2 Network Architecture.doc

8/11/2019 2 Network Architecture.doc

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Network Architecture

GSM Network ArchitectureA GSM network is made up of multiple components and interfaces that facilitate sending andreceiving of signaling and traffic messages. It is a collection of transceivers, controllers,switches, routers, and registers.

A Public Land Mobile etwork !PLM " is a network that is owned and operated b# one GSMservice provider or administration, which includes all of the components and e$uipment asdescribed below. %or e&ample, all of the e$uipment and network resources that is owned andoperated b# 'ingular is considered a PLM .

Mobile Station (MS)(he Mobile Station !MS" is made up of two components)


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Mobile Equipment (ME) (his refers to the ph#sical phone itself. (he phone must be able tooperate on a GSM network. *lder phones operated on a single band onl#. ewer phones aredual+band, triple+band, and even $uad+band capable. A $uad+band phone has the technicalcapabilit# to operate on an# GSM network worldwide.

ach phone is uni$uel# identified b# the International Mobile Equipment Identity (IMEI) number. (his number is burned into the phone b# the manufacturer. (he IM I can usuall# be

found b# removing the batter# of the phone and reading the panel in the batter# well.

It is possible to change the IM I on a phone to reflect a different IM I. (his is known as IM Ispoofing or IM I cloning. (his is usuall# done on stolen phones. (he average user does not havethe technical abilit# to change a phone-s IM I.

Subscriber Identity Module (SIM) + (he SIM is a small smart card that is inserted into the phone and carries information specific to the subscriber, such as IMSI, TMSI, Ki !used forencr#ption", Service Provider ame !SP ", and Local Area Identity !LAI". (he SIM can alsostore phone numbers !MSIS " dialed and received, the Kc !used for encr#ption", phone books,and data for other applications. A SIM card can be removed from one phone, inserted intoanother GSM capable phone and the subscriber will get the same service as alwa#s.

ach SIM card is protected b# a /+digit Personal Identification umber !PI ". In order to unlock a card, the user must enter the PI . If a PI is entered incorrectl# three times in a row, the card

blocks itself and can not be used. It can onl# be unblocked with an 0+digit Personal 1nblocking2e# !P12", which is also stored on the SIM card.

Base Transceiver Station (BTS)Base ranscei!er Station (B S) + (he 3(S is the Mobile Station-s access point to the network.It is responsible for carr#ing out radio communications between the network and the MS. It


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handles speech encoding, encr#ption, multiple&ing ! TDMA", and modulation4demodulation ofthe radio signals. It is also capable of fre$uenc# hopping. A 3(S will have between 5 and 56(ransceivers !(78", depending on the geograph# and user demand of an area. ach (78represents one A7%' .

*ne 3(S usuall# covers a single 59: degree sector of an area. 1suall# a tower with ; 3(Ss willaccommodate all ;6: degrees around the tower.


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Um Interface

Base Station Controller (BSC)

Base Station "ontroller (BS") + (he 3S' controls multiple 3(Ss. It handles allocation of radiochannels, fre$uenc# administration, power and signal measurements from the MS, and handoversfrom one 3(S to another !if both 3(Ss are controlled b# the same 3S'". A 3S' also functionsas a >funneler>. It reduces the number of connections to the Mobile Switching Center !MS'" andallows for higher capacit# connections to the MS'.

A 3S' m# be collocated with a 3(S or it ma# be geographicall# separate. It ma# even becollocated with the Mobile Switching 'enter !MS'".

Base Station "ontroller


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The interface between the BTS and the BSC is known as the AbisInterface

Abis Interface

The Base Transceiver Station (BTS) and the Base Station Controller(BSC) together make up the Base Station System (BSS).

Base Station System


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Mobile Switc in! Center (MSC)Mobile Switchin# "enter (MS") + (he MS' is the heart of the GSM network. It handles callrouting, call setup, and basic switching functions. An MS' handles multiple 3S's and alsointerfaces with other MS'-s and registers. It also handles inter+3S' handoffs as well ascoordinates with other MS'-s for inter+MS' handoffs.

Mobile Switc in! Center

(he interface between the 3S' and the MS' is known as the A Interface

A Interface


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"ateway Mobile Switc in! Center ("MSC)(here is another important t#pe of MS', called a Gatewa# Mobile Switching 'enter !GMS'".(he GMS' functions as a gatewa# between two networks. If a mobile subscriber wants to placea call to a regular land line, then the call would have to go through a GMS' in order to switch tothe Public Switched (elephone etwork !PS( ".

"ateway Mobile Switc in! Center

%or e&ample, if a subscriber on the 'ingular network wants to call a subscriber on a (+Mobilenetwork, the call would have to go through a GMS'.


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Connections Between Two #etwor$s

(he interface between two Mobile Switching 'enters !MS'" is called the E Interface

% Interface

&ome 'ocation e!ister (&' )$ome %ocation &e#ister ($%&) + (he


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isitor 'ocation e!ister

%ocation Area "ode (%A")

A LA' is a fi&ed+length code !two octets" that identifies a location area within the network. ach

Location Area is serviced b# a =L7, so we can think of a Location Area 'ode !LA'" beingassigned to a =L7.

%ocation Area Identity (%AI)An LAI is a globall# uni$ue number that identifies the countr#, network provider, and LA' ofan# given Location Area, which coincides with a =L7. It is composed of the Mobile 'ountr#'ode !M''", the Mobile etwork 'ode !M '", and the Location Area 'ode !LA'". (he M''and the M ' are the same numbers used when forming the IMSI.

Location Area Identit# !LAI"

"ell Global Identi ication ("GI)(he 'GI is a number that uni$uel# identifies a specific cell within its location area, network, andcountr#. (he 'GI is composed of the M'', M ', LAI, and 'ell Identit# !'I"


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Cell "lobal I*entity

(he =L7 also has one other ver# important function) the assignment of a (emporar# MobileSubscriber Identit# !(MSI". (MSIs are assigned b# the =L7 to a MS as it comes into itsLocation Area. (MSIs are onl# allocated when in cipher mode.

(he interface between the MS' and the =L7 is known as the B Interface and the interface between the =L7 and the


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7easons include the IM I being listed as stolen or cloned or if the handset is malfunctioning ordoesnt have the technical capabilities to operate on the network.

(he gra# list is a list of IM Is that are to be monitored for suspicious activit#. (his could includehandsets that are behaving oddl# or not performing as the network e&pects it to.

(he white list is an unpopulated list. (hat means if an IM I is not on the black list or on the gra#

list, then it is considered good and is >on the white list>.

(he interface between the MS' and the I7 is called the ! Interface .

%+,i-ment I*entity e!ister

A,t entication Center (A,c)Authentication "enter (Au") + (he Au' is responsible for generating the necessar# cr#ptovariables for authentication and encr#ption on the network. (hese variables are the 7A ,S7 S, and 2c. (he Auc also stores the 2i for each IMSI on the network. Although it is notre$uired, the Auc is normall# ph#sicall# collocated with the


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A,t entication Center

(here is one last interface that we haven-t discussed. (he interface between the

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