PacComm TINY-2 - Channel Use and Timing

PACKET
RADIO
PROTOCOL
Channel
Use
and
Timing
eREJ
Used
to
request
retransmission
of
frames
starting
from
“x”,
Missed
frames
are
detected
by
receiving
a
frame
number
different
from
that
expected.
eDM
Sent
in
response
to
any
frame
received
other
than
a
connect
req
est
(SABM)
when
the
packet
controller
is
disconnected.
Sent
in
response
to
an
SABM
whenever
the
packet
controller
can’t
connect
to
the
re
esting
user,
e.g.,
if
the
packet
controller
is
already
connected
or
CONOk
is
OEF.
eSABM
Set
Asynchronous
Balanced
Mode
-
initiates
a
connect.
eDISC
Initiates
a
disconnect.
eUA
Sent
to
acknowledge
receipt
of
an
SABM
or
DISC.
eFRMR
Sent
when
an
abnormal
condition
occurs,
i.e.,
the
control
byte
received
is
undefined
or
not
proper
protocol
at
the
time
received.
eUI
An
I
frame
without
a
frame
number.
It
is
not
acknowledged.
Channel
Use
and
Timing
.
The
following
discussions
mention
timing
parameters
which
are
set
by
various
commands.
An
important
part
of
any
/
packet
radio
protocol
is
the
means
b
which
many
stations
make
efficient
use
of
an
RF
channe
achieving
maximum
throughput
with
minimum
interference.
The
basis
for
this
time
domain
multiplexing
is
Carrier-Sensed
Multiple
Access
(CSMA)
with
collision
detection
and
collision
avoidance.
CSMA
means
simply
that
no
station
will
transmit
if
the
frequen-
cy
is
in
use.
The
packet
controller
continually
monitors
for
the
resenice
of
packet
frame
flags
on
frequency
and
transmits
only
if
there
is
no
activity
present.
In
order
to
make
detection
of
a
busy
channel
more
reliable,
the
packet
controller
sends
an
audio
signal
(continuous
flags)
any
time
the
transmitter
is
keyed
and
apacket
is
not
being
sent,
as
during
the
transmitter
keyup
delay
f
clay
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
packet
controller
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
rer
Operating
Manual,
4th
Ed.
©
PacComm,
1990
43