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CALL DROP

In mobile communication, call drop means loss of call due to certain reasons after the

Traffic Channel (TCH) is allocated to particular ms and the user is in conversation and

active call suddenly ends without the terminate indication which causes a lot ofinconvenience to users, is one of the faults that are complained most.

There are three types of call drop: -1.RF loss call drop2Handover failure call drop;

3.LAPD call drop.RF Loss call drop : -

RF loss call drop generally happens due to two reasons: -

Downlink failure

Uplink failure

According to GSM specification, MS has a timer S (T100), when MS starts the call, the timer is

assigned with an initial value (radio link timeout) that is broadcasted on BCCH. When MS cannot

correctly decode a SACCH message S is decreased by 1 and 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.

Generally the call drop is due to:-

Interference: - If there is a interference from the co-frequency or from adjacent frequency

then sometimes call drop happens.

Timing Advance: - If sometimes the TA is restricted then beyond that TA call drop happens.

RX Level: - If sometimes at long distance or in indoor where the signal strength is low so due

to low level call drop happens.

Over shooting of coverage: - If there is over coverage then due no definition of hand over call

drop happens.

Link budget:- If difference between the uplink and downlink is so much then call drop

happens.

CIC check: - If the CIC value is not proper on A link on MSC side then sometimes call drop

happens.

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Antenna problem: - If some times the antenna is faulty then call drop happens.

RX level access min: - If Rx level access min is so more then call drop happens

NCC permit

Faulty Hardware

Hand over call drop: -

If MS receives Assignment message (BSC starts relevant handover control timer) but

fails to handover to the destination cell and does not return to the original cell either MS

fails to occupy the destination cell channel and sendsHandover Complete orAssignment

Complete message, neither MS returns to the original cell channel and sends HandoverFailure or AssignmentFailure message. At this moment, the handover control timer of

BSC will be timeout and event as a handover failure call drop.

Hand over call drop happens generally due to some reasons : -

Neighbor missed

Ping pong of handovers

Due to Interference

Improper definitions of neighbors.

There are three types if BSC handover timers: -

Timer for inter BSC = T8

Timer for Inter cell Intra BSC = T3103

Timer for Intra cell = T3107

Normally the handover call drops are on highways

LAPD Call drop: -

LAPD meaning: when LAPD link is broken, the call-in-progress on the carrier will be

interrupted. Count when BSC receives the LAPD link broken message.

LAPD call drop generally due to

Transmission Problem(BER)

OMCR Alarm (PCM alarm)

BTS side

BSC side