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The Internet Radio Linking Project. Presentation for the Triangle Linux Users Group by Jim Price WW4M April 10, 2003. Portions of this presentation courtesy of KD4RAA, VE3SY, VE7LTD, VK3JED. Overview. What is Amateur Radio? Ham Radio Activities IT and Amateur Radio

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the internet radio linking project

The Internet RadioLinking Project

Presentation for the Triangle Linux Users Groupby Jim Price WW4M

April 10, 2003

Portions of this presentation courtesy of KD4RAA, VE3SY, VE7LTD, VK3JED

  • What is Amateur Radio?
  • Ham Radio Activities
  • IT and Amateur Radio
  • Ham Radio Internet Linking
  • The IRLP Network
  • IRLP Hardware
  • IRLP Software
  • Using IRLP
  • Listening
  • For More Info...
  • Demonstration
what is amateur radio
What is Amateur Radio?
  • Licensedas apublic serviceby the Federal Communications Commission
  • voluntarynon-commercialcommunications, with an emphasis on providingemergency communications
  • advancementof the radio art
  • advancing communication and technical skillsand provide trained operators, technicians and electronics experts
  • promoteinternational goodwill
  • 680,000 hams in the U.S., 18,000 in NC

Celebrity Hams

Entertainers: Chet Atkins WA4CZD, Ronnie Milsap WB4KCG, Marlon Brando FO5GJ, Patty Loveless KD4WUJ, Gary Shandling KD6OY, Burl Ives KA6HVA, Arthur Godfrey K4LIB, Priscilla Presley N6YOS

Joe Walsh WB6ACU


.A huge number of politicians and royals including:Gov. George Pataki N2ZCZ, Sen. Barry Goldwater K7UGA, Spain’s King Juan Carlos EA0JC, Jordan's King Hussein JY1 and Queen Noor JY2, Argentina’s Pres. Carlos Menem LU1SM, India’s Rajiv Gandhi VU2RG

Famous and infamous hams: most astronauts, Hugh Downs KD6WUS, Art Bell W6OBB, Kevin Mitnick N6NHG, and the author of The Joy of Sex, Dr. Alex Comfort KA6UXR

Walter Cronkite KB2GSD


Celebrity non-Hams

Forest WhitakerPhenomenon

Dennis Quaid

amateur radio activities



Amateur Radio Activities
  • “Dxing” (long distance contacts)
  • Contesting
  • Amateur Radio Satellites
  • Digital Modes
  • HF (shortwave)
  • VHF/UHF and repeaters
  • Morse code, analog voice
  • Public Service/ Emergencies
  • Home-building equipment
it and amateur radio
IT and Amateur Radio
  • Computer-Controlled Modes
    • Packet Radio
    • Narrow Band Teletype (e.g. PSK-31)
    • Amateur Television
    • IEEE 802.11b WiFi / HSMM
  • GPS & APRS
  • Amateur Radio Satellites
  • Internet Radio Linking
  • Remote Bases and Web Radio
  • Digital Signal Processing
  • Software Defined Radio
  • Spread Spectrum Technology
  • Contact Logging Software
  • Antenna Design


Radio or Repeater

Distant Radio or Repeater


Radio Link





441 MHzdown

446 MHzup

  • Repeater: A station that simultaneously retransmits the transmission of another station over a wider area on another frequency
ham radio internet linking
Ham Radio - Internet Linking

Radio-only Access

  • IRLP (linux)

PC -or- Radio Access:

  • eQSO (Windows)
  • iLink (Windows)
  • WIRES (Windows)
  • EchoLink (Windows or linux)
the internet radio linking project irlp
The Internet Radio Linking Project (IRLP)
  • Uses the Internet to link distant radio sites
  • Gives global coverage to normally localized VHF and UHF frequencies
  • Enables minimally equipped stations to communicate globally
  • Allows end user control of links via their radio’s DTMF (Touchtone) keypad

The Internet Radio Linking Project (IRLP)

Created by Dave Cameron VE7LTD of Vancouver, BC  

Dave was frustrated with the unreliable operation of Windows-based Iphone and turned to linux.

Iphone and other systems use VOX. IRLP uses COS.IRLP is radio-access only.


IRLP Growth

November 1998 - 2

December 1999 +10= 12June 2000 +20 = 32December 2000 +29 = 61June 2001 +61 = 122December 2001 + 199 = 321June 2002 + 236 = 557December 2002 +263 = 820

database records 1/1/03


IRLP Network

445 United States154 Canada

54 Australia + New Zealand

49 Europe

12 Caribbean + Bermuda 6 Japan

3 Southern Africa

2 Ecuador, Mexico

1 India

1 Antarctica

active nodes 4/1/03


IRLP is based on Speak Freelyfor Linux.

Speak Freely for linux is Open Source, while Speak Freely for Windows is GPL.

Linux offered the best in reliability, programmability, efficiency, and functionality. 

IRLP currently ships with RH 7.3 and will run on a 486 DX100 or better computer(init=3).

typical irlp node
Typical IRLP Node
  • Standard PC, P100 or better
  • Linux (Red Hat 7.3)
  • SB16 ISA soundcard
  • Custom IRLP computer/radio control interface
  • Radio
  • Dedicated internet connection

Custom Red Hat CD-ROM

Boot Diskette

Custom IRLP Software

IRLP Control Board


DB9 IRLP to Radio Interface

LPT1 to IRLP Jumper Cable


DB9 for radio inter-face

DB25 for Data Control from LPT1 & IRLP

+12VDC from PC

v.2 IRLP card

indicators on v 2 control board
Indicators on v.2 Control Board

– DTMF Decode Sense – Carrier Operated Squelch Sense

– PTT out to Link Radio or Controller


100MB Switch

10MB HubLinksys Router

DualBand 2M/440Control Xcvr

KD4RAANode 4260 / 4270Configuration

4270 Link Radio 4260 Link Radio

Audio Equalizer and Broadcast Leveling

what happens during a call

What Happens During a Call?

Notes from Dave Cameron, VE7LTD

dtmf decode
DTMF Decode
  • DTMF program monitors COS and DTMF
  • Once detected DTMF sequence passed to the decode script
  • Decode script checks custom_decode for matches
  • Assuming a call is decoded, call script is started with the node number as the argument


call script
Call Script
  • Best server is determined by using find_best_server script
  • Best server is asked for latest IP of node being called
  • If IP received is different from IP in hosts file, a new hosts file is d/l from best server
  • Irlp_call is started, and a TCP connection is made to the called node on port 15425


irlp call irlp answer
  • Remote node starts irlp_answer in response to TCP call on port 15425
  • PGP security performs a dual challenge to ensure calling node is an IRLP node
  • Codec (GSM/ADPCM) is determined
  • Irlp_call and irlp_answer start speak freely software on UDP ports 2074 and 2075


during the call
During the Call
  • Irlp_call and irlp_answer send keepalives in the background. If keepalive fails, the connection drops (every 15 sec)
  • Irlp_call and irlp_answer keep open info channel to pass dtmf regeneration info, disconnect/timeout message


  • Disconnecting node uses TCP info channel to send disconnect message. Both nodes run the off script
  • Unexpected drop in the TCP connection prompts reset of IRLP node
  • If the timeout elapses, disconnect is sent



NCSU Bookstores:

Sending Audio

Audio streaming is enabled when the link radio receives a signal with COS and keys the IRLP interface card.

The sound card receives the radio’s audio and creates a continuous mono 8-bit digital stream of raw audio at 8000Hz (64k bps).

Speak Freely’s sfmike program compresses the audio stream by a factor of two (32k bps) using an audio compression algorithm (codec).


NCSU Bookstores:

The audio is split into packets, which are transmitted over port 2074 using a User Datagram Protocol (UDP) stream. UDP does NOT confirm the reception of packets - it "fires and forgets"


NCSU Bookstores:

Receiving Audio

Speak Freely’s sfspeaker receives the packets on port 2075 and rejoins them into an 8-bit ULAW stream.


Next the ULAW stream is uncompressed back into an 8-bit raw stream of audio.


The raw audio is streamed through the digital to analog (D/A) converter (the output device of your sound card).


The IRLP interface card keys the radio when the audio stream is present (live -- no buffering).


NCSU Bookstores:

Audio Compression



Jason Woodard,


NCSU Bookstores:

  • ADPCM(Adaptive Differential Pulse Code Modulation)
  • only 32 kb/s - more efficient than the PCM codec used by WAV, AIFF and CD audio (and is used on the Sony Mini Disc)
  • cuts the data rate from 8000 to 4000 bps
  • superior sound quality vs. GSM

NCSU Bookstores:

  • GSM(Global System for Mobile Communications)
  • requires only 13 kb/s bandwidth
  • reduces the data rate from 8000 to 1650 bps, which makes a 28.8 Kb modem usable
  • serves 71% of the total digital wireless market and provides good quality speech

NCSU Bookstores:

ADPCM quantizes the difference between the sampled signal and a prediction. If the prediction is accurate, the difference between the real and predicted samples will have a lower variance than the real speech samples and will be accurately quantized with fewer bits than needed for the original.

At the decoder the quantized difference signal is added to the predicted signal to give the reconstructed speech signal. Performance is aided by using adaptive prediction and quantization, so that the predictor and difference quantizer adapt to the changing characteristics of the sampled speech.


NCSU Bookstores:

GSM uses a Regular Pulse Excited (RPE) codec. Input is split into 20ms frames, each with 8 short term predictor coeffiecients. Frames are further split into four 5ms sub-frames, each with a delay and gain for a long term predictor. After short and long term filtering, the residual signal for each sub-frame is decimated into three possible excitation sequences, each 13 samples long. The sequence with the highest energy is chosen as the best rep-resentation of the excitation sequence, and each pulse in the sequence has its amplitude quantized with three bits.

At the decoder the reconstructed excitation signal is fed through the long term and short term synthesis filters to reconstruct the speech. A postfilter improves perceptual quality


Ports Required

2074 - 2093 Audio (bi-directional UDP)

15425-7 IRLP Control/Update (TCP)

Outbound ports used: 80 (http) for updates

873 or 8873 (rsync) for downloading updates

10000 (for IP determination)

parallel port set to "standard" or "compatible" mode, not to ECP, EPP or bi-directional


Some IRLP Details

512-bit bi-directional PGP authentication ensures connections only with other IRLP nodes.

Redundant servers support IRLP-BIND(DHCP isn’t a problem) as well as download of pgp key rings, software updates, and station ID wavfiles.

IRLP accepts commands from keyboard or via DTMF (dual tone multi frequency, i.e. “Touchtones”)


IRLP and the GPL

“I had released code under the GPL, and it led to nothing but 1000's of emails criticising my code, asking for features, wondering why I built this that way, etc... I got tired of answering questions.”

“There is a strong commercial potential for this product, and I did not want to hand several 1000 hours worth of code to the private sector just to be replicated.”

“Closed code maintains a standard, which open releases would compromise.”


irlp admin
IRLP Admin
  • FCC regulations prohibit unlicensed people using amateur frequencies.
  • IRLP links are accessible only on amateur radio frequencies within range of a node. No direct Internet access.
  • OpenSSH used for remote administration.
  • Linux packages updated automatically (up2date, autorpm, apt).
  • Cron jobs to automate functions
irlp scripting
IRLP Scripting
  • The IRLP software is composed largely of bash shell scripts.
  • Flexible: easily customized to suit local needs.
  • Almost anything that can be run from the console can be controlled via radio.
    • Morse Code ID using MIDI or wav (per FCC legal requirements)
    • Node status (link off/on and where linked to)
    • Time of day (talking clock)
    • Weather reports, Amber Alerts
    • Local Announcements
    • Download and play radio news programs from internet
    • Random dialing
    • *69 / “Call Waiting”
irlp advantages
IRLP Advantages
  • User Flexibility - Custom Scripts
  • Accessible only by radio
  • Security
  • Stability of linux OS
  • Superior audio; COS (not VOX)
  • Cost: linux is free, will run on old PCs
  • Continues to function if servers fail
two ways to make contacts
Two ways to make contacts



Reflectors(like chat rooms -- multiple nodes connected together, hearing audio from one node at a time)

irlp reflectors
IRLP Reflectors
  • Enable multiple nodes to link together into a network – number limited only by bandwidth.
  • Running a reflector requires bandwidth (32 kbps per connected node) to handle the multiple data streams. Most reflectors are hosted by ISPs (like Inflow) who often donate the bandwidth.

Raleigh Reflector

Established for coordinated response to severe weather events and for public service

Links NC repeaters beyond the range of conventional RF linking systems

During Hurricane Lily, linked stations in Louisiana with the NationalHurricane Center in Florida

Has linked stations from Georgia to Maryland to track several winter storms

Simulcast a ham radio contact between the International Space Station and Chapel Hill middle school students

Participated in the Boy Scouts’ JOTA event

impact of internet linking on amateur radio
Impact ofInternet Linking on Amateur Radio
  • Level of amateur activity has increased dramatically
  • Amateurs who have been inactive for a long time are coming back on the air
  • Amateur Radio is becoming more appealing to today’s Internet-oriented youth
  • New opportunities for experi-mentation.
the future
The Future
  • Internet linking is already becoming commonplace – over 800 IRLP nodes on the air.
  • Advances in technology will improve performance of links.
  • Technology can be ported to high-speed microwave and satellite links.
  • Enhanced global slack.
listening to irlp
Listening to IRLP
  • Live Demonstration
  • Monitoring IRLP Reflector 2 via Live365 streaming audio (linked from
  • Recorded conversation with Antarctica, and
  • Recorded school contact with the International Space Station (linked from
  • local frequencies via radio or scanner
how to become a ham radio operator
How to Become a Ham Radio Operator
  • Radio amateurs are licensed by the FCC, after passing examinations in radio theory, electronics, regulations, and optionally, Morse Code.
  • Examinations are conducted locally by individuals or clubs almost every week and cost about $12 (10-year renewals are free)
  • Many radio clubs run study courses. 1-on-1 tutoring, book and online stuyding, and taking mock online exams are other study options.
more info
More Info
  • IRLP in Raleigh
  • -
  • IRLP in General -
  • What is Amateur Radio? -
  • Raleigh Amateur Radio Society -
  • American Radio Relay League -