Also known as: Winlink, Winlink 2000, WL2K
Winlink (formally Winlink 2000 / WL2K) is a worldwide radio email system that lets a licensed amateur send and receive standard email — with attachments — through a radio link when the internet is unavailable. A user’s client connects over the air to a Radio Message Server (RMS) gateway, which is itself internet-connected; the gateway relays the message into and out of the global email system.1 The over-the-air hop can use several data modes — most commonly Pactor and VARA on HF, or AX.25 packet on VHF — so Winlink is best understood as the network and message layer riding on top of whichever modem the conditions allow.
How it works
Winlink is a store-and-forward system with a hub-and-spoke topology. Redundant central Common Message Servers (CMS) hold each user’s mailbox; a mesh of volunteer RMS gateways provides the RF on-ramps. To send mail, a client (Winlink Express is the standard one) composes messages offline, then opens a radio session to a reachable gateway, exchanges compressed and error-controlled data, and disconnects — the gateway forwards to the CMS, and from there to the wider email system. Incoming mail is queued until the user next connects.
Two features make it practical over poor links. First, messages are compressed and protocol-framed (the B2F/FBB protocol) so a short on-air session moves a lot of text. Second, the ARQ data modes underneath — Pactor’s HF ARQ, the OFDM-based VARA modems, or the open ARDOP mode — automatically request retransmission of any block that fails its checksum, so email arrives intact even across a fading HF path. On VHF/UHF, plain 1200/9600-baud packet over AX.25 fills the same role at local range.
Relevance to SDR
Winlink matters in the wider RF world chiefly for emergency and off-grid communications: served agencies, sailors, and expeditions use it to pass formatted email when infrastructure is down. On a waterfall its component modes are recognisable — Pactor’s structured bursts, or VARA’s OFDM block of subcarriers — and monitoring/decoding them means decoding the underlying modem, since the email payload is compressed and, for Pactor, carried by a proprietary protocol.
GopherTrunk does not decode Winlink or its data modes. Pactor is proprietary, VARA is a closed OFDM modem, and none of them are digital land-mobile trunking, which is GopherTrunk’s focus. Winlink is documented here as context for the amateur HF/VHF data ecosystem that surrounds the signals GopherTrunk does target, not as a supported protocol.