Dropped Call(TCH Drop-SDCCH Drop)
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 criterions 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.Excessive Timing 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 registered 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 registered
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
Dropped Call due to Sudden Drop
On circuit switch service, when a call is abnormally disconnected, a Clear Message
with cause code Call Control be treated as normal Disconnection is sent to the MSC –
named Clear Request Message.
Refer to Ericsson system, the following Urgency condition is checked at that time and
the relevant counter is incremented as a consequence:
1.
2.
3.
4.
Excessive TA
Low Signal Strength
Bad Quality
Sudden Drop
As named, straight forward meaning for the dropped call is described for the first
three items. However, Sudden Drop is quite not easy to understand.
Sudden Loss is drops that have not been registered as bad quality, signal strength,
timing advance. The term Sudden Loss is used because if the network cannot establish
a connection with the lost MS after a pre-defined period, the sudden loss counter is
incremented if the last reported measurement from the MS does not fulfill any of the
reasons
mentioned.
A connection is marked as sudden loss if none of the three types of urgency states
(that is excessive TA, low signal strength or bad quality) are indicated and the locating
procedure indicates missing measurement results from the MS.
Drops due to ‘Other’ reasons are generally associated with hardware problems and
disturbances; number of drops due to ‘Other’ reasons is obtained by subtracting the
drops with known reasons from the total number of drops.
Main contributors in sudden and other TCH drop:
•
•
•
•
•
•
•
Very sudden and severe drop in signal strength, such as when subscribers enter
into buildings, elevators, parking garages, etc.
Very sudden and severe occurrence of interference or bad quality.
MS running out of battery during conversation.
Handover Lost.
BTS HW faults.
Synchronization or Abis link fault (transmission faults).
MS Faults.
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.
Probable Reasons of SDCCH Congestion
---Low Availability
Action: Check SDCCH Availability. Check if the channels are manual, control or
automatic blocked.
---Increasing Traffic Demand
The high traffic could be related to an occasional event or due to a long term growth.
Action: Check if short term traffic growth. Make trend comparisons. Check if
combined SDCCH is used. Check SDCCH dimensioning.
---Bad use of Adaptive configuration of Logical Channels
By using the Adaptive configuration of logical channels feature, the basic SDCCH
configuration in a cell will be under-dimensioned. If this feature is not used correctly,
it will cause SDCCH congestion.
Action: Check if ACSTATE is on. Check parameters related to Adaptive configuration of
logical channels
---Long Mean Holding Time
If the mean holding time is long, this generates a higher traffic load.
Action: Check SDCCH Mean Holding Time
---Too Frequent Periodic Registration
Action: Check Random Access Distribution. Check the timer T3212 in the BSC and the
parameters
---BTDM and GTDM in the MSC
Solution: Decrease the periodic registration.
---Location Area Border Cell
If the cell is situated on a misplaced Location Area border, this means that
unnecessary many normal LUs are performed.
Action: Check site position and location area border. Check Location Update
Performance. Check parameter CRH etc.
---Extensive SMS Usage
Extensive SMS usage increases the SDCCH traffic and could cause congestion if badly
dimensioned SDCCH channels.
Action: Check SMS activity.
---Cell Broadcast Used
Action: Check if Cell Broadcast is active. .If active, check if it is used by the operator.
---IMSI Attach/Detach in Use.
An introduction of IMSI attach/detach will increase the traffic on SDCCH. However, the
benefits are that the paging success rate will increase. The recommendation is to use
Attach/Detach.
---Cell Software File Congestion
Action: Check SAE setting. High Ratio of Random Accesses
Action: Check Random Access performance
SDCCH drop reasons
1)High intreference of freq.,like co-channel
2)SDCCH time slot faulty
3)Poor TRX DL quality
4)Hardware fault like antenna or duplexer malfunction
5)May be feeder cable and connectors are faulty.
6)Site taking calls from a very far distance.
GSM Mobile Terminated Call
1. The PSTN subscriber dials the MS’s telephone number (MSISDN), the MSISDN is
analyzed in the PSTN, which identifies that this is a call to a mobile network
subscriber. A connection is established to the MS’s home GMSC. The PSTN sends
an Initial Address message (IAM) to the GMSC.
2. The GMSC analyzes the MSISDN to find out which HLR, the MS is registered in,
and queries the HLR for information about how to route the call to the serving
MSC/VLR. The HLR looks up the MSISDN and determines the IMSI and
the SS7 address for the MSC/VLR that is servicing the MS. The HLR also checks
if theservice, “call forwarding to C-number” is activated, if so, the call is
rerouted by the GMSC to that number.
3. The HLR then contacts the servicing MSC/VLR and asks it to assign a MSRN to
the call. [MSRN - Mobile Station Routing Number].The MSC/VLR returns an
MSRN via HLR to the GMSC.
4. The GMSC sends an Initial Addressing message (IAM) to the servicing MSC/VLR
and uses the MSRN to route the call to the MSC/VLR. Once the servicing
MSC/VLR receives the call, the MSRN can be released and may be made
available for reassignment.
5. The MSC/VLR then orders all of its BSCs and BTSs to page the MS. Since the
MSC/VLR does not know exactly which BSC and BTS the MS is monitoring, the
page will be sent out across the entireLocation Area(LA).
6. When the MS detects the paging message to the BTS’s in the desired LA. The
BTS’s transmit the message over the air interface using PCH. To page the MS,
the network uses an IMSI or TMSI valid only in the current MSC/VLR service
area.
7. When the MS detects the paging message, it sends a request on RACH for a
SDCCH.
8. The BSC provides a SDCCH, using AGCH.
9. SDCCH is used for the call set-up procedures. Over SDCCH all signaling
preceding a call takes place. This includes: Marking the MS as “active” in the
VLR. Authentication procedure (Start ciphering, Equipment identification).
10. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS
are told to tune to the TCH. The mobile phone rings. If the subscriber answers,
the connection is established.
GSM IDENTITY NUMBERS(IMSI,TMSI,CGI,MSRN,IMEI)
GSM identities
The GSM network is complex and consists of the Switching System (SS) and the Base
Station System (BSS). The switching system, which consists of HLR, MSC, VLR, AUC and
EIR, interfaces both the Base Station System and also other networks like PSTN/ISDN,
data networks or other PLMNs.
In order to switch a call to a mobile subscriber, the right entities need to be involved.
It is therefore important to address them correctly. The numbers used to identify the
identities in a GSM/PLMN network is described in this chapter. See also Figure 56.
Numbering plans are used to identify different networks. For a telephone number in
the PSTN/ISDN network, numbering plans E.164 is used.
Mobile Station ISDN Number (MSISDN)
The MSISDN is a number which uniquely identifies a mobile telephone subscription in
the public switched telephone network numbering plan. According to the CCITT
recommendations, the mobile telephone number or catalogue number to be dialled is
composed in the following way:
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
A National Destination Code is allocated to each GSM PLMN. In some countries, more
than one NDC may be required for each GSM PLMN. The international MSISDN number
may be of variable length. The maximum length shall be 15 digits, prefixes not
included.
Each subscription is connected to one Home Location Register (HLR).
The length of the MSISDN depends on the structure and numbering plan of each
operator, as an application of CCITT recommendation E.164.
The following is an example of dialling a GSM subscriber.
International Mobile Subscriber Identity (IMSI)
The IMSI is the information which uniquely identifies a subscriber in a GSM/PLMN.
For a correct identification over the radio path and through the GSM PLMN network, a
specific identity is allocated to each subscriber. This identity is called the
International Mobile Subscriber Identity (IMSI) and is used for all signalling in the
PLMN. It will be stored in the Subscriber Identity Module (SIM), as well as in the Home
Location Register (HLR) and in the serving Visitor Location Register (VLR).
The IMSI consists of three different parts:
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
MSIN = Mobile Subscriber Identification Number (max 10 digits)
According to the GSM recommendations, the IMSI will have a length of maximum 15
digits.
All network–related subscriber information is connected to the IMSI. See also Figure
56.
Mobile Station Roaming Number (MSRN)
HLR knows in what MSC/VLR Service Area the subscriber is located. In order to provide
a temporary number to be used for routing, the HLR requests the current MSC/VLR to
allocate and return a Mobile Station Roaming Number (MSRN) for the called subscriber,
see Figure 56.
At reception of the MSRN, HLR sends it to the GMSC, which can now route the call to
the MSC/VLR exchange where the called subscriber is currently registered.
The interrogation call routing function (request for an MSRN) is part of the Mobile
Application Part (MAP). All data exchanged between the GMSC - HLR - MSC/VLR for the
purpose of interrogation is sent over the No. 7 signalling network.
The Mobile Station Roaming Number (MSRN), according to the GSM recommendations,
consists of three parts:
MSRN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
Note: In this case, SN is the address to the serving MSC.
Temporary Mobile Subscriber Identity (TMSI)
The TMSI is a temporary number used instead of the IMSI to identify an MS. It raises
the subscriber’s confidentiality and is known within the serving MSC/VLR-area and
changed at certain events or time intervals. The structure of the TMSI may be chosen
by each administration but should have a maximum length of four octets (8 digits).
International Mobile station Equipment Identity (IMEI)
The IMEI is used for equipment identification. An IMEI uniquely identifies a mobile
station as a piece or assembly of equipment. (See IMEI, chapter 5.)
IMEI = TAC + FAC + SNR + sp
TAC = Type Approval Code (6 digits), determined by a central GSM body
FAC = Final Assembly Code (2 digits), identifies the manufacturer
SNR = Serial Number (6 digits), an individual serial number of six digits uniquely
identifying all equipment within each TAC and FAC
sp = spare for future use (1 digit)
According to the GSM specification, IMEI has the length of 15 digits.
Location Area Identity (LAI)
LAI is used for location updating of mobile subscribers.
LAI = MCC + MNC + LAC
MCC = Mobile Country Code (3 digits), identifies the country. It follows the same
numbering plan as MCC in IMSI.
MNC = Mobile Network Code (2 digits), identifies the GSM/PLMN in that country and
follows the same numbering plan as the MNC in IMSI.
LAC = Location Area Code, identifies a location area within a GSM PLMN network. The
maximum length of LAC is 16 bits, enabling 65 536 different location areas to be
defined in one GSM PLMN.
Cell Global Identity (CGI)
CGI is used for cell identification within the GSM network. This is done by adding a Cell
Identity (CI) to the location area identity.
CGI = MCC + MNC + LAC + CI
CI = Cell Identity, identifies a cell within a location area, maximum 16 bits
Base Station Identity Code (BSIC)
BSIC allows a mobile station to distinguish between different neighboring base
stations.
BSIC = NCC + BCC
NCC = Network Colour Code (3 bits), identifies the GSM PLMN.
Note that it does not uniquely identify the operator. NCC is primarily used to
distinguish between operators on each side of border.
BCC = Base Station Colour Code (3 bits), identifies the Base Station to help distinguish
between BTS using the same BCCH frequencies
Location Number (LN)
Location Number is a number related to a certain geographical area, as specified by
the network operator by ”tying” the location numbers to cells, location areas, or
MSC/VLR service areas.
The Location Number is used to implement features like Regional /Local subscription
and Geographical differentiated charging.
GSM Mobile Originating Call Flow
Mobile User calling a Land Line Subscriber.
1. MS
after
dialing
a
number
&
pressing
SEND
key,
sends Channel
Request(Chan_Req) message on RACH to ask for a signalingchannel (Radio
Resources). [RACH - Random Access channel]
2. The BSC allocates a Traffic Channel(TCH) using AGCH. TCH allocation assigns a
specific Frequency & a Timeslot on that frequency. [AGCH - Access Grant
Channel]
3. The MS sends a call setup request through SDCCH, to the MSC/VLR. [SDCCH
- slow dedicated control channel]. Over SDCCH, all signaling takes place.
This includes: marking the MS status as active in the VLR
4. Then comes Authentication Procedure which includes Ciphering (The channel is
ciphered so as to protect the call), Equipment Identification, etc.
5. The MSC/VLR instructs the BSC to allocate an Idle TCH (this message contains
the dialed digits and other information needed for call establishment). The BTS
and MS are told to tune to the TCH.
6. The MSC allocates a voice circuit on one the digital trunks between the MSC
and the BSS.
7. MSC informs the BSS about the allocated voice circuit. The call is also switched
from signaling to voice.
8. The BSS notifies the Mobile about the changeover to voice mode.
9. The MSC routes the call and sends the call towards the called subscriber.
10. The PSTN indicates to the MSC that it has received all the digits and the called
subscriber is being rung.
11. The MSC informs the mobile that the called subscriber is beingalerted via a
ring.
12. The called subscriber answers the call.
Drive Testing
The Purpose of Drive Testing
Drive testing is principally applied in both the planning and optimisation stage of
network
development. However, there are other purposes for which drive testing can be used:
•To provide path loss data for initial site survey work
•To verify the propagation prediction during the initial planning of the network.
•To verify the network system parameters, as defined in the EG8: GSM/DCS
System-Specific Parameters.
•To provide the initial test parameters used in Benchmarking (as defined in the
“Analysis” section of the Network Performance and Monitoring Guideline).
•To verify the performance of the network after changes have been made e.g.
When a new TRX is added; the removal or addition of a new site; any power
Adjustments or changes to the antenna; any changes in clutter or traffic habits
such as the addition of new roads etc.
•To measure any interference problems such as coverage from neighboring
Countries.
•To locate any RF issues relating to traffic problems such as dropped or blocked
calls.
•To locate any poor coverage areas.
•To monitor the network against a slow degradation over time, as well as
Monitoring the network after sudden environmental conditions, such as gales
or electrical storms.
•To monitor the performance of a competitor’s network.
When to Drive Test
Drive testing can take place during the day or at night and is dependant upon the
Operator’s requirements and subscriber habits.
Drive testing during the day will mimic the conditions as seen by subscribers, but may
clog up the network if call analysis is being performed.
Drive testing during the night will allow a greater area to be surveyed due to the
reduction
in vehicular congestion. It will also allow for certain test signals to be transmitted and
tested, particularly when setting up a new site, without interrupting normal
operation.
However, night-time testing does not mimic the conditions experienced by
subscribers.
For planning purposes, drive testing is typically performed at night and for
maintenance
purposes, drive testing is performed during the day.
Where to Drive Test
Some areas of a network will have greater performance problems than others. Drive
testing should not be uniform throughout the whole network, but should be weighted
towards areas where there are significant RF problems.
There may be other areas of the network that require temporary coverage during a
certain
time of the year e.g. an exhibition centre or a sports stadium. These areas should be
examined and planned in greater detail.
It is important that a drive test is documented. This is specified by the Operator and
can
either take the form of creating a new item of documentation or filling in an existing
document. All documentation will be passed to Analysts and Engineers, who will need
accurate records of any test work carried out.
----Route Plans
The area to be drive tested is ascertained before leaving the office. There are three
levels
of drive testing depending on the purpose of the test:
Primary Route: This includes all major roads, highways and throughfares and should be
given priority to all other roads when conducting a coverage test, unless a new site is
put
into service for a specific objective.
Secondary Route: This includes all streets, by-streets and compounds, where
accessible,
such as a University Campus. Secondary routes are used in areas where problems have
been located during a primary route test and further investigation is needed.
Miscellaneous Routes: This includes in-building and non-access routes to vehicles such
as shopping malls, golf courses, airports, hotels, conference centres etc.
A route is prepared by photocopying a map and highlighting the route to be driven. For
primary routes, a map of scale no less than 1:20,000 should be used, and a map of
scale
1:10,000 is recommended for secondary routes. It is recommended that the route is
marked in a contiguous circuit, taking account of one-way streets at this stage.
A drive test should be planned in both directions, where possible, and at the same
speed.
This minimises any errors and checks the point of handovers and cell dimensioning. For
new sites that are being tested, it is recommended that the transceiver is forced to
camp
onto the cell (forbidding any handovers) in order to ascertain the full coverage of the
cell.
The test should be re-driven with any forced handovers removed.
Layer 1 Messages
Other Layer 1 criteria that is useful for field measurements include:
C1 • criteria
ARFCN of Serving Cell - • (TCH in dedicated mode, BCCH in idle mode))
Time • Slot (TS)
Layer 3 Messages
All Layer 3 messages should be collected where possible. Layer 3 Messages are used by
Analysts to determine more accurately the cause of a problem within the network.
Some field test equipment can perform basic analysis of particular Layer 3 messages
during data collection. This enables certain conditions such as call classification or
handovers to be flagged to the survey technician.
Call Classification
In principle there are five call classifications, some of which can be sub-divided
further.
Good Calls: These are calls that are successfully placed on the network and
maintained
for the required duration.
Dropped Calls: These are calls that are successfully placed on to the network but are
terminated without authorisation. Using Layer 3 Messages, these calls can be subdivided
into:
End • User Hang-up
System Hang-up •
Other •
Blocked Calls: These are calls that cannot be placed on to the network. Again, using
Layer 3 messages, these can be sub-divided as follows:
System • Busy
End User Engaged •
No • Service
Other •
Roamed Calls: These are calls that are successfully placed on another network.
Roamed
calls may also be good calls or dropped calls.
Noisy Calls: These are calls which have been successfully completed for the duration
of
the call but which experienced a number of noise bursts that a subscriber may find
intolerable. The threshold for determining the level of poor audio is programmed
during
the set-up of the test.
In GSM, this particular classification is very difficult to determine with great accuracy.
It
should be noted that it is not enough to monitor just the RxLEV and the RxQUAL.
Troubleshooting
No Data Collected
Occasionally, the equipment fails to trigger the collection device to save the data to
file.
Check all cables •
Ensure • the Processing Unit is powered
Re-start • the laptop computer
Re-start the equipment •
Re-drive • the test.
No Positional Information Collected
If data is collected using GPS only, it may be possible that satellite reception was lost
during a drive through a tunnel etc. It is important that back-up equipment is used,
such as
a Dead-Reckoning device, since a GPS receiver will re-transmit the last known position
until it receives an update. If the vehicle moves without GPS cover, the data will be
inaccurate and cannot be analysed.
Check the GPS antenna • cable to the receiver
Drive to an open area • and ensure that the GPS system is working correctly
If • required, install a back-up positional device to safeguard against lost GPS
Coverage Holes
If there are patches of poor coverage in unexpected areas, it may indicate the fringes
of a
coverage hole. It is important to re-drive this particular area.
Complete • a route plan using secondary roads as far as possible
Make • notes of any buildings / obstructions that may cause shadowing
Take • note of pedestrian / vehicular habits in the area
Dropped Calls
Dropped calls can be caused by either RF environments or incorrect system
parameters.
The following data should be checked to ensure that it has been collected properly.
Layer 3 Messages •
Neighbour • Cell List (BA Table)
RxLEV (Server • & Neighbour)
RxQUAL (Server • & Neighbour)
Finally, ensure that the automatic setting for the call length is not shorter than that
for the
timer monitoring for unauthorised call drop-outs. The setting should be a minimum of
30 seconds.
Handover Problems
Handover problems are generally caused by inaccurate settings of the handover
boundary.
This can cause ping-ponging, where the server will keep changing, and congestion at
the
switch. Check the following.
The transceiver antenna • is fitted correctly
Collection of Layer 3 • Messages
Collection of Neighbour • Cell List (BA Table)
Collection of Scanning • Information
Collection of Cell • Identities
Collection of T.Adv for • the Serving Cell
Also, ensure that the collection of data from the new serving cell immediately after
the
handover has occurred (particularly RxLEV and RxQUAL) is not timed to occur prior to
the-synchronisation of the transceiver itself.
If a particular serving cell can be isolated as a potential cause of handover problems,
slowly drive around the cell in a radius of around 500m - 1km, checking when
handovers
occur.
Blocked Calls / System Busy
If calls are repeatedly classified as blocked, it is recommended that the drive test is
temporarily halted in order to try and locate the cause.
Check • that the number called is fully functional
Check • that there is adequate coverage from the expected serving BTS
Check • the equipment transceiver is functioning correctly by using an ordinary
mobile to call the office
If all appears • functional, try to place calls through an alternative BTS. If this
succeeds, inform the office immediately and re-suspend the drive test.
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.
KPI Introduction
1. CSSR (CALL SETUP SUCCESS RATE)
Definition: Rate of calls going until TCH successful assignment
2. SCR (SUCCESSFULL CALL RATE)
Definition: Rate of calls going until normal release that is not interrupted by SDCCH DROP,
neither by assignment failures, and neither by CALL DROP.
3. CALL DROP RATE (CDR)
Definition: Rate of all losses of TCH connections during a call in relation to the number of
successful Call Setups
4. HOSR (HAND OVER SUCCESS RATE)
Definition: Successful internal and external outgoing handovers of total number of internal and
external outgoing handover attempts
5. PSR (PAGING SUCCESS RATE)
Definition: Rate of successful paging attempts of total number of paging attempts.The formula
is based on NSS point of view (based on MSC or LAC)
6. LOCATION UPDATE SUCCESS RATE
Definition: Successful location update
attempts of total number of location update attempts. The formula is based on NSS point of
view.
7. SDCCH BLOCK RATE
Definition: SDCCH congestion of total number of SDCCH seizure attempts
8. SDCCH DROP RATE
Definition: Dropped SDCCH connections of total number of SDCCH connections without TCH
congestion.
9. TCH ASSIGNMENT BLOCK RATE
Definition: Rate of TCH unsuccessful seizures during assignment procedure due to congestion
10. TCH Assignment Failure Rate (exclude blocking)
Definition: Rate of RTCH seizure failed (system + radio) during normal assignment procedure
over the total amount of RTCH request for normal assignment procedure
11. EMD (Erlang Minute per Drop)
Definition: Total of Erlang minutes (TCH occupation) in one period measurement per drop call
(after TCH Assignment).
12. TCH Availability
Definition: Available TCH of total number of defined TCH
13. RACH Success Rate
Definition : Rate of Successful RACH over the total number of channel required message
received
TCH ASSIGNMENT SUCCESS RATE
Probable Reason:
1) No dominant serving cell: The serving cell cannot cope with the TCH traffic.
2) Severe congestion on TCH: Failing TCH allocation for assignment or handover due to
congestion
3) Low signal strength for call access: The signal strength might be higher on the BCCH
than on the TCH.
4) Interference: Disturbance on SDCCH or target TCH
5) Faulty transceiver: Faulty equipment
The following procedure should be performed for TCH Assignment analysis:
For TCH assignment success rate, the first thing, check the TCH Time Congestion.
If there is congestion on TCH, it is recommend doing the dimensioning and adding TRU
based on carried TCH traffic demand.
If there is no congestion on TCH, check the output power of the BTS. If the output
power is low, increase the output power.
If the output power is ok, check the faulty BTS by extracting BTS error log.
If hardware fault found, swap or repair HW.
Perform drivetests to check the coverage and received RxLEV.
If no dominant cell or similar signal strengths of a few cells found during drivetests, it
is recommended to add BTS.
If there is no problem on the dominant cell, check the interference whether cochannel or adjacent channel.
Check the disturbance whether it is on SDCCH or target TCH. If disturbance found,
improve the frequency plan.
Mostly, the problems of low TCH assignment are TCH availability and interference.