Capacity Plus

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Capacity Plus is a MOTOTRBO network topology where the timeslots (and logical channels) of one or more repeaters are trunked. This allows radios to share the logical channels, resulting in less system access time and increased channel capacity for a given quality of service.

Rather than the user selecting which channel to communicate on, the repeaters and radio allocate a timeslot to the call.

The fixed network infrastructure in a Capacity Plus system can consist of up to eight repeaters all located at one physical RF site and supporting a large number of users in the system coverage area. For radio-to-server data intensive applications, up to 12 data revert repeaters can be added to pass larger volumes of GPS.

Capacity Plus does not require a Trunking Site Controller as all the intelligence required to manage call processing is built into the repeaters themselves.

It is a proprietary implementation from Motorola based on the core part of the DMR Tier 2 standards.[1]

Multisite Capacity Plus

Multisite Capacity Plus is a multi-site trunked system consisting of up to eight repeaters per site and up to 15 sites. For data intensive applications, up to 12 data revert repeaters per site can be added to pass radio to server data such as GPS. Like Capacity Plus, all the intelligence to manage call setup and management is built into the repeaters themselves.

Since there are no Trunking Site Controllers or Central Switch, Talkgroup Calls are allocated to sites on a static basis. This means that a talkgroup call will be streamed to a RF site no matter if there are talkgroup members (radios) on that site or not.

See Multisite Capacity Plus.

Rest Channel[1]

All idle radios (i.e. radios that neither party to a call nor transmitting) monitor one system allocated timeslot known as a Rest Channel. Any new calls will start on the Rest Channel. At the start of a call, the Rest Channel host repeater selects the next available timeslot - which may be on another repeater - to be the new Rest Channel. At the same time, the Rest Channel becomes a Voice Channel (sometimes called a Traffic Channel) and passes the call.

At the start of a call, the current Rest Channel host will inform the idle radios about the new Rest Channel. This means that the radios which are not participating in the call will move to the new Rest Channel. Radios which will be party to the call being set up, will remain on - what is now - the Voice Channel; unmute and pass audio.

If the current Rest Channel is using the last available timeslot (i.e. the other available channels are in use) and a call is setup, all idle radios not party to that call will remain on the now busy timeslot. These radios will indicate that the system is busy and the LED will flash yellow. If all timeslot (channels) on the system are busy and a radio user initiates a call, then the radio will generate a so-called bonk tone to indicate that the system is busy. As soon as a timeslot becomes free, any idle radios are informed, and move to the free timeslot - which will also become the new Rest Channel host.

At the end of a voice call, the repeater also broadcasts the status of all other available channels. This triggers any radio on the channel to move to the current Rest Channel or to a timeslot (channel) where a Talkgroup Call of interest is active.

Unsupported Features[1]

Capacity Plus (Single Site) supports all MOTOTRBO digital features except the following:

  • Scan. Capacity Plus only supports Talkgroup Scan which allows the radio to look for activity on multiple talkgroups on the system. This is not scan, like on conventional where frequencies are searched for specific activity.
  • Emergency Revert Channel. Capacity Plus does not support a Revert Channel for emergency because it is a trunked system and having a dedicated channel for certain call types would be counterintuitive.
  • IP Site Connect. Capacity Plus Single Site is a single site system and therefore does not support features related to IP Site Connect configuration such as wide-area coverage and automatic roaming. However, a radio can be programmed with multiple channels in multiple zones, one of which could be a Capacity Plus Single Site system, another an IP Site Connect System, and others could be MOTOTRBO conventional channels or analogue conventional channels.
  • Impolite Call Setup. Capacity Plus only supports impolite Emergency Call and impolite transmissions (group members can transmit over an ongoing call). A new call always starts on an idle channel and therefore, a radio does not start a non-Emergency Call impolitely.
  • Talkaround mode. A radio can have a talkaround personality but in Capacity Plus Single Site mode, there is no talkaround option.
  • Channel Busy Indication. Mnitoring is an important part of using a conventional radio system, where a radio stays on a channel. In Capacity Plus a radio moves from one Rest Channel to another. Most of the Rest Channels are in an idle state and therefore, monitoring is not necessarily needed.
  • Data Packet Fragmentation. Capacity Plus does not fragment a data packet before transmitting it over the air. Thus, the size of an IP datagram (including IP and UDP headers) should be less than the maximum size of the Packet Data Unit. The value of the Packet Data Unit is a CPS programmable parameter with a maximum size of 1500 bytes.
  • Transmit Interrupt. Capacity Plus does support Voice Interrupt, Emergency Voice Interrupt, Remote Voice Dekey, and Data Over Voice Interrupt which are slightly different from Transmit Interrupt which is a conventional mode feature.

Further Reading


  1. 1.0 1.1 1.2 MOTOTRBO System Planner 68007024085-NY retrived 09.06.2022