LTE439: SGW Relocation with X2
Connection
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LTE439: SGW Relocation with X2 Connection
Table of Contents
1 LTE439: SGW Relocation with X2 Connection .......................................................................... 6
1.1 LTE439 benefits ................................................................................................................. 6
1.2 LTE439 functional description ........................................................................................ 6
1.3 LTE439 system impact ..................................................................................................... 9
1.4 LTE439 reference data ................................................................................................... 10
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LTE439: SGW Relocation with X2 Connection
List of Figures
Figure 1
4
Path switch ....................................................................................................................... 8
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LTE439: SGW Relocation with X2 Connection
List of Tables
Table 1
Table 2
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LTE439 summary of changes ......................................................................................... 6
New counters introduced by LTE439 ......................................................................... 11
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LTE439: SGW Relocation with X2 Connection
1. LTE439: SGW Relocation with X2 Connection
Benefits, requirements, functionality, system impact, reference data of the feature.
Table 1: LTE439 summary of changes
Date of change
Section
Change description
24.05.2017
LTE439 reference
data
Table 1 LTE439 hardware and software
requirements has been updated - the following
changes have been introduced:
• Flexi Zone BTS release version from FL16 and TL16 to
"Not supported"
• Flexi Zone Access Point release version from FL16
and TL16 to "Not supported"
The LTE439: SGW Relocation with X2 Connection feature extends the functionality of the X2
handover (HO). A serving gateway (S-GW) can be changed during the HO's completion phase
without changing the mobility management entity (MME). In addition, the eNB supports the
release of a list of radio access bearers (E-RABs) indicated by the MME.
1.1 LTE439 benefits
The LTE439: SGW Relocation with X2 Connection feature provides the following benefits:
a possibility to reduce the call drop rate (the UE's call continues even if the MME switches
the S-GW)
the S-GW relocation without changing the MME during the X2 HO
a UE stays connected even if some E-RABs need to be released during the X2 HO
1.2 LTE439 functional description
In general, an HO means that there is a change of radio connection between a network and a
UE which is already in a connected mode. HOs can be requested by the eNB to other cells on
the same LTE carrier (intra-frequency HO) or to LTE cells on other carriers (inter-frequency
HO) or to other cells of a different RAT (inter-RAT HO). HOs in LTE are network-controlled
and UE-assisted. Resources are prepared in a target cell before a UE is commanded to move
to this cell. Data packets whose reception cannot be acknowledged during the U-plane path
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LTE439: SGW Relocation with X2 Connection
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LTE439: SGW Relocation with X2 Connection
switch are retransmitted.
The X2 HO consists of three main stages:
1. HO preparation stage
2. HO execution stage
3. HO completion stage
The LTE439: SGW Relocation with X2 Connection feature is an enhancement of the LTE53:
Intra and Inter eNB Handover with X2 feature. The novelty introduced with the LTE439: SGW
Relocation with X2 Connection feature is based on two functionalities that are independent
of each other. They can be, however, triggered by one common S1AP: PATH SWITCH
REQUEST ACKNOWLEDGE message. The new concepts are:
1. UL path switch
If the MME advises switching the UL path by providing new pairs of UL transport
addresses and a tunnel endpoint identifier (TEID) for all or some of the UE's E-RABs, then
the eNB reconfigures the transport layer to use the new addresses instead. According to
3GPP, all E-RABs of one UE must be served by one common S-GW. The procedure for that
is as follows:
target eNB sends the S1AP: PATH SWITCH REQUEST message to the MME with a new
TEID for downlink (DL) data
MME reconfigures transport paths in the evolved packet core (EPC)
new target S-GW sends DL data to the target eNB using IP addresses and DL TEIDs
provided in the S1AP: PATH SWITCH REQUEST message
MME responds with the S1AP: PATH SWITCH REQUEST ACKNOWLEDGE message
containing new pairs of IP addresses and TEIDs for UL data (located in the target SGW)
eNB switches a path for UL data to be sent to the target S-GW
The MME can provide pairs of UL IP address and TEID only for some of the E-RABs. If the
MME provides IP addresses with a different IP version from the one previously used, the
eNB reacts as in the case of a path-switch failure (for a detailed path-switch view, see
Path switch). That is why, both UL and DL must use the same IP version, for example, IPv4
or IPv6.
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LTE439: SGW Relocation with X2 Connection
Figure 1: Path switch
2. E-RAB release
According to 3GPP, the purpose of the E-RAB release procedure is to enable the release
of already established E-RABs for a given UE. The procedure uses UE-associated
signalling.
If the MME advises releasing a subset of E-RABs allocated during the HO preparation,
then the eNB starts the radio resource control (RRC) connection reconfiguration
procedure to release the respective data radio bearer (DRB) towards the UE. In contrast
to a regular E-RAB release procedure, no non-access stratum protocol data unit (NAS
PDU) is transferred to the UE. The procedure for that is the following:
target eNB sends the S1AP: PATH SWITCH REQUEST message to the MME to inform
about new IP addresses and DL TEIDs (for E-RAB to be switched in DL)
MME responds with the S1AP: PATH SWITCH REQUEST ACKNOWLEDGE message,
containing the E-RAB To Be Released List information element (IE), to indicate that
MME/S-GW released some of the bearers
eNB starts an RRC connection release procedure to release respective E-RABs towards
the UE and to modify configuration for the remaining E-RAB (if needed)
As a result, the UE is connected to the target cell with a reduced number of E-RABs.
The HO described in the LTE439: SGW Relocation with X2 Connection feature has the same
preparation stage as the one already known via the X2 interface. The only difference takes
place during the HO completion phase, in which the S-GW change is done (it is not changed
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during the execution phase). In any case, the target eNB must be able to connect to the
source S-GW, serving the UE in the source cell, during the HO execution.
The eNB supports the following HO scenarios via the X2 interface in combination with the SGW relocation:
inter-eNB, intra-frequency
inter-eNB, inter-frequency band (if enabled)
inter-eNB, intra-frequency band with different center frequency (if enabled)
1.3 LTE439 system impact
LTE439: SGW Relocation with X2 Connection impact on features and interfaces.
Interdependencies between features
The following features must be activated before activating the LTE439: SGW Relocation with
X2 Connection feature:
LTE53: Intra and Inter eNodeB Handover with X2
LTE735: RRC Connection Re-establishment
LTE1617: RLF Triggered Handover
With the LTE439: SGW Relocation with X2 Connection feature, it rarely happens that the last
QCI1 is released (and it is the MME which decides which E-RAB should be released). This
impacts the features whose configuration depends on the presence of QCI1. The interaction
concerns the following features:
LTE42: DRX in RRC Connected Mode
LTE64: Service-based Handover Thresholds
LTE473: Extended DRX Settings
LTE585: Smart DRX
LTE872: SRVCC to WCDMA
LTE873: SRVCC to GSM
LTE907: TTI Bundling
LTE1140: Intra-frequency Load Balancing
LTE1406: Extended VoLTE Talk Time
LTE1469: Dynamic UE Measurement Configuration
LTE1929: UL Semi-persistant Scheduling
LTE2112: Dedicated Mobility Thresholds for SRVCC
LTE2430: QCI Establishment Triggered Protection Timer
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LTE439: SGW Relocation with X2 Connection
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LTE439: SGW Relocation with X2 Connection
Impact on interfaces
The LTE439: SGW Relocation with X2 Connection feature impacts interfaces as follows:
S1
new IEs are supported within the S1AP: PATH SWITCH REQUEST ACKNOWLEDGE
message (E-RAB To Be Switched in Uplink List and E-RAB To Be
Released List)
Impact on network management tools
The LTE439: SGW Relocation with X2 Connection feature has no impact on network
management tools.
Impact on system performance and capacity
The LTE439: SGW Relocation with X2 Connection feature has no impact on system
performance or capacity.
1.4 LTE439 reference data
LTE439: SGW Relocation with X2 Connection requirements, measurements and counters,
and sales information.
For reference data descriptions, see Single RAN System/SRAN Reference Data
Requirements
For supported hardware and software, see Single RAN Features/List of Features and
Documents/List of Features Supported in Single RAN.
Alarms
There are no alarms related to the LTE439: SGW Relocation with X2 Connection feature.
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BTS faults and reported alarms
There are no faults related to the LTE439: SGW Relocation with X2 Connection feature.
Commands
There are no commands related to the LTE439: SGW Relocation with X2 Connection feature.
Measurements and counters
Table 2: New counters introduced by LTE439
Counter ID
Counter name
Measurement
M8006C277
E-RAB releases in Handover
Completion phase at target
cell
LTE EPS Bearer
M8006C278
QCI1 E-RAB releases in
Handover Completion phase
at target cell
LTE EPS Bearer
M8006C279
eNB initiated E-RAB releases
due to a failed Handover
Completion phase at target
cell
LTE EPS Bearer
M8006C280
eNB initiated QCI1 E-RAB
releases due to a failed
Handover Completion phase
at target cell
LTE EPS Bearer
Key performance indicators
There are no key performance indicators related to the LTE439: SGW Relocation with X2
Connection feature.
Parameters
There are no parameters related to the LTE439: SGW Relocation with X2 Connection
feature.
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LTE439: SGW Relocation with X2 Connection
Sales information
For licensing information, see 5G/System Descriptions/5G Licenses. Remember that for 5G
features, you also need to configure the Use Case Level Activation Parameters (UCLAPs). For
more information, see .
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LTE439: SGW Relocation with X2 Connection