System Feature Overview 23
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.
NOTE: 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.
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 will
occur and it is unclear which source will be 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.
When operating in Capacity Plus or Linked Capacity Plus modes, the impolite operation is
supported only in Emergency Calls.
2.2.3.2 Polite to All Operation (Admit Criteria 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 each other’s transmissions.
However, when this option is used, any strong signal on the channel blocks other users from
transmitting.
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