INTRODUCTION
The X2 interface is used for direct communication of neighbor eNBs and handover between eNBs. The X2 handover skips the steps comparing to S1 handover and reduces the total handover time. In addition, it may reduce the HO time
exchanging the HO messages instead of exchanging the handover messages between eNBs through MME depending on backhaul network structure. Also, the X2 interface is for exchanging load information between neighbor eNBs.
The X2 interface has control plane and user plane. The control plane connect X2 and AP via the SCTP protocol and make it possible to exchange signaling messages such as X2 handover, load information, and interference information.
The user plane uses GTP tunnels to forward the user data from the source eNB to the target eNB at handover.
When a neighbor cell is added to eNB, the eNB automatically sets up X2
connection with eNB which includes the target cell. The IP address of target eNB is required to set up X2 connection, use the Automatic Neighbor Relation (ANR) function to find the steps for getting the IP address.
The X2 connection is a SCTP-based between eNBs in the X2 application layer.
The X2 interface management function includes all procedure such as setup and monitoring the X2 connection, processing errors, and resetting to manage the X2 connection.
BENEFIT
The operator manages the signalling associations between eNBs, surveying X2 interface, and recovering from errors.
Efficient usage of the radio resources can be provided.
DEPENDENCY AND LIMITATION
Limitation
Maximum 256 X2 connections are supported.
The X2 based handover between Home eNBs is allowed if no access control at
FEATURE DESCRIPTION
X2 AP Setup
The X2AP setup procedure is for setting up the X2 interface between two eNBs for the first time. Assuming that eNB 1 triggers X2 setup, the X2 AP setup procedures for successful case is as follows.
1 The eNB1 sends its global eNB ID, served cell information, neighbor
information, MultibandInfoList, and GU group ID list information to eNB2 using the X2 Setup Request message. (In the perspective of HeNB, eNB 1 shall contain the CSG ID IE in the X2 SETUP REQUEST message for each CSG or hybrid cell)
2 The eNB2 receives the X2 Setup Request message and stores the information contained in it in appropriate locations. Then, eNB2 sends its global eNB ID, served cell information, neighbor information, and GU group ID list
information to eNB1 using the X2 Setup Response message. (In the perspective of HeNB, eNB2 shall contain the CSG ID IE in the X2 SETUP RESPONSE message for each CSG cell or hybrid cell. The eNB receiving the IE shall take this information into account when further deciding whether X2 handover between the source cell and target cell may be performed.)
The X2 AP setup procedure for unsuccessful case is as follows:
Samsung eNB2 sends X2 Setup failures to the eNB1 if:
received PLMN is not supported or
received ECGI is not eNB2's ECGI
1 The eNB1 receives the X2 setup failure message from eNB2.
2 The eNB1 waits as long as Time To Wait as included in the X2 setup failure message and then resends the X2 setup request message to eNB2.
X2 AP Reset
If an abnormal failure occurs with the X2 interface between two interacting eNBs, X2AP Reset procedure is performed to reconcile the resources between the two eNBs.
The X2 AP reset procedure is as follows:
1 The eNB1 sends the X2 Reset Request message to eNB 2.
2 The eNB2 sends the X2 Reset Response message to eNB1. If there are any procedures which eNB 1 is carrying out via the X2 Interface, eNB2 stops all of them and performs the Call Release procedure for the call.
Samsung eNB sends X2 Reset Request message to its neighbor eNBs when the cell of eNB is going to be released.
If eNB1 could not receive X2 Reset Response message, it does not resend X2 Reset Request message and there is no further actions
Keep Alive between eNBs
The eNB and neighbor eNB can monitor X2 connection by exchanging SCTP HEARTBEAT/ HEARTBEAT ACK messages defined by SCTP protocol.
HEARTBEAT message is periodically transmitted and the period is configured as HEART_BEAT_INTERVAL. When transmitting HEARTBEAT message, eNB delivers the current time in the Heartbeat Information field, which is also included in the HEARTBEAT ACK message so that the sender and receiver can calculate the Round Trip Time (RTT).
In case of SCTP connection is disconnected, all active calls will be disconnected.
Note that idle mode UEs are not maintained in eNB and HERATBEAT message is defined by SCTP layer.
When HEARTBEAT ACK message is not received, eNB tries to retransmit HEARTBEAT message periodically. The maximum number of retransmission is configured as NUM_PATH_RE_TX. The period of retransmission is „Heartbeat Retransmission Interval‟ in the below figure and calculated as
HEART_BEAT_INTERVAL + RTO + RTO*[-0.5, 0.5], where RTO is increased as exponential backoff if the previous HEARTBEAT message is unanswered. The initial, minimum and maximum values are configured as RTO_INITIAL,
RTO_MIN and RTO_MAX. If HEARTBEAT ACK is not received after all the retransmission, the link status is considered as abnormal.
In case of X2 setup procedure, eNB transmits INIT message to establish SCTP association. If it fails to get the response of INIT ACK message, eNB retransmits INIT message after one second. If it goes unanswered, eNB repeats this procedure with the period of CONNECT_INTERVAL until SCTP setup is successful as described in the below figure.
The operator could manage the neighbor eNB link status as follows:
locked: Cancels the relevant X2 handover procedure if there is any current X2 handover call, and blocks a new X2 handover out.
unlocked: Normally processes the X2 Handover.
shuttingDown: Normally processes the relevant X2 handover procedure if there is any current X2 handover call, and blocks a new X2 handover out.
To recover X2 connection, operator can perform the following actions through LSM. (Refer the detail to the system operation part)
Turn OFF/ON x2 connection with each neighbor eNB manually.
Send SCTP ABORT message only to neighbor eNBs which current X2 status is enable.
Send SCTP ABORT message to all neighbor eNBs regardless of the current X2 interface status.
SYSTEM OPERATION
How to Activate
Pre-condition
SCTP connection is established and operational state is normal.
Key Parameters
The maximum number of X2 neighbor eNB is 256 or 512.
The following table shows the several system parameters of each neighbor eNB information:
CRTE-NBR-ENB/DLT-NBR-ENB/CHG-NBR-ENB/RTRV-NBR-ENB
Parameter Description
STATUS This parameter indicates the validity of the neighbor eNB.
N_EQUIP: The information is determined as invalid.
EQUIP: The information is determined as valid.
This parameter must be set accurately since it determines the X2 link and handover execution.
NO_X2 This parameter determines whether to execute X2 link setup with the neighbor eNB.
False: X2 link setup with the neighbor eNB is executed.
True: X2 link setup with the neighbor eNB is not performed.
The parameter must be set accurately for X2 link setup to be determined by the setting.
NO_HO This parameter determines whether HO is possible with the neighbor eNB.
False: Handover is done with the neighbor eNB.
True: Handover is not done with the neighbor eNB.
The parameter must be set accurately for Handover to be executed as determined by the setting.
ENB_ID This parameter indicates the eNB ID of the neighbor eNB to which the neighbor cell belongs. Depending on the neighbor eNB type, the entry must be made in 20 bits for Macro eNB ID, and 28 bits for Home eNB. This information is used during handover.
The eNB ID of the neighbor eNB must be entered accurately. If the information does not match, the Handover will not be executed.
ENB_TYPE This parameter is the eNB type of the neighbor eNB.
Macro_eNB: Macro eNB.
Home_eNB: Home eNB.
ENB_MCC This parameter is the PLMN information (MCC) that represents the neighbor eNB. Enter a 3-digit number whose each digit ranges from 0 to 9.
ENB_MNC This parameter is the PLMN information (MNC) that represents the neighbor eNB. Enter a 2-digit or 3-digit number of whose each digit ranges from 0 to 9.
IP_VER IP address version indicating the IP address of a neighboring eNB. All neighboring eNB IP version information must be the same.
IPV4: Indicates IPV4 address.
IPV6: Indicates IPV6 address.
NBR_ENB_IPV4 This parameter indicates the IP version 4 address of the neighbor eNB. This information is used during X2 Link setup for the SCTP connection setup.
Accurately set the information to ensure proper X2 setup.
NBR_ENB_IPV6 This parameter indicates the IP version 6 address of the neighbor eNB. This information is used during X2 Link Setup for the SCTP connection setup.
Accurately set the information to ensure proper X2 setup.
SECONDARY_NBR_ENB_
IPV4
This parameter indicates the secondary IPv4 address of the neighbor eNB.
This information is used during SCTP multi-homing connection setup.
SECONDARY_NBR_ENB_ This parameter indicates the secondary IPv6 address of the neighbor eNB.
Parameter Description
IPV6 This information is used during SCTP multi-homing connection setup.
ADMINISTRATIVE_STATE This parameter is the neighbor eNB link status information.
locked: Cancels the relevant X2 handover procedure if there is any current X2 handover call, and blocks a new X2 handover out.
unlocked: Normally processes the X2 Handover.
shuttingDown: Normally processes the relevant X2 handover procedure if there is any current X2 handover call, and blocks a new X2 handover out.
REMOTE_FLAG This parameter indicates whether the neighbor eNB is managed by the same EMS or a different EMS.
False: Neighbor eNB is managed by the same EMS.
True: Neighbor eNB is managed by a different EMB.
The SCTP protocol manages using several system parameters for time interval of heartbeat message broadcast, re-broadcasting times of heartbeat or data message, and initial broadcast timeout value for Round Trip Time (RTO), minimum re-broadcast timeout value for RTO, maximum re-re-broadcast timeout value for RTO, and init message broadcast time interval for re-connection trial. Details are shown in below table.
CHG-SCTP-PARA/RTRV-SCTP-PARA
Parameter Description
HEART_BEAT_INTERVAL This parameter is the interval of transmitting a heartbeat message. A heartbeat message is transmitted to check the validity of a link in the SCTP protocol.
NUM_PATH_RE_TX This parameter is the re-transmission count by which the SCTP path will be failure if the response to the heartbeat or SCTP data is not received for more than NUM_PATH_RE_TX.
RTO_INITIAL This parameter is the value used to calculate the SCTP round trip time (RTT). It is the initial value of the Retransmission TimeOut which is used before the RTT value is measured through packet transmission.
RTO_MIN The minimum value of the Retransmission TimeOut used to calculate the SCTP Round Trip Time (RTT).
RTO_MAX The maximum value of the Retransmission TimeOut used to calculate the SCTP Round Trip Time (RTT).
CONNECT_INTERVAL This parameter is the timer value to transmit the init msg. to attempt SCTP connection periodically if a SCTP link is not set up for a peer node which is equipped to the PLD.
NUM_ASSOC_RE_TX The number of times to retransmit heartbeat messages or SCTP data messages in order to check whether the SCTP link is normal. The SCTP Association is disconnected if responses to the heartbeats or SCTP data messages are not received during specified (NUM_ASSOC_RE_TX) attempts.
Monitoring SCTP and X2 state of neighbor eNB are possible using
RTRV-X2-Parameter Description
NBR_ENB_ID This parameter is the ID of the neighbor eNB.
SCTP_STATE This parameter is the Stream Control Transmission Protocol (SCTP) status. It is the physical connection status between the eNBs.
disable_SD_PlmnTg_MIS: shutdown by PLMN TGID (TunnelGroupId) setting mismatch
disable_SD_PlmnTg_UA: shutdown by undecidable PLMN TGID
disable_SD_PlmnVr: shutdown by VR (Virtual Route) ID deletion.
disable_SD_NoX2: shutdown by NO_X2 setting
disable_SD_SonAnr: shutdown by one-way neighbor addition
disable_ABT_Inact: abort by SCTP INIT
disable_OOS: out of service (all case without above case)
X2AP_STATE This parameter is the X2AP status. It is the logical connection status between eNBs. If SCTP is disabled, X2AP cannot be enabled.
Counters and KPIs
There are no related counters or KPIs.
REFERENCE
[1] 3GPP TS36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2
[2] 3GPP TS36.413 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 application protocol (S1AP)
[3] 3GPP TS36.423 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP)