Is There Call Re-Establishment in VoLTE?
July 4, 2019

Call re-establishment is essential in VoLTE calls to avoid RTP timeouts. It’s also recommended to audit call re-establishment parameters for both QCI = 1 and QCI = 9.

The UE may experience a connection failure such as when a radio link failure (RLF) occurs. When the RLF happens, the dedicated bearer may become lost and then the VoLTE call may be dropped.
The UE attempts to re-establish a connection with the Network and/or the dedicated bearer. If the reestablishment fails, the UE enters a radio resource control (RRC) idle state in which the network releases the dedicated bearer. With no dedicated bearer, the VoLTE call drops.  

In general, the call re-establishment procedure is the same for both QCI = 9 and QCI = 1 from air interface perspectives. However, the investigation is recommended to see if QCI = 1 is impacted by any other issues.

It is essential for VoLTE to target quick Radio Link Failure (RLF) with a stable re-establishment procedure to ensure VoLTE continuity with less mute time. This can be done with RLF parameters and good RF planning, a similar concept to the legacy AMR situation in 3G, where carriers target quick RLF and stable re-establishment to avoid a high mute time and one of the callers ending the call.

eNB controls the Call Reestablishment, However, when re-establishment is not fully supported, source cell sends “RRC Conn Reestablishment Reject”, otherwise, the reestablishment succeeds after eNB send “RRC Conn Reestab”.

In 3GPP, RLF parameters can be set differently in order to distinguish RLF timers for services like QCI = 1 and
QCI = 9.
According to the 3GPP (36.331), RLF parameters T301, T310, N310, T311, and N311 can be set to distinguish between the SIB2 RLF timers and the Connected mode RLF timer. 

RLF Parameters for QCI=1 [3GPP 36.331]

Parameter Value  Comments
T311 1000
Maximum time allowed for the UE to find a suitable E-UTRA cell or a cell using another RAT after radio link failure is declared.
T301 600 to 1000 ms The typical duration between a re-establishment request and reject messages.
T310 1000 ms Recovery timer before RLF is declared → target is to drop quickly and re-establish quickly.
It can be set to lower value for VoLTE to do quicker re-establishment rather than higher mute time.
N310 5

Specifies the maximum number of successive out-of-sync indications from L1.
Can be reduced if RLF is declared sooner and re-establishment is stable.

N311 2 Indicates the maximum number of successive “in-sync” indications received from L1.
If re-establishment is unstable, then set N311 = 1 to delay RLF as RTP interruption may be higher due to the UE going to Idle if re-establishment does not succeed.

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