GSM call re-establishment timer

[Lotrus]

Hi,
I'm learning how call re-establshment works in GSM and can't find detailed information about procedure. That's how i see it:
When AMR RLT expires on MS, it starts search the suitable cell, and when it finds it, sends CM Re-establishment Request to network.
When AMR RLT expires on network side, it starts reestablishment timer on MSC. If it expires, network stops holding resourses for that MS.

I have next qustions:
1) Is there some timer on BSC, or it is only on MSC?
2) I can't find what timer MS uses to check, is there any point to send re-establishment request, or it's too late.

[akram]

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[JCarlos21]

Hi Lotrus,

Please find next the procedures for the Call Re-Establishment on GSM network:

Call Re-Establishment Procedure
1) After the MM connection failure indication is reported to the CM entity, if the MS receives at least one request for MM connection re-establishment from CM, it will initiate the call re-establishment procedure. If several CM entities request for re-establishment, only one re-establishment procedure will be initiated.

2) After the CM sends the request for the re-establishment of MM connection, MM sublayer sends a request for the establishment of RR connection and enters the WAIT FOR REESTABLISH state. This request includes an establishment cause and a CM re-establishment request. When the RR sublayer indicates a RR connection is established (the CM re-establishment request message has been sent through the Um interface), the MM sublayer starts T3230 and indicates to all the CM entities that the MM connection is under construction. The MM sublayer stays in WAIT FOR REESTABLISH state.

The CM Re-establishment Request message contains the MS identity (IMSI or TMSI), Classmark 2, and encrypted sequence number.
Whether the CM entity can request for re-establishment depends on protocol discriminator (PD).

3) After receiving the CM re-establishment request, the network analyzes the request type and starts the MM program or RR program. The network can start the classmark enquiry program to obtain more information about the MS encryption ability. The network can also decide to perform the authentication procedure or ciphering mode setting procedure.

4) When the RR sublayer indicates the ciphering mode setting procedure is over or the CM SERVICE ACCEPT message is received, the MM connection is re-established. The T3230 stops and informs all the CM entities related to the re-establishment to enter the MM CONNECTION ACTIVE state.

5) If the network cannot connect the re-establishment request to the current MS call, it sends the CM SERVICE REJECT with the reject cause to the MS.
The reject cause (value) includes unidentifiable call (#38), unidentifiable IMSI (# 4), unauthorized ME (# 6), network failure (#17), congestion (#22), unsupported service (#32), and temporary service failure (#34)。

6) After receiving the CM SERVICE REJECT, the MS stops T3230 and releases all MM connections and RR connections. If the reject cause if #4, the MS deletes the TMSI, LAI, and CKSN in SIM card, and changes the status from “updating” into “no updating”, and then enters the “WAIT FOR NETWORK COMMAND” state. The location updating will be initiated after the RR release.

If the reject cause is #6, the MS deletes the TMSI, LAI, and CKSN in SIM card, and changes the status from "updating" into “roaming inhibit”. The SIM is regarded invalid until the MS is switched off or the SIM card is pulled out.

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Please refer to this thread, Post #4 specifically for T3230 timer on MS:

http://www.finetopix.com/showthread.php?16471-GSM-Timer-T3230

I hope this could help you buddy.

Best regards,

Jc

[Lotrus]

If i understood correctly, in step 1) network sends request to MS to initiate call re-establishment procedure. That's interesting. While i was testing this feature with TEMS Investigation, i saw that MS initiated this procedure, but didn't see any requests from network. So i was sure, that it is MS initiates procedure right after cell update. How can i see this request "if the MS receives at least one request for MM connection re-establishment from CM, it will initiate the call re-establishment procedure"?

[JCarlos21]

If i understood correctly, in step 1) network sends request to MS to initiate call re-establishment procedure. That's interesting. While i was testing this feature with TEMS Investigation, i saw that MS initiated this procedure, but didn't see any requests from network. So i was sure, that it is MS initiates procedure right after cell update. How can i see this request "if the MS receives at least one request for MM connection re-establishment from CM, it will initiate the call re-establishment procedure"?

Hi Lotrus,

It is indeed. You're right by saying that MS initiates Call Re-establishment procedure. The request that it's mentioned above is related to CM sub-layer from layer 3 protocol in charge of Call Control.

While using TEMS Investigation you should see Layer 3 Messages window, under MM protocol, "CM Re-establishment Request" message.

To help you to see more clearly this procedure, please find attached a diagram showing what was described above.

36541

I hope this information can help you.

Best regards,

Jc

[Lotrus]

I have too low reputation to open attachments. I understand this procedure in common, but my questions still actual:

1) Is there some timer on BSC, or it is only on MSC? - I found reestablishment timer on network side only on MSC. is it only one timer for reestablishment from network side?
2) I can't find what timer MS uses to check, is there any point to send re-establishment request, or it's too late. - For example dropped call happened and MS camped on suitable cell only after 1 minute/1 hour/1day of searching. How it understands when it should send "CM Re-establishment Request" message and when it is too late? I suppose some timer should exist, but can't find what exactly.

[JCarlos21]

Hi Lotrus,

Responding to your exact questions, here it goes:

1) Correct. No timer resides on BSC.
2) After MS identify Radio Link Failurer and expiry of "S" Counter initiliazed to a network defined value (increased by 2 when good SACCH arrives, decreased by 1 when bad SACCH) The discrimination here would be if the cell is suitable and Call Re-Establishment is allowed. By definition of 3GPP standard (TS 45.008) MS is under no circumstances allowed to access a cell to attempt call re-establishment later than 20 seconds after finding a RLF.

The timer that is involved on MS side is the T3230 though, which is initialized after receiving IMMEDIATE ASSIGNMENT and sending back CM RE-ESTABLISHMENT REQUEST, but this step goes after finding a cell to attempt to.

I hope this can help you my friend.

Best regards,


I have too low reputation to open attachments. I understand this procedure in common, but my questions still actual:

1) Is there some timer on BSC, or it is only on MSC? - I found reestablishment timer on network side only on MSC. is it only one timer for reestablishment from network side?
2) I can't find what timer MS uses to check, is there any point to send re-establishment request, or it's too late. - For example dropped call happened and MS camped on suitable cell only after 1 minute/1 hour/1day of searching. How it understands when it should send "CM Re-establishment Request" message and when it is too late? I suppose some timer should exist, but can't find what exactly.

[asifhaider919]

For Nokia MSC the timer on MSS side is : MAX WAIT TIME FOR RE-ESTABL REQUEST> is the timer ( default value is 10 seconds ) on the MSC BSSAP

[Lotrus]

Thank you! That was exactly what I was searching.