MCPTT latency requirements

Mission critical communication has strict performance requirements for the push-to-talk (PTT) voice calls. The groups calls should be of course established as soon as possible, but the most important thing is that floor control is 'instant' and mouth-to-ear voice delay is minimal during mission critical PTT calls.

3GPP 22.179 specifies some key performance indicators (KPI), and the interesting ones are:
  • MCPTT access time (KPI 1)
  • Mouth-to-ear latency (KPI 3)
The figure below (from 22.179) illustrates the definitions of these two KPIs. The same specification defines also target values for these KPIs. The exact definition includes some probabilities and assumptions, but in order to keep it simple we need just to remember that KPI 1 should be less than 300 ms and KPI 3 should be also less than 300 ms.


One might ask whether LTE networks can guarantee such low latency targets. Well, these figures are actually coming from the existing 'oldish' legacy systems like TETRA. LTE is the latest commercial cellular technology so such end-to-end latency requirements give actually a lot of buffer for loaded network elements and mobile devices to forward packets and process signaling & voice within the required limits.

To get some perspective for the end-to-end (E2E) latency in LTE networks and Internet I have below two examples based on the Speedtest using iPhone6 over commercial LTE network in Finland. The E2E latency from Espoo to a server in Paris (distance 1900 km) was 52 milliseconds and to the closest server in Helsinki 14 ms. (Note: Speedtest reports the lowest ping, not an average)

 

Although the Speedtest latency and throughout results indicate that the LTE cell was quite empty, we can anyway notice that LTE radio, IP networks and optical transmission are able to guarantee low latency even over long distances with best effort data service. When we take into account that optimized MCPTT service will use prioritized GBR (guaranteed bit rate) bearers for voice media and floor control, the LTE based networks are not going to be any limit for high performance MCPTT service.

If there are any issues with mouth-to-ear latency or floor control performance, the most likely problems are in UE client and application server (AS) implementations. Assuming that media processing in AS does not perform any transcoding, then MCPTT UE product is probably the key component to achieve the MCPTT KPI requirements.

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