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Subel. L10
22 questions
Subelement L10

Digital Transmission Techniques.

Section L10

What digital code consists of elements having unequal length?

  • Baudot
  • ASCII
  • Correct Answer
    Varicode
  • AX.25

Varicode is an encoding method where ASCII characters are represented by bit patterns ranging from 1 to 10 bits in length. Varicode is used for PSK31 ("Phase Shift Keying, 31 Baud"). ASCII (American Standard Code for Information Interchange) or Baudot (RTTY) rely on a precise number of evenly-timed bits. AX.25 is a protocol used for packet transmission.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Open Systems Interconnection (OSI) model standardizes communications functions as layers within a data communications system. Amateur digital radio systems often follow the OSI model in structure. What is the base layer of the OSI model involving the interconnection of a packet radio TNC to a computer terminal?

  • Correct Answer
    The physical layer
  • The link layer
  • The network layer
  • The transport layer

Key words: TNC to COMPUTER INTERCONNECTION. Physical layer (for example, RS-232): the electrical, mechanical, procedural and functional specifications for moving data across a physical medium, including modulation, establishing and terminating connections. The Data Link layer packages data bits into frames, provides error-free transfer of frames including physical addressing, network topology, error notification. The Network layer (for example, Internetwork Protocol) uses logical addresses to route frames through a network of links. The Transport layer (for example, TCP Transmission Control Protocol) provides the end-to-end control, ensures that data is complete and properly sequenced ( e.g., retransmissions ).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is the purpose of a Cyclic Redundancy Check (CRC)?

  • Lossy compression
  • Error correction
  • Lossless compression
  • Correct Answer
    Error detection

"A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data" (Wikipedia). A CRC is computed at the originating end and sent with the message; the receiving end does the same calculation, a mismatch in the CRC indicates that the payload was damaged. The CRC manages error detection, other mechanisms can provide error correction.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is one advantage of using ASCII rather than Baudot code?

  • Correct Answer
    It includes both upper and lower case text characters in the code
  • ASCII includes built-in error correction
  • ASCII characters contain fewer information bits
  • The larger character set allows store-and-forward

With 5 bits, Baudot can accommodate 32 combinations ( 2 exponent 5 ) per character set; one set represents uppercase letters A through Z, the other are figures 0 through 9 plus other symbols. The original ASCII (American Standard Code for Information Interchange) used seven bits to accommodate 128 combinations; enough for lowercase and uppercase letters, all digits and other symbols. So called "extended ASCII" uses 8 bits for a total of 256 combinations, this adds accented letters.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What type of error control system is used in AMTOR ARQ (Mode A)?

  • Correct Answer
    The receiving station automatically requests repeats when needed
  • The receiving station checks the frame check sequence (FCS) against the transmitted FCS
  • Each character is sent twice
  • Mode A AMTOR does not include an error control system

AMTOR is similar to RTTY but with error correction added; a special 7-bit code is used. Amateur Teleprinting Over Radio (AMTOR) can run under two modes. Mode B ( FEC - Forward Error Correction ): characters are sent twice in groups of five in consecutive blocks. Mode A ( ARQ - Automatic Repeat reQuest ): characters are transmitted in blocks of 3, receiving station returns a positive acknowledgement or a request to resend. [ Frame Check Sequence is part of the AX.25 Packet protocol, no relation to AMTOR ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What error-correction system is used in AMTOR FEC (Mode B)?

  • The receiving station checks the frame check sequence (FCS) against the transmitted FCS
  • Correct Answer
    Each character is sent twice
  • Mode B AMTOR does not include an error-correction system
  • The receiving station automatically requests repeats when needed

AMTOR is similar to RTTY but with error correction added; a special 7-bit code is used. Amateur Teleprinting Over Radio (AMTOR) can run under two modes. Mode B ( FEC - Forward Error Correction ): characters are sent twice in groups of five in consecutive blocks. Mode A ( ARQ - Automatic Repeat reQuest ): characters are transmitted in blocks of 3, receiving station returns a positive acknowledgement or a request to resend. [ Frame Check Sequence is part of the AX.25 Packet protocol, no relation to AMTOR ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

APRS (Automatic Packet Reporting System) does NOT support which one of these functions?

  • Correct Answer
    Automatic link establishment
  • Two-way messaging
  • Telemetry
  • Amateur-specific local information broadcast

Key word: NOT. "APRS is a digital communications information channel for Ham radio. (...) As a single national channel (...), it gives the mobile ham a place to monitor for 10 to 30 minutes in any area, at any time to capture what is happening in ham radio in the surrounding area. Announcements, Bulletins, Messages, Alerts, Weather, and of course a map of all this activity including objects, satellites, nets, meetings, hamfests, etc. (...) APRS also supports global call sign-to-call sign messaging (...)" (www.aprs.org Bob Bruninga WB4APR). Automatic Link Establishment (ALE) is a standard for systems capable of automatically selecting a band and frequency from a list of channels for HF communications with a given similarly-equipped station.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Which algorithm may be used to create a Cyclic Redundancy Check (CRC)?

  • Convolution code
  • Lempel-Ziv routine
  • Correct Answer
    Hash function
  • Dynamic Huffman code

"A hash function is any algorithm that maps data of arbitrary length to data of a fixed length. (...) The values returned by a hash function are called hash values, hash codes, hash sums, checksums or simply hashes" (Wikipedia). A convolutional code is a type of error-correcting code (e.g., Viterbi or Reed-Solomon). Lempel-Ziv and Dynamic Huffman coding are lossless data compression algorithms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The designator AX.25 is associated with which amateur radio mode?

  • Correct Answer
    packet
  • RTTY
  • ASCII
  • spread spectrum speech

Packet radio adheres to the AX.25 protocol. AX.25 is derived from the X.25 networking protocol: one notable difference is the use of call signs as addresses. AX.25 uses a Frame-Check Sequence for error detection. The Frame-Check sequence (FCS) is a sixteen-bit number calculated by both the sending and receiving stations of a frame. Comparing the received FCS with a locally computed one permits detecting corruption in transit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How many information bits are included in the Baudot code?

  • Correct Answer
    5
  • 7
  • 8
  • 6

With 5 bits, Baudot can accommodate 32 combinations ( 2 exponent 5 ) per character set; one set represents uppercase letters A through Z, the other are figures 0 through 9 plus other symbols. The original ASCII (American Standard Code for Information Interchange) used seven bits to accommodate 128 combinations; enough for lowercase and uppercase letters, all digits and other symbols. So called "extended ASCII" uses 8 bits for a total of 256 combinations, this adds accented letters.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How many information bits are included in the ISO-8859 extension to the ASCII code?

  • Correct Answer
    8
  • 7
  • 6
  • 5

With 5 bits, Baudot can accommodate 32 combinations ( 2 exponent 5 ) per character set; one set represents uppercase letters A through Z, the other are figures 0 through 9 plus other symbols. The original ASCII (American Standard Code for Information Interchange) used seven bits to accommodate 128 combinations; enough for lowercase and uppercase letters, all digits and other symbols. So called "extended ASCII" uses 8 bits for a total of 256 combinations, this adds accented letters.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What term describes a wide-band communications system in which the RF carrier varies according to some predetermined sequence?

  • Time domain frequency modulation
  • Correct Answer
    Spread spectrum communication
  • Amplitude-companded single sideband
  • AMTOR

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is the term used to describe a spread spectrum communications system where the centre frequency of a conventional carrier is changed many times per second in accordance with a pseudorandom list of channels?

  • Time-domain frequency modulation
  • Frequency companded spread spectrum
  • Correct Answer
    Frequency hopping
  • Direct sequence

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What term is used to describe a spread spectrum communications system in which a very fast binary bit stream is used to shift the phase of an RF carrier?

  • Correct Answer
    Direct sequence
  • Frequency hopping
  • Phase companded spread spectrum
  • Binary phase-shift keying

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Frequency hopping is used with which type of transmission?

  • Packet
  • RTTY
  • Correct Answer
    Spread spectrum
  • AMTOR

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Direct sequence is used with which type of transmission?

  • AMTOR
  • Packet
  • RTTY
  • Correct Answer
    Spread spectrum

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Which type of signal is used to produce a predetermined alteration in the carrier for spread spectrum communication?

  • Frequency-companded sequence
  • Quantizing noise
  • Random noise sequence
  • Correct Answer
    Pseudo-random sequence

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is it difficult to monitor a spread spectrum transmission?

  • It varies too quickly in amplitude
  • The signal is too distorted for comfortable listening
  • Correct Answer
    Your receiver must be frequency-synchronized to the transmitter
  • It requires narrower bandwidth than most receivers have

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is frequency hopping spread spectrum?

  • The carrier is frequency-companded
  • The carrier is phase-shifted by a fast binary bit stream
  • Correct Answer
    The carrier frequency is changed in accordance with a pseudo-random list of channels
  • The carrier is amplitude-modulated over a wide range called the spread

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is direct-sequence spread spectrum?

  • The carrier is amplitude modulated over a range called the spread
  • The carrier is frequency-companded
  • The carrier is altered in accordance with a pseudo-random list of channels
  • Correct Answer
    The carrier is phase-shifted by a fast binary bit stream

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why are received spread-spectrum signals so resistant to interference?

  • The high power used by a spread-spectrum transmitter keeps its signal from being easily overpowered
  • Correct Answer
    Signals not using the spectrum-spreading algorithm are suppressed in the receiver
  • The receiver is always equipped with a special digital signal processor (DSP) interference filter
  • If interference is detected by the receiver, it will signal the transmitter to change frequencies

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How does the spread-spectrum technique of frequency hopping work?

  • Correct Answer
    The frequency of an RF carrier is changed very rapidly according to a particular pseudo-random sequence
  • If interference is detected by the receiver, it will signal the transmitter to change frequency
  • If interference is detected by the receiver, it will signal the transmitter to wait until the frequency is clear
  • A pseudo-random bit stream is used to shift the phase of an RF carrier very rapidly in a particular sequence

Spread-spectrum transmission relies on a wide range of frequencies rather than a single one to reduce the effects of noise and interference. It requires several megahertz of bandwidth. Frequency-Hopping Spread-Spectrum changes the carrier frequency a number of times per second in a given pattern. Direct-Sequence Spread-Spectrum uses a pseudo-random bit pattern, many times faster than the data stream, to impress Phase Shift Keying on a carrier. For proper demodulation, the receiver must synchronize itself with the incoming stream. Noise and interference do not follow the same pre-agreed patterns and are thus effectively ignored.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

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