ARQ-E
A synchronous shortwave data system with automatic error correction.
ARQ-E is a full-duplex synchronous data communications system used for sending data over shortwave radio. It employs an automatic repeat request (ARQ) mechanism to ensure data integrity, requesting retransmission when errors are detected. The system is also known as ARQ-1000 duplex or ARQ-1000D.
- Also known as
- ARQ-1000 duplex, ARQ-1000D
- Baud rates
- 48, 64, 72, 86, 96, 144, 192 baud
- Character bits
- 7 bits (5 data bits + 1 identification bit + 1 parity bit)
- Parity
- odd parity
- Alphabet base
- CCITT Telegraph Alphabet No. 2 (Baudot)
- Variants
- ARQ-E3 (ITA3 alphabet, CCIR 519), ARQ-N (no marking pattern)
Lore & Background
ARQ-E extends the five-bit Baudot alphabet by adding an identification bit at the start of each character to distinguish normal characters from function signals. Of the 32 possible combinations this identification bit enables, only three are used: α (start polarity), β (steady stop polarity), and RQ (retransmit request). A stream of β characters is sent when there is no data to transmit. A seventh bit provides odd parity, allowing the receiver to detect transmission errors and request repeats via the return channel.
After the seven-bit character is formed, the polarity may be inverted in a marking pattern, with every fourth or every eighth character having its bits transposed. A five-character cycle variant exists for use with in-line encrypters. The bits are then serialized and modulated onto a radio carrier using frequency-shift keying at standard baud rates.
When an error is detected, an RQ symbol is sent along with a repetition of the last characters in the cycle. Upon receiving an RQ, the transmitter sends another RQ and repeats characters starting from the flagged one. The number of repeated characters depends on the cycle length: three in the four-character cycle, seven in the eight-character cycle, and three after two RQ characters in the five-character cycle. Accurate position in the sequence is critical for encrypted streams.
Reader's Guide
ARQ-E represents a robust method for reliable data transmission over the challenging shortwave radio environment. Its use of synchronous transmission eliminates the need for start and stop bits, maximizing throughput, while the automatic repeat request mechanism ensures data integrity without requiring forward error correction. The system's adaptation of the Baudot alphabet with additional identification and parity bits demonstrates a practical evolution of telegraphy for digital data. The inclusion of idle signals (α and β) maintains synchronization during idle periods. Variants such as ARQ-E3 and ARQ-N show the system's flexibility, with ARQ-E3 using a different alphabet (ITA3) and ARQ-N omitting the marking pattern. The availability of decoding software from both professional and hobbyist sources has made ARQ-E accessible for monitoring utility transmissions, preserving its relevance in modern spectrum analysis.
Did You Know?
- ARQ-E is also called ARQ-1000 duplex or ARQ-1000D.
- The system uses an idle signal (β) sent continuously when there is no data to transmit.
- A marking pattern inverts bits every fourth or eighth character, with a five-character cycle for encrypted use.
- Standard baud rates include 48, 64, 72, 86, 96, 144, and 192 baud.
Frequently Asked Questions
What is ARQ-E in shortwave data communications?
ARQ-E is a full-duplex synchronous data-link protocol designed for transmitting telegraph-style data over shortwave radio channels. It guarantees delivery by automatically requesting a retransmission whenever the receiving station detects a corrupted character, making it a self-correcting link without human intervention.
What baud rates can ARQ-E operate at?
The system supports seven selectable speeds: 48, 64, 72, 86, 96, 144, and 192 baud. Operators choose the rate based on channel conditions, with the lower settings reserved for noisy or weak-signal paths.
How is each character structured in ARQ-E?
Every transmitted character occupies exactly seven bits: five data bits carrying the symbol, one identification bit, and one odd-parity bit used for error detection. The data bits follow the CCITT Telegraph Alphabet No. 2 (Baudot) code, giving the system its classic telegraph character set.
What are the known variants of ARQ-E?
Two principal variants exist: ARQ-E3, which substitutes the ITA3 alphabet under CCIR 519 for the standard Baudot set, and ARQ-N, which removes the marking-pattern requirement from the protocol. Both retain the core ARQ retransmission logic of the base system.
Why do shortwave operators consider ARQ-E important?
Because it combines full-duplex operation with automatic error recovery, ARQ-E lets two stations exchange data simultaneously over a single shortwave channel without manual check-and-acknowledge exchanges. Its also-known-as names ARQ-1000 duplex and ARQ-1000D reflect its long service life in military and diplomatic HF networks.
More in Radio Modulation Modes, Part 2 1-24
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