Understanding MCPTT Performance via the Standard 4 KPIs

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This article will explain how to measure the performance of Mission Critical Push To Talk (MCPTT) and how the standards based definitions for the four KPI metrics (Key Performance Indicators) describe this performance. We will also talk about the different interpretations of access time versus grant time. These KPIs are used to measure the performance and responsiveness of MCPTT services End-End in a lab or across networks and place expected thresholds on them to help ensure they allow first responders to clearly and instantly communicate with their colleagues in the heat of an emergency event.

Basically the four MCPTT KPI’s are:

Hear Beep Before Speaking—Access Time
- When the PTT button is pressed, what is the time lag before I should start speaking
- You will get a beep which is your indication to start speaking. However, the MCPTT may send the access beep before the voice path is open right through the network to your recipient party. Therefore, often they miss the first few syllables you speak.
Mouth-to-Ear Latency
- Once the voice path is available, we need to know (Mouth-to-Ear) Latency. This is the one-way delay from speech entering the microphone to emanating from the recipient’s speaker.
- Learn full measurement information here.
  The target value for this KPI is less than 200 milliseconds for one-way delay.
Voice Quality
- The quality of the speech (known as voice quality) is the third KPI. some specs call for voice quality/Intelligibility. These are completely different measurements.
  - Speech Quality—is the clarity of the speech and is impacted by:
    1. Network impairments such as packet loss and jitter
    2. The quality of the radio connection
    3. Codec impairments or the implementation of the codec on the mobile device or other Apps being used at the same time
  - Speech Quality is measured using POLQA or PESQ.
  - Speech Quality measurement does not include a measurement of audio latency which is measured separately.
  - Speech Quality is quantified in a MOS scale of 1–5.
- Intelligibility—is a measure of how the transmitted speech can be heard by the receiving party. In conditions of high background noise, it becomes difficult to distinguish between consonants and fricatives and intelligibility suffers. For example, we can’t recognize the difference between cat, bat and mat.
  - Intelligibility is measured using a MRT—Mean Rhyme Test, such as ABC-MRT.
KPI 4 Has to do with a third person, or more, joining the PTT Talk Group.

NIST talks about access probable retention time i.e. the probability of one of the MCPTT participant legs being dropped from the PTT Group conference call. Whereas 3GPP defines KPI 4 as:

The Late call entry time i.e. time to enter an ongoing MCPTT Group call measured from the time that a user decides to monitor such an MCPTT Group Call, to the time when the MCPTT UE’s speaker starts to play the audio

The 3GPP TS 22.179 specification provides the specific details on how the KPIs measurements are defined

Explanation—Forming a Group MCPTT Conference Call vs 1-to-1 Call

Basically, the signaling of the network has allocated the resources from the MCPTT servers through the Radio Access Network (RAN) and the SBCs etc. to set up the talk group between 2 parties, say Alice and Bob. When a third party, Catherine, wants to join that same Talk Group, this requires less resources from the network and therefore End to End Access Time should be quicker.

The target value for this KPI is less than 2 seconds for Emergency Group Calls and Imminent Peril Calls, and less than 5 seconds for other MCPTT calls.

In practice, you can imagine this needs to be tested as different mechanisms are used to signal the imminent peril calls versus normal MCPTT calls. Plus, in a congested network and low bandwidth situations, these performance parameters will vary drastically.

Distinguishing Access Time versus Grant Time

Flow chart showing access time and grant time.

The 3GPP Technical Specification TS 22.179 defines two key performance indicators (KPIs) related to the time it takes for a user to participate in an MCPTT call: Access Time and Grant Time.

3GPP defines MCPTT access time as:

The time between when an MCPTT User requests to speak (normally by pressing the MCPTT control on the MCPTT UE) and when this user gets a signal to start speaking

Definitions of KPIs are dependent on the technology to which they are applied. For example, LMR P.25 trunk radio systems versus LTE Public Safety Broadband Networks (PSBN).

Do not assume that when the originating user hears the beep of the grant tone, that the voice path is open and first utterances will be received by the listener. This has to be tested for each combination of vendor equipment, technology used and network load conditions.

Access Time (KPI 1)

Access Time measures the time between when a user requests to speak (normally by pressing the PTT button on their radio) and when they get a beep to start speaking. This KPI focuses on the user experience and reflects the responsiveness of the system to their request. It includes the time for the user’s device to send the request, the network to process the request, and the network to send the signal back to the user.

Grant Time

Grant time measures the time it takes from when the network receives the user’s call request to when the network sends a grant message back to the user. This is not perceived by the user so cannot be a User Experience (UX) KPI. instead, it informs engineers of a more confined performance of the core network and as such has value in improving that performance.

Grant time is typically a smaller component of the access time.

The only Complete Solution for Automated MCPTT KPI’s Testing and Measurement

Opale Systems MultiDSLA is perfect for complete performance analysis of all 4 x MCPTT KPI’s however defined. MultiDSLA has:

Ability to detect tones of varying frequencies over a dynamic range of 110dB, more than enough for these measurements
Task Lists for easy test script construction—to ensure you can do any MCPTT measurement regardless of differences between standards
Automation built-in
Ability to connect with and control PTT on all radios and mobile devices
APIs for integration with your test automation harness
…and the best measurement accuracy

In this case a grant tone is a succession of 3 frequencies (500, 1000 and 2000 Hz in this example). MultiDSLA supports 2 tracks for configuring the wait for tone event on track one.