7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
1/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
1
Contents
1 Introduction 3
2 BSC overload 5
2.1 BSC overload conditions 6
2.2 BSC overload reactions 7
2.3 Timer control of overload reduction 8
2.4 Traffic reduction algorithm 10
3 MSC overload 15
4 BTS overload 19
4.1 BTS overload conditions 20
4.2
Traffic reduction algorithm 22
4.3 Simultaneous BTS and BSC overload 24
4.4 Parameters for overload handling 25
5 Traffic reduction algorithm 27
5.1 Preemption, queuing 28
Overload handling
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
2/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks2
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
3/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
3
1 Introduction
The aim of the overload control mechanism is to preserve the traffic handlingcapabilities of the mobile base station system (BSS) under adverse conditions.
The overload control mechanism is implemented in the BSC and consists of trafficdetection mechanism and defensive actions.
Traffic detection mechanismconsists of overload messages coming either fromthe various parts of the network (BTS and/or MSC) of from the BSC itself (overloaddetection).
Defensive actionsaimed at reducing both mobile originating (MOC) and/or mobileterminating (MTC) calls (overload handler); the strategy is here presented and follows
the GSM recommendation 08.08.
GSM rec 08.08gives the following definition of the general overload handlingstrategy: The overload causing traffic is reduced in several timer guarded steps untilthe overload is prevented. Otherwise, if for some time no overload is indicated, thetraffic is increased again in timing steps until full load has been resumed.
The time-out values of the timers, number of steps, amount and type of trafficreduced in each step, overload recognition and threshold parameters are allconsidered implementation dependent and have not been specified.
Essentially there are two ways to reduce overload:
reducing the mobile terminating traffic by discarding paging messages,
reducing the mobile originating traffic by discarding CHANNEL REQUIRED orCHANNEL REQUEST messages (dependent on which NE detects the overload).
To reduce overload in moderate way, incoming traffic is not completely rejected, butreduced by several steps of escalation controlled by timers.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
4/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks4
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
5/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
5
2 BSC overload
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
6/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks6
2.1 BSC overload conditions
The following overload conditions are recognized:
Tx buffers for CCS7 links are congested
Buffering is a CCS7 level 2 function handled by PPXL. Congestion may occurbecause of high traffic volume or frequent retransmission (in case of badtransmission quality).
Percentage of busy level 3 radio registers to handle incoming call requestsexceeds a hard-coded threshold of 90 %
For dedicated transactions (e.g. MOC, MTC, SMS, incoming handover, location
update), a (release-dependent) number of level 3 call processing transactionregisters is available.
Real time TDPC load exceeds the threshold OVLSTTHR
The database parameter OVLSTTHR determines the start of TDPC real timeprocessor overload, OVLENTHR gives the end of this overload condition.
Note: This overload condition is only detected if the database flag BSCOVLH is set to"TRUE".
Lack of TDPC memory detected
If the memory occupation of TDPC exceeds 70 %, the overload condition starts. Forvalues below this hard-coded threshold, the overload condition has ended.Oscillations are avoided through the usage of timer T18.
Real time processor load of PPXX exceeds 70 %
The threshold for PPXX processor overload is hard-coded to 70 %.
Lack of PPXX memory resources
If the usage of the PPXX memory goes above the hard-coded value of 70 %, thisoverload condition is detected.
Overflow in BSC paging queue
PAGING messages from MSC are queued in the BSC buffer (the number of bufferedpagings depends on number of the LACs that the BTSs are grouped in and on CCCHconfiguration).
After successful delivery of PAGING COMMAND via Abis, the PAGING messagesare removed from the paging queue.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
7/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
7
2.2 BSC overload reactions
For all overload conditions (with the exception of "Overflow in BSC Paging Queue"),the overload actions taken by the BSC are
alarm message "BSC Overload Detected" (incl. specific overload cause value) issent to RC/LMT/event log file
BSSMAP OVERLOAD message is sent to MSC (with cause and without cellidentifier what means general overload situation)
mobile terminating traffic (PAGING messages from MSC) as well as
mobile originating traffic (CHANNEL REQUIRED messages from BTS) are
discarded.
For the overload cause " BSC Paging Queue Overflow", neither mobile originatingnor mobile terminating traffic is reduced (directly, e.g. BSC will discard just thosemessages that have arrived in the BSC and can not be put in the paging queue).
However, the following alarm messages are issued:
"BSC Paging Queue Overload" is sent to RC/LMT/event log file and
BSSMAP OVERLOAD message is sent to MSC.Therefore, mobile terminating traffic is reduced indirectly by the MSC.
2.2.1 Mechanism for reduction of MOC and MTC traffic
In the BSC overload handling is based on "overload level" mechanism.
The "overload level" determines to which extent the originating or terminating traffic isreduced. It is realized as a counter variable which is automatically increased if after adefined observation period the overload conditions still persist. When the overloadprocess receives the first trigger (BSC overload detected) the BSC overload level isset to 1 and starts overload handling (control and reduction) via timers.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
8/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks8
2.3 Timer control of overload reduction
When the BSC detects an overload situation, it performs the following actions:
start timers T2 and T18,
set overload level to "1",
send "OVERLOAD" message to MSC,
send "overload detected" message to RC/LMT/event log file.
Timer T2 is fixed to 2 sec.
If after the expiry of T2, the overload condition is still present, the variable"overload level" is increased by one step.
If the overload condition has disappeared (at expiry of T2), the variable isdecreased by one step.
After the overload level has reached the level "0" again and the timer T18 expires,the overload condition has ended. If T18 expires before the overload level "0" hasbeen reached it is restarted.
T2: Overload level de-/escalation timer
T18: Overload O&M alarm observation timer
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
9/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
9
To MSC: OVERLOAD
Overload level = 1
Overload level = 2
If overload still present:
T 2 (2 sec) T 18
To O&M: overload detected
Overload
Detected
To MSC: OVERLOAD
If overload still present:To MSC: OVERLOAD
Overload level = 3
If overload NO more present:
Overload level = 2
If T 18 expires foroverload level > 0
T18 is restarted
If overload NO more present:
Overload level = 1
If overload NO more present:
Overload level = 0
Overload
End
To O&M: overload ceased
BSC
Fig. 1 Overload level management
The overload de-/escalation timer T2is fixed to 2 sec. If an overload condition isstill present after expiry of T2, the variable "overload level" is increased. If anoverload condition has disappeared after expiry of T2, the variable "overload level" isdecreased.
The overload observation timer T18is set in the BSC timer package:
DB Name Range
(default)
Unit Meaning
BSC T17 0...254
(HLFSEC-20).
HLFSEC, SEC5 Timer started when anMSC Overloadmessage is received
BSCT18
GSM 08.08
0...254
(HLFSEC-60).
HLFSEC, SEC5 Overload observation
timer
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
10/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks10
2.4 Traffic reduction algorithm
Mobile originating traffic is reduced by discarding CHANNNEL REQUIRED messages(on Abis) in a specific cycle.
Mobile terminating traffic is reduced by discarding PAGING messages (from Ainterface) in a same specific cycle.
The extent to which these messages are discarded depends on the current overloadlevel as shown in the figure 2.
The BSC administers only one overload level variable which determines the extent ofdiscarding both CHANNNEL REQUEST and PAGING messages.
CHANNEL REQUIRED messages with the two following establishment causes are
notdiscarded: "EMERGENCY CALL" and
"ANSWER TO PAGING".
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
11/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
11
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig. 2 Paging and Channel Required messages discarding cycle
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
12/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks12
2.4.1 BSC paging queue overflow
PAGING messages are discarded due to lack of space in the BSC paging queuesavailable for one LAC.
When the percentage of discarded paging messages has exceeded the thresholddefined by PAGQOVLIND in the last second
an alarm BSC Paging Queue Overload is sent to RC/LMT/event log file and
BSSMAP OVERLOAD message is sent to MSC.
In addition, the two following timers are started:
the BSC paging queue overload observation timer T1 (fixed at 1 sec). This timeralso defines the frequency of the BSSMAP OVERLOAD message sending
the Overload O&M alarm observation timer T18.
If during the runtime of T1, PAGING messages are discarded (again), T18 isrestarted (after expiry of T1) and a BSSMAP OVERLOAD message is sent to MSCagain.
If no PAGING messages are discarded during runtime of T1, only T1 is restarted andobservation continues.
If no PAGING messages are discarded during the runtime of T18, when it expires thetimer T1 is stopped and the alarm is ceased.
DB Name Object Range
(default)
Meaning
PAGQOVLIND BSC 0...100
(0, disabled)
Paging queue overloadindication
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
13/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
13
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
To MSC: OVERLOAD
If PAGING msg. discarded:
T 1 (1 sec) T 18
To O&M:BSC Paging Queue Overload
PAGING msg.
from MSCdiscarded
To MSC: OVERLOAD
If PAGING msg. discarded:
If NO PAGING msg. discarded:
If PAGING msg. are
discarded, T1 andT18 are restarted
Overload End
To O&M: BSC Paging Queue
Overload ceased
BSC
If NO PAGING msg. discarded:
If NO PAGING msg. discarded:
If NO PAGING msg.
are discarded, only
T1 is restarted
To MSC: OVERLOAD
Fig. 3 Overload handling algorithm in case of BSC Paging Queue Overflow
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
14/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks14
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
15/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
15
3 MSC overload
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
16/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks16
BSC overload reaction due to MSC overload is controlled with the database flagMSCOVLH.
MSC overload is signaled with the BSSMAP OVERLOAD message from MSC.
BSC reactions to MSC overload message differ in comparison with the BSC overloaddetected, i.e. :
BSC sends an alarm message "MSC overload detected" to RC/LMT/event log file,
BSC will not reduce mobile terminating traffic as the reduction of it is performed bythe MSC already
BSC starts to reduce mobile originating traffic (i.e. systematically discardsCHANNEL REQUIRED messages).
When the BSC receives the first BSSMAP message OVERLOAD from the MSC, thetimers T17 and T18 are started, the overload level is set to 1and the first step oforiginating traffic reduction is performed as the reduction of terminating traffic isperformed by MSC. The alarm "MSC Overload detected" is output.
As long as T17 is running, another MSC overload messages are ignored. If at leastone overload message was received from the MSC in the time period between T17expiry and T18 expiry, the overload level is increased by 1 when T18 expires andboth timers are restarted. If no further OVERLOAD message is received from theMSC, the expiry of T18 triggers the decrease of the load level by one step.
When the overload level has reached the value 0 and timer T18 expired for the lasttime, MSC overload alarm is cleared.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
17/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
17
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
To BSC: MSC OVERLOAD
Overload level = 1
The first step of traffic
reduction
T 17 T 18
To O&M: MSC overload detected
MSC Overload
Detected
To BSC: MSC OVERLOAD
Massage is ignored
If overload still present:To BSC: MSC OVERLOAD
If T 18 expires for
overload level > 0, T17
and T 18 are restartedIf overload NO more present:
Overload level = 1
If overload NO more present:
Overload level = 0
Overload
End
To O&M: MSC overload ceased
BSC
To BSC: MSC OVERLOAD
Overload level = 2
The second step of traffic
reduction
Fig. 4 MSC Overload handling
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
18/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks18
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
19/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
19
4 BTS overload
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
20/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks20
4.1 BTS overload conditions
The BTS overload conditions are recognized by the BSC when some of the followingmessages are received from the BTS:
PCH OVERLOAD message (cause "CCCH overload") from BTS via Abis/RSL
In the BTS two slots (i.e. PAGING REQUEST messages) for each paging group areavailable. The OVERLOAD message is sent if PAGING COMMANDs (from Abis)cannot be placed in a paging queue and therefore the BTS discards them.
DELETE INDICATION message from BTS via Abis/RSL
For each configured CCCH the BTS has an AGCH queue with 16 slots where each
IMMEDIATE ASSIGNMENT COMMAND or IMMEDIATE ASSIGNMENT REJECTmessage requires one place. How fast this is emptied depends on the volume ofpaging messages and the amount of CCCH blocks reserved for AGCH. Already atthe MEDIUM load of the AGCH queue an Overload Indicator on16 bits is issued fromthe BTS towards the BSC to report load history of entries in the queue.
For the AGCH messages discarding in the BTS two scenarios have to bedistinguished in the BSC:
the BSC receives DELETE INDICATION messages without IE 'timeout'
the BSC receives DELETE INDICATION messages with IE 'timeout'.
Within the first scenario the receipt of the DELETE INDICATION message without IE'timeout' is the indication for an AGCH overload situationin the BTS. The DELETEINDICATION message is sent by the BTS to the BSC if an IMMEDIATEASSIGNMENT/REJECT message can not be placed in the respective AGCH queueand, if BTS overload handling is enabled (BTSOVLH=TRUE), leads to output thealarm message 'AGCH QUEUE CONGESTION DETECTED' towards the O&Moutput media.
Within the second scenario the receipt of the DELETE INDICATION message with IE'timeout' is the indication of an IMMEDIATE ASSIGNMENT/REJECT message expirywhile waiting to be transmitted. AGCH messages are not transmitted if they havebeen delayed for too long due to signaling processing and AGCH queuing. In otherwords, IMMEDIATE ASSIGNMENT/REJECT message can be rejected by the BTSimmediately after reception if due to long Abis link propagation or congestion systemcalculates that when message can be scheduled for transmission it will expire. Also,the message will be rejected immediately before its transmission via the Um due tochanges of the actual condition of the queues at the moment of the delivery.
Whenever the BSC receives a DELETE INDICATION message with 'timeout', itindicates this event by the alarm message ' MESSAGES DISCARDED DUE TOTIMEOUT' towards the O&M output media (RC, LMT).
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
21/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
21
Abis LAPD signaling overload UL
PPXL modules have layer 2 buffers for LAPD messages. All transmitted LAPD
messages are buffered until they are positively acknowledged by the remote end (inthis case from the BSC). In case of excessive signaling volume or frequent re-transmission buffer congestion may occur.
When the BTSE has detected an overload situation on the LAPD link based on theLAPD load threshold SLAPDOVLTH, it sends the O&M message LAPD OVERLOADtowards the BSC (only valid for BTSEs of generation BTSplus). If the parameterDLAPDOVL is set to TRUE, this message is the trigger for the BSC to start trafficreduction measures.
If the Abis overload situation has been detected for a particular BTSM, the BSCregards all BTS belonging to this BTSM as overloaded.
BTS detects RACH overload
In case the percentage of busy RACH exceeds the threshold TCCCHLDI, the BTSsends CCCH LOAD INDICATION message to BSC (periodicity PCCCHLDI).
No special overload actions are taken neither by BTS nor BSC.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
22/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks22
4.2 Traffic reduction algorithm
BTS overload handling works similar to BSC overload handling - reducing mobileterminating traffic by discarding PAGING messages and mobile originating traffic bydiscarding CHANNEL REQUIRED messages.
The difference is that OVERLOAD message contains optional IE "Cell Identifier" if theoverload to be reported only concerns specific BTS. Therefore BSC starts themechanism for the traffic reduction towards the affected cell only.
The differences between BSC and BTS overload handling are:
BTS overload handling is restricted to the BTS affected
two different thresholds are administered for discarding of CHANNEL REQUIRED
messages ("OVLD_LEVEL_UPLINK") and PAGING messages("OVLD_LEVEL_DOWNLINK").
PCH overload (after OVERLOAD message from BTS) incrementsOVLD_LEVEL_DOWNLINK only.
AGCH overload, i.e. the receipt of a DELETE INDICATION message (BTS->BSC),does not lead to any overload defense action by the BSC.
if PPXL Abis LAPD signaling overload is detected, both OVLD_LEVEL_UPLINKand OVLD_LEVEL_DOWNLINK are set to the max value ("10").
4.2.1 PCH overload
After the message OVERLOAD "CCCH Overload" has been received by the BSC, ifthe BTS overload handling is enabled (BTSOVLH=TRUE), the BSC outputs:
the alarm message 'BTS OVERLOAD HANDLING STARTED DUE TO PCHQUEUE CONGESTION' with overload cause 'CCCH overload' towards the O&Moutput media like RC, LMT, event log file, etc.,
the BSSMAP message OVERLOAD with cell identity and overload cause 'CCCH
overload' to the MSC,
the mobile terminating traffic is reduced by discarding PAGING messages for theaffected BTS based on the current value of OVLD_LEVEL_DOWNLINK (startingvalue for the variable after the first Abis overload message is "0").
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
23/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
23
4.2.2 AGCH overload
After a message DELETE INDICATION has been received by the BSC, if the BTSoverload handling is enabled (BTSOVLH=TRUE), the BSC outputs:
the alarm message 'AGCH QUEUE CONGESTION DETECTED' or 'AGCHMESSAGES DISCARDED DUE TO TIMEOUT' with overload cause 'AGCHoverload' towards the RC, LMT, event log file, etc.,
the BSSMAP OVERLOAD message with cell identity and overload cause 'CCCHoverload' to the MSC.
No any overload defense action is taken by the BSC. Instead, the discarding ofIMMEDIATE ASSIGNMENT or/and IMMEDIATE ASSIGNMENT EXTENDED
messages is entirely left to the BTS.
4.2.3 Abis LAPD signaling overload
After the Abis LAPD signaling overload condition is detected by the BSC, if thedownlink LAPD overload is enabled (DLAPDOVL=TRUE), the BSC outputs:
the alarm message 'BTS OVERLOAD HANDLING STARTED DUE TO UPLINKLAPD LINK CONGESTION' with overload cause 'AGCH overload' towards theRC/LMT/ event log file,
the BSSMAP message OVERLOAD with cell identity and overload cause 'CCCHoverload' to the MSC.
An algorithm for the reduction of the mobile terminating and originating trafficwhich discards PAGING messages and CHANNEL REUIRED message for theaffected BTS in correspondence with the current value ofOVLD_LEVEL_DOWNLINK starts.
Note: On the first detection of the LAPD overload condition (Abis message LAPDOVERLOAD message received), the variable OVLD_LEVEL_DOWNLINK starts withthe value '1'.
4.2.4 RACH overload
After BTS sends CCCH LOAD INDICATION, no overload defense actions areperformed at all.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
24/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks24
4.3 Simultaneous BTS and BSC overload
In case of BTS and BSC overload occurring simultaneously, the discarding ofmessages for the BTS affected is based on the maximum value of the variablesrelating to BSC overload and relevant BTS overload (OVLD_LEVEL_UPLINK orOVLD_LEVEL_DOWNLINK).
CHANNEL REQUIRED messages with cause "answer to paging" are treateddifferently:
If BSC is discarding PAGING messages coming via A interface (due to BSC orBTS overload) then CHANNEL REQUIRED messages with cause "answer topaging" are not discarded.
If no PAGING messages coming via A interface are discarded (no BSC or BTS
overload condition), the channel filter applies to all CHANNEL REQUIREDmessages except for the messages relating to emergency calls.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
25/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
25
4.4 Parameters for overload handling
SpecificationName
DB Name Object Range
(Default)
Meaning
T3122
GSM 04.08
T3122 BSC 0...255 sec
(5)
Timer to bar a new accessfor a certain time afterunsuccessful access (e.g.no SDCCH available).
EN_BSC_OVL_HAND
BSCOVLH BSC TRUE,
FALSE
(TRUE)
Flag to enable/disableBSC overload handling.
EN_MSC_OVL_HAND
MSCOVLH BSC TRUE,
FALSE
(TRUE.)
Flag to enable/disableMSC overload handling.
EN_BTS_OVL_HAND
BTSOVLH BSC TRUE,
FALSE
(TRUE)
Flag to enable/disableBTS overload handling.
OVL_START_THRESH
OVLSTTHR BSC 7000... 10000(9500)
Threshold for TDPCprocessor (in BSC)overload detection: 10000= 100 %
OVL_END_THRESH
OVLENTHR BSC 7000... 10000(8500)
Threshold for TDPCprocessor (in BSC) belowwhich the sending ofoverload messages isstopped: 10000 = 100 %
CCCH_LOAD_IND_PERIOD
PCCCHLDI BTS 0...255
Unit: 1sec
(255)
Period for sending theCCCH load indicationmessage from BTS toBSC.
CCCH_LOAD_THRESH
TCCCHLDI BTS 0...100
Unit: 1%
(100)
CCCH load threshold, ifthis threshold is exceededthe BTS informs the BSCusing the CCCH loadindication message.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
26/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks26
DB Name Object Range (Default) Meaning
FLAPDOVL BTSM 10100Unit: 1%
(80-60-10)
First LAPD overload threshold,represents the first load levelthresholds of the BTSE RadioSignaling Links (LPDLRs). Itconsists of three fields:
- firstLevelUpperThreshol
- firstLevelLowerThreshold
- firstLevelReductionStep.
SLAPDOVL BTSM 10100
Unit: 1%
(90-80)
Second LAPD overload threshold
defines the second load levelthresholds of the BTSE RadioSignaling Links (LPDLRs). Itconsists of two fields:
- secondLevelUpperThreshold
- secondLevelLowerThreshold.
DLAPDOVL BTSM TRUE, FALSE
(TRUE)
Downlink LAPD Overloadenables or disables theprocedure that detects thedownlink LAPD overload. If theBTSE has detected an overloadsituation on the LAPD link basedon the LAPD load thresholdsSLAPDOVLTH it sends the O&Mmessage LAPD OVERLOADtowards the BSC.
LAPDOVLT BTSM 160
(10)
Unit: 1s
LAPD overload timer defines thetime to pass between twoconsecutive LAPD OVERLOADindication messages.
LAPDOVLT0 BTSM 0,510,0
(1)
Unit:1s
LAPD overload timer 0 definesthe time to pass between twoconsecutive LAPD OVERLOADchecks.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
27/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
27
5 Traffic reduction algorithm
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
28/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks28
5.1 Preemption, queuing
This feature allows the assignment of a TCH when, due to the unavailability ofresources, no unused channel can be found within the serving cell. Three mainprocedures using a subscriber Enhanced Multi-level Precedence and Pre-emption(eMLPP) priorities and a call priority are possible:
Preemption, which is a means of providing TCH resources for high priority TCHrequests
Directed Retry, which causes the handover from a SDCCH in one cell to a TCH inanother cell (described in chapter ''Design of radio cells'')
Queuing, which allows the queuing of TCH requests on a per cell and call prioritybasis.
An extended queuing mechanismWireless Priority Service (WPS) based onpriority class of the subscribers that have no pre-emption capability but the VeryImportant Persons (VIP) service, in particular during the busy hours, is introduced inBR8.0.
WPS feature is specified to further differentiate the call establishment probability ofVIP calls based on the subscribed priority without relying exclusively on pre-emption
as eMLPP.
In case of unavailability of resources, two cases can be foreseen:
Assignment Request
Handover Request.
In case of Assignment Request, Preemption is tried first: BSC looks for a call in thecell with a priority lower than the new TCH request and, then moves the lower prioritycall to another cell performing a Forced Handover (or a Forced Release if HO fails)
in order to free the TCH. The freed TCH is then assigned to the new request. If nocall with a lower priority is using a TCH or a call with the same priority or higher isalready waiting (queued) Directed Retry is performed. If still no free TCH is available,the queuing procedure is carried out, storing the TCH request in the cell queue basedon their priority.
In case of Handover Request, Preemption is tried first. If Preemption is notsuccessful, the Directed Retry procedure will be skipped and Queuing is carried out.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
29/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
29
Preemption feature gives to the operator the possibility to differentiatesubscriptions per user class (e.g. different contracts for high priority class such aspolice and other emergency body, business people and children)
Queuing and Direct Retry features improve the network access and, in general,network QoS
In GSM different priorities (eMLPP, call event and MS category) and access classes(ordinary subscribers get access classes 0-9, 10 is emergency call, while classes 11-15 are special high priority subscribers like police, PLMN operator ...) are merged intothe MSC Priority Information Element being (optionally) conveyed in the ASS REQ orHO REQ message from MSC to BSC. The priority information element contains thefollowing entries:
Priority Information Element
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
spare PCI PRIORITY LEVEL QA PVI
PCI (preemption capability indicator)
It indicates if the preemption shall be applied or not.
PRIORITY LEVEL
It contains the priority of the call; GSM supports the prioritization of calls using14 priority levels (1 is the highest and 14 is the lowest priority)
QA (queuing allowed indicator)
It is used to perform the queuing procedure.
PVI (preemption vulnerability indicator)
It indicates if this connection might be preempted by another allocation request.
The Preemptive Capability Indicator (PCI) applies to the allocation of resources for anevent (e.g. ASS REQ, HO REQ) and indicates whether the event is able to trigger thepreemption of radio resources.
The Preemptive Vulnerability Indicator (PVI) applies for the entire duration of aconnection and indicates whether the connection may become a target ofpreemption.
The Queuing Allowed Indicator (QA) is used to decide on a per call basis if queuingshall be applied or not.
The Priority Level (PL) is subdivided in 14 different levels, Priority Level 1 being thehighest value.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
30/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks30
5.1.1 Preemption
The entire procedure is shown in the following figure. This flow-chart basicallyindicates which algorithms and which checks are necessary in which network entity(BTS, BSC or MSC) and the message flow that has to be conveyed between thenetworks entities. It can be seen immediately that, in the best case, an assignment ofa TCH in a neighbor cell takes place.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
31/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
31
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flowchart of Preemption
ASS_REQ / HO_REQ (no TCH available)
Set PCI=0, PVI=0,
QA=1, PL=14
N
PCI = 1 ?
N
Y
Other queued calls with
PCI=1 and higher or
equal priority ?
Y
Other active call with
PVI=1 and lower PL
N
Successful ?Forced Release of the
lower PL call
BSC assigns the free TCH
Y
N
Y
Y
N
Directed Retry
Procedure
Does the message
contain the priority I.E.?
Send Forced Handover
for the lower PL call
Fig. 5 Flowchart of preemption
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
32/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks32
5.1.1.1 BSC Database Parameters for Preemption
EN_PREEMPTION
DB Name Object Range
(default)
Unit Description
EPRE BTS ENABLEDDISABLED
(DISABLED)
Parameter to enable/disable preemption
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
33/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
33
5.1.2 Wireless Priority Service
Wireless Priority Service (WPS) provides priority access to authorized subscribersletting them gain priority access to the next available radio channel when requestingWPS calls without restricting the publics ability to access the same network. TheBSC receives an Assignment Request or HO request message from the MSC thatcontains the priority information element assigned to the subscriber.
WPS applies 5 different levels of priority to the subscribers; these priority levels aredefined in the HLR and are taken into account in the event of congestion.
The meaning of the WPS priority level based on the mobile access class and eMLPPpriority, which is mapped onto the GSM08.08 priority and the queuing allowed (QA)on the A-interface is shown in the table.
Implementation
dependent, in the range
of 7-14
40 - 9Public Users
63125 (lowest)
5213 and 124
4113 and 123
3014 and 13 and 122
2B14 and 13 and 121 (highest)
GSM08.08 Priority
Level
eMLPP Priority
Level
Access ClassWPS User Priority
Assignment
Fig. 6 WPS user priorities
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
34/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks34
5.1.3 Queuing mechanism
The new queuing mechanism introduces two separate queuing areas:
WPS queue
Public queue
Such approach is known as PURQ-AC algorithm. The algorithm manages systemsbehavior in the event of call arrival as well as call departure.
Call arrival
When new call arrives the algorithm is checking whether radio resource is available.If so, the call is served immediately otherwise the call is queued in either WPS queue
or public queue depending on users subscription.
Call departure
In general the queued calls are served before new arriving ones.
The decision about order of preference (i.e. WPS queue will be served before publicqueue or inversely) is made first. It is determined dynamically on the basis ofmaximum allowed WPS share specified by means of appropriate attributes(parameter WPSPREF).
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
35/38
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
36/38
Overload handling
MN1789EU12MN_0001
2007 Nokia Siemens Networks36
5.1.3.1 BSC Database Parameters for Queuing
The flag to enable queuing (EQ) has to be set as TRUE, otherwise all queuing
parameters for WPS are meaningless.Existing parameters for queuing are modified and additional for WPS are added. It ispossible to set the
length of the queue (QL),
maximum time for Queuing the ASSIGNMENT REQUEST (T11),
maximum time for queuing the HO request (TQHO)
for the WPS and the public queues separately.
DB Name Object Range
(Default)
Unit Meaning
EQ BTS ENABLED/
DISABLED
(DISABLED)
Parameter used to enable/ disable the queuingfeature.
BSCT11PUB BSC 0254
(HLFSEC-16)
HLFSEC=0.5 s
SEC5=5s
Maximum queuing time for
ASS_REQ in publicqueue.
BSCT11WPS BSC 0254
(HLFSEC-16)
HLFSEC=0.5 s
SEC5=5s
Maximum queuing time forASS_REQ in WPS queue.
BSCTQHO
GSM 08.08
BSC 0254
(HLFSEC-20)
HLFSEC=0.5 s
SEC5=5s
Maximum allowed queuingtime for incominghandover.
BSCTQHOPUB BSC 0254
(HLFSEC-16)
MS=100ms
HLFSEC=0.5 s
SEC5=5s
Maximum queuing time forHO_REQ in public queue.
BSCTQHOWPS BSC 0254
(HLFSEC-16)
HLFSEC=0.5 s
SEC5=5s
Maximum queuing time forHO_REQ in WPS queue.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
37/38
Overload handling
MN1789EU12MN_0001 2007 Nokia Siemens Networks
37
DB Name Object Range
(Default )
Unit Description
QLPUB BTS 1100
(50)
Maximum number of TCHrequests that can be queuedin the public area.
QLWPS BTS 1..100
(0 means WPSdisabled )
Maximum number of TCHrequests that can be queuedin the WPS area.
LWWPSPRI BTS 1...10
(NULL)
Lowest WPS priority levelthat is considered in the
queue.WPSPREF BTS Counter Cycle
2...10
(4)
Number WPS
1 3
(1)
WPS Preference, number ofcalls per complete cycle andconsecutively served WPScalls.
Assignment request Queuing Timers
BCST11PUB/ BCST11WPS
Start: ASS_REQ is queued, BSC sends QUEUE_IND to MSC
Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.
Action expiry: remove ASS_REQ from queue, send CLEAR_REQ to MSC
HO request Queuing Timers
Start: HO_REQ is queued, BSC sends QUEUE_IND to MSC
Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.
Action expiry: remove HO_REQ from queue, send HO_FAIL to MSC
With the WPSPREF the portion of WPS calls that is served out of the queue isdefined.
7/13/2019 02 Mn1789eu12mn 0001 Overload Handling
38/38
Overload handling