Channel Access
Channel Access (sometimes referred to as TX Admit Criteria or Busy Channel Lockout) is a setting in a MOTOTRBO radio that prevents a radio user from transmitting while another user is using the channel. Without Channel Access, it would be possible for radio users to (un)intentionally block (or jam) the transmissions of other radio users. Channel Access is set up in the radio using the CPS or Radio Management.
In all system topologies except Capacity Max
Channel Access dictates under what conditions a radio is allowed to initiate a transmission on a channel. The channel access rules are governed in part by the mobile and portable radios. It is the radio’s responsibility to assess the state of the system, and utilize its channel access rules to decide whether to grant the call to the user.[1]
In repeater systems, it is the repeater’s responsibility to:
- Identify if a channel is busy, or
- Identify if a channel is idle, or
- Inform for which radio the channel is reserved.
The repeater does not block or deny any channel access from radios on its system, but will not repeat transmissions from another system.
There are two main types of channel access in a MOTOTRBO system: Polite and Impolite access. In the CPS/RM, channel access is referred to as the Admit Criteria. MOTOTRBO supports the following Admit Criteria:
- Always: This criteria is often referred to as “Impolite” channel access, and can be applied to analogue and digital channels.
- Channel Free: This criteria is often referred to as “Polite to All”, and can be applied to analog and digital channels.
- Color Code Free: This criteria is sometimes referred to as “Polite to Own Color Code” or “Polite to Own System”, and is applied only to digital channels.
- Correct PL: This criteria is sometimes referred to as “Polite to Other System”, and is applied only to analogue channels. The radio checks for a PL match prior to allowing a transmission.
Channel access methods must be specified for each channel in the radio CPS. The TX (Transmit) parameters for each defined channel contains an “Admit Criteria” selection that must be set to one of the values described above.
All these channel access options govern how standard group voice calls and Private Calls access the system. Not all transmission types utilize these settings. For example, emergency voice calls always operate impolitely. This gives emergency voice calls a slightly higher priority over existing traffic on the channel. Data calls are always polite. Since a data call can be queued and retried, its priority is considered lower than voice.
Note that a “polite” radio user attempting a voice call will be polite to data, but an impolite user may not.
Control messages (used for signaling features) are also always polite. The exception is the emergency alarm. Emergency alarms are sent with a mix of impolite and polite channel access, in order to optimize the likelihood of successful transmission.
When the Admit Criteria is either Channel Free or Correct PL, a configurable RSSI threshold is provided per channel in the radio. If the received signal strength is less than the configured RSSI threshold, the signal is considered as an interference and the radio gets channel access when the user initiates a new call. However, if the received signal strength is greater or equal to the configured threshold, the channel is considered busy and the radio does not get channel access when the user initiates a new call. It is the responsibility of the site planner or the service provider to set the RSSI Threshold to an appropriate value considering the RF interference and also ensure that the desired signal strength is more than the configured threshold. The default value of RSSI Threshold is -124 dBm. The configurable range is between -124 dBm to -80 dBm. When a value of -124 dBm is selected, subscriber does not get channel access if carrier activity is detected due to interference on the channel when the user initiates a new call.
A value of -124 dBm is very sensitive to even the slightest RF interference.
When operating in IP Site Connect mode, the repeaters also check the channel for interference before transmitting. This is required since even though the source radio checks the channel at one site, it does not mean there is no interference at another site. Therefore, a repeater checks for over-the-air interference before waking up and transmitting. The repeater always acts with an Admit Criteria of Channel Free and has a configurable signal strength threshold. Although one site may be busy, the other non-busy sites continue with the call.
Impolite operation
Impolite operation is also known as the admit criteria of "Always".
When configured for impolite operation, a radio does not check for an idle channel prior to allowing a transmission. From the user’s perspective, the radio simply transmits when the PTT is pressed. However, on a digital repeater channel, the radio checks if the repeater is hibernating. Transmission does not proceed, if the repeater is hibernating and the radio is unable to wake it.
It is very important to note that when a radio is utilizing impolite operation, it is possible that it is transmitting on top of another user’s transmission. This causes RF contention at the target. When RF contention occurs between digital transmissions, it is impossible to predict which signal is usable. If one transmission is much stronger than the other, it is received instead of the weaker signal. But in most cases, the two transmissions on the same frequency and time slot results in both transmissions being unusable. Thus, it is recommended that only disciplined users are granted the right to use impolite operation. Further, those impolite users are encouraged to utilize the busy channel LED on their radio to determine, if the channel is idle prior to transmitting.
Impolite operation on a IP Site Connect system
When operating in IP Site Connect mode, it is important to understand that impolite channel access only occurs at the local site. If a call is taking place on the IP Site Connect system, and the original source of that call is at the same site as the interrupting “impolite” radio, RF contention occurs and it is unclear which source is successful. If the original source of the call is at a different site from the interrupting radio, the original call continues at all other sites except where the interrupting radio is located.
Impolite operation on a Capacity Plus system
When operating in Capacity Plus, impolite operation is supported only in Emergency Calls.
Polite to All Operation
Polite to all operation is also known as the admit critearia of "Channel Free".
When configured for Polite to All operation, the radio checks if channels are idle or busy, prior to allowing a transmission. The radio is polite to all analog or digital transmissions, another system’s transmission, or other traffic on your system. This option is often used, when there are neighboring communications systems, to prevent radio users from disrupting other transmissions. However, when this option is used, any strong signal on the channel blocks other users from transmitting.
Polite to Own Digital System Operation
This criteria applies only to digital channels, and also known as the admit criteria of "Color Code Free".
When configured for Polite to Own Digital System operation, the radio checks for an idle or busy channel, prior to allowing a transmission. This operation is similar to the Polite to All operation with exception that the radio is not polite to analog systems or other system transmissions. It is only polite to other traffic in its own system. This option is often used when there are no neighboring communications systems, or when there is no concern about interfering with radios in neighboring communication systems.
Polite to Other Analog System Operation
This criteria applies only to analogue channels, and also known as the admit criteria of "Correct PL".
When configured for Polite to Other Analogue System operation, the radio checks for an Idle or busy channel, prior to allowing a transmission. This operation is similar to the Polite to All operation with exception that the radio is not polite to analogue systems with the same PL. It is polite to other system transmissions. The radio checks for a PL match prior to allowing a transmission.
Polite, Impolite or Voice Interrupt In A Call
This is also known as "In Call Criteria", and applies only when the radio is participating in an active call.
The radio can optionally allow others that are part of the call to transmit impolitely (Always), to automatically clear the channel using the Voice Interrupt feature prior to beginning the voice transmission (Voice Interrupt), or to follow the previously configured channel access (Follow Admit Criteria). If configured for an In Call Criteria of Always, the user receives a Talk Permit Tone when they press the PTT while receiving a transmission for them. In other words, a radio has the ability to transmit over another user while listening to their transmission. However, when this happens, the other party does not stop transmitting and therefore RF contention can occur which may corrupt both transmissions. The In Call Criteria of Voice Interrupt is an alternative to the In Call Criteria of Impolite.
The Voice Interrupt option has advantages including the ability to avoid the previously described RF contention issue by clearing the channel prior to beginning a transmission, which yields a higher probability of successfully communicating with the intended target radio(s), as compared with the RF contention encountered with impolite transmissions. However, Voice Interrupt has disadvantages including a longer channel access time when an interruption is necessary, due to the signaling having to complete the interruption and handoff.
If configured for an In Call Criteria of Voice Interrupt, the radio user receives a Talk Permit Tone when PTT is pressed while receiving an interruptible voice transmission and the channel is successfully cleared down. In other words, a radio user has the ability to clear the channel of another user’s interruptible voice transmission before beginning their own voice transmission when both radios are participating in the same voice call (for example, both are members of the same group during a Group Call, or both are participating in the same Private Call). Depending on the radio’s CPS configuration, the radio user whose transmission was interrupted may or may not receive a Talk Prohibit Tone until the user releases the PTT. If the channel is not successfully cleared down, the user typically receives a Channel Busy Tone until the PTT is released.
For the Voice Interrupt feature to operate consistently, all radios using the channel should be provisioned with the ability to be interrupted. However, not all need to be provisioned with the Voice Interrupt capability.
If some radios are provisioned without the ability to be interrupted (for example, normally desirable for a supervisor’s radio), then those transmissions cannot be interrupted and the radio user receives a Channel Busy tone if the Voice Interrupt feature is attempted while receiving an uninterruptible voice transmission.
If configured for Follow Admit Criteria and the previously configured channel access (Admit Criteria) is set to either Channel Free or Color Code Free, the user receives a Transmit Denial Tone when they press the PTT while receiving a transmission for them. Users must wait until the user stops transmitting and call hangtime starts before they are granted a transmission. Utilizing the Channel Free Tone helps train users from transmitting too early. Although a setting of Always may be useful for speeding up conversations for well disciplined users, it may cause undisciplined users to “step over” other users.
Therefore, it is recommended that most users are provisioned with an In Call Criteria of Follow Admit Criteria.
Repeater Wake-up Provisioning
When there is no inbound traffic for a specified duration (Subscriber Inactivity Timer), the repeater stops transmitting and enters an inactive state. In this inactive state, the repeater is not transmitting, but instead it is listening for transmissions. When the user or radio needs to transmit through the repeater, the radio sends a wake-up message to the repeater.
Upon receiving the wake-up message, the repeater activates and begins transmitting idle messages. The radio then synchronizes with the repeater before it begins its transmission. The repeater wake-up sequence is configurable within the radio. The number of wake-up attempts (“TX Wakeup Message Limit“) and the time between the attempts (“TX Sync Wakeup Time Out Timer”) may be altered if required to operate with other vendor’s systems. It is recommended that these values remain at default while operating on MOTOTRBO systems.
In Capacity Max
In Capacity Max, there is only one Channel Access method: Polite to own.[2]
The radio does not need to check if the channel is free, as this is done by the system. It only needs to check for invalid activity on the channel (interference).
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.
For Emergency, a radio is able to interrupt an ongoing transmission if the target Talkgroup for emergency is busy.
Channel Access for Capacity max is set up in Radio Management.