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MFJ-1278B MULTI-MODE PACKET RADIO PROTOCOL
through the same digipeaters to the originating station. Since the digipeated packets are not
acknowledged to the digipeater, an unsuccessful transmission must be retried from scratch by
the originating station. In order to help alleviate the congestion of the frequency that tends to
result when digipeated packets suffer collisions, the digipeater is given first shot at the
frequency every time it becomes clear. Other stations, instead of transmitting as soon as they
hear the channel clear, must wait a short time (DWAIT). This restriction applies to all
stations except the digipeater, which is permitted to transmit relayed packets immediately.
This prevents digipeated packets from suffering collisions except on transmission by the
originating station.
A special time delay (RESPTIME) is used as the minimum wait time prior to transmitting
acknowledgment frames, to prevent TNCs accepting data at high speed from the
asynchronous port from colliding with acknowledgment frames when fewer than
MAXFRAME packets are outstanding. The receiving TNC will wait long enough before
sending the ACK so that it will hear the data packet which would have caused the collision,
thus avoiding a fairly frequent source of delay in versions of AX.25 prior to 2.0.
CHANNEL FLOW CONTROL
Flow control of data through the link is determined by the rate at which data is being supplied
to a sending TNC and accepted from a receiving TNC.
A TNC receiving data from the link will send an RNR when the next I frame successfully
received will not fit into the buffer for output to the serial port.
Whenever a TNC transmitting data received from the serial port over the link runs out of
temporary buffer space, the serial port will be halted by an XOFF character or CTS signal. In
the MFJ-1278B implementation this happens whenever there are 7 packets built and less than
210 characters left in the buffer for input from the serial port.
When the TNC receiving data from the link clears out its buffers, it sends an RR to the
transmitting TNC. In order to guard against the possibility of the RR being lost and the link
becoming permanently locked, the transmitting TNC will periodically re-transmit the packet
that provoked the RNR. The receiving TNC will continue to respond with RNR until it can
accept the packet.