5G NR - System Information Block 16 (SIB16)

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.2.2.4.17, 6.3.1
Direction	gNB -> UE	Message Type	Broadcast System Information

Full message name	5G NR - System Information Block 16 (SIB16)
Protocol	RRC
Technology	5G
Direction	gNB -> UE
Interface	Uu
Signaling bearer / channel	Broadcast transport / BCCH-DL-SCH
Typical trigger	Broadcast when the cell needs to expose slice-based cell reselection priorities.
Main purpose	Provides slice-aware reselection priorities so the UE can apply the correct idle or inactive mobility behavior for slicing-oriented deployments.
Main specification	3GPP TS 38.331, 5.2.2.4.17, 6.3.1
Release added	Release 17
Procedures where used	System Information Acquisition, Idle / Inactive Mobility with Slicing
Related timers	SIB16 does not use a dedicated UE-specific transaction timer
Related cause values	SIB16 does not carry reject causes

What is System Information Block 16 (SIB16) in simple terms?

System Information Block 16 (SIB16) is an NR broadcast system information block used to provide slice-based cell reselection information.

Provides slice-aware reselection priorities so the UE can apply the correct idle or inactive mobility behavior for slicing-oriented deployments.

Why this message matters

SIB16 tells the UE how to apply slice-based reselection priorities when it is idle or inactive.

Where this message appears in the call flow

System Information Acquisition

Call flow position: Read as additional SI after the basic access and mobility context is already known.

Typical state: UE is camped and expanding its idle mobility rules.

Preconditions:

MIB and SIB1 have been acquired.

Next likely message: UE applies slice-based reselection priorities locally

Idle / Inactive Mobility with Slicing

Call flow position: Used when the UE performs cell reselection with slice-related priorities.

Typical state: UE is not in a dedicated transaction.

Preconditions:

The serving cell broadcasts SIB16.

Next likely message: Cell reselection decision

Call flow position

Previous message(s): MIB, System Information Block 1 (SIB1), System Information Block 3 (SIB3)

Next message(s): Idle or inactive cell reselection using slice-aware priorities

Message direction and transport

Sender and receiver: gNB -> UE

Interface: Uu

Domain: Access-side radio control and slice-aware idle mobility information

Signaling bearer: Broadcast transport

Logical channel: BCCH-DL-SCH

Transport / encapsulation: RRC system information carried on BCCH-DL-SCH after MIB and SIB1 acquisition.

Security context: Broadcast information. It is cell-common and not protected by dedicated AS security.

Message Structure Overview

SIB16 is a slice-aware idle mobility SIB.
The main engineering question is whether the broadcast reselection priorities match the intended slicing behavior.

ASN.1 for 5G NR - System Information Block 16 (SIB16)

SystemInformation-IEs ::= SEQUENCE {
  sib-TypeAndInfo SEQUENCE (SIZE (1..maxSIB)) OF CHOICE {
    sib16-v1700 SIB16-r17,
    ...
  },
  ...
}

SIB16-r17 ::= SEQUENCE {
  freqPriorityListSlicing-r17 FreqPriorityListSlicing-r17 OPTIONAL,
  lateNonCriticalExtension OCTET STRING OPTIONAL,
  ...
}

How to read this ASN.1

The practical payload is freqPriorityListSlicing-r17. Engineers use it to explain why the UE prefers or avoids certain reselection targets in slicing-aware deployments.

Decoded Message Dump

BCCH-DL-SCH-Message
  message: c1
    systemInformation
      criticalExtensions
        systemInformation
          sib-TypeAndInfo
            sib16
              freqPriorityListSlicing-r17: present

How to read this dump

The main question is whether slice-based reselection priorities are present and consistent with deployment intent.

Important Information Elements

IE	Required	Description
`freqPriorityListSlicing-r17`	Optional	Slice-based frequency reselection priorities used for idle or inactive mobility decisions.

Detailed field explanation

`freqPriorityListSlicing-r17`

Slice-based frequency reselection priorities used for idle or inactive mobility decisions.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

What to check in logs and traces

Confirm the UE acquired the additional SI carrying SIB16.
Check whether freqPriorityListSlicing-r17 is present.
Correlate SIB16 with observed idle or inactive reselection behavior.

Common Issues and Troubleshooting

UE reselection behavior does not match slicing expectations.

Likely cause: SIB16 may be missing or its slice-based priorities may not match the intended configuration.

What to inspect: Check freqPriorityListSlicing-r17 and compare it with the frequencies the UE prefers.

Next step: Compare SIB16 with SIB3 and SIB4 before concluding the issue is only in the UE.

LTE / 5G / Variant Comparison

SIB16 versus SIB3 / SIB4

SIB3 and SIB4 provide normal reselection rules, while SIB16 adds slice-based priority behavior.

FAQ

What is SIB16 in 5G NR?

SIB16 is System Information Block 16, used for slice-based cell reselection information.

Why is SIB16 useful?

It explains slice-aware idle and inactive mobility behavior.

Related message pages

Related Procedures

5G Mobility Registration Update

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.