Channel Element Dimensioning Guideline for SMS

Ericsson Internal TECHNICAL REPORT 1 (4)

Prepared (also subject responsible if other) No.

EAB/FJW/SM Klas Berggren EAB/FJW-08:1566 Uen Approved Checked Date Rev Reference

EAB/FJW/SM [Klas Berggren] 2008-10-21 A

1

Channel Element dimensioning guideline for SMS

1 General

Number of Channel Elements (CE) needed for SMS depend on the following factors: SMSs sent and received per RBS during busy hour, the time each SMS allocates SRB and required Grade of Service (GoS).

This guideline is for the SRB rate 13.6 kbit/s. It assumes that an SMS experiencing congestion is not retried, neither on UL nor on DL.

2 Allocation time on SRB

SMSs are utilizing CE resources defined for SRB. This guideline assumes that allocation time on SRB is the same as the time between RRC connection request and RRC connection release complete as seen from the UE, and has been measured to 2.0 s.

3 Method

Erlang B assumes longer channel allocations than 2.0 s, and that calls have different duration. When the call duration is fixed and short the call attempts can be modeled as being Poisson distributed in time, see appendix.

Input:

nSMSpersub , the number of SMS/subscriber/busy hour

nsubsperRBS , number of subscribers on the RBS

Step 1: Calculate the number of SMS/busy hour/RBS

nSMS= nSMSper sub * nsubsperRBS

Step 2: UL/DL separation and addition of Soft Handover factor

Channel elements are allocated separately for UL and DL.

nSMS =nSMS_DL + nSMS_UL

SRB can be in soft handover. Thus, as in the case for other services, a soft handover factor is needed. This factor is included in table 1 below and set to 30%.





2

Step 3: Finding required number of CEs

Given Poisson distributed SMS call attempts, an SRB allocation time of 2.0s and a GoS of 1,2 or 5%, Table 1 gives required CEs versus SMS/RBS/hour. It should be separated in UL and DL.

No. of CE

No.of SMS/RBS/hour 1% GoS

No. of SMS/RBS/hour 2% GoS

No. of SMS/RBS/hour 5% GoS

1 205 297 4912 603 785 11323 1139 1406 18914 1769 2117 27275 2471 2892 36186 3224 3714 45477 4022 4578 55108 4855 5472 64999 5717 6392 7511

10 6604 7338 854011 7514 8300 958512 8443 9281 1064313 9389 10278 1171814 10351 11286 1280215 11327 12307 1388716 12313 13340 1499517 13313 14386 1610318 14316 15438 1722419 15341 16504 1834620 16366 17570 1948121 17404 18650 2061622 18456 19730 2176623 19509 20824 2291524 20561 21918 2406425 21627 23026 2522726 22707 24133 2639027 23773 25241 2755328 24853 26349 2871629 25947 27484 2989330 27041 28606 3107031 28135 29741 3224732 29243 30876 3343833 30350 32012 3462934 31458 33147 3582035 32566 34296 3701036 33687 35446 3820137 34809 36595 3939238 35930 37758 4059639 37066 38907 4180140 38187 40070 43006

Table 1 Required number of CEs for UL or DL. Soft handover factor is included. The calculation method behind table 1 is given in the appendix.





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4 Channel elements for signaling

SMS as well as connection setups of all other services use SRB. This means that the method and calculations for SMS can be used for all signaling (RRC connection setups). Tests in lab has shown that the SRB allocation for non-SMS services varies between 1.1 and 1.4 seconds depending on service, number of radio links and others. This means that Table 1 can be used for estimating the CE need for all signaling, but will give a slight over-estimate. However, different SRB allocation time for different services would complicate the method quite much.





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APPENDIX

1 Channel element for SRB supports 1 Erlang, or 1 simultaneous user during 1 hour. Given that an SMS allocates the SRB 2.0 s, 1 CE thus can ideally support 3600/2=1800 SMS/hour if they never occur at the same time. But SMS call attempts are Poisson distributed in time, and the probability for congestion having 1 CE equals the probability having more than 1 simultaneous user. Thus, for 1 CE:

=

=

=x

x

xm

congestion xe

p m2 !

.=

=

=

1

0 !.

1x

x

xm

xe m

Where x is supported number of simultaneous SMS users, or equally, available no of CEs m is the total occupation time of the SRB, defined as *=m where

is number of SMS call attempts during busy hour, is allocation time of SRB=2.0 s

For a known number of available CE, congestionp will be a function of only.

Correspondingly, for 2 CEs:

=

=

=x

x

xm

congestion xe

p m3 !

.=

=

=

2

0 !.

1x

x

xm

xe m

And for 3 CEs:

=

=

=x

x

xm

congestion xe

p m4 !

.=

=

=

3

0 !.

1x

x

xm

xe m

GeneralAllocation time on SRBMethodChannel elements for signaling

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Channel Element Dimensioning Guideline for SMS