Packet radio

Packet radio is a digital mode and protocol used for digital communication over amateur radio. Packet radio is connection (session) based, and supports a store and forward model of message passing.

AX.25

AX.25 is a packet protocol based on the X.25 protocol and fine tuned for amateur radio. It is widely used for data transfer in amateur radio, with the most common applications being APRS and Winlink on VHF and UHF.

Modulation for AX.25 is typically 300 baud FSK for HF or 1200 baud AFSK on FM for VHF/UHF. For HF radios that do not support FSK, AFSK on SSB can be used instead. On VHF or above, 9600 baud FSK is also supported using specially modified radios or dedicated data ports.

AX.25 is a data link layer protocol. Other protocols (such as APRS and TCP/IP) may be encapsulated in AX.25 packets.

A modified version of AX.25, known as FX.25, also supports forward error correction and is backwards compatible with AX.25. This provides higher decode rates of packets in poor signal conditions at the expense of a lower effective data rate due to the additional data required for air correction.

Terminal Node Controller (TNC) and KISS

To avoid the user having to implement the packet protocol in every software that needs to use it, a terminal node controller (TNC) can be used. A standard or fully implemented TNC contains protocol layers between the radio and the serial port intended for human or higher-level protocol use, allowing the TNC to take care of the details of formatting packets for the protocol, managing collision detection and repeat requests, and other low-level protocol functions.

Many of these TNCs will still provide access to low-level details of the protocol. The commands used to interact with the TNC are also often different between different manufacturers and devices. In cases where low-level access is not required and a simple, universal interface is desirable, the KISS protocol may be implemented. KISS mode is intended for direct TNC to computer software interface, where the software in use on the host computer will need control of packet contents, but not low-level details. This has the advantage of simplifying the programming for the user software, but removes the ability to optimize how the packets are sent, which could potentially reduce the data transfer rate.

Digipeaters

A digipeater is a radio which is designed to forward packets between local stations. This enables two stations that cannot directly communicate with each other to route their traffic through the digipeater and enable them to make a connection. Typically, a digipeater is on a high tower (much like a voice repeater) to cover a large area, although it could be any system. A mobile radio could be used as a digipeater to relay for a nearby low power handheld.

Packets can be routed through a specific digipeater or sequence of digipeaters to attempt to reach a specific station. Alternatively, the packet can be relayed by any digipeater that hears it, with a limit on how many times the packet can be retransmitted. This is most common when there is no specific destination station, such as with APRS.

Many radios with built-in TNCs will contain options to allow them to function as digipeaters, as well as some TNC software such as Dire Wolf.

Internet Gateway (IGate)

An internet gateway is a radio which forwards packets received over RF to the Internet, and in many cases, also forwards packets received over the Internet to RF. This allows packet radio networks to span across large distances where there may be no intermediate digipeaters, and allows access to services such as email, texting, weather forecasts, and more over RF. IGates can also be configured to serve as digipeaters.

Bulletin Board Systems (BBSs)

A packet radio BBS can store messages in mailboxes and hold messages for forwarding to other stations at a later time. They can also provide access to other services that can be accessed via terminal, such as chat rooms or even text-based games.

Required Equipment

To participate in packet radio, the following equipment is required:

• Radio
• TNC
• Terminal or program

A computer can replace the terminal with any serial communications program or a packet-specific program for a specific use case such as file transfer, sending email, or others.

Traditionally, TNCs were a separate piece of hardware, but for modern applications, they may be contained within the radio or replaced with a soundcard and software.

Software

TNC Software

AGWPE (AGW Packet Engine)

website includes multiple Microsoft Windows packet radio programs including sound card TNC emulation

Dire Wolf

A software TNC for AX.25 and FX.25 utilizing the computer's sound card instead of specialized hardware. Without any additional software, it can perform as:

• APRS GPS Tracker
• Digipeater
• Internet Gateway (IGate)
• APRStt gateway

It can also be used as a virtual TNC for other applications.

LinPaK

Linux packet radio and email system

Linux AX.25 kernel driver

built into linux, see http://en.tldp.org/HOWTO/AX25-HOWTO/

mixw

supports multiple digital modes

soundmodem

user mode soundcard TNC for all platforms (Windows, Linux, Solaris)

TNT and DPBox

Linux full featured TNC including host mode BBS software and clients

Application Software

PinPoint APRS

A Windows APRS client, providing a map for received position reports, the ability to send and receive position reports and messages, and several other functions.

Winlink Express

A software that allows access to Winlink, an RF email service. Formerly known as RMS Express, formerly known as Winlink 2000 or WL2K.

External links

• Wikipedia:Packet radio
• http://www.tapr.org/tapr/html/pktf.html
• http://ka1fsb.home.att.net/
• Linux links
  • HAMSOFT Linux Software for the Hamradio Community
  • Linux Amateur Radio AX.25 HOWTO]
  • FBB Packet-radio BBS mini-HOWTO
  • Netrom-Node mini-Howto set up AX.25 and FBB in a netrom
• Packet Radio and Similar Digital Radio Modes