暴走日报
Interoperability in LTE
By V. Srinivasa Rao, Senior Architect & Rambabu Gajula, Lead Engineer
千古奇观Published , March 12, 2010
Introduction
With the fast-changing mobile landscape and convergence in all aspects of telecommunications, “amless handover” is important for any technology to succeed. Operators and consumers both benefit from amless handover in terms of cost effectiveness, enhanced features, location independence and ea of u, not only within a Long Term Evolution (LTE) network but also between networks including UMTS, GSM and CDMA.
In this paper we briefly touch upon the procedures executed by the ur equipment (UE) and the various network elements to provide the handover rvices requested by the UE. We cover Intra-LTE and LTE to/from UMTS handovers.
Objectives of Handover Procedures
1) It is important that QoS is maintained, not just before and after a handover, but during the handover as
well.
2) Handover shall not drain the UE battery power.
3) Service continuity shall be maintained (i.e., minimal handover latency).
4) Seamless handoff to 3G / 2G / CDMA technology.
There are two ways a handoff can be decided:
牛黄解毒片的禁忌∙Network Evaluated: the network makes the handover decision
∙Mobile Evaluated: the UE makes the handoff decision and informs the network about it. In this instance, the final decision will be made by the network bad upon on the Radio Resource
Management.
In 3G and LTE networks, a hybrid approach is ud to decide on the handover. In this ca, the UE will assist in the handoff decision by measuring the neighboring cells and reporting the measurements to the network, which in turn decides upon the handoff timing and the target cell/node. The parameters to measure and the thresholds for reporting are decided by the network.
In LTE there are three types of handovers:
∙Intra-LTE: Handover happens within the current LTE nodes (intra-MME and Intra-SGW)
∙Inter-LTE: Handover happens toward the other LTE nodes (inter-MME and Inter-SGW)
∙Inter-RAT: Handover between different radio technology networks, for example GSM/UMTS and UMTS
We’ll look at Intra-LTE handovers with X2AP signaling and S1AP signaling first, then Inter-RAT handovers in LTE (i.e., handover between LTE and UMTS).
Intra-LTE (Intra-MME / SGW) Handover Using the X2 Interface:
This procedure is ud to handover a UE from a source eNodeB (S-eNB) to a target eNodeB (T-eNB)
using the X2 interface when the Mobility Management Entity (MME) and Serving Gateway (SGW) are unchanged. It is possible only if direct connectivity exists between the source and target eNodeB’s with the X2 interface.
The X2 handover procedure is performed without Evolved Packet Core (EPC) involvement, i.e. preparation messages are directly exchanged between the S-eNB and T-eNB. The relea of the resources at the source side during the handover completion pha is triggered by the T-eNB. The message flow is shown in Figure 1 followed by the description.
Figure 1: Intra-LTE (Intra-MME / SGW) Handover Using the X2 Interface
1. A data call is established between the UE, S-eNB and the network elements. Data packets are transferred
to/from the UE to/from the network in both directions (DL as well as UL)
2. The network nds the MEASUREMENT CONTROL REQ message to the UE to t the parameters to
measure and t thresholds for tho parameters. Its purpo is to instruct the UE to nd a measurement report to the network as soon as it detects the thresholds.
3. The UE nds the MEASUREMENT REPORT to the S-eNB after it meets the measurement report criteria
communicated previously. The S-eNB makes the decision to hand off the UE to a T-eNB using the
handover algorithm; each network operator could have its own handover algorithm.
4. The S-eNB issues the RESOURCE STATUS REQUEST message to determine the load on T-eNB (this is
optional). Bad on the received RESOURCE STATUS RESPONSE, the S-eNB can make the decision to proceed further in continuing the handover procedure using the X2 interface.
5. The S-eNB issues a HANDOVER REQUEST message to the T-eNB passing necessary information to
prepare the handover at the target side (e.g., UE Context which includes the Security Context and RB Context (including E-RAB to RB Mapping) and the Target cell info).
6. The T-eNB checks for resource availability and, if available, rerves the resources and nds back the
HANDOVER REQUEST ACKNOWLEDGE message including a transparent container to be nt to the UE as an RRC message to perform the handover. The container includes a new C-RNTI, T-eNB curity
algorithm identifiers for the lected curity algorithms, and may include a dedicated RACH preamb
le and possibly some other parameters (i.e., access parameters, SIBs, etc.).
7. The S-eNB generates the RRC message to perform the handover, i.e, RRCCONNECTION
RECONFIGURATION message including the mobilityControlInformation. The S-eNB performs the
蓝屏代码f4
necessary integrity protection and ciphering of the message and nds it to the UE.
8. The S-eNB nds the eNB STATUS TRANSFER message to the T-eNB to convey the PDCP and HFN
status of the E-RABs.
9. The S-eNB starts forwarding the downlink data packets to the T-eNB for all the data bearers (which are
being established in the T-eNB during the HANDOVER REQ message processing).
10. In the meantime, the UE tries to access the T-eNB cell using the non-contention-bad Random Access
Procedure. If it succeeds in accessing the target cell, it nds the RRC CONNECTION
RECONFIGURATION COMPLETE to the T-eNB.
11. The T-eNB nds a PATH SWITCH REQUEST message to the MME to inform it that the UE has changed
cells, including the TAI+ECGI of the target. The MME determines that the SGW can continue to rve the UE.
12. The MME nds a MODIFY BEARER REQUEST (eNodeB address and TEIDs for downlink ur plane for
the accepted EPS bearers) message to the SGW. If the PDN GW requested the UE’s location info, the MME also includes the Ur Location Information IE in this message.
13. The SGW nds the downlink packets to the target eNB using the newly received address and TEIDs
折扣怎么算(path switched in the downlink data path to T-eNB) and the MODIFY BEARER RESPONSE to the M
ME. 14. The SGW nds one or more “end marker” packets on the old path to the S-eNB and then can relea any
ur plane / TNL resources toward the S-eNB.
15. The MME responds to the T-eNB with a PATH SWITCH REQ ACK message to notify the completion of the
handover.
16. The T-eNB now requests the S-eNB to relea the resources using the X2 UE CONTEXT RELEASE
message. With this, the handover procedure is complete.
Intra-LTE (Intra-MME / SGW) Handover Using the S1 Interface:
The S1-bad handover procedure is ud when the X2-bad handover cannot be ud –e.g., no X2 connectivity to the target eNodeB; by an error indication from the T-eNB after an unsuccessful X2-bad handover; or by dynamic information learnt by the S-eNB using the STATUS TRANSFER
procedure. The S-eNB initiates the handover by nding a Handover required message over the S1-MME reference point. The EPC does not change the decisions taken by the S-eNB.
The availability of a direct forwarding path is determined in the S-eNB (bad on the X2 connectivity with the T-eNB) and indicated to the source MME. If a direct forwarding path is not available, indirect forwarding will be ud. The source MME us the indication from the S-eNB to determine whether to apply indirect forwarding or not. The message flow is depicted in Figure 2 followed by the description of the procedures.
辞职信模版
Intra-LTE Handover (Intra-MME / SGW)
Figure 2: Intra-LTE (Intra-MME / SGW) Handover Using the S1 Interface
∙As mentioned in the previous ction, bad on the MEASUREMENT REPORT from the UE, the S-eNB decides to Handover the UE to another eNodeB (T-eNB). The handover procedure in this ction is very similar to that in the previous ction (Intra-LTE Handover Using the X2 Interface), except the involvement of the MME in relaying the handover signaling between the S-eNB and T-eNB.
∙There are two differences here:
o No need for the PATH SWITCH Procedure between the T-eNB and MME, as MME is aware of the Handover.
o The SGW is involved in the DL data forwarding if there is no direct forwarding path available between the S-eNB and T-eNB.
∙Once the Handover is complete, the MME clears the logical S1 connection with the S-eNB by initiating the UE CONTEXT RELEASE procedure.如数
Inter-MME Handover Using the S1 Interface (without changing S-GW)
In an inter-MME handover, two MMEs are involved in the handover, the source MME (S-MME) and target MME (T-MME). The S-MME controls the S-eNB and the T-MME controls the T-eNB; both MMEs are connected to the same SGW. This handover is triggered when the UE moves from one MME area to another MME area.
Figure 3: Inter-MME Handover (Intra-SGW)
1. As mentioned in the previous ction (Intra-MME / SGW handover), bad on the MEASUREMENT
REPORT from the UE, the S-eNB decides to handover the UE to another eNodeB (T-eNB). The handover procedure in this ction is very similar to that in the previous ction except for the involvement of two MMEs coordinating the handover signaling between the source and target eNodeBs.
2. The S-MME us GTP signaling to communicate the handover signaling to the T-MME and vice versa.
The FORWARD RELOCATION procedure in GTP-C is being ud here.
1千克是多少吨3. After receiving the S1 HANDOVER REQUIRED, the S-MME detects that the target cell requested for
handover belongs to another MME and initiates the GTP FORWARD RELOCATION REQ message to the T-MME.
4. The T-MME creates the S1 logical connection toward the T-eNB and nds the S1 HANDOVER REQ on
it.
5. The T-eNB prepares the requested resources and responds with a HANDOVER REQ ACK to the T-MME.
6. The T-MME nds a GTP FORWARD RELOCATION RESP to the S-MME, to notify the resource
rervation at the T-eNB. From this point onwards, the interaction between the S-MME and S-eNB is very similar to the S1-bad Intra-MME / SGW handover described in the previous ction.
7. DL data packets are forwarded from the S-eNB to T-eNB via the SGW during the handover as the SGW is
not changed here.
8. Once the T-eNB detects the UE in its area, it notifies the T-MME with a S1 HANDOVER NOTIFY
message.
9. The T-MME notifies the completion of the handover to the S-MME with a GTP FORWARD RELOCATION
COMPLETE NOTIFY message.
