GSM basics - T3101 Timer

This hub is related to GSM radio frequency optimization. If you are not familiar on this subject then I do not recommend further reading....

T3101 (Default 3s)

When: immediate assignment process

Timer T3101 is used to set the interval between the start time when an IMM ASS message is sent by BSC and the stop time when an EST IND message is reported to BSC. BSC releases the seized SDCCH if no EST IND message is received within the time specified by timer T3101.

Normal flow

MS → RACH → BSC

MS ← IMM ASS ← BSC (start counting the timer)

MS → EST IND → BSC (stop the timer)

Abnormal flow

1). MS → RACH → BSC

     MS  X  IMM ASS ← BSC (start counting the timer)

In this case IMM ASS message is not received by the MS. Then BSC will release the seized SDCCH after waiting the time specified by timer T3101.

2). MS → RACH → BSC

     MS ← IMM ASS ← BSC (start counting the timer)

     MS  X  EST IND  X  BSC

In this case, ESTIND message is not sent by the MS after receiving IMM ASS message. The BSC will release seized SDCCH channel as above case.

3). MS → RACH → BSC

     MS ← IMM ASS ← BSC (start counting the timer)

     MS → EST IND  X  BSC

In this case ESTIND message is sent by MS. But BSC does not receive ESTIND message. The seized SDCCH channel will release as above cases.