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2G/3G interoperability in Releases B7, B8, B9 and B10

Alcatel-Lucent File Reference Date Edition PageFFUV5DE4.doc v 2 3DC 21144 0049 TQZZA 09/10/2007 04 1

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Functional Feature Description


This document covers following features:

In B7 Release:

15 48 10 UMTS to GSM handover

15 48 20 GSM to UMTS cell re-selection

30 28 10 GPRS to UMTS cell re-selection triggered by MS

In B8 Release

30 28 12 UMTS search deactivation during GPRS transfer

In B9 Release

15 48 30 Load based 3G handover filtering

15 48 40 GSM to UMTS handover

15 48 50 SI2Quater scheduling

In B10 Release

15 48 54 Usage of CPICH/RSCP for GSM to UMTS cell re-selection

15 48 58 Fast 3G re-selection at 2G call release

15 48 62 Compressed Inter-RAT Info Element

15 48 32 Full 2G load reporting to UTRAN

15 48 42 2G to 3G handover based on 2G load and service criteria

30 28 14 2G to 3G cell reselection when NC2 is activated

30 04 50 Allow restriction of NC2 to Rel-4 MS

15 48 84 2G to TD-SCDMA cell re-selection


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1. GLOSSARY

Abbreviations

2G Second Generation

3G Third Generation

BCCH Broadcast Control Channel

BSC Base Station Controller

BSS Base Station Sub-System

BSSMAP BSS Mobile Application Part

BTS Base Transceiver Station

FDD Frequency Division Duplex

GERAN GSM/EDGE Radio Access Network

GPRS General Packet Radio Service

GSM Global System for Mobile communications

HPLMN Home PLMN

IE Information Element

IMSI International Mobile Subscriber Code.

MS Mobile Station

MSC Mobile-services Switching Centre, Mobile Switching Centre

OMC-R Operations & Maintenance Centre Radio

PBCCH Packet Broadcast Control Channel

PLMN Public Lands Mobile Network

RAT Radio Access Technology

RNC Radio Network Controller

UE User Equipment

TD SCDMA Time Division Synchronous Code Division Multiple Access

UMTS Universal Mobile Telecommunication System

UTRAN UMTS Terrestrial Radio Access Network


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2. INTRODUCTION

This document presents the Alcatel BSS features in relation with 3G interoperability. It describes

the related B7 features as well as the evolutions brought by the B8, B9 and B10 releases.

It is organized as follows:

- Section 3 gives the rationale for UMTS inter-working features,

- Section 4 provides the description of the related BSS features,

- Section 5 details the operation and maintenance aspects.


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3. RATIONALE FOR 2G/3G INTEROPERABILITY BSS FEATURES

In large parts of the world, GSM / GPRS is the main mobile system: This is the 3G islands in the

2G sea model. Most of the UMTS terminals are bi-mode 2G/3G terminals and 3G subscribers are

also 2G subscribers.

It is then very important for the Operator to be provided with all the necessary features to ensure

efficient traffic /service sharing between 2G and 3G.

3G towards 2G

In this direction, 2G/3G inter-working means ensuring service continuity at the edge of the UMTS

coverage.

The following features can be identified:

- 3G to 2G cell re-selection: Having no impact on the BSS (in this direction, cell re-selection is

an UTRAN matter only), this feature will not be further detailed,

- 3G to 2G handover.

2G towards 3G

Allowing a mobile to go (back) to 3G is important, because:

- From the End User perspective:- He has paid for 3G services,

- From the 2G Operator perspective:

- If he is also the 3G Operator of the End User, he does not want to keep needlessly in

his 2G layer 3G capable terminals and he wants to satisfy his customer with enhanced

3G services,

- If he is not the 3G Operator of the End User (e.g. national roaming, where a 2G

incumbent operator offers its 2G coverage to a 3G new comer), he does not want to

keep needlessly in its 2G layer those 3G capable terminals, as soon as they can be

handled by this 3G network.

- From the 3G Operator perspective:- He wants to get the maximum revenues from its 3G subscribers.

2G to 3G interoperability in idle mode aspects are taken into account through cell re-selection

means.

In a first approach, a solution could have been to rely only on the HPLMN re-selection procedure.

However this approach, which has the advantage of simplicity, has two very important drawbacks:


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- The case where the Operator has the same PLMN code for its 2G and 3G networks can not be

handled.

- Even if the Operator has selected 2 different PLMN codes for its 2G and 3G networks, the

HPLMN procedure is very slow (not better than 6 minutes, default value 60 minutes).

Therefore 3G selection shall be based on a real cell re-selection capability, comparable to what

already exists for 2G to 2G cell re-selection. The following features cover these aspect:

- GSM to UMTS cell re-selection (15 48 20),

- GPRS to UMTS cell re-selection triggered by MS (30 28 10),

- 2G to TD-SCDMA cell re-selection (15 48 84).

The Alcatel-Lucent system allows to parameterize the 2G to 3G cell re-selection features so that

MS can re-select 3G as soon as a suitable 3G cell is found. Thanks to this 3G camping strategy,

the End User takes benefit from a quicker access time to 3G services, as its terminal is already

camping in a 3G cell when the CS call or the PS session starts.

In Dedicated Mode, handover from a GSM cell to UMTS can also be useful (see feature 15 48 40).


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4. FEATURE DESCRIPTION

4.1 UMTS to GSM handover

The feature provides the GSM network with the ability to support an incoming handover from a

UMTS cell.

As a synthesis, the already existing GSM handover procedures are almost completely re-used and

the handover is seen by the BSS as a GSM inter-BSC handover.

As a consequence, the feature relies mainly on the support of modifications brought to the

existing interfaces (e.g.: the BSSMAP messages coming from a 3G MSC include a new cell

identifier).

4.2 GSM to UMTS Cell Re-selectionGPRS to UMTS cell re-selection triggered by

MS

As seen from the BSS, the feature consists in broadcasting in the concerned GSM cells information

on surrounding UMTS cells.

These information are:

- The list of frequencies used in the surrounding UMTS cells,

- The radio parameters required for re-selecting a UMTS cell (see Parameters section).

They are broadcast on BCCH, in SI2 ter.

With these information the MS is able to operate a blind cell re-selection (Synchronizing with a

UMTS frequency, the terminals is able to detect all its associated scrambling codes (one UTRAN

FDD cell can be derived from one UMTS frequency plus one scrambling code)).

It has to be particularly noted that the blind search technique simplifies 2G/3G operation. Indeedthere is no need for exchanging scrambling codes between the 2G and the 3G networks. This

advantage holds all the more true when the 2G and the 3G Operators are different, a typical

example being 2G/3G national roaming (a 2G operator offering its 2G coverage to one or several

3G newcomers).


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The feature is controlled by a parameter EN_2G_TO_3G_CELL_RESELECTION, on a per cell basis1,

4.3

UMTS search deactivation during GPRS transfer

The feature (30 28 12) is available from the B8 release onwards.

2G to 3G cell re-selection has no impact on MS performance when the MS is in idle mode.

This is not the case when the MS performs a GPRS transfer, for two reasons:

- The measurements that a bi-mode MS performs on the surrounding UMTS frequencies may

disturb an on going GPRS transfer.

- Cell re-selections triggered between a GSM and a UMTS cell take quite a long time and

induce data transfer interruption highly perceived by the end user.

To solve this issue, the Alcatel-Lucent BSS allows forbidding any GPRS to UMTS cell re-selection for

an MS performing a GPRS transfer.

Behavior of the feature when NC2 is not activated

If the Qsearch parameter (broadcast in System Information) is different from never search 3G

cells, each time a bi-mode terminal enters the Packet Transfer mode, the BSS sends a Packet

Measurement Order message to this MS indicating that it shall not monitor anymore 3G cell (and

thus shall not perform any cell re-selection towards a 3G cell).

The MS reverts by itself to the behavior specified in the System Information (MS authorized to

perform 3G search) at READY timer expiry.

Behavior of the feature when the NC2 is activated

In that case, to inhibit 3G measurements and re-selections, the feature uses the Packet

Measurement Order message already sent to put the MS in NC2 mode. No additional message is

sent.

Two possibilities are offered to revert to original behavior, depending of NC2 parameterization.

1Before B9MR4, this parameter is on a per BSS basis.


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- At the end of the data transfer: in that case, the BSS uses the same Packet Measurement

Order message to make revert the MS in NCO and to re-authorize 3G search.

- At Ready timer expiry: In that case, the MS reverts alone to NC0 mode and 3G search

activated mode simultaneously.

Please refer to NC2 functional feature description for more details on NC2 implementation.

4.4 2G to 3G cell reselection when NC2 is activated

4.4.1

30 28 14: 2G to 3G cell re-selection in PTM when NC2 is activated

This B10 feature (30 28 14) allows Dual mode MS performing a packet transfer to reselect a 3G cell

even when the NC2 feature is activated.

To do so, such MS in such cells are kept in NC0 at the start of the packet transfer.

The feature allows to not prevent Dual Mode MS from re-selecting a 3G cell even during a packet

transfer, while keeping the benefits of the NC2 feature for all other cases.

This behavior corresponds to a new value of the EN_2G_TO_3G_CELL_RESELECTION parameter.

As this parameter is settable on a per cell basis, the feature can be restricted to 2G cells having

good 3G neighborhoods.

4.5 SI2 quater scheduling

From the B9 release (feature 15 48 50), the 3G neighborhood information are conveyed in

SI2quater on BCCH instead of SI2 ter.

The SI2 quater coding is more efficient than SI2 ter then it allows the MSs to acquire and to refresh

more quickly their knowledge of the 3G neighboring cells information.

SI2quater message are sent on the Extended BCCH if the Extended BCCH has been enabled by the

Operator, otherwise on the Normal BCCH.

4.6 Usage of CPICH/RSCP for GSM to UMTS cell re-selection

This feature (15 48 54) is available from B10 onwards.


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In a W-CDMA system, CPICH (Common Pilot Channel) Ec/No characterizes the interference level

and is mostly determined by the system load, whereas CPICH RSCP (Received Signal Code Power)

gives an indication about coverage (received level).

Up to 3GPP Rel-4, the MS considered only CPICH Ec/No for 2G to 3G cell reselection. This lead to

potential 2G to 3G cell reselection failures when the target 3G cell was lightly loaded (high CPICH

Ec/No), but provided poor coverage (low CPICH RSCP).

This can happen for example in deep indoor cases, where the in building penetration loss affects in

the same way Ec and No (Ec/No = constant) while receiving level decreases.

From 3GPP Rel-5 new parameters FDD_RSCPmin can be broadcast in System Information messages,

so that the MS can use both CPICH Ec/N0 and RSCP criteria for cell reselection. An other new

parameter (FDD_Qmin_Offset) can also be broadcast.

FDD_RSCPmin gives the MS the minimum threshold of RSCP for UTRAN FDD cell re-selection.

FDD_Qmin_Offset is used to put some dBs in offset to FDD_Qmin value: the threshold is now

(FDD_Qmin - FDD_Qmin_Offset). This allows the MS to use less constraining conditions on

the Ec/No criteria, knowing that it can now also use a new criterion on RSCP.

The BSS broadcasts those parameters in SI2 Quarter.

As defined in the Standards, 3GPP Rel-5 or later MS are obliged to use the new parameters. 3GPP

Rel-4 (or below) MS can compute RSCP level automously.

This feature can be activated/deactivated by the operator from the the OMC using the parameters

EN_CPICH_2G_3G_Reselection.

4.7 Fast 3G re-selection at 2G call release

This feature (15 48 58) is available from B10 onwards.

The Fast 3G re-selection at 2G call release feature improves the cell reselections procedure

towards 3G after a 2G call release, through a reduction of the signaling load in 2G.

The initial strategy of many 2G/3G Operators seems to be 3G-camping, to allow Dual Mode MS to

start a circuit call or a packet session in 3G as fast as possible. In this strategy, Dual Mode MS

camp on 3G as soon as a suitable 3G cell is discovered.


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According to previous 3GPP specifications, when a MS releases a call in a 2G cell, it shall camp on

this cell. This induces an unnecessary LA/RA Updates in case the call terminates in a cell belonging

to a LA different from the one of the initial cell and having 3G neighborhoods.

Starting with 3GPP Rel-6, a new Information Element Cell Selection Indicator after Release of allTCHs and SDCCH is defined in the CHANNEL RELEASE message, indicating to the MS where

camping shall be tried after Channel Release.

This new IE may contain 2G or 3G cell/frequency list. For this feature, the Alcatel-Lucent BSS

places in the IE the list of the 3G frequencies surrounding the 2G cell where the call has

terminated.

This feature is activated by the Operator at the OMC interface using the EN_FAST_3G_Reselection

parameter.

4.8 Load based 3G handover filtering

The feature (15 48 30) is available from the B9 release onwards.

The feature increases the global efficiency of the network through load balancing improvements

between 2G and 3G.

Indeed it may happen that 3G to 2G handovers impact 2G load at a point that 2G only MS may not

be served anymore.

To avoid this situation, the BSS can be configured in such a way that it accepts only the handovers

requested for Emergency reasons when the 2G target cell is too much loaded: then handovers

requested for non Emergency (non time critical) reasons) are rejected.

A cell is considered as too much loaded when its current load exceeds the value of the

THR_CELL_LOAD_3G_REJECT parameter.

An incoming 3G handover is seen as requested for Emergency reasons if the handover cause IE of

the BSSAP Handover Request message has one of the following values:

- Uplink quality,

- Uplink strength,

- Downlink quality,

- Downlink strength,

- Distance.


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An incoming 3G handover rejection is signaled by the BSS through the HANDOVER FAILURE message

with the cause no radio resource available. In addition this message contains an indication of

the load of the 2G cell.

As defined in 3GPP R5, the cell load is given in the UL and DL Cell Load Information IEs encoded

in the Inter-System Information Transparent Container which is transparently routed by the MSC

towards the serving RNC. Within this structure, Alcatel provides the Cell Capacity Class and

Cell Load information. The UL and DL information are identical. The Cell Capacity Class is an

O&M parameter changeable at the OMC-R.

Then it is up to the UTRAN to decide on the usage of these informations (e.g. stop sending non

critical handover towards this 2G cell for a while, in order to avoid unnecessary additional

signaling load in both 2G and 3G systems).

The feature Load based 3G handover filtering is activated in a cell when the

THR_CELL_LOAD_3G_REJECT parameter is below 100%.

4.9 Full 2G load reporting to UTRAN

This B10 feature (15 48 32) enhances the feature 15 48 30.

The BSS provides cell load information to the RNC also in case the handover coming from a 3G cell

is accepted, in the HO Request Ack message.This feature improves the 3G knowledge of the 2G

load. Then the UTRAN is in a better position to decide whether or not it can try a non-critical

handover to 2G.

4.10 GSM to UMTS Handover

This feature (15 48 40) is available from B9.

Handover from GSM to UMTS can be useful, for example at the border of UMTS coverage in order

to unload a bit more GSM cells.

When the feature is activated, the BSS needs to broadcast in the concerned GSM cells information

on surrounding UMTS cells. All information on the candidate 3G cells are transmitted to multimode

mobiles, through SACCH channels.


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Unlike cell re-selection, blind handover is not possible; therefore the 3G cells have to be

identified in an unambiguous way (see parameters in Operation & Maintenance Chapter).

A 3G cell is defined through its MCC, MNC, LAC, CI, and RNC.

In each 2G cell where the feature is activated, information on neighbor 3G cells is broadcast; it

encompasses the Radio Frequency, Scrambling Code and Diversity of these cells. It also indicates

the measurements that multimode mobiles must report to the BSS.

The MS report measurements (Ec/No) which are averaged in a sliding window and compared to a

threshold in the BSS.

A handover is systematically triggered for multimode MS which report a UTRAN cell with a

sufficient signal strength.

When several 3G cells are reported and candidate for handover, the 3G cell with the best radio

conditions is chosen.

Handover to 3G is always prioritized regarding handover to 2G.

Handover to 3G is supported for TCH only (no handover to 3G from SDCCH ).

In addition, the BSS handles the Service Handover IE (information element).

- The MSC may inhibit the handover to 3G for some MS: In this case, the MSC informs the BSSthrough the Service Handover IE, transmitted in the ASSIGNMENT REQUEST and in the

HANDOVER REQUEST messages.

- If this element indicates Handover to UTRAN shall not be performed, then the BSS does

not send any measurement information message to that MS, then avoiding the MS to search

3G cells.

4.11 2G to 3G handover based on 2G load and service criteria

This B10 feature (15 48 42) improves the 2G to 3G handover feature (15 48 40).

The decision to trigger a handover to 3G is refined, based on an extended analysis of the SERVICE

HANDOVER field of the ASSIGNMENT REQUEST message, and on the load of the 2G serving

cell:When the SERVICE HANDOVER field is MS Shall not be handed over to 3G or MS should not

be handed over to 3G, then the MS is kept in the 2G serving cell.


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When the SERVICE HANDOVER field is MS should be handed over to 3G, then the MS is

handed over to 3G.

When the SERVICE HANDOVER field is missing or received with an unexpected value:

For this case, 2 new parameters are introduced: THR_CELL_LOAD_3G_REQ and

EN_2G_3G_HO_NO_SERVICE_HO.

If the Serving GSM cell is not loaded, the MS is kept in this cell (The HO to 3G is not

performed),

If the Serving GSM cell is loaded, the parameter EN_2G_3G_HO_NO_SERVICE_HO

handles the choice between staying in GSM and triggering a handover to 3G.

Based on MSC indication given at call set-up, the feature allows for a better load sharing between2G and 3G cells.

4.12 Compressed Inter-RAT HO INFO element

The MS UTRAN Classmark message sent by MS at call set-up contains the INTER RAT HANDOVER IE

(Radio Bearer and UE radio access capability) used by the RNC in case of 2G to 3G handover.

The BSC places it in the HANDOVER REQUIRED message sent to the MSC:

Either via the Source RNC to target RNC transparent information element in case of

intersystem handover to 3G,

Or via the Old BSS to New BSS information element in case of inter-BSS handover, to be

ready for a potential subsequent handover to 3G from this new BSS.

In 3GPP Rel-5, a compressed format of this IE has been introduced. This possibility is implemented

in the B10 release (15 48 62).

As the BSS does not decompress this element before transmitting it to the UTRAN, the feature is

usable only if the UTRAN supports the compressed format. This is why it is activated by the

Operator at the OMC interface using the EN_Compressed_Inter_RAT_Info parameter.

The BSS indicates in the IMMEDIATE ASSIGNMENT message whether the MS shall use the legacy

format or the compressed format.

A reduction of call set-up time of around 700 ms is expected for Dual Mode MS.


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4.13 2G to TD-SCDMA cell re-selection

Based on information provided on BCCH, this B10 feature (15 48 84) allows Dual Mode GSM/TD-

SCDMA mobiles to re-select a TD-SCDMA cell.

The Feature behavior is quite similar to the FDD case.

It is restricted to the Chinese market.


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5. OPERATION AND MAINTENANCE

5.1 Main Parameters

Parameter name Description Instance

Cell re-selection

EN_2G_TO_3G_CELL_RESELECTION Can take 4 values:

- 2G to 3G cell reselection deactivated.

- 2G to 3G cell reselection activated.

- 2G to 3G cell reselection activated, but 3G search is

forbidden to MS performing a GPRS transfer (From B8).

- 2G to 3G cell reselection activated even if NC2 is

activated in the cell (Network_Control_Order >=2)(From B10)

BSS

2G Cell

from

B9MR4

FDD_ARFCN_LIST - Broadcast on BCCH

- List of FDD UMTS frequencies to be monitored by

terminals

BSS

Qsearch - Broadcast on BCCH

- Signal level of the GSM serving cell under which a

terminal shall begin to look for UMTS cells.

- Signal level of the GSM serving cell over which a

terminal shall begin to look for UMTS cells.

- Specific values enable also to always enable or alwaysdisable the search of UMTS cells in the terminals.

Used also for interoperability with TD-SCDMA (B10)

2G Cell

FDD_Qoffset - Broadcast on BCCH

Usage by terminal: UMTS cell can be selected by the MS if

received level in UMTS cell is higher than the level

received in serving GSM and neighbor GSM cells (RLA_C or

RLA_P) + this offset.

2G Cell

FDD_Qmin - Broadcast on BCCH

Usage by terminal: UMTS cell can be selected if

Signal/Noise (Ec/No) is higher than this threshold.

2G Cell

FDD_Qmin_Offset Applies an offset to FDD_Qmin value. (B10) 2G Cell

FDD_RSCPmin Minimum threshold of RSCP for UTRAN FDD cell re-

selection. (B10)

2G Cell

EN_CPICH_2G_3G_Reselection Defines the CPICH measurements used for 2G to 3G cell

reselection: EC/No only or EC/No and RSCP. (B10)

2G Cell

EN_FAST_3G_Reselection Enables corresponding feature. (B10) 2G Cell

EN_Compressed_Inter_RAT_Info Enables corresponding feature. (B10) 2G Cell


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Load based 3G handover filtering

THR_CELL_LOAD_3G_REJECT Load threshold (%) determining whether a 3G to 2G

handover request shall be accepted or rejected (From B9)

2G Cell

CELL_CAPACITY_CLASS Capacity class of the cell, measured on a linear scale,

ranging from 1 to 100 (From B9)

2G Cell

Handover to 3G

Info transmitted to MS on SI

FDD_ARFCN(n) UTRA Absolute Radio Frequency Channel Number 3G Cell

SCRAMBLING_CODE_3G(n) Primary Scrambling Code as defined in 3GPP TS 25.213 3G Cell

EN_3G_DIVERSITY(n) This parameter indicates if diversity is applied for the

cell

3G Cell

FDD_MULTIRAT_REPORTING Number of cells from 3G FDD that shall be included in the

list of strongest cells or in the measurement report.

BSS

FDD_REPORTING_THRESHOLD Threshold (Always, 6 dB 36 dB, never) above which theMS shall report the measurements.

2G Cell

FDD_REPORTING_OFFSET Offset (0 dB, 6 dB 42 dB) that the MS shall apply to the

measurements before reporting.

2G Cell

QSEARCH_C Same goal as QSEARCH, used in dedicated mode 2G Cell

HO parameters

EN_3G_HO Flag to enable/disable the Handover to UMTS 2G Cell

A_ECNO_HO Averaging interval for Ec/No 2G Cell

THR_ECNO Ec/No Threshold above which a handover to UTRAN will

be triggered

2G Cell

THR_CELL_LOAD_3G_REQ 2G serving cell threshold over which a HO to 3G is

attempted if the flag EN_2G_3G_HO_NO_SERVICE_HO is

set. (From B10)

2G Cell

EN_2G_3G_HO_NO_SERVICE_HO See above (from B10) 2G Cell

Interoperability with TD-SCDMA

EN_2G_TO_3GTDD_CELL_RESELECTI

ON

Enables the GSM to UTRAN TDD cell reselections and

defines the 3G search mode for MS in GMM Ready state

(B10)

4 values, as for GSM to UTRAN FDD reselection

2G Cell

TDD_ARFCN_LIST List of up to 3 TDD UARFCN BSC

TDD_Qoffset Usage by terminal: TDSCDMA cell can be selected by the

MS if received level in TD-SCDMA cell is higher than the

level received in the serving and neighbor GSM cells

(RLA_C or RLA_P) + this offset.

2G Cell


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Alcatel-Lucent File Reference Date Edition PageFFUV5DE4 doc v 2 3DC 21144 0049 TQZZA 09/10/2007 04 18

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