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8/2/2019 9.Call Process
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Call Process
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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.
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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 BTSs 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.
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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
allocationmessage 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.
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Initialization (3)
When MS correctly receives its initialized
allocation, it will, according to channel description,
adjust itself to this channel, set up a signaling
transmission link, and send the first initializedmessage on the dedicated channel, including
subscribers identification number (such as IMSI),
cause for the current access, registration, and
authentication. If BSC has no idle channels to
allocate, BSC will send to MS the immediate
allocation rejected message.
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Subscriber Status (1)
Mobile stations are normally in one of
thefollowing 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.
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Subscriber Status (2)
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.
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Subscriber Status (3)
MSC will send the location updating request to
this subscribers 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.
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Subscriber Status (4)
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.
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Subscriber Status (5)
MSC will first determine whether the original LAI
is within its own service area. If yes, MSC needs
only to modify this subscribers 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 subscribers IMSI.
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Subscriber Status (6)
HLR will record the MSC number that
sends this request, then send back the
location updating acknowledgment. MSCmakes the attached mark on the
subscribers IMSI in VLR, records the LAI,
and sends back to MS the locationupdating accepted message. MS then
modifies the LAI in the SIM card.
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Subscriber Status (7)
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.
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Subscriber Status (8)
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.
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Subscriber Status (9)
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.
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Subscriber Status (10)
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.
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Periodic Registration (1)
After MS sends the IMSI
detachedmessage 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.
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Periodic Registration (2)
In such cases, when this subscriber is paged,
the system will continusously send paging
messages, thus ineffectively wasting lots ofradio 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.
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Location Updating (1)
BS
BS
MSC-A
MSC-B
VLR
VLR
HLRMS
MS
(1)
(3)
(2)
(4)
(9)
(5)
(6)
(7)(8)
(10)
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Location Updating (2)
(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.
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Location Updating (3)
(3)(4) MS sends the location updating request
with the message of Im 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).
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Location Updating (4)
(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 (ifTMSI is re-
assigned, it is sent together to MS).
(10) Notifying the original VLR to delete subscriber
data related to this MS.
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Outgoing call flow from MS to PSTN (1)
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Outgoing call flow from MS to PSTN (2)
(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.
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Outgoing call flow from MS to PSTN (3)
(3) Authentication of the mobile stations
identification number; if encryption is
needed, then it sets the encryption mode andenter the call setup starting phase.
(4) Service channel allocation.
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Outgoing call flow from MS to PSTN (4)
(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.
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Incoming Call Flow from PSTN to MS(1)
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Incoming Call Flow from PSTN to MS(2)
(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.
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Incoming Call Flow from PSTN to MS(3)
(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.
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Incoming Call Flow from PSTN to MS(4)
(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,
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Incoming Call Flow from PSTN to MS(5)
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.
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PSTN GMSC MSC/VLR
HLR
MS1 6
4 32 5
1.MSISDN 2. MSISDN 3. IMSI 4. MSRN
5. MSRN 6. MSRN
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6
34
52
9
78101
HLR/AUC
MSC1/VLR2 MSC2/VLR2
MS1 MS2
MS-MS Call