Huawei Dinamic SDCCH

1

www.huawei.com

Copyright © 2008 Huawei Technologies Co., Ltd. All rights reserved.

Dynamic SDCCH

2

Page2Copyright © 2008 Huawei Technologies Co., Ltd. All rights reserved.

Contents

1. HW II Channel Assignment Algorithm

2. TCH/SDCCH Conversion

3


MSC/VLR

TCE

SGSN

BSC

Ater/sub

AbisAbis

GbA BSS

Um

Air Interface Dimensioning

TRX:0mainBCCH

SDCCHTCHTCHTCHTCHTCHTCH

TRX:1TCHTCHTCHTCHTCHTCHTCHTCH

Air Interface DimensioningWhen setting up cells, it has to be decided, how many TCH and how manySDCCH are needed. Therefor a detailed traffic model is neccessary takinginto account the number of users and their behaviour.As a result it can be decided how many TRX are to be installed and howmany time slots should be set up as TCH Full or Half or both (ChannelCombinations I to III according to GSM specification). And of course howmany SDCCH Channel Combinations (Combination V combined with 4 SDCCH or one or several Combination VII with 8 SDCCH each) are needed.

4


Calculation of necessary TCH

TtBHCAA m*

=

e.g. 400 subscriberswhere 1/2 try to establish a

call during busy hour

Mean Holding Time depending on e.g:• subscriber profile• price per minute

Resulting A => Erlang B table for accepted Blocking e.g. B = 1% => n = 18 TCH => 3 frequencies needed

Erls

sA 103600

180*200==

Example of TCH dimensioningOn the air interface multiples of 8 timeslots have to be allocated where combinations of one or several signaling channels and up to 8 traffic channels (only non-BCH frequency) are possible. So the operator would first estimate the traffic channels needed because they take up more of the resource.Usually an initial traffic model shows the TCH traffic only in mErl per user in the cell, so it is irrelevant how many users actually tried to establish a call. A refined traffic model could show busy hour call attempts per user and mean holding time. The result should show the same (e.g. initial: 400 subscribers, 25 mErl each => 10 Erl; refined: see figure).For a start it seems reasonable to round up to numbers dividable by 7 (8 timeslots one for BCH and/or SDCCH, 7 for the TCH). If the resulting value is considered too high for whatever reason, the calculation can be based on TCH half rate instead of full rate. Then 14 channels will fit on one carrier. Whether actually one control channel per frequency is needed will be estimated in the next chapter.Values according to Erlang B calculation:A= Offered TrafficTm= Mean Holding TimeBHCA= Busy Hour Call AttemptsT=Observation Period

5


Calculation of necessary SDCCH

SDCCH traffic per user in the busy hour from standard or

PM data refined traffic model

Resulting A => Erlang B table for accepted Blocking e.g. B = 1% => n = 6 SDCCH => Channel Combination V. is not sufficient (compare Register 2). =>at least IV. and VII. are needed => 8 SDCCH => 3.13 Erl at 1 % Blocking

ErlErlusersA 6.1004.0400 =•=

Example of SDCCH DimensioningThe SDCCH capacity can be estimated similarly. One major difference is though that SDCCH are used for different reasons. A traffic channel TCH is used only for calls (packet or voice). The SDCCH is used for Call Control, Session Management, Location Management, SMS. So their usage is not directly related to TCH usage and also the usage attempts per busy hour is not as easily to obtain.But again a traffic model gives the amount of SDCCH traffic per user in the cell for the busy hour (e.g. the GSM standard model specifies 4 mErl per user in busy hour). If the value is at the border of having to set up a new timeslot for SDCCH or if the SDCCH is expected to change a lot, it might be useful to set up an alternating TCH/SDCCH channel. In this configuration the system decides, whether SDCCH or TCH capacity is needed and switches the timeslot between these modes accordingly.

6


MSC/VLR

TCE

SGSN

BSC

Ater/sub

AbisAbis

GbA BSS

Um

Air Interface Dimensioning

TRX:0mainBCCH

SDCCHSD-TCH

TCHTCHTCHTCHTCH

TRX:1TCHTCH

SD-TCHTCHTCHTCHTCHTCH

Dynamic Customer BehaviourBut what happens if the customer behaviour is not well known or changesover time of day, week, month, year? Well then the Huawei II TCH/SDCCH conversion feature can dynamically change the behaviour of a full time slot. TCH or SDCCH be can changed dynamically, but of course not withinmilliseconds, but at least within minutes to what the customers require in thisvery moment:

•SDCCH dynamic adjustment optimizes the usage of the traffic channels and signaling channels, reduces the SDCCH channel blocking and mitigates the impact on system performance caused by SDCCH initial configuration. •The adjustment process is triggered every time before channel assigning, but the adjusted SDCCH can be only used in the next occupying request. •Only support the dynamic adjustment between TCH/F and SDCCH.

Of course, the SDCCH can also switch back to TCH, but should not do so toooften in order to avoid ping pong between TCH and SDCCH. This behaviourcan be set with the feature‘s parameter set as we‘ll see later.

7


TCH to SDCCH ConversionConversion Trigger:

Idle SDCCH < Idle SD Thrsh.

Resource Check Ongoing?

SDCCH + 8 <Cell SD Maximum?

ISIdle TCH/F + idle (TCH/H)/2 ≤ 4

ANDIdle TCH/F + idle (TCH/H)/2 ≤ No of TRX?

Stop Select suitable TCH/F

System Busy?

SDCCH conversionallowed?

Other SDCCH conversion ongoing?

No

Yes

Yes

No

Yes

Yes

Yes

No

Yes

No

No

No

SD DynamicAllowed Yes/No

Cell SD Max0…fixed SDCCH+8…255

After the SDCCH dynamic adjustment process is triggered, dynamic adjustment can be started under the following conditions:

•There should not be a resource check ongoing (usually at night during low traffic hour).•The BSC internal flow control level should be less than 0 (i.e. the BSC is not in overload).•Dynamic adjustment of other SDCCH in the cell should not be in progress (one conversion at a time).•“SD Dynamic Allocation Allowed” should be “Yes” (feature enabled).•The number of the SDCCHs +8 should be less than “Cell SD Maximum”. •IF the number of the cell idle TCH/F + (the number of the cell idle TCH/H)/2 ≤ 4 AND ≤ the number of the TRXs in the cell THEN the dynamic SDCCH adjustment is not allowed. ELSE allowed. Then a suitable TCH/F should be selected for conversion.

Parameter: Cell SD Maximum•Range:0~255•Default: Configured SDCCHs + 8 (i.e single dynamic)•Description: The maximum number of SDCCHs in the cell.

Parameter: SD Dynamic Allocation Allowed

•Range: Yes, No•Default: Yes•Description: Indicates whether SDCCH dynamic allocation is allowed.

8


TRX:4TCH/FTCH/FTCH/FTCH/FTCH/FTCH/FTCH/HTCH/H

TCH/F Selection

TRX:0mainBCCH

SDCCHTCH/FTCH/HTCH/HTCH/FTCH/FTCH/F

TRX:1SDCCHSDCCHTCH/FTCH/FTCH/FTCH/HTCH/HTCH/H

TRX:2SDCCHTCH/HTCH/HTCH/HTCH/HTCH/HTCH/HTCH/H

No

Fixed

P-GSM 900 E-GSM 900 DCS1800

No No

TRX:3SDCCHTCH/FTCH/FTCH/FTCH/FTCH/FTCH/HTCH/H

No

No

Yes

Fixed Fixed Fixed

Same as BCCH

Same Band but more than8 SDCCH

All Half Rate Different Band

Different Band butP-GSM is sub setof E-GSM so all TCH/F may beselected

Reason

TCH/F SelectionIt is mandatory to select a TCH/F for conversion to SDCCH, since only fulltime slots can be converted to Combination VII. So TCH/H has to beconverted into TCH/F (if dynamic TCH is supported) or to be discarded by thealgorithm.

Additionally, the full rate TCH (TCH/F) has to meet the following conditions:

•The band class of the frequency should be the sub set of or the same as the band class of the BCCH frequency (see figure). •The number of SDCCHs on the TRX should be no more than eight (i.e. TCH/SDCCH conversion can only take place if at the most one timeslot uses Combination VII at the moment).•The channel should be fault free (i.e. in the occupied state or idle state).•In case of a multi TRXs cell, the full rate TCH channel must not be on the BCCH carrier.

9


TCH/SDCCH Conversion

Outer CellP-GSM900

Inner cellE-GSM900

Outer CellP-GSM900

Inner cellDCS1800

Non layered CellP-GSM 900

Free TCH/F Occupied TCH/F Free TCH/F in outer cell Occupied TCH/F in outer cellFree TCH/F in inner cell Occupied TCH/F in inner cell

•Free TCH/F in outer cell Occupied TCH/F in outer cell•The inner cell channels can not be converted

TCH/F PrioritisationIf there are several TCH/F available the algorithm my prioritize according to the following:

Non concentric cell•Free TCH/F Occupied TCH/F

Non 900/1800 hybrid concentric cell•Free TCH/F in outer cell Occupied TCH/F in outer cellFree TCH/F in inner cell Occupied TCH/F in inner cell

900/1800 hybrid concentric cell•Free TCH/F in outer cell Occupied TCH/F in outer cell•The inner cell channels can not be converted because they are not on the same frequency band

10


TCH/SDCCH Conversion

Yes No

NoYes

No

Yes

3 second timertimes out

Idle SDCCH number >N1+8?

Yes No

C-3 C+12

C<=0?

Whether there isSDCCH not swtiched

back?

Whether swtichingback is possible?

Select an SDCCHto Switch back

Restart the 3second timer

Stop the 3second timer

Restart the 3second timer

Start

End

N1 = Idle Thrsh:0..2..63

C = TCH Min. Recovery Time:60...3600 seconds

SDCCH to TCH Switch BackThe system decides “whether there is a suitable SDCCH to switch back” according to the following conditions:

•All the SDCCHs of the timeslot are occupied or idle (i.e. fault free)•If it is not a concentric cell, select randomly an SDCCH adjusted from a full rate TCH.•If it is a concentric cell, select first the SDCCH adjusted from a full rate TCH on the outer subcell. If there is none, select the SDCCH adjusted from a full rate TCH on the inner subcell.

Parameter: TCH Min. Recovery Time•Range:60~3600•Unit: Second•Default: 600•Description: The minimum duration for the SDCCH to be switched back to TCH.

Parameter: Idle SD Thrsh

•Range:0~63•Default:2•Description: When the number of the idle SDCCH is smaller than this parameter, the TCH can be adjusted to SDCCH.

11


Summary

In this course, we have learned

TCH/SDCCH conversion.

Application should be flexibly depending on the real

environment

12

Thank youwww.huawei.com

Documents

Huawei Dinamic SDCCH