Huawei Dinamic SDCCH

Dynamic SDCCH www.huawei.com

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

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Contents 1. HW II Channel Assignment Algorithm 2. TCH/SDCCH Conversion


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Air Interface Dimensioning MSC/VLR

A

Gb

SGSN

TCE

BSS

BSC

Ater/sub

Abis

Abis

Um

TRX:0 mainBCCH SDCCH TCH TCH TCH TCH TCH TCH


TRX:1 TCH TCH TCH TCH TCH TCH TCH TCH

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

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Calculation of necessary TCH A=

BHCA * tm T

e.g. 400 subscribers where 1/2 try to establish a call during busy hour

A=

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

200 *180 s = 10 Erl 3600 s

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


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Example of TCH dimensioning On 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 Traffic Tm= Mean Holding Time BHCA= Busy Hour Call Attempts T=Observation Period

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Calculation of necessary SDCCH SDCCH traffic per user in the busy hour from standard or PM data refined traffic model

A = 400 users • 0.004 Erl = 1.6 Erl 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 Copyright © 2008 Huawei Technologies Co., Ltd. All rights reserved.

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Example of SDCCH Dimensioning The 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.

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Air Interface Dimensioning MSC/VLR

A

Gb

SGSN

TCE

BSS

BSC

Ater/sub

Abis

Abis

Um

TRX:0 mainBCCH SDCCH SD-TCH TCH TCH TCH TCH TCH


TRX:1 TCH TCH SD-TCH TCH TCH TCH TCH TCH

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Dynamic Customer Behaviour But what happens if the customer behaviour is not well known or changes over 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 within milliseconds, but at least within minutes to what the customers require in this very 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 too often in order to avoid ping pong between TCH and SDCCH. This behaviour can be set with the feature‘s parameter set as we‘ll see later.

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TCH to SDCCH Conversion Conversion Trigger: Idle SDCCH < Idle SD Thrsh.

Yes

Cell SD Max 0…fixed SDCCH+8…255

Resource Check Ongoing?

No

SDCCH + 8 < Cell SD Maximum?

No Yes

Yes

System Busy? No No

Yes

SDCCH conversion allowed? SD Dynamic Allowed Yes/No Yes

Yes

IS Idle TCH/F + idle (TCH/H)/2 ≤ 4 AND Idle TCH/F + idle (TCH/H)/2 ≤ No of TRX?

Other SDCCH conversion ongoing? No

No

Stop


Select suitable TCH/F

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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.

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TCH/F Selection P-GSM 900

TRX:0 Fixed

No

Reason

mainBCCH Fixed SDCCH TCH/F TCH/H TCH/H No TCH/F TCH/F TCH/F

Same as BCCH

E-GSM 900

TRX:1 SDCCH SDCCH TCH/F TCH/F TCH/F TCH/H TCH/H TCH/H

Same Band but more than 8 SDCCH

TRX:2 Fixed

No

SDCCH TCH/H TCH/H TCH/H TCH/H TCH/H TCH/H TCH/H

Fixed

Yes

No

All Half Rate


DCS1800

TRX:3

TRX:4

SDCCH TCH/F TCH/F TCH/F TCH/F TCH/F TCH/H TCH/H

TCH/F TCH/F TCH/F TCH/F TCH/F TCH/F TCH/H TCH/H

No

Different Band but P-GSM is sub set of E-GSM so all TCH/F may be selected

Different Band

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TCH/F Selection It is mandatory to select a TCH/F for conversion to SDCCH, since only full time slots can be converted to Combination VII. So TCH/H has to be converted into TCH/F (if dynamic TCH is supported) or to be discarded by the algorithm.

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 time slot 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.

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TCH/SDCCH Conversion Non layered Cell P-GSM 900

Inner cell E-GSM900

Outer Cell P-GSM900

Free TCH/F in outer cellÎ Occupied TCH/F in outer cellÎ

Free TCH/FÎ Occupied TCH/F

Free TCH/F in inner cellÎ Occupied TCH/F in inner cell

Inner cell DCS1800

Outer Cell P-GSM900 •Free TCH/F in outer cellÎ Occupied TCH/F in outer cell •The inner cell channels can not be converted


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TCH/F Prioritisation If there are several TCH/F available the algorithm my prioritize according to the following: Non concentric cell •Free TCH/FÎ Occupied TCH/F Non 900/1800 hybrid concentric cell •Free TCH/F in outer cellÎ Occupied TCH/F in outer cellÎ Free TCH/F in inner cellÎ Occupied TCH/F in inner cell 900/1800 hybrid concentric cell •Free TCH/F in outer cellÎ Occupied TCH/F in outer cell •The inner cell channels can not be converted because they are not on the same frequency band

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TCH/SDCCH Conversion Start

3 second timer times out

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

Yes

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

Idle SDCCH number >N1+8?

No

C-3

C+12

Yes

Yes

C<=0?

Whether there is SDCCH not swtiched back?

Whether swtiching back is possible?

No

No Restart the 3 second timer

No

Stop the 3 second timer

Yes Select an SDCCH to Switch back

Restart the 3 second timer

End


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SDCCH to TCH Switch Back The 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.

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Summary z

In this course, we have learned

z

TCH/SDCCH conversion.

Application should be flexibly depending on the real environment


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Thank you www.huawei.com

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Huawei Dinamic SDCCH

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