T319 RRC Timer in 5G NR

The T319 RRC Timer plays a critical role in 5G NR during the RRC Connection Resume procedure. The Radio Resource Control (RRC) protocol layer relies on timers like T319 to ensure that the User Equipment (UE) does not get stuck in a pending state when attempting to communicate with the Next Generation NodeB (gNB).

In this post, we will explore the exact trigger conditions, expiration behavior, and relevant ASN.1 code underlying the T319 timer based on 3GPP Specification TS 38.331.

What is the T319 Timer?

The T319 timer acts as a watchdog for the RRC Resume Procedure. When a UE in RRC_INACTIVE state wants to transition back to RRC_CONNECTED, it sends a resume request. To prevent the UE from waiting indefinitely if the network drops the message or fails to respond, the UE starts the T319 timer.

T319 Basic Operations

Handling of Failure to Resume RRC Connection (T319 Expiry)

If the gNB does not respond within the configured T319 duration, the timer expires. According to 3GPP TS 38.331 Section 5.3.13.5, the UE shall:

1. If timer T319 expires:
   • If the UE supports multiple CEF (Connection Establishment Failure) reporting:
     • Check if the UE is not in SNPN access mode and has failure information available in VarConnEstFailReport matching the RPLMN.
     • OR if the UE is in SNPN access mode and has failure info matching the registered SNPN identity.
   • If the cell identity of the current cell is not equal to the one stored in measResultFailedCell and if maxCEFReport-r17 has not been reached:
     • Append the VarConnEstFailReport as a new entry in the VarConnEstFailReportList.
   • Finally, the UE typically aborts the RRC connection resume procedure and falls back to RRC_IDLE.

T319 ASN.1 RRC Configuration

In the UE-TimersAndConstants Information Element (IE) within the RRC specifications, the network explicitly configures the duration of T319.

asn.1
UE-TimersAndConstants ::= SEQUENCE {
    t319    ENUMERATED {ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000},
    -- ...
}

The network has the flexibility to set T319 to a value as short as 100 milliseconds or as long as 2 seconds, depending on the cell's expected processing delays and radio conditions.

What about Small Data Transmission (SDT)?

For Small Data Transmission (SDT), 3GPP introduces a variant known as T319a:

• Start: Upon transmission of RRCResumeRequest or RRCResumeRequest1 when the resume procedure is initiated for SDT.
• Stop: Upon reception of RRCResume, RRCSetup, RRCRelease, RRCReject message or upon failure to resume RRC connection for SDT.

asn.1
t319a    ENUMERATED {ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000} OPTIONAL

Note: If t319a-r18 is absent, the UE applies the value configured in t319a-r17.

There is also a separate timer definition used for Layer 2 UE-to-Network (U2N) remote relays:

asn.1
t319-RemoteUE-r17    ENUMERATED {ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000} OPTIONAL

If this field is absent, the generic T319 timer value applies to the remote UE.