Ham Radio 2000/Ham Radio 2000.iso/ham2000/packet/p_tapr/117notes/softdisk.117

Text File | 1990-03-10 | 32KB | 857 lines

TAPR TNC 2 Software Release Notes ............ Release 1.1.7


The correct sign-on message for this release of TNC 2  soft-
ware is:

     Tucson Amateur Packet Radio TNC 2
     AX.25 Level 2 Version 2.0
     Release 1.1.7   2/11/90 - 32K RAM
     Checksum $51
     cmd:

TAPR  TNC2  Software Release 1.1.7 is a major  release  with
numerous enhancements.

Significant changes and enhancements include:

Prioritized Acknowledgment (PriACK) protocol due to N7CL.

Improved channel access methods (similar to "persistence").

Better  compatibility when running AX25 V1 (T3 may  be  dis-
abled).

A filter to inhibit monitoring of NET/ROM, ROSE, TCP/IP  and
other networking packets.

Improved  transmit  timers  (time-  and/or  character-based)
along with quick-release of PTT at the end of a frame.

Pre-frame  sync  of  zeroes rather than  flags  for  reduced
TXDELAY (the other station can lock on to your transmissions
faster).

No dead-time between frames in a multi-frame transmission.

Improved  KISS  operation in full duplex  for  MicroSat  and
other applications.

Improved buffer-full handling over normal AX.25 practice for
less channel congestion.

Improved modem demodulator calibration facilities.

Additional error (Health) counters.

Finally,  the  locations of the default  parameters  in  the
EPROM, and their format, will be documented for those of you
who  prefer  a custom EPROM rather than trusting  bbRAM  for
remote site locations.

Summary of New Commands and Messages

New defaults since 1.1.6

     CHECK    12
     DWAIT    33
     FRACK     8
     RESPTIME  0
     SCREENLN  0
     TXDELAY  33

New commands since 1.1.6

     ACKPRIOR  ON
     ACKTIME   14
     CHECKV1   OFF
     DEADTIME  33
     FIRMRNR   OFF
     MNONAX25  OFF
     RXCAL          (immediate command)
     SLOTS     3
     STATUS         (immediate command)
     TXDELAYC  2

New Health counters since 1.1.6

     ASYFRERR
     ASYQOVER
     RCVDRNR
     RXABORT
     RXLENERR
     RXRESYNC
     SENTRNR

Other changes

A "diddling" TXDELAY/TXDELAYC interval replaces the familiar
"flagging"  keyup  time.   Zeroes  are  sent  for   TXDELAY,
followed by the number of flags specified in TXDELAYC.

RS-232 characters with framing errors are ignored.

DAYTIME should be much more accurate.

Protocol handling fixes, and many smaller fixes to  numerous
to detail.

Complaints welcome, via mail or CIS only, to:

                     Howard Goldstein N2WX
                     5201-23rd Ave N
                     St. Petersburg, FL 33710-5174
                     USA

                     CIS:  75006,702


NOTE:   TNC 2 SOFTWARE SOURCE CODE and TNC FIRMWARE
                   Copyright (c) 1985-1990
                           Systek
                    All rights reserved.

Reproduction  or translation of any part of this work beyond
that  permitted by Sections 107 or 108  of  the 1976  United
States  Copyright Act (or its legal successor)  without  the
express  permission  of  the  copyright  owner  is  unlawful
except as noted below.   Requests for permission to copy  or
for further information should be addressed to:

                Tucson Amateur Packet Radio
                        PO Box 12925
                      Tucson AZ  85732
                    ATTN TNC 2 SOFTWARE


ACkprior ON|OFF                     Default: ON
                                           Display Group: L

Parameters:

     ON        Enables Prioritized acknowledgments.

     OFF       Acknowledgments have the same priority as any
               other frames on the channel.

When  ACKPRIOR is ON, acknowledgments have  priority.   This
means  that  is  your  TNC receives a  frame  that  it  must
acknowledge  (ACK), it will immediately send the ACK if  the
channel  is  clear.   If other  stations  are  running  with
ACKPRIOR ON, they will not access the channel until you have
had a chance to send the ACK, even if they cannot hear  your
transmissions.

When OFF, acknowledgments are queued up for transmission the
same as any other frame.



ACKTime n                         Default: 14
                                           Display Group: T

Parameters:

     n         0-250 in 10 mSec increments.

ACKTIME  specifies  the time required to send an RR  or  RNR
response  frame  (an acknowledgment) to an I  frame  at  the
radio port data rate.  This corresponds to about 510 mSec at
1200  bps with 8 digipeaters and about 140 mSec at 1200  bps
with  no  digipeaters.  The default of 140  mSec  (ACKT  14)
allows for no digipeaters.



CHECKV1 ON|OFF                    Default: OFF
                                          Display Group: T

Parameters:

     ON        Enables  CHECKtime  (T3) when  running  AX.25
               Level  2  Version 1.0  protocol.   This  will
               result   in  an  automatic  disconnect   when
               packets  haven't flowed between this TNC  and
               the  remote  TNC for CHECKtime.   See  CHECK,
               above.

     OFF       Disables   the  CHECKtime  timer   (T3)   and
               requires   the   operator   to   initiate   a
               disconnect  sequence.  This is in  accordance
               with AX.25 Level 2 Version 1.0 procedures.

When  ON,  the  CHECK (T3) timer is  used  to  automatically
disconnect  an  AX.25  Level 2 Version 1.0  link  when  data
hasn't  flowed for CHECK time.  Note that T3 is not  defined
for Version 1.0 operation.  See CHECK, above.

When  OFF, T3 is ignored and normal Version  1.0  procedures
apply.

NOTE:     Many operators prefer using Version 1.0  protocol,
          especially on marginal or noisy links in order  to
          eliminate  some of the overhead of AX.25  Level  2
          Version 2.0 protocol.



FIRMRnr ON|OFF                     Default: ON
                                           Display Group: L

Parameters:

     ON        When this TNC's buffers fill, an RNR is sent.
               When  the buffers are no longer full,  an  RR
               command frame is sent.

               When  this TNC receives an RNR, it  will  not
               poll  the sending TNC until [T0*10  seconds].
               T0 = CHECKtime*8 unless CHECKtime*8 is <30 or
               >253 in which case T0 = 12.

     OFF       When this TNC's buffers fill, an RNR is  sent
               as a response to an I frame.

               When this TNC receives an RNR, it will resend
               the data every FRACKtime (T1) until an RR  is
               received.

When  OFF,  standard  AX.25 Level  2  Version  2.0  protocol
procedures are used.  If a TNC sending data receives an RNR,
meaning the remote TNC cannot accept more data, the  sending
TNC will continuously re-send the data every T1 (FRACK) time
until an RR is received, meaning the remote TNC accepted the
data.   This often results in a lot of  unnecessary  channel
congestion.

When  FIRMRNR  is ON, this TNC will stop sending  data  upon
receipt  of an RNR.  If the remote TNC does not send  an  RR
before  [T0] (see above), this TNC will poll the remote  TNC
to  be sure it is still active.  If this TNC sent  the  RNR,
then  it  will send an RR when its buffers have  cleared  to
allow more data to be received.

NOTE:     The remote TNC should be capable of FIRMRNR opera-
          tion.  This mode is operable with all TNCs  having
          a  FIRMRNR settable to ON, and with  most  network
          nodes.   If  the  remote TNC  is  not  capable  of
          FIRMRNR  operation,  and if the channel  is  poor,
          there  will be additional delays in  getting  data
          passed between the TNCs.



MNonax25 ON|OFF                    Default: OFF
                                          Display Group: M

Parameters:

     ON        Monitors all frames with valid CRC.

     OFF       Monitors  AX.25 Level 2 Protocol frames  with
               no higher-level protocols (PID = F0).

When  OFF,  only  AX.25  Level 2  protocol  frames  will  be
displayed.   Any packets with level 3 or level  4  protocols
will  be  inhibited.   Thus, NET/ROM,  TCP/IP,  ROSE,  etc.,
networking frames will not be displayed.  These frames often
have  binary values buried in them which  cause  undesirable
responses on a normal screen display.

When  ON,  any  packet or frame with a  valid  CRC  will  be
displayed regardless of protocol.



RXCAL
                                         Display Group: NONE

RXCAL  is  an  immediate command.  It is  used  for  precise
alignment of the demodulator.  Briefly, after the  modulator
tones  are set using the normal CALIBRATE  routines,  analog
loopback  is  entered and RXCAL issued.  You then  tune  the
demodulator  until  the CON and STA LEDs are  both  lit,  or
slowly   alternate.    See  the  section   New   Calibration
Procedures for details on using this command with a TAPR TNC
2.



SLots n                          Default: 3
                                            Display Group: L

Parameters:

     n         0-127

SLOTS  specifies the number of "slots" from which to  choose
when  deciding to access the channel.  For example, SLOTS  3
means  there are three slots, each having a  probability  of
1/3 to be selected.  Each slot is DEADTIME long.  If SLOTS 0
is chosen, the TNC will act as if SLOTS 1 had been selected.

Other  implementations  of this idea of channel  access  use
"PERSISTENCE"  to decide the probability of  channel  access
when  the channel is clear and "PPERSISTENCE" to  enable  or
disable this feature.

A  low  value means a greater chance of attempting  to  send
data  when  the channel is clear, and a higher  value  means
less chance.  Normal AX.25 practice is to have every station
jump  on the channel when it goes clear, virtually  assuring
collisions and retries.

SLOTS  and its relation to the more common PPERSISTENCE  and
PERSISTENCE commands are given below:

SLOTS     PPERSISTENCE   PERSISTENCE    Probability to XMIT

   1           OFF            255       Jump on = 100%
   2           ON             127       50%
   3           ON              85       33%
   4           ON              63       25%
   5           ON              51       20%

and so forth.

A  typical  value would be 2 or 3 for a channel with  a  few
other  users and 5 to 7 for a fairly busy  channel.   Higher
values than about 10 imply the channel is too busy and other
frequencies should be used if available.

NOTE:     SLOTS is independent of ACKPRIOR.



STATUS
                                         Display Group: NONE

STATUS is an immediate command.  It returns the acknowledged
status  of the current outgoing packet link buffer.  If  all
outgoing  packets  are  acknowledged,  STATUS  returns   'No
Outstanding packets', otherwise it returns the  'Outstanding
packets'   message.    Note   that   this   command    works
independently of the RS-232 flow-control state of the TNC.

This command may be useful for special applications, like if
you  can't see the STATUS LED on the TNC, or you  want  your
computer to watch the status...



TXDELAYC n                     Default: 2
                                           Display Group: T

Parameters:

     n         0-120  specifying additional  transmit  delay
               time  added to TXdelay in terms of  CHARACTER
               TIME  at  the current radio port  data  rate.
               Flags  will  be sent during this time,  so  a
               value of 1 is required as a minimum.

TXDELAYC  is  used  to  fine-tune  transmit  delay  after  a
transmit command is issued and before data is sent.  Used in
conjunction  with  TXDELAY, it allows a link to be  set  for
maximum performance.

TXDELAY should be used to account for the time it takes  the
radio  (the slower of this TNC's radio and the remote  TNC's
radio) to switch between receive and transmit and allow  the
receiving  unit's  data carrier detect  (DCD)  circuitry  to
respond.   "0"  bytes are sent to allow the  remote  TNC  to
synchronize to the data stream sent during TXDELAY.

TXDELAYC then sends flags to start the frame.  If TXDELAYC 0
is  selected,  the TNC will act as if TXDELAYC  1  had  been
chosen.

With this method of keying up, a user may operate at various
HDLC data rates without having to reset TXDELAY if the  same
radios are used.  The summation of TXDELAY and TXDELAYC will
automatically adjust the keyup time for the varying rates.


New Health Counters


A number of counters have been added to TNC 2.  All of  them
are  16  bits wide, and are ALWAYS initialized  to  0000  on
power up or "RESTART".

These counters are displayed using the HEALTH option of  the
DISPLAY command.

     o-  ASYFRERR:  Increases  when a character received  on
                    the serial port has a framing error.   A
                    large count may indicate problems in the
                    serial  link  between the  TNC  and  the
                    computer or terminal.

     o- ASYQOVER:   Increases  when  the TNC's  async  input
                    buffer overruns.  A count greater than 0
                    indicates  the  host  computer  is   not
                    responding properly to flow control from
                    the  TNC.  It also means that data  have
                    been  lost, so you should take steps  to
                    find  out why this is happening and  get
                    it fixed!

     o- RCVDRNR:    Increases when this TNC receives an  RNR
                    frame.  A large count is normal.

     o- RXABORT:    Increases  when an aborted frame is  re
                    ceived from the HDLC controller.   Large
                    counts are not uncommon.

     o- RXLENERR:   Increases  when a received frame  has  a
                    length  error and indicates the TNC  has
                    temporarily become confused.  It may  be
                    a result of a bug in the SIO chip and  a
                    count above 0 is no cause for alarm.

     o- RXRESYNC:   Indicates  the number of times  the  TNC
                    has recovered from RXLENERR  conditions.
                    In  firmware  prior to  1.1.7,  the  TNC
                    would  simply  hang until  RESTARTed  or
                    RESET.

     o- SENTRNR:    Increases  when  this TNC sends  an  RNR
                    frame.  This is useful  for  determining
                    the   effectiveness   of   the   FIRMRNR
                    routines.

New Calibration Procedures

Modem Calibration

The TAPR TNC 2 contains a versatile,  user configurable  FSK
modem that is suitable for both 1200 baud packet use as well
as lower data rate use, such as 200 Hz shift 300 baud use on
HF links.  Calibration of the modem is usually only required
when  the  unit is initially put into operation or when  you
want to change the modem characteristics.   You may want  to
check  the calibration if you have difficulty connecting  to
other  packet  stations,  but recalibration is normally  not
necessary.

Calibration  of the TNC on-board modem  is  straightforward.
First, determine the data rate at which you wish to operate.
The  required shift may then be determined,  followed by the
actual  FSK frequencies to use.   While you may select  from
any tones in the normal audio range,  there are a few simple
rules that may help.

 1.  The TNC only supports 300 baud and 1200 baud  operation
     of the radio link using the on-board modem.   While the
     modem is capable of operating at other data rates,  the
     TNC  data-rate  generator circuit only supplies  clocks
     for 300,  1200 and 9600 baud (9600 baud is for use with
     an external modem).

 2.  Most  1200 baud operation uses 1000 Hz shift and  tones
     of  1200 Hz and 2200 Hz.  These are the  default  tones
     for which the TNC modem is optimized, using the  compo-
     nents supplied in the kit.  However, 800 Hz shift  with
     tones of 1300 Hz and 2100 Hz may provide improved oper-
     ation.   This tone-pair is compatible with  TNCs  using
     1000 Hz shift modems.

 3.  Most  HF operation below 28 MHz uses 300 baud,  200  Hz
     shift FSK.   TAPR recommends 1600 Hz and 1800 Hz as the
     modulator tone frequencies, but any pair of frequencies
     with  this separation,  modulating an SSB  transmitter,
     will  work.   If your demodulation center frequency  is
     1700 Hz,  your transmissions will be readable  whenever
     you  are able to copy the other station,  regardless of
     his actual modulation frequencies.   Incidentally,  the
     sideband may be chosen at will due to the nature of the
     data, although most stations conventionally use LSB.

In  the  example below we will set up the modem  for  normal
1200 baud operation with 800 Hz shift.

The  software  calibration routine  compares  the  frequency
being  calibrated against a reference signal.   The  desired
frequency  is  specified by setting a count,  n,  using  the
command CALSET.

For the modulator, determine the calibration count according
to the following formula:

          n = (525,000 / f) + 1

where  f is the desired frequency  in  Hertz.   Substituting
1300 and 2100 Hz, we end up with:

          n = 405 (for 1300 Hz)
          n = 251 (for 2100 Hz)

To  set  the  PLL demodulator, we  use  the  RXCAL  command,
explained below.

Modulator Adjustment

The  next step is to preset the trimpots for the  modulator.
When adjusting the trimpots,  you should hear a soft "click"
when  the  element  is adjusted to the end  of  its  travel.
Turning  the  element  past this point will  not  result  in
damage to the trimpot;  however, it is suggested that you do
not  make  a practice of turning the adjustment  screw  past
this  point  too much.   Use a small-tipped  screwdriver  or
special trimpot adjusting tool when adjusting the trimpots.

Preset  R77  20 turns counter-clockwise (CCW) or  until  you
hear the element "click" at full CCW rotation.  R77 controls
the  higher of the two modulator output tones and this  sets
it to its maximum frequency.

Preset  R78  20 turns clockwise (CW) or until you  hear  the
element "click" at full CW rotation.  R78 controls the lower
of  the  two modulator output tones and this sets it to  its
lowest frequency.

Turn  off the power to the TNC and remove it from its  cabi-
net.   To protect the battery backed-up RAM,  be sure to set
the TNC on a non-conducting surface -- a sheet of paper will
do.   Attach your terminal and follow the standard  power-up
procedure.  When you see the command mode prompt, enter

          cmd:CALSET 405

or  use the value appropriate for the lower modulator  tone.
Next, enter

          cmd:CALIBRA

This  starts  the software calibration routine.   The  "com-
mands" to the calibration routine are single characters, and
require no carriage return.

Install  a push-on jumper at JMP9 pins 1 and 6 (the top  two
pins of JMP9  nearest  C44) and  another  push-on  jumper at
JMP4.

NOTE:     During  calibration the STA and CON LEDs are  used
          as  frequency  indicators.  When the  STA  LED  is
          glowing,  the  frequency is too  high,  go  lower.
          When the CON LED is glowing, the frequency is  too
          low, go higher.

Press the K key on your computer.  Either the STA or CON LED
should illuminate.   Next,  press the space bar on your key-
board and verify that the other LED (CON or STA) illuminates
in  place  of  the one illuminated in  the  preceding  step.
Press the space bar again until the CON LED is illuminated.

Slowly rotate the adjustment screw on trimpot R78 in the CCW
direction until both the STA and CON LEDs  illuminate.   The
LEDs  are a bit slow to respond,  so be careful when the STA
LED begins to flicker.   You will probably notice a  portion
of the adjustment when both LEDs illuminate; set the adjust-
ment  screw to the center of this band.   If you go too far,
only the STA LED will be illuminated; in this case, turn the
adjustment screw CW.

When  both LEDs are steadily illuminated,  press the Q  key.
Both  LEDs  should extinguish and the  command  mode  prompt
should reappear.  The lower tone is now calibrated.

To calibrate the upper tone, enter

          cmd:CALSET 251

or  use the value appropriate for the higher modulator tone.
Next, enter

          cmd:CALIBRA

Now press the K key.   Either the CON or the STA LED  should
illuminate.  Tap the space bar until the STA LED glows.

Slowly  rotate the adjustment screw of trimpot R77 CW,  fol-
lowing the instructions above for setting R78.  Continue the
adjustment until both the STA and CON LEDs glow steadily.

When  both LEDs are steadily illuminated,  press the Q  key.
Both  LEDs  should extinguish and the  command  mode  prompt
should reappear.   Remove the jumpers at JMP9 pins 1 & 6 and
at JMP4.

Demodulator Adjustment

NOTE:     Do  not remove the jumper at JMP8 for  demodulator
          calibration!

To calibrate the PLL demodulator, preset R79 20 turns clock-
wise  (CW) or until you hear the element "click" at full  CW
rotation.   Place  a jumper across JMP9 pins 3  and  4  (the
lowest two pins -- these are the two pins nearest the  silk-
screen legend "C43").  Install a push-on jumper at JMP4  (to
disable  the transmitter PTT watchdog timer) and another  at
JMP7 (to establish "analog loopback").  Enter

          cmd:RXCAL

at the computer or terminal keyboard.

Now press the K key.  The STA LED should illuminate.

Slowly rotate the adjustment screw of R79 CCW until both the
STA  and CON LEDs glow.  This is a very difficult  condition
to achieve, partly because the indicator is quite  sensitive
and  partly because the LEDs lag the adjustment by a  second
or  two.   However,  if you establish the  position  of  the
trimpot  where the LEDs toggle and leave R79 set  to  within
about 1/4 turn of that position there will be no degradation
in performance.

Type Q and note that the command mode prompt reappears.  Re-
move  the jumpers at JMP4, JMP7 and JMP9.  Your TNC 2  modem
is now calibrated to the tones you have selected.

Other Considerations

If  you are operating at a data rate other than  1200  baud,
the  loop timing components on your demodulator will not  be
optimum.  The table below lists the component values for the
two most common FSK pairs as well as CALSET parameters.

               Modem Calibration Parameters.

           Data rate      1200 baud      300 baud
           Shift           800 Hz        200 Hz
           Center Freq    1700 Hz        1700 Hz
           Upper Freq     2100 Hz        1800 Hz
           Lower Freq     1300 Hz        1600 Hz

           C54            2200 pF        0.01 uF
           R93            47.5 k         226 k

           CALSET Parameters

           High Tone (R77) 251           293
           Low Tone  (R78) 405           329

     NOTE:  Components used for R93 and C54 should have
     temperature-stable characteristics.

Finally,  it  is best to match the IF bandwidth of your  re-
ceiver to the data rate you will operate.   For 300 baud  HF
operation,  a  filter of about 500 Hz bandwidth is  optimum;
any  CW filter wider than this should be better than an  SSB
bandwidth filter!

Prioritized Acknowledgment - An Overview

THE IDEA

The  idea  behind the  prioritized  acknowledgment  (PriACK)
protocol is quite simple.  Acknowledgments (ACKs) get prior-
ity  access  to  the  channel so that  time  is  not  wasted
retrying packets that have already been correctly copied  by
the remote TNC.  In normal AX.25 practice, for a variety  of
reasons, the ACK is often not received within the time limit
defined by the FRACK (T1) protocol timer.

In  fact, because "channel access" methods are  not  clearly
defined  in the AX.25 Level Two Version 2.0  protocol  docu-
ment,  the  present protocol is more likely  to  synchronize
collisions  with acknowledgment packets than with any  other
type of packet!

THE PROBLEM

When  a channel becomes clear, if your TNC has something  to
send, it will immediately send it.  Unfortunately, if anyone
else's TNC has something to send, it will also "jump on" the
channel  as  soon as it becomes clear.  The result  is  col-
lisions  and retries.  Only after a collision does the  pre-
sent  protocol suggest using random backoff.  Thus, an  ack-
nowledgment to a just-received packet is almost assured of a
collision-and-retry sequence, even thought the TNC correctly
received the packet.  This will usually cause the TNC  which
is awaiting the ACK to time out and re-send the data.

To this collision synchronization mechanism, AX.25 Level Two
Version  2.0 adds a propensity to cause even ACKs which  are
not  from  hidden terminals, and thus  less  susceptible  to
collision,  to be delayed beyond even generous  FRACK  timer
settings when the channel gets busy.

Once  FRACK  times  out, even if the ACK  finally  makes  it
through  before  the retry is sent, the original  packet  is
retried  anyway.  This wastes a lot of time which  could  be
better  used clearing the channel of some of the  legitimate
offered load.

This  feature of AX.25 Level Two protocol accounts for  much
of  the abysmal performance of the currently popular  NETROM
and  THENET  nodes when used with  omnidirectional  antennas
systems.

HOW IT WORKS

PriACK  protocol  avoids the above problems by  giving  ACKs
priority access to the channel.  It does this in such a  way
that  even  ACKs coming from hidden  terminals  are  usually
protected from collision.

Standard  AX.25  protocol gives a limited  version  of  this
priority access only to digipeated frames through the  DWAIT
timer.

Ack prioritization works with slotted channel access in  the
following way:

1.   Response frames (ACKs) are always sent immediately upon
     receipt  of a valid packet.  Ultimately, not  even  DCD
     will  be checked for sending an ACK.  However, in  this
     release DCD will still hold an ACK off the channel.

2.   Stations  queued up to access the channel  but  waiting
     for a channel busy condition (DCD true) to clear,  will
     start a slotted access procedure only after enough time
     for  a response frame to clear the channel  has  trans-
     pired (ACKTIME).  This is true even if the TNC  waiting
     to use the channel cannot hear the ACK being sent.

3.   Slot  time windows (DEADTIME) are set large  enough  to
     ensure the local TNC will be able to accurately  deter-
     mine  if another detectable station has started  trans-
     mitting in any slot preceding the slot selected by  the
     local TNC.  This prevents two TNCs which have  selected
     adjacent slots from colliding.

As  you can see, under this protocol there will never  be  a
condition where an ACK is delayed from being sent beyond the
FRACK  timer limitation.  However, the FRACK timer is  still
active  and  must be set to a value that is long  enough  to
allow  time for (MAXFRAME * PACLEN) + ACKWAIT to expire  be-
fore  FRACK  does.  This time will depend on the  radio  and
radio  channel data rate in use.  The TNC knows that  if  it
doesn't see the ACK immediately when expected, it is probab-
ly never going to see it.  (See discussion of new parameters
below for definition of ACKWAIT.)

Forcing a channel access delay for all stations on the chan-
nel  which are waiting for a chance to transmit allows  ACKs
from hidden terminals to get back to the expecting  station.
This clears that traffic from the offered channel load.   If
the  packet was indeed copied and ACKed, further retries  of
the same information will not be necessary.

NEW PARAMETERS

There   are  several  new  parameters  needed   for   PriACK
operation.   See the command description elsewhere  in  this
document for their meanings and settings.

1.  ACKPRIOR

2.  DEADTIME

3.  ACKTIME

4.  SLOTS

5.  ACKWAIT (not directly user settable)

This  timer  actually  consists  of the  sum  of  ACKTIME  +
DEADTIME.   It  represents the time the queued up  TNC  will
wait before transmitting if it has selected the first  slot.
This  allows time for a hidden station's ACK to be  sent  on
the channel.

It  is  split into two timers because it has  two  unrelated
components.  One is related only to the data rate being used
on the channel (ACKTIME).  The other is related only to  the
absolute  time  the radios and TNC DCD circuits  require  to
determine that a transmitter is active after PTT is asserted
(DEADTIME).

OTHER RELATED PARAMETERS

Several of the "standard" TNC parameters will affect  PriACK
operation.   Most  of these are parameters in  TNCs  on  the
channel which are not using PriACK.

FRACK

FRACK  must  be  set  correctly in  any  TNC  regardless  of
protocol.

FRACK should be set to a value which allows time for the TNC
to  send  its packet and receive the ACK.  This  value  will
depend  on the settings being used for PACLEN and the  radio
port  data rate.  This is because, in many TNCs,  the  FRACK
timer  starts at the beginning of the packet rather than  at
the  end!  Thus, it is best to set FRACK to its  worst  case
value for whatever data rate you are using on the radio port
and still allow for MAXFRAME 7 PACLEN 255 packets to be sent
and  allow other non-PriACK stations to collide.   This  way
you  can make adjustments to PACLEN without having to  worry
about interaction with the setting of FRACK.

FRACK  should be set to 8 for 1200 baud work and 16 for  300
baud operation.

RESPTIME

RESPTIME should be set to 0 in your TNC when running PriACK.
With  proper  DCD  operation, RESPTIME 0 will  work  in  any
event.

If there are some radios on the packet channel with  abysmal
transmit-to-receive  recovery  times that cannot  copy  your
ACKs,  you may have to increase your TXDELAY or use a  value
of  RESPTIME greater than 0 to accomodate them  until  those
stations can replace their slow radios with more  acceptable
ones.

DWAIT

DWAIT  is ignored when running PriACK. However, when  PRiACK
is  off, or if you are using a TNC which does not  yet  have
PriACK capability, DWAIT should be set equal to DEADTIME, or
33 (330 mSec) for the default.

TXDELAY

TXDELAY should be set to allow for the slowest radio in  the
channel.  It should be the same as or greater than  DEADTIME
and  DWAIT.   The default of 33 (330 mSec) is  adequate  for
most work.

INITIAL PARAMETER SETTINGS SUMMARY

The settings for use at 1200 baud on a VHF FM channel are:

ACKPRIOR ON
ACKTIME  14
DEADTIME 33
DWAIT    33
FRACK     8
MAXFRAME  1-7  depending on channel quality
PACLEN   64-255 depending on channel quality
RESPTIME  0    (If  you  require a nonzero  value  for  this
               parameter   in   order  to   prevent   ACKing
               individual packets in a MAXFRAME greater than
               1  blast,  your DCD circuit is  not  working.
               Please get it fixed.)
SLOTS     3
TXDELAY  33

The  settings to use for 300 baud work on a linear  mode  HF
channel are:

ACKPRIOR ON
ACKTIME  52
DEADTIME  8
DWAIT     8
FRACK    16
MAXFRAME  1
PACLEN   32 to 128 depending on channel quality
RESPTIME  0
SLOTS     3
TXDELAY   8