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GSM Issue-CM Service Re-Establishment Failed as network releases channel

The said issue is most commonly seen in GSM network where in call drop happens so often due to CM Service RE-Establishment failure. This results when network releases the channel.

ID(HEX) Name of mobility management Direction Description
28/68 CM REeStablishment REQuest MS(UE) to BTS An option in GSM is to allow for a call reestablishment in case of a dropped connection. Here first a CHANNEL REQUEST has to be sent to the BTS and then it is tried with the CM_RES_REQ to reestablish an RR connection for the still existing and active MM and CC connection.

GSM Timer T3230 starts- At the transmission of CM service request OR CM Re-Establishment Request.
GSM Timer T3230 Stops-when mobile receives CM service accept/CM service reject
When it times out-the call is cleared by UE(MS)
Default value-15 seconds

CM RE-ESTABLISHMENT REQUEST message content

• MM Protocol Discriminator
• Skip indicator
• CM RE-ESTABLISHMENT REQUEST message type
• Ciphering key sequence number
• spare half octet
• MS classmark
• Mobile identity
• Location Area ID

If there is no issue on the radio link and this issue is observed first thing need to be analyzed is that whether network or cell supports re-establishment or not.

If the network do not have capabilities to associate CM RE-establishment request with the any originating call from MS, the CM SERVICE REJECT is transmitted back with specified cause for reject.
#38 "call cannot be identified"

HEX VALUE REJECT CAUSE DESCRIPTION
# 4 IMSI unknown in VLR
#6 illegal ME
#17 network failure
#22 congestion
#25 not authorized for this CSG
#32 service option not supported
# 34 service option is temporarily Out of Order

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