Field Guide · protocol

Also known as: VDL Mode 2, VDL2, VDLM2

VDL Mode 2 (VHF Data Link Mode 2) is a digital aircraft datalink that carries ACARS and ATN traffic over VHF using differential 8-PSK (D8PSK) at 31,500 bps — roughly thirteen times the throughput of plain 2400 bps VHF ACARS.1 It is the mainstream high-rate VHF bearer for airline datalink today, moving the same operational and FANS messages far faster than the legacy MSK channel.

D8PSK · 3 bits/symbol 31.5 kbps VHF AVLC frames ground station CSMA access · 25 kHz channels · 136.975 MHz CSC
VDL Mode 2 keys three bits per symbol with differential 8-PSK at 31.5 kbps, framing data in AVLC over carrier-sense VHF channels.

Overview

VDL Mode 2 keeps ACARS’s message applications but replaces the physical and link layers. The physical layer is differentially encoded 8-PSK at 10,500 symbols/s (31.5 kbps); the link layer is AVLC (Aviation VHF Link Control), an HDLC-derived framing with addressing and acknowledgements. Media access is CSMA — stations listen before transmitting and back off on a busy channel, sharing each 25 kHz channel among many aircraft and ground stations.

Technical characteristics

Property Value
Band VHF aeronautical (Common Signalling Channel 136.975 MHz)
Channel spacing 25 kHz
Modulation Differential 8-PSK (D8PSK)
Symbol rate 10,500 symbols/s
Bit rate 31,500 bps
Link layer AVLC (HDLC-derived)
Media access CSMA (carrier-sense multiple access)
Payloads ACARS-over-AVLC (AOA) and ATN/OSI

The move to 8-PSK packs three bits into every symbol, and differential encoding removes the need to resolve absolute carrier phase — the receiver only tracks phase changes, which simplifies demodulation on a fading VHF channel. AVLC’s acknowledgements make the link reliable, unlike the fire-and-forget style of some legacy ACARS blocks.

History

VDL Mode 2 was standardised by ICAO in Annex 10 and the AEEC in ARINC 631 to relieve saturation of the 2400 bps ACARS channels as datalink use grew. Deployment accelerated through the 2000s and 2010s as datalink service providers rolled out ground networks, first for ACARS-over-AVLC and later for ATN Baseline traffic.

Deployment

VDL Mode 2 is now the primary VHF datalink for most airline aircraft, operated by ARINC and SITA ground networks worldwide, with 136.975 MHz reserved as the common signalling channel. It coexists with legacy VHF ACARS and with the HF and satcom bearers such as HFDL used over oceans.

Decoding it with GopherTrunk

Not decoded. VDL Mode 2 is an aviation VHF datalink outside GopherTrunk’s land-mobile trunking and 1090 MHz ADS-B focus. Its D8PSK signal is receivable with a modest SDR and open decoders, but GopherTrunk does not implement the D8PSK/AVLC chain. This page frames it honestly alongside its predecessor ACARS and the underlying modulation 8-PSK.

Sources

  1. VHF Data Link — Wikipedia, for VDL Mode 2’s differential 8-PSK physical layer, 31.5 kbps rate, AVLC link layer, CSMA access, the 136.975 MHz common signalling channel, and its ACARS/ATN payloads. ↩

See also