Call Process
Introduction
Though basically similar, the call process of mobile
subscribers 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 is
needed, 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 the
network. 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. It
starts 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 channel
SDCCH 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 service
application, such as a call, sending one short
message.
• After BTS completes the activiation of the
designated channel, BSC sends the “immediate
allocation”message on AGCH via BTS, including
the description of the SDCCH channel assigned by
BSC to MS, TA, the maximum initialization
transmission power and the access random
reference value.
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 initialized
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
allocate, BSC will send to MS the “immediate
allocation rejected” message.
Subscriber Status (1)
• Mobile stations are normally in one of
thefollowing states: idle, switched off, and
busy. So, the network should handle
accordingly against these 3 states.
1) When the MS powers on, the network
labels it with “attached” flag.
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 demand
access to the network, send to MSC the
“location updating request” message, and
notify the GSM system that this is a new
subscriber in this location area.
Subscriber Status (3)
• 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 in
VLR, and then will send to MS the “location
updating accepted” message.
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 rather
switched on after switch-off, in which case
the received LAI is different from the LAI
on the SIM card, then it will also
immediately send the “location updating
request” to MSC.
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 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 “location
updating request” to the corresponding HLR
according to H1H2H3 in this subscriber’s IMSI.
Subscriber Status (6)
• HLR will record the MSC number that
sends this request, then send back the
“location updating acknowledgment”. MSC
makes the “attached” mark on the
subscriber’s IMSI in VLR, records the LAI,
and sends back to MS the “location
updating accepted” message. MS then
modifies the LAI in the SIM card.
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 MSC
needs only to mark this subscriber with the
“attached” flag.
Subscriber Status (8)
2) MS switches off, and gets “detached” from
the network
• When MS turns off the power supply, it
sends 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.
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 the
network, so no more paging messages need
to be sent to page this subscriber.
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 socalled 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 service
coverage 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 socalled perioidc registration.
Location Updating (1)
(4)
(5)
(3)
MS
BS
(2)
MSC-A
(6)
(9)
(8)
(7)
(1)
VLR
MS
BS
HLR
MSC-B
(10)
VLR
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 location
area identification is different from the
currently used location area identification.
Location Updating (3)
(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).
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 (if TMSI is reassigned, 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)
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 random
access channel.
(2) The setup process to set up signaling
connection between the mobile station MS
and the mobile service switching center
MSC.
Outgoing call flow from MS to PSTN (3)
(3) Authentication of the mobile station’s
identification number; if encryption is
needed, then it sets the encryption mode and
enter the call setup starting phase.
(4) Service channel allocation.
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 the
call connection acknowledgment signal to the
mobile station.
(6)The called subscriber offhooks to reply, in which
case a response (connection) message is sent to the
mobile station, thus entering the ultimate call
session phase.
Incoming Call Flow from PSTN to MS(1)
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 called
mobile subscriber.
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.
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 the
above outgoing call flow till the mobile
station rings,
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 to
signal calling and called parties enter final
call session.
Thanks