Drop Calls in GSM

If the radio link fails after the mobile sends the Service Connect Complete Message then it is considered a dropped call. Dropped call analysis can consume a considerable amount of time. Using good post–processing analysis tools, the root cause of some of the drops can be determined from mobile data alone. However, there will be cases where the cause cannot be reliably confirmed unless system data is also used.

Calls often drop when strong neighbors suddenly appear. When the MS is suddenly confronted with a strong new signal, or when the signal it is using takes a sudden deep fade, it will have poor C /I and high forward FER. The call will drop unless it gets help quickly.

Using a post– processing tool, display a map of the locations of dropped calls that exhibit symptoms of poor coverage. Verify this type of drop is not occurring in good– coverage areas. If so, suspect and investigate hardware at the serving site.

Another technique is to examine the dropped call message files and identify the BTS from which the sync channel message is received immediately after each drop. This could be achieved by analyzing Layer 3 messages in log files or running traces from NMS/OSS.

Drop calls can be classified by looking to their orientations:

• TCH radio drops are the drops that occurred due to summation of radio and ABIS reasons.
• TCH non–radio drops are the drops that occurred due to summation of network management, BSCU reset, BTS fail, LAPD failure, user and A interface.
• TCH handover drops are the drops that occurred in handover phase while the call tries back to old serving channel but fails and drops. These drops may occur due to RF, ABIS and A interface reasons.

General Reasons for Drop Calls are as follows:

• Drop Call due to Low Signal Strength.
• Drop Call due to Missing Neighbor.
• Drop Call due to Bad RX Quality.
• Drop Call due to Interference.
• Drop Call due to Handover Failures.

General Recommendations:

• Defining missing neighbor relations.
• Proposing new sites or sector additions.
• Proposing antenna azimuth changes.
• Proposing antenna tilt changes.
• Proposing antenna type changes.
• Re–tuning of interfered frequencies.
• BSIC changes.
• MHA/TMA adds.
• Changing power parameters.
• Adjusting Handover margins.