Gsm Timers 3n2k6k
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GSM Timers Timer Name
Description
Value
4 SACCH multiframes. T100 RADIO-LINK- Detects the presence of the radio link by detecting SACCH frames every That is 1.92 seconds if TIMEOUT 480 ms. the SACCH is completely absent. T200 Data link timer
Used for re-transmission on the data link. The value varies depending on the message type.
155 ms for FACCH
T301 Alerting (ringing) timer
Timer used to limit the amount of time a has to answer a call.
20 seconds
T303 Mobility Management connection timer
Time the network waits after sending a CM SERVICE REQUEST until receiving a response. This occurs before initiating call clearing procedures towards the MS.
10 seconds
T305 Release timer
Time the network waits after transmitting a DISCONNECT message until receiving a RELEASE message.
10 seconds
Time the network waits after transmitting a DISCONNECT message while T306 In-band tones in-band tones/announcements are provided, until receiving a RELEASE release timer message.
10 seconds
Time the network waits after sending a RELEASE message until receiving T308 Release timer a RELEASE COMPLETE message. This occurs before re-transmitting the RELEASE or releasing the Mobility Management connection.
10 seconds
T310 Call proceeding timer
Time the network waits after receiving a CALL CONFIRMED message until receiving a ALERTING, CONNECT, or DISCONNECT message before initiating clearing procedures towards the MS.
10 seconds
T313 Connect acknowledge timer
Time the network waits after transmitting a CONNECT message until receiving the CONNECT ACKNOWLEDGE message before performing clearing procedures with the MS.
10 seconds
T323 Modify complete timer
Time the network waits after sending a MODIFY message during call mode changes, until receiving a MODIFY COMPLETE or MODIFY REJECT message before initiating call clearing procedures.
10 seconds
T3101 Immediate assignment timer
Time the network waits after sending the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message until the main signalling link is established before releasing the newly allocated channels.
1 second
T3103 Handover timer
Time the network waits after transmitting a HANDOVER COMMAND message until receiving HANDOVER COMPLETE or HANDOVER FAILURE or the MS re-establishes the call before the old channels are released. If the timer expires and the network has not received a correctly decoded L2 (format A or B) or TCH frame, then the newly allocated channels are released.
2 seconds
T3105 Physical information repetition timer
Time the network waits after sending the PHYSICAL INFORMATION message until receiving a correctly decoded L2 (format A or B) or TCH frame. This occur before re-transmitting the PHYSICAL INFORMATION message or releasing the newly allocated channels.
50 ms
Time the network waits after transmitting an ASSIGNMENT COMMAND T3107 Channel message until receiving the ASSESSMENT FAILURE message or the MS assignment timer re-establishes the call before releasing the old and the new channels.
3 seconds
T3109 Signaling Time the network waits after sending the CHANNEL RELEASE message disconnection timer before disconnecting the signalling link.
5 seconds
T3111 Channel deactivation after disconnection timer T3113 Paging timer
Time the network waits after disconnecting the signalling link before deactivating the channel. Time the network waits after transmitting the PAGING REQUEST message until receiving the PAGING RESPONSE message. This occurs before re-transmitting the PAGING REQUEST (if the maximum number of
500 ms 5 seconds
Timer Name
Description
Value
re-transmissions have not been exceeded). T3212 Location update timer
The location update timer is set to zero, periodic location update by the MS are disabled. If the MS camps to the BCH and decodes a new MCC or MNC from the one it last camped on, it should perform a location update.
zero = infinite time
T3250 TMSI reallocation timer
Time the network waits after sending the TMSI REALLOCATION COMMAND until receiving TMSI REALLOCATION COMPLETE. This occurs before aborting the procedure and releasing the Radio Resource connection.
5 seconds
T3260 Authentication response timer
Time the network waits after an AUTHENTICATION REQUEST until receiving AUTHENTICATION RESPONSE. This occurs before aborting the procedure and releasing the Radio Resource connection.
5 seconds
GSM Frame Erasure Rate (FER) Measurement Description This section is only applicable to the lab applications and is not applicable to GPRS or EGPRS. You can use the GSM Frame Erasure Rate (FER) measurement to the mobile station's reference sensitivity for control channels. How is the FER Measurement Made? The test set measures FER by sending a Layer 3 message that does not require a Layer 3 response from the mobile station. It does require acknowledgment in the form of an RR frame from the mobile station. When the test set does not receive the RR frame in acknowledgment, it retransmits the Layer 2 message. The test set counts the number of times it resends Layer 2 messages. The test set uses an MM Information message with all the optional fields omitted for the Layer 3 message. You can make the Frame Erasure Rate Measurement on a full-rate FACCH channel (FACCH/F) or a half-rate FACCH channel (FACCH/H). Operating Considerations The FER measurement can only be performed in Active Cell Operating Mode. The connection type must be Auto. FER Measurement Parameters
•
Samples to Test - The number of samples to be taken by the measurement.
•
Minimum Frame Interval (FACCH/F)- The minimum interval between FACCH frames (full rate) being sent to the mobile station.
•
Minimum Frame Interval (FACCH/H)- The minimum interval between FACCH frames (half rate) being sent to the mobile station.
•
Trigger Arm
•
Measurement Timeout
FER Measurement Results
•
Integrity Indicator -
•
Frames Sampled - The count of samples tested.
•
Frames Erased - The count of frames requiring retransmission by the test set.
Frame Erasure Rate - The ratio of Frames Erased to Frames Sampled 0 comments Links to this post GSM Timers-Network side
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. 0 comments Links to this post GSM Timers-MS side GSM Timers
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.
0 comments Links to this post Basic Antenna Definitions Beamwidth - Defined by –3dB power points on both vertical and horizontal planes. - Usually affects the physical size of the antenna. Gain - Defined as the power output relative to an isotropic antenna (Gain 0dB) or Dipole antenna (Gain 2.2dB). Front-to-Back Ratio - Defined as the amount of power in Front direction relative to Back direction. - Usually approximately 20-25dB.
Polarization - Electromagnetic wave consists of both an E Field and H Field. Polarisation usually refers to the direction of the Electric field relative to the intended direction of use for the antenna. Downtilt - Downtilt is required to focus max.power where signal is desired (Coverage). -Downtilt is required to prevent interference to other coverage areas (Interference). 0 comments Links to this post Timing Advance With Calculation A Timing Advance (TA) is used to compensate for the propagation delay as the signal travels between the Mobile Station (MS) and Base Transceiver Station (BTS). The Base Station System (BSS) assigns the TA to the MS based on how far away it perceives the MS to be. Determination of the TA is a normally a function of the Base Station Controller (BSC), bit this function can be handled anywhere in the BSS, depending on the manufacturer.
Time Division Multiple Access (TDMA) requires precise timing of both the MS and BTS systems. When a MS wants to gain access to the network, it sends an access burst on the RACH. The further away the MS is from the BTS, the longer it will take the access burst to arrive at the BTS, due to propagation delay. Eventually there comes a certain point where the access burst would arrive so late that it would occur outside its designated timeslot and would interfere with the next time slot.
Access Burst As you recall from the TDMA Tutorial, an access burst has 68.25 guard bits at the end of it.
This guard time is to compensate for propagation delay due to the unknown distance of the MS from the BTS. It allows an access burst to arrive up to 68.25 bits later than it is supposed to without interfering with the next time slot.
68.25 bits doesnt mean much to us in the sense of time, so we must convert 68.25 bits into a frame of time. To do this, it is necessary to calculate the duration of a single bit, the duration is the amount of time it would take to transmit a single bit. Duration of a Single Bit As you recall, GSM uses Gaussian Minimum Shift Keying (GMSK) as its modulation method, which has a data throughput of 270.833 kilobits/second (kb/s). Calculate duration of a bit.
So now we know that it takes 3.69µs to transmit a single bit.
Propagation Delay Now, if an access burst has a guard period of 68.25 bits this results in a maximum delay time of approximately 252µs (3.69µs × 68.25 bits). This means that a signal from the MS could arrive up to 252µs after it is expected and it would not interfere with the next time slot.
The next step is to calculate how far away a mobile station would have to be for a radio wave to take 252µs to arrive at the BTS, this would be the theoretical maximum distance that a MS could transmit and still arrive within the correct time slot.
Using the speed of light, we can calculate the distance that a radio wave would travel in a given time frame. The speed of light (c) is 300,000 km/s.
So, we can determine that a MS could theoretically be up to 75.6km away from a BTS when it transmits its access burst and still not interfere with the next time slot.
However, we must take into that the MS synchronizes with the signal it receives from the BTS. We must for the time it takes for the synchronization signal to travel from the BTS to the MS. When the MS receives the synchronization signal from the BTS, it has no way of determining how far away it is from the BTS. So, when the MS receives the syncronization signal on the SCH, it synchronizes its time with the timing of the system. However, by the time the signal arrives at the MS, the timing of the BTS has already progressed some. Therefore, the timing of the MS will now be behind the timing of the BTS for an amount of time equal to the travel time from the BTS to the MS.
For example, if a MS were exactly 75.6km away from the BTS, then it would take 252µs for the signal to travel from the BTS to the MS.
The MS would then synchronize with this timing and send its access burst on the RACH. It would take 252µs for this signal to return to the BTS. The total round trip time would be 504µs. So, by the time the signal from the MS arrives at the BTS, it will be 504µs behind the timing of the BTS. 504µs equals about 136.5 bits.
The 68.25 bits of guard time would absorb some of the delay of 136.5 bits, but the access burst would still cut into the next time slot a whopping 68.25bits.
Maximum Size of a Cell In order to compensate for the two-way trip of the radio link, we must divide the maximum delay distance in half. So, dividing 75.6km in half, we get approximately 37.8 km. If a MS is further out than 37.8km and transmits an access burst it will most likely interfere with the following time slot. Any distance less than 37.8km and the access burst should arrive within the guard time allowed for an access burst and it will not interfere with the next time slot. In GSM, the maximum distance of a cell is standardized at 35km. This is due mainly to the number of timing advances allowed in GSM, which is explained below.
How a BSS Determines a Timing Advance
For each 3.69µs of propagation delay, the TA will be incremented by 1. If the delay is less than 3.69µs, no adjustment is used and this is known as TA0. For every TA, the MS will start its transmission 3.69µs (or one bit) early. Each TA really corresponds to a range of propagation delay. Each TA is essentially equal to a 1-bit delay detected in the synchronization sequence.
In order to determine the propagation delay between the MS and the BSS, the BSS uses the synchronization sequence within an access burst. The BSS examines the synchronization sequence and sees how long it arrived after the time that it expected it to arrive. As we learned from above, the duration of a single bit is approximately 3.69µs. So, if the BSS sees that the synchronization is late by a single bit, then it knows that the propagation delay is 3.69µs. This is how the BSS knows which TA to send to the MS.
The Distance of a Timing Advance When calculating the distances involved for each TA, we must that the total propagation delay s for a two-way trip of the radio wave. The first leg is the synchronization signal traveling from the BTS to the MS, and the second leg is the access burst traveling from the MS to the BTS. If we want to know the true distance of the MS from the BTS, we must divide the total propagation delay in half. For example, if the BSS determines the total propagation delay to be 3.69µs, we can determine the distance of the MS from the BTS.
We determined earlier that for each propagation delay of 3.69µs the TA is inceremented by one. We just learned that a propagation delay of 3.69µs equals a one-way distance of 553.5 meters. So, we see that each TA is equal to a distance of 553.5 meters from the tower. Starting from the BTS (0 meters) a new TA will start every 553.5m.
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Excessive Timing Advance (TA) Another Problem Excessive Timing Advance (TA) Drop call due to excessive TA happens when the TA value at drop call connection is higher than the cell parameter TALIM (TADROP > TALIM) and from this counter TFDISTA is incremented.
Location Parameter setting
Probable Reason High sites or sites next to water pick up traffic from far away Very low TALIM setting, which would indicate a ‘false’ excessive timing advance
How to analyze: · Check cell parameter MAXTA and TALIM. If it covers far coverage, it is possible to setting of the cell parameters MAXTA and TALIM to a higher value (for e.g. MAXTA=63, TALIM=62) · If the cell is really covering far away from the site, other options are reducing the coverage by down tilting the antennas, reducing antenna height, changing antenna or reducing output power · If it is a rural area and need to cover a larger area, Extended Range feature might be useful to be considered. Other Reason Drop due other reason equal to total number of drops subtracts all drops with reason. If any of the above drop reason didn’t meet the criteria, the reason for drop will be in the ‘Other Reason’.
H/W fault Disturbance Parameter Setting Mobile Station Interference
Probable Reason Hardware Problem (Managed Object in BTS) Link/ Transmission disturbance problem Wrongly defined setting (for e.g. LAC – Location Area Code) MS problem Interference problem (Uplink)
How to analyze: · Check the BTS error log for hardware faults. (run commands: RXELP & RXMFP to look the hardware faults log) · Check if ICM is indicating uplink interference in the cell.
· Check with O&M regarding transmission problems, HW problems and service affecting maintenance work during the time period. The average cell downtime and TCH availability should also be check. It might be intermittent link connection. · Check object type MOTS, which is based on drop on Timeslot (TS) in order to find faulty devices.
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0 comments Links to this post Congestion Analysis Hi all This is second topic for today that is "Congestion Analysis" a well known word for Telecom professionals .
Traffic congestion is one of the major network problems in a mobile system. A high congestion deteriorates the overall performance of the network and should be minimized. · 1: Short term growth If the high traffic related to an occasional event, like sports event, fairs, conference, a temporary solution might be considered. · 2: Long term growth If there is a long-term growth the network capacity has to grow according to the demand. Type of Congestion The congestion analysis begins by identifying if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may mean that the only way to get rid of the congestion is to add more physical capacity in of transceivers or sites. Consider how many channels that are allocated in the cell. If possible, expand the capacity with new transceivers, otherwise a new site must be implemented. Frequency planning schemes such as MRP and FLP could be used to relieve congestion. Microcells could be used to take traffic in severe congested areas. SDCCH Congestion
In R8, the time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does.
The flowchart below, explains a general approach to investigate SDCCH Congestion. The next section describes the action points in this flowchart. The reference to each action point is indicated on the flow chart as well.
Let me know your suggestions and Happy learning 0 comments Links to this post Low Signal Strength Analysis lets starts todays topic that is Low signal strength analysis What could be the probable cause of low signal while you drive or optimize. First see the following flow chat and try to understand the things
that Low Signal strength is one of the reason of drop call. It can be indicated by many calls disconnected at low signal strength by subscriber, drop calls due to excessive TA, poor handover performance and poor call setup performance. What could be the probable reasons Probable Reason Poor BSC High LOWSSDL & LOWSSUL will give Exchange more drop reason due to SS and this might Property setting not show the actual drop. It is because drop due to SS is more priority than Quality. No dominant cell Cell might be isolated or standalone. Antenna tilt & Too much downtilt sometimes might not orientation cover a larger area and the subscriber might lose the SS. Output Power Low output power might cause smaller border cell. Just try to observed what could be the right cause :-
The following procedure should be performed for low signal strength analysis: 1: Identify the baseline requirement of design and BSC exchange property (setting for LOWSSUL/LOWSSDL). 2:
Check the value for LOWSSDL & LOWSSUL. If it is higher than ACCMIN, change the parameter to a reasonable value since the drop reason will be more priority to SS compared to Quality. 3: Check the site position, antenna direction, position etc. This is to ensure the possible location is open to interference (open water environment) or isolated. Good map is needed for this. 4: Check if the site is sectorized or Omni. If it is Omni, set the cell into sectorized cell. 5: Check if the signal strength is uplink or downlink limited. Mostly, It is designed to be downlink limited. 6: Check the coverage cover expected area from the planet. If it is not, check the antenna tilt and orientation. Change the direction or tilt if it is too much downtilt or pointing to a wrong direction. 7: Sometime, low output power might cause low SS. Check output power and if it is low, increase the output power. 8: Check cell whether it has hotspots from drivetests. If found, adding new site is recommend. 9: In order to check power distribution, run Cell Traffic Recording (CTR) to that particular cell. 10: Check if the cell has indoor coverage problem. If yes, add micro site instead. Need Your suggestions and doubts and let me know if problem is still there... happy learning 0 comments Links to this post TCH Drop Analysis 1. Radio Link Time-Out
Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel.
2. Layer 2 Time-Out
If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.
3. Release Indication When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received. 4. MSC Time-Out Normal Release: If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH. Reject (only SDCCH): If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release). 5. Assignment to TCH Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:
a. There must be a TCH channel available, i.e. no congestion b. The locating algorithm must have received at least one valid measurement report. If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC. TCH Drop reason (1) The classification of TCH Drop Reasons are arranged in the order of priority: 1.ExcessiveTiming Advance 2.Low Signal Strength 3.Bad Quality 4.Sudden Loss of Connection 5.Other Reasons Excessive Timing Advance The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is commonly apparent to isolated or island sites with a wide coverage area. Action: Check if the cell parameter TALIM is < "63" Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.
TCH Drop Reasons (2) Low Signal Strength on Down or Uplink or Both Links The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and LOWSSUL are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason.
Action: Check coverage plots. Check output power. Check power balance and link budget.
Check if Omni site. Check antenna configuration & type. Check antenna installation. Perform drive tests & site survey. Check TRX/TS with high CONERRCNT. Solution: Add a repeater to increase coverage in for example a tunnel. Change to a better antenna (with higher gain) for the base station. Add a new base station if there are large coverage holes. Block/Deblock TRX TCH Drop Reasons (3) Poor Quality on Down or Uplink or Both Links The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and BADQUL (expressed in DTQU) are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD Quality BL will pegged. Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH. Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality.
Action: Check C/I and C/A plots. Check Frequency Plan (Co-BCCH or Co-BSIC Problem). Check MAIO, HOP, HSN parameters. Check FHOP if correctly configured (BB or SY). Check for External Interference. Perform drive tests. Solution: Change BCCH frequency. Change BSIC. Change MAIO, HOP, HSN. Change FHOP. Record RIR or on-site Frequency Scanning to identify source of interference. Use available radio features.
TCH Drop Reasons (4) Sudden Loss of Connection Drops due to Sudden Loss are drops that have not been ed as low signal strength, excessive timing advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement results from the MS. There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very sudden and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission faults), and MS Faults. Action: Check BTS Error Logs, Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis). Check for DIP Slips. Check LAPD Congestion. Correlate Handover Lost to Drops due to Sudden Loss Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Change CONFACT or increase Transmission Capacity Investigate HO Lost Problem TCH Drop Reasons (5) TCH Drops due to Other Reasons TCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad Quality and Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated with hardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost. Action: Check BTS Error Logs. Check Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis).
Check for DIP Slips. Correlate Handover Lost to Drops due to Other Reasons Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Investigate HO Lost Problem
Problem reason of drop in SDCCH Low Signal Strength on Down or Uplink The reason for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure. Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error log Solution: Add new sites. Increase output power. Repair faulty equipment. Poor Quality on Down or Uplink Action: Check C/I and C/A plots. Check frequency plan. Perform drive tests. Solution: Change frequency. Use available radio features. Too High Timing Advance Action: Check if the cell parameter TALIM is < style="font-weight: bold;">Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for cochannel cells. Mobile Error Some old mobiles may cause dropped calls if certain radio network features are used. Another reason is that the MS is damaged and not working properly. Action: Check MS fleet. Solution: Inform operator. Subscriber Behavior Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing illadvised locations to attempt calls, etc. Action: Check customer complaints and their MS. Battery Flaw When a subscriber runs out of battery during a conversation, the call will be ed as dropped call due to low signal strength or others.
Action: Check if MS power regulation is used. Check if DTX uplink is used. Congestion on TCH The SDCCH is dropped when congestion on TCH. Action: Check TCH congestion Solution: Increase capacity on TCH or using features like Assignment to another cell, Cell Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc HOSR Analysis Probable Reasons of Bad Handover Performance ---Neighboring Cell Relation Action:Add neighbor cell relation.
---Missed measurement frequencies in BA-list Action:Check measurement frequencies list.
---Permitted Network Color Code problem Action:Check NCC Permitted
---HW faults. Action: Check BTS error log.
---Blocking on Target Cell Action:Remove Blocking on Tager Cell
---Congestion A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers. Action: Check TCH congestion.
---Timer Expire After MS is Lost The MS never answers the base station. Action: Check coverage. Check interference.
---Link Connection or HW Failure Action: Check BTS error log. Perform site visit. Perform link performance measurements.
---Bad Antenna Installation Action: Perform site survey and check antenna installation. Check antenna cabling.
---Many Neighbors Defined Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC. Action: Check number of definitions. ---Delayed Handover Decision A delayed handover decision can be due to congestion in the target cell. Action: Check handover parameters. ---Wrong Locating Parameter Setting Action: Check locating parameters. ---Bad Radio Coverage Action: Check coverage plots. ---High Interference, Co-Channel or Adjacent The potential handover candidate is disturbed by interference. Outgoing handover due to bad uplink quality may indicate interference from co-channel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel base station. Action: Check interference. Check if many handovers are performed due to downlink or uplink bad quality. ---Receiver Antenna Problem or RBS HW problems (in candidate cell) Action: Check antenna installation. Check RBS HW and Error log of the target cell ---Poor Inter-MSC/BSC Handover Performance For outer or external cell, wrong definitions in either MSC or BSC may be reason for the problem. Action: Check inter-MSC/BSC handover performance. ---Incorrect Down Tilt Action: Perform site survey and check antenna installation. Solution: Correct antenna tilting
Description
Value
4 SACCH multiframes. T100 RADIO-LINK- Detects the presence of the radio link by detecting SACCH frames every That is 1.92 seconds if TIMEOUT 480 ms. the SACCH is completely absent. T200 Data link timer
Used for re-transmission on the data link. The value varies depending on the message type.
155 ms for FACCH
T301 Alerting (ringing) timer
Timer used to limit the amount of time a has to answer a call.
20 seconds
T303 Mobility Management connection timer
Time the network waits after sending a CM SERVICE REQUEST until receiving a response. This occurs before initiating call clearing procedures towards the MS.
10 seconds
T305 Release timer
Time the network waits after transmitting a DISCONNECT message until receiving a RELEASE message.
10 seconds
Time the network waits after transmitting a DISCONNECT message while T306 In-band tones in-band tones/announcements are provided, until receiving a RELEASE release timer message.
10 seconds
Time the network waits after sending a RELEASE message until receiving T308 Release timer a RELEASE COMPLETE message. This occurs before re-transmitting the RELEASE or releasing the Mobility Management connection.
10 seconds
T310 Call proceeding timer
Time the network waits after receiving a CALL CONFIRMED message until receiving a ALERTING, CONNECT, or DISCONNECT message before initiating clearing procedures towards the MS.
10 seconds
T313 Connect acknowledge timer
Time the network waits after transmitting a CONNECT message until receiving the CONNECT ACKNOWLEDGE message before performing clearing procedures with the MS.
10 seconds
T323 Modify complete timer
Time the network waits after sending a MODIFY message during call mode changes, until receiving a MODIFY COMPLETE or MODIFY REJECT message before initiating call clearing procedures.
10 seconds
T3101 Immediate assignment timer
Time the network waits after sending the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message until the main signalling link is established before releasing the newly allocated channels.
1 second
T3103 Handover timer
Time the network waits after transmitting a HANDOVER COMMAND message until receiving HANDOVER COMPLETE or HANDOVER FAILURE or the MS re-establishes the call before the old channels are released. If the timer expires and the network has not received a correctly decoded L2 (format A or B) or TCH frame, then the newly allocated channels are released.
2 seconds
T3105 Physical information repetition timer
Time the network waits after sending the PHYSICAL INFORMATION message until receiving a correctly decoded L2 (format A or B) or TCH frame. This occur before re-transmitting the PHYSICAL INFORMATION message or releasing the newly allocated channels.
50 ms
Time the network waits after transmitting an ASSIGNMENT COMMAND T3107 Channel message until receiving the ASSESSMENT FAILURE message or the MS assignment timer re-establishes the call before releasing the old and the new channels.
3 seconds
T3109 Signaling Time the network waits after sending the CHANNEL RELEASE message disconnection timer before disconnecting the signalling link.
5 seconds
T3111 Channel deactivation after disconnection timer T3113 Paging timer
Time the network waits after disconnecting the signalling link before deactivating the channel. Time the network waits after transmitting the PAGING REQUEST message until receiving the PAGING RESPONSE message. This occurs before re-transmitting the PAGING REQUEST (if the maximum number of
500 ms 5 seconds
Timer Name
Description
Value
re-transmissions have not been exceeded). T3212 Location update timer
The location update timer is set to zero, periodic location update by the MS are disabled. If the MS camps to the BCH and decodes a new MCC or MNC from the one it last camped on, it should perform a location update.
zero = infinite time
T3250 TMSI reallocation timer
Time the network waits after sending the TMSI REALLOCATION COMMAND until receiving TMSI REALLOCATION COMPLETE. This occurs before aborting the procedure and releasing the Radio Resource connection.
5 seconds
T3260 Authentication response timer
Time the network waits after an AUTHENTICATION REQUEST until receiving AUTHENTICATION RESPONSE. This occurs before aborting the procedure and releasing the Radio Resource connection.
5 seconds
GSM Frame Erasure Rate (FER) Measurement Description This section is only applicable to the lab applications and is not applicable to GPRS or EGPRS. You can use the GSM Frame Erasure Rate (FER) measurement to the mobile station's reference sensitivity for control channels. How is the FER Measurement Made? The test set measures FER by sending a Layer 3 message that does not require a Layer 3 response from the mobile station. It does require acknowledgment in the form of an RR frame from the mobile station. When the test set does not receive the RR frame in acknowledgment, it retransmits the Layer 2 message. The test set counts the number of times it resends Layer 2 messages. The test set uses an MM Information message with all the optional fields omitted for the Layer 3 message. You can make the Frame Erasure Rate Measurement on a full-rate FACCH channel (FACCH/F) or a half-rate FACCH channel (FACCH/H). Operating Considerations The FER measurement can only be performed in Active Cell Operating Mode. The connection type must be Auto. FER Measurement Parameters
•
Samples to Test - The number of samples to be taken by the measurement.
•
Minimum Frame Interval (FACCH/F)- The minimum interval between FACCH frames (full rate) being sent to the mobile station.
•
Minimum Frame Interval (FACCH/H)- The minimum interval between FACCH frames (half rate) being sent to the mobile station.
•
Trigger Arm
•
Measurement Timeout
FER Measurement Results
•
Integrity Indicator -
•
Frames Sampled - The count of samples tested.
•
Frames Erased - The count of frames requiring retransmission by the test set.
Frame Erasure Rate - The ratio of Frames Erased to Frames Sampled 0 comments Links to this post GSM Timers-Network side
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. 0 comments Links to this post GSM Timers-MS side GSM Timers
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.
0 comments Links to this post Basic Antenna Definitions Beamwidth - Defined by –3dB power points on both vertical and horizontal planes. - Usually affects the physical size of the antenna. Gain - Defined as the power output relative to an isotropic antenna (Gain 0dB) or Dipole antenna (Gain 2.2dB). Front-to-Back Ratio - Defined as the amount of power in Front direction relative to Back direction. - Usually approximately 20-25dB.
Polarization - Electromagnetic wave consists of both an E Field and H Field. Polarisation usually refers to the direction of the Electric field relative to the intended direction of use for the antenna. Downtilt - Downtilt is required to focus max.power where signal is desired (Coverage). -Downtilt is required to prevent interference to other coverage areas (Interference). 0 comments Links to this post Timing Advance With Calculation A Timing Advance (TA) is used to compensate for the propagation delay as the signal travels between the Mobile Station (MS) and Base Transceiver Station (BTS). The Base Station System (BSS) assigns the TA to the MS based on how far away it perceives the MS to be. Determination of the TA is a normally a function of the Base Station Controller (BSC), bit this function can be handled anywhere in the BSS, depending on the manufacturer.
Time Division Multiple Access (TDMA) requires precise timing of both the MS and BTS systems. When a MS wants to gain access to the network, it sends an access burst on the RACH. The further away the MS is from the BTS, the longer it will take the access burst to arrive at the BTS, due to propagation delay. Eventually there comes a certain point where the access burst would arrive so late that it would occur outside its designated timeslot and would interfere with the next time slot.
Access Burst As you recall from the TDMA Tutorial, an access burst has 68.25 guard bits at the end of it.
This guard time is to compensate for propagation delay due to the unknown distance of the MS from the BTS. It allows an access burst to arrive up to 68.25 bits later than it is supposed to without interfering with the next time slot.
68.25 bits doesnt mean much to us in the sense of time, so we must convert 68.25 bits into a frame of time. To do this, it is necessary to calculate the duration of a single bit, the duration is the amount of time it would take to transmit a single bit. Duration of a Single Bit As you recall, GSM uses Gaussian Minimum Shift Keying (GMSK) as its modulation method, which has a data throughput of 270.833 kilobits/second (kb/s). Calculate duration of a bit.
So now we know that it takes 3.69µs to transmit a single bit.
Propagation Delay Now, if an access burst has a guard period of 68.25 bits this results in a maximum delay time of approximately 252µs (3.69µs × 68.25 bits). This means that a signal from the MS could arrive up to 252µs after it is expected and it would not interfere with the next time slot.
The next step is to calculate how far away a mobile station would have to be for a radio wave to take 252µs to arrive at the BTS, this would be the theoretical maximum distance that a MS could transmit and still arrive within the correct time slot.
Using the speed of light, we can calculate the distance that a radio wave would travel in a given time frame. The speed of light (c) is 300,000 km/s.
So, we can determine that a MS could theoretically be up to 75.6km away from a BTS when it transmits its access burst and still not interfere with the next time slot.
However, we must take into that the MS synchronizes with the signal it receives from the BTS. We must for the time it takes for the synchronization signal to travel from the BTS to the MS. When the MS receives the synchronization signal from the BTS, it has no way of determining how far away it is from the BTS. So, when the MS receives the syncronization signal on the SCH, it synchronizes its time with the timing of the system. However, by the time the signal arrives at the MS, the timing of the BTS has already progressed some. Therefore, the timing of the MS will now be behind the timing of the BTS for an amount of time equal to the travel time from the BTS to the MS.
For example, if a MS were exactly 75.6km away from the BTS, then it would take 252µs for the signal to travel from the BTS to the MS.
The MS would then synchronize with this timing and send its access burst on the RACH. It would take 252µs for this signal to return to the BTS. The total round trip time would be 504µs. So, by the time the signal from the MS arrives at the BTS, it will be 504µs behind the timing of the BTS. 504µs equals about 136.5 bits.
The 68.25 bits of guard time would absorb some of the delay of 136.5 bits, but the access burst would still cut into the next time slot a whopping 68.25bits.
Maximum Size of a Cell In order to compensate for the two-way trip of the radio link, we must divide the maximum delay distance in half. So, dividing 75.6km in half, we get approximately 37.8 km. If a MS is further out than 37.8km and transmits an access burst it will most likely interfere with the following time slot. Any distance less than 37.8km and the access burst should arrive within the guard time allowed for an access burst and it will not interfere with the next time slot. In GSM, the maximum distance of a cell is standardized at 35km. This is due mainly to the number of timing advances allowed in GSM, which is explained below.
How a BSS Determines a Timing Advance
For each 3.69µs of propagation delay, the TA will be incremented by 1. If the delay is less than 3.69µs, no adjustment is used and this is known as TA0. For every TA, the MS will start its transmission 3.69µs (or one bit) early. Each TA really corresponds to a range of propagation delay. Each TA is essentially equal to a 1-bit delay detected in the synchronization sequence.
In order to determine the propagation delay between the MS and the BSS, the BSS uses the synchronization sequence within an access burst. The BSS examines the synchronization sequence and sees how long it arrived after the time that it expected it to arrive. As we learned from above, the duration of a single bit is approximately 3.69µs. So, if the BSS sees that the synchronization is late by a single bit, then it knows that the propagation delay is 3.69µs. This is how the BSS knows which TA to send to the MS.
The Distance of a Timing Advance When calculating the distances involved for each TA, we must that the total propagation delay s for a two-way trip of the radio wave. The first leg is the synchronization signal traveling from the BTS to the MS, and the second leg is the access burst traveling from the MS to the BTS. If we want to know the true distance of the MS from the BTS, we must divide the total propagation delay in half. For example, if the BSS determines the total propagation delay to be 3.69µs, we can determine the distance of the MS from the BTS.
We determined earlier that for each propagation delay of 3.69µs the TA is inceremented by one. We just learned that a propagation delay of 3.69µs equals a one-way distance of 553.5 meters. So, we see that each TA is equal to a distance of 553.5 meters from the tower. Starting from the BTS (0 meters) a new TA will start every 553.5m.
The TA becomes very important when MS switches over to using athe normal burst order to transmit The normal must transmit burst does with not more have precise the 68.25 timing. bits With of guard guard time. time The ofto normal 8.25 only normal has 8.25 burst bits can ofonly guard be slot. received time, so MS the up to MS 30.44µs transmits late and not more interfere than 15.22µs with thethe after next itatime is supposed slot. Because then ofbits, itburst the will two-way interfere trip with ofinthe the next radio time signal, if data. the
Excessive Timing Advance (TA) Another Problem Excessive Timing Advance (TA) Drop call due to excessive TA happens when the TA value at drop call connection is higher than the cell parameter TALIM (TADROP > TALIM) and from this counter TFDISTA is incremented.
Location Parameter setting
Probable Reason High sites or sites next to water pick up traffic from far away Very low TALIM setting, which would indicate a ‘false’ excessive timing advance
How to analyze: · Check cell parameter MAXTA and TALIM. If it covers far coverage, it is possible to setting of the cell parameters MAXTA and TALIM to a higher value (for e.g. MAXTA=63, TALIM=62) · If the cell is really covering far away from the site, other options are reducing the coverage by down tilting the antennas, reducing antenna height, changing antenna or reducing output power · If it is a rural area and need to cover a larger area, Extended Range feature might be useful to be considered. Other Reason Drop due other reason equal to total number of drops subtracts all drops with reason. If any of the above drop reason didn’t meet the criteria, the reason for drop will be in the ‘Other Reason’.
H/W fault Disturbance Parameter Setting Mobile Station Interference
Probable Reason Hardware Problem (Managed Object in BTS) Link/ Transmission disturbance problem Wrongly defined setting (for e.g. LAC – Location Area Code) MS problem Interference problem (Uplink)
How to analyze: · Check the BTS error log for hardware faults. (run commands: RXELP & RXMFP to look the hardware faults log) · Check if ICM is indicating uplink interference in the cell.
· Check with O&M regarding transmission problems, HW problems and service affecting maintenance work during the time period. The average cell downtime and TCH availability should also be check. It might be intermittent link connection. · Check object type MOTS, which is based on drop on Timeslot (TS) in order to find faulty devices.
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0 comments Links to this post Congestion Analysis Hi all This is second topic for today that is "Congestion Analysis" a well known word for Telecom professionals .
Traffic congestion is one of the major network problems in a mobile system. A high congestion deteriorates the overall performance of the network and should be minimized. · 1: Short term growth If the high traffic related to an occasional event, like sports event, fairs, conference, a temporary solution might be considered. · 2: Long term growth If there is a long-term growth the network capacity has to grow according to the demand. Type of Congestion The congestion analysis begins by identifying if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may mean that the only way to get rid of the congestion is to add more physical capacity in of transceivers or sites. Consider how many channels that are allocated in the cell. If possible, expand the capacity with new transceivers, otherwise a new site must be implemented. Frequency planning schemes such as MRP and FLP could be used to relieve congestion. Microcells could be used to take traffic in severe congested areas. SDCCH Congestion
In R8, the time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does.
The flowchart below, explains a general approach to investigate SDCCH Congestion. The next section describes the action points in this flowchart. The reference to each action point is indicated on the flow chart as well.
Let me know your suggestions and Happy learning 0 comments Links to this post Low Signal Strength Analysis lets starts todays topic that is Low signal strength analysis What could be the probable cause of low signal while you drive or optimize. First see the following flow chat and try to understand the things
that Low Signal strength is one of the reason of drop call. It can be indicated by many calls disconnected at low signal strength by subscriber, drop calls due to excessive TA, poor handover performance and poor call setup performance. What could be the probable reasons Probable Reason Poor BSC High LOWSSDL & LOWSSUL will give Exchange more drop reason due to SS and this might Property setting not show the actual drop. It is because drop due to SS is more priority than Quality. No dominant cell Cell might be isolated or standalone. Antenna tilt & Too much downtilt sometimes might not orientation cover a larger area and the subscriber might lose the SS. Output Power Low output power might cause smaller border cell. Just try to observed what could be the right cause :-
The following procedure should be performed for low signal strength analysis: 1: Identify the baseline requirement of design and BSC exchange property (setting for LOWSSUL/LOWSSDL). 2:
Check the value for LOWSSDL & LOWSSUL. If it is higher than ACCMIN, change the parameter to a reasonable value since the drop reason will be more priority to SS compared to Quality. 3: Check the site position, antenna direction, position etc. This is to ensure the possible location is open to interference (open water environment) or isolated. Good map is needed for this. 4: Check if the site is sectorized or Omni. If it is Omni, set the cell into sectorized cell. 5: Check if the signal strength is uplink or downlink limited. Mostly, It is designed to be downlink limited. 6: Check the coverage cover expected area from the planet. If it is not, check the antenna tilt and orientation. Change the direction or tilt if it is too much downtilt or pointing to a wrong direction. 7: Sometime, low output power might cause low SS. Check output power and if it is low, increase the output power. 8: Check cell whether it has hotspots from drivetests. If found, adding new site is recommend. 9: In order to check power distribution, run Cell Traffic Recording (CTR) to that particular cell. 10: Check if the cell has indoor coverage problem. If yes, add micro site instead. Need Your suggestions and doubts and let me know if problem is still there... happy learning 0 comments Links to this post TCH Drop Analysis 1. Radio Link Time-Out
Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel.
2. Layer 2 Time-Out
If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.
3. Release Indication When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received. 4. MSC Time-Out Normal Release: If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH. Reject (only SDCCH): If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release). 5. Assignment to TCH Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:
a. There must be a TCH channel available, i.e. no congestion b. The locating algorithm must have received at least one valid measurement report. If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC. TCH Drop reason (1) The classification of TCH Drop Reasons are arranged in the order of priority: 1.ExcessiveTiming Advance 2.Low Signal Strength 3.Bad Quality 4.Sudden Loss of Connection 5.Other Reasons Excessive Timing Advance The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is commonly apparent to isolated or island sites with a wide coverage area. Action: Check if the cell parameter TALIM is < "63" Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.
TCH Drop Reasons (2) Low Signal Strength on Down or Uplink or Both Links The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and LOWSSUL are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason.
Action: Check coverage plots. Check output power. Check power balance and link budget.
Check if Omni site. Check antenna configuration & type. Check antenna installation. Perform drive tests & site survey. Check TRX/TS with high CONERRCNT. Solution: Add a repeater to increase coverage in for example a tunnel. Change to a better antenna (with higher gain) for the base station. Add a new base station if there are large coverage holes. Block/Deblock TRX TCH Drop Reasons (3) Poor Quality on Down or Uplink or Both Links The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and BADQUL (expressed in DTQU) are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD Quality BL will pegged. Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH. Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality.
Action: Check C/I and C/A plots. Check Frequency Plan (Co-BCCH or Co-BSIC Problem). Check MAIO, HOP, HSN parameters. Check FHOP if correctly configured (BB or SY). Check for External Interference. Perform drive tests. Solution: Change BCCH frequency. Change BSIC. Change MAIO, HOP, HSN. Change FHOP. Record RIR or on-site Frequency Scanning to identify source of interference. Use available radio features.
TCH Drop Reasons (4) Sudden Loss of Connection Drops due to Sudden Loss are drops that have not been ed as low signal strength, excessive timing advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement results from the MS. There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very sudden and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission faults), and MS Faults. Action: Check BTS Error Logs, Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis). Check for DIP Slips. Check LAPD Congestion. Correlate Handover Lost to Drops due to Sudden Loss Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Change CONFACT or increase Transmission Capacity Investigate HO Lost Problem TCH Drop Reasons (5) TCH Drops due to Other Reasons TCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad Quality and Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated with hardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost. Action: Check BTS Error Logs. Check Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis).
Check for DIP Slips. Correlate Handover Lost to Drops due to Other Reasons Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Investigate HO Lost Problem
Problem reason of drop in SDCCH Low Signal Strength on Down or Uplink The reason for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure. Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error log Solution: Add new sites. Increase output power. Repair faulty equipment. Poor Quality on Down or Uplink Action: Check C/I and C/A plots. Check frequency plan. Perform drive tests. Solution: Change frequency. Use available radio features. Too High Timing Advance Action: Check if the cell parameter TALIM is < style="font-weight: bold;">Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for cochannel cells. Mobile Error Some old mobiles may cause dropped calls if certain radio network features are used. Another reason is that the MS is damaged and not working properly. Action: Check MS fleet. Solution: Inform operator. Subscriber Behavior Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing illadvised locations to attempt calls, etc. Action: Check customer complaints and their MS. Battery Flaw When a subscriber runs out of battery during a conversation, the call will be ed as dropped call due to low signal strength or others.
Action: Check if MS power regulation is used. Check if DTX uplink is used. Congestion on TCH The SDCCH is dropped when congestion on TCH. Action: Check TCH congestion Solution: Increase capacity on TCH or using features like Assignment to another cell, Cell Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc HOSR Analysis Probable Reasons of Bad Handover Performance ---Neighboring Cell Relation Action:Add neighbor cell relation.
---Missed measurement frequencies in BA-list Action:Check measurement frequencies list.
---Permitted Network Color Code problem Action:Check NCC Permitted
---HW faults. Action: Check BTS error log.
---Blocking on Target Cell Action:Remove Blocking on Tager Cell
---Congestion A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers. Action: Check TCH congestion.
---Timer Expire After MS is Lost The MS never answers the base station. Action: Check coverage. Check interference.
---Link Connection or HW Failure Action: Check BTS error log. Perform site visit. Perform link performance measurements.
---Bad Antenna Installation Action: Perform site survey and check antenna installation. Check antenna cabling.
---Many Neighbors Defined Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC. Action: Check number of definitions. ---Delayed Handover Decision A delayed handover decision can be due to congestion in the target cell. Action: Check handover parameters. ---Wrong Locating Parameter Setting Action: Check locating parameters. ---Bad Radio Coverage Action: Check coverage plots. ---High Interference, Co-Channel or Adjacent The potential handover candidate is disturbed by interference. Outgoing handover due to bad uplink quality may indicate interference from co-channel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel base station. Action: Check interference. Check if many handovers are performed due to downlink or uplink bad quality. ---Receiver Antenna Problem or RBS HW problems (in candidate cell) Action: Check antenna installation. Check RBS HW and Error log of the target cell ---Poor Inter-MSC/BSC Handover Performance For outer or external cell, wrong definitions in either MSC or BSC may be reason for the problem. Action: Check inter-MSC/BSC handover performance. ---Incorrect Down Tilt Action: Perform site survey and check antenna installation. Solution: Correct antenna tilting
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