T305 timer is started at the MSC/MS when DTAP message DISCONNECT is sent, this will guard the DISCONNECT procedure run correctly in T305 period.

T308 timer is started at the MSC/MS when DTAP message RELEASE is sent after the DISCONNECT procedure, this timer guards how long the RELEASE procedure runs.

(GSM 04.08 section 5.4 call clearing)

T305 and T308 at MSC should be set to no less than 11 seconds to guarantee that the layer 2 T200 is timing out N200 times on the air interface, reducing these timers to less than 11 seconds will pull them below the related BSS timers[T200*(N200+1) in this case] at the airinterface that are used to ensure a successful transmission of a message (DISCONNECT and RELEASE in this case) to/from the mobile over the air interface. Normally the higher level MSC timers would be set longer than the related lower level times in the BSS, using a strategy that lower level timers should expire first.

Setting T305 and T308 to less than 11 seconds here is forced the higher level MSC timers to expire before the BSS timers[T200*(N200+1) and link_fail(only for MSC issuing DISCONNECT/RELEASE first)] when there are air interface transmission problems (RF interference) for DISCONNECT and RELEASE signalling, and as a result the BSS statistics SD/TCH drop doesn't get a chance to peg for T200*(N200+1).

The end user experience is probably not that relevant here as they were trying to clear the call anyway. The end user would see no difference.

However the network operator could be fooled here though thinking that he has a clean RF interface with a lower drop call rate, when in fact there could be RF interference issues to be resolved. The low MSC timers are just masking it.

T305- This parameter is used for the MSOFTX3000 to interwork with the BSC. After the MSOFTX3000 sends the Disconnect message to the BSC, the T305 timer is started on the basis of the preset time. If no response is returned after the timer times out, the MSOFTX3000 sends the Release message to the BSC. Value range: 1-60 (in the units of seconds) Default value: 30 (in the units of seconds) If you do not set this parameter, the global T305 timer set in the system is adopted. T308-This parameter is used for the MSOFTX3000 to interwork with the BSC. After the MSOFTX3000 sends the Release message to the BSC, the T308 timer is started on the basis of the preset time. If no response is returned after the timer times out, the MSOFTX3000 resends the Release message and restarts the T308 timer. If the T308 timer times out again, the MSOFTX3000 sends the Release Complete message to the BSC. Value range: 1-60 (in the units of seconds) Default value: 10 (in the units of seconds) If you do not set this parameter, the global T308 timer set in the system is adopted.