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 Sp�cial Mobile
(GSM) to study and develop a pan-European public land mobile system. The
proposed system had to meet certain criteria:
- 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
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. 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 , 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.
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 system. This is done by providing
functional and interface descriptions for each of the functional entities defined in the
system.
Table of contens
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
B-channel bit rate of 64 kbps to be practically achieved.
Using the ITU-T definitions, 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).
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.
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.
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.
Multipath equalization
At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars,
airplanes, etc. Thus many reflected signals, each with a different phase, can reach an
antenna. Equalization is used to extract the desired signal from the unwanted
reflections. It works by finding out how a known transmitted signal is modified by
multipath fading, and constructing an inverse filter to extract the rest of the desired
signal. This known signal is the 26-bit training sequence transmitted in the middle of
every time-slot burst. The actual implementation of the equalizer is not specified in
the GSM specifications.
Table of contens
Frequency hopping
The mobile station already has to be frequency agile, meaning it can move between a
transmit, receive, and monitor time slot within one TDMA frame, which normally are
on different frequencies. GSM makes use of this inherent frequency agility to
implement slow frequency hopping, where the mobile and BTS transmit each TDMA
frame on a different carrier frequency. The frequency hopping algorithm is broadcast
on the Broadcast Control Channel. Since multipath fading is dependent on carrier
frequency, slow frequency hopping helps alleviate the problem. In addition,
co-channel interference is in effect randomized.
Table of contens
Discontinuous transmission
Minimizing co-channel interference is a goal in any cellular system, since it allows
better service for a given cell size, or the use of smaller cells, thus increasing the
overall capacity of the system. Discontinuous transmission (DTX) is a method that
takes advantage of the fact that a person speaks less that 40 percent of the time in
normal conversation, by turning the transmitter off during silence periods. An
added benefit of DTX is that power is conserved at the mobile unit.
The most important component of DTX is, of course, Voice Activity Detection. It
must distinguish between voice and noise inputs, a task that is not as trivial as it
appears, considering background noise. If a voice signal is misinterpreted as noise,
the transmitter is turned off and a very annoying effect called clipping is heard at the
receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often,
the efficiency of DTX is dramatically decreased. Another factor to consider is that
when the transmitter is turned off, there is total silence heard at the receiving end, due
to the digital nature of GSM. To assure the receiver that the connection is not dead,
comfort noise is created at the receiving end by trying to match the characteristics of
the transmitting end's background noise.
Table of contens
Power control
There are five classes of mobile stations defined, according to their peak transmitter
power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to
conserve power, both the mobiles and the Base Transceiver Stations operate at the
lowest power level that will maintain an acceptable signal quality. Power levels can
be stepped up or down in steps of 2 dB from the peak power for the class down to a
minimum of 13 dBm (20 milliwatts).
The mobile station measures the signal strength or signal quality (based on the Bit
Error Ratio), and passes the information to the Base Station Controller, which
ultimately decides if and when the power level should be changed. Power control
should be handled carefully, since there is the possibility of instability. This arises
from having mobiles in co-channel cells alternatingly increase their power in response
to increased co-channel interference caused by the other mobile increasing its power.
This in unlikely to occur in practice but it is (or was as of 1991) under study.
Table of contens
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.
The signalling protocol in GSM is structured into three general layers,
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, 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.
Table of contens
Handover
In a cellular network, the radio and fixed links required are not permanently allocated
for the duration of a call. Handover, or handoff as it is called in North America, is the
switching of an on-going call to a different channel or cell. The execution and
measurements required for handover form one of basic functions of the RR layer.
There are four different types of handover in the GSM system, which involve
transferring a call between:
- Channels (time slots) in the same cell
- Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC),
- Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and
- Cells under the control of different MSCs.
The first two types of handover, called internal handovers, involve only one Base
Station Controller (BSC). To save signalling bandwidth, they are managed by the
BSC without involving the Mobile services Switching Center (MSC), except to notify
it at the completion of the handover. The last two types of handover, called external
handovers, are handled by the MSCs involved. An important aspect of GSM is that
the original MSC, the anchor MSC, remains responsible for most call-related
functions, with the exception of subsequent inter-BSC handovers under the control of
the new MSC, called the relay MSC.
Handovers can be initiated by either the mobile or the MSC (as a means of traffic
load balancing). During its idle time slots, the mobile scans the Broadcast Control
Channel of up to 16 neighboring cells, and forms a list of the six best candidates for
possible handover, based on the received signal strength. This information is passed
to the BSC and MSC, at least once per second, and is used by the handover algorithm.
The algorithm for when a handover decision should be taken is not specified in the
GSM recommendations. There are two basic algorithms used, both closely tied in
with power control. This is because the BSC usually does not know whether the poor
signal quality is due to multipath fading or to the mobile having moved to another
cell. This is especially true in small urban cells.
The 'minimum acceptable performance' algorithm gives precedence to power
control over handover, so that when the signal degrades beyond a certain point, the
power level of the mobile is increased. If further power increases do not improve the
signal, then a handover is considered. This is the simpler and more common method,
but it creates 'smeared' cell boundaries when a mobile transmitting at peak power
goes some distance beyond its original cell boundaries into another cell.
The 'power budget' method uses handover to try to maintain or improve a certain
level of signal quality at the same or lower power level. It thus gives precedence to
handover over power control. It avoids the 'smeared' cell boundary problem and
reduces co-channel interference, but it is quite complicated.
Table of contens
Location updating
A powered-on mobile is informed of an incoming call by a paging message sent over
the PAGCH channel of a cell. One extreme would be to page every cell in the
network for each call, which is obviously a waste of radio bandwidth. The other
extreme would be for the mobile to notify the system, via location updating messages,
of its current location at the individual cell level. This would require paging messages
to be sent to exactly one cell, but would be very wasteful due to the large number of
location updating messages. A compromise solution used in GSM is to group cells
into location areas. Updating messages are required when moving between location
areas, and mobile stations are paged in the cells of their current location area.
The location updating procedures, and subsequent call routing, use the MSC and two
location registers: the Home Location Register (HLR) and the Visitor Location
Register (VLR). When a mobile station is switched on in a new location area, or it
moves to a new location area or different operator's PLMN, it must register with the
network to indicate its current location. In the normal case, a location update message
is sent to the new MSC/VLR, which records the location area information, and then
sends the location information to the subscriber's HLR. The information sent to the
HLR is normally the SS7 address of the new VLR, although it may be a routing
number. The reason a routing number is not normally assigned, even though it would
reduce signalling, is that there is only a limited number of routing numbers available
in the new MSC/VLR and they are allocated on demand for incoming calls. If the
subscriber is entitled to service, the HLR sends a subset of the subscriber information,
needed for call control, to the new MSC/VLR, and sends a message to the old
MSC/VLR to cancel the old registration.
For reliability reasons, GSM also has a periodic location updating procedure. If an
HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the
database up to date would cause overloading. Therefore, the database is updated as
location updating events occur. The enabling of periodic updating, and the time
period between periodic updates, is controlled by the operator, and is a trade-off
between signalling traffic and speed of recovery. If a mobile does not register after
the updating time period, it is deregistered.
A procedure related to location updating is the IMSI attach and detach. A detach lets
the network know that the mobile station is unreachable, and avoids having to
needlessly allocate channels and send paging messages. An attach is similar to a
location update, and informs the system that the mobile is reachable again. The
activation of IMSI attach/detach is up to the operator on an individual cell basis.
Table of contens
Authentication and security
Since the radio medium can be accessed by anyone, authentication of users to prove
that they are who they claim to be, is a very important element of a mobile network.
Authentication involves two functional entities, the SIM card in the mobile, and the
Authentication Center (AuC). Each subscriber is given a secret key, one copy of
which is stored in the SIM card and the other in the AuC. During authentication, the
AuC generates a random number that it sends to the mobile. Both the mobile and the
AuC then use the random number, in conjuction with the subscriber's secret key and a
ciphering algorithm called A3, to generate a signed response (SRES) that is sent back
to the AuC. If the number sent by the mobile is the same as the one calculated by the
AuC, the subscriber is authenticated .
The same initial random number and subscriber key are also used to compute the
ciphering key using an algorithm called A8. This ciphering key, together with the
TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is
XORed with the 114 bits of a burst (the two 57 bit blocks). Enciphering is an option
for the fairly paranoid, since the signal is already coded, interleaved, and transmitted
in a TDMA manner, thus providing protection from all but the most persistent and
dedicated eavesdroppers.
Another level of security is performed on the mobile equipment itself, as opposed to
the mobile subscriber. As mentioned earlier, each GSM terminal is identified by a
unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in
the network is stored in the Equipment Identity Register (EIR). The status returned in
response to an IMEI query to the EIR is one of the following:
- White-listed
The terminal is allowed to connect to the network.
- Grey-listed
The terminal is under observation from the network for possible problems.
- Black-listed
The terminal has either been reported stolen, or is not type approved (the correct type
of terminal for a GSM network). The terminal is not allowed to connect to the
network.
Table of contens
Call routing
Unlike routing in the fixed network, where a terminal is semi-permanently wired to a
central office, a GSM user can roam nationally and even internationally. The
directory number dialed to reach a mobile subscriber is called the Mobile Subscriber
ISDN (MSISDN), which is defined by the E.164 numbering plan. This number
includes a country code and a National Destination Code which identifies the
subscriber's operator. The first few digits of the remaining subscriber number may
identify the subscriber's HLR within the home PLMN.
An incoming mobile terminating call is directed to the Gateway MSC (GMSC)
function. The GMSC is basically a switch which is able to interrogate the subscriber's
HLR to obtain routing information, and thus contains a table linking MSISDNs to
their corresponding HLR. A simplification is to have a GSMC handle one specific
PLMN. It should be noted that the GMSC function is distinct from the MSC function,
but is usually implemented in an MSC.
The routing information that is returned to the GMSC is the Mobile Station Roaming
Number (MSRN), which is also defined by the E.164 numbering plan. MSRNs are
related to the geographical numbering plan, and not assigned to subscribers, nor are
they visible to subscribers.
The most general routing procedure begins with the GMSC querying the called
subscriber's HLR for an MSRN. The HLR typically stores only the SS7 address of the
subscriber's current VLR, and does not have the MSRN (see the location updating
section). The HLR must therefore query the subscriber's current VLR, which will
temporarily allocate an MSRN from its pool for the call. This MSRN is returned to
the HLR and back to the GMSC, which can then route the call to the new MSC. At
the new MSC, the IMSI corresponding to the MSRN is looked up, and the mobile is
paged in its current location area .
Table of contens
Conclusion and comments
On this web-page I have tried to give an overview of the GSM system.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 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 . 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 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 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.
Contact author with any comments or inquiries.
© Kevin Doyle 1998.
All Rights Reserved.
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