VHF digital link (VDL) Mode 2 is an air-ground data link specified in the ICAO documents “Annex 10 Volume III - Communication Systems” and” Manual on VHF Digital Link (VDL) Mode 2”.
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Frequency range |
VHF |
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Frquency range |
VHF, 136.975 MHz (std. transmission frequency) |
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Operation modes |
CSMA |
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Modulation |
D8PSK |
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Symbol rate |
31.5 kBd |
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Receiver settings |
BW = 15-30 kHz |
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Input(s) |
IF |
VDL-M2 uses D8PSK (Differentially encoded 8-Phase Shift Keying) modulation scheme operating at a bit rate of 31.5 kbps in a 25 kHz frequency band. The data is transmitted in the form of short bursts. The maximum number of data bits at a single packet is 217 -1 bits, which corresponds to a packet length of approximately 4 seconds.
Each packet carries a header field, which contains a 48 bits synchronization sequence and a packet length field. The packet length field is protected with a (25, 20) block code, and the remaining data bits are protected with a systematic fixed-length Reed Solomon (255, 249) 28-ary code.
Channel access is achieved using the carrier sense multiple access (CSMA) algorithm and the data link service sublayer uses the aviation VHF link control (AVLC) protocol. AVLC is an extension of the HDLC standard which is specified by the following ISO documents: ISO 3309, ISO 4335, ISO 7809, and ISO 8885.
The AVLC packets start and end with a special flag byte (0x7E) and include a 9 bytes long AVLC header after the start flag and 2 bytes long CRC field before the end flag. The header contains the 24-bit ICAO aircraft/ground station addresses of the sending and receiving terminals and one byte link control field indicating the type of the packet, e.g., INFO, Receive Ready (RR), Exchange Identity (XID), TEST, and Selective Reject (SREJ).
In HEX output mode, the whole AVLC packet is printed bytewise as hex.
In ITA5-US mode the AVLC frame types and 24-bit ICAO aircraft / ground station addresses are decoded and printed. For INFO frames, the data field is decoded correspondingly if it is an ACARS packet, and is printed in hex format (0xXX) if it is an ATN packet. For XID frames, the parameters IDs are mapped to their names, and corresponding values are printed as hex. For other frame types, the whole data field is printed as hex.
It is recommended to decrease the “Gain” manually until locating the bursts first visually in FFT Direct view with a 48 kHz bandwidth. The polarity should be set also correctly according to the output polarity of the receiver.
From Options | Display Mode, Valid Frames Only, All Frames or Indicate Errorneous Frames may be selected.