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This is the 6pack-mini-HOWTO, written by

Andreas Könsgen DG3KQ

Internet: ajk@iehk.rwth-aachen.de

AMPR-net: dg3kq@db0pra.ampr.org

AX.25:    dg3kq@db0ach.#nrw.deu.eu

Last update: April 7, 1998

1. What is 6pack, and what are the advantages to KISS?

6pack is a transmission protocol for data exchange between the PC and

the TNC over a serial line. It can be used as an alternative to KISS.

6pack has two major advantages:

- The PC is given full control over the radio

  channel. Special control data is exchanged between the PC and the TNC so

  that the PC knows at any time if the TNC is receiving data, if a TNC

  buffer underrun or overrun has occurred, if the PTT is

  set and so on. This control data is processed at a higher priority than

  normal data, so a data stream can be interrupted at any time to issue an

  important event. This helps to improve the channel access and timing

  algorithms as everything is computed in the PC. It would even be possible

  to experiment with something completely different from the known CSMA and

  DAMA channel access methods.

  This kind of real-time control is especially important to supply several

  TNCs that are connected between each other and the PC by a daisy chain

  (however, this feature is not supported yet by the Linux 6pack driver).

- Each packet transferred over the serial line is supplied with a checksum,

  so it is easy to detect errors due to problems on the serial line.

  Received packets that are corrupt are not passed on to the AX.25 layer.

  Damaged packets that the TNC has received from the PC are not transmitted.

More details about 6pack are described in the file 6pack.ps that is located

in the doc directory of the AX.25 utilities package.

2. Who has developed the 6pack protocol?

The 6pack protocol has been developed by Ekki Plicht DF4OR, Henning Rech

DF9IC and Gunter Jost DK7WJ. A driver for 6pack, written by Gunter Jost and

Matthias Welwarsky DG2FEF, comes along with the PC version of FlexNet.

They have also written a firmware for TNCs to perform the 6pack

protocol (see section 4 below).

3. Where can I get the latest version of 6pack for LinuX?

At the moment, the 6pack stuff can obtained via anonymous ftp from

db0bm.automation.fh-aachen.de. In the directory /incoming/dg3kq,

there is a file named 6pack.tgz.

4. Preparing the TNC for 6pack operation

To be able to use 6pack, a special firmware for the TNC is needed. The EPROM

of a newly bought TNC does not contain 6pack, so you will have to

program an EPROM yourself. The image file for 6pack EPROMs should be

available on any packet radio box where PC/FlexNet can be found. The name of

the file is 6pack.bin. This file is copyrighted and maintained by the FlexNet

team. It can be used under the terms of the license that comes along

with PC/FlexNet. Please do not ask me about the internals of this file as I

don't know anything about it. I used a textual description of the 6pack

protocol to program the Linux driver.

TNCs contain a 64kByte EPROM, the lower half of which is used for

the firmware/KISS. The upper half is either empty or is sometimes

programmed with software called TAPR. In the latter case, the TNC

is supplied with a DIP switch so you can easily change between the

two systems. When programming a new EPROM, one of the systems is replaced

by 6pack. It is useful to replace TAPR, as this software is rarely used

nowadays. If your TNC is not equipped with the switch mentioned above, you

can build in one yourself that switches over the highest address pin

of the EPROM between HIGH and LOW level. After having inserted the new EPROM

and switched to 6pack, apply power to the TNC for a first test. The connect

and the status LED are lit for about a second if the firmware initialises

the TNC correctly.

5. Building and installing the 6pack driver

The driver has been tested with kernel version 2.1.90. Use with older

kernels may lead to a compilation error because the interface to a kernel

function has been changed in the 2.1.8x kernels.

How to turn on 6pack support:

- In the linux kernel configuration program, select the code maturity level

  options menu and turn on the prompting for development drivers.

- Select the amateur radio support menu and turn on the serial port 6pack

  driver.

- Compile and install the kernel and the modules.

To use the driver, the kissattach program delivered with the AX.25 utilities

has to be modified.

- Do a cd to the directory that holds the kissattach sources. Edit the

  kissattach.c file. At the top, insert the following lines:

  #ifndef N_6PACK

  #define N_6PACK (N_AX25+1)

  #endif

  Then find the line

  int disc = N_AX25;

  and replace N_AX25 by N_6PACK.

- Recompile kissattach. Rename it to spattach to avoid confusions.

Installing the driver:

- Do an insmod 6pack. Look at your /var/log/messages file to check if the

  module has printed its initialization message.

- Do a spattach as you would launch kissattach when starting a KISS port.

  Check if the kernel prints the message '6pack: TNC found'.

- From here, everything should work as if you were setting up a KISS port.

  The only difference is that the network device that represents

  the 6pack port is called sp instead of sl or ax. So, sp0 would be the

  first 6pack port.

Although the driver has been tested on various platforms, I still declare it

ALPHA. BE CAREFUL! Sync your disks before insmoding the 6pack module

and spattaching. Watch out if your computer behaves strangely. Read section

6 of this file about known problems.

Note that the connect and status LEDs of the TNC are controlled in a

different way than they are when the TNC is used with PC/FlexNet. When using

FlexNet, the connect LED is on if there is a connection; the status LED is

on if there is data in the buffer of the PC's AX.25 engine that has to be

transmitted. Under Linux, the 6pack layer is beyond the AX.25 layer,

so the 6pack driver doesn't know anything about connects or data that

has not yet been transmitted. Therefore the LEDs are controlled

as they are in KISS mode: The connect LED is turned on if data is transferred

from the PC to the TNC over the serial line, the status LED if data is

sent to the PC.

6. Known problems

When testing the driver with 2.0.3x kernels and

operating with data rates on the radio channel of 9600 Baud or higher,

the driver may, on certain systems, sometimes print the message '6pack:

bad checksum', which is due to data loss if the other station sends two

or more subsequent packets. I have been told that this is due to a problem

with the serial driver of 2.0.3x kernels. I don't know yet if the problem

still exists with 2.1.x kernels, as I have heard that the serial driver

code has been changed with 2.1.x.

When shutting down the sp interface with ifconfig, the kernel crashes if

there is still an AX.25 connection left over which an IP connection was

running, even if that IP connection is already closed. The problem does not

occur when there is a bare AX.25 connection still running. I don't know if

this is a problem of the 6pack driver or something else in the kernel.

The driver has been tested as a module, not yet as a kernel-builtin driver.

The 6pack protocol supports daisy-chaining of TNCs in a token ring, which is

connected to one serial port of the PC. This feature is not implemented

and at least at the moment I won't be able to do it because I do not have

the opportunity to build a TNC daisy-chain and test it.

Some of the comments in the source code are inaccurate. They are left from

the SLIP/KISS driver, from which the 6pack driver has been derived.

I haven't modified or removed them yet -- sorry! The code itself needs

some cleaning and optimizing. This will be done in a later release.

If you encounter a bug or if you have a question or suggestion concerning the

driver, feel free to mail me, using the addresses given at the beginning of

this file.

Have fun!

Andreas