1 introduction to gsm.doc

7/27/2019 1 Introduction to GSM.doc

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Introduction to GSM

Introduction

GSM is an acronym that stands for Global System for Mobile Communications. The original

french acronym stands for Groupe Spcial Mobile. It was originally developed in 1984 as astandard for a mobile telephone system that cold be sed across !rope.

GSM is now an international standard for mobile service. It offers high mobility. Sbscribers can

easily roam worldwide and access any GSM networ".

GSM is a digital celllar networ". #t the time the standard was developed it offered mch higher

capacity than the then$crrent analog systems. It also allowed for a more optimal allocation of

the radio spectrm% which therefore allows for a larger nmber of sbscribers.

GSM offers a nmber of services inclding voice commnications% Short Message Service

&SMS'% fa(% voice mail% and other spplemental services sch as call forwarding and caller I).

*rrently there are several bands in se in GSM. 4+, M-% 8+, M-/% 9,, M-% 18,, M-%

and 19,, M- are the most common ones.

Some bands also haveExtended GSM (EGSM)bands added to them% increasing the amont of

spectrm available for each band.

GSM ma"es se of 0reency )ivision Mltiple #ccess &0)M#' and Time )ivision Mltiple

#ccess &T)M#'.

Uplinks/Downlinks & Reverse Forward

GSM allows for se of dple( operation. !ach band has a freency range for the plin" &cell

phone to tower' and a separate range for the downlin" &tower to the cell phone'. The plin" is

also "nown as theReverseand the downlin" is also "nown as theForward. In this ttorial% I will

se the terms plin" and downlin".


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Uplink and Downlink

Frequency Division Multiple Access (FDMA)

GSM divides the allocated spectrm for each band p into idividal carrier freencies. *arrier

separation is 2,, "h. This is the 0)M# aspect of GSM.

Absolute Radio Frequency !annel "u#ber (ARF")

The #30* is a nmber that describes a pair of freencies% one plin" and one downlin". The

plin" and downlin" freencies each have a bandwidth of 2,, "-. The plin" and downlin"

have a specific offsetthat varies for each band. The offset is the freency separation of the

plin" from the downlin". !very time the #30* increases% the plin" will increase by 2,, "h

and the downlin" also increases by 2,, "h.

#n #30* has an allowed bandwidth of 2,, "-% which corresponds e(actly to the carrier

separation. The freency of the #30* refers to its center freency. If an #30* has a

freency of 914.8, M-% then it occpies the freency space from 914.5 M- to 914.9 M-

&2,, "- total'. 3emember that this is the allocatedbandwidth. 6ecase of the natre of themodlation method &GMS7' and data rate sed in GSM% the actal physical bandwidth will be

abot 1+.4 "-. The nsed bandwidth for each #30* acts as a bffer between other #30*

to avoid interference.

ote: #lthogh GSM operates in dple( &separate freencies for transmit and receive'% the mobile station does not

transmit and receive at the same time. # switch is sed to toggle the antenna between the transmitter and receiver.


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The following table smmaries the freency ranges% offsets% and #30*s for several poplar

bands.

$%M ands

The following diagram illstrates an #30* with paired plin" and downlin" freencies for

#30* 1 in the GSM 9,, band.

alculatin' Uplink/Downlink Frequencies

The following is a way to calclate the plin" and downlin" freencies for some of the bands%

given the band% the #30*% and the offset.

$%M

;p < 89,., = * , .2'

)own < ;p = 4+.,

e(ample:

Given the #30* 52% and we "now the offset is 4+M- for the GSM9,, band:

;p < 89,., = &52 ,.2'

;p < 89,., = &14.4'

;p < 9,4.4, M-


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)own < ;p = >ffset

)own < 9,4.4, = 4+.,

)own < 949.4, M-

The plin"?downlin" pair for GSM9,, #30*52 is 9,4.4,?949.4, &M-'

ote that channel , &89,., M-' is sed as a gard band in GSM$9,, and is not sable for

traffic.

-ere are the formlas for !GSM9,,% )*S18,,% and @*S19,,:

*$%M

;p < 89,., = * ,.2' * ,$124'

;p < 89,., = &*$1,24',.2' * 95+ $ 1,2'

)own < ;p = 4+.,

otice that the extendedportion of the !$GSM band actally occrs below the reglar band

in freency. #30* 954$1,2 occr from 88,., M- to 889.8, M-. The reglar GSM &,$

124' occrs from 89,., M- to 914.8 M-.

#30* 954 &88,., M-/' is sed as a gard band and so is not sable for traffic. #30* ,

&89,., M-'% which is sed as a gard band in reglar GSM$9,,% is available for se as a traffic

channel in !$GSM since this channel happens to be in the middle of this band and a gard

channel is not necessary there.

D%+,

;p < 151,., = &* $ +11' .2'

)own < ;p = 9+.,

-%+

;p < 18+,., = &* $ +12' .2'

)own < ;p = 8,.,


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$%M ARF" +

Numbering System (Identifiers)

Mobile %ubscriber I%D" (M%I%D")

The MSIS) is the sbscriberAs phone nmber. It is the nmber that another person wold

dial in order to reach the sbscriber. The MSIS) is composed of three parts:

*ontry *ode &**'

ational )estination *ode &)*'

Sbscriber mber &S'

M%I%D"

Country Code (CC)$ This is the international dialing code for whichever contry the MS is

registered to.

National Destination Code (NDC)$ In GSM% an )* is assigned to each @BM. In some

cases% a @BM may need more than one )*.


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Subscriber Number (SN)$ This is a nmber assigned to the sbscriber by the service provider

&@BM'.

The combination of the )* and the S is "nown as the ational &significant' Mobile mber.

This nmber identifies a sbscriber within the GSM @BM.

"ational (si'ni.cant) Mobile "u#ber

International Mobile %ubscriber Identity (IM%I)

The IMSI is how the sbscriber is identified to the networ". It niely identifies the

sbscriber within the GSM global networ". The IMSI is brned into the SIM card and is paired

with an MSIS).

The IMSI is composed of three parts:

Mobile *ontry *ode &M**'

Mobile etwor" *ode &M*'

Mobile Sbscriber Identification mber &MSI'

IM%I

Mobile Country Code (MCC)$ This nmber identifies which contry the sbscriberAs networ"

is in. It has digits.

Mobile Network Code (MNC)$ This nmber identifies the home GSM @BM of the sbscriber&*inglar% T$Mobile% etc.'. It has 2 or digits. Some networ"s may have more than one M*

allocated to it. The crrent version of the IT; allocation for the M** can be fond here

Mobile Subscriber Identification Number (MSIN)$ This nmber niely identifies a ser

within the home GSM networ".
http://www.itu.int/itudoc/itu-t/ob-lists/icc/e212_685.htmlhttp://www.itu.int/itudoc/itu-t/ob-lists/icc/e212_685.html


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e#porary Mobile %ubscriber Identity (M%I)

The TMSI is a 2$bit nmber &4 octets' that is temporarily assigned to a MS and is sed on

the networ" in lie of the IMSI. It is designed to protect the privacy of the sbscriber and prevent

the IMSI from being discovered. The CB3 will assign the TMSI to a MS when it registers in that

Bocation #rea. The networ" may also reire the CB3 to assign a new TMSI to a MS

periodically or even every time it completes a transaction. The TMSI is stored on the SIM card.

The TMSI is always assigned when in cipher mode. &traffic is encrypted'. *lic" hereto read

more abot athentication and encryption.

International Mobile *quip#ent Identity (IM*I)

The IM!I niely identifies the Mobile !ipment &the phone itself'. It is essentially a

serial nmber that is brned into the phone by the manfactrer. The crrent format for the IM!I

is composed of three parts:

Type #llocation *ode &T#*' $ 8 digits

Serial mber &S3' $ D digits

*hec" )igit &S@' $ 1 digit

IM*I

Type llocation Code (TC)$ This nmber niely identifies the model of a wireless device.It is composed of 8 digits. ;nder the new system &as of #pril 2,,4'% the first two digits of a T#*

are theReporting Body dentifier&36I' of the GSM approved grop that allocated this model

type.

Serial Number (SN!)$ This nmber is a manfactrer defined serial nmber for the model of

wireless device.

C"eck DigitThis nmber is a chec" digit "nown as a!u"n #"ec$ %igit. It is sed to ensre that

the first 14 digits were transmitted?received correctly.

>n many devices the IM!I nmber can be retrieved by entering E,DE
http://www.gsmfordummies.com/encryption/encryption.shtmlhttp://www.gsmfordummies.com/encryption/encryption.shtml


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For#er IM*I %tructure

@rior to #pril% 2,,4 the IM!I had a different strctre:

Type #llocation *ode &T#*' $ D digits

0inal #ssembly *ode &0#*' $ 2 digits

Serial mber &S3' $ D digits

*hec" )igit $ 1 digit

For#er IM*I %tructure

#s of #pril 2,,4% the se of the 0#* was no longer reired. The crrent practice is for the T#*for a new model to get approved by national reglating bodies% "nown as theReporting Body

dentifier.

International Mobile *quip#ent Identity/%o0tware 1ersion

(IM*I%1)

This is a newer form of the IM!I that omits the Spare digit at the end and adds a 2$digit

Software &ersion 'uber (S&')at the end. The SC identifies the software version that the

wireless device is sing. This reslts in a 1D$digit IM!I.

Type #llocation *ode &T#*' $ 8 digits

Serial mber &S3' $ D digits Software Cersion mber &SC' $ 2 digits

IM*I%1

Mobile %tation Roa#in' "u#ber (M%R")

The MS3 is a nmber that is sed to rote calls to the Mobile switching *enter &MS*' thatis associated with the CB3 that the MS is registered with.

@rocess: Fhen someone places a call to a MS the networ" will ery the -ome Bocation

3egister &-B3' to find ot which CB3 the MS is crrently registered with. The -B3 will send a

reest to the CB3 of the MS indicating a call setp. The CB3 generates the MS3 for the call

and sends it bac" to the -B3. The networ" then passes the MS3 to the MS* that is originating


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the call. The originating MS* then contacts the desired MSAs MS* and ses the MS3 to setp

the call.

The format of the MS3 is similar to the MSIS). It will start with the ** and be followed by

the )*. The remaining digits are allocated however the networ" wants to. Some form of

seential nmbering is often employed. It is important to remember that the MS3 is assigned

to a calland not to a MS itself. The MS3 is essentially a reference nmber assigned to a call

that is so that the MS*?CB3 "nows which MS the call is for and to enable call setp. 0or that

reason% the MS3 is sometimes dbbed Mobile StationRoutingnmber.

2inks

The strctres of all of the nmbers mentioned here are covered in detail in G@@ specification

2.,,

This website is an e(cellent resorce for loo"ing p and decoding MSIS)s% IMSIs% and IM!Is.

http:??www.nmberingplans.com?
http://www.3gpp.org/ftp/Specs/html-info/23003.htmhttp://www.3gpp.org/ftp/Specs/html-info/23003.htmhttp://www.numberingplans.com/http://www.3gpp.org/ftp/Specs/html-info/23003.htmhttp://www.3gpp.org/ftp/Specs/html-info/23003.htmhttp://www.numberingplans.com/

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