9 Reasons for Long Call Setup Time in VoLTE


The other day I called a colleague and he answer HELLO on the first ring tone. I perceived it as if it was even less than 2 seconds or so. 

Granted the call quality was great. I was amazed and surprised at the same time.  Voice over LTE (VoLTE) not only brings in better voice quality. It also results in least amount of call setup time as well compared to other conventional and non-VoLTE related voice solutions.  User perception of voice call setup time through VoLTE has resulted in faster connectivity versus conventional solutions. 

This results​ in Call Setup Delay becoming one of the major key performance indicator ( KPI ) in case of VoLTE from end user perspective. Long setup delay can make poor user experience about VoLTE call. 

As a performance engineer and optimization engineer, it is important to understand what could be the causes for long call setup delay in LTE, so that optimization can be performed accordingly.  Below we will talk about some of the reasons that could cause long call setup time for VoLTE

Call Setup delay

Call setup time is the time from the moment VoLTE user initiates the call until the user receives notification about the called party being alerted. This call type termed as VoLTE to VoLTE, and call setup time is measured entirely from the UE, which originates the call. 

Typical Call Setup Time is 3 to 5 seconds.  

What could be the possible causes for long call setup time?

In order for VoLTE Optimization engineers and performance engineers to investigate what is causing long call setup time. You can continue to read further. The reasons for long call setup time could possibly be due to any of the following reasons. 

  1. If RACH procedure is not successful. It can result in long call setup times. This reasoning of long setup on account of RACH procedure is only applicable in case of Idle mode for both originating and terminating calls. 
  2. In case of Idle mode ---- for terminating VoLTE calls can result in longer call setup time due to poor settings of paging cycle, delayed paging, or just because of non-optimized default paging settings. 
  3. RF conditions can affect and hinder the setup of Dedicated Radio Bearer which can result in delay of the call setup time. 
  4. Using features for the call setup such as early media, call forwarding, network location , pre-condition etc. can result in delaying the call setup time.
  5. If the network is already congested and other VoLTE users are already active it will result in delaying the setup of other VoLTE calls.
  6. Re-transmission of signalling messages on account of poor conditions or other reasons can also result in delay for call setup time.
  7. If VoLTE is not supported in the entire LTE coverage , this would result in other voice solutions such as either CS Fall back or  SRVCC etc. resulting in longer call setup times. 
  8. A VoLTE call setup is a team effort of different network entities. For a successful VoLTE call setting up  of each network portion i.e., RAN, CORE, IMS, Transport etc should work in unison for call setup. Therefore, any of Timers misalignment on any of the network entity will result in longer call setup delay. Hence, all timers in different network entities should be optimized across all domains.
  9. The overall call latency also depends upon the IP backhaul latency and RAN backhaul latency. If any of this latency is higher. It can result in longer call setup time as well. 

Shortest volte call setup?

Ideally, the shortest VoLTE latency is achieved when both the UEs involved are in RRC Connected state. 

Longest volte call setup?

Ideally, the longest VoLTE latency is achieved when both the UEs involved are in RRC Idle state. (not connected)

What did you experience as the major causes for longer VoLTE call setup time factors in your market? Please share in the comments below. Thanks 🙂