MS NETWORK CONNECTION SEQUENCE
When an MS powers on within network coverage, it starts by scanning all frequencies within
its allocated band (e.g. 124 for standard GSM). It measures the received power on each of
these frequencies and placed them in order.
The MS then selects and listens out on the strongest RF level carrier for a frequency correction
burst which is transmitted on the control channel of a BCCH carrier. This is to initially
achieve frequency synchronisation with the transmitting BTS.
its allocated band (e.g. 124 for standard GSM). It measures the received power on each of
these frequencies and placed them in order.
The MS then selects and listens out on the strongest RF level carrier for a frequency correction
burst which is transmitted on the control channel of a BCCH carrier. This is to initially
achieve frequency synchronisation with the transmitting BTS.
Having achieved frequency synchronisation; the MS listens on the SCH for frame
synchronisation information. The SCH channel provides frame timing, the current frame
number and BSIC information.
Once frame synchronisation is achieved, the MS starts to read and decode the additional
information being transmitted on the BCCH. This includes the adjacent cell list, minimum
received signal strength, the LAI and beacon frequencies from surrounding cells.
The MS then continues to monitor the PCH for incoming call paging requests, sends periodic
location updates and maintains a record of surround cell signal strengths.
If the MS fails to detect either the FCCH or the SCH, it will reselect the next highest RF carrier
level from its measured list and repeats the detection process.
synchronisation information. The SCH channel provides frame timing, the current frame
number and BSIC information.
Once frame synchronisation is achieved, the MS starts to read and decode the additional
information being transmitted on the BCCH. This includes the adjacent cell list, minimum
received signal strength, the LAI and beacon frequencies from surrounding cells.
The MS then continues to monitor the PCH for incoming call paging requests, sends periodic
location updates and maintains a record of surround cell signal strengths.
If the MS fails to detect either the FCCH or the SCH, it will reselect the next highest RF carrier
level from its measured list and repeats the detection process.
IMSI ATTACH PROCEDURE
IMSII Attttaacchh
• Mobile camps on to best serving BTS
• Mobile sends IMSI to MSC
• MSC/VLR is updated in HLR
• Subscriber data including current location
area is added to local VLR
• MSC and HLR carry out authentication
check - challenge and response using Ki
• Optionally EIR checks for status of mobile
(white/grey/black)
The IMSI attach procedure is used by the MS to indicate that it is has adopted the active
(power-on) state within the network. The IMSI attach is also performed as part of the location
updating procedure.
The basic IMSI attach procedure is illustrated above and described below, with each of the
stages identified on the diagram:
Stage 1
The MS sends a message to BSS on the RACH requesting a channel allocation. The BSS
responds with a ‘Immediate Assignment’ message on the AGCH. This message assigns a
SDCCH channel to the MS.
Stage 2
On assignment of the SDCCH channel, the MS sends and IMSI attach message over the
SDCCH to the MSC/VLR relayed via the BSS. This informs the MSC/VLR of the MS’s IMSI.
This information may also be updated in the HLR which provides subscriber profile data to
the VLR is it does not already have it.
Stage 3. Security procedures are activated including authentication and (optionally) and
IMEI check with the EIR.
Stage 4
Assuming successful authentication, the VLR responds to the MSC with an ‘IMSI Attach
Acknowledge’ message which is forwarded by the MSC, via the BSS, to the MS.
Stage 5
The MSC also sends a ‘Clear Command’ message to the MS over the SDCCH in order to
release the dedicated resources used to effect the IMSI attach
(power-on) state within the network. The IMSI attach is also performed as part of the location
updating procedure.
The basic IMSI attach procedure is illustrated above and described below, with each of the
stages identified on the diagram:
Stage 1
The MS sends a message to BSS on the RACH requesting a channel allocation. The BSS
responds with a ‘Immediate Assignment’ message on the AGCH. This message assigns a
SDCCH channel to the MS.
Stage 2
On assignment of the SDCCH channel, the MS sends and IMSI attach message over the
SDCCH to the MSC/VLR relayed via the BSS. This informs the MSC/VLR of the MS’s IMSI.
This information may also be updated in the HLR which provides subscriber profile data to
the VLR is it does not already have it.
Stage 3. Security procedures are activated including authentication and (optionally) and
IMEI check with the EIR.
Stage 4
Assuming successful authentication, the VLR responds to the MSC with an ‘IMSI Attach
Acknowledge’ message which is forwarded by the MSC, via the BSS, to the MS.
Stage 5
The MSC also sends a ‘Clear Command’ message to the MS over the SDCCH in order to
release the dedicated resources used to effect the IMSI attach
Stage 6. The VLR assigns a Temporary Mobile Subscriber Identity (TMSI) to the MS and
informs it that it is attached to the network. This function creates a ‘Mobility Management
(MM) Context’.
informs it that it is attached to the network. This function creates a ‘Mobility Management
(MM) Context’.
IMSI DETACH PROCEDURE
IMSI Detach
• Explicit:
• Mobile informs MSC it is switching off
• HLR stores last location area for mobile
• VLR records that mobile is no longer available on
network
• Mobile powers down
• Implicit
• VLR forces IMSI Detach due to no response
The IMSI detach procedure is invoked if the MS is deactivated either by powering down or if
the SIM is removed or forced by the network.
There are two causes of an IMSI detach:
· Explicit: The MS informs the network that it is detaching.
· Implicit: The MSC has not been able to contact the MS for a pre-determined amount
of time and forces an IMSI detach.
The detach procedure is essentially the reverse of the IMSI attach procedure and is as follows:
Stage 1
The MS sends a ‘Channel Request’ message on the RACH to the BSS. The BSS assigns a an
SDCCH channel, informing the MS over the AGCH.
Stage 2
The MS sends a ‘IMSI Detach Indication’ message to the BSS which identifies the MS using its
TMSI. The BSS forwards this message to the MSC which in turn updates its VLR using a
‘Detach IMSI’ signalling message.
Stage 3
The VLR informs the HLR of the change using a ‘Deregister Mobile Subscriber’ signalling
message. The HLR responds to the VLR with a ‘Deregistration Accepted’ message.
the SIM is removed or forced by the network.
There are two causes of an IMSI detach:
· Explicit: The MS informs the network that it is detaching.
· Implicit: The MSC has not been able to contact the MS for a pre-determined amount
of time and forces an IMSI detach.
The detach procedure is essentially the reverse of the IMSI attach procedure and is as follows:
Stage 1
The MS sends a ‘Channel Request’ message on the RACH to the BSS. The BSS assigns a an
SDCCH channel, informing the MS over the AGCH.
Stage 2
The MS sends a ‘IMSI Detach Indication’ message to the BSS which identifies the MS using its
TMSI. The BSS forwards this message to the MSC which in turn updates its VLR using a
‘Detach IMSI’ signalling message.
Stage 3
The VLR informs the HLR of the change using a ‘Deregister Mobile Subscriber’ signalling
message. The HLR responds to the VLR with a ‘Deregistration Accepted’ message.
Stage 4
The VLR notifies the MSC of this acceptance with an ‘Acknowledge IMSI Detachment’
message. The MSC does not notify the MS as, by this stage, the MS may well be disconnected.
Stage 5
The MSC sends a ‘Clear Command’ message to the BSS to release the assigned SDCCH
resources. The BSS responds with a ‘Clear Complete’ message which completes the IMSI
Detach procedure.
The VLR notifies the MSC of this acceptance with an ‘Acknowledge IMSI Detachment’
message. The MSC does not notify the MS as, by this stage, the MS may well be disconnected.
Stage 5
The MSC sends a ‘Clear Command’ message to the BSS to release the assigned SDCCH
resources. The BSS responds with a ‘Clear Complete’ message which completes the IMSI
Detach procedure.
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