Also known as: Phil Karn, KA9Q
Phil Karn (amateur radio callsign KA9Q) is an American engineer and radio amateur whose work bridges the internet and radio: he devised Karn’s algorithm for TCP round-trip estimation, wrote the influential KA9Q packet-radio networking software, and authored open-source forward-error-correction libraries used across the SDR and amateur communities.1 His career spans professional communications engineering at companies such as Bell Labs, Bellcore, and Qualcomm, and decades of volunteer contribution to amateur digital radio.
Life and work
Karn was born in 1956 and earned degrees in electrical engineering from Cornell and Carnegie Mellon. As a professional he worked on data communications at Bell Labs and Bellcore and later on cellular systems at Qualcomm. In parallel, as radio amateur KA9Q, he became one of the central figures in the amateur packet-radio movement of the 1980s and 1990s.1
His KA9Q Network Operating System (“NOS”) was a landmark: a self-contained TCP/IP stack that ran on ordinary personal computers and carried internet protocols over amateur radio links using the AX.25 data-link protocol. For many operators it was their first hands-on TCP/IP implementation, and it demonstrated that packet-switched internetworking could run over slow, noisy radio channels.
Contribution
In professional networking, Karn is known for Karn’s algorithm, a simple but important rule for measuring TCP’s round-trip time: retransmitted segments are excluded from round-trip samples, because you cannot tell whether an acknowledgement matches the original transmission or the retransmission. The rule prevents corrupted timing estimates and is standard in TCP implementations.1
For radio, Karn’s most enduring gift to the SDR world is a set of carefully optimised, freely licensed forward-error-correction routines: fast Viterbi decoders for convolutional codes, Reed-Solomon encoders and decoders, and related building blocks. These libraries have been reused across amateur and open-source projects — from deep-space telemetry experiments to software radios — because they are correct, efficient, and unencumbered.
Legacy
Karn’s combination of professional rigor and open contribution made him one of the quiet architects of amateur digital radio. His NOS software seeded a generation of packet operators, his TCP algorithm is embedded in the internet’s transport layer, and his FEC code continues to be pulled into new SDR projects whenever a developer needs a proven Viterbi or Reed-Solomon implementation. He also became a prominent advocate for strong cryptography and open standards. His work is a reminder that the same coding theory protects data whether it travels over fiber or over a fading radio path.