System Feature Overview 21
68007024085 April 2011
• Correct PL: This criteria is sometimes referred to as “Polite to Other System”, and is
applied only to analog 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 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 will check 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. Note that
although one site may be busy, the other non-busy sites will continue with the call.
2.2.3.1 Impolite Operation (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 will not proceed, if the repeater is hibernating and the radio is unable to wake it.
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