Monday, June 04, 2012

CSA - GSM Timing advance and absolute delay

The notion of GSM Timing advance and absolute delay as considered in relation to cell site analysis (CSA) requires not only understanding the reason and purpose for these linked but seperate elements to be measured, but equally being aware of the components identified to make the measurements.

Within the scope of the work relating to mobile communications forensics and evidence examiners cannot create or make up the measurements and components but should look, as a starting point, to the GSM technical standards for guidance from which the examiner could identify a qualifying statement how GSM Timing advance and absolute delay may be performed and then qualify the guidance given in the testing schedules defined within GSM conformance testing.

Importantly, the defintion Timing advance (TA) is a time offset in bits as sent to the MS by the BS. The MS shall advance its transmissions to the BS by the timing advance relative to 3 timeslots behind transmissions received from the BS. The absolute delay is the delay between a common burst reference point within the received and the transmitted RF burst.

To determine the measurements the components used in normal or dummy bursts is, as mentioned above, the common burst reference point and defined as the transition from bit 13 to bit 14 of the midamble.

For an access burst the common burst reference point is defined to be the transition from bit 48 to bit 49 of the burst.

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