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.

Comments

Post a Comment

Popular posts from this blog

Mission Critical Push To Talk and EPS bearers

Image
Mission critical push to talk architecture defines several reference points and uses multiple protocols over different EPS bearers. 23.179 specifies that UE connets to MCPTT specific APN in order to use MCPTT service. It is defined that SIP signaling uses QCI 69 bearer. Additionally HTTP is used for signaling. 23.179 defines also that QCI 8 or better is used for HTTP messaging. Voice media is transmitted over secure RTP either using unicast GBR QCI65 bearer or optionally in multicasting downlink media over MBMS GBR QCI65 bearer. Same bearer (unicast or multicast) is used also floor control. Following figure depicts how these MCPTT reference points can be mapped to unicast and multicast bearers. Although the 23.179 states that SIP and HTTP can be mapped to different unicast bearers that have different QoS, it can be also interpreted that QCI69 is better than QCI8 and therefore it should be possible to carry both SIP and HTTP signalling over the default QCI69 bearer. After LT...
Read more

APNs in MCPTT smartphone

Image
LTE smartphones provide mobile broadband connection to Internet. The prerequisite is that the UE has correct settings, which depend on the mobile operator. Nowadays device vendors and operators have made the use of mobile data so easy that usually users do not need to do any manual configuration. Each smartphone has an internet APN (Access Point Name) configuration, which matches with the operator's network configuration. In general certain APN configuration allows UE to connect (when included in subscription) to certain IP network called packet data network (PDN). So, smartphones know operator's internet APN setting, which enables UE to connect to Internet. Operators have also other APNs and respective PDNs, which offer operator specific services. Some of those services are standardized. On old, still existing service is MMS (Multimedia Messaging Service). MMS is supported by all smartphones and probably automatically configured in all devices and networks. The question is...
Read more

Public Safety prioritization

Image
LTE QoS and prioritization for mission critical communication Public safety (PS) users have the most stringent requirements for network & service accessibility and service quality. Various mechanisms can be used to fulfill these requirements in different network conditions. Those mechanisms must be able to differentiate both users and applications based on certain priorities.  PS users can have different priority levels, which can also change dynamically. For example paramedic engaged in emergency mission has a higher priority than policemen on a lunch break. LTE networks can be shared serving both PS users and commercial subscriber with same resources (e.g. same carriers), in which case there is also a need to prioritize PS users over regular subscribers. Key reason for adopting LTE for mission critical communication is of course broadband capability for data applications. At the same time LTE certainly supports also voice calls (both group calls & one-to-one ca...
Read more