SDCCH Usage and Dimension Ing

SDCCH Usage and Dimensioning

ALCATEL File Reference Date Edition PageSdcch_dimensioning.doc 3DF 01903 0010 PGZZA 18/01/2002 01 1/35

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SiteVélizy Mobile Networks Division

PROFESSIONAL CUSTOMER SERVICES

Originator(s)

E. SALOMON

E. BEAUMONT

SDCCH USAGEAND

DIMENSIONING

METHODS

DomainDivisionRubricType

Distrib. Codes

Network Design and ArchitectureMND/BU BSS/PCS/NMO/NDA

Methods

RELEASED

PREDISTRIBUTION:

MND/PCS Vélizy: Ch. Blachier MND/SYT Vélizy: A. FreulonMND/PCS/NOD Vélizy: F. Jarreau MND/ND Vélizy: A. Van GyselMND/BR AME OPERATIONS: Vélizy H. Forgeoux

ABSTRACT:This document describes the management of the SDCCH channel and a methodology to dimension thisresource.

APPROVALS

Name

App.

Date

Ch. BLACHIER C. BRECHTMANN




This page is left intentionally blank




SDCCH DIMENSIONING




TABLE OF CONTENT

1 HISTORY.......................................................................................................................................................................................6

2 REFERENCE DOCUMENTS ..................................................................................................................................................7

3 SCOPE.............................................................................................................................................................................................8

4 SDCCH PRESENTATION........................................................................................................................................................94.1 SIGNALLING CHANNELS DESCRIPTION................................................................................................................................... 94.2 SDCCH USAGE...................................................................................................................................................................... 10

5 SDCCH PROCEDURE.............................................................................................................................................................125.1 SDCCH CHANNEL ACTIVATION ............................................................................................................................................ 125.2 SERVICE REQUEST ................................................................................................................................................................... 16

5.2.1 Traffic Channel..............................................................................................................................................................165.2.2 SMS..................................................................................................................................................................................175.2.3 Location Update............................................................................................................................................................195.2.4 IMSI attach/detach........................................................................................................................................................20

5.3 SDCCH CHANNEL RELEASE .................................................................................................................................................. 20

6 SDCCH DIMENSIONING......................................................................................................................................................226.1 CALL MIX .................................................................................................................................................................................. 226.2 SDCCH HOLDING TIME AND SDCCH USAGE PER CALL................................................................................................... 236.3 NUMBER OF CALLS................................................................................................................................................................... 246.4 SDCCH TRAFFIC..................................................................................................................................................................... 246.5 NUMBER OF SDCCH TS......................................................................................................................................................... 256.6 TOOL.......................................................................................................................................................................................... 266.7 RECOMMENDATIONS............................................................................................................................................................... 27

7 IMPACT ON ABIS RESOURCE...........................................................................................................................................29

7.1 OPTIMISATION OF THE ABIS RESOURCE ............................................................................................................................... 297.2 SDCCH ALLOCATION AND RESPONSE TIME....................................................................................................................... 30

7.2.1 Maximum number of SDCCH......................................................................................................................................307.2.2 Compatibility with Half-Rate Mode...........................................................................................................................317.2.3 Throughput .....................................................................................................................................................................31

7.3 ABIS INTERFACE RECOMMENDATION................................................................................................................................... 31

8 SDCCH DIMENSIONING EXAMPLE...............................................................................................................................32

9 CONCLUSION ...........................................................................................................................................................................33

10 GLOSSARY.................................................................................................................................................................................34

01 18/01/2002 Evolium/R&D MCD/NS_UMTS/PCS/NDAEd DATE CHANGE NOTE APPRAISAL AUTHORITY ORIGINATOR




TABLE OF TABLES

Table 1: Alcatel Reference call mix......................................................................................................22Table 2: SDCCH Holding Time...........................................................................................................23Table 3: Inputs of the SDCCH dimensioning tool...............................................................................26Table 4: Outputs of the SDCCH dimensioning tool ............................................................................27Table 5: Recommended SDCCH values..............................................................................................27Table 6: Multiplexed Channel Block presentation...............................................................................29Table 7: Maximum SDCCH number per MCB type ............................................................................30Table 8: MCB scheme compliance with Dual Rate .............................................................................31

TABLE OF FIGURESFigure 1: Radio link establishment for Mobile Originated/ Terminated Call ......................................13Figure 2: Signalling to traffic channel transfer....................................................................................17Figure 3: SMS transmission on SDCCH channel.................................................................................18Figure 4: Release of the SDCCH channel ...........................................................................................21Figure 5: Handling of traffic ................................................................................................................25




1 HISTORY

Ed. Prop. Date Author Comments

01 01 20/12/01 NDA team Creation




2 REFERENCE DOCUMENTS

[1] 3BK 02974 AAAA TQZA ALCATEL 900/1800 BSS System description[2] 3BK 10204 0511 DTZZA Dynamic SDCCH allocation[3] 8BL 00601 0007 TQZZA BSS – Definition of service indicators[4] 3DF 00993 7000 PGZZA Optimum Location Area Planning[5] 3BK 11203 0057 DSZZA Transmission Functional Specification Release B7[6] 3BK 11203 0059 DSZZA Abis Signalling Load B7[7] 3BK 11203 0040 DSZZA BSS Telecom Traffic model[8] “Réseaux GSM”, 5ème édition revue et augmentée, X. Lagrange, Ph. Godlewski, SamiTabbane, Hermes science publication[9] http://www.gsm-support.org/[10] 3DC 21126 0001 TQZZA Functional Feature Description – Immediate AssignmentExtended in Release B5




3 SCOPE

With the growing interest for Short Message Service (SMS), a precise dimensioning of the signallingresources on GSM air interface is a necessity.

Since the first SMS, sent in December 1992 on the Vodaphone Network, from a personal computerto a mobile phone, the SMS market has first grown smoothly to reach nowadays a tremendousdevelopment speed. By May 2001, 15 billion SMS messages were being sent each month, including150 millions in France alone, representing already an important source of income (almost 10%) foroperators. New ranges of application, like taxed SMS, are to be launched in a near future providingmore services to the customer and more revenues to the operators.

In China, one of the largest markets of mobile telephony in the world in terms of subscribers, thetraffic generated by the SMS doubles every month. The Chinese users of portable send on averagethree SMS per month. Globe Telecom, one of the Filipinos mobile operators, treats more than 300messages per month per subscriber, thus underlining the strong potential of the Chinese market.

Mobile messaging is even evolving beyond text by taking a development path from SMS to EMS(Enhanced Message Service) to MMS (Multimedia Messaging Service) where text, sound, animation,and pictures will be supported.

Defining too many SDCCH will lead to a lack of TCH or PDCH resources, and incomes aregenerated from circuit switch or packet switch user data. On the contrary, a lack of SDCCH channelswill result in a high blocking rate for SDCCH channels, with deteriorated TCH allocationconsequences. Once again, it implies a decrease of revenue for the operator.

The optimal number of SDCCH channels depends mainly on the duration of the calls, the number ofLocation updates and the SMS use.

The first part of the document analyses how the Stand-alone Dedicated Control Channel (SDCCH) isimplied in those events. Then, an analysis of the SDCCH dimensioning process is performed. Finally,a user-friendly tool developed to support SDCCH dimensioning is presented with a set ofrecommended SDCCH configurations based on Alcatel typical call-mix.




4 SDCCH PRESENTATION

4.1 Signalling Channels Description

In the BTS, every Transceiver (TRX) serves eight basic physical radio channels using a Time DivisionMultiple. Every physical radio channel is defined by the TRX serving this channel and by the TSnumber (TS 0 ...TS 7). If frequency hopping is not used, a one-to-one relationship exists betweenphysical channels and Radio Frequency (RF) carrier frequencies.

The signalling channel type can be defined using a 3 letters abbreviation:• BCCH - Broadcast Control Channel• CBCH - Cell Broadcast Channel• CCCH - Common Control Channel• FCCH - Frequency Correction Channel• SACCH - Slow Associated Control Channel• SCH - Synchronisation Channel• SDCCH - Standalone Dedicated Control Channel

Those signalling channels are combined in the TRX TS:

• BCC: Main BCCH/CCCH of the Cell• FCCH+SCH+BCCH+CCCH

• CBC: Combined BCCH, i.e. main BCCH/CCCH combined with 4 SDCCH channels• FCCH+SCH+BCCH+CCCH+SDCCH/4+SACCH

• CCC: Multiple CCCH (BCCH+CCCH) - not yet used• SDC: Specifies that the physical channel concerned carries 8 SDCCH channels together with their

SACCHs• SDCCH/8+SACCH/8

• TCH: Traffic Channel (can be used as a full rate or half rate channel)• BLO: Blocked Channel due to radio resources• CBH: Main combined BCCH with Cell Broadcast Channel

• FCCH+SCH+BCCH+CCCH+SDCCH/3+SACCH/3+CBCH• SDH: Specifies that the physical channel concerned carries 7 SDCCH channels together with their

SACCHs with Cell Broadcast Channel• SDCC/7+SACCH/7+CBCH




There are two types of applications for the GSM Cell Broadcast feature:• The applications where the information broadcast relates more or less to the MS operation in the

Network. This type of application is driven directly by the network/operator. These applicationssuch as Home zone indication, charging rate indication or Network condition indication, arevalue-added features for the operator.

• The application where the operator offers the cell Broadcast facility for use by the entities externalto the GSM network. These application such as road traffic information, public safety,advertisement etc, can be a source of additional revenue for the operator.

The two types of applications described can coexist.

The GSM Cell Broadcast reduces the number of available SDCCH channels per TS from 8 to 7. Thisfeature can impact from one to two SDCCH TS. Hence, low impact is foreseen for cells with largeSDCCH configuration.

One SDCCH channel offers a throughput of 800 bit/s.

4.2 SDCCH USAGE

Call set-up is required to establish connection between a Mobile Station (MS) and the Network Sub-System (NSS). The NSS is responsible for managing the connection with the correspondent. Differenttypes of call are defined in the GSM specifications and imply different usage of the radio channels.

The three basics types of call are presented in the following table.

Type of Calls Description-Mobile Management Calls These calls, e.g. location update, are used by the system to gather MS

information. The exchanges are protocol messages only; therefore,only a signalling channel is used.

Service Calls These calls, e.g. SMS and Supplementary Service (SS) calls, pass smallamounts of information. Therefore, only a signalling channel is used.

User Traffic Calls These calls to a correspondent, e.g. speech or data calls (inconnected mode), can pass large amounts of information. Thereforethey require more than a signalling channel and used a trafficchannel.

These channels are associated with FACCH/SACCH. An SDCCH is always assigned for call set-up,even if a TCH is later required for the call. Therefore, SDCCH is implied in the three types of calldescribed above.

The directed retry feature, which can be seen as a special kind of intercell handover that would beapplied during the call set-up process if a radio cell is congested by traffic load, is not considered inthe following dimensioning process.




GPRS introduction has no impact on the SDCCH load. Packet data channels set-ups and RoutingArea updates are realised via AGCH or PAGCH (if Master PDCH is introduced) and then viadedicated GPRS channels.However, AGCH load and consequently Location Area and Routing Area designs will need to bemonitored carefully, especially when GPRS traffic will take off without Master PDCH implemented yet.




5 SDCCH PROCEDURE

5.1 SDCCH channel activation

The MS initiates a call (MOC) by sending a channel_request message, with a Random AccessInformation Value (REF). The REF includes an establishment cause and a random number, used forauthentication. It is transmitted on the RACH channel. The RACH channel is associated with theCCCH channel, which the MS is monitoring while in idle mode. In case of Mobile Terminated Call(MTC), the same procedure is followed except that the channel_request message comes in reply to apaging_request command. The establishment cause field of the REF specifies:

• An emergency call• Call re-establishment• Response to paging• MS originated speech call• Location update• Service call (SMS, etc…)

The MS notes the random number and frame number associated with each channel_requestmessage. These are used by the MS to recognise the response sent from the BSS. This response issent on the AGCH, which can be monitored by several MSs. The MS decodes all messages sent onthis AGCH and only accepts a message with a random number and frame number matching one ofthe last three requests sent.




MS BTS BSC MSC

Channel Request (RACH)

REFChannel Required

REF+RFN+TA

Channel Activation

TA+SDCCH+power

SDCCH Allocation

Channel Activation Ack

Immediate Assign command

TA+SDCCH+power+RFN+REF

REF Storedin MSmemory

MS comparesmessage withREF in memory

SABM

cm+Service Request

Switch toSDCCH Establish Indication

cm+Service Request

SCCP Connection Requestcm+Service Request

SCCP Connection Confirm

UA

Service Request

Service Request mustmatch original sentby MS in the SABM

Immediate Assignment AGCH

TA+SDCCH+RFN+REF

Paging

+cell list IEPaging command

CCCH TS+paging groupPaging request

TMSI/IMSI

Mob

ile

Term

ina

ted

Mob

ile

Ori

gin

ate

d+

Term

ina

ted

REF: Random Access Information ValueRFN: Reduced Frame NumberTA: Timing AdvanceCm: ClassmarkPower: MS power, BTS powerSABM: Set Asynchronous Balanced ModeService Request: Initial layer 3 message including the MS identity and classmarkUA: Unnumbered Acknowledgement

Figure 1: Radio link establishment for Mobile Originated/ Terminated Call

The MS continues to transmit channel_request messages until it receives a response. If no response isreceived before the MS has retransmitted a predefined number of retries, the MS:

• Displays a network error message for all calls except location updates• Performs automatic reselection for location update calls. This means that the MS attempts

random access on a different cell.




On receipt of the channel_request message from the MS, the BTS sends a channel_required messageto the BSC. This message contains the random number sent by the MS, and the timing advancemeasured by the BTS.The BSC checks the channel_required message to ensure it can accept the request. It allocates anSDCCH channel if one is available.

The BSCs then sends a channel_activation message to the BTS. It also set a timer to wait for anacknowledgement from the BTS, indicating that it is ready to activate the channel. The channelactivation message contains:

• A description of the SDCCH to be used• The Timing Advance (TA)• MS and BTS power commands. The MS and BTS power are set to the maximum allowed

in the cell.The BTS initiates the physical layer resources for the channel and sets the LAPDm contentionresolution ready for the first MS message on the SDCCH. It then sends achannel_activation_ackowledgement message to the BSC. The BSC stops then its guard timer.

The BSC builds and sends an immediate_assign_command message reiterating the informationgiven in the channel_activation command. This message also includes the random number andframe number of the original MS request to which the BSC is replying. It also instructs the BTS toinform the MS of the SDCCH channel assignment. The BSC starts a guard timer for the MS torespond.

The BTS sends the immediate_assignment message to the MS on the AGCH.

The MS checks the random number and frame number in the immediate_assignment message. If itmatches those from its last three channel_request sent, the MS switches to the indicated SDCCH andsets its timing advance to the value indicated in the immediate_assignment message.

The first layer 2 frame sent on the SDCCH is a standard LAPDm type frame, known as the SetAsynchronous Balanced Mode (SABM). On the air interface, it establishes the LAPDm connection withthe BTS. This frame can also contain layer 3 messages.

The MS starts its LAPDm connection and sends a layer 3 message in its first frame. The BTS uses thismessage for contention resolution. The BTS sends an acknowledgement to the MS containing thesame layer 3 message. Therefore, only the MS that sent the message can accept the response of theBTS and consider itself connected.




For an MS originated call, the layer 3 message from the MS contains:• An information Element (IE) indicating:

Ø CM service request (speech, SMS, emergency call)Ø Location updating request (Location updating procedure)Ø IMSI detach indication (MS power off)Ø CM re-establishment request (after a failure)

• The MS identity• The MS classmark

The network uses this message to decide which call negotiation procedures are required and whetherto assign a TCH.The BTS sends an establish_indication message to the BSC to indicate that the MS has connected.The BSC stops the guard timer, extracts the classmark information and initiates an SCCP connectionwith the MSC.

The BSC sends an SCCP_connection_request message to the MSC. The MSC replies with anSSCP_connection_confirm message. This message can contain a classmark request or a cipher modecommand.

The signalling link is then established between the MS and the MSC.

SDCCH set-up is then involved in the three types of services enlighten above:• Service request (call set-up, SMS, …)• Location update• IMSI attach/detach




5.2 Service Request

5.2.1 Traffic Channel

The call set-up procedure has been established via SDCCH, as presented above. However, in case ofcall traffic or data traffic in connected mode, the establishment of the communication requires theallocation of a dedicated traffic channel (TCH).

The TCH allocation is realised simultaneously to the BSC/MSC and BTS/MSC resource allocation.Thus, the MSC reserves a BSC/MSC resource on the A interface and sends a TCHassignment_request message to the BSC.

The BSC checks the availability of TCH resources and if available sends a channel_activation to theBTS, containing the references of the TCH to be used. The BTS replies with achannel_activation_ackowledgement message to the BSC.

The BSC can now send a commutation order to the MS via an assignment_command message. Thismessage contains a complete description of the TCH to be used.

The MS commutes then on the new channel and re-establishes a layer 2 connection with the BTS bysending a SABM message. The signalling SABM message is sent on the FACCH (TCH resourcestolen). This message is acknowledged via an UA message.

The MS sends then an assignment_complete message to the BSC.

The BSC can then release the SDCCH resource.




MS BTS BSC MSC

TCH Allocation

Physical context confirm

TA+power

Assign command

TCH

SABM (FACCH)Release SDCCH

UA (FACCH)

SACCH

TA+power update

Assignment Request

Channel type+cm

Physical context request

Channel Activation AckPower+TA

Channel Activation

TCH+TA+cipher+DTX+power

Assign command (SDCCH)

TCH

Assignment complete

Assignment complete (FACCH)

Initiate SDCCH Release

DTX: Discontinuous TransmissionFACCH: Fast Associated Control ChannelSACCH: Slow Associated Control Channel

Figure 2: Signalling to traffic channel transferAn authentication and ciphering procedure might take place before releasing the SDCCH channel toensure that the subscriber identification (IMSI, TMSI) and IMEI are valid and to encrypt the rest of thedata exchanged.

5.2.2 SMS

SMS transmission on the Air interface is realised via SDCCH when the mobile is in idle mode. Theexisting SACCH is used if the mobile is already transmitting.




MS BTS BSC MSC

SABM

cm+Service Request

Switch toSDCCH

Establish Indicationcm+Service Request

SCCP Connection Requestcm+Service Request

SCCP Connection Confirm

UA

Service Request

SDCCH channel allocation

Authentication and CipheringSABM

SAPI=3

UA

SAPI=3

Establish Indication

SAPI=3

SMS Control Protocol - Data

SMS Control Protocol - Ack

SMS Control Protocol - Data

SMS Control Protocol - Ack

Initiate connection Release

SAPI: Service Access Point Identifier

Figure 3: SMS transmission on SDCCH channelThe number of bytes for SMS cannot be estimated because it depends on the message content. Thus,it is necessary to consider an average SMS length.




5.2.3 Location Update

A powered-on mobile is informed of an incoming call by a paging message sent over the PCHchannel. One extreme would be to page every cell in the network for each call, which would beobviously a waste of radio bandwidth. The other extreme would be for the mobile to notify thesystem, via location updating messages, of its current location at the individual cell level, but it wouldbe very wasteful due to the large number of location updating messages issued. A compromisesolution used in GSM is to group cells into Location Areas (LA), and mobile stations are paged in thecells of their current LA.

The Location Update (LU) procedure is always initiated by the MS. Location Update is performed afterthe call has finished. Reasons for location update include:

• Periodic updateThe MS performs periodic LU after a lack of signalling activity for a specific time. If thetimer expires, the MS initiates a LU, even if it has not changed of LA. This periodic locationupdate procedure has been introduced for reliability reasons. If an HLR or MSC/VLR fails,to have each mobile register simultaneously to bring the database up to date would causeoverloading. Therefore, the database is updated as location-updating events occur. Theenabling of periodic updating, and the time period between periodic updates, iscontrolled by the operator, and is a trade-off between signalling traffic and speed ofrecovery. The duration of the MS timer is defined by the network and sent to the MS assystem information messages on the BCCH. This time can range from 6 minutes up to 25hours. If a mobile does not register after the updating time period, it is deregistered.

• Change of location areaThe MS detects a new Location Area Identity (LAI) in the BCCH when the current call hasfinished. The MS camps on a cell with a different location area code to the one in the MSmemory. The MS initiates then a LU procedure. This saves the system performing severalLU if the MS is handed over several times during a call.

In both cases, the MS initiates the LU procedure by sending a channel_request message indicatingthat the call is for a location update (please see Figure 1, page 13).

The BSS assigns a dedicated SDCCH channel and establishes a signalling link between the MS andthe MSC.

When a signalling path is established, the MS sends the LAI of the old cell on which it was camped tothe MSC. The new VLR interrogates the old VLR for authentication and subscriber information. TheBSS adds the cell identity of the MS current location to the message sent to the MSC. The NSS storesthis information in either its HLR or its VLR.




5.2.4 IMSI attach/detach

A procedure related to location updating is the IMSI attach and detach. A detach lets the networkknow that the mobile station is unreachable (mobile off), and avoids having to needlessly allocatechannels and send paging messages. An attach (mobile on) is similar to a location update, andinforms the system that the mobile is reachable again.

IMSI attach/detach is a mobility feature, which primarily concerns the MSC and the MS. Usedtogether with the periodic location update procedure, IMSI attach/detach allows the network toprovide more efficient control and use of the resources.

For example, if a mobile-terminated call arrives for an MS, which is detached, the MSC knows thatthe MS is inactive and does not need to initiate a paging request. For the BSS, this can reduce loadon the PCH.

Initiation of the IMSI attach/detach procedure is controlled by a parameter in the BSS,Attach_detach_allowed. When this parameter is set, the BSS broadcasts system information on all cellindicating that the network supports the IMSI attach/detach feature.

MSs, which have successfully connected and logged themselves onto the network are then obliged toperform IMSI attach/detach procedure.

5.3 SDCCH channel release

The release of the SDCCH channel is initiated by the MSC sending a clear_command message to theBSC. The BSC acknowledges the command and initiates the release of the dedicated channel. Itsends a layer 3 channel_release message to the MS to release the connection and a deactivate_sacchmessage to stop the BTS sending messages on the SACCH channel.

The MS releases then the layer 2 connection by sending a DISC message. The BTS acknowledges thismessage by sending a UA and transmit to the BSC a release_indication message.

The BSC sends then a RF_channel_release message to the BTS. The BTS can now completely releasethe dedicated channel and acknowledges the command by transmitting aRF_channel_release_acknowledge to the BSC.




MS BTS BSC MSC

Channel Release

Release Indication

Return to BCCH channel

UA (SDCCH)

Clear Command

DISC (SDCCH)

Deactivate SACCHChannel Release (SDCCH)

RF Channel ReleaseClear Complete

RF channel Release Ack

Figure 4: Release of the SDCCH channel




6 SDCCH DIMENSIONING

We have seen in the previous chapter that the SDCCHs are used for signalling during call setups,short message services and location updates (directed retry not presented here). This chapter focuseson the SDCCH dimensioning. It defines what are the parameters of interest to perform such adimensioning based on the SDCCH procedure presented in chapter 5 above.

6.1 Call mix

We have seen that the SDCCH are implied in different procedures:• Location update: periodic, new updating, IMSI attach/detach• SMS (Short Message Service, originating or terminated)• Originating and Terminating calls

Directed retry procedure should be considered, but it is similar to TCH queuing procedure (SDCCHholding time is increased) and it is not taken into account in the SDCCH dimensioning methodologyand tool presented below.

To realise a dimensioning of the SDCCH resources, it is necessary to quantify the different types oftransaction. It will lead to define a call mix. The typical B7 Alcatel call mix, given at busy hour, is thefollowing:

Mean TCH holding time 50 secondsMobile Orig. Call 60% percentMobile Term. Call 40% percentLocation Update 3 per callIMSI Attach 0.5 per callIMSI Detach 0.5 per callOrig. SMS PtP 0.3 per callTerm. SMS PtP 0.7 per call

TRAFFIC MIX @ BH PER CALL

Table 1: Alcatel Reference call mix

Call mix varies related to:• Cell: The TCH traffic induced by subscribers and average number of LU/call are cell

dependent• Hour: By default the dimensioning is always realised for the busy hour• Period: Traffic intensity and distribution may be modified according to seasons/ special

events, …

Any SDCCH dimensioning study is based on the call mix values, which might be provided either bymarketing department (new network), measured (on existing network) or derived from assumptions.




The measurement might be realised either via A or Abis interface monitoring or using tools such asAnaQOS or RNO.

On one hand, trace measurements provides more detailed data, such as Mobile terminated SMS orImsi attach values. Those data must be deduced from Imsi detach and Mobile originated SMS values,when working with RNO or AnaQOS.

On the other hand, RNO or AnaQOS measurements can be realised on several days or even weekswhereas trace measurements are restricted to a short period of time.

Whatever the measurement method used, the SDCCH congestion rate indicator allows detectingnon-accurate signalling channel dimensioning.

By default, we introduce three cell profiles based on the position of the cell compared to the LocationArea Border:

• Normal signalling• High signalling• Very high signalling

All the call mix parameters presented above are the same for the three types of profiles, except theratio LU_RATE = (LU + Imsi attach + Imsi detach) / TCH call, which is different for each profile:

• Normal signalling: LU_RATE = 1 (cell which is not at a Location area border)• High signalling: LU_RATE = 4 (cell which includes a Location area border but with no specific

peak of LU)

• Very high signalling: LU_RATE = 8 (cell which includes a Location area subject to peaks of LU)

6.2 SDCCH holding time and SDCCH usage per call

To dimension the number of SDCCH channels needed, the first step is to compute the SDCCH trafficin Erlang. To do so, we need to process the SDCCH holding time. The SDCCH Holding Times (HT)have been measured for the different network events presented in the SDCCH PROCEDURE chapter.Those measurements have been realised with Alcatel test-bed. The average SDCCH holding timeand SDCCH usage per call are then deduced depending on the traffic mix parameter.

Procedure MOC MTC LU IMSI attach IMSI detach SMS SDCCH average HT (s) SDCCH usage per call (s)SDCCH HT per procedure (ms) 3300 2850 4400 2970 2565 6500 - -Alcatel call mix (LU+SMS) 0.6 0.4 3 0.5 0.5 1 4.26 25.59No LU, No SMS 0.6 0.4 0 0.5 0.5 0 2.94 5.89No LU, SMS 0.6 0.4 0 0.5 0.5 1 4.13 12.39LU, No SMS 0.6 0.4 3 0.5 0.5 0 3.82 19.09

Table 2: SDCCH Holding TimeThose values can also be retrieved using RNO or AnaQOS.




6.3 Number of calls

Knowing the SDCCH traffic per call, it is now necessary to obtain the number of calls per hour. Thisvalue is based on the average call duration and on the traffic of the cell. The following formula isused:

callpertrafficcelltheintraffic

Ncalls__

___=

The traffic per call and traffic in the cell can be measured. For dimensioning purposes, it could bebetter to dimension directly the number of SDCCH for the maximum traffic that the cell can handle.In this case, the Erlang B formula, based on the number of available traffic TS and the blockingprobability, gives the traffic in the cell.

For instance, with 4 TRX, 29 TS reserved for voice traffic and a 2% blocking probability we obtain amaximum traffic of 21.04 Erlang. The typical Alcatel call duration is 50 s (14 mErlang). It gives thefollowing number of calls at busy hour, Ncalls:

1503014.0

04.21==Ncalls calls per hour in a 4 TRX cell.

6.4 SDCCH traffic

We can now derive the SDCCH traffic in the cell by multiplying the SDCCH usage per call by thenumber of calls. The following formula gives the total SDCCH traffic in the cell:

3600_

_usageSDCCH

NcallstrafficSDCCH ×=

Following the example presented above, we obtain:

ErlangtrafficSDCCH 17.53600

39.121503_ =×= at busy hour

for a profile with no LU but with SMS (fifth row of Table 2: SDCCH Holding Time).




6.5 Number of SDCCH TS

We want to determine the number of resources necessary to carry this traffic according a blockingprobability pbk. The Erlang B formula is used to derive the number of channels necessary given thetraffic to handle and the blocking probability.Erlang B traffic model is based on the following assumptions:

• An infinite number of calling sources• Random traffic arrival pattern (Poisson distribution)• Blocked calls are cleared• Hold times exponentially distributed

The third assumption, how to handle lost calls, is not exact when dealing with SDCCH traffic. Thefigure below depicts the three options available when the station you are calling does not answer:

Lost Calls Cleared (LCC)Lost Calls Held (LCH)Lost Calls Delayed (LCD)

Figure 5: Handling of traffic

The LCC option assumes that once a call is placed and the server (network) is busy or not available,the call disappears from the system. In essence, you give up and do something different. This is theassumption made by the Erlang B law.

The LCH option assumes that a call will be in the system for the duration of the hold time, regardlessof whether or not the call was placed. In essence, you continue to redial for as long as the hold timebefore giving up. Recalling, or redialing, is an important traffic consideration. Suppose 200 calls areattempted. Forty receive busy signals and attempt to redial. That results in 240 call attempts, a 20%increase. The trunk group is now providing an even poorer GoS than initially thought.

The LCD option means that once a call is placed, it remains in a queue until a server is ready tohandle it. Then it uses the server for the full holding time. This assumption is the one used by theErlang C law.




Assuming that the lost calls will clear the system (Erlang B) tends to understate the number of trunksrequired; on the other hand, LCH overstates the number.

Using the Erlang B formula for dimensioning SDCCH channel is not exact since a Location Update,for instance, would not be tried only once. It is therefore a kind of queuing system and not a total losssystem even if the immediate assignment reject failure, implemented in our BSS, prevents the MS torepeat its SDCCH request in case of SDCCH congestion. Anyway, one must keep in mind that the useof the Erlang B formula understates the number of SDCCH channels required.

We obtain finally:

),_( bkptrafficSDCCHErlangBNchannels =

For our previous example, with a pbk equal to 0.5%, it gives:

15)005.0,17.5( == ErlangBNchannels channelsNow that we have processed the number of channels, we can derive the number of TS needed. If theTS are of the SDC type (please see 4.1 above), 8 channels are available per TS. Thus, to cope with15 channels, it will be necessary to implement 2 TS.

6.6 Tool

An Excel tool, entitled “SDCCH_dim”, has been developed to allow a rapid computation of theSDCCH configuration needed based on a precise call-mix. The inputs, enlighten in green, are thefollowing:

Measured cell traffic 21.04 ErlangMean TCH holding time 50 secondsNb voice TRX in cell 4 TRXTCH required blocking probability 2.00% percentSDCCH Required blocking probability 0.50% percent Mobile Orig. Call 60% percent Mobile Term. Call 40% percent Location Update 0 per call IMSI Attach 0.5 per call IMSI Detach 0.5 per call Orig. SMS PtP 0.3 per call Term. SMS PtP 0.7 per call

TRAFFIC MIX @ BH

inp

uts

Combined

Not combined

None

1 SDH

2 SDH

BCCH

SDH

Table 3: Inputs of the SDCCH dimensioning toolBy default, the traffic in the cell (in blue) is the maximum allowed by the blocking probability and itsconfiguration. Note that since GPRS traffic has no impact on SDCCH dimensioning, the traffic to betaken into account is the one provided only by the circuit resources. If any GPRS dedicated TRXs exist,they should not be taken into account for SDCCH dimensioning.




After entering your input in the table, you have to choose:• Whether the BCCH is in combined or not combined mode (Option buttons in BCCH group

box)

• Whether the cell broadcast is introduced and with one or two channels (Option buttons inSDH group box)

Then, you have to press the Process button at the right hand side of the Excel sheet.

Process

The outputs of the tool, enlighten in light yellow, are the following:Nb TCH TS 29 TSNb SDH TS 0 TSNb SDC TS 2 TSTotal SDCCH TS 2 TSProcessed SDCCH blocking probability 0.0550% percentMaximum cell traffic 21.04 Erlang

outp

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Table 4: Outputs of the SDCCH dimensioning tool

6.7 Recommendations

In this chapter, the recommended number of SDCCH channels based on the typical Alcatel call mixand the three types of profiles defined in paragraph 6.1 are presented. The table below gives thosevalues.

TRX BCCH combined Normal High Very high1 Yes 12 12 121 No 8 8 82 Yes 12 12 202 No 8 16 163 No 8 16 244 No 16 24 325 No 24 32 -6 No 24 32 -7 No 24 40 -8 No 32 40 -9 No 32 48 -

10 No 32 48 -11 No 40 56 -12 No 40 64 -13 No 40 64 -14 No 48 72 -15 No 48 72 -16 No 48 80 -

nu

mb

er o

f S

DC

CH

ch

ann

els

LU Profiles

Table 5: Recommended SDCCH values




Remarks:• SDCCH HTs have been set to 4s for all transactions except SMS where the holding time has

been set to 7s. This corresponds to values less optimistic than those measured under optimalconditions in Alcatel premises (presented paragraph 6.3 above).

• Very High Signalling profile should not be implemented on cells with more than 4 TRX to avoidTCU overload.

• Immediate assignment extended should reduce the CCCH load and might allow preventingCCCH congestion even for high traffic cells. See Immediate assignment Extended in Release B5functional feature description [10] for condition of application. SMS usage as well, which implieslong SDCCH holding time, should less impact CCCH load than classical voice traffic.

• DR TRE: the number of needed SDCCH is to be increased as DR TREs handle up to two timesthe number of calls handled by FR TREs. The number of SDCCH is obtained by multiplying by 2the number of SDCCH for FR TRE.

• The maximum number of SDCCH per TRX is:- For a FUMO: 12 SDCCH- Other TRE: 24 SDCCH

• Those maximum values are also function of the Abis multiplexing scheme aspresented on paragraph7 below




7 IMPACT ON ABIS RESOURCE

The Abis interface is impacted by the SDCCH allocation realised at Air interface level. The RSLchannels at Abis level carry the SDCCH load. To do so, the RSL throughput has to be compliant withthe number of SDCCH implemented on the TRXs.

The statistical 64K multiplexing is a new feature of the B6.2 release. The goal of this new feature istwofold:

• To optimise the physical resource employment (the Abis Time slot and PCM) in order toreduce the cost of Abis transmission

• To allow more SDCCH resources allocation and better response time than the static RSL 16K

7.1 Optimisation of the Abis resource

The first point, optimisation of the physical resource, is obtained by multiplexing from 1 to 4 RSL andthe OML of a BTS on 1 Abis TS at 64kbit/s.When the signalling statistical multiplexing is used, the Abis channels may be seen as a group ofMultiplexed Channel Block (MCB). A MCB connects 1, 2 or 4 TRX of a single BTS equipmentto a single TCU. A TCU may handle up to 4 MCB according to the limitation of 32 TCH per TCU.Each MCB is made of 1 signalling channel (64 kbit/s) and 2 to 8 traffic TS.The number of TRX multiplexed within the MCB is defined according to the signalling trafficsupported by the signalling channel.The possible MCBs used over Abis are:

MCB type TS used on Abis interface DescriptionMCB 64/4 9 One signalling channel at 64 kbit/s and 4 TRXMCB 64/2 5 One signalling channel at 64 kbit/s and 2 TRXMCB 64/1 3 One signalling channel at 64 kbit/s and 1 TRX

Table 6: Multiplexed Channel Block presentation

Hence, the MCB 64/4 and MCB64/2 allow both to carry 12 TRX (maximum number of TRX in oneBTS cabinet) on 1 Abis link with respectively, 27 TS and 30 TS occupied.




7.2 SDCCH Allocation and Response time

The static 16K has several restrictions:

• Each TRX is carrying a maximum of one SDCCH TS.• Not compatible with the Half-Rate mode.• Maximum throughput of the RSL channels is 16 kbit/s.

The statistical 64K allows overcoming those restrictions.

7.2.1 Maximum number of SDCCH

With the statistical 64K-multiplexing scheme, the number of SDCCH channels implemented per TRXis limited to the maximum value allowed per TRX, that is to say:

• For a FUMO: 12 SDCCH• Other TRE: 24 SDCCH

One should only pay attention that the sum of the SDCCH channels corresponding to the multiplexedRSL is below or equal to 32 (limit of the TCUC), as presented in the following table.

MCB type Configuration SDCCH #MCB 64/2 2 TRX FR, BCCH 32MCB 64/4 4 TRX FR, BCCH 32

MCB 64/2 2 TRX DR, BCCH 32

Table 7: Maximum SDCCH number per MCB type

Hence, the MCB64/2 allows implementing up to 2 SDDCH TS per TRX. Moreover, it is notrecommended for 16K static multiplexing to set BCCH and SDCCH on the same TRX. No suchlimitation exits with statistical multiplexing, especially if MCB64/2 is applied. However, TCU load stillneeds to be checked at BSC side.




7.2.2 Compatibility with Half-Rate Mode

As stated below, MCB64/2 and MCB64/1 are fully compatible with half-rate feature.

MCB type CompatibilityMCB 64/4 Full Rate onlyMCB 64/2 Dual Rate or Full RateMCB 64/1 Dual Rate or Full Rate

Table 8: MCB scheme compliance with Dual Rate

7.2.3 Throughput

For 16K static multiplexing the throughput is fixed and equal to 16 kbit/s. In the statisticalmultiplexing mode, one, two or 4 RSL share a 64 kbit/s channel, which implies an instantaneousthroughput 4 times higher than the static multiplexing scheme.

Hence, statistical multiplexing 64K shall be better than the static multiplexing at 16 kbit/s. Forinstance, power control response time shall be better when statistical scheme is implemented.

Hence, MCB64/2 is strongly advised for the following configuration:• The operator wants to concentrate the SDCCH on a few TRXs• Multi-band cell (possibility to have SDCCH of the outer zone on the same MCB)• Cell with very high signalling load (LA border, high SMS) or cell with more than 6 TRX FR

(high CCCH load)• Cell with GPRS, without Master PDCH, and with very good performances required (congested

RSL will slow down the TBF establishment)• High paging rate (LA > 100 000 users or more than 28 paging/s/cell)

7.3 Abis Interface recommendation

Statistical 64K should always be preferred to static multiplexing. It offers better RSL throughput whilstlimiting the number of Abis TS used compared to the 'no multiplexing' scheme.If star configuration is implemented MCB64/2 ('high load') should be preferred to MCB64/4. Itrepresents the best trade-off between Abis TS occupancy and response time.




8 SDCCH DIMENSIONING EXAMPLE

In this chapter, we present an example of SDCCH dimensioning for an Asian-Pacific operator. Thereference call mix is the following:

Measured cell traffic 25.53 ErlangMean TCH holding time 50 secondsNb voice TRX in cell 5 TRXTCH required blocking probability 2.00% percentSDCCH Required blocking probability 0.50% percent Mobile Orig. Call 60% percent Mobile Term. Call 40% percent Location Update 7 per call IMSI Attach 0.5 per call IMSI Detach 0.5 per call Orig. SMS PtP 1 per call Term. SMS PtP 1.5 per call

TRAFFIC MIX @ BH

inpu

ts

The number of SMS per call is 2.5 times above the reference B7 Alcatel call-mix. We consider a non-combined cell with a very high profile (Imsi Attach+Imsi Detach+LU=8) and 5 voice TRX installed.Cell Broadcast is not implemented. We obtain the following figures:

Nb TCH TS 34 TSNb SDH TS 0 TSNb SDC TS 5 TSTotal SDCCH TS 5 TSProcessed SDCCH blocking probability 0.4849% percentMaximum cell traffic 25.53 Erlang

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In this case, one SDCCH channel per TRX should be implemented.

The MCB64/2 is recommended to carry the high load of signalling, especially on the first TRX(CCCH+SDCCH).




9 CONCLUSION

With the rapid take off of SMS usage, a precise monitoring of the SDCCH resources is mandatory.

New ranges of application, like taxed SMS, will be launched in a near future and will require an SMSquality of service follow-up to justify their cost to the user. The basement of such a QoS monitoring isthe right dimensioning of the SDCCH channels, which carry the SMS flows.

Anticipating SDCCH resources understatement can be realised if a close follow-up of the SDCCHcongestion indicator is realised with e.g. RNO warning reports. At this stage, the tool presented inthis document allows performing a precise re-dimensioning of the precious circuit resources.

The best trade-off between voice circuit and signalling channels will be provided.

Finally, a particular attention must be focused on the Abis multiplexing scheme and TCU load toavoid jeopardising a good Air interface dimensioning either with an unsuitable Abis throughput orwith a high TCU load.




10 GLOSSARY

A A interface is between BSC and MSC

Abis Abis interface is between BTS and BSC

AC Alternative Current

BCCH Broadcast Control Channel

BIE Base station Interface Equipment

BSC Base Station Controller

BSS Base Station Subsystem

BTS Base Transceiver Station

CCCH Common Control Channel

CIC Channel Identity Code

CM ClassMark

CPRA Common Processor A

DCN Digital Cross-connect Network

DR Dual Rate

DRFU Dual Rate Frame Unit

DTC Digital Trunk Controller

DTX Discontinuous Transmission

EFR Enhanced Full Rate

FACCH Fast Associated Control Channel

FR Full Rate

G703 Physical/electrical characteristics of hierarchical digital interfaces

Gb Telecommunication interface between MFS and SGSN

GCH GPRS Channel (Ater interface)

GPRS General Packet Radio Service

GPU GPRS Processing Unit

GSL GPRS Signalling Link

HLR Lome Location Register

IMEI International Mobile Equipment Identity

IMSI International Mobile Subscriber Identity

LAPD Link Access Protocol on D channel

MD Multi Drop

MFS Multi-BSS Fast packet Server

MS Mobile Station




MSC Mobile Switching Centre

NE Network Element

NSS Network Sub-System

O&M Operation and Maintenance

PSTN Public Switched Telephone Network

PDCH Packet Data Channel (Air and Abis Interface)

QoS Quality of Service

REK Range Extension Kit

RSL Radio Signalling Link

RTC Remote Tuneable Combiner

SABM Set Asynchronous Balanced Mode

SACCH Slow Associated Control Channel

SDCCH Stand Alone Dedicated Control Channel

SGSN Serving GPRS Support Node

SM Sub Multiplexer

SS7 Signaling System number 7

SUM Station Unit Module

SUMP Station Unit Module PCM

TA Timing Advance

TC TransCoder

TCH Traffic Channel

TCUA Terminal Control Unit A

TCUC Terminal Control Unit C

TMA Tower Mounted Amplifier

TMSI Temporary Mobile Subscriber Identity

TRAU Transcoder/Rate Adaptation Unit

TRCU Transcoder Rate adaptation Control Unit

TRX Transmitter / Receiver (logical view)

TS Time slot

TSU Terminal Sub Unit

UA Unnumbered Acknowledgement

VLR Visitor Location Register

WBC Wide Band Combiner

END OF DOCUMENT

SDCCH Usage and Dimension Ing

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