GSM-Call-Flows (1).ppt

8/14/2019 GSM-Call-Flows (1).ppt

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GSM Call Flow



2

GSM Call Flow Call processing means all steps which set up,

maintain, and then end a call. According to the Telecom Glossary put out by

the American ational Standard !or Telecommunications, call processing means" 1. The se#uence o! operations per!ormed by a

switching system !rom the acceptance o! anincoming call through the !inal disposition o!

the call. 2. The end$to$end se#uence o! operationsper!ormed by a networ% !rom the instant a callattempt is initiated until the instant the callrelease is completed. . . .



&

GSM Call Flow contd.., The !irst part to mobile call processing

is initiali'ation. (t)s what happens when

you !irst turn on your phone. *ou get a connection to a nearby cell

site, then the cellular networ% chec%syour account.

(! you ha+e a +alid telephone numberand your account is good then your callproceeds. et)s ta%e this step by step.



-

GSM Call Flow contd.., *ou turn on your phone. Assume that

you)re in your home location.

First o! all a connection is to beestablished with near by TS. (t is notpossible to ma%e a call unless yourmobile has a lin% to a cell site.

So a connection to cellular system is tobe established which means that a!re#uency is needed to transmit on.



/

GSM Call Flow contd.., So the mobile tries to !ind out broadcast

channels. Again, the CC0 is not a dedicated radio

!re#uency. (t is rather a channel within the bitstream carried by any o! the !re#uencies in acell.

A base station)s roadcast Control Channel

continuously sends out identi!ying in!ormationabout its cell site and helps the mobile to getsome in!ormation about the cell site.

For the initial period mobile acts as a recei+erchec%ing !or a signal !rom any base station

with in the range.



GSM Call Flow contd.., The mobile scans the a+ailable

!re#uencies and measures the recei+edle+el on each channel.

Finally the GSM system decides whichcell has to handle the mobile stationwhich is usually the cell site deli+ering

the highest signal strength to themobile. As a net step, the mobile recei+es

corresponding bursts on FCC0 and SC0

and synchroni'es with the cell site.



3

ocation Management (n+ol+es two basic operations

4aging Search by system to trac% the mobile MSC broadcasts message Target replies in the uplin% channel

5pdate Mobile sends update message on the

uplin% channel when it changes itslocation



6

Location management in GSM ishandled by Mobility Management layer

in the GSM protocol architecture. A powered-on mobile is informed of an

incoming call by a paging message.

If the paging is done exactly for onecell, then networ re!uires updatingeach time it changes the cell whichincrease the burden on the system



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ecause o! that a compromisesolution used in GSM is to group

cells into “Location areas” 5pdating messages are re#uired

when the mobile station mo+esbetween location areas.

Also mobile stations are paged inthe cells o! their current locationarea.



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MS StatesAn MS can be in one o! the !ollowing states"

Detached: MS is powered o!!

Attached: MS power is on An attached MS can be"

Idle: MS has no dedicated channel allocated andlistens to CC0 and 4C0

Active: MS has a dedicated connection to thenetwor%

Changing !rom idle to acti+e mode can be a result o!location updating, call setup, short message trans!eretc.,



11

Location updating 9hen mobile powered ‘ON’

4er!orms update indicating it:s

IMSI( International Mobile SubscriptionId )

The abo+e procedure called ; IMSIAttach Procedure ;

9hen mo+es to new ocation Area or a

<i!!erent 4M 5pdate message sent to new MSC/VLR (! MS authori'ed in the new MSC/VLR

then subscriber:s 0= updates the current location sends a message to the old MSC/VLR to

:



12

9hen a mobile station is power o!! itper!orms an (MS( detach procedure in

order to tell the networ% that it is nolonger a+ailable.

Rei!tratio":

This is the process in which an MSin!orms a networ% that it is attached.



1&

(MS( Attach 9hen an MS is switched on, the (MS(

attach procedure is eecuted. Thisin+ol+es the !ollowing steps"

1. The MS sends an (MS( attachmessage to the networ% indicating thatit has changed state to idle.

2. The >= determines whether there isa record !or the subscriber alreadypresent. (! not, the >= contacts thesubscriber:s 0= !or a copy o! the

subscription in!ormation.



1-

(MS( Attach contd.., &. The >= updates the MS status

to idle.

-. Ac%nowledgement is sent to theMS.



1/

Mobile ?riginated Call

@M?C1. The MS uses =AC0 to as% !or a signaling

channel.

2. The SC allocates a signaling channel, using

AGC0.&. 9hen the channel is allocated, it sends its

(MS( and >= is signed as busy.

-. The MS sends a call set$up re#uest +ia S<CC0

to the MSCB>=. ?+er S<CC0 all signalingpreceding a call ta%es place. This includes" Mar%ing the MS as ;acti+e in the >= The authentication procedure Start ciphering

D#uipment identi!ication



1

Sending the called party:s number to thenetwor%

Chec%ing i! the subscriber has the ser+ice

;arring o! outgoing calls acti+ated/. The MSCB>= instructs the SC to

allocate an idle TC0. The TS and MSare told to tune to the TC0.

. The MSCB>= !orwards the called partynumber to an echange in the 4ST,which establishes a connection to thesubscriber.

3. (! the called subscriber answers, the

connection is established.



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M?C @Call set$up MS to4ST



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Mobile Terminated Call@MTC

The maEor di!!erence between an MTCand M?C is that in a call to an MS the

eact location o! the mobile subscriberis un%nown.

There!ore, the MS must be located usingpaging be!ore a connection can be

established. elow is the description o! the call set$

up procedure !or a call !rom a 4ST

subscriber to a mobile subscriber.



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MTC contd..,

1. The 4ST subscriber %eys in the MS:stelephone number @MS(S<.

The MS(S< is analy'ed in the 4ST, which

identi!ies that this is a call to a mobile networ%subscriber.

A connection is established to the MS:s homeGMSC.

ow the call has entered the dialled party:snetwor% 2. The GMSC analy'es the MS(S< and #ueries

the 0= !or in!ormation about how to route thecall to the ser+ing MSCB>=.



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MTC contd..,

&. The 0= translates MS(S< into (MS(,and determines which MSCB>= is

currently ser+ing the MS. The 0= also chec%s i! the ser+ice, ;Call

!orwarding to Cnumber is acti+ated, i!so, the call is rerouted by the GMSC to

that number. -. The 0= re#uests an MS= !rom the

ser+ing MSCB>=.



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MTC contd..,

/. The MSCB>= returns an MS= +ia0= to the GMSC.

. The GMSC analyses the MS= androutes the call to the MSCB>=.

3. The MSCB>= %nows which A the MSis located in. A paging message is sentto the corresponding SC.



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MTC contd..,

6. The SC:s distribute the pagingmessage to the TS in the desired A.

7. 9hen the MS detects the pagingmessage, it sends a re#uest on =AC0!or a S<CC0.

18. The SC pro+ides a S<CC0, usingAGC0.

11. S<CC0 is used !or the call set$up

procedures.



2&

MTC contd..,

?+er S<CC0 all signaling preceding acall ta%es place. This includes"

Mar%ing the MS as ;acti+e in the >= The authentication procedure

Start ciphering

D#uipment identi!ication 12. The MSCB>= instructs the SC to

allocate a TC0.



2-

MTC contd..,

The TS and MS are told to tune to the TC0. The mobile phone rings. (! the

subscriber answers, the connection isestablished.



2/

Call to MS !rom 4ST@MTC



2

Mobile ?riginated Call@M?C

PSTN GMSC

VLR

BSS

MSC

MS 1

2

6 5

3 4

9

10

7 8

1, 2" connectionre#uest

&, -" securitychec%

/$6" chec%resources @!reecircuit

7$18" set up call

Mobile Terminated Call



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Mobile Terminated Call@MTC

PSTNcalling

stationGMSC

HLR VLR

BSSBSSBSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

13

1610 10

11 11 11

14 15

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1" calling a GSM subscriber

2" !orwarding call to GMSC

&" signal call setup to 0=

-, /" re#uest MS= !rom >=" !orward responsible MSC to

GMSC

3" !orward call to current MSC

6, 7" get current status o! MS

18, 11" paging o! MS

12, 1&" MS answers

1-, 1/" security chec%s

1, 13" set up connection



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MTCBM?C

BTSMS

paging request

channel request

immediate assignment

paging response

authentication requestauthentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acno!ledge

data"speech e#change

BTSMS

channel request

immediate assignment

ser$ice request

authentication requestauthentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acno!ledge

data"speech e#change

MTC MOC



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Se#uence o! ?perations !or an(ncoming Call

PSTN/ISDN

GMSC

HLR

MSC

VLR

BS

1

2

3

4

6 7

8

9

10

5

MSC/VLR whereGMSC/HLR o MS!"ho#e e$wor% MS &" '(rre$)* )o'+$e,

-&.(re 6

Se(e'e o er+$&o" or + I'o#&. C))

MS



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Call !low when MS is inroaming



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illing 4rocedures

GSM uses Time <i+ision Multiple Access@T<MA technology to di+ide allchannels to eight time slots !or carrying

data. 6 time slots pro+ides 6 callssimultaneously. ut one slot isdedicated to controlling.

At the end o! each call MSC produces a

C<= and they are stored in a !ile andthis !ile is sent to illing Gateway@G9.

G9 con+erts the data intostandarti'ed !ormat and route them to

the SCS.



&2

illing 4rocedures @Cont:d

SCs produces the bill related to thein!ormation stored in C<=. These in!ormationsare

(! the target number is a constant or a mobilephone

(! it is a mobile phone, its 'one location The duration o! the call

(! there is a special campaign applied to targetnumber

y considering all these things, SCs preparesa bill.



&&

=oaming in the etwor%

Cha"i" Cell! #ithi" a" LA: MS:s are constantly mo+ing around in the

cellular networ%. The MS location in!ormation stored in the >=

is the A.

(! an MS changes cells within a A, the networ%

is not updated.



&-

The MS %nows that the new cell belongsto the same A by listening to the CC0in the new cell.

The CC0 broadcasts the cell:s A(. The MS compares the last A( recei+ed

with the new A(.

(! they are the same, it means that theMS has not changed A)s and does notneed to in!orm the networ%.



&/

ocation 5pdating @SameMSCB>=

(! an MS detects a change in A( onthe CC0, it in!orms the networ%.

9hen the MS sends the ocation5pdating message, the MSCB>=determines whether it is an MS,

which is already registered, or i! itis an MS +isiting !rom anotherMSCB>=.



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ocation 5pdating @sameMSCB>=

MS TS SC MSCB>=

ocation 5pdate

=e#uest

Authentication

Ciphering

ocation 5pdate Accept







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ocation 5pdating @ew MSCB>=contd..,

1. Authentication is per!ormed when it is success!ul, the>= chec%s its database to determine whether or not ithas a record !or this MS$subscription.

2. 9hen the >= !inds no record !or the MS, it sends a

re#uest to the subscriber:s 0= !or a copy o! the MS$subscription. &. The 0= passes the in!ormation to the >= and

updates its location in!ormation !or the subscriber. The0= instructs the old >= to delete the in!ormation ithas about the MS subscription.

-. The >= stores its subscription in!ormation !or the MSincluding the latest location and status @idle. The >=sends ac%nowledgement to the MS.



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ocation 5pdate

MS NewMSC

NewVLR

HLR 1),VLR

Lo'+$&o 42,+$e

Re/(e"$ 42,+$e Lo'+$&o

3re+ 42,+$e Lo'+$&o

C+'e) Lo'+$&o

C+'e) Lo'+$&o

3'% I"er$ S("'r&er

D+$+

I"er$ S("'r&er

D+$+ 3'%

42,+$e Lo'+$&o3'%

42,+$e Lo'+$&o

3re+ 3'%

Lo'+$&o 42,+$e

3''e2$

Lo'+$&o 42,+$e 12er+$&o

-&.(re 5



-1

ocation 5pdating, type4eriodic =egistration

4eriodic registration is a !eature which!orces MS:s to send a registration

message to the networ% at prede!inedinter+als.

(! an MS should miss such a registration,the networ% will mar% the MS as

detached. This may occur i! an MS is out o! the

area o! co+erage and ensures that

needless paging is not per!ormed.





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<etaching !rom the etwor%

IMSI Detach: (MS( detach enables the MS to indicate

to the networ% that it is switched o!!. At power o!!, the MS sends an (MS(detach message to the networ%.

?n reception, the 0= mar%s the

corresponding (MS( as detached. o ac%nowledgement is sent to the MS.



--

(mplicit <etach

(! the MS sends an (MS( detach message to thesystem and the radio lin% #uality is poor, thesystem might not be able to decode the

in!ormation. ecause no ac%nowledgment is sent to the MS,

no !urther attempt is made. (n this case, the system still regards the MS as

attached. (! periodic registration is in use, the system will

soon determine that the MS is detached. The >= then per!orms an implicit detach,

mar%ing the MS as detached.



-/

MS 4urging

MS purging is used to in!orm the0= that the >= is about to

remo+e a subscriber record !romthe >=.



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ocating

An MS continuously measures signalstrength and #uality on it:s own cell andsignal strength on the CC0 carriers o!

the neighboring cells. The measurements are carried out on

the downlin% while MS is in acti+e mode. The measurement results are sent to

the TS on SACC0 at regular inter+als. The ser+ing TS measures signal

strength and #uality on the uplin%.



-3

ocating contd..,

The measurements !rom the TS andMS are sent to the SC in the !orm o!measurement reports.

ased on these reports, the SCdecides i! a hando+er is necessary andto which cell.

This is called locating. As soon as a neighboring cell is

considered to be better than the ser+ingcell, a hando+er is attempted.



-6



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Types o! 0ando!! contd..,

There are se+eral types o! hando+er,including"

(ntra$TS hando+er 0ando+er between cells controlled by

the same SC @(nter TSB(ntra SC 0ando+er between cells controlled by

di!!erent SC:s, but the same MSCB>=

@(ntra MSC 0ando+er between cells controlled by

di!!erent MSCB>=s @(nter MSC



/8

(ntra$Cell 0ando+er

A special type o! hando+er is the intra$cell hando+er.

(t is per!ormed when the SC considers

the #uality o! the connection too low,but recei+es no indication !rom themeasurements that another cell wouldbe better.

(n that case the SC identi!ies anotherchannel in the same cell which mayo!!er a better #uality, and the MS isordered to retune to it.



/1

0ando+er between Cellscontrolled by Same SC

9hen per!orming a hando+er betweentwo cells controlled by the same SC,

the MSCB>= is not in+ol+ed. 0owe+er, the MSCB>= will be in!ormed

when a hando+er has ta%en place.



/2

(ntra SC 0ando+er

BTSold

%&measurement

result

BSC

'&Lin esta(lishment

MS

%&measurement

report

H) decision

BTSne!

%&ch& acti$ation

%& ch& acti$ation ac

*&H) command

*&H) command

+&H) complete

+&H) complete

,&Release TCH

-& H) .ccess



/&

(ntra SC 0ando!! contd..,

1. The SC orders the new TS toacti+ate a TC0.

2. The SC sends a message to the MS,+ia the old TS, containing in!ormationabout the !re#uency and time slot tochange to and also the output power touse. This in!ormation is sent to the MSusing FACC0.

&. The MS tunes to the new !re#uency,and transmits hando+er accessin!ormation to new TS.



/-

(ntra SC 0ando!! contd..,

-. 9hen the new TS detects thehando+er bursts, it sends in!ormation

about TA. This is also sent +ia FACC0. /. The MS sends a 0ando+er Complete

message to the SC +ia the new TS.

. The SC tells the old TS to release

the old TC0.



//

(ntra MSC 0ando+er

9hen another SC is in+ol+ed in ahando+er, the MSCB>= must also be

in+ol+ed to establish the connectionbetween the two SC:s.





( t MSC 0 d



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(ntra MSC 0ando+ercontd..,

/. MS tunes to the new !re#uency andtransmits hando+er access bursts in thecorrect time slot.

. The new TS sends in!ormation about TA.

3. MS sends a 0ando+er Completemessage to MSC +ia the new SC.

6. MSC sends the old SC an order torelease the old TC0. 7. The old SC tells the old TS to

release the TC0.



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GSM-Call-Flows (1).ppt