A sensor uploads a report and then has nothing to send for several hours. Does it need to keep listening for paging during that time? If the application can tolerate being unreachable, UE can request MICO mode from the 5G core network.

MICO stands for Mobile Initiated Connection Only. In basic MICO operation, UE does not need to monitor paging while CM-IDLE, and AMF treats it as unreachable. UE remains registered and can initiate communication when it has data, signalling or a registration update to perform.

The name can be misleading. MICO does not prohibit downlink data. UE can receive downlink communication while connected, and optional enhancements can preserve a limited period of paging reachability after release. This page uses the Release 18 versions listed in References.

• What Does MICO Change ?
• High-level MICO Sequence
• Negotiation and NAS Information
  • Request, Acceptance and Re-negotiation
  • RAAI and SPRTI in the MICO Indication
• When Can the Network Reach UE ?
  • Basic MICO without Active Time
  • MICO with Active Time
  • MICO with Extended Connected Time
• Registration Area and T3512
  • All-PLMN Registration Area
  • Ordinary and Strictly Periodic T3512
• Worked Example: Report, Response and Sleep
• MICO, PSM, eDRX and NTN Coverage Gaps
• Restrictions and Trace Checks
• References

What Does MICO Change ?

MICO changes the agreement between UE and AMF about reachability. The network does not keep trying to page a basic MICO UE while it is CM-IDLE. UE can therefore avoid paging monitoring and may deactivate Access Stratum activity under the applicable NAS rules.

This is a 5GS NAS feature over 3GPP access. It is not a new RRC state or a satellite-only mechanism. NTN devices can use it, but terrestrial devices with similar traffic patterns can also benefit.

The table separates registration, connection and reception. These are different questions when reading a modem log.

An application that requires an immediate unsolicited command cannot assume basic MICO will meet that requirement. The application needs a reachable interval, a UE-initiated check-in strategy or another suitable operating mode.

High-level MICO Sequence

UE first requests MICO during registration. AMF decides whether to allow it, based on factors such as subscription, network policy and expected UE behavior. After acceptance, the connection state and any negotiated Active Time determine reachability.

Figure 1 shows basic MICO without Active Time. The NAS messages pass through NG-RAN. Authentication, security and other registration exchanges are omitted to keep the MICO decision visible.

Figure 1. Basic MICO allows UE to remain registered without paging reachability in CM-IDLE.

UE indicates its preference in Registration Request. AMF accepts by including the MICO indication in Registration Accept. A request alone is not permission to use the mode. UE may use MICO only if the latest registration procedure provided the required acceptance.

Later, UE can initiate signalling to send a report. It can then receive an application response while connected. UE selects the applicable procedure, such as registration or Service Request, based on its current state and reason for access.

Negotiation and NAS Information

MICO is negotiated between UE and AMF, rather than enabled by an RRC DRX timer. The accepted configuration can differ from UE's preference. Check Registration Accept as well as Registration Request when reviewing a trace.

Request, Acceptance and Re-negotiation

UE requests MICO during initial registration or a subsequent registration procedure. UE and AMF re-negotiate it at later registrations. If UE wants Active Time, it requests that value together with MICO.

AMF does not activate MICO when UE has not requested it. While UE is connected, AMF can trigger registration to change the arrangement using the UE Configuration Update procedure. It cannot rely on an ordinary page to reach an already-unreachable idle MICO UE.

Presence of the MICO indication has different significance in Registration Request, Registration Accept and Configuration Update Command. In the last case, it participates in requesting re-negotiation. Do not interpret every occurrence as a fresh acceptance of idle-mode sleep.

RAAI and SPRTI in the MICO Indication

The MICO indication is a type 1 NAS information element. It carries registration-area and timer-related flags. Neither flag is a standalone "MICO enabled" switch; message context and the presence of the IE matter.

The following descriptions follow TS 24.501 clause 9.11.3.31. They describe the decoded fields rather than a synthetic encoded NAS packet.

For example, a Registration Accept containing MICO indication with RAAI = 0 can still accept MICO. It simply does not allocate the entire PLMN as the registration area. Treating RAAI = 0 as "MICO rejected" would give the wrong result.

When Can the Network Reach UE ?

The answer depends on whether UE has basic MICO, MICO with Active Time, or an extended connected interval. These options solve different application-response problems and consume different amounts of energy.

Basic MICO without Active Time

When UE enters CM-IDLE without negotiated Active Time, AMF treats it as unreachable for mobile-terminated communication. UE need not monitor paging and may stop AS activity under the conditions specified by NAS.

Basic MICO does not promise that every downlink packet waits in a buffer until the next report. AMF rejects the relevant downlink delivery request while UE is unreachable; other network or application functions may provide deferred delivery. SMS, location services and data delivery have their own handling.

UE can initiate activity for pending uplink data or signalling, a registration-related change, or periodic registration. T3512 is not the only possible wake-up trigger. The ability to resume activity also depends on finding usable network coverage.

MICO with Active Time

Active Time gives the network a temporary paging opportunity after connection release. When negotiated, UE starts T3324 on entering the applicable idle mode, and AMF considers UE reachable while the corresponding Active Time runs.

UE follows its applicable paging DRX/eDRX schedule during that interval. Active Time is therefore not continuous receiver activity. If UE becomes connected before expiry, UE and AMF stop the active timer. On a later transition back to idle, the negotiated interval starts again.

After expiry, UE can enter the no-paging MICO behavior. Under the NAS conditions in TS 24.501 clause 5.3.6, it may deactivate AS and enter 5GMM-REGISTERED.NO-CELL-AVAILABLE. That NAS substate does not establish that the physical cell disappeared.

MICO with Extended Connected Time

Extended Connected Time addresses a response that is expected after UE initiates communication. AMF can maintain the connection and provide NG-RAN with a minimum time to keep UE in RRC_CONNECTED despite inactivity.

This differs from Active Time. Extended Connected Time operates while connected; Active Time preserves paging reachability after transition to idle. A connected UE can still use configured connected-mode DRX, so the extended interval does not necessarily imply continuous PDCCH monitoring.

For example, a sensor uploads a report and the server needs several seconds to generate a response. Keeping the connection available may deliver that response in the same interaction. The gain in responsiveness must be compared with the energy and resource cost.

Registration Area and T3512

Power saving also depends on how often UE must update registration. MICO can change registration-area allocation and supports a strictly periodic timer option. Neither option removes the need to follow mobility restrictions and registration rules.

All-PLMN Registration Area

Because basic idle MICO UE is not paged, paging-area size need not constrain its registration area in the same way. If the AMF serving area covers the whole PLMN, policy and subscription can allow an all-PLMN registration area.

This is an option, not a requirement. With a smaller area, UE checks whether it is still within that area when it has mobile-originated data or signalling. If necessary, it performs Mobility Registration Update before initiating that communication.

When Active Time is allocated, AMF should avoid an all-PLMN registration area so that temporary reachability does not require paging across the entire PLMN. RAAI should therefore be interpreted together with Active Time and the assigned tracking-area information.

Ordinary and Strictly Periodic T3512

Without the strictly periodic indication, T3512 is reset and started on transition from connected to idle over 3GPP access. It stops when UE becomes connected. Intervening communication can therefore move the next periodic update time.

With negotiated strictly periodic operation, T3512 starts after registration completes and continues across connected/idle transitions. If it expires while UE is idle, UE performs periodic registration. If it expires while UE is connected, UE resets and restarts it under the NAS rule.

Do not infer that a one-hour T3512 value always means a registration exactly one hour after connection release. Record the accepted SPRTI setting and actual timer start events. AMF has corresponding monitoring and implicit deregistration handling.

Worked Example: Report, Response and Sleep

Assume registration has accepted MICO with T3324 = 60 seconds and ordinary T3512 = 3600 seconds. UE finishes a report and enters CM-IDLE at t = 0. Assume continuous coverage and no intervening connection until the periodic update.

Figure 2 separates paging reachability from the periodic registration deadline. The radio-monitoring schedule inside Active Time is not drawn. UE also needs preparation time to acquire service before performing an update.

Figure 2. Active Time offers a short response window; ordinary T3512 continues toward the next periodic registration while UE is idle.

A downlink response arriving at t = 20 seconds can trigger paging during Active Time. Delivery still waits for the applicable paging occasion and access procedure. If UE connects, the timers no longer follow the no-intervening-connection timeline shown in the figure.

A command arriving at t = 120 seconds cannot reach UE by ordinary paging after Active Time has expired. If deferred delivery is supported and the command is retained until the next update, the nominal wait is 3600 - 120 = 3480 seconds, plus access and delivery time.

An uplink alarm at t = 600 seconds can cause earlier activity. Under ordinary timer operation, entering connected stops T3512; the later return to idle starts it again. Strictly periodic operation would behave differently, so these two cases should not be mixed in a log analysis.

MICO, PSM, eDRX and NTN Coverage Gaps

These mechanisms can all reduce radio activity, but they make different promises about downlink reachability. Start with the core network and UE state before comparing timer values or describing them as equivalent sleep modes.

For NR NTN, AMF can consider expected unavailability when configuring power-saving behavior. A device must still recover coverage and perform the applicable return procedures. See NTN Paging and Power Saving and Discontinuous Coverage.

Restrictions and Trace Checks

MICO is optional and requires network acceptance. It is not requested over non-3GPP access under the cited NAS procedure. Emergency-service restrictions also prevent the ordinary power-saving rules from being applied without regard to reachability needs.

When an emergency PDU session is successfully established after MICO was enabled, UE and AMF locally disable MICO. Re-enabling it requires acceptance in a later registration procedure. UE also allows the applicable implementation-specific interval for emergency callback before requesting MICO again.

The following checks help distinguish a correct no-paging interval from a failed delivery procedure.

For the wider exchanges, see Registration and Paging. A missing page during the accepted no-paging interval is expected behavior, not sufficient evidence of a radio problem.

References

The relevant clauses of these Release 18 specifications support the explanations. Diagrams and timer values are illustrative, not captured NAS messages. No ASN.1 listing is needed for the NAS information elements discussed here.

• 3GPP TS 23.501 V18.12.0 : Clauses 5.4.1.3 and 5.31.7.3 to 5.31.7.5; MICO, Extended Connected Time, Active Time and periodic registration.
• 3GPP TS 24.501 V18.11.1 : Clauses 5.3.6, 5.3.7, 5.5.1 and 9.11.3.31; NAS behavior, timers, registration and MICO indication.