2.call process

7/31/2019 2.Call Process

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Call Process



Introduction

Though basically similar, the call process of mobilesubscribers and that of ordinary fixed subscribers are

different in the following aspects:

• Before a mobile subscriber originates a call, he should

first input the number, ensure that no modification isneeded, and then send the call.

• Before the number is sent out and the call is connected,

there is some additional information that should be

transferred between mobile stations (MS) and thenetwork. Such operations are automatically performed

by the equipment, with no need for user interference,

but it results in a certain delay.



Initialization (1)

• Initialization is a random access process. Itstarts from MS which sends a “channel

request” message on RACH.

• After receiving this message, BTS notifies

BSC, and attaches BTS’s estimation of the

transmission delay (TA) from this MS to BTS

and the cause for the current access.

• BSC will select an idle and dedicated channelSDCCH to notify BTS to activate it according

to the access cause and the current

information.



Initialization (2)• Access causes mainly include: location updating;

response to a paging call; and subscriber serviceapplication, such as a call, sending one short

message.

• After BTS completes the activiation of thedesignated channel, BSC sends the “immediate

assignment”message on AGCH via BTS, including

the description of the SDCCH channel assigned by

BSC to MS, TA, the maximum initializationtransmission power and the access random

reference value.



Initialization (3)

• When MS correctly receives its initialized

assignment, it will, according to channel

description, adjust itself to this channel, set up a

signaling transmission link, and send the firstinitialized message on the dedicated channel,

including subscriber’s identification number (such

as IMSI), cause for the current access, registration,

and authentication. If BSC has no idle channels to

assignment, BSC will send to MS the “immediate

assignment rejected” message.



Subscriber Status (1)

• Mobile stations are normally in one of the

following states: idle, switched off, and

busy. So, the network should handleaccordingly against these 3 states.

1) When the MS powers on, the network

labels it with “attached” flag.




• If it is the first time MS is switched on, in

which case no original LAI can be found in

its SIM card, it will immediately demandaccess to the network, send to MSC the

“location updating request” message, and

notify the GSM system that this is a newsubscriber in this location area.




• MSC will send the “location updating request” to

this subscriber’s HLR according to the H1H2H3

message in IMSI sent by this subscriber. HLR will

record the MSC number that sends this request(M1M2M3), and send back the “location updating

acknowledgment” message to MSC. By now,

MSC will take this MS as activated, and make the

“attached” mark on the IMSI of this subscriber inVLR, and then will send to MS the “location

updating accepted” message.




• The SIM card of MS records this location

area identifier (LAI). If it is not the first

time that MS is switched on, but ratherswitched on after switch-off, in which case

the received LAI is different from the LAI

on the SIM card, then it will alsoimmediately send the “location updating

request” to MSC.




• MSC will first determine whether the original LAI

is within its own service area. If yes, MSC needs

only to modify this subscriber’s LAI in VLR,make “attached” mark on its IMSI, and send LAI

to MS in the “location updating accepted”

message. MS will modify the LAI in the SIM card.

If not, then MSC should send the “locationupdating request” to the corresponding HLR

according to H1H2H3 in this subscriber’s IMSI.




• HLR will record the MSC number that

sends this request, then send back the

“location updating acknowledgment”. MSCmakes the “attached” mark on the

subscriber’s IMSI in VLR, records the LAI,

and sends back to MS the “locationupdating accepted” message. MS then

modifies the LAI in the SIM card.




• In case MS is first switched off and then

switched on, and the received LAI is the

same as that in the SIM card, then MSCneeds only to mark this subscriber with the

“attached” flag.




2) MS switches off, and gets “detached” from

the network • When MS turns off the power supply, itsends the last message to the network,

including the detachment processing request.

After MSC receives the message, it will

notify VLR to make the corresponding

“detached” mark on the IMSI of this MS.




• HLR in this case has got no notice that this

subscriber is already detached from the

network. So, when this subscriber is paged,and when HLR requests MSC/VLR for

MSRN, MSC/VLR will notify HLR that

this subscriber is already detached from thenetwork, so no more paging messages need

to be sent to page this subscriber.




3) MS busy • In this case, the network assigns to MS one

traffic channel to send voice or data, and

mark this subscriber as “busy”. When MS

moves around, it must have the ability to

switch to other channel, and this is the so-called handover.



Periodic Registration (1)

• After MS sends the “IMSI

detached”message to the network, due to

poor radio link quality or severe fading, the

GSM system can not correctly decode

messages, and might still regard this MS as

in the attached status. In another case, MS is

switched on, but it moves outside the servicecoverage area, but the GSM system still

regards MS as in the attached status.



Periodic Registration (2)

• In such cases, when this subscriber is paged,

the system will continusously send paging

messages, thus ineffectively wasting lots of radio resources. To solve this problem, the

GSM system takes the compulsory

registration measure, e.g., requiring MS to

register once every 30 minutes (which can

be determined by the carrier). This is so-

called perioidc registration.



Location Updating (1)

BS

BS

MSC-A

MSC-B

VLR

VLR

HLR MS

MS

(1)

(3)

(2)

(4)

(9)

(5)

(6)

(7)(8)

(10)




(1) MS moves from one area (belonging to the

coverage of MSC-B) to another area

(belonging to the coverage of MSC-A).(2) By detecting the broadcasting information

sent persistently by the base station BS, MS

finds out that the newly received locationarea identification is different from the

currently used location area identification.




(3)(4) MS sends the location updating request

with the message of “I’m here” via this base

station to MSC-A.

(5) MSC-A sends the location updating

message that contains the MSC-A identifier

and MS identification number to HLR (the

authentication or encryption calculation

process will start from here, though not

shown in the diagram).




(6) HLR sends back the response message, including

all the related subscriber data.

(7)(8) Subscriber data registration in the visited VLR.(9) Sending related location updating response

message via the base station to MS (if TMSI is re-

assigned, it is sent together to MS).

(10) Notifying the original VLR to delete subscriber

data related to this MS.



Outgoing call flow from MS to PSTN (1)




(1) Within the service cell, once the mobile

subscriber dials, the mobile station will

request the base station for the randomaccess channel.

(2) The setup process to set up signaling

connection between the mobile station MSand the mobile service switching center

MSC.




(3) Authentication of the mobile station’s

identification number; if encryption is

needed, then it sets the encryption mode andenter the call setup starting phase.

(4) Service channel allocation.




(5) Adopting the No.7 signaling user part ISUP/TUP

to set up a channel from the fixed network

(ISDN/PSTN) to the called subscriber, send

ringing to the called subscriber, and send back thecall connection acknowledgment signal to the

mobile station.

(6)The called subscriber offhooks to reply, in which

case a response (connection) message is sent to themobile station, thus entering the ultimate call

session phase.



Incoming Call Flow from PSTN to MS(1)




(1) Through the No.7 signaling user part

ISUP/TUP to enter MSC(GMSC) and

receive a call from the fixed network (ISDN/PSTN).

(2) GMSC requests HLR for the MSC address

(i.e., MSRN) visited by the related calledmobile subscriber.




(3) HLR requests the visited VLR to assign

MSRN which is assigned and notified by

VLR to HLR in each call.(4) After GMSC obtains MSRN from HLR, it

can re-search for routes to set up connection

to the visited MSC.(5)(6) The visited MSC obtains related

subscriber data from VLR.




• (7)(8) MSC sends paging messages to the

mobile station through all base stations( BS)

in the location area.

• (9)(10) The mobile station of the called

mobile subscriber sends back the paging

response messages, then carries out the same

steps of (1), (2), (3), (4) as shown in theabove outgoing call flow till the mobile

station rings,




then sends back the call connection

acknowledgment signal (omitted in the

diagram) to the calling subscriber.• (11) The mobile subscriber offhooks to

answer, thus the response (connection)

message is sent back to the fixed network tosignal calling and called parties enter final

call session.

2.call process

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