Network & Telecom

Connected Mode Mobility

Q: What is the difference between connected mode and idle mode mobility?

In connected mode the network is in control. The UE has an active RRC connection and bearers, so the serving cell configures the UE with measurement objects and reporting events, then the source cell decides when to hand the UE over to a target cell and signals the handover command. In idle mode the UE is in control: it performs autonomous cell reselection based on broadcast priorities and offsets, and the network only learns of the change at the next tracking area update or when a connection is established. Connected mode mobility therefore preserves active sessions with interruption times of a few tens of milliseconds, while idle mode reselection has no session to preserve and can take seconds.

Q: How does the A3 event trigger a handover in connected mode?

The A3 event fires when a neighbor cell becomes offset better than the serving cell. The entering condition is Mn + Ofn + Ocn - Hys > Mp + Ofp + Ocp + Off, where Mn and Mp are the measured RSRP of the neighbor and serving cells in dBm, Ofn/Ofp are frequency-specific offsets, Ocn/Ocp are cell-specific offsets, Hys is the hysteresis (typically 1 to 3 dB), and Off is the A3 offset (often 2 to 4 dB). The condition must hold continuously for the time-to-trigger window (40 to 640 ms) before the UE sends a measurement report. The serving eNB or gNB then issues the RRC handover command. Hysteresis and time-to-trigger together suppress reports caused by short fades and prevent ping-pong between two similar-strength cells.

Q: What causes handover interruption time and how is it minimized?

Interruption time is the gap during which the UE cannot send or receive user data while it detaches from the source cell, achieves uplink synchronization with the target via random access, and resumes data on the target. Baseline LTE intra-frequency handover interruption is roughly 30 to 50 ms, dominated by RACH and downlink synchronization. Inter-frequency handover adds measurement gap overhead and is longer. 5G NR reduces this with mechanisms such as RACH-less handover, make-before-break dual-connectivity handover, and Layer 1/Layer 2 triggered mobility, which can cut the interruption to near zero by maintaining the source link until the target link is ready. Conditional handover further hides preparation latency by pre-configuring the target so execution is instantaneous once a threshold is met.

/kə-nek-təd mohd moh-bil-i-tee/

Once a mobile device holds an active RRC connection, the network rather than the device steers it between cells. This network-controlled procedure configures the UE with neighbor measurements, evaluates reporting events such as A3 and A5, and issues an explicit handover command to move the active bearers to a stronger target cell. Hysteresis and time-to-trigger are applied to each A3 event so that brief fades do not cause spurious or ping-pong handovers, holding the per-handover data interruption to roughly 30 to 50 ms for intra-frequency LTE moves and near zero for 5G NR make-before-break execution. Connected mode mobility is what keeps a live voice or data session intact as a user travels across a cellular network.

Category: Network & Telecom

Interruption: 30 to 50 ms (LTE intra-freq)

Time-to-Trigger: 40 to 640 ms

How the Network Steers an Active Connection

In LTE and 5G NR, a device in the RRC_CONNECTED state has active radio bearers and an established context in the serving base station. Because that context cannot simply follow the device autonomously, the serving cell takes responsibility for mobility. It sends the UE a measurement configuration listing the carrier frequencies and neighbor cells to monitor, the filtering coefficients to apply, and the reporting events that should generate a measurement report. The UE measures reference signal received power and quality on the serving and neighbor cells, applies Layer 3 filtering, and reports back. The serving eNB or gNB then decides whether to keep the UE, change its measurement configuration, or hand it over.

A handover decision is followed by a preparation phase in which the source cell signals the target over the X2 or Xn interface, the target admits the bearers and allocates resources, and the source sends the UE an RRC reconfiguration message carrying the target cell identity and a dedicated random access preamble. The UE detaches from the source, performs random access on the target, and resumes data. During the brief gap between leaving the source and synchronizing to the target, user data cannot flow; minimizing this interruption is a central goal of every mobility enhancement, from RACH-less handover to dual-connectivity make-before-break.

The decision itself is governed by measurement events. The most common is the A3 event, where a neighbor becomes a configured offset stronger than the serving cell. Companion events include A2 (serving cell drops below a threshold, often used to start inter-frequency measurements) and A5 (serving cell below threshold 1 while a neighbor exceeds threshold 2). Tuning the offsets, hysteresis, and time-to-trigger trades off handover responsiveness against the risk of ping-pong, where a UE oscillates between two cells of similar strength.

Measurement Event and Handover Equations

A3 event entering condition (neighbor offset better than serving):
M_n + Of_n + Oc_n − Hys > M_p + Of_p + Oc_p + Off

A3 event leaving condition:
M_n + Of_n + Oc_n + Hys < M_p + Of_p + Oc_p + Off

Layer 3 filtered measurement:
F_n = (1 − a) × F_n−1 + a × M_n, a = (1/2)^(k/4)

Where M_n, M_p = measured RSRP of neighbor and serving cell (dBm); Of, Oc = frequency- and cell-specific offsets (dB); Hys = hysteresis (1 to 3 dB); Off = A3 offset (2 to 4 dB); the condition must hold for the time-to-trigger; k = filterCoefficient. Example: with Of and Oc both 0, Off = 3 dB and Hys = 1 dB, the neighbor at M_n = −82 dBm and serving at M_p = −88 dBm give M_n − M_p = 6 dB > Off + Hys = 4 dB, so the report fires after the TTT expires.

Connected Mode vs. Idle Mode Mobility

Attribute	Connected Mode (Handover)	Idle Mode (Reselection)
RRC state	RRC_CONNECTED	RRC_IDLE / RRC_INACTIVE
Decision authority	Network (source cell)	UE (autonomous)
Trigger	A3 / A5 measurement report	Broadcast priorities & S-criteria
Active session	Preserved (bearers moved)	None to preserve
Typical latency	30 to 50 ms (intra-freq)	Seconds (no urgency)
Signaling	X2/Xn + RRC reconfiguration	None until next TAU
Ping-pong control	Hysteresis + time-to-trigger	Reselection timer T_reselection

Common Questions

Frequently Asked Questions

What is the difference between connected mode and idle mode mobility?

In connected mode the network is in control: the UE has an active RRC connection and bearers, so the serving cell configures measurement events, then the source cell decides when to hand the UE over and signals the handover command. In idle mode the UE is in control, performing autonomous cell reselection on broadcast priorities, and the network only learns of the change at the next tracking area update. Connected mode preserves active sessions with tens-of-milliseconds interruption; idle reselection has no session to protect and can take seconds.

How does the A3 event trigger a handover in connected mode?

The A3 event fires when a neighbor becomes offset better than the serving cell: M_n + Of_n + Oc_n − Hys > M_p + Of_p + Oc_p + Off, with Hys around 1 to 3 dB and Off around 2 to 4 dB. The condition must hold for the time-to-trigger window (40 to 640 ms) before the UE sends a measurement report, after which the eNB or gNB issues the RRC handover command. Hysteresis and time-to-trigger together suppress reports from short fades and prevent ping-pong between similar-strength cells.

What causes handover interruption time and how is it minimized?

Interruption is the gap where the UE cannot pass data while it detaches from the source, achieves uplink sync with the target via random access, and resumes on the target. Baseline LTE intra-frequency handover runs roughly 30 to 50 ms; inter-frequency adds measurement gap overhead. 5G NR cuts this with RACH-less handover, make-before-break dual connectivity, and Layer 1/2 triggered mobility, approaching zero interruption by holding the source link until the target is ready. Conditional handover hides preparation latency by pre-configuring the target.

Related Terms

← Connected Mode DRX Connection Density →