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.

GSM Timers

GSM Timers

In the thread Cell Site Analysis-Call Analysis it highlighted the range of Cause Failures for mobile calls. The overview it provided can be quite helpful, but behind those Cause Failures there can be a range of Timers and some of them can be the reason a Cause Failure occurs (positive or negative outcome). For example we can see that timer T3216 (below) in essence relates to the failure of a Immediate Assignment Request, but the "root cause" of the failure can infact be due to SDCCH congestion or poor radio link, such as: interference, coverage restriction or radio path imbalance. Understanding the "Causes for the cessation or loss of mobile communication" requires more than knowing the Cause Code or Timer but all the "root cause" behind them.

The Timer table below provides a useful but not exhaustive list. It essential to keep monitoring the GSM and 3GPP standards. Finally, it is important to recognise that Timers have different durations dependent upon when the timer is applicable. For instance, for radio resources management the durations are often denoted in seconds and some timers are in milliseconds.

However, other timer durations (expiration) are used for internal operation for devices such as mobile telephone or SIM and can be in minutes and in some instances hours. An example of the latter can be the elementary file EFHPLMN (7F206F31) - see GSM11.11. The Timer is set in decimal-digit increments e.g. 01, 02, 03 and so on. Each increment represents a value of n-minutes which the standard GSM0211 refers to as 6 minutes, but commonly rapid updates can cause drain on the mobile telephone's battery it is understood that n-minutes can be 30-minutes. The maximum the timer can be set for is 8-hours. The timer value is network operator dependent, which means either timer method may be used.

Timers and counters for radio resource management

Timers on the mobile station side
T3122: This timer is used during random access, after the receipt of an IMMEDIATE ASSIGN REJECT message.Its value is given by the network in the IMMEDIATE ASSIGN REJECT message.

T3124: This timer is used in the seizure procedure during a hand-over, when the two cells are not synchronized.Its purpose is to detect the lack of answer from the network to the special signal. Its value is set to 675 ms if the channel type of the channel allocated in the HANDOVER COMMAND is an SDCCH (+ SACCH); otherwise its value is set to 320 ms.

T3126:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the immediate assignment procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T. The maximum value of this timer is 5 seconds.

T3128:This timer is started when the mobile station starts the uplink investigation procedure and the uplink is busy.It is stopped at receipt of the first UPLINK FREE message. At its expiry, the uplink investigation procedure is aborted. The value of this timer is set to 1 second.

T3130:This timer is started after sending the first UPLINK ACCESS message during a VGCS uplink access procedure.It is stopped at receipt of a VGCS ACCESS GRANT message.At its expiry, the uplink access procedure is aborted.The value of this timer is set to 5 seconds.

T3110:This timer is used to delay the channel deactivation after the receipt of a (full) CHANNEL RELEASE. Its purpose is to let some time for disconnection of the main signalling link. Its value is set to such that the DISC frame is sent twice in case of no answer from the network. (It should be chosen to obtain a good probability of normal termination (i.e. no time out of T3109) of the channel release procedure.)

T3134:This timer is used in the seizure procedure during an RR network commanded cell change order procedure. Its purpose is to detect the lack of answer from the network or the lack of availability of the target cell. Its value is set to 5 seconds.

T3142:The timer is used during packet access on CCCH, after the receipt of an IMMEDIATE ASSIGNMENT REJECT message. Its value is given by the network in the IMMEDIATE ASSIGNMENT REJECT message.

T3146:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during a packet access procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message during a packet access procedure, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the packet access procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T are defined in section 3.3.1.2. The maximum value of this timer is 5 seconds.

T3164:This timer is used during packet access using CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message. It is stopped at the transmission of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expire, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

T3190:The timer is used during packet downlink assignment on CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message or of an PDCH ASSIGNMENT COMMAND message when in dedicated mode.It is stopped at the receipt of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expiry, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

Timers on the network side
T3101:This timer is started when a channel is allocated with an IMMEDIATE ASSIGNMENT message. It is stopped when the MS has correctly seized the channels. Its value is network dependent. NOTE: It could be higher than the maximum time for a L2 establishment attempt.

T3103:This timer is started by the sending of a HANDOVER message and is normally stopped when the MS has correctly seized the new channel. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the HANDOVER COMMAND, plus the value of T3124, plus the maximum duration of an attempt to establish a data link in multiframe mode.)

T3105:This timer is used for the repetition of the PHYSICAL INFORMATION message during the hand-over procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.

T3107:This timer is started by the sending of an ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly seized the new channels. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the ASSIGNMENT COMMAND message plus twice the maximum duration of an attempt to establish a data link multiframe mode.

T3109:This timer is started when a lower layer failure is detected by the network, when it is not engaged in a RF procedure. It is also used in the channel release procedure. Its purpose is to release the channels in case of loss of communication. Its value is network dependent. NOTE: Its value should be large enough to ensure that the MS detects a radio link failure.

T3111:This timer is used to delay the channel deactivation after disconnection of the main signalling link. Its purpose is to let some time for possible repetition of the disconnection. Its value is equal to the value of T3110.

T3113:This timer is started when the network has sent a PAGING REQUEST message and is stopped when the network has received the PAGING RESPONSE message. Its value is network dependent. NOTE: The value could allow for repetitions of the Channel Request message and the requirements associated with T3101.

T3115:This timer is used for the repetition of the VGCS UPLINK GRANT message during the uplink access procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.

T3117:This timer is started by the sending of a PDCH ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly accessed the target TBF. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the PDCH ASSIGNMENT COMMAND message plus T3132 plus the maximum duration of an attempt to establish a data link in multiframe mode.

T3119:This timer is started by the sending of a RR-CELL CHANGE ORDER message and is normally stopped when the MS has correctly accessed the new cell. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent.NOTE: It could be higher than the maximum transmission time of the RR_CELL CHANGE ORDER, plus T3134, plus the maximum duration of an attempt to establish a data link in multiframe mode.

T3141:This timer is started when a temporary block flow is allocated with an IMMEDIATE ASSIGNMENT message during a packet access procedure. It is stopped when the mobile station has correctly seized the temporary block flow. Its value is network dependent.