Sunday, November 03, 2013

Directed Retry

A fundamental and vital goal of any mobile communication network is to maintain communications between the network and the mobile station (MS), whether the MS is dwelling in an area or on the move. To assist the aims and objectives GSM is commonly known to use 'Handover' for which there is a specific GSM standard TS03.09 [cf W-CDMA see 3GPP TS23.009].

The assumption being made for these cause values is that the MS is seeking to obtain a service for speech calls

│7 6 5│ 4 3 2 1│ │

│0 0 0│0 0 0 0│ │Radio interface message failure │

│0 0 0│0 0 0 1│ │Radio interface failure │

│0 0 0│0 0 1 0│ │Uplink quality │

│0 0 0│0 0 1 1│ │Uplink strength │

│0 0 0│0 1 0 0│ │Downlink quality │

│0 0 0│0 1 0 1│ │Downlink strength │

│0 0 0│0 1 1 0│ │Distance │

│0 0 0│0 1 1 1│ │O and M intervention │

│0 0 0│1 0 0 0│ │Response to MSC invocation │

│0 0 0│1 0 0 1│ │Call control │

│0 0 0│1 0 1 0│ │Radio interface failure, reversion to old channel │

│0 0 0│1 0 1 1│ ││

│0 0 0│1 1 0 0│ │Better Cell │

│0 0 0│1 1 0 1│ │Directed Retry │

│0 0 0│1 1 1 0│ ││

│0 0 0│1 1 1 1│ │Traffic

Key and germane to handover being successful is that operators can use various handover techniques controlled by handover triggering algorithms. These triggers activiate when detection mechanisms identify propagation or network conditions at the existing cell or for the target cell where neither meet a set criteria for usage. One such condition is referred to by Professor Sami Tabbane in Management of Radio Mobility: The Handover Procedure - Intercell and Intra-BSC Handover "A handover that is triggered for reasons of traffic loading and occurs during call setup is called directed retry." 

Examiners are expected to know about Directed Retry, to take account of its possibility when conducting CSA (cell site analysis) investigations and understand its influence and impact on evidence record in call records and associated cell data. A point of contention in evidence for often arises where a defendant states "I was not at the location claimed by the prosecution but was in a different area". Invariably this receives a response "Why does your mobile use the radio coverage from a particular sector (azimuth) from a particular fixed mast (BTS)?" Directed retry makes possible the scenario of having a mobile phone in an adjacent cell from the one shown in the call records. Directed Retry is not a trigger simply triggering every few minutes but arises as Professor Tabbane records, due to traffic loading at the time of call setup.

A mistake that experts and investigators could make would be to ignore the existence of Directed Retry and, even more problematical, not to have asked the question was Directed Retry active at cell/BSC level at the material time of the calls, apart from any intervention within the network.

GSM standards make Directed Retry explicit that which might be implicit to for a GSM radio location area. This logically raises questions how can Directed Retry be configured and activated? Mobile network radio equipment manufacturers offer the capability in their equipment for mobile network engineers to radio fine tune post-installation, and the parameters that can be fine tuned are the Handover triggers of which Directed Retry is one such trigger:

As each equipment manufacturer vary the way fine tuning may be implemented using a GUI to input the trigger parameters is one methiod. Another is to incorporate data into the .mdb or .xls file which has been scripted to produce e.g. an .xml output for uplifting to the radio base station database. This means Directed Retry can be checked that it is active in a particular GSM radio location area. Furthermore, due to continuing radio fine tuning updates to the trigger parameters can occur and older versions of .mdb/.xls maybe recovered from archive.

Experts and Investigators will need to be aware of the triggers Directed Retry (DR) and Forced Directed Retry (FDR) and identify when, in a mobile network, either of these triggers would be implemented and activated for the radio network. This equally means tracking down the equipment manufacturers that offer one form or another or both forms of Directed Retry.

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