SDCCH Dimentioning Guidelines

8/9/2019 SDCCH Dimentioning Guidelines

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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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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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1 IntroductionOn 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 arealso 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 bylooking 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 SDCCHThe 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 measuredtime 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: 3.5 seconds.

2.4 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.


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 theset-up for a regular call.

MS originated call Holding time: 2.7 seconds

MS terminated call Holding time: 2.9 seconds

2.7 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: 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


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 dimensioningAn 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 configurationfor 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 anavailable 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 trafficload 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 isbeneficial 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: 3

Cell Broadcast used: NoEstimated SDCCH load: 5 mE/subscriber

Estimated TCH load 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/TCH-

ratio

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: 2 %

SDCCH/4: 1% (1/2 of the TCH GOS)

all other SDCCH configurations: 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 estimationsMost 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 persubscriber 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: 1 per hour and active subscriber

Fraction of active subscribers: 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 =>

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

4.2 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, theSDCCH 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: 2 per hour and active subscriber

Fraction of active subscribers: 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-upObviously, 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 * (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%.


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 ontheoretical 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-, Inner-

and Border cells), the total SDCCH load per subscriber can be achieved by

adding the contributions from each event.

Event Average Inner Border

Location updating: 0.4 0 1.2 mE/subscriber

IMSI attach/detach: 1.8 1.8 1.8 mE/subscriber

Periodic registration: 0.8 0.8 0.8 mE/subscriber

Call set-up: 0.9 0.9 0.9 mE/subscriber

SMS p-p: 1.0 1.0 1.0 mE/subscriberSupplementary

services: 0.2 0.2 0.2 mE/subscriber

Total: 5.1 4.7 5.9 mE/subscriber

20 % safety-

margin 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: 6.1 / 25 = 24 %

Inner: 5.6 / 25 = 23 %

Border: 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 configurationsIn 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 anSDCCH/8. For further information regarding paging load and Location Area

dimensioning, see Ref. 3.

6 Dimensioning based on STS-dataAn 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 SDCCHload 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. Theoptimum 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 References1. 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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4/100 56-FCU 101 206 Uen Rev A 1997-01-15 17(23)

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: 2 %

SDCCH/4: 1 % (1/2 of the TCH GOS)

all other SDCCH configurations: 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

Number of

SDCCH

subchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

Number of

SDCCHsubchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-ratio

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

Number of

SDCCH

subchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

Number of

SDCCH

subchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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 configurationNumber of SDCCH

subchannels

Capacity,SDCCHs

Number of TCHs

Capacity,TCH

Max.SDCCH/TCH-

ratio

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%)

2 * SDCCH/8 + SDCCH/4 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

Number of

SDCCH

subchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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%)

2 * SDCCH/8 + SDCCH/4 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

SDCCH configuration

Number of

SDCCH

subchannels

Capacity,

SDCCHs

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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%)

2 * SDCCH/8 + SDCCH/4 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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3 * SDCCH/8 + SDCCH/4 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)


Location Area.

The figures in parenthesis are for configurations where Cell broadcast is used.

Table B1

1 TRX

SDCCH configuration

Number of

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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

SDCCH

subchannels

Capacity,

Signalling

SDCCH +

TCH

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-

ratio

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%)

2 * SDCCH/8 + SDCCH/4 20 (19) 14.3 (13.6) 45 35.6 41% (39%)

Table B7

7 TRXs

SDCCH configuration

Number of

SDCCH

Capacity,

Signalling

SDCCH +

Number of

TCHs

Capacity,

TCH

Max.

SDCCH/TCH-


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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%)

2 * SDCCH/8 + SDCCH/4 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.


Location Area.

Table C1

No Cell Broadcast

Number of TRXs SDCCH configuration,

Average Cell

SDCCH configuration, Inner

Cell

SDCCH configuration,

Border Cell


Average cell,

Immediate ass. on TCH

1 SDCCH/4 SDCCH/4 SDCCH/4 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 + SDCCH/4 2 * SDCCH/8 2 * SDCCH/8 + SDCCH/4 2 * SDCCH/8

7 2 * SDCCH/8 + SDCCH/4 2 * SDCCH/8 + SDCCH/4 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


Inner Cell


Border Cell


Average Cell,Immediate ass. on TCH


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 2 * SDCCH/8 + SDCCH/4 SDCCH/4 + SDCCH/8

6 2 * SDCCH/8 + SDCCH/4 2 * SDCCH/8 + SDCCH/4 2 * SDCCH/8 + SDCCH/4 2 * SDCCH/8


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7 3 * SDCCH/8 2 * SDCCH/8 + SDCCH/4 3 * SDCCH/8 2 * SDCCH/8

8 3 * SDCCH/8 3 * SDCCH/8 3 * SDCCH/8 + SDCCH/4 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 2 * SDCCH/8 + SDCCH/4 20 45 35.6

7 2 * SDCCH/8 + SDCCH/4 20 53 43.1

8 3 * SDCCH/8 24 60 49.6

9 3 * SDCCH/8 24 68 57.2

10 3 * SDCCH/8 + SDCCH/4 28 76 64.9

Documents

SDCCH Dimentioning Guidelines