Dale Wykoff, N5EIA,

and

David Dickey, KN4TEM

​

PART 1

Introduction to AX.25

Digital Modes Overview

Motivation: Why not just use phone or CW?

• Quantity of information
• Can’t make a code for everything, e.g., Q-Codes
• Noise resilient
• Data
• Multimedia
• Images
• Movies
• Large corpus of text
• Precise locations
• EMR (medical records)
• Remote Commands
• Computer Networking
• The physical layer of the OSI model

​

What is a digital mode?

Binary is just another number system, and these numbers are either data or commands.

Digital Mode Examples

• AX.25
• PSK31/61
• RTTY
• JT-65
• JT-4
• JT-9
• FT-8
• FT-4
• QRA-64
• ISCAT
• MSK144
• WSPR
• SSTV
• ATV

• FAX
• CW
• Trunked
• P25
• OpenSky
• TETRA
• Olivia
• Contestia
• Domnino
• Hellschreiber
• FSQ
• IKFP
• MT63
• QPSK

​

• 8PSK
• PSKR
• THOR
• Wefax
• Naxtex/Sitor B
• Digital Voice
• D-Star
• DMR
• System Fusion
• FreeDV
• 802.11
• OFDM
• DSSS
• LTE

Image taken from sarc.org.au

Examples:��

• RTTY
• SSTV
• PSK-31
• Hellschreiber
• Thor
• AX.25

See : DigitalModes (w1hkj.com)

For our purposes, think of digital information, being conveyed over the air by first converting it into sound and broadcasting the sound as you do your own voice.

​

The receiving end can take the audio and turn it back into a stream of bits.

Where can we use digital modes?

Behold, the Band Plans!

What is available to who?

Where can we use digital modes?

Example regional band plan for 70 cm band

Example regional band plan for 2m band

These conventions exist for every band and can vary by geography.

Restrictions

Legal:

Physical:

• FCC symbol rate limitations
• Parts 97.305(c) and 97.307(f)
• Depend on band
• The lower the frequency, the lower the bandwidth
• Power limitations

• Shannon-Hartley theorem
• A given channel may only carry so much information
• Nyquist sampling
• Take samples of the waveform at a certain rate
• More power does not always create a better channel
• Constructive and destructive interference

How can we use digital modes?

• Fundamentally, we must pipe these digital sounds into the radio
• TNC's / External Sound Cards
• Example made by Dale Wykoff N5EIA
• Signalink
• Raspberry Pi hats
• TNC-X
• Many others
• USB connections Yaesu, Icom CI-V connections
• Software
• SoundModem
• FLDIGI
• WSJT-X
• Digipan
• PSK
• Winlink Express / PAT Winlink / RMS Express
• Flarq
• MMSSTV

• Too Many!
• Let’s start with something simple!
• Let’s start with something that has lots of applications we can use!
• Must comply with FCC restrictions
• Remember dial-up?
• Amateur Radio already has an algorithm that meets all of these criteria

Enter AX.25

• You may have heard this referred to as “Packet Radio”
• Maybe you remember the “bad old days”
• Much easier and cheaper today
• Easier for hearing impaired
• Digital economy already > 15% of global gdp
• Number of IT developers doubles every five years
• Applications are easier and cheaper to create
• Computers and computer networks are now cheap and ubiquitous
• Watch for trends such as “New Packet Radio”
• Potentially even open CDMA techniques
• 70 cm will be faster than 2m band
• The higher the frequency the faster you can send data

Applications

• Winlink (send emails)
• APRS (texting and gps)
• Bulletin Board Systems
• Peer to Peer (P2P) applications
• AMPRNet (Amateur Packet Radio Network)
• File transfer
• SSH and Telnet
• Games
• HTTP
• Make your own!

AX.25 functionality for Linux:

• Ubuntu Manpage: gen_packets - Generate audio file for AX.25 frames.
• Ubuntu Manpage: atest - Decode AX.25 frames from an audio file.
• Ubuntu Manpage: axcall - make an AX.25, NET/ROM or Rose connection.
• axlisten: monitor AX.25 traffic - Linux Man Pages (1) (systutorials.com)
• ax25d: ax25d configuration file. - Linux Man Pages (5) (systutorials.com)
• Direwolf
• LinBPQ
• Packetlet (sms)
• Can route axlisten to inetd or httpd
• Set up AX.25 for TCP/IP: Configuring an AX.25 interface for TCP/IP (tldp.org)
• Implement your own AX.25 linux based applications:
• AX.25 Client and Server Programming (sdf.org)

PART 2

OPERATING WITH WINLINK

USING A

BAOFENG UV-5R RADIO

​

THE SET UP

​

USING A BAOFENG UV-5R

​

​

​

AND AN

​

HP WINDOWS 7 LAPTOP

AX.25 SOUND MODEM SETUP

​

2010 COPYRIGHT UZ7HO

​

VERSION 1.13

​

​

​

TROUBLESHOOTING

-Make sure that all conections are good.

-Make sure that correct power is getting to the INTERFACE board

and PREAMPLIFIER.

-Make sure that the volume on the BAOFENG transceiver is correct.

This should be about ¼ above OFF.

-Check the Directional Antenna orientation and connections.

Use volt-ohm meter and/or Oscilloscope when available.

Use the SOUNDMODEM calibration generator to send tones (transciever TX OFF)

Use another Tranciever to check for transmissions.

​

PART 2

INTERFACE BOARD

DETAILS

AND

OPERATION

​

THE TNC

(TERMINAL NODE CONTROLLER)

​

1. TYPICAL STAND-ALONE MODEL

- microprocessor,

-firmware,

-modem,

-DUMB terminal (monitor, keyboard)

-connections to transciever

(audio in / out, PTT)

-designed to implement AX.25 networking

-complete - needed no other equipment

​

SOFTWARE TNC

- home computer

- software programs

-interface to transciever

-a modem

​

OTHER DIGITAL MODES

APRS

SSTV

PEER-TO-PEER WINLINK

ETC.

​