Gsm Call Flow

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  • Topic: Mobile phone, GSM, Base station
  • Pages : 12 (1533 words )
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  • Published : April 17, 2013
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GSM Call Flow (GSM Originating Call)
Cell
Mobile Station
User
Mobile

Mobile Network
Base Stations
NSS
BSS
MSC VLR

Fixed Network
PSTN
PSTN

EventStudio System Designer 4.0
13-Sep-08 21:38 (Page 1)

LEG: GSM Mobile Originated Call
This sequence diagram was generated with EventStudio System Designer 4.0 (http://www.EventHelix.com/EventStudio). Copyright © 2008 EventHelix.com Inc. All Rights Reserved. The EventStudio source files for this document can be downloaded from http://www.eventhelix.com/call-flow/gsm-call-setup.zip.

This scenario describes the call setup for a GSM originating call. A mobile user calling a land line subscriber is covered here. Copyright © 2000-2008 EventHelix.com Inc. All Rights Reserved. The user keys in the phone number for

the landline subscriber and ..
.. presses the Send button

Dial the called person's number

Send Button
Begin RR Connection Establishment

Call related information needs to be transported from the mobile phone to the Mobile Switching Center (MSC). This requires the establishment of a Radio Resource (RR) connection to MSC. The first phase of the call setup just sets up this RR connection. RR CHANNEL REQUEST

RACH

RR connection establishment is
triggered by sending the Channel
Request message. This message
requests the Base Station System (BSS)
for allocation for radio resources for the
RR connection setup. The mobile now
waits for an assignment on the Access
Grant Channel (AGCH). At this point the
mobile is listening to the AGCH for a
reply.

Note: The RR CHANNEL REQUEST is sent on a Random Access Channel (RACH). This is a slotted aloha channel that can be used at random, without any coordination between the mobiles. Any mobile can transmit on this channel whenever it wishes. If two mobiles transmit on the channel at the same time, their messages will be lost in a collision. The mobiles will detect the collision via a timeout and retransmit the message after a random back off.

allocate
TCH

RR IMMEDIATE ASSIGNMENT
AGCH, Radio_Resource = (TCH, Frequency,
Timeslot), Time Correction, Frequency Correction

Apply the time and frequency corrections

Tune to the frequency and timeslot

RR SABM + MM CM SERVICE REQUEST
TCH, SAPI = 0

The BSS allocates a Traffic Channel
(TCH) to the mobile. The TCH allocation
assigns a specifies a frequency and a
timeslot on that frequency. After the
mobile receives this message, the
mobile shall only use the specified
resources for communication with the
mobile network.
The BSS transmits the radio resource
assignment to the Mobile via the AGCH
channel. The message also contains the
time and frequency corrections. The time
corrections allow the mobile to time it's
transmissions so that they reach the
BSS only in the specified slot. The
frequency corrections correct for the
Doppler shift caused by the mobile's
motion.
Adjust the frequency and timing based
on the advice from the BSS. This step is
required so that transmissions from the
mobile reach the base station at the
precise time and with the correct
frequency.
The mobile detunes from the AGCH and
tunes to the specified radio channel.
This is the first message that is sent
after tuning to the channel. The Mobile
initiates a LAPm connection with the
BSC by sending a Set Asynchronous
Balanced Mode (SABM) message. The
service request message meant for the
MSC is also sent in this message.

GSM Call Flow (GSM Originating Call)
Cell
Mobile Network
Mobile Station
Base Stations
NSS
User
Mobile
BSS
MSC VLR
RR UA

Fixed Network
PSTN
PSTN

TCH, SAPI = 0

SCCP CONNECTION REQUEST + MM CM SERVICE REQUEST
SS7

Check subscriber authentication

EventStudio System Designer 4.0
13-Sep-08 21:38 (Page 2)

The BSS replies with Unnumbered
Acknowledge (UA) to complete the
LAPm setup handshake
The BSS receives the CM Service
Request message from the mobile and
forms a "BSSMAP COMPLETE LAYER 3...
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