SDCCH Dimensioning

SDCCH DIMENSIONING GUIDELINE CME 20 R6 Contents 1 Introduction ..................................................................... 3 2 Events that require an SDCCH......................................... 3 2.1 Location Updating............................................................................. 3 2.2 IMSI attach/detach ........................................................................... 4 2.3 Periodic registration.......................................................................... 4 2.4 Call set-up ....................................................................................... 4 2.5 Short Message Service point to point (SMS p-p)................................. 5 2.6 Facsimile group 3 set-up................................................................... 5 2.7 Supplementary services .................................................................... 5 2.8 False Accesses................................................................................ 6

3 SDCCH dimensioning ...................................................... 7 3.1 SDCCH configurations ...................................................................... 7 3.2 SDCCH Grade Of Service................................................................. 8 3.3 Immediate assignment on TCH .......................................................... 8 3.4 SDCCH / TCH ratio......................................................................... 10

4 Traffic estimations ......................................................... 11 4.1 Location Updating........................................................................... 11 4.2 IMSI attach/detach ......................................................................... 12 4.3 Periodic registration........................................................................ 12 4.4 Call set-up ..................................................................................... 13 4.5 SMS p-p ........................................................................................ 13 4.6 Facsimile group 3 set-up................................................................. 13 4.7 Supplementary services .................................................................. 13 4.8 False Access ................................................................................. 14 4.9 Safety margin................................................................................. 14

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SDCCH DIMENSIONING GUIDELINE, CME 20 R6

4.10 Total SDCCH load ........................................................................ 14 4.11 SDCCH / TCH ratio....................................................................... 15

5 Recommended configurations .......................................15 6 Dimensioning based on STS-data..................................15 6.1 Cells with no congestion.................................................................. 16 6.2 Cells with congestion ...................................................................... 16 6.3 Cells using Immediate assignment on TCH........................................ 16

7 References ......................................................................16

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SDCCH DIMENSIONING GUIDELINE

1

Introduction On the Stand alone Dedicated Control CHannel (SDCCH), different signalling and transmission activities take place. This document is a guideline how to select a suitable SDCCH configuration. The different events that requires an SDCCH are briefly described. The SDCCH holding times for each event are also included. When having estimated the SDCCH load, there are tables how to, depending on the number of TRXs in the cell, choose a suitable SDCCH configuration. The SDCCH traffic estimations in this document are based on a CME 20 traffic model used by Ericsson. The SDCCH needs vary substantially between networks due to subscriber behavior and parameter settings. The best way to use these estimations is to understand the calculations involved in the dimensioning of the SDCCH and re-make them for every particular network with its unique input parameters. An optimum SDCCH configuration, for every cell, can only be achieved by looking at cell statistics, i.e. STS counters. Therefore, this document also describes how to select an SDCCH configuration based on STS data. An SDCCH dimensioning based on STS data follows the same principles as SDCCH dimensioning based on traffic estimations.

2

Events that require an SDCCH The following procedures have an effect on the SDCCH load: • Mobility Management procedures, i.e. Location Updating and Periodic Registration. • Radio Resources management procedures, i.e. IMSI attach/detach and Call set-up. • Subscriber Services, i.e. SMS p-p, Fax set-up and Supplementary Services. The values for the Holding time of the SDCCH come from time measurements performed by Ericsson, although some assumptions have been made.

2.1

Location Updating Every time an MS passes a Location Area border, a Location Updating is performed. The approximate time that an SDCCH is occupied during a Location Updating is Holding time:

3.5 seconds

which is the average measured time (3.0 seconds) plus a margin that accounts for the time between the arrival of the Channel Release Acknowledge message at the BSC and the time before the channel is available again.

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2.2

IMSI attach/detach The IMSI detach procedure enables the MS to indicate to the network that it is about to become inactive. The IMSI attach is performed when the MS is turned on in the same Location Area as it was when it sent the IMSI detach message. If the Location Area has changed, a normal Location Updating is performed. IMSI attach uses the same location updating procedure as Location Updating, i.e. IMSI attach Holding time:

3.5 seconds

The IMSI detach procedure consist of the IMSI Detach Indication message sent from the MS to the network. No authentication is performed and no acknowledge message is sent to the MS. We have assumed IMSI detach Holding time:

2.9 seconds

which is the average time for Location Updating minus the average measured time for authentication (0.6 seconds).

2.3

Periodic registration To avoid unnecessary paging of an MS in case the MSC never got the IMSI detach message and to update the registers holding information about whether the MS is attached/detached, there is an other type of location updating called Periodic registration. Periodic registration uses the same procedure as Location Updating, i.e. Holding time:

2.4

3.5 seconds.

Call set-up When a connection is to be established, a channel for signalling has to be allocated. The authentication, ciphering mode initiation and set-up signalling are performed on the SDCCH. The estimated time that the SDCCH is occupied with the performance of a call set-up differs slightly between MS originated and MS terminated calls. MS originated call Holding time: 2.7 seconds MS terminated call Holding time: 2.9 seconds The figures are average measured times (2.2 and 2.4 seconds respectively) plus a margin that mainly accounts for the time between the arrival of the Channel Release Acknowledge message at the BSC and the time before the channel is available again.

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2.5

Short Message Service point to point (SMS p-p) The SMS p-p is a text message service supported by CME 20 R5. This service provides the transmission of an SMS between a message handling system (service centre) and a mobile station. The transmission of an SMS p-p normally takes place on the SDCCH. If a TCH already is allocated, when assigning a channel for the SMS p-p, the SMS transmission takes place on the allocated TCH, using the SACCH. The time that the transmission of an SMS p-p holds the SDCCH is the sum of the set-up time for the SMS transmission and the time it takes to transfer the message itself. The holding time for the set-up is slightly shorter than the time for a normal call set-up since less signalling is involved. The holding time of the SDCCH is based on measurements, and accounts for the time between the Channel Activation message and the Channel Release message. Holding time:

6.2 seconds

The measurements were performed for MS terminated SMS p-p, but is considered to be valid also for MS originated SMS.

2.6

Facsimile group 3 set-up The Facsimile group 3 service allows the connection (send and/or receive) of CCITT group 3 fax apparatus to the MSs in a GSM PLMN. The set-up for a fax transmission takes place on the SDCCH. It can be assumed that the set-up of a fax transmission holds the SDCCH as long as the set-up for a regular call. MS originated call Holding time: MS terminated call Holding time:

2.7

2.7 seconds 2.9 seconds

Supplementary services The use of supplementary services is optional for the operator. Some of the services could be basic while others could be available to the subscriber by express subscription. Examples of supplementary services are Line Identification Presentation, Call forwarding, Barring of calls and Multi-Party Service. The estimated SDCCH holding time for the activities related to control of the Supplementary Services is given by measurements: Holding time:

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0.9 seconds.

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2.8

False Accesses When a Channel Request message is received by the system, an SDCCH is allocated by sending an Immediate Assignment message. If it happens that the channel request was caused by radio disturbances, no more messages are received from the non-existing MS and the system waits a certain amount of time before performing a disconnection. The time that a False Access holds the SDCCH is estimated to Holding time:

5.8 seconds

which is the average measured time (5.3 seconds) plus a margin that mainly accounts for the time between the arrival of the Channel Release Acknowledge message at the BSC and the time before the channel is available again.

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3

SDCCH dimensioning An SDCCH dimensioning means selecting the number of time slots in a cell that are going to be used for signalling. The generated SDCCH and TCH traffic in a cell are the inputs for the SDCCH dimensioning. For a given number of TRXs, and an estimated SDCCH / TCH ratio, a suitable SDCCH configuration for that cell can be chosen.

3.1

SDCCH configurations

3.1.1

SDCCH/8 8 subchannels for signalling is mapped on one time slot. Each assigned SDCCH/8 results in one less TCH for that cell.

3.1.2

SDCCH/4 (Combined SDCCH and BCCH) There is a possibility to map 4 subchannels for signalling on the time slot used for the BCCH. As a result of combining the SDCCH with the BCCH, the paging capacity on the BCCH is reduced.

3.1.3

Cell Broadcast The Cell Broadcast service provides the transmission of an SMS from a message handling centre to all MSs in the serving area of the BTS. When the Cell Broadcast service is active in a cell, one signalling subchannel is replaced by one Cell Broadcast Channel, CBCH.

3.1.4

Possible configurations The optional Cell Broadcast service and the possibility to have combined SDCCH and BCCH result in four possible SDCCH-configurations. Each TRX can have one out of these SDCCH configurations defined: • SDCCH/4: The SDCCH is combined with the BCCH in time slot 0 on the BCCH carrier. This SDCCH configuration provides 4 subchannels for signalling. Only one SDCCH/4 can be defined for each cell. • SDCCH/8: This SDCCH configuration provides 8 subchannels for signalling. Up to 16 SDCCH/8 can be defined for each cell. • SDCCH/4 including CBCH: If one subchannel is replaced by a CBCH, the SDCCH/4 configuration provides 3 subchannels for signalling. • SDCCH/8 including CBCH: If one subchannel is replaced by a CBCH, the SDCCH/8 configuration provides 7 subchannels for signalling. An SDCCH/4 is automatically allocated at time slot 0 on the BCCH carrier. For SDCCH/8, the time slot number can be specified, but the system automatically chooses which carrier to use. In case there are several SDCCH/8 defined, they are assigned different frequencies but with the same time slot number.

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3.2

SDCCH Grade Of Service The SDCCH dimensioning is a compromise between SDCCH blocking rate and TCH capacity. A connection for speech or data requires an SDCCH for the call set-up signalling and a TCH for the remainder of the call. In order to have a successful call set-up, there has to be an available SDCCH as well as an available TCH. The SDCCH and the TCH are equally important for the completion of a call, but since the SDCCHs use the physical channels more efficiently, the SDCCHs should be dimensioned for a better grade of service compared to the TCHs. A rule of thumb is that the SDCCH GOS should be dimensioned for no more than 1/4 of the TCH GOS, i.e. if the SDCCH exceeds 1/4 of the TCH GOS, a configuration with more SDCCHs should be used. However, when using the configuration with only four signalling channels, i.e. the SDCCH/4 configuration, an SDCCH GOS of TCH GOS/2 should be allowed. The rule of thumb stated above is illustrated by an example: Assume that a cell is dimensioned for a GOS of 2 % on the TCH. If a configuration with 4 subchannels for signalling is used (only one SDCCH/4), the SDCCH should not be dimensioned for a GOS in excess of : Max. SDCCH GOS = 1/2 * 2 % = 1 %. For all other SDCCH configurations, the SDCCH should not be dimensioned for a GOS in excess of : Max. SDCCH GOS = 1/4 * 2 % = 0.5 %.

3.3

Immediate assignment on TCH In CME 20 R6, it is possible to perform the initial signalling (Immediate assignment) on a TCH. In case of Immediate assignment on TCH, the Channel Administration processing (see Ref. 1) assigns a TCH for signalling instead of an SDCCH. The Channel Administration allows seven different channel allocation strategies (CHAPs). The TCH first strategy decreases the signalling load on SDCCH significantly, thus enabling the possibility to use a combined SDCCH in most situations. However, the traffic load on TCH will in this case increase substantially. The SDCCH first strategy means that an SDCCH always is allocated if there is an idle SDCCH and in case there is congestion on SDCCH, the signalling is performed on a TCH instead. This possibility will simplify the dimensioning of SDCCH substantially, since the resources are used more efficiently and the trunking loss is reduced to a minimum.

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3.3.1

SDCCH dimensioning When all SDCCHs are occupied, the excessive call set-ups, Location Updatings etc. are performed on TCH instead. This means that the traffic load on TCH increases, but in this case it is possible to use a configuration with more available TCHs. Thus, there is a trade-off between the overflow traffic load on TCH, originating from Immediate assignment on TCH, compared to the extra capacity that can be added on TCH by reducing the number of time slots reserved for SDCCH. The trunking gain for configurations with a small number of SDCCHs is substantial. In this paper, the breakpoint when it is beneficial to add one SDCCH/8 is used. It is assumed that SDCCH/4 either is always used or not used at all in a Location Area (paging load considerations). This means that when extra capacity on SDCCH is required, one SDCCH/8 is added. The breakpoint is defined when 0.5 Erlang1 of the signalling traffic is carried by TCH. If the traffic load exceeds the breakpoint, one extra SDCCH/8 have to be added. When the overflow traffic on TCH is equal or less than 0.5 Erlang, it is beneficial to use a TCH extra instead of adding an SDCCH/8, since the total TCH capacity increases with almost one Erlang. Recommendations for initial dimensioning of the SDCCH configuration when using Immediate assignment on TCH is presented in Appendix B & C. The GOS, or percentage of the time that all SDCCH channels are occupied at the breakpoint, is presented in Table 1.

Table 1

Percentage of time when all SDCCHs are occupied at the breakpoint for different configurations of SDCCH. SDCCH/4 18 %. SDCCH/8 9 %. SDCCH/4 + SDCCH/8 6 %. 2 * SDCCH/8 4,5 %. SDCCH/4 + 2 * SDCCH/8 3,5 %. The SDCCH should not be dimensioned for an SDCCH occupancy percentage in excess of these figures. If the GOS on SDCCH becomes higher, it is beneficial to add an SDCCH/8. Example: Suppose that the traffic on SDCCH is 4 Erlang in a cell, that combined SDCCH should not be used and that Immediate assignment on TCH is not used. In this case it is necessary to use two SDCCH/8 in order to meet the demand for a GOS less than 0,5 %. The capacity on TCH is 14 Erlang if there are three TRXs in the cell.

1 The breakpoint is defined at 0.5 Erlang since the overflow traffic on TCH is more bursty and have higher variance than Erlang distributed traffic. This means that the overflow traffic on TCH decreases the total capacity on TCH with slightly more than 0.5 Erlang.

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If Immediate assignment on TCH, SDCCH first, is used, it is possible to use only one SDCCH/8. The GOS on SDCCH is in this case 3 % which yields that the overflow traffic on TCH is 3 % * 4 Erlang = 0.12 Erlang. The capacity of the TCHs in the cell is now 14.9 - 0.12 ≈ 14.7 Erlang, which means that the capacity in the cell is increased with 0.7 Erlang. Note that in the case when Immediate assignment on TCH is used, there is no congestion at Immediate assignment at all. There could of course be congestion both on SDCCH and TCH, but in this case, 100 % of the resources are used in the cell.

3.4

SDCCH / TCH ratio When dimensioning the SDCCH, the main factor is the SDCCH traffic and TCH traffic ratio, i.e. SDCCH-load / TCH-load. For every combination of SDCCH configuration and number of TRXs, there is a breakpoint in terms of the SDCCH / TCH ratio where the SDCCH GOS exceeds the rule of thumb stated in chapter 3.2. A complete list of tables, including every (reasonable) combination of SDCCH configuration, number of TRXs and their "breakpoint values" (given in percentage) is included in appendix A. The selection of which SDCCH configuration to use is illustrated by an example: Assume a situation with the following data: Number of TRXs in cell: Cell Broadcast used: Estimated SDCCH load: Estimated TCH load

3 No 5 mE/subscriber 20 mE/subscriber.

The SDCCH / TCH ratio = 5 / 20 = 25% After having calculated the SDCCH / TCH ratio, find the correct table in appendix A or B based on the number of TRXs in the cell (in this case table A3). 3 TRXs

SDCCH configuration

Number of SDCCH subchannels

Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

0.87 (0.46)

23

15.8

5.5% (2.9%)

SDCCH/8

8 (7)

2.7 (2.2)

22

14.9

18% (15%)

SDCCH/8 + SDCCH/4

12 (11)

5.3 (4.6)

22

14.9

36% (31%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

21

14.0

58% (53%)

Table 1. Table A3 from appendix A The configuration SDCCH/4 + SDCCH/8 has a maximum SDCCH / TCH ratio of 36 %, which is sufficient. Note that this configuration has a

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combined SDCCH / BCCH. If a combined BCCH can not be used (e.g. due to excessive paging load), the 2 * SDCCH/8 configuration must be used. The tables are calculated with the following GOS. TCH: SDCCH/4: all other SDCCH configurations:

2% 1% (1/2 of the TCH GOS) 0.5 % (1/4 of the TCH GOS)

Other values of the GOS will affect the capacity. However, the TCH GOS only has a marginal effect on the maximum SDCCH / TCH ratio as long as the relation between the SDCCH GOS and TCH GOS are kept the same. This means that these tables also can be used for dimensioning cells with other congestion levels than 2%. The values given in parentheses are for Cell broadcast. The configurations shaded grey are the recommended configurations for an average cell, Cell broadcast not used, according to the traffic estimations in chapter 4.

4

Traffic estimations Most of the traffic estimations in this chapter comes a from a CME 20 traffic model used by Ericsson. The figures differ substantially between different networks. Therefore, the figures should be substituted with the values given by the operator, if available. The figures also vary between cells. If possible, the SDCCH dimensioning should be followed up by looking at STS-data (see chapter 5). Knowing the SDCCH holding times, with a given number of performances during busy hour for every procedure, the generated SDCCH traffic per subscriber can be calculated as follows: For each type of procedure, multiply the number of performances per busy hour and subscriber and the holding time of the channel. By dividing the result with 3.6, the procedures contribution to the SDCCH load in mErlang / subscriber is achieved. For some of the events, the number of performances per busy hour, is given per active subscriber. An active subscriber is a subscriber whose MS is attached to the network, i.e. in idle or busy mode.

4.1

Location Updating The Location Updating contribution to SDCCH load very much differ between an inner cell, and a cell at the border of a Location Area. Furthermore, if the cell is at the border of a location area, the number of Location Updatings in the cell depends on the traffic situation. For example, a highway crossing a Location Area border results in a larger number of Location updatings in those Location Area border cells. The figures given in the CME 20 Traffic model for Location Updating are

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Number of Location Updatings: Fraction of active subscribers:

1 per hour and active subscriber 40%

In these estimations, three types of cells have been considered: • Average cell: This type of cell has the average number of Location updatings given in the CME 20 Traffic model. • Inner cell: This type of cell is considered to have no Location Updatings at all. • Border cell: This type of cell is considered to have three times the average number of Location Updatings. The number of Location Updatings in the border cell comes from the assumption that there are twice as many inner cells as border cells, were virtually all the Location Updatings are performed. 3.54 seconds holding time =>

4.2

Average:

1 * 40% * 3.5 / 3.6 = 0.4 mE per subscriber

Inner:

0 mE per subscriber

Border:

3 * 0.4 = 1.2 mE per subscriber

IMSI attach/detach The attach and detach procedures are optional to the operator. If used, the number of performances is highly dependent on the subscriber behaviour. The figures given in the CME 20 Traffic model for IMSI attach/detach are Number of IMSI attach: 1 per hour and subscriber Number of IMSI detach: 1 per hour and subscriber 3.5 and 2.9 seconds Holding time respectively => (1 * 3.5 + 1 * 2.9) / 3.6 = 1.8 mE per subscriber

4.3

Periodic registration The implementation of the Periodic Registration is optional. If used, the time between registrations is a choice of the operator as well as. Consequently, the SDCCH load due to Periodic registration vary substantially between operators. If the cell parameter T3212 (see Ref. 2) is set to half an hour, the number of Busy Hour Call attempts, BHCA, for Periodic registration are Number of periodic registrations: Fraction of active subscribers:

2 per hour and active subscriber 40%

3.5 seconds Holding time => 2 * 40% * 3.5 / 3.6 = 0.8 mE per subscriber

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The rate of periodic updatings is actually over-estimated since no Periodic registration is performed half an hour after a Location Updating or Mobile Terminated call.

4.4

Call set-up Obviously, the number of call set-ups depend on the subscriber behaviour and differs considerably from one network to another. The figures given in the CME 20 Traffic model for call set-up are MS Originated traffic (including no B-answer): 0.8 BHCA MS Terminated traffic (including no B-answer): 0.3 BHCA. 2.7 seconds and 2.9 seconds holding time for MS originated and MS terminated calls respectively => (0.8 * 2.7 + 0.3 * 2.9) / 3.6 = 0.9 mE/subscriber

4.5

SMS p-p The number of SMS p-p transmissions is highly dependent on the subscriber behavior, but also how the operator uses this service. As an example, SMS can be used for delivering internal network notices, e.g. voice-mail. In these estimations, it is assumed that all SMS p-p traffic is sent on the SDCCH. The figures used for the SMS p-p traffic estimations are: MS Terminated traffic:

0.5 BHCA

MS Originated traffic:

0.1 BHCA.

For MS terminated traffic, the figure comes from the CME 20 Traffic model. For the MS originated traffic, the figure is an assumption based on data from existing networks. 6.2 seconds Holding time => 6.2 * (0.1 + 0.5) / 3.6 = 1.0 mE/subscriber

4.6

Facsimile group 3 set-up In the Ericsson traffic model, the number of fax transmissions is small and will only have a marginal effect on the total SDCCH load. Therefore, Facsimile group 3 set-up will not be taken in considerations in the SDCCH traffic estimations in this document.

4.7

Supplementary services In the CME 20 Traffic model, the number of activities related to control of the Supplementary Services is

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Number of activation's, deactivation's, etc.: 0.3 per hour and active subscriber Fraction of active subscribers: 40%. 0.9 seconds Holding time => 2 * 40% * 0.9 / 3.6 = 0.2 mE per subscriber

4.8

False Access The number of False Accesses is related to the number of SDCCHs in the cell rather than being traffic dependent. The additional SDCCH load due to false accesses is probably in most cases marginal, and will not be taken into consideration in the SDCCH traffic estimations in this document.

4.9

Safety margin When selecting an SDCCH configuration, especially if the data is based on theoretical estimations, there should always be a "safety margin". A successful call-setup requires an available SDCCH as well as an available TCH. A configuration with one TCH less than an optimised configuration will have a smaller impact on the total GOS compared to a shortage of SDCCHs. This is, however, not the case when using Immediate assignment on TCH (See chapter 3.3). In this document, a safety margin of 20 % is added to the estimated SDCCH load. Since the SDCCH dimensioning is based on the SDCCH / TCH ratio, the unreliability of the TCH load must be included in this margin.

4.10

Total SDCCH load For the three different types of cells stated in chapter 4.1 (i.e. Average-, Innerand Border cells), the total SDCCH load per subscriber can be achieved by adding the contributions from each event.

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Event

Average

Inner

Border

Location updating: IMSI attach/detach: Periodic registration: Call set-up: SMS p-p: Supplementary services:

0.4 1.8 0.8 0.9 1.0

0 1.8 0.8 0.9 1.0

1.2 1.8 0.8 0.9 1.0

mE/subscriber mE/subscriber mE/subscriber mE/subscriber mE/subscriber

0.2

0.2

0.2

mE/subscriber

Total:

5.1

4.7

5.9

mE/subscriber

20 % safetymargin added:

6.1

5.6

7.0

mE/subscriber

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4.11

SDCCH / TCH ratio The SDCCH / TCH ratio is the ratio between the estimated SDCCH load, calculated in chapter 4.10, and the estimated TCH load, which is given in the CME 20 Traffic model. TCH load:

25 mE / subscriber

SDCCH / TCH ratio = Average: Inner: Border:

6.1 / 25 = 24 % 5.6 / 25 = 23 % 7.0 / 25 = 28 %

Experience from live GSM networks shows that the SDCCH / TCH ratio differs quite a lot between different networks and different time intervals in a network. This depends, amongst others, on the parameter setting, the subscriber behaviour, the size of the Location Areas and the services provided by the network operator. The SDCCH / TCH ratio has shown to be, in most cases, between 10 % and 40 %.

5

Recommended configurations In Appendix C, Table C1 and C2 contain the recommended SDCCH configurations, based on the SDCCH / TCH ratio calculated in chapter 4.11. Six different types of cells have been considered; Average, Inner and Border cells, with and without Cell broadcast. Note that in large Location Areas with high paging load, the use of combined SDCCH should not be used. In this case the SDCCH/4 must be replaced by an SDCCH/8. For further information regarding paging load and Location Area dimensioning, see Ref. 3.

6

Dimensioning based on STS-data An estimation of the SDCCH load, as described in chapter 4, is based on assumptions regarding the subscriber behaviour. Furthermore, the SDCCH need can differ considerably between different cells. Example of factors which might differ between cells are generated traffic per subscriber, Location Area borders and subscriber mobility. A more accurate dimensioning is achieved by using cell statistics, i.e. STS counters. The STS counters provide statistics regarding SDCCH load, SDCCH congestion, TCH load and TCH congestion. When there is congestion on SDCCH or TCH, the availability of the channels should also be checked. It is common that there are blocked channels or hardware problems not discovered until there is congestion in the cell, since in this case, this cell is more thoroughly evaluated. Statistics of the traffic load and congestion should be collected during busy hour when the traffic reach the peak levels.

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6.1

Cells with no congestion As long as there is no congestion in either the TCH nor the SDCCH, the SDCCH configuration is not critical. However, the cell should still be dimensioned for a future increase in traffic. With no statistics of the congestion levels, the dimensioning has to be based on statistics regarding the SDCCH load and TCH load. It is reasonable to assume that the SDCCH / TCH ratio is not going to differ that much when the overall traffic in the cell increase. The SDCCH / TCH ratio can be calculated using the statistics from the STS counters. A suitable SDCCH configuration can be selected using the tables in appendix A, as described in chapter 3.4.

6.2

Cells with congestion The most accurate SDCCH dimensioning is achieved by looking at the congestion level for the TCHs and the SDCCHs for the specific cell. The optimum configuration is achieved by selecting a configuration with as many TCHs as possible, without letting the SDCCH GOS exceed 1/4 of the TCH GOS (with only one SDCCH/4: 1/2 of the TCH GOS). This procedure can be applied regardless of the TCH congestion level in the cell.

6.3

Cells using Immediate assignment on TCH When using Immediate assignment on TCH, the SDCCHs could be dimensioned for a much higher utilisation compared to the case without using Immediate assignment on TCH. The trunking gain is in most cases substantial and yields a higher capacity on TCH. There are STS counters that provide the percentage of the time when all channels are occupied. These counters should be monitored for SDCCH and TCH and the SDCCH dimensioning should be performed according to the figures for GOS in section 3.3.1.

7

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References 1.

User Description, Channel Administration, 4/1551-FCU 101 201 Uen

2.

Radio Network Parameters and Cell Design Data, 3/100 56-FCU 101 201 Uen

3.

Location Area Dimensioning Guideline, 5/100 56-FCU 101 206 Uen Rev A

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Appendix A Table A1 - A7 are tables for selecting SDCCH configurations as described in chapter 3. The tables are calculated with the following GOS. TCH: SDCCH/4: all other SDCCH configurations:

2% 1 % (1/2 of the TCH GOS) 0.5 % (1/4 of the TCH GOS)

Note that SDCCH/4 should not be used in case of high paging load in the Location Area. The figures in parenthesis are for configurations where Cell broadcast is used. Table A1 1 TRX

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

0.87 (0.46)

7

2.9

30% (16%)

SDCCH/8

8 (7)

2.7 (2.2)

6

2.3

120% (94%)

Table A2 2 TRXs

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

0.87 (0.46)

15

9.0

9.7% (5.1%)

SDCCH/8

8 (7)

2.7 (2.2)

14

8.2

33% (27%)

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

14

8.2

65% (56%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

13

7.4

109% (100%)

Table A3 3 TRXs

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

0.87 (0.46)

23

15.8

5.5% (2.9%)

SDCCH/8

8 (7)

2.7 (2.2)

22

14.9

18% (15%)

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

22

14.9

36% (31%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

21

14.0

58% (53%)

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Table A4 4 TRXs

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/8

8 (7)

2.7 (2.2)

30

21.9

12% (10%)

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

30

21.9

24% (21%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

29

21.0

39% (35%)

2 * SDCCH/8 + SDCCH/4

20 (19)

11.1 (10.3)

29

21.0

53% (49%)

Table A5 5 TRXs

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

38

29.1

18% (16%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

37

28.3

29% (26%)


20 (19)

11.1 (10.3)

37

28.3

39% (36%)

3 * SDCCH/8

24 (23)

14.2 (13.4)

36

27.3

52% (49%)

Table A6 6 TRXs

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

46

36.5

15% (13%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

45

35.6

23% (21%)


20 (19)

11.1 (10.3)

45

35.6

31% (29%)

3 * SDCCH/8

24 (23)

14.2 (13.4)

44

34.7

41% (39%)

Table A7 7 TRXs

18(23)

SDCCH configuration


Capacity, SDCCHs

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4 + SDCCH/8

12 (11)

5.3 (4.6)

54

44.0

12% (10%)

2 * SDCCH/8

16 (15)

8.1 (7.4)

53

43.1

19% (17%)


20 (19)

11.1 (10.3)

53

43.1

26% (24%)

3 * SDCCH/8

24 (23)

14.2 (13.4)

52

42.1

34% (32%)

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28 (27)

17.4 (16.6)

52

42.1

41% (39%)

Appendix B The dimensioning in this section is based on the case when Immediate Assignment on TCH, SDCCH first, is used. Table B1 - B7 are tables for selecting SDCCH configurations as described in chapter 3. The tables are calculated with the assumption that the limit for the capacity is reached on SDCCH when 0.5 Erlang of the signalling traffic is served by TCH. Max. SDCCH/TCH - ratio is calculated as: (signalling on SDCCH + TCH) / (Capacity on TCH - 0.5 Erlang) Note that SDCCH/4 should not be used in case of high paging load in the Location Area. The figures in parenthesis are for configurations where Cell broadcast is used. Table B1 1 TRX

SDCCH configuration


Capacity, Signalling SDCCH + TCH

Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

2.8 (2.2)

7

2.9

117% (92%)

SDCCH/8

8 (7)

5.5 (4.8)

6

2.3

306% (267%)

Table B2 2 TRXs

SDCCH configuration



Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

2.8 (2.2)

15

9.0

33% (26%)

SDCCH/8

8 (7)

5.5 (4.8)

14

8.2

71% (62%)

Table B3 3 TRXs

SDCCH configuration



Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

2.8 (2.2)

23

15.8

18% (14%)

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SDCCH/8

8 (7)

5.5 (4.8)

22

14.9

38% (33%)

SDCCH/4 + SDCCH/8

12 (11)

8.3 (7.8)

22

14.9

58% (54%)

Table B4 4 TRXs

SDCCH configuration



Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/4

4 (3)

2.8 (2.2)

31

22.8

13% (10%)

SDCCH/8

8 (7)

5.5 (4.8)

30

21.9

26% (22%)

SDCCH/4 + SDCCH/8

12 (11)

8.3 (7.8)

30

21.9

39% (36%)

2 * SDCCH/8

16 (15)

11.3 (10.5)

29

21.0

55% (51%)

Table B5 5 TRXs

SDCCH configuration



Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/8

8 (7)

5.5 (4.8)

38

29.1

19% (17%)

SDCCH/4 + SDCCH/8

12 (11)

8.3 (7.8)

38

29.1

29% (27%)

2 * SDCCH/8

16 (15)

11.3 (10.5)

37

28.3

41% (38%)

Table B6 6 TRXs

SDCCH configuration



Number of TCHs

Capacity, TCH

Max. SDCCH/TCHratio

SDCCH/8

8 (7)

5.5 (4.8)

46

36.5

15% (13%)

SDCCH/4 + SDCCH/8

12 (11)

8.3 (7.8)

46

36.5

23% (22%)

2 * SDCCH/8

16 (15)

11.3 (10.5)

45

35.6

32% (30%)


20 (19)

14.3 (13.6)

45

35.6

41% (39%)

Number of TCHs

Capacity, TCH

Max. SDCCH/TCH-

Table B7 7 TRXs

SDCCH configuration

20(23)

Number of SDCCH

Capacity, Signalling SDCCH +

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subchannels

TCH

ratio

SDCCH/8

8 (7)

5.5 (4.8)

54

44

13% (11%)

SDCCH/4 + SDCCH/8

12 (11)

8.3 (7.8)

54

44

19% (18%)

2 * SDCCH/8

16 (15)

11.3 (10.5)

53

43.1

27% (25%)


20 (19)

14.3 (13.6)

53

43.1

34% (32%)

Appendix C Table C1 and Table C2 are lists of the recommended SDCCH configurations according to the traffic estimations in chapter 4. Note that SDCCH/4 should not be used in case of high paging load in the Location Area. Table C1 No Cell Broadcast Number of TRXs

SDCCH configuration, Average Cell

SDCCH configuration, Inner Cell

SDCCH configuration, Border Cell

SDCCH configuration, Average cell, Immediate ass. on TCH

1

SDCCH/4

SDCCH/4

SDCCH/4

SDCCH/4

2

SDCCH/8

SDCCH/8

SDCCH/8

SDCCH/4

3

SDCCH/8 + SDCCH/4

SDCCH/8 + SDCCH/4

SDCCH/8 + SDCCH/4

SDCCH/8

4

2 * SDCCH/8

SDCCH/8 + SDCCH/4

2 * SDCCH/8

SDCCH/8

5

2 * SDCCH/8

2 * SDCCH/8

2 * SDCCH/8

SDCCH/4 + SDCCH/8

6


2 * SDCCH/8


2 * SDCCH/8

7



3 * SDCCH/8

2 * SDCCH/8

8

3 * SDCCH/8

3 * SDCCH/8

3 * SDCCH/8

SDCCH/4 + 2 * SDCCH/8

Table C2 Cell Broadcast used Number of TRXs

SDCCH configuration, Average Cell

SDCCH configuration, Inner Cell

SDCCH configuration, Border Cell

SDCCH configuration, Average Cell, Immediate ass. on TCH

1

SDCCH/8

SDCCH/8

SDCCH/8

SDCCH/4

2

SDCCH/8

SDCCH/8

SDCCH/8 + SDCCH/4

SDCCH/4

3

SDCCH/8 + SDCCH/4

SDCCH/8 + SDCCH/4

SDCCH/8 + SDCCH/4

SDCCH/8

4

2 * SDCCH/8

2 * SDCCH/8

2 * SDCCH/8

SDCCH/4 + SDCCH/8

5

2 * SDCCH/8

2 * SDCCH/8


SDCCH/4 + SDCCH/8

6




2 * SDCCH/8

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21(23)


22(23)

7

3 * SDCCH/8


3 * SDCCH/8

2 * SDCCH/8

8

3 * SDCCH/8

3 * SDCCH/8


SDCCH/4 + 2 * SDCCH/8

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Table C3 is a quick reference for estimating the capacity in a cell for different number of TRXs. The table can be used for approximate estimations regarding capacity when data to do a more accurate SDCCH dimensioning is not provided. The capacity estimations can be used at an early stage in the cell planning process, e.g. for estimating the capacity using a nominal cell plan. The SDCCH configurations are based on the traffic estimations in chapter 4, and corresponds the "average cell" in table C1 (no cell broadcast). Table C3 No Cell Broadcast Number of TRXs

SDCCH configuration,

Number of signalling subchannels

Number of TCHs

TCH capacity (Erlang at 2% GOS)

1

SDCCH/4

4

7

2.9

2

SDCCH/8

8

14

8.2

3

SDCCH/8 + SDCCH/4

12

22

14.9

4

2 * SDCCH/8

16

29

21.0

5

2 * SDCCH/8

16

37

28.3

6


20

45

35.6

7


20

53

43.1

8

3 * SDCCH/8

24

60

49.6

9

3 * SDCCH/8

24

68

57.2

10


28

76

64.9

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SDCCH Dimensioning

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