User's Manual

001-5008-000 rev5_2.doc Page 58
The RTS threshold parameter specifies how large a packet must be before the unit will use
RTS/CTS handshaking in the over-the-air protocol. A value of 0 means the ViPR will always
use over-the-air RTS/CTS handshaking. A value equal to the RF_MTU (OTA maximum
transmit unit) means the ViPR will never use RTS/CTS handshaking. A value of 128 means
the ViPR will use RTS/CTS for packets larger than 128 bytes.
Note: This should not be confused with RTS/CTS for RS232 Serial ports.
7.1.2 Carrier Sense Level Threshold
Default = -110 dBm. This is the threshold the ViPR uses to determine whether a received
RF signal is a valid message or unwanted noise. If an RF level higher than the Carrier
Sense Level Threshold is detected, the ViPR will attempt to decode the signal and will not
transmit until the RF level drops. Outgoing data will be buffered until the channel is
available. The carrier sense may be raised to prevent false carrier sense detection if the
network is installed in a noisy environment. In certain situations where the ambient RF
noise level is very low, the carrier sense level threshold can be lowered to gain extra receive
sensitivity. (The ViPR's specified receive sensitivity depends upon the channel
bandwidth/speed being used. Refer to the product specification in Appendix A for details.)
7.1.3 Listen Before Transmit
Default = Enabled (listen to noise and data). The ViPR radio has the ability to receive on
the Rx frequency to determine if the RF channel is busy. When the RF channel is busy the
ViPR can buffer any data that needs to be sent over the air and will transmit when the RF
channel is free. There are three modes available in the ViPR for the Listen Before Transmit
feature. They are described in the following paragraphs.
Enabled (listen to noise and data): This is the default mode for the ViPR. The ViPR will
monitor the RF level on the receive channel. When the received level is above the carrier
sense threshold the ViPR will try to receive and decode any and all messages from remote
ViPRs. In this mode, the ViPR will wait to transmit any data until the received level falls
below the carrier sense threshold. The received level will rise above the carrier sense
threshold due to several scenarios:
1) The ViPR is receiving valid data
2) The ViPR is not receiving data because two or more ViPRs are transmitting at the
same time causing a collision
3) The ViPR is not receiving data because the RF level is right at or below data
sensitivity or
4) There is interference from another RF system or electrical devices on the
frequency that the ViPR is operating on.
In any of these scenarios, the ViPR waits to transmit any data until the RF level falls below
the carrier sense threshold. This ensures that the data will have the best chance of
reaching its destination.
Enabled (listen to data only): In this mode, the ViPR will monitor the RF level on the
receive channel. When the received level is above the carrier sense threshold the ViPR will
try to receive and decode any and all messages from remote ViPRs.