MFJ-1270C/1274C MANUAL PACKET PROTOCOL
Page 230
activity, frame length, whether the packet is being digipeated, and other time-
related factors. If no ACK is received, the packet must be re-sent. If the
unACKed frame was lost due to a collision, the assumption is that there is at
least one other packet station out there that also lost a frame and will probably
have exactly the same criterion for deciding when to retry the transmission as
this station is using.
In order to avoid a second collision, the collision avoidance protocol calls for the
stations retrying transmissions to wait a random time interval after hearing the
frequency become clear before they key their transmitters. There must be
enough different random wait times to provide a reasonable chance of two or
more stations selecting different values. The difference between adjacent time
values must be similar to the keyup time delay of typical stations on the
frequency. This is the time lapse after a station keys its transmitter before other
stations detect its presence on the channel, and is a function of the keying
circuitry of the transmitter and the signal detection circuitry of the receiver. We
have chosen the random time to be a multiple (0-15) of the transmitting station's
keyup delay (
TXDELAY
). This is reasonable if one's own keyup delay is
similar to that of other stations on the channel.
One other factor must be taken into consideration in optimizing data throughput.
The currently implemented link protocols provide for relaying (digipeating) of
packets. The acknowledgment procedure for such packets is that the relay
station simply repeats packets without acknowledgment to the sending station.
The receiving station sends its ACK back 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
