PacComm TINY-2 - Page 51

PACKET
RADIO
PROTOCOL
Channel
Use
and
Timing
reasons
for
non-receipt
of
the
packet,
but
the
packet
controller’s
response
is
based
on
the
assumption
of
a
collision.
After
transmitting
a
packet,
the
packet
controller
waits
a
“reasonable”
lengthof
time
(FRack)
for
an
acknowledgment.
“Reasonable”
is
determined
by
the
link
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
dif-
ferent
random
wait
times
to
provide
a
reasonable
chance
of
two
or
more
stations
selecting
different
values.
The
difference
be-
tween
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 optimizin;
data throughput. The currently implemented link protoco'!
provide for relaying (digipeating) of packets. The acknow-
ledgment 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 ex-
cept on transmission by the originating station.
44 Operating Manual, 4th Ed. © PacComm, 1990