Friday, June 18, 2010

Cell Site Analysis - Call Analysis

Cell Site Analysis - Call Analysis

An element of Cell Site Analysis is Call Analysis. Evidence seen at court is often on the basis that the call records show a call which is implied that the call was started and end in the normal way - that being the user is assumed to have pressed the end key to terminate the call.

I have mentioned before in other threads how reading the GSM Standards requires consideration before undertaking work in mobile telephone evidence:

GSM Timers (see above)
Cell Site Analysis (see below)

The list below is to illustrate some of the causes for output that need to be considered regarding cessation or loss of a mobile communication. It is not exhautive as can be noted by reading the Standard.

GSM Standard GSM04.08
Digital cellular telecommunications system;
Mobile radio interface;
Layer 3 specification
(GSM 04.08)

Causes for the cessation or loss of mobile communication

Annex F (informative): GSM specific cause values for radio resource management
This annex is informative.
Cause value = 0 Normal event;
indicates that the channel is released because of a normal event or that an assignment or handover is successfully, and normally, completed.

Cause value = 1 Abnormal release, unspecified;
indicates that the channel is released because of an abnormal event without specifying further reasons.

Cause value = 2 Abnormal release, channel unacceptable;
indicates that the channel type or channel characteristics are not acceptable.

Cause value = 3 Abnormal release, timer expired;
indicates that the release is caused by a timer expiry.

Cause value = 4 Abnormal release, no activity on the radio path;
indicates that some supervisory function has detected that the channel is not active.

Cause value = 5 Pre-emptive release;
indicates that the channel is released in order to be allocated to a call with priority (e.g. an emergency call).

Cause value = 8 Handover impossible, timing advance out of range;
indicates that a handover is unsuccessful because the target BTS is beyond the normal range and the target BTS would not accept an out of range timing advance.

Cause value = 10 Frequency not implemented
indicates that the MS does not have the capability to operate on (at least one of) the requested frequency(ies).

Cause value = 65 Call already cleared;
indicates that a handover is unsuccessful because the connection has been released by the network or the remote user.

Cause value = 101 No cell allocation available;
indicates that an assignment or handover is unsuccessful because the MS has no current CA.

Annex G (informative): GSM specific cause values for mobility management
This annex is informative.
G.1 Causes related to MS identification
Cause value = 4 IMSI unknown in VLR
This cause is sent to the MS when the given IMSI is not known at the VLR.

G.2 Cause related to subscription options
Cause value = 13 Roaming not allowed in this location area
This cause is sent to an MS which requests location updating in a location area of a PLMN which does not offers roaming to that MS in that Location Area.

G.3 Causes related to PLMN specific network failures and congestion
Cause value = 17 Network failure
This cause is sent to the MS if the MSC cannot service an MS generated request because of PLMN failures, e.g. problems in MAP.

Cause value = 22 Congestion
This cause is sent if the service request cannot be actioned because of congestion (e.g. no channel, facility busy/congested etc.)

The purpose of identifying issues like this is to show that Cell Site Analysis is not about going to specific geographical locations, taking radio test measurements and then making a conclusion about a mobile telephone's location.

For instance, fairly recently evidence before a criminal court dealt with a mobile call that started on a cell site (Mast) and although the general location for the mobile 'phone had at least another seven (7) Masts within 1Km to 7Km of each other and the mobile 'phone in which to handover for a mobile call, the call was in fact handled and ended (dropped after a short period of time) on a Mast some 25Km away. This could generate an impression that the mobile telephone was travelling at impossible speeds in a vehicle between two locations in order to use each Mast for the start and end of the mobile call.

In reality, the fact that closer Masts to the mobile telephone were not used, does present the conundrum why were these Masts were excluded from use for a Mast much, much further away? Were these seven Masts all out of action at the same time, or did something else occur? This is the job of the investigator to find out when conducting Cell Site Analysis.

What a credit though to the GSM mobile telephone network displaying its amazing versatility in endeavouring to hold onto and maintain a mobile call.

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