WIT2410
©
2000- 2005 Murata
Inc
10
M-2410-0000 Rev G
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.
2.2.5. Full Duplex Communication
From an application perspective, the WIT2410 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 a packet 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.
2.2.6. 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 WIT2410 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 WIT2410 simply tries again on the next channel. Even if two
thirds of the band is unusable, the WIT2410 can still communicate reliably.
Data input to the WIT2410 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.
All of this error detection and correction is transparent to the user application. All the
user application sees is error-free data from the modem. However, if the ARQ mode is
disabled, transmissions with errors are discarded, and missing data detection will be the
