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Introduction

IMSI Attach

IMSI Detach

Location Area Update

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To get an idea about idle mode behavior (GSMprocedures/events) those are -

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IMSI Attach

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A MS performs an IMSI Attach when it comes onto a network. If a

phone is just powering up, it will contact the network and perform anIMSI Attach, letting the network know where the MS is.

IMSI attach processes are-

1. Channel Request2. IMSI Verification / Request Triplets

3. Authentication

4. Encryption

5. Location Update

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IMSI Attach Processes

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a. The MS will send a Channel Request (CHAN_REQ) message to the BSS on

the RACH.

b. The BSS responds on the AGCH with an Immediate Assignment

(IMM_ASS_CMD)message and assigns an SDCCH to the MS.

c. The MS immediately switches to the assigned SDCCH and sends a Location

Update Request (LOC_UPD_REQ)to the BSS. The MS will send either anIMSI or a TMSI to the BSS.

d. The BSS will acknowledge the message. This acknowledgement only tells the

MS that the BTS has received the message, it does not indicate the location

update has been processed.

1. Channel Request

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IMSI Attach Processes

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a. The BSS forwards the Location Update Request to the MSC/VLR.

b. The MSC/VLR forwards the IMSI to the HLR and requests verification of the IMSI

as well as Authentication Triplets (RAND-Random Number, SRES- Authentication

Response, Kc-Session Key).

c. The HLR will forward the IMSI to the Authentication Center (AuC) and request

authentication triplets.d. The AuC generates the triplets and sends them along with the IMSI, back to the

HLR.

e. The HLR validates the IMSI by ensuring it is allowed on the network and is

allowed subscriber services. It then forwards the IMSI and Triplets to the

MSC/VLR.

2. IMSI Verification / Request Triplets

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IMSI Attach Processes

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a. The MSC/VLR stores the SRES and the Kc and forwards the RAND to the BSS

and orders the BSS to authenticate the MS.

b. The BSS sends the MS an Authentication Request (AUTH_REQ)message to

the MS. The only parameter sent in the message is the RAND.

c. The MS uses the RAND to calculate the SRES and sends the SRES back to the

BSS on the SDCCH in an Authentication Response (AUTH_RSP). The BSSforwards the SRES up to the MSC/VLR.

d. The MSC/VLR compares the SRES generated by the AuC with the SRES

generated by the MS. If they match, then authentication is completed

successfully.

3. Authentication

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IMSI Attach Processes

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a. The MSC/VLR forwards the Kc for the MS to the BSS. The Kc is NOT sent across the Air Interface to the

MS. The BSS stores the Kc and forwards the Set Cipher Mode (CIPH_MOD_CMD) command to the MS.

The CIPH_MOD_CMD only tells the MS which encryption to use (A5/X), no other information is included.

b. The MS immediately switches to cipher mode using the A5 encryption algorithm. All transmissions are now

enciphered. It sends a Ciphering Mode Complete (CIPH_MOD_COM) message to the BSS.

c. The MSC/VLR sends a Location Updating Accept (LOC_UPD_ACC) message to the BSS. It also

generates a new TMSI for the MS. TMSI assignment is a function of the VLR. The BSS will either send the

TMSI in the LOC_UPD_ACC message or it will send a separate TMSI Reallocation Command message(TMSI_REAL_CMD).In both cases, since the Air Interface is now in cipher mode, the TMSI is not

compromised.

d. The MS sends a TMSI Reallocation Complete message (TMSI_REAL_COM) up to the MSC/VLR.

e. The BSS instructs the MS to go into idle mode by sending it a Channel Release (CHAN_REL) message.

The BSS then de-assigns the SDCCH.

f. The MSC/VLR sends an Update Locationmessage to the HLR. The HLR records which MSC/VLR the

MS is currently in, so it knows which MSC to point to when it is queried for the location of the MS.

4. Encryption

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IMSI Attach Processes

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a. The MS sends a TMSI Reallocation Complete message (TMSI_REAL_COM)

up to the MSC/VLR.

b. The BSS instructs the MS to go into idle mode by sending it a Channel Release

(CHAN_REL)message. The BSS then deassigns the SDCCH.

c. The MSC/VLR sends an Update Locationmessage to the HLR. The HLR

records which MSC/VLR the MS is currently in, so it knows which MSC to pointto when it is queried for the location of the MS.

5. Location Update

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IMSI Detach

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A MS will perform an IMSI Detach when it powers off. An IMSI Detach

message informs the network that the MS is no longer on the networkand should not be paged. In most networks, the MSC/VLR will initiate

an IMSI Detach if the MS has not been active on the network for a

specified period of time.

IMSI detach processes are-

1. Channel Request

2. IMSI Detach Request

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IMSI Detach Processes

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a. 1. The MS requests a channel in the same manner it does in the Location

Update and IMSI Attach. The MS sends a Channel Request message on the

RACH.

b. The BSS replies on the AGCH and assigns the MS a SDCCH.

c. The MS sends an IMSI Detach Indication (IMSI_DET_IND) message to the

BSS on the SDCCH. The BSS forwards the message to the MSC/VLR

1. Channel Request

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IMSI Detach Processes

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a. The MSC/VLR sends a Location Cancel Request to the HLR.

b. The HLR marks the IMSI as detached and removes any pointers for the IMSI

from its registry. It then sends a Location Cancel Acknowledgment message

to the MSC/VLR.

2. IMSI Detach Request

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Location Area Update

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A MS will need to update its location whenever it moves to a tower

that is serviced by a different VLR then the one it is currently on. AnMS c an move from BTS to BTS without ever telling the network, as

long as it is within the same location area. Once it moves to a new

location area, it is required to inform the network.

If the MS moves to another Location Area then the processes are-

1. Channel Request

2. Gaining VLR requests data from Losing VLR

3. Location Update

4. Updating the Registers

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Location Area Update Processes

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a. The MS requests a channel by sending a Channel Request (CHAN_REQ)

message on the RACH.

b. The BTS responds by sending an Immediate Assignment Command

(IMM_ASS_CMD) message on the AGCH.

c. The MS switches to the assigned SDCCH and replies with a Location Update

Request (LOC_UPD_REQ). Included in the LOC_UPD_REQ is the TMSI theMS is currently using as well as the Location Area Identifier (LAI) of the VLR it

is leaving.

d. The BTS acknowledges receipt of the message.

1. Channel Request

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Location Area Update Processes

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a. The BSS forwards the Location Update Request to the gaining MSC/VLR.

b. The gaining MSC/VLR does not recognize the TMSI/IMSI of the MS, so it

contacts the losing MSC/VLR that corresponds to the LAI that was provided by

the MS. The new MSC/VLR requests the subscriber data for the given TMSI.

c. The gaining MSC/VLR will then authenticate the MS. There are two ways this

could occur. First, the losing MSC/VLR may have forwarded any sets of tripletsthat it was retaining for the MS. The gaining MSC/VLR would then just use the

next set of triplets. Second, the gaining MSC/VLR could contact the HLR and

request authentication triplets from the AuC and proceed with authentication

that way.

2. Gaining VLR requests data from Losing VLR

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Location Area Update Processes

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a. Once the MS has been authenticated and is in Cipher Mode, the MSC/VLR

sends a Location Update Accept (LOC_UPD_ACC) message through the

BSS to the MS. The LOC_UPD_ACC may have a TMSI assignment in it,

otherwise the TMSI will be assigned in a TMSI_REAL_CMD message.

b. The MS will respond with a TMSI Reallocation Complete (TMSI_REAL_COM)

message indicating it has received the TMSI.c. The BSS then sends the MS a Channel Release (CHAN_REL) message

instructing it to go into idle mode. The BSS then un-assigns the SDCCH. As far

as the MS is concerned, the location update has been completed.

3. Location Update

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Location Area Update Processes

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a. The Gaining MSC/VLR sends an Update Location message to the HLR. The

HLR updates its records to point to the gaining MSC/VLR when it is asked for its

location.

b. It also passes on subscriber information for the MS to the gaining MSC/VLR.

c. The HLR sends a Cancel Location message to the losing MSC/VLR.

d. The losing MSC/VLR deletes the MS's record and also releases the TMSI forreassignment. Then it sends a Cancel Location Result message back to the

HLR, confirming the cancellation.

4. Updating the Registers

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References

http://gsmfordummies.com/tdma/logical.shtml

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http://gsmfordummies.com/tdma/logical.shtmlhttp://gsmfordummies.com/tdma/logical.shtml

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