The Random access request message (8-bit "Channel Request" message on RACH) contains information on why the Mobile Station (MS) wishes to establish connection with the network (i.e. 3-bit "Establishment Cause") and a random discriminator (i.e. 5-bit "Random Reference number"). However, it does not contain any identifier to identify the MS. Therefore it is possible that at the same time, more than one MS transmit a Channel Request message with the same content (i.e. same Establishment Cause with same Random Reference number on same time slot). So the network on receiving this request processes it, reserve resources and transmits Immediate Assignment message. Considering that the MSs receive this message and the data in the message corresponds to the content of Channel Request message transmitted by the respective MSs, the MSs assumes that a dedicate resource is reserved for it. This situation is referred to as "Contention Resolution" in GSM and the problem is solved by a contention resolution procedure.
The solution is the following: The MSs transmit a data link layer SABM frame containing a Layer 3 service request message (i.e. CM Service Request, Location Updating Request, IMSI Detach, Paging Response or CM Re-establishment Request). The service request message contains the identifier of the MS. The data link layer of the MS stores the content of this frame to perform contention resolution. The network returns this service request message in a data link layer UA (Unnumbered Acknowledgment) frame. The data link layer of the MS compares the content of the UA frame information field, i.e. of the service request message, with the content of the stored message. If the contents are not identical, the MS comes to a conclusion that its reservation failed. Then the MS terminates its traffic on said dedicated channel and eventually restarts a new reservation operation for a dedicated channel. If the contents are identical, the MS comes to a conclusion that the reservation succeeded and continues traffic on said dedicated channel.
The solution is the following: The MSs transmit a data link layer SABM frame containing a Layer 3 service request message (i.e. CM Service Request, Location Updating Request, IMSI Detach, Paging Response or CM Re-establishment Request). The service request message contains the identifier of the MS. The data link layer of the MS stores the content of this frame to perform contention resolution. The network returns this service request message in a data link layer UA (Unnumbered Acknowledgment) frame. The data link layer of the MS compares the content of the UA frame information field, i.e. of the service request message, with the content of the stored message. If the contents are not identical, the MS comes to a conclusion that its reservation failed. Then the MS terminates its traffic on said dedicated channel and eventually restarts a new reservation operation for a dedicated channel. If the contents are identical, the MS comes to a conclusion that the reservation succeeded and continues traffic on said dedicated channel.
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