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 - 8.1.4.2 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.

CSA: Directed Retry Can Alter Mobile Phone's Location

CSA: Directed Retry Can Alter Mobile Phone's Location

Class│Value│
0 0 0│1 1 0 1│ Directed Retry

The accuracy or inaccuracy of cell site analysis testing measurements largely depends upon what has been considered and there are indeed many points to consider. One handover (HO) procedure, if it is included within a mobile network's radio-availability and traffic-flow arsenal, is called Directed Retry (DR). The GSM and 3GPP standards refers to this procedure.



What is Directed Retry (DR)?
Directed retry has adjustable parameters in order to define thresholds that once passed can trigger DR. When DR is set as Not Use it is inactive. Once set to Use the default value is set until the parameter is adjusted. That is to say a 'value' that is set as default can be modified in response to condtions eg quality of service (QoS) or traffic observations. A manufacturer of the say the BSS may provide recommended values, but it might be the OMC-R or BSS engineering team may require to make their own determination about values for internal or external handover procedures.

Use DR enables for example the BSS to move a mobile phone's communications to another cell (Mast or sector of a Mast) prior to call set up. That can be for an outgoing or incoming communication.

DR may be triggered by, for instance, due to 'congestion' and therefore may require internal or external handover procedures to combat that traffic condition. An outcome is that a mobile phone that receives service from the current serving cell (maybe the Mast is seen as closer to the mobile phone as well) is handed over to a cell that originates from a Mast that could be eg:

- some distance from the mobile phone's actual location
- coverage from a adjacent Mast in an area
- etc

This is one of many radio cases that when conducting radio test measurments a 2G/3G passive radio detection device and its readings may not record the appropriate network messages and thus mis-inform their users attempts in assessing a mobile phone's general location when conducting cell site analysis, as the device's readings may be incomplete. The Cell ID obtained from a call detail record (CDR) can only reflect the antenna identities on a fixed-positioned Mast and that a mobile phone has had its communications routed to and from the network using a particular Mast (so to speak). It doesn't automatically follow that the Cell ID confirms the general local area in which the mobile phone was actually located without certain radio data and other necessary checks being made.