SRVCC Handover to UTRAN
USER DESCRIPTION
120/1553-HSC 105 50/1 Uen D2
Copyright
© Ericsson AB 2012. All rights reserved. No part of this document may be
reproduced in any form without the written permission of the copyright owner.
Disclaimer
The contents of this document are subject to revision without notice due to
continued progress in methodology, design and manufacturing. Ericsson shall
have no liability for any error or damage of any kind resulting from the use
of this document.
Trademark List
All trademarks mentioned herein are the property of their respective owners.
These are shown in the document Trademark Information.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Contents
Contents
1
Introduction
1
1.1
Basic Characteristics
1
1.2
Benefits
2
1.3
Additional Information
2
2
Feature Operation
5
2.1
Network Requirements
5
2.2
Handover for UE with Voice Bearers
6
2.3
Feature Operation Sequence Diagram
7
2.4
Process Steps
8
3
Parameters
11
3.1
Introduced Parameters
11
3.2
Affected Parameters
11
3.3
Related Parameters
11
4
Network Impact
13
4.1
Capacity
13
4.2
Performance
13
4.3
Configuration Impact
13
5
Associated Features and Affected Functions
15
5.1
Prerequisite Features
15
5.2
Affected Features
15
5.3
Related Features
15
5.4
Affected System Functions
15
6
Performance
17
6.1
KPIs
17
6.2
Counters
17
6.3
Events
17
7
O&M Information
19
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
SRVCC Handover to UTRAN
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Introduction
1
Introduction
This document describes the SRVCC Handover to UTRAN feature and its main
benefits and impacts in the LTE RAN.
In the event of poor coverage in the serving cell, the SRVCC Handover to
UTRAN feature enables the eNodeB to use handover as the mechanism to
transfer User Equipment (UE) with voice bearers from LTE to WCDMA; with the
voice bearers being handed over to the Circuit Switched (CS) domain.
This feature is closely related to the Coverage-Triggered WCDMA IRAT
Handover feature, in the sense that the same trigger (poor coverage) handover
procedure is used. The main difference is that only voice bearers are
included in the handover and will end up in the CS domain, while with the
Coverage-Triggered WCDMA IRAT Handover feature, all bearers are included
in the handover and will end up in the Packet Switched (PS) domain. When
both features, SRVCC Handover to UTRAN and Coverage-Triggered WCDMA
IRAT Handover, are enabled, voice bearers end up in the CS domain and PS
bearers (also included in the handover) end up in the PS domain.
If the UE PS bearers are not transferred to the WCDMA target cell in the
handover that transfers the voice bearers, the PS bearers can be reestablished
in the target cell once the UE has arrived there.
1.1
Basic Characteristics
This section describes the basic characteristics of the feature.
Feature name:
SRVCC Handover to UTRAN
Product identity:
See Feature Overview
Replaces:
N/A
Dependencies
This feature requires the following RAN features to be active:
•
Coverage-Triggered WCDMA Session Continuity
•
Coverage-Triggered WCDMA IRAT Handover
This feature must be active to be able to handover PS bearers when UE
having voice bearers as well as PS bearers must be handed over to
WCDMA. If the Coverage-Triggered WCDMA IRAT Handover feature is not
active, only the UE voice bearers are handed over to the WCDMA cell.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
1
SRVCC Handover to UTRAN
This feature affects no other feature.
1.2
Benefits
This section describes the benefits of this feature.
The benefits of the SRVCC Handover to UTRAN feature are the following:
•
LTE Voice over IP (VoIP) session transition to the CS domain in a WCDMA
cell when poor coverage is detected in the serving cell
•
Enhanced control over how UE with voice bearers are transferred between
frequencies and cells
•
Using handover to transfer UE between the nodes of different technologies
enables the target node (RNC) to use different admission criteria than
would be used at a plain connection setup
This makes it possible to improve the probability that a service, for example
VoIP is not interrupted in a network with high load.
1.3
Additional Information
More information about this feature and related topics can be found in the
following documentation:
•
Coverage-Triggered WCDMA Session Continuity
This document describes mainly how measurements are configured and
started as well as how candidate frequencies for mobility are chosen based
on priorities set according to requirements of UE that have voice bearers.
•
Coverage-Triggered WCDMA IRAT Handover
This document describes mainly how handover of PS bearers to WCDMA
is handled when poor coverage is detected by the UE.
2
•
3GPP TS 25.331 Radio Resource Control (RRC); Protocol Specification
•
3GPP TS 25.413 UTRAN Iu interface Radio Access Network Application
Part (RANAP) signalling
•
3GPP TS 23.401 General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access
•
3GPP TS 36.331 Evolved Universal Terrestrial Radio Access Network
(E-UTRAN); Radio Resource Control (RRC); Protocol Specification
•
3GPP TS 36.133 Evolved Universal Terrestrial Radio Access (E-UTRA);
Requirements for support of radio resource management
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Introduction
•
3GPP TS 36.306 Evolved Universal Terrestrial Radio Access (E-UTRA);
User Equipment (UE) radio access capabilities
•
3GPP TS 36.214 Evolved Universal Terrestrial Radio Access (E-UTRA);
Physical layer; Measurements
•
3GPP TS 23.216 Single Radio Voice Call Continuity (SRVCC); Stage 2
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
3
SRVCC Handover to UTRAN
4
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Feature Operation
2
Feature Operation
This section describes the SRVCC Handover to UTRAN feature in more detail,
including network configuration requirements and operation flows.
The SRVCC Handover to UTRAN feature is very similar to the
Coverage-Triggered WCDMA IRAT Handover feature, but with the main
difference that when the handover procedure is started, an indication is
included that Single Radio Voice Call Continuity (SRVCC) should be used. The
indication on the use of SRVCC tells the Mobility Management Entity (MME)
that the UE voice bearers must be transferred to the CS domain in the target
cell. In addition, when deciding on what target cell to choose for the SRVCC
handover, an explicit check is made on whether the cell is SRVCC capable.
2.1
Network Requirements
The following must be fulfilled at feature activation:
•
License keys must be activated to operate both the Coverage-Triggered
WCDMA Session Continuity and SRVCC Handover to UTRAN features.
The WcdmaSessionContinuity Managed Object (MO) and the
SRVCCtoUTRAN MO hold the licensing parameters. Details on the
process required to load and activate a license can be found in License
Management.
•
The Coverage-Triggered WCDMA Session Continuity feature must be
configured correctly, especially regarding prioritizing frequencies for use
by UE with voice bearers. Details on how to do this can be found in
Coverage-Triggered WCDMA Session Continuity.
•
There must be UE in the network that have the capability to do SRVCC
Handover to WCDMA. This is indicated in the SRVCC operation
possible information element in the INITIAL CONTEXT SETUP
REQUEST S1 message or in the HANDOVER REQUEST S1/X2 message
described in TS 36.413 and TS 36.423.
•
The core network (MME) that handles the eNodeB where the UE
is connected must support SRVCC. This is indicated in the SRVCC
operation possible information element in the INITIAL CONTEXT
SETUP REQUEST S1 message or in the HANDOVER REQUEST S1/X2
message described in TS 36.413 and TS 36.423.
In addition, some neighbor WCDMA system must support the SRVCC
procedure.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
5
SRVCC Handover to UTRAN
2.2
Handover for UE with Voice Bearers
The SRVCC Handover to UTRAN feature uses the Event A2 (serving cell
becomes worse than threshold) measurement process to detect poor coverage
in the serving cell. This is enabled by the Coverage-Triggered WCDMA
Session Continuity feature. At reception of an Event A2 measurement report,
the eNodeB determines what other frequencies (LTE or other RAT) are target
candidates for transferring the UE.
With the SRVCC Handover to UTRAN feature, a clear distinction is made
between UE having only data bearers and UE that have at least one voice
bearer.
The criteria for telling whether a UE has voice bearers is for the UE to have at
least one bearer with a Quality of Service Class Identifier (QCI) profile with the
value of QCI=1.
Note:
Even if the UE has a bearer with a QCI value (other than QCI=1) that
correspond to a voice profile, the SRVCC procedure is not or cannot
be applied to such a bearer. This is because (in accordance with TS
23.216) the core network (MME) is only considering bearers with QCI=1
as voice bearers. Only these will be transferred to the CS domain of
the target when the SRVCC procedure is applied for a certain UE.
At the detection of poor coverage in the source cell (that is, the reception of an
event A2 measurement report), if the UE has at least one bearer with QCI=1, if
the highest prioritized frequency that the UE can handle (has capabilities for) is
a WCDMA frequency, the RBS can do one of the following two actions:
•
Immediately initiate an SRVCC handover for the UE to a cell on the target
WCDMA frequency that is known by the RBS to cover the source cell
completely. This is referred to as ‘‘blind SRVCC handover to WCDMA’’.
•
Start an Event B2 (serving becomes worse than threshold1 and
inter-RAT neighbor becomes better than threshold2) measurement in
the UE to determine whether there is a cell on this WCDMA frequency that
is covering the UE. Wait until the UE sends a report for this measurement
and initiate an SRVCC handover for the UE to one of the cells received
in the measurement report. This is referred to as ‘‘measurement based
SRVCC handover to WCDMA’’
As can be seen from the above options, the eNodeB will never perform the
action ‘‘release with redirect’’ for UE that has voice bearers. If no appropriate
target is found where the UE can be handed over (that is, there is no specific
cell that can be prepared in advance), the UE will be kept in the serving cell
until new measurement reports are processed and possible new targets for
handover can be found.
6
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Feature Operation
Note:
If, at the time a decision to transfer the UE to a WCDMA cell through
SRVCC, the UE has PS bearers, the aim is to, in the same handover
procedure, transfer even the data bearers to the target WCDMA cell.
The necessary conditions for this are the following:
•
The target cell is simultaneously CS and PS handover capable.
•
The Coverage-Triggered WCDMA IRAT Handover feature is
operable in the eNodeB.
If at least one of the above conditions is not fulfilled, the handover
related message sent to the core network will indicate that only the
voice bearers must be transferred to the target WCDMA cell.
2.2.1
Outcome of an SRVCC Handover
The outcome of an SRVCC Handover is dependent solely on whether the UE
voice bearers are successfully transferred to the target WCDMA cell:
2.3
•
If all the UE voice bearers are successfully handed over to the target
WCDMA cell then the outcome of the SRVCC Handover will be success.
The successful outcome is regardless of whether any of the UE PS (data)
bearers, if any, were successfully transferred to the WCDMA cell, that is, if
the voice bearers are successfully handed over to the WCDMA cell, SRVCC
Handover will have a successful outcome despite the fact that at least one
of the UE PS bearers was not successfully handed over to the WCDMA cell.
•
If at least one of the UE voice bearers is not successfully handed over
to the target WCDMA cell, the outcome of the SRVCC Handover will be
failure. Failure is regardless of whether any of the UE PS bearers (or all)
could have been successfully transferred to the WCDMA cell, that is, if
at least one of the voice bearers is not successfully handed over to the
WCDMA cell, an SRVCC Handover will have a failure despite all the UE PS
bearers could have been successfully handed over to the WCDMA cell.
Feature Operation Sequence Diagram
The SRVCC Handover to UTRAN feature operates as follows:
1. The source RBS configures an Event A2 (serving cell becomes worse than
threshold) measurement in the UE as described in Coverage-Triggered
WCDMA Session Continuity.
2. When the Event A2 measurement report is received from the UE by
the serving RBS, the RBS determines the set of candidate frequencies
where the UE (having voice bearers) can be transferred, as described in
Coverage-Triggered WCDMA Session Continuity.
3. If any of the candidate frequencies is a WCDMA frequency, the RBS
checks whether it has information about a cell on this frequency that is fully
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
7
SRVCC Handover to UTRAN
covering the source cell and that supports SRVCC, and if so selects this
cell as the target for (blind) handover. The fact that a cell is covering the
source cell is expressed by using the coverageIndicatorparameter
described in Table 2.
4. If none of the WCDMA frequency candidates has a cell that is configured
to fully cover the source cell and the UE or no such cell has support for
SRVCC, and the eNodeB is configured to start measurements when the
UE reports poor coverage (Event A2), an Event B2 measurement (serving
becomes worse than threshold1 and inter-RAT neighbor becomes
better than threshold2) for each of the candidate WCDMA frequencies
will be configured in the UE. When an Event B2 measurement report is
received from the UE, the RBS investigates the reported cells and selects a
target cell to which it is possible to make SRVCC handover. If none of the
reported cells can be used for SRVCC handover, the UE will continue to
stay connected to the serving LTE cell.
5. If a target cell was selected for SRVCC handover, the RBS prepares and
executes the handover attempt.
This is done in the same way as is described in Coverage-Triggered
WCDMA IRAT Handover, with the following two additions:
•
The handover preparation HANDOVER REQUIRED S1 message
contains an indication that SRVCC has to be used, that is, the SRVCC
HO Indication information element is included in the message as
described in TS 36.413.
•
If the UE has PS (data) bearers as well and the Coverage-Triggered
WCDMA IRAT Handover feature is operable in the eNodeB, the
eNodeB requests that the data bearers are also handed over to the
target using the same handover procedure. This is done by setting the
value of SRVCC HO Indication information element to CS and PS.
The events sent from the RBS during SRVCC handover preparation contain
information saying that it is a ‘‘SRVCC handover’’, see Table 6.
2.4
Process Steps
This section describes the main process steps for the feature flow. The process
steps for the feature SRVCC Handover to UTRAN are very similar to the
process steps for the Coverage-Triggered WCDMA IRAT Handover feature
described in Coverage-Triggered WCDMA IRAT Handover.
2.4.1
Configure for SRVCC Handover to WCDMA
Regardless of whether the eNodeB executes a blind or measurement-based
SRVCC handover, a WCDMA cell can be used as a target cell for the handover
only if the following parameter is set to a value different from the default value:
lac
8
Location area code
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Feature Operation
Must be sent to the MME to correctly identify the target
MSC/SGSN that serves the target WCDMA RNC.
Furthermore, if the UE has data bearers, and the candidate target cell
has the srvccCapability attribute set to value PS_AND_CS and the
Coverage-Triggered WCDMA IRAT Handover feature is operable in the
eNodeB, for the handover to PS domain to work correctly the following
parameter must be set to a value that is different from the default value:
rac
Routing area code
Must be sent to the MME to uniquely be able to identify
the target SGSN in the target location area of the target
WCDMA cell.
If the value of the srvccCapability parameter in the MO instance
corresponding to the selected target cell is CS_ONLY or the Coverage-Triggered
WCDMA IRAT Handover feature is inoperable, no PS handover is performed
for the UE data bearers, if any.
2.4.2
Configure for Blind and Measurement Based SRVCC Handover to
WCDMA
The operations needed to configure a eNodeB for blind and measurement-based
SRVCC Handover to UTRAN are the same as the operations required for
configuring WCDMA handover and are described in Coverage-Triggered
WCDMA IRAT Handover. The only difference from the description in
Coverage-Triggered WCDMA IRAT Handover is that regardless of whether a
candidate cell for blind or measurement based SRVCC handover is found, no
release with redirect will be performed for the UE having voice bearers.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
9
SRVCC Handover to UTRAN
10
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Parameters
3
Parameters
This section describes parameters introduced by the SRVCC Handover to
UTRAN feature and parameters affected by activating the feature.
3.1
Introduced Parameters
Table 1 describes the parameters introduced by the feature.
Table 1
Introduced Parameters
Parameter
Description
srvccCapability
SRVCC capability of the external cell
voicePrio
Prioritizes UTRAN frequencies among all frequencies related to
the cell for UE in connected mode
Sets the priority to which frequency to perform mobility to for a
voice session. The value 0 means the lowest priority. The value
7 means the highest priority. The value -1 means the frequency
is excluded
Prioritizes EUTRAN frequencies among all frequencies related to
the cell for UE in connected mode
voicePrio
Sets the priority to which frequency to perform mobility to for a
voice session. The value 0 means the lowest priority. The value
7 means the highest priority. The value -1 means the frequency
is excluded
3.2
Affected Parameters
The implementation of this feature affects no parameters.
3.3
Related Parameters
Table 2 describes parameters that somehow affect the feature.
Table 2
Parameters affecting the feature
Parameter
Description
coverageIndicator
This parameter is used to identify WCDMA cells that fully cover the
current LTE cell and are thus available for blind SRVCC Handover.
lac
This parameter is required in the HANDOVER REQUIRED S1
message that is sent as part of the SRVCC Handover preparation
procedure. The parameter is used to correctly identify the
MSC/SGSN that serves the target WCDMA RNC.
rac
When the UE has data bearers to be handed over to WCDMA, this
parameter is included in the HANDOVER REQUIRED S1 and is
used to correctly identify the SGSN in the target location area of
the WCDMA cell.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
11
SRVCC Handover to UTRAN
12
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Network Impact
4
Network Impact
This section describes how the feature impacts the network functions and
capabilities.
4.1
Capacity
This section provides information on the impact of the feature on network
capacity.
The feature ensures that network capacity is enhanced by improving the
possibility that the UE is served by the best available frequency or cell within
the network.
4.2
Performance
The SRVCC Handover to UTRAN feature has no impact on performance. This
is because there is no comparable alternative feature or solution that would
transfer UE with voice bearers from an LTE cell to a WCDMA cell.
4.3
Configuration Impact
The SRVCC Handover to UTRAN feature provides enhanced active mode
mobility between LTE and WCDMA cells in the operator network when UE has
voice bearers. When the feature is active, a UE having voice bearers can be
directed to do a SRVCC handover to a WCDMA cell while maintaining the voice
session and, under some conditions, even maintaining the data sessions.
Correct configuration of the SRVCC Handover to UTRAN feature is required to
ensure the following:
•
That the UEis transferred to a WCDMA cell for which the UE has coverage
and that can support the UE voice session in the CS domain.
•
That an Event B2 (serving becomes worse than threshold1 and
inter-RAT neighbor becomes better than threshold2) measurement is
configured in the UE whenever there are several candidate target cells,
none of which covers the source cell completely.
•
That the UE makes the SRVCC handover directly, without an Event B2
measurement, whenever there is (at least) one SRVCC capable target cell
which is known to cover the source cell completely.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
13
SRVCC Handover to UTRAN
14
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Associated Features and Affected Functions
5
Associated Features and Affected
Functions
This section describes how the SRVCC Handover to UTRAN feature affects
other features and functions.
5.1
Prerequisite Features
Table 3 lists prerequisite features to the SRVCC Handover to UTRAN feature,
that is, they must be active prior to introducing the SRVCC Handover to UTRAN
feature.
Table 3
Feature
Description
Coverage Triggered WCDMA Session
Continuity
The WCDMA Session Continuity, Coverage-Triggered must be
active to enable use the SRVCC Handover to UTRAN feature.
Note:
5.2
Prerequisite Features
If a UE has PS or data bearers and an SRVCC capable target WCDMA
cell is found that supports even the handover of the PS bearers, the
Coverage-Triggered WCDMA IRAT Handover feature must also be
active as a prerequisite for performing handover for the PS bearers
as well.
Affected Features
This feature affects no other feature.
5.3
Related Features
This feature is related to the Coverage-Triggered WCDMA IRAT Handover
feature in the sense that this feature must be active to enable handover of
PS bearers when a UE having voice bearers as well as PS bearers must be
handed over to WCDMA.
5.4
Affected System Functions
This feature affects no system functions.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
15
SRVCC Handover to UTRAN
16
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
Performance
6
Performance
This section describes performance indicators, counters, and events associated
with the SRVCC Handover to UTRAN feature.
6.1
KPIs
Table 4 lists the main Key Performance Indicators (KPIs) associated with the
SRVCC Handover to UTRAN feature.
Table 4
6.2
KPIs
KPI
Description
Mobility success rate
More information can be found in Key Performance Indicators under
‘‘Mobility’’.
Counters
Table 5 lists the counters associated with the SRVCC Handover to UTRAN
feature.
Table 5
Counters
Counter
Description
pmHoSrvccPrepAttWcdma
Number of SRVCC attempts to WCDMA
pmHoSrvccPrepSuccWcdma
Number of successful SRVCC attempts to WCDMA
More information about counters can be found in Managed Object Model RBS.
6.3
Events
Table 6 lists the events associated with the SRVCC Handover to UTRAN
feature.
Table 6
Events
Event
Event Parameter
Description
INTERNAL_PROC_HO_PRE
P_S1_OUT
EVENT_PARAM_SRVCC_TYPE
Indicates if it is an SRVCC handover
and what kind of SRVCC
See SRVCC HO Indication in S1 AP
3GPP 36.413.
More information about events can be found in PM Event List.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
17
SRVCC Handover to UTRAN
18
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
O&M Information
7
O&M Information
This section provides Operation and Maintenance (O&M) information for the
SRVCC Handover to UTRAN feature.
A license and parameter setting activates the SRVCC Handover to UTRAN
feature. Details of the license activation process can be found in License
Management.
120/1553-HSC 105 50/1 Uen D2 | 2012-11-26
19