Connected mode

LTE handover

LTE connected-mode handover: RRCConnectionReconfiguration with mobilityControlInfo and timer T304.

Overview

In E-UTRA the network commands the handover with an RRCConnectionReconfiguration that includes mobilityControlInfo. After receiving it the UE accesses the target PCell at the first available RACH occasion (an asynchronous handover), or uses the RACH-less (rach-skip) options that TS 36.331 defines.

The inter-eNodeB signalling (X2AP or S1AP) and the EPC procedures are defined in TS 36.423, TS 36.413 and TS 23.401. Read those for the network-side messages.

Call flow

1
UEtoSource eNB
MeasurementReport
2
Source eNBtoUE
RRCConnectionReconfiguration (mobilityControlInfo)
The UE starts T304.
3
UEtoTarget eNB
Random access, then RRCConnectionReconfigurationComplete

Timers and parameters

Name	Where	Role
T304	UE	Handover supervision. LTE values run from 100 ms up to 10 s.

Full parameter and timer lookup →

Specifications

Spec	What it covers
TS 36.331	RRC handover procedure and mobilityControlInfo.
TS 36.423	X2AP, inter-eNodeB signalling.
TS 36.413	S1AP, signalling between eNodeB and MME.
TS 23.401	EPS architecture, including handover procedures.

Failure handling

Radio link failure and RRC re-establishment

Out-of-sync handling with N310, N311 and T310, then re-establishment supervised by T311 and T301.

4G5G

Idle mode

Cell selection and reselection

How an idle or inactive UE selects a cell by the S-criterion and keeps the best one by priority and ranking.

4G5GNTNIoT

Connected mode

5G NG-based (N2) handover

Handover whose preparation is signalled through the AMF over N2 instead of directly between gNBs.