Capacity Max

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Capacity Max is a DMR Tier 3 trunked radio system based on MOTOTRBO. It uses the European Telecommunications Standards Institute (ETSI) Digital Mobile Radio (DMR) Tier 3 standard.

A trunked system allows the radio terminals to share the system channels thereby allowing a large number of talk groups to communicate over a comparatively low number of radio channels which in turn effectively increases the utilization of system and allocated spectrum. Unlike Capacity Plus, Capacity Max makes use of a dedicated control channel - one TDMA timeslot per RF site.

In a control channel based system, the idle radio terminals wait on the current control channel and move to a traffic channel (sometimes called a payload channel) when there is a call of interest.


A Capacity Max system must have at least:

  • One CMSS (Capacity Max System Server), with a licensed Trunking Controller.
  • One RF site containing at least one trunking repeater with a Capacity Max system license
  • One or more switches and routers to provide IP connectivity to the above equipment.
  • Radio terminals with a Capacity Max license.
  • Although not contiguous to network operation, a PC with MOTOTRBO Radio Management is needed to configure the CMSS, repeaters and radio terminals.
The SLR5500; SLR8000; Radios and the CMSS.

System Operation

Open Mode and Advantage Mode

Capacity Max increases the capacity (for example, the number of calls per unit of time) of a radio system beyond what is supported by the DMR trunking protocol. To achieve this, the Capacity Max infrastructure offers the following two modes:

  • Open System Mode This mode uses DMR trunking protocol and therefore it supports both MOTOTRBO and non-MOTOTRBO radios. In this mode, a Capacity Max system provides more capacity when it is working with MOTOTRBO radios since the following two features are not supported by other vendors:
  1. Registration on second slot[1]: A MOTOTRBO radio uses the second slot of the control channel for registration and authentication, if the second slot of the control channel is not in a call. A Capacity Max system allocates a call to the second slot of the control channel only if no idle channels are available. This frees both inbound and outbound of the control channel for initiation of other services.
  2. Arbitration during a call: During a call hang time, radios at different sites may try to initiate their transmission almost at the same time. The system needs to arbitrate among them. MOTOTRBO radios on a Capacity Max system do the arbitration over the trunked channels. This frees the control channel.
  • Advantage Mode This mode provides more capacity than the Open System Mode. All the capacity enhancements of Open System Mode with MOTOTRBO radios are also available in Advantage Mode. It achieves higher capacity by compressing the announcement of ongoing calls over the outbound of the control channel.[1][2] Advantage Mode uses proprietary over the air signaling and therefore does not support non-MOTOTRBO radios. Some of the features (for example, faster “late entry” to an ongoing call), which use proprietary messages over the air are only available in this mode.

Other than Open System Mode and Advantage Mode, a Capacity Max radio has a third mode (Open Radio Mode), which is used by the radio when it is working in a Non-Motorola DMR Tier 3 system. In this Open Radio Mode, a radio does not use any proprietary features and will thertefore work on a system which is compliant with ETSI TS 102-361-4 v1.7.1[3] or later.

Control Channel

Capacity Max control channel is based on the DMR trunking protocol. The DMR trunking protocol requires at least one logical channel to be assigned as a control channel. The control channel has an inbound communication path for transmissions from radios, known as the inbound control channel. A radio randomly accesses the inbound control channel to request access to the system. The required resources (for example, channels) are then allocated by the system. The system informs the radios of the allocated resources over an outbound communication path, known as the outbound control channel. All the radios that are not participating in a call listen to the outbound control channel. The allocated resources are released and can be subsequently allocated to other requests. This method of sharing channels is known as trunking, and the channels are referred to as trunked channels.

Dedicated Control Channel or Shared Control Channel

The Capacity Max system transmits continuously over a dedicated control channel. One dedicated control channel can support many trunked channels. This mode of operation provides the highest performance and throughput.

Independent operators can share frequencies in a band. The telecommunications regulators may require the channel to de-key and yield use of the channel to the co-channel users when not in use. You should access the channel asynchronously since co-operative sharing of channel is not possible in most of the cases. In this mode, the shared channel remains inactive, the repeater is de-keyed, and a radio or the system uses the shared channel politely.

Capacity Max system supports both dedicated and shared channel as a control channel.

The shared control channel reduces the maximum call capacity of a site and slows down the roaming of a radio to the site. A Capacity Max system does not support composite control channel. In a composite control channel, the control channel reverts to a trunked channel if no other trunked channels are available. A composite control channel is useful for a site, where only few frequency pairs are available.

Active Control Channel

The DMR Trunking protocol allows a site to operate with one or two control channels. Two control channels increases the capacity of the site, but requires the radios at the site to be sub-divided into two partitions. This allows load sharing between the two control channels. The load sharing is not effective for calls, which require participation of radios from both the partitions. This is because the system must inform the allocated resources to radios on the outbound path of both control channels. Thus, a Capacity Max system supports one control channel at each site. To increase the capacity of its control channel, a Capacity Max system off-loads certain uses (for example, Registration and Authentication, Arbitration during hang times) of control channels to trunked channels. To reduce configuration, the control channel of a Capacity Max system is always on the first slot of a physical channel.

Multiple Control Channels

Capacity Max allows up to four channels to be designated as the candidate control channels. To offer radio services, a site should have at least one repeater that is a candidate control channel. Multiple candidate control channels help, when the active control channel fails. In the event of the active control channel failure, one of the remaining candidate control channels is selected as the next control channel.

Preferred Control Channel

Capacity Max allows one or more candidate control channel to be the “Preferred Control Channel”. A non-preferred candidate control channel acts as a control channel, only if the site has no “Preferred” candidate control channel. The preferred channel having the lowest “Repeater Id” is selected as the control channel, when there are multiple preferred control channels. The “Preferred” option is useful, when a frequency pair is more suitable (for example, less interference) as the control channel over other candidate control channels.

Shared Control Channel

Radio Management allows setting the candidate control channel attribute to "dedicated" and "shared". The default attribute is the dedicated control channel.

The following combinations of the two attributes are allowed:

  • Preferred and dedicated
  • Preferred and shared
  • Non-preferred and dedicated
  • Non-preferred and shared

The candidate control channels at a site may be a mix of dedicated and shared channels. The shared control channel repeater goes to sleep, that is no transmission over the air when the following scenarios happen:

  • The second slot is idle without call or registration of radio or user is in progress over the second slot.
  • The first slot has no activities such as grants, Acks, and announcements taking place.

During sleep, the shared control channel repeater monitors for interference (both inbound and outbound). The shared control channel repeater makes itself ‘unavailable’ when interference happens. This triggers selection of another candidate control channel if available as the next control channel. If the selected control channel is engaged in a call, the on-going call is terminated including Emergency and All Call. The occurrence of such call dropping is because there is on-going interference on the control channel and the system is heavily loaded.[4]

The probability of dropping a call can be reduced by configuring the repeaters:[5]

  • There are no “Preferred” candidate control channels.
  • The IDs of repeaters having candidate control channels are lower than the IDs of the trunked repeaters.
  • The “Preference Level” of candidate control channels repeaters are lower, that is of a numerically higher value than the “Preference Level” of the trunked repeaters.
  • The “Preference Level” of a candidate control channel repeater with lower ID is lower than the “Preference Level” of a candidate control channel repeater with higher ID.

For an example, refer to the following table:

Role Candidate CC Candidate CC Candidate CC Candidate CC Trunked Trunked
Repeater ID 1 2 3 4 5 6
Preference Level 5 (Least) 4 3 2 1 (Most) 1

During sleep, the shared control channel repeater periodically wakes up and transmits beacons, which helps a radio in roaming to the site and also staying on the site during sleep. The beacon interval is a system-wide parameter and is configurable from 1 - 10 seconds. The beacon duration is fixed at 240 ms. A lower beacon interval reduces time to roam but it also reduces the sharing of the channel.

The existing periodic verification applies on the candidate control channels when the active control channel is in site trunking mode. When a radio finds a shared candidate control channel in the system trunking mode, it starts using this shared control channel as the active control channel. A sleeping shared control channel transmits for a short duration (beacon). The radio takes a longer time, which is approximately five minutes if the beacon interval is two seconds to find the shared control channel.

Control Channel Rotation

A MOTOTRBO repeater is capable of performing the role of a control channel continuously without increasing its failure rate (for example, failure of power amplifier). Thus, the Capacity Max system does not move the control channel periodically.

  1. In a Capacity Max system, the control channel moves from repeater A to repeater B, only if:
  2. Repeater A fails and there is no hardware redundancy; or
  3. Both repeater A and its redundant repeater fail; or

Repeater A is not the preferred control channel repeater, and repeater B which is the preferred control channel repeater, powers on or has recovered from a failure.[6]

Capacity Max Voice and Data Services Over Trunked Channels

Call processing is the primary function of Capacity Max and any other radio system. A call is initiated either over the air (that is, by a radio) or over the wire (that is, by a device such as a voice or data gateway). In Capacity Max, a call is initiated over the air either on the control channel or on the revert channels. This section describes only calls initiated over the air on a control channel. A Capacity Max system supports multiple types of calls. This section focuses on the parts of the life cycle that apply to multiple call types.

Channel Acquisition and Registration

Before initiating a service at a site, a radio must acquire the control channel of the site, and be successfully registered at the site (except when the site is in Site Trunking mode). A radio registers with the system:

  • On power-on
  • On roaming to a site, which is not in site-trunking mode
  • On changing the personality (that is knob position), where the personality is one of the following:
  1. The two personalities are from two different systems
  2. The two personalities have different TX members.
  • When the radio has not received a response for its request within the number of hours configured as the Inactivity Check time
  • On change in the subscribed Talkgroups

Successful registration requires the following conditions:

  • Subscriber Access Control (SAC) should have an entry for the radio.
  • The radio should not be disabled in the SAC. • The site should be in the list of the Valid Sites of the radio in the SAC.
  • Authentication is required by the system with the following conditions:
  1. The radio has the right key.
  2. The SAC has the radio physical serial number (PSN) (For MOTOTRBO radio only).
  • The radio is in the coverage area of the control channel of the site, that is, the radio is receiving the outbound transmission over the control channel.
  • The color code and the SYScode in the outbound transmission match those configured in the radio. An unregistered radio does not unmute to a voice or data call.

Initiation of Request for Services

A radio initiates all requests for services over a control channel by randomly accessing the control channel. After acquiring a control channel and successfully registering, a radio that needs to initiate a service accesses the control channel as specified in the random access procedure defined in the DMR Tier 3 standard[3]. The main purpose of the random access procedure is to reduce collisions caused by simultaneous accesses of the control channel by multiple radios. The random access procedure also minimizes access delays and maximizes throughput under heavy traffic loads.

An attempt at random access may fail under any of the following conditions:

  • The time slot, which was randomly selected by the radio for accessing the control channel, is no longer available for random access. When the infrastructure solicits a response from a radio, it transmits a packet data unit (PDU) on the outbound channel. In order to prevent a collision occurring between this solicited response and a random access transmission by another radio, the infrastructure prohibits any random access transmissions in the given timeslot.
  • Another radio randomly accesses the same timeslot. Subject to the relative signal strength of the transmissions, one or both transmissions may be lost over the air.
  • The radio’s signal is corrupted by noise.
  • The request from the radio is received over-the-air but is lost over the wireline network in the infrastructure.

To reduce the probability of failure of random access, a Capacity Max radio makes multiple attempts with random delays in between. The maximum number of attempts for requesting emergency services is higher than the one for requesting non-emergency services. Due to multiple attempts and random delay in between them, a random access may take a long time. Capacity Max puts an upper limit (approximately ten seconds) on the duration of the random access procedure.

Radio users are not required to keep the push-to-talk (PTT) button pressed during the service request process. Momentarily pressing the PTT button is sufficient.

Qualification of Service Requests

A Capacity Max system qualifies all requests for services. On receipt of a service request from a radio, the infrastructure verifies the source and target (if any) of the request. The source (that is, the requesting radio) must satisfy all of the following conditions:

  • The requesting radio is present in the SAC.
  • The requesting radio is enabled in the SAC.
  • The requesting radio is registered.
  • The requesting radio is permitted to be on its current site in the SAC.
  • The requesting radio is permitted to initiate the requested service in the SAC.

The target and the conditions that the target must satisfy depend upon the type of the service and are described in articles on specific call types.

Service Setup Using All Start

A Capacity Max system sets up a service using “All Start” method. A Capacity Max system starts a qualified service request only when at least one trunked channel is available at all the sites that are associated with the service at that time. This method may delay the start of a service (when trunked channels are not available at one or more associated sites), but guarantees that the all the associated sites participate in the service.

  • For an individual call, the associated site(s) is/are the site(s) where the source and target radios are present.
  • For a talkgroup call, the associated site(s) consist of the following:
    1. The sites that a system administrator has statically associated with the talkgroup by configuring in the radio management application.
    2. The sites where at least one radio has affiliated for the talkgroup.
    3. The site where the call request is being initiated.

The system checks the availability of channels only at the sites that are present in the system at that time. When a site is isolated from the rest of the system, the system does not include the site in the associated sites.

Service Setup Using Fast Start

Fast Start can be used to set up a talkgroup voice service and it is available in both Advantage Mode and Open System Mode. With Fast Start, a Capacity Max system starts a qualified group voice service as long as the source site has channel resources for the call. The call is placed at every associated site that has a channel resource available at that time. The associated sites that do not have a channel resource immediately available join the call on a late entry basis later if a channel resource becomes available before the call end. If no channel resource becomes available prior to call end, the call is not heard at the site. When a resource becomes available at a site, the system checks the Fast Started calls associated with this site. If there is more than one such calls, the system selects one of them and allocates the resource to the selected call. Then the site joins the selected Fast Started call. The selection is based on the priority of the Fast Started calls. The order of priority in a descending order is Emergency Calls, All Calls, Pre-emption Calls, Normal Calls. Having the same priority, Fast Started calls are processed by using the first-come-first-served principle.

For a normal group voice call that involves a VRC gateway site, if all of the talkpath licenses have been occupied, the call can be started without the VRC gateway site. The VRC gateway site can late entry to join the call if the talkpath license becomes available later. In a fast start configuration, the group voice call may not be delivered to all the associated sites and the call initiator does not know if all the group members (at various sites) received the call.

The Fast Start method applies to all types of group voice calls including: All Call, Broadcast Call, Emergency Call, Priority call, normal talkgroup call, and the Telephone group call. It does not apply to individual voice calls or data calls.

A Capacity Max system supports the following three system level configurations:

  • Disabled. This is the default configuration. The system sets up a group voice service by using the All Start method.
  • System Wide. The system sets up a group voice service by using the Fast Start method. All RF sites and VRC gateway sites in the system are Fast Start sites.
  • Site Wide. With this option selected, the user can use the Radio Management to configure the list of Fast Start sites in the Capacity Max Site Parameters. The system applies Fast Start rules when allocating resources at the associated sites if the site was configured to be a Fast Start site.

Service Request Queuing or Pre-empt an Ongoing Call

If a trunked channel is not available at one or more sites associated with a service request, one of the following scenarios occurs:

  • The service request is queued and the source radio is informed.
  • In following cases, on-going calls are pre-empted and the channel is allocated to the service request.
    1. Emergency Voice Call and All Call follows the “All Start” method. If a trunked channel is not available at the associated site, a busy trunked channel (busy with anything besides an
    2. Emergency or All call) is assigned to the call. The request is rejected if all the channels at the site are busy with Emergency or All calls.

A Capacity Max system allows a radio user to request for preemption if required for its service request. For more details, see Allowing Request for Preemption.

When a trunked channel becomes available, the system checks the service requests in queue for the availability of all the required channels. If there are more than one such service requests, then the system selects one of them and allocates channels to the selected service request. The selection is based on the priority of the service request, which is the maximum of the priority of the initiating radio, and the priority of the target (a radio or a talkgroup).

A Capacity Max system allows the system owner to configure a priority for each radio and each talkgroup.

Service requests having the same priority are processed using the first-come-first-served principle.

A service request waits in the queue until all required channels become available. This has the following implications:

  • Between two service requests having the same priority, the request with fewer required channels is likely to be processed first.
  • During heavy usage, requests remain in the queue longer, and the radio may receive the grant after its waiting period is over. When this occurs, the service initiating radio does not initiate transmission, and the allocated channels are wasted for few seconds. A system owner can configure the duration of the wait period (that is, the TP_Timer) for which a radio waits for the grant of its queued request.
  • Radios in a heavily loaded system should be configured with a greater TP_Timer (a system-wide parameter).

Some of the advantages of queuing in a loaded system are that it improves channel utilization, and a radio user is not required to press PTT multiple times. A disadvantage is that in a heavily loaded system, the recent requests have to wait for the older requests. Queing increases the delay in processing a request. In many scenarios, the delay is not desirable. For this reason, a Capacity Max system allows its system owner to disable or enable the queuing. When queuing is disabled, the system does not use priority of radios or talkgroups, and the TP_Timer could be small (around 4 seconds).

When channels are assigned to a talkgroup request in the queue, all queued requests for the same talkgroup and same call type (voice or data) are removed from the queue. A radio can have only one service request in the queue at any time. When the radio makes a new service request, the previous service request is removed from the queue.

Allowing Request for Pre-emption

A Capacity Max system allows a call initiator that is a radio user, the option board in the SU, a non-IP peripheral connected to the SU, a dispatcher, or a data application to pre-empt an ongoing call in favor of its next service request. The system pre-empts only if there is no idle channel at the RF site. A radio user can request for pre-emption using a programmable button that moves the selection in a cyclic fashion between Normal and High. The change is applicable to user initiated call that is Voice call, Text message over trunked channel, Job Ticket, but not location or telemetry. The selection resets to its default value when the call ends.

The Radio Management allows user to set/reset a flag in the Subscriber Access Control (SAC) of a radio including Voice application and Data Gateway. Only if the flag permits, the system allows the request for pre-emption. The site allows the request for pre-emption from all radios during Site Trunking and when SAC is unavailable.

Radio Management allows user to set/reset a flag for each talkgroup. If the flag is set then a call to a specified Talkgroup can pre-empt an ongoing call, if there is no idle channel at a RF site. Call request is queued if there is no channel with lower priority calls than the call request. Out of all the lower priority calls, the channels with the lowest priority calls are selected for pre-emption. The radio which is transmitting over the air at the selected channel is forced to stop the transmission.

Note that Emergency and All Calls are never pre-empted.

The order of priority in descending order is Emergency Calls and All Calls, Preemption Calls then Normal Calls.

Channel Allocation

A Capacity Max system allocates channels to a service request according to the system owner’s preference.

A Capacity Max system allows system owners to configure their preferences for usage of a physical trunked channel (both slots) in a repeater. The guidelines for assigning the preference level for a trunked channel are:

  • Exclusively licensed channels should have higher preference than the shared channels.
  • Within shared channels, the preference is inversely proportional to the co-channel user activity.

When a service request is selected for allocation of channels, the system assigns a trunked channel at a site according to the following rules.

  • Within all ‘Idle’ trunked channels at that site, the high preference trunked channel is assigned before the low preference trunked channel.
  • Within same preference level, trunked channels are assigned in round robin method.
  • The trunked channel on the second slot of a control channel repeater is assigned only when no other idle channel is available at the site. The second slot of a control channel is used by MSI radios for registration.
Retention of Ongoing Calls

Any changes in the system data do not affect the ongoing calls. To set up a call, a Capacity Max system uses several data sets such as the Subscriber Access Control (SAC) and static associations between sites and talkgroups. Any change in the data sets after the allocation of channels to a call does not affect the call. For example, during a call, if the system owner disables the source or destination of the call, the call does not stop.

Late Entry to Voice Talkgroup Calls

A radio can enter a voice talkgroup call if it was not on the control channel at the time of call initiation. This condition may occur when the radio is at another site, out of coverage, in fade, transmitting over a revert channel, or participating in another call. A Capacity Max system supports late entry for voice talkgroup call only. The probability of having late entry for an Individual call is small because an Individual call starts after acknowledgment from the

target radio/user.

To facilitate late entry, a site announces periodically all the ongoing voice talkgroup calls over the control channel. The announcements are more frequent (that is, late entries are approx. 50% less late) in Advantage mode than in Open System mode of a Capacity Max system. In Advantage mode, the lateness can be further shortened by approximately 50%, if the talkgroups’ IDs are less than 1024.

Priority Monitor Feature

For radios participating in a call over trunked channels, ongoing Emergency Calls, All Calls, and calls for priority talkgroups are announced periodically over the busy trunked channels. The announcements are more frequent (that is, late entries are less late) if the emergency talkgroups IDs are less than 1024. On receiving an announcement, a radio moves to the announced call if the announced call is of interest and has higher priority than its current call.

Floor Arbitration of Trunked Channels

A call is made of multiple transmissions, where the maximum time gap between two consecutive transmissions is called call hang time. A Capacity Max system allows its system owner to configure three hang times, one each for talkgroup voice call, individual voice call, and emergency voice call.

During a call, a radio accesses the trunked channel by being polite to its own color code (that is, a radio cannot talk over other radios in a call) except in case of Voice Interrupt, and during a telephone call a radio is impolite to the telephone. The radios participating in the call stay on the trunked channel during the call hang time. A radio can initiate a transmission during the call hang time. In a call involving multiple sites, radios at different sites can initiate voice transmissions near simultaneously. A Capacity Max system uses a floor arbitration algorithm, which selects one of the radios for transmission.

The floor arbitration algorithm is a proprietary feature and is used only by Motorola Solutions radios when they are operating in a Capacity Max system. For non-Motorola Solutions radios, some transmissions have more than one radio transmitting over each other. The probability of this increases as the number of non-Motorola Solutions radios increases.

In some trunked radio systems, radios that need to transmit during hang time move to the control channel and request permission. The system arbitrates with any other requests received from other radios for the same call and provides a grant or reject. The disadvantage is that the request for every transmission reduces the inbound capacity (that is, the number of calls initiated per hour) of the control channel. For example, if a call has on the average 2.5 transmissions and the inbound capacity is 12000 random accesses per hour, the request for every transmission reduces the capacity to 4800 calls per hour.

Call Termination

A Capacity Max system allows a radio user to terminate a call.

A call is terminated if one or more of the following conditions occur:

  • A radio can initiate a “call end request” during hangtime on the trunk channel where the call is ongoing.
    1. In a talkgroup call, only the call initiating radio (that is, a radio whose request for the call was granted) can initiate a call end request.
    2. In an Individual call, either the source radio or the target radio can initiate a call end request.

This feature improves the channel utilization, especially in case of a long call hangtime.

  • The Call Hangtime has expired because no radio initiated transmission during the call hangtime.
  • The Time Out Timer (TOT) of either the radio or the repeater has expired.


  1. 1.0 1.1 The control channel of a Capacity Max system is always on the first slot.
  2. The outbound control channel becomes the dominant factor in restricting the capacity, when more and more calls are multi-site calls.
  3. 3.0 3.1
  4. It is suggested to detect outbound interference by having a mobile radio (one for each shared control channel) connected to the repeater through GPIO line.
  5. The candidate control channel of lowest ID is selected as the next control channel.
  6. The selection of a candidate control channel as the next control channel is independent of the channel being "dedicated" or "shared".