By: Randy Richmond
Will Land Mobile Radio (LMR) ever go away? Will cellular Push-To-Talk (PTT) solutions, such as 3GPP Mission Critical PTT (MCPTT), replace LMR? Opinions vary, but one thing is certain, the LMR technology known as Project 25 (P25) is the standard against which public safety users in North America compare all alternative solutions.
The 3rd Generation Partnership Project (3GPP), the global cellular standards body, has used today’s LMR capabilities as the baseline requirements for its own MCPTT standards. In keeping with the 2012 Spectrum Act legislation, the National Institute of Standards and Technology (NIST) conveyed the USA’s requirements to 3GPP, which were well documented in a series of reports from the National Public Safety Telecommunications Council (NPSTC). In developing these reports, NPSTC heavily relied on the capabilities of P25.
Whether today’s P25 users anticipate an eventual end to their LMR systems or not, many are planning or have begun augmenting their P25 systems with MCPTT because it’s now offered by multiple carriers in North America. To do so, it’s helpful to understand the key differences between P25 and MCPTT capabilities, which are outlined below. The capabilities referenced are those enabled by P25 standards developed by the Telecommunications Industry Association (TIA), and MCPTT open standards as developed by 3GPP.
The comparison is extremely useful if you plan to offload non-critical users from a P25 system to MCPTT, as you’ll want to know whether the features LMR users have been accustomed to will be supported on MCPTT. And if you have plans to interwork your P25 and MCPTT solutions for LMR/LTE interoperability, only capabilities that exist on both networks will be able to make that traversal.
High Level Comparison
Table 1: High Level Comparison
| Aspect | TIA 102 P25 | 3GPP MCX | NOTES |
| Number of Groups supported | Up to 65,534 | Virtually unlimited | |
| Number of Users supported | Up to 9,999,998 | Virtually unlimited | |
| Number of Users per Group supported | Up to 9,999,998 | No technical limit. But the practical limit may be a few hundred on unicast networks (no limit on multicast eMBMS networks) | Few multicast MCPTT networks currently exist |
| Voice Calls | Yes (AMBE+2 narrowband vocoder) | Yes “MCPTT” (AMR-WB wideband vocoder with other vocoder options) | MCPTT’s AMR-WB vocoder has noticeably better fidelity compared to P25 |
| Simultaneous Media Streams | One voice stream per SU/radio | Multiple streams per UE/device | MCPTT enables monitoring multiple groups at the same time |
| Data | Yes – Narrowband, Slow Speed (9600 bps) | Yes – “MCDATA”. Broadband, High Speed | Used for text/SDS, location, file transfer and mobile CAD |
| Video Calls | No | Yes – “MCVIDEO” (Private & Group) | |
| Priority & Preemption | Yes (trunking only) | Yes | |
| Emergency Levels | 2 (Normal, Emergency) | 3 (Normal, Imminent Peril, and Emergency) | |
| Dynamic Prioritization | No | Yes | Authorized users can remotely change the priority of designated MCPTT users/groups |
| Power Levels | Portables – typically 5 watts+
Mobiles – typically 50 watts+ |
User equipment (UE)/handsets – typically 0.2 watts
High Power UE (HPUE) – up to 1.25 watts |
3GPP Band 14 HPUE are now available, but generally not as handheld devices |
| Off Network Communications | Yes – “Direct Mode/Talk Around” (FDMA conventional) | Yes – “ProSe” (Proximity Services – limited range due to low power level) | 3GPP ProSe is not yet generally available |
| Encryption | Yes (optional) | Yes | |
| Over the Air Programming (OTAP) | Yes (optional) | Yes | |
| Over the Air Rekeying (OTAR) | Yes (optional) | Yes |
Detailed Comparison
Table 2 below provides a more detailed comparison, including standard P25 capabilities and whether they translate to 3GPP MCPTT capabilities. The feature name used in this table is the common name for the P25 feature used in the TIA standards documents (specific P25 vendors may have different names for them). Where a different name for the equivalent feature is used for MCPTT, it is shown in quotation marks in the MCPTT column.
Table 2: Detailed Comparison
| Project 25 Feature Name[1] | TIA 102 P25 | 3GPP MCPTT |
| Talker ID (aka PTT ID) | Yes – numeric with optional text alias | Yes – textual with optional supplemental info |
| Groups Calls | Yes – Confirmed or Unconfirmed Calls[2] | Yes[3] – Chat or Pre-arranged[4], Pre-established or On-demand[5] |
| Emergency Group & Individual Calls | Yes | Yes3 |
| Emergency Alert (w/o voice) | Yes | Yes |
| Emergency Cancel | Yes | Yes |
| Individual Calls (aka, Unit-to-Unit Call) | Yes – half or full duplex, with or without Availability Check[6]
|
Yes3 – “Private Call” (with or without Floor Control[7], Auto or Manual commencement[8], Pre-established or On-demand5) |
| Discrete Listening (aka, Private Call Eavesdropping) |
Yes | No |
| Group Regrouping | Yes (not yet available over ISSI/CSSI) | Yes |
| User Regrouping | Yes | Yes |
| Radio Unit Monitor (RUM) | Yes (timed only) | Yes – “Ambient Listening” (On/Off, monitored audio may be sent as Private or Group) |
| Call Alert | Yes | Yes – “Private Call Back Request” |
| Dispatcher Override & Preemption | Yes | Yes |
| Priority | Yes (trunking only) | Yes |
| Preemption | Yes (trunking only) | Yes |
| Announcement Group Call (to multiple groups) | Yes | Yes – “Broadcast Group Call” |
| Broadcast Call (one way) | Yes (trunking only) | Yes – “Group Broadcast Call” |
| System Call (aka, All Call) | Yes | Yes3 – “User Broadcast Group Call” |
| Radio Inhibit (aka, Radio Disable) | Yes | Yes – “Remote Device Disable” |
| Radio Un-inhibit (aka, Radio Enable) | Yes | Yes – “Remote Device Re-Enable” |
| Radio Check | Yes | Yes “Presence” |
| Text Messaging to Individuals | Yes | Yes – “Short Data Service – SDS” |
| Location Conveyance from Field Radios to the Network | Yes | Yes |
Capabilities Unique to MCPTT
Although P25 does have some advantages over MCPTT (e.g., off network operation and subscriber unit power levels), 3GPP MCPTT has a number of capabilities not found in open standard P25 systems, such as those shown in Table 3below.
Table 3: Unique MCPTT Capabilities
| 3GPP MCX Feature Name | Feature Description |
| Imminent Peril Group Call | A group call with priority between normal and emergency (chat type only4). |
| First-To-Answer Private Call Invitation | A private call invitation sent to a group of users. The first to answer enters into a 1-to-1 private call with the initiator. |
| Adhoc Group Creation | The ability for authorized users (typically dispatchers) to dynamically create a new group (3GPP may be rescinding this capability for security reasons). |
| User Group Query | The ability to remotely discover the group affiliation of a specific user. |
| Remote User Group Invitation/ Assignment (aka, “REGA”) | The ability for authorized users (typically dispatchers) to remotely invite or assign a user to affiliate with a specific group. |
| Priority Dynamic Uplift/Downgrade | The ability for an authorized user (typically a dispatcher) to dynamically change the priority of an individual user or group. |
| Remote User/UE Call Initiation/Termination | The ability for an authorized user (typically a dispatcher) to remotely initiate or terminate a private or group call of another user/group. |
| Broadcast Group Calls (two way) | A two-way broadcast group call. This may be with pre-established or on-demand establishment5. |
| SDS Text Messaging to Groups | Sending text messages to a group (including broadcasts to multiple groups). |
| SDS Text Message Delivery Confirmation | Confirmation that a private text message has been delivered to the destination device. |
| SDS Text Message Read Notification | Confirmation that a private text message has been read by the targeted user. |
| MCData File Distribution | The ability to attach files (e.g., audio clips, video clips, location coordinates) to private and group text messages, with or without mandatory download. |
| Location Conveyed with Voice/Text Media | The ability to convey the originator’s location along with the originator’s media (voice/text/video). |
| Location Conveyance from/to Field Radios | The ability to convey the originator’s location to other users, including dispatch consoles. |
What Capabilities Can Your Providers Support?
Note that few manufacturers implement all the capabilities supported by the standards. In addition, few system owners (including cellular network operators) enable all the capabilities offered by the manufacturers. Some manufacturers choose to achieve comparable capabilities, using non-standard, proprietary methods, while others choose to implement capabilities not supported by the standards (which of course are also proprietary). Therefore this capability survey may not reflect what is actually supported by your P25 LMR system or your MCPTT cellular provider. But at the very least, the comparison can serve as a starting point for discussion with your LMR and MCPTT suppliers to see what they can actually support, and when they will be able to support it.
Footnotes
1For a definition of the P25 features see the DHS Statement of Project 25 User Needs (SPUN): http://www.project25.org/images/stories/ptig/10-06-2020_P25-SPUN_FINAL-r1_508c.pdf
2P25 Group Call Confirmation: Confirmed vs Unconfirmed: A confirmed call or data transfer attempts to secure RFSSs, sites, and users before starting the transmission. An unconfirmed call or data transfer makes no particular effort to guarantee the participation of particular RFSSs, sites, or users.
3These MCPTT capabilities are also available in 3GPP ProSe (off-network mode).
4 MCPTT Group Call Type – Pre-arranged vs Chat: Group Call types can either be pre-arranged or chat. When a pre-arranged Group Call is initiated by one of the group members, all other preconfigured/affiliated group members are automatically invited to the call. Designated users in the group can enter or exit the call for the duration of the group call. When a Chat Group Call is initiated, all group members are not automatically invited to the call. To participate in a Chat Group Call, a group member has to explicitly join the call. Any group member can join and leave a Chat Group Call at any time. Emergency and Imminent Peril Group Calls are both permitted as Pre-arranged Group Calls and the Chat Group Call types.
5MCPTT Establishment – Pre-established vs On-Demand: Private and Group Call establishment can either be pre-established or on-demand. Pre-established calls are always on (session active) which eliminates any initial setup time for the first talker in the call. On-demand calls activate the session with the first caller, and end the session when a participant presses the end button or the session times out.
6P25 Individual Call Availability Check: With vs Without: With Availability Check, the P25 network first determines whether the target P25 subscriber unit is on network (powered on, in range) before initiating an Individual Call to the radio. Calls delivered with Availability Check are like Automatic Commencement calls. Calls delivered without Availability Check are like Manual Commencement calls (see other footnote).
7MCPTT Floor Control – With vs Without: Private Calls may be with or without floor control. With floor control the call is half duplex. Without floor control the call may be full duplex. Group Calls are always with floor control (half duplex).
8 MCPTTCommencement Mode – Automatic vs Manual: Private Calls may commence at the target user’s equipment (UE) automatically (audio delivered to the UE’s speaker with no action by the target user), or manually (by the target user pressing an “answer” button – aka, “hooked answer”). Group Calls always use Automatic commencement.
Randy Richmond is a critical communications industry veteran and professional engineer. For 3 decades has been involved in various public safety communications standards and organizations, including NENA, APCO, TIA, PSTA, NPSTC, and the NFPA. Randy is Zetron’s former Standards and Regulatory Specialist (retired) and in 2018 was a principle investigator for a DHS SBIR contract reporting on the state of LMR/LTE interoperability.
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