recommendation Q.931, and
manages Supplementary Services and the Short Message Service.
Signalling between the different entities in the fixed part of the network, such as
between the HLR and VLR, is accomplished throught the Mobile Application Part
(MAP). MAP is built on top of the Transaction Capabilities Application Part
(external link) (TCAP, the top layer of Signalling System Number 7. The
specification of the MAP is quite complex, and at over 500 pages, it is one of the
longest documents in the GSM recommendations [16].
Figure 3. Signalling protocol structure in GSM
6. Conclusion and comments
In this paper I have tried to give an overview of the GSM system. As with any
overview, and especially one covering a standard 6000 pages long, there are
many details missing. I believe, however, that I gave the general flavor of GSM
and the philosophy behind its design. It was a monumental task that the original
GSM committee undertook, and one that has proven a success, showing that
international cooperation on such projects between academia, industry, and
government can succeed. It is a standard that ensures interoperability without
stifling competition and innovation among suppliers, to the benefit of the public
both in terms of cost and service quality. For example, by using Very Large Scale
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Integration (VLSI) microprocessor technology, many functions of the mobile
station can be built on one chipset, resulting in lighter, more compact, and more
energy-efficient terminals.
Telecommunications are evolving towards personal communication networks,
whose objective can be stated as the availability of all communication services
anytime, anywhere, to anyone, by a single identity number and a pocketable
communication terminal [25]. Having a multitude of incompatible systems
throughout the world moves us farther away from this ideal. The economies of
scale created by a unified system are enough to justify its implementation, not to
mention the convenience to people of carrying just one communication terminal
anywhere they go, regardless of national boundaries.
The GSM system, and its sibling systems operating at 1.8 GHz (called DCS1800)
and 1.9 GHz (called GSM1900 or PCS1900, and operating in North America),
are a first approach at a true personal communication system. The SIM card is a
novel approach that implements personal mobility in addition to terminal mobility.
Together with international roaming, and support for a variety of services such as
telephony, data transfer, fax, Short Message Service, and supplementary
services, GSM comes close to fulfilling the requirements for a personal
communication system: close enough that it is being used as a basis for the next
generation of mobile communication technology in Europe, the Universal Mobile
Telecommunication System (UMTS).
Another point where GSM has shown its commitment to openness, standards
and interoperability is the compatibility with the Integrated Services Digital
Network (ISDN) that is evolving in most industrialized countries, and Europe in
particular (the so-called Euro-ISDN). GSM is also the first system to make
extensive use of the Intelligent Networking concept, in in which services like 800
numbers are concentrated and handled from a few centralized service centers,
instead of being distributed over every switch in the country. This is the concept
behind the use of the various registers such as the HLR. In addition, the
signalling between these functional entities uses Signalling System Number 7, an
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international standard already deployed in many countries and specified as the
backbone signalling network for ISDN.
GSM is a very complex standard, but that is probably the price that must be paid
to achieve the level of integrated service and quality offered while subject to the
rather severe restrictions imposed by the radio environment.
IEC tutorial on Intelligent Networks is here: http://www.iec.org/online/tutorials/in/
IEC tutorial on International Intelligent Networks is here:
http://www.iec.org/online/tutorials/intern_in/
Although dealing with conventional cellular, this IEC tutorial will give you a good
understanding of the Wireless Intelligent Network or WIN:
http://www.iec.org/online/tutorials/win/
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