MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL PACKET PROTOCOL
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DISC Initiates a disconnect.
UA Sent to acknowledge receipt of an SABM or DISC.
FRMR Sent when an abnormal condition occurs, i.e., the control byte
received is undefined or not proper protocol at the time received. UI frames UI
An I frame without a frame number. It is not acknowledged.
Channel Use and Timing Functions
The following discussions mention timing parameters which are set by various
commands. These timing functions are also discussed in Chapter 5.
An important part of any packet radio protocol is the means by which many
stations make efficient use of an RF channel, achieving maximum throughput
with minimum interference. The basis for this time domain multiplexing is
C arrier- S ensed M ultiple A ccess (CSMA) with collision detection and collision
avoidance.
CSMA means simply that (as every Amateur knows) no station will transmit if
the frequency is in use. The MFJ-TNC continually monitors for the presence of
an audio data carrier on frequency and transmits only if there is no carrier. (The
RF carrier is not normally detected; however, an input is available on the MFJ-
TNC radio interface connectors to allow such an input.) In order to make
detection of a busy channel more reliable, the MFJ-TNC sends an audio signal
(continuous flags) any time the transmitter is keyed and a packet is not being
sent, as during the transmitter keyup delay (
TXDELAY ), or while a slow audio
repeater is being keyed ( AXDELAY ).
By itself, CSMA is not enough to insure a minimum, or even low, interference
rate, due to the likelihood of simultaneous keyup by two or more stations. This
is where collision detection and collision avoidance come in. The MFJ-TNC
detects a collision by the absence of an ACK from the station it is sending to.
The receiving station does not acknowledge the frame that suffered the collision,
since either the FCS was incorrect or the packet was not heard. There are other
possible reasons for non-receipt of the packet, but the MFJ-TNC 's response is
based on the assumption of a collision.
After transmitting a packet, the MFJ-TNC waits a "reasonable" length of time
(
FRACK ) for an acknowledgment. "Reasonable" is determined by the link