Murata Electronics Corporation 8 6/2/2021
The above calculations are provided as a means of estimating the capacity of a multipoint
WIT2420 network. To determine the precise amount of capacity, you can actually set up the
radio system and then query the maximum data length from one of the remotes in control
mode to discover its exact setting. Divide this number by the hop duration as above to get
the remote's exact capacity.
2.2.4. Full Duplex Communication
From an application perspective, the WIT2420 communicates in full duplex. That is, both
the user application and the remote terminal can be transmitting data without waiting for the
other to finish. At the radio level, the base station and remotes do not actually transmit at the
same time. If they did, the transmissions would collide. As discussed earlier, the base
station transmits a synchronization signal at the beginning of each hop followed by up to
three packets of data. After the base station transmission, the remotes will transmit. Each
base station and remote transmission may be just part of a complete transmission from the
user application or the remote terminal. Thus, from an application perspective, the radios are
communicating in full duplex mode since the base station will receive data from a remote
before completing a transmission to the remote.
2.2.5. Error-free Packet Transmission Using ARQ
The radio medium is a hostile environment for data transmission. In a typical office or
factory environment, 1% - 2% of the 2.4GHz frequency band may be unusable at any given
time at any given station due to noise, interference or multipath fading. For narrowband
radio systems (and also many spread spectrum radio systems which use direct sequence
spreading), this would imply a loss of contact on average of over 30 seconds per hour per
station. The WIT2420 overcomes this problem by hopping rapidly throughout the band in a
pseudo-random pattern. If a message fails to get through on a particular channel, the
WIT2420 simply tries again on the next channel. Even if two thirds of the band are
unusable, the WIT2420 can still communicate reliably.
Data input to the WIT2420 is broken up by the radio into packets. A 24-bit checksum is
attached to each packet to verify that it was correctly received. If the packet is received
correctly, the receiving station sends an acknowledgment, or ACK, back to the transmitting
station. If the transmitter doesn't receive an ACK, at the next frequency hop it will attempt to
send the packet again. When ARQ is enabled, the transmitting radio will attempt to send a
packet packet attempts limit times before discarding the packet. A value of 00H disables
ARQ. When it is disabled, any transmission received with errors is discarded. It is the
responsibility of the user application to track missing packets. A second parameter, ARQ
Mode, allows the choice between using ARQ to resend unsuccessful transmissions or always
sending a transmission packet attempts limit times regardless of the success or failure of any
given transmission.
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