The following are main types of call drop:
1． RF loss call drop (i.e. radio link fault call drop);
2． Handover failure call drop;
3． LAPD call drop.
3.1.1 RF Loss Call Drop
The RF loss call drop is shown in Fig. 3.1 1.
Fig. 3.1 1 RF Loss Call Drop
The RF loss call drop is divided into two parts: uplink and downlink.
1． Downlink failure
According to GSM specification, MS has a timer S (T100), when MS starts the call, the timer is assigned with an initial value (i..e. radio_link_timeout) that is broadcasted on BCCH.
When MS cannot correctly decode a SACCH message (four SACCH BLOCK), S is decreased by 1; when MS correctly decodes a SACCH message, S is increased by 2.
S will not exceed the initial value defined by radio_link_timeout. When S is zero, MS will abandon the radio resource connection and enter idle mode, and a call drop occurs.
2． Uplink failure
The uplink failure parameter monitored by system is link_fail.
When the site cannot correctly decode a SACCH message, the timer in HDPC (the timer’s maximum value is defined by link_fail) is decreased by 1; when the site correctly decodes a SACCH message, the timer is increased by 2 (the timer’s value will exceed the value defined by link_fail).
When the timers’ value is zero, the site stops transmitting downlink SACCH and starts the rr_t3109 timer (rr_t3109>T100). When T100 of MS is timeout, MS returns to idle mode and call drop occurs. The site releases the radio channel when the rr_t3109 timer is timeout. Besides, BSC has to send a Clear request message to MSC.
Either uplink failure or downlink failure will cause stopping sending SACCH to the opposite end, and this will start the radio resource releasing procedure of the opposite end. Each time link_fail occurred on TCH will be counted as a RF_LOSSES_TCH.