Справочник Пользователя для AeroComm Corporation 4424200
3/23/2006
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5.2.3 Random Back Off
Random Back Off – If multiple LT4424 transceivers try to send packets out over the RF at the exact
same time, the packets will collide and will not be received by the intended receiver. In fact, if after a
collision occurs, both transceivers retry at the same time, the retry will also fail. To avoid further
collisions, a transceiver can be programmed to wait a random number of packet times (hops) before
resending its data. The amount of randomness is controlled by this parameter. Keep in mind that
selecting a larger value for Random Back Off will increase the overall latency of the LT4424. The
latency calculation becomes:
same time, the packets will collide and will not be received by the intended receiver. In fact, if after a
collision occurs, both transceivers retry at the same time, the retry will also fail. To avoid further
collisions, a transceiver can be programmed to wait a random number of packet times (hops) before
resending its data. The amount of randomness is controlled by this parameter. Keep in mind that
selecting a larger value for Random Back Off will increase the overall latency of the LT4424. The
latency calculation becomes:
Worst Case Latency = 8 ms Hop * # of retries * Maximum Random Value
[multiply by 16 ms if using Full Duplex mode]
Latency is a very important consideration when using a wireless device. The LT4424 has a 256 byte
interface buffer. If, due to latency, the radio cannot send the data out over the RF as fast as data is
coming into the radio over the serial interface, the buffer will eventually fill up. If data continues coming
into the radio once the buffer is full, the buffer will overflow and the new incoming data will be lost. It is
strongly recommended that the radio host monitor the CTS pin to avoid this situation. The transceiver
asserts this pin high as the buffer is filling to signal the OEM Host to stop sending data. The transceiver
will take CTS Low once the buffer becomes less full.
interface buffer. If, due to latency, the radio cannot send the data out over the RF as fast as data is
coming into the radio over the serial interface, the buffer will eventually fill up. If data continues coming
into the radio once the buffer is full, the buffer will overflow and the new incoming data will be lost. It is
strongly recommended that the radio host monitor the CTS pin to avoid this situation. The transceiver
asserts this pin high as the buffer is filling to signal the OEM Host to stop sending data. The transceiver
will take CTS Low once the buffer becomes less full.
Random Backoff Settings:
• 00h – Wait 1 packet time, then retry (Random Back Off is disabled)
• 01h – Wait 1 – 2 packet times, then retry
• 03h – Wait 1 – 4 packet times, then retry
• 07h – Wait 1 – 8 packet times, then retry
• 0Fh – Wait 1 – 16 packet times, then retry
• 1Fh – Wait 1 – 32 packet times, then retry
• 3Fh – Wait 1 – 64 packet times, then retry
• 7Fh – Wait 1 – 128 packet times, then retry
• FFh – Wait 1 – 256 packet times, then retry
•
• 01h – Wait 1 – 2 packet times, then retry
• 03h – Wait 1 – 4 packet times, then retry
• 07h – Wait 1 – 8 packet times, then retry
• 0Fh – Wait 1 – 16 packet times, then retry
• 1Fh – Wait 1 – 32 packet times, then retry
• 3Fh – Wait 1 – 64 packet times, then retry
• 7Fh – Wait 1 – 128 packet times, then retry
• FFh – Wait 1 – 256 packet times, then retry
•
5.3 D
UPLEX
M
ODE
In Half Duplex mode, the LT4424 will send a packet out over the RF when it can. This can cause
packets sent at the same time by a Server and a Client to collide with each other over the RF. To
prevent this, Full Duplex Mode can be enabled. Though the RF hardware is still technically half
duplex, it makes the radio seem full duplex. This can cause overall throughputs to be cut in half.
packets sent at the same time by a Server and a Client to collide with each other over the RF. To
prevent this, Full Duplex Mode can be enabled. Though the RF hardware is still technically half
duplex, it makes the radio seem full duplex. This can cause overall throughputs to be cut in half.
Note: All transceivers on the same network must have the same setting for Full Duplex.