Overview of the GSM system and protocol architecture

Ovewiew

The

GSM

System

and

Protocol Architecture

We can use

GSM

as a basic framework to define and develop

the standards for handling the mobility-specific functions

next-generation

PCNs.

Moe Rahnema

lobal system for mobile telecom-

munication (GSM) comprises the

CEFT-defined standardization of

the services, functional/subsystem

interfaces, and protocol archi-

tecture, based on the use of

worldwide standards produced by CCI7T and CCIR,

for a pan-European digital land mobile system

primarily intended to serve users in motor vehi-

cles. The digital mobile radio networks, for which

GSM represents the European standards, pro-

vide powerful message signaling capabilities that

facilitate and enhance roaming,compared to the first

generation analogue systems, through automatic

network location detection and registration.

GSM provides terminal mobility, with person-

al mobility provided through tbe insertion of a

subscriber identity module (SIM) into the GSM net-

work (mobile station). The SIM carries the personal

number assigned to the mobile user. The GSM-based

cellular mobile networks are currently in widespread

use in Europe. At the present time, the next gen-

eration of personal communication services

(PCS) beyond GSM is also being considered.

These third generation systems, known as univer-

sal personal communication networks (PCN) will be

using lower power handsets to provide personal

mobility to pedestrians, aswell. The PCS low-power

handsets are expected to eliminate the need to

have different handsets for wide-area (cellular)

and local (cordless) applications. The universal PCS

will also provide a higher quality of personal-service

mobility across the boundaries of many different net-

works (mobile and fixed, wide- and local-area).

Many network capabilities, however, such as

mobility management, user security protection, and

resource allocation, addressed in GSM, are also some

of the critical requirements and issues in UPC net-

works of the future. GSM is expected to play a

major role in the specification of the standards

for UPC. In the United Kingdom, PCN is already

being designed and deployed with close adher-

ence to the GSM standards other than the differ-

ent operating frequencies (GSM operates at

900

MHz and the United Kingdom PCN operates at

1800

MOE

RAHNEMA

is a

principal communication

engineer at Motorola Satellite

Communications.

MHz). Generally, GSM may be viewed as a frame-

work for studying the functions and issues that

are specific to cellular type personal communication

networks, whatever the means of implementation

might be.

In applying and extending GSM to the next gen-

eration personal communication networks, how-

ever, one should be careful in differentiating some

of the implementation specifics unique to the GSM

network architecture and application from the func-

tions and issues that would be more or less gener-

ally applicable and relevant to cellular networking.

It is with this point in mind that the reader should

view GSM as a framework or platform on which

to build his or her vision of how GSM may be used

as a guide to design and build the next generation

networks. In that regard, a good understanding of

the GSM standards and network functions is

essential for the professional working on the next

generation personal communication networks. This

article is intended to assist with this objective.

The

Cellular

Concept

ellular mobile communication is based on the

concept of frequency reuse. That is, the limit-

ed spectrum allocated to the service is partitioned

into, for example, N non-overlapping channel

sets, which are then assigned in a regular repeat-

ed pattern to a hexagonal cell grid. The hexagon

is just a convenient idealization that approximates

the shape of a circle (the constant signal level

contour from an omnidirectional antenna placed

at the center) but forms a grid with no gaps or

overlaps. The choice ofN is dependent on many trade-

offs involving the local propagation environment,

traffic distribution, and costs. The propagation envi-

ronment determines the interference received from

neighboring co-channel cells which in turn gov-

erns the reuse distance, that is, the distance

allowed between co-channel cells (cells using the

same set of frequency channels).

The cell size determination is usually based on

the local traffic distribution and demand. The more

the concentration of traffic demand in the area,

0163-6804/93/$03.00 19930

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1993

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the smaller the cell has to be sized in order to avail

the frequency set to a smaller number of roaming

subscribers and thus limit the call blocking proba-

bility within the cell. On the other hand, the

smaller the cell is sized, the more equipment will

be needed in the system as each cell requires the nec-

essary transceiver and switching equipment,

known as the base station subsystem

(BSS),

through which the mobile users access the net-

work over radio links. The degree to which the

allocated frequency spectrum is reused over the cel-

lular service area, however, determines the spectrum

efficiency in cellular systems. That means the

smaller the cell size, and the smaller the number

of cells in the reuse geometry, the higher will be

the spectrum usage efficiency. Since digital mod-

ulation systems can operate with a smaller signal

to noise (i.e., signal to interference) ratio for the

same service quality, they, in one respect, would allow

smaller reuse distance and thus provide higher spec-

trum efficiency. This is one advantage the digital cel-

lular provides over the older analogue cellular radio

communication systems. The interested reader may

refer to

[1,2]

for the details on spectrum efficien-

cy analysis in cellular network.

It isworthmentioning that thedigitalsystems have

commonly used sectored cells with 120-degree or

smaller directional antennas to further lower the

effective reuse distance. This allows a smaller

number of cel!s in the reuse pattern and makes a

larger fraction of the total frequency spectrum avail-

able within each cell. Currently, research is being

done on implementing other enhancements such

as the use

dynamic channel assignment strate-

gies for raising the spectrum efficiency in certain

cases, such as high uneven traffic distribution

over cells.

The Network Infrastructure

he cellular concept

networking is based on the

superposition of a distributed star type net-

work architecture on the existing fixed landline tele-

phony communication infrastructure. The basic

network architecture is illustratedin Fig.

The tele-

phony network is used to provide not only the

communication links between a mobile user and

a fixed landline user, but also to provide the con-

nectivity between the mobile users roaming in remote-

ly located cells or in the domain of mobile networks

operated by different service providers. The

BSSs,

provide management of the radio resources,

and the switching between the radio channels and

the TDM slots on their connections with the

mobile switching center (MSC). MSCs link groups

of neighboring

BSSs

through point-to-point land-

line or microwave-based El trunks. The MSC

acts as the nerve center of the system. It controls

call signaling and processing, and coordinates the

handover

the mobile connection from one

base station to another as the mobile roams

around. Each MSC is in turn connected to the

local public switched telephony network (PSTN,

or ISDN) to provide the connectivity between the

mobile and the fixed telephony users, as well as the

necessary global connectivity among the MSCs of

the cellular mobile network. This is intended to make

it possible for any mobile user to communicate

with any other mobile

fixed telephony user in

the world. Thus, the global connectivity provided

Figure

Cellular network infrastructure.

by the existing landline telephony infrastructure

is used to link up the cellular mobile subscribers

throughout the world.

Direct links between certain “local” MSCs

may also be provided

allow the communication

between two mobile users to bypass the telepho-

ny network when there is considerable traffic

flow between the mobile users roaming in the

areas under the coverage of those MSCs. Thus,

the communication path between any two mobile

users roaming under the coverage of two “local”

MSCsmayormaynot switch throughthepublictele-

phony network. It depends on the connectivity

provided between the two MSCs. The MSC may also

connect to public data networks (PDN), such as

the packet-switched networks, to provide the mobiles

with access to data services.

defines anumber of networkdatabases that

are used in performing the functions of mobil-

ity management and call control in a public land

mobile network (PLMN). These elements include

the location registers consisting of the home loca-

tion register (HLR), and the visiting location reg-

ister

(JXR),

the equipment identity register (EIR),

and the authentication center (AC). The HLR main-

tains and updates the mobile subscriber’s loca-

tion and his or her service profile information.

The VLR maintains the same information local-

ly, where the subscriber is roaming. The VLR is

defined as a stand-alone function (see followingpara-

graph), but is usually viewed by vendors as part of

the MSC. These registers are called service con-

trol points (SCP) in the terminology used in intel-

ligent networking (IN). The EIR is used to list

the subscribers’ equipment identities, which are used

for identification of unauthorized subscriber equip-

ment, and hence denial of service by the network.

The AC provides the keys and algorithm for

maintaining the security

subscriber identities, and

for encrypting information passed over the air inter-

face. The MSC is equipped with a service switch-

ing point (SSP) module which is used to query

the databases such as a location register to identi-

where a mobile subscriber is located and what

his or her service profile is, for the routing, and

processing of calls to (or by) the subscriber.

The GSM specifications have defined logically

separate functions and standard interfaces for each

the databases, to allow each function to be imple-

mented on a physically separate network compo-

nent. The interfaces are specified via the mobile

application part (MAP) that uses the transaction

Network Databases and

Standardization

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1993

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GSM,

the radio

channels are

based

TDM

structure

that is

implemented

multiple

subbands

frequency

(TDkW

FDMA).

capability applicationspart (TCAP) of (SS7). These

are all elements of an IN. GSM is considered an

IN application and GSM providers are consider-

ing the GSM implementation as experience in

intelligent networking.

Numbering Plan

he numberingconsistsof at least one international

ISDN number allocated to either the mobile sub-

scriber, if the mobile is card operated,

to the mobile

station, otherwise. The mobile station ISDN (MSIS-

DN) conforms to the CCIlT E.164 recommenda-

tion, and should, in each country, comply to that

country’s ISDN numbering plan. The MSISDN num-

ber basically consists of a country code (CC), a

“national destination code (NDC), which speci-

fies a PLMN within that country, and a subscriber

number (SN). This structure

shown in Fig. 2.

The MSISDN number is used for dialing by a

calling subscriber from the PSTN/ISDN, and is used

toroute thecall to thegateway MSCofthe GSMnet-

work. The GSM MSC then uses the MSISDN to inter-

rogate the appropriate HLR for the re-routing

information required to extend the call

the mobile’s

visiting MSC.

The rerouting information is specified by the

mobile station roaming number (MSRN) which is

obtained from the HLR and is used to progress the

call

the called mobile. The MSRN

a tempo-

rary number, allocated by the VLR (associated with

the mobile’s visiting MSC) and sent

the mobile’s

HLR either on location update (discussed in a

later section) or on a per call basis. The MSRN

has the same structure as the MSISDN numbers

in the visiting location area where it is allocated.

For

provision of mobile packet data service; a

mobile international data number conforming to

CCITT recommendation

121 may be specified.

GSM recommendation 03.70 discusses the require-

ments for the numbering intenvorking functions

required in this case.

Addressing and Call Routing

The MSISDN number is used for the routing of

calls within the PSTNIISDN networks. The details

of call routing requirements are discussed in

GSM recommendation 03.04. The following

paragraphs provide a summary discussion of pos-

sible scenarios involved in call routing.

National Calls from the Fixed Network

A local

transit exchange, when receiving a call

destined for a mobile, recognizes the NDC, and

routes the call to a gateway MSC. The gateway

MSC performs the HLR query for the MSRN,

which it then uses to reroute the call.

International Calls from the

Fixed Network

When a local or transit exchange receives an inter-

national call and recognizes the international pre-

fix, it routes the call to the nearest ISC. The ISC

recognizes that the NDC indicates a PLMN. If it can

support HLR query (i.e.,

it has TCAP signaling

connectivity to the HLR) it queries the HLR and

receives the called subscriber’s roaming number and

routes the call to the visiting MSC. If not, it routes

the call to the ISC of the home PLMN of the

called subscriber.

NDC SN

I I

SN NDC

Figure

The

structure

for

the

GSM

MSISDN.

National Calls from Within the PLMN

Whenalocalexchange(MSC)receivesacalldestined

for a mobile, it queries the mobile’s HLR for the roam-

ing number of the mobile. On receipt of the MSRN,

it routes the call

the called mobile’s visiting MSC.

Addressing Other Components of a

PLMN

Other components of a PLMN, which may be

addressed for the routing of various signaling

messages, are the MSCs, and the location regis-

ter& If these elements are addressed from within

the same PLMN, the SS7 point codes (PC) can be

used. Otherwise, for interPLMN routing, global

titles (GT) derived, for instance, from the mobile

country code (MCC) and the national destination

codes (NDC) are used.

Radio Channel Structure in

GSM

GSM, the radio channels are based on a TDMA

structure that is implemented on multiple frequency

subbands (TDMAIFDMA). Each base station is

equippedwith a certain number of these preassigned

frequencyhime channels.

CEPT has made available

two

frequency bands

to be used by the GSM system. These are: 890-915

MHz for the direction mobile to base station, and

935-960 MHz for the direction base station to

mobile terminal. These bands are divided into

124 pairs of carriers spaced by 200 kHz, startingwith

the pair 890.2 MHz. Each cell site has a fixed

assignment of a certain number

carriers, rang-

ing from only one to usually not more than

channels.The cell ranges in size from

to several km.

The assigned spectrum of 200 kHz per chan-

nel is segmented in time by using a fixed alloca-

tion, time-division multiple access (TDMA) scheme.

The time axis is divided into eight time slots of

length 0.577 ms. The slots numbered from time

slot 0 to 7 form a frame with length 4.615 ms.The

recurrence of one particular time slot in each

frame makes up one physical channel.

The TDMA scheme uses a gross bit rate of about

270 kb/s (with a Gaussian minimum shift keying

modulation, GMSK) and requires sophisticated

adaptive receiver techniques to cope with the trans-

mission problems caused by multipath fading.The

TDMA factor

in combination with a carrier

spacing of 200 kHz would correspond to the earli-

er analog system using single-channel per-carrier with

a 25 kHz carrier spacing. The GSM digital system

allowed operation at lower carrier

interference

(CII) ratio by using the gains provided by digital voice

compression along with channel coding (powerful

error correction). The reduced CII ratio in turn

allowed the use of shorter channel reuse dis-

tances to achieve spectrum efficiencies competi-

tive to that achieved by the analog systems.

The TDMA structure is applied in both the for-

ward (base station

mobile) and the reverse (mobile

to base station) directions. The numbering, however,

is staggered

three time slots, toprevent the mobile

station from transmitting and receiving at the

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same time. These time slots are used to carry

user, and signaling

control information in

bursts. The bursts are slightly shorter than the

slots, namely .546 ms, to allow for burst timing align-

ment errors, delay dispersion on the propagation

path, and for smoothswitchon/off ofthe transmitter.

GSM defines a variety of traffic and signal-

ing/control channels

different bit rates. These

channels are assigned to logical channels derived

from multiframe structuring of the basic eight

slotted TDMA frames just discussed. For this

purpose, two multiframe structures have been

defined: one consisting of 26 time frames (result-

ing in a recurrence interval of 120 ms), and onecom-

prising 51 time frames

(or

236 ms).

The 26 multiframe is used to define traffic

channels (TCH), and their slow and fast associat-

ed control channels (SACCH and FACCH) that

carry link control information between the mobile

and the base stations. The TCH have been defined

to provide

six

different formsof services, that is, full-

rate speech

data channels supporting effective bit

rates of 13 kb/s (for speech), 2.4,4.8, and

9.6

kb/s;

and the half-rate channels with effective bit-rates

6.5

(for speech) and kb/s, 2.4 kb/s, and 4.8 kb/s

for data (note that the gross bit rates on these

channels are higher due to required channel coding,

22.8 kb/s for full-rate speech). The full-rate

TCHs are implemented on 24 frames of the mul-

tiframe, with each TCH occupying one time slot from

each frame: The SACCH is implemented on

frame

(numberedfromO),providingeight

SACCH

channels, one dedicated to each of the eight TCH

channels. Frame 25 in the multiframe is currently

idle and reserved to implement the additional

eight SACCH required when half-rate speech chan-

nels become a reality. The FACCH is obtained

on demand by stealing from the TCH, and is used

by either end for signaling the transfer character-

istics of the physical path, or other purposes such

as connection handover control messages. The steal-

ing of a TCH slot for FACCH signaling is indi-

cated through a flag within the TCH slot.

The 51-frame multiframe has a more complex

structure and we will refer the reader to GSM

Recommendation

05.0

for the specific positions

of the various logical channels in the multiframe.

The 51-frame structure, however, is used to derive

the following signaling and control channels.

SDCCH

Stand-alone dedicated control chan-

nel is used for the transfer of call control signal-

ing to and from the mobile during call setup. Like

the TCHs, the SDCCH has its own SACCH and

is released once call setup is complete.

BCCH

Broadcast control channel is used in

the BSS to mobile direction to broadcast system

information such as the synchronization parame-

ters, available services, and cell ID. This channel

is continuously active, with dummy bursts substi-

tuted when there is no information to transmit,

because its signal strengths are monitored by mobiles

for handover determination.

SCH

Synchronization channel carries informa-

tion from the BSS for frame synchronization.

FCCH

Frequency control channel carries infor-

mation from the

BSS

for carrier synchronization.

CCCH

Common control channels are used for

transferring signaling information between all

mobiles and the BSS for call origination and call-

paging functions. There are three common con-

trol channels:

PCH: paging channel used to call (page) a mobile

from the system.

RACH: random access channel used by the mobiles

trying to access the system. The mobiles use

the slotted Aloha scheme over this channel for

requesting a DCCH from the system at call ini-

tiation.

AGCH: access grant channel used by the sys-

tem to assign resources to a mobile such

a DCCH

channel.

Note that the AGCH and the PCH are never used

byamobile at the same time, and therefore are imple-

mented on the same logical channel. All the con-

trol signaling channels, except the SDCCH, are

implemented on time slot

in different TDMA

frames of the 51 multiframes using a dedicated

RF carrier frequency assigned on a per cell basis.

The multiframe structure for the SDCCH and its

associated slow associated control channel

(SACC)

implemented on one of the physical chan-

nels (TDM slots and RFcarriers) selected by the sys-

tem operator.

Mobility Management

obility management is concernedwith the func-

tions of tracking the location of roaming

mobiles and registering the information in appro-

priate network elements, and handling connec-

tion handoffs for users in the communication process.

These functions qe discussed in the following

sections.

Connection HandoHs

This may be done between channels in the same

cell, between channels in different cells under the

same

gSS

coverage, or between cells under the

coverage of different BSSs, and even different

MSCs. In GSM, the BSS may autonomously han-

dle the connection handoffs in the same cell,

between cells under its own coverage. This is called

internal connection handoffs. The MSC is involved

in managing connection handoffs that need to

take place between cells under coverage

two

different BSSs. These are called external connec-

tion handoffs. When the BSS indicates that an exter-

nal handover is required, the decision of when

and whether an external handover should occur

is then taken by the MSC. The MSC uses the signal

quality measurement information reported by the

mobile stations (MSs) which are pre-processed at

the BSS for external handover determination.

The original MSC handling a call will always

keep control of the call in an external handover

to a different and even a subsequent MSC.

When the

BSS

performs an internal connec-

tion handoff, it informs the MSC at the comple-

tion of the process. The need for a connection handoff

may be indicated by the mobile user, through

messaging on the FACH, for instance, or by the

BSS as it keeps tracking the quality of the signals

received. The

BSS

monitors the quality of the

radio signal received and also transmits such

results to the MSC who keeps a more global view

on the radio channels belonging to its BSSs. The

Common

control

channels

are

used for

transfem'ng

signaling

infomation

between

all

mobiles and

the

BSS

for

call

origination

and call-

paging

functions.

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1993

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Figure

GSMprotocol architecture,

Figure

LAPDm

“addressfield” format.

MSC may also initiate the need for a connection

handoff for traffic reasons in an attempt tq bal-

ance out the traffic load in the network.

Handling of Location Information

Location information is maintained and used by

the network to locate the user for call routing pur-

poses. The network registers tQe user’s location

in a register called the user’s, HLR, which is asso-

ciated with an MSC located in the PLMN, to which

the user is subscribed. Each

BSS

keeps broadcast-

ing, on a periodic basis, the cell identities on the

“broadcast control channels” of the cells under its

coverage. The mobiles within each cell keep mon-

itoring such information.

changes in location

are detected (from the last information recorded

by them), they each report the new location to

the

BSS

which routes it to the VLR, of the MSC

towhichit

isconnected.TheVLR,inturn,sendsthe

location information to the user’s HLR, where it

is also recorded. In the meantime, the HLR

directs the old VLR to delete the old visiting

location of the mobile from its data base, and also

sends a copy of the user’s service profile to the

new VLR. Location updating is performed by the

mobility management (MM) protocol sublayer

that will be discussed later in the article.

Call Routing and Signaling

call may be initiated by a mobile user to anoth-

er mobile

a fixed landline user,

in reverse,

by a fixed landline user to amobile.

For

routing acall

to a mobile user, however, the network signaling

needs to first locate the mobile. We will illustrate this

for thecasewhen acall isinitiatedby alandline user,

and then comment on the scenario in which the

call

initiated by a mobile to another mobile.

When the call is initiated by a mobile to a land-

line user, the procedure is rather straightforward.

In the case of a call initiated by a landline user,

the PSTN may use the mobile station ISDN num-

ber, MSISDN, to route the call to the closest Gate-

way MSC within the mobile’s PLMN. The GMSC in

turn uses the MSISDN to interrogate the mobile’s

HLRfor the routing information required to extend

the call to the visiting MSC of the mobile at the

time.Thisv,isiting MSC (or more specifically the,VLR

within the local MSC) is identified in the mobile’s

HLR by the MSRNwhich specifies the visiting MSC.

The MSRN is a temporary number allocated by

the VLR and sent to the HLR on location updat-

ing, or call initiation. The MSRN should have the

same structure as the MSISDN numbers in the VLR

area where it is allocated. The VLR then initiates

the paging procedure and the MSC pages the mobile

station with a paging broadcast to all

BSSs

of the

location area, as the exact base station area of the

mobile may not be known. After paging response,

the current

BSS

is located. The RR and MM con-

nections are established, during which both

authenticationoftheuser (for accesstothenetwork),

as well as cipher mode setting are performed.

The VLR then sends the required parameters

for

call setup to the MSC, and may also assign the mobile

a new TMSI for the call. The MSC sends asetup mes-

sage to the mobile station.

The mobile station, on receiving the set-up

message performs acompatibility check and returns

a call-confirmed message to the network, which may

include the bearer capability of the mobile sta-

tion. The

BSS

may at this point assign a traffic

channel, TCH, to the call, or may assign it at a

later stage, the latest being on receipt of the

“connect message” from the mobile station. If

user alerting is carried out at the MS, an alerting mes-

sage is sent to the calling subscriber. When, the

subscriber answers the call, the MS sends a con-

nect message, which at the network side initiates the

completion of the traffic channel allocation and

switch through of the connection. The connect

message

progressed to the calling subscriber.

The network also sends an acknowledgement to

the MS, that enters the active state.

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Figure

LAPDm

general frame format.

Protocol layering

Architecture

he GSM protocol architecture used for the

exchange

signaling messages pertaining to

mobility, radio resource, and connection manage-

ment functions is shown in Fig.

The protocol

layering consists of the physical layer, the data

link layer, and the Layer

It is noted to the

OSI-minded reader to be careful in not confusing

the Layer

protocol functions defined by GSM with

what is normally defined to be the Layer

func-

tions in the

OS1

model. The GSM Layer

proto-

cols are used for the communication of network

resource, mobility, code format and call-related man-

agement messages between the various network enti-

ties involved. Since, in the

OS1

model, some

these

functions are actually provided by the higher lay-

ers, the term “message layer” may be a more

appropriate term for refering to the Layer

in GSM.

The message !ayer (Layer

protocol is made

of three sublayers called the resource manage-

ment (RR) implemented over the link between

the MS and the

BSS,

the mobility management (MM),

and connection management (CM) sublayers

providing the communication between the MS

and the MSC. Layer

also implements the mes-

sage transport part (MTP), level

and the sig-

naling connection control part of the CCIn SS7

the link between the

BSS

and the MSC (the A

interface) to provide the transport and address-

ing functions for signaling messages belonging to the

various calls routed through the

MSC.In

dis-

cussing the functionality provided by the Layer

in the GSM protocol stack, particular attention should

be paid to not confuse the details of this layer’sfunc-

tionality with what is commonly provided by the

Layer

of the

OS1

protocol stack.

GSM, the

CM,

and MM sublayers, for instance, provide

actually some of the functionalities which are

realized by the transport, the session, and the

presentation layers of

OSI,

as will be seen later. The

functions of each protocol layerisublayer is discussed

in some detail in the following.

Physical Layer

The physical layer on the radio link was discussed in

the section on radio channel structure. The traffic

channels

the landside are formed fromTDM slots

implemented on 2.048 Mb/s links

(El

trunks).

The signaling channels are basically logically mul-

tiplexed

an aggregate of the TDM slots.

Link Layer on the Air Interface

The data link layer over the radio link (connect-

ing theMS to

theBSS)isbas'edonaLAPD-like pro-

tocol, labeled LAPDm, that has been modified

for operation within the constraints set by the

radio path.

particular, LAPDm uses no

flags (and therefore

bit stuffing) for frame delim-

Address

Control

Length

Information

Fill

field field indicator field

bits

itation. Instead, frame delimitation in LAPDm is

done by the physical layer that defines the trans-

mission frame boundaries. LAPDm uses a “Length

Indicator” field to distinguish the information

carrying field from fill-in bits used to fill the

transmission frame. LAPDm uses an address

field to carry the service access point identifier,

(SAPI),

bits in this case2 which LAPD also

uses to identify the user of the service provided

by the protocol. When using command/control

frames, the SAPI identifies the user for which

a command frame is intended, and the user trans-

mitting a response frame. The format for the address

field is shown in Fig. 4. The 2-bit link protocol

discriminator (LPD)

used to specify a particu-

lar recommendation of the use of LAPDm, the

C/R is a single bit which specifies a command or

response frame as used in LAPD, and a 1-bit extend-

ed address (EA)

used to extend the address

field to more than one octet (the EA bit in the

last octet of the address should be set to

other-

wise to

0).

The 8-bit is reserved for future uses.

LAPDm uses a control field as is used in

LAPD to carry sequence numbers, and to specify

the type of frame. LAPDm uses three types of frames

used for supervisory functions, unnumbered

information transfer and control functions (unac-

knowledged mode), and numbered information

transfer (multiframe acknowledged mode) as

used in LAPD. LAPDm uses no cyclic redundan-

cy check bits for error detection. Error correction

and detection mechanisms are, instead, provided by

a conbination

block and convolutional coding

used (in conjuctionwith bit interleaving) in thephys-

ical layer. The general frame format for LAPDm

is shown in Fig.

Link Layer on the

interface

the terrestrial link connecting the

BSS

to the

MSC (the A interface), the MTP level

of the

SS7protocol is used to provide the

OS1

Layer 2func-

tions of reliable transport for the signaling messages,

such as recovery from transmission errors through

error detection and retransmission.

Message layer Protocols and

Functions

Radio Resource (RR) Management

Sublayer

The RRmanagement sublayer terminates at the

BSS

and performs the functions of establishing physi-

cal connections over the radio for the purpose of

transmitting call-related signaling information such

as the establishment of signaling and traffic chan-

nels between a specific mobile user and the

BSS.

The

RR management functions are basically imple-

mented in the

BSS.

DPDm

is a

PD -like

protocol that

has been

modified

for

operation

within the

constraints

set

the

radio pass.

h‘

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1993

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Location

updating

is the

procedure

for

keeping

the network

informed

where

the mobile

is roaming.

Mobility Management Sublayer (MM)

The MM sublayer is terminated at the MSC and

the related messages from or to the MS are

relayed transparently in the BSS using the DTAP

process. The MM sublayer provides functions

that can be classified into three types of proce-

dures. These are called the MM specific procedures,

the MM common procedures, and the MM con-

nection-related procedures. These procedures

are discussed in the following.

MM Connection Related Procedures

These are the procedures used to establish, main-

tain, and release a MM connection between the MS

and the network (MSC) over which an entity of

the connection management (CM) sublayer can

exchange information with its peer. More than

one MM connection may be active at the same

time to serve multiple CM entities. Each CM

entitywithin the MS will have its

own

MM connection,

and each connection is identified by the protocol dis-

criminator, and a transaction identifier within the

related signaling messages exchanged. The trans-

action identifier is sort of analogous to the call

reference used by ISDN to identify signaling mes-

sages from different calls

the

channel. Thus

parallel calls can be supported by the same MS which

are then identified by a different value for the

transaction identifier parameter. Establishment

of a MM connection requires that no MM-specif-

ic procedure (discussed later) be active.

The MM connections provide services to the

different entities of the upper connection man-

agement (CM) sublayer which currently consist

of the call control (CC), the short messagc ser-

vices (SMS), and the call-independent supple-

mentaryservices

(SS).

AnMMconnection is initiated

by a CM service request message which identifies

the requesting CM entity and the type of service

required of the MM connection. The services

provided by the MM connections include such things

as enciphering (for privacy of user information), and

authentication (of the users-access to the network

and the service requested) which would be actual-

lyprovided by the presentation, and application lay-

ers in the OS1 framework. Each of these services

would involve the exchange of multiple messages

between the MS and the network before the required

MM connection is established and the requesting

entity within the CM sublayer is notified.

Mobility Management Specific

Procedures

The MM specific procedures do not set up an

MMconnection.Theycanonlybeinitiatedwhenno

other MM-specific procedure is running, andno MM

connection is established. These procedures

consist of location updating, and the IMSI attach

procedures. These are discussed in the following.

Location Updating

Location updating is the procedure for keeping the

network informed of where the mobile is roaming.

Location updating is always initiated by the mobile

station on either detecting that it is in a new loca-

tion area by periodically monitoring the location

information broadcast by the networkon the broad-

cast channel, and comparing it to the information

previously stored in its memory,

by receiving an

indication from the network that it is not known

in the VLR upon trying to establish an MM con-

nection. Anytime, the network updates the mobile's

location, it sends

itanupdated"temporarymobi1esub-

scriber identification" (TMSI), in ciphered mode,

which is stored in the MS and used for subsequent

mobile identification in paging and call initiating

operations. The purpose of using the TMSI as

opposed to the user's IMSI is to keep the subscriber's

identity confidential on the radio link. The

TMSI

has

no GSM- specific structure, and has significance only

within the location area assigned. The TMSI has

tobe combinedwith thelocation areaidentifier

(LAI)

to provide for unambiguous identification outside

the area where it is assigned.

lMSl Attach

zhe IMSI attach procedure is the complement of the

IMSI detach procedure, a function of the MM

common procedures (discussed later). Both of these

procedures are network options whose necessity

of usage are indicated through a flag in the sys-

tem information broadcast on the BCCH chan-

nel. The IMSI detachiattach procedures mark the

MS as detachediattached in the VLR (and option-

ally

the HLR) on MS power down or power up

or subscriber information module (SIM) removed or

inserted (The IMSI detach disables the location

updating function to prevent unnecessary signal-

ing overhead

the network). Any incoming

calls, in that case, are either rejected

forward-

ed as may be specified by the user). The IMSI is used

to indicate the IMSI as active in the network.

This procedure is invoked

an IMSI is activated

a MS (power up, or SIM insertion) in the cov-

erage area of the network,

an activated MS enters

the network's coverage area from outside. The IMSI

attach procedure is then performed only

the stored

location area at the time is the same as the one

being broadcast on the BCCH channel of the

serving cell. Otherwise, a normal location updat-

ing procedure is invoked regardless of whether the

network supports IMSI attachidetach procedures.

MM Common Procedures

The MM common procedures can be initiated at any

time while a dedicated radio channel exists between

the network and the MS. They do not se't up an

MM connection, but can be initiated during an

MM specific procedure,

while an MM connection

is in place. The MM Common procedures consist

of IMSI detach, TMSI reallocation, and authenti-

cationiidentification. These are discussed next.

lMSl Detach

The IMSI detach procedure is invoked by the mobile

station to indicate inactive status to the network.

No response

acknowledgement is returned to

the MS by the network on setting the active flag

for the IMSI.

The IMSI detach procedure is not started if at the

time a

MM-specific procedure

is active. In that case,

the IMSI detach procedure is delayed,

possible

until the MM-specific procedure is finished, oth-

erwise the IMSI detach request is omitted.

If at the time of a detach request, a radio con-

nection is in existence between the MS and the

network, the MM sublayer will release any ongo-

ing MM connections before the MM detach indi-

cation message is sent.

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April 1993

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TMSI Reallocation

The purpose of TMSI reallocation is to provide iden-

tity confidentiality. That is, to protect the user

from being identified and located by an intruder.

This procedure must be performed at least at each

change of the MSC coverage area. Reallocation

in any other case is left to the network operator.

If the TMSI provided by a mobile station is

unknown in the network; for instance, in the case

of a data base failure, the MS has to provide its

IMSI on request from the network. In this case

the identification procedure has to be performed

before the TMSI procedure can be initiated.

Authentication

The purpose of the authentication procedure is

to let the network verify the identity provided by

the userwhen requested, and to provide a new cipher-

ing key to the mobile station. The caseswhen authen-

tication procedures should be used are defined in

GSM Recommendation 02.09. The authentica-

tion procedure is always initiated and controlled

by the network.

Identification

This procedure is used by the network to request

a mobile station to provide specific identification

parameters to the network, such as the user’s

international mobile subscriber or equipment

identifiers(IMS1 or IMEI). The mobile station should

be ready to respond to an identity request mes-

sage at any timewhile RRconnection exists between

the mobile and the network.

Connection Management

Sublayer (CM)

he CM sublayer terminates at the MSC and con-

tains entities that currently consist of CC includ-

ing call-related supplementary services, SMS, and

call independent supplementary services support

(SS).

Once a MM connection has been established,

the CM can use it for information transfer. The

CCentityuses the CCI’ITQ.931 protocol, withminor

modifications, for the communication of call con-

trol-related messages between the MS and the MSC.

The SMS is a GSM-defined service that providesfor

speedy packet mode (“connectionless”) commu-

nication of messages up to 140 bytes between the MS

and a third party service center. These messages can

be sent

received by the mobile stationwhile avoice

or data call is in the active or inactive state. It is accept-

able, however,

the service is aborted while a

call is in a transitional state such as handover or busy-

to-idle. The service center is responsible for the

collection, storage, and delivery of short mes-

sages, and is outside the scope of GSM.

BSS

Application Part

(BSSAP)

he BSS, in addition to providing thechannel switch-

ing and aerial functions, performs radio resource

management, and intenvorking functions between

the data link protocols used

the radio and the

BSS-MSC side for transporting signaling-related

messages. These functions are provided by the

BSS Management Application Process (BSSMAP),

and the Direct Transfer Application Process (DTAP).

The BSSMAP is used to implement all proce-

dures between the MSC and the BSS that require

interpretation and the processing of information

related to single calls, and resource manage-

ment. Basically, the BSSMAP is the process with-

in the BSS that controls radio resources in

response to instructions from the MSC (in that

sense, the BSSMAP represents the RR sublayer

to the MSC).

For

instance, the BSSMAP is used

in the assignment and switching

radio channels

at call setup, and handover processes.

The DTAP process is used for the transparent

transfer

MM/CM signaling messages between the

MS and the MSC. That is, the DTAP function

provides the transport level protocol intenvorking

function for transferring Layer 3 signaling messages

from and to the MS to and from the MSC with-

out any analysis of the message contents.

Signaling Transport Protocols

he CCITT

SS7

MTP and SCCP protocols are

used to implement both the data link and the

Layer 3 transport functionsfor carrying the call con-

trol and mobility management signaling messages

the BSS-MSC link. The MM and CM sublayer

signaling information from the mobile station is rout-

ed over signaling channels (such as the DCCH,

SACCH, FACCH) to the BSS from where they are

transparently relayed through the DTAP process

to an SCCP, of CCITT

SS7

type logical channel,

assigned for that call, on the BSS-MSC link for trans-

mission to the peer CC entity in the MSC for pro-

cessing. Similarly, any call signaling information

initiated by the MSC

the SCCP connection is

relayed through the DTAP process in the BSS to the

assigned signaling’ channel, using the LAPDm

datalink protocol, for delivery to the mobile station.

The interworking between the Layer 2 proto-

col on the radio side and the

SS7

the BSS-MSC

link is provided by a distribution data unit within

the information field of the SCCP. These param-

eters a’re known as the discrimination, and the data

link connection identifier (DLCI) parameters.

The discrimination parameter (currently dedicat-

ed one octet) uses a single bit to address a message

either to the DTAP or the BSSMAP processes. The

DLCIparameter (sized one octet) is made up of two

subparameters that identify the radio channel type

(such as the DCCH, SACCH, FACCH), and the

“Service Access Point Interface”(SAP1) value (in the

LAPDm protocol) used for the message on the radio

link. The SCCP provides for the logical multiplexing

of signaling information from different calls onto

the same physical channel (such as a single 64 kb/s

slot of a 2.048

Mb/s

trunk)

the BSS-MSC link.

For

each call supported by a BSS, an SCCP logical

connection is established on the BSS-MSC link. Any

information pertaining to a specific call flows through

its associated SCCP connection and that is how

signaling information exchange pertaining to

different calls are identified in the

BSS

or MSC.

The connectionless service mode of the SCCP

is also supported for the transfer of OA&M relat-

ed messages as well as BSSMAP messages that

do not pertain to any specific call (Note that BSSMAF’

messages pertaining to specific calls, such as hand-

off messages, are transmitted using the SCCP

connection established for the call). The SCCP rout-

ing function uses the Subsystem Number

(SSN)

The authenti-

cation proce-

dure allows

the network

to veri8 the

identity pro-

vided

the

user ,when

requested,

and topro-

vide a new

ciphering key

to the mobile

station.

h‘

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1993

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The optimum

size

for

the

paging area

determined

a proper

balance

between the

costs

paging and

the costs

location

updates.

in the Service Information Octet (SIO) within the

MTP level

message to distinguish messages

addressed to the OA&M function from those

addressed to either the DTAP or the BSSMAP appli-

cation parts. The high-level address translation capa-

bility of the SCCP, known as global title translation,

may then be used to provide additional address-

ing capabilities such as use of E. 164 numbering

for addressing different OA&M entities. The

global title translation feature of the SCCP also pro-

vides the MSC the capability to address signaling

messages to remote MSCs that may be located in

a different PLMN.

The interworking functions between the CM, MM

and BSSMAP entities and the corresponding

entitiesof the

SS7

(i.e., the ISDN-UP), MAP, SCCP,

and the transactions capabilities application part

(TCAP) is provided by the MSC.

Paging

and present it in some logical and well-related

format. I have tried my best, however, to achieve this

goal in this article.

This article was meant to provide a concise, brief,

but adequately detailed description of the GSM sys-

tem and protocol architecture that can serve as a

quick, rather self-contained conceptual frame-

work for extending and relating the mobility-specific

functions of the next generation personal com-

munication networks to the GSM network functions,

and the protocols used to achieve them. Finally, a

list of references have been provided for any

more detailed information on the issues addressed

in the article.

Acknowlegements

The author would like to thank Bomber Bishop

and

David Leeper from Motorola, and Prapeep

Sherman from AT&T for their careful reading of

the original manuscript and for providing useful

comments.

aging messages for mobiles are sent via the

BSSMAP'to the BSS as a connectionless mes--

sage through the SCCP/MTP. The paging mes-

sage may include the mobile's IMSI in order to

allow derivation of the paging population num-

ber. A single paging message transmitted to the BSS

may contain a list of cells in which the page is to be

broadcast. The larger the paging area is defined, the

lower the frequency of location updatesand hence

the associated traffic overhead on the network.

On the other hand, large paging areas result in

increased use of transmitting power as well as the

radio resources (channels). Therefore, the optimum

size for the paging area (location area) is detem-

ined by a proper balance between the costs-of

paging and the costs of location updates.

The paging messages received from the MSC are

stored in the BS, and corresponding paging messages

are transmitted over the radio interface at the appro-

priate time. Each paging message relates to only one

mobile station and the BSS has to pack the pages into

the relevant 04.08 paging messagz(inc1ude Layer

information). Once a paging message is broad-

cast over the radiochannel(s), if a response message

is received from the mobile, the relevant signaling

connection is set up towards the MSC and the

page response message is passed to the MSC.

Summary Remarks

he description of the GSM network functions,

system architecture and protocols are spread

over a large number of GSM do.cuments, each

which contains many details with some of the crit-

ical issues and highlights covered within those details.

Therefore, it is not an easy task to extract out

some of the crucial concepts and design specifics,

References

[11 W.C.Y. Lee, "Spectrum Efficiencyin Cellular,"lEE€Trans on Veh. Tech.,

[21 W.C.Y. Lee, "Spectrum Efficiency and Digital Cellular."38th

/E€€

I31 GSM Recommendation 04.03, "MS-855 Interface: Channel Struc-

[41 GSM Recommendation 05.01, "Physical Link Layer on the Radio

I51 GSM Recommendation 05.02, "Multiplexing and Multiple Access

[61 Conference Proceedings. Digital Cellular Radioconference, Hagen FRG,

[71 GSM Recommendation002.02, "Bearer Services Supported bya PLMN."

181 GSM Recommendation 09.01, "General Aspects on PLMN Inter-

(91 GSM Recommendation 03.04, "Signaling Requirements Related to

[lo1 GSM Recommendation 08.02, "855-MSC Interface-Interface Principles."

[l 11 GSM Recommendation

04, "BSS-MSC Layer 1 Specifications."

11 21 GSM Recommendation

08.06,

Signaling Transport Mechanisms

[131 GSM Recommendation 09.02, "Mobile Application Part (MAP)

I141 GSM Recommendation

08.08.

"855-MSC Layer

Specifications."

[151 GSM Recommendation 04.08, "Mobile Radio Interface-Layer

vol.

38. no. 2, May 1989.

Veh. Tech. Conf. Records,

pp.643.

June 1988..

tures and Access Capabilities."

Path" (General Description).

on the Radio Path."

Oct. 1988.

working."

Routing of Calls to Mobile Subscribers."

for BSS-MSC Interface."

Specification.''

Specifications."

Biography

MO€

RnhNFMn

received

oegree in engmeer ng science

from

the

Jniversity

Kentucky at -exington in 1978 with honors He received

the M

degree dnd the more advance0 engineermg degree in Avion

ics

from

MI1 n 1981

From

1983

1984 he tabght and stbdied

com

mbnication scjencesat Northeastern Universdyfrom which healso receivea

the Engineer degree m electrical and computer engineer ng with Ph

eve coursework He worked as a senior communlcation deslgn eng

neer at nf net in Andover. Mass

from

1984 to 1985 where he designed

thed gitalslgna

processingfirmwarefora4800baud

modem

From

1985

1989

heworkeoasamemberofthetechnicalstaff

atGTE Laboratories

ana developed a new system architecture for fast packet switching

oasedon theslotted ring concept (pbbtished inl€EE TransaciionsonCom

munrcations April 1990) From 1989

1991 ne workea

a prNnci

pa engineerat

Arinconthedesignandanalys

sofatrlgroundcommun cation

networks for the airlines industry He pned Motorola as a princlpa

communlcatton engineer in 1992, and since has been working on the

d bm satellite prolect

His

mterests nclude wireless networks com

mbn catnon systems and digital slgnal processing

100

IEEE

Communications Magazine

April

1993

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