G2E12 from the General License Course Section 2.4, Digital Modes:
Q. How does the receiving station respond to an ARQ data mode packet containing errors?
A. Terminates the contact
B. Requests the packet be retransmitted
C. Sends the packet back to the transmitting station
D. Requests a change in transmitting protocol
First, let’s define “data mode packet” and “ARQ.” Then we’ll see just what this question is all about.
As you probably recall from Technician Class studies, a data mode or digital mode is one of several methods of communicating with digitally encoded information. Different protocols are available for different digital modes – a protocol is simply a set of rules for encoding, sending, receiving, and decoding information. For example, a data mode protocol may be used to encode alphanumeric characters typed into a computer keyboard as sets of 1s and 0s, otherwise known as a binary code. The 1s and 0s may be represented by a pair of audio tones, one tone for each of the two digits. The two tones are transmitted over the air in alternating patterns of the sets of 1s and 0s that represent various keyboard characters. The receiving station decodes the tones back into the binary digit representations and displays the decoded keyboard characters. The protocol precisely details the binary code representations for each character, the timing and sequence by which the codes are transmitted and received, and other specifics about the nature of the digital exchange. Email, text, telemetry, data files, or position information may be communicated with digital codes in this manner.
A very common type of protocol uses packets of digits. A packet is a formatted package (a predefined sequence) of digital information that includes the message content being sent along with additional information that is necessary to implement the protocol and to ensure accurate data transmission is achieved between stations. A digital packet will typically include a “header” section first that provides the message routing data or address, error correction information, and other packet control and sequencing data. Following the header section is the data section or “payload.” This is the message content in the packet, such as the binary sequence representing keyboard characters. Finally, the “trailer” section of the packet provides information about the message status and additional error detection information. To send a lengthy email or text message, many packets may be used, each carrying only a part of the payload message. The packet payloads must be sequenced in order to accurately reproduce the entire email message at the receiving station.
Sometimes a digital packet is not received properly due to interference or poor conditions. Some of the information within the packet is then lost or scrambled, and the packet payload cannot be properly integrated with the others by the receiving station. What’s a receiving station to do? Send an ARQ! Automatic Repeat Query.
An ARQ is transmitted back to the sender by the packet receiving station when a packet in the expected sequence is missing or contains errors. The ARQ essentially says to the sending station, “Hey, uh, I missed something here in packet 42042. Can you resend that one please?” When the ARQ is received back at the packet sending station, that station will again transmit packet 42042. When the packet receiving station gets the accurate version of packet 42042, it will make sure that it gets sequenced properly right in between 42041 and 42043, and thereby promote the regurgitating of a properly sequenced and coherent email or text message on the receiving station’s computer screen.
There are multiple ways of effecting ARQ, and it is defined by the particular protocol being used in a digital radio mode. By definition, a transmitting protocol that uses ARQ to ensure accurate messaging transmits “ARQ data mode packets,” as referenced in question G2E12. Now you know how the receiving station responds to an ARQ data mode packet containing errors:
The answer to General Class question G2E12, “How does the receiving station respond to an ARQ data mode packet containing errors?” is “B. Requests the packet be retransmitted.”
Related Questions: G2E13, G8B11