2.4GHZ wifi uses 2.412-2.467 GHz (2412-2467MHz) in North America (2.412-2.462 GHz in the USA), so be careful when transmitting in the 2.4GHz range to not interfere with people's wifi.

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Silly Hint: a low power signal is just a whisper (WSPR)

WSPR (often pronounced as 'whisper') is an acronym for Weak Signal Propagation Reporter and was designed by Joe Taylor (also creator of JT-65 and FT-8) to test propagation of HF frequency radio waves by sending low-power signals and logging decoded signals from other locations to WSPRnet. Stations do not respond to decoded signals, thus it acts as a one-way beacon.

The incorrect options, Olivia, PSK31, and SSB-SC, are all two-way communication modes.

Weak Signal Propagation Reporter (WSPR)

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When forming a packet for transmission the information is organized into a specific order so that the message segments may be properly reassembled and decoded after all parts are received.

Each data packet contain several fields, including the Header, which contains routing and handling information.

The packet also includes an address field indicating where the message is going; a control field, indicating the type of data; a frame number which indicates the section order; the data field; and the frame check or cyclic redundancy check field.

For more info see Wikipedia: Packet Radio

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The Baudot code is a character set used with some radioteletype (RTTY) transmissions. This code assigns each letter of the alphabet with a 5 bit code, with stop and start bits for each character.

Hint: Baudot has 5 letters plus an additional letter (6 total)

For more info see Wikipedia: Baudot code

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Hint: NACK = Not ACKnowledges so “retransmit”

PACTOR protocol is a modulated packet data mode that uses an ARQ, or Automatic Repeat Request, system for error controls. The receiving station will send a message letting the transmitter know that either ACK, the message received corrrectly and is ACKnowledged, or NAK, that the message has errors or is NOT ACKnowledged. In which case, the receiver is requesting that the packet be re-transmitted.

For more info see Wikipedia: PACTOR

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Hint. Excessive attempts drops the line, like a phone.

It will only try a certain number of times before it terminates the connection. This is called a timeout.

PACTOR combines the bandwidth efficiency of packet radio with the error-correction (CRC) and automatic repeat request (ARQ) of AMTOR.

WINMOR is a radio transmission protocol intended to be used in the Winlink 2000 Global Radio E-mail System by amateur radio operators, marine radio stations, and radio stations in isolated areas.[1] WINMOR will complement the PACTOR modes in the high frequency portion of the Winlink system.

These are internet protocols which have safeguards built in which will drop excessive requests to prevent a DoS situation.

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Hint ARQ = Automatic Repeat Request

When transmitting using the packet system for data transmissions, a mode of operation for automatic error control is ARQ which stands for Automatic Repeat ReQuest.

If the receiving station gets a message containing errors the operator will request that the packet be retransmitted. The packet message will re-transmit automatically until the receiver sends a message confirming that the message was received correctly, or the system times out.

For more info see Wikipedia: Automatic Repeat Request.

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Hint: Remember PSK31 uses Varicode for bauds (31 bauds)

Upper case letters use longer Varicode signals and thus slow down transmission

PSK31 uses Varicode.

The idea behind varicode is that symbols often used for transmission like the character "e" is given a smaller length of data, whereas obscure characters like "^" are assigned longer length codes to encode them.

Upper case letters are given longer codes compared to their lower case companions since lower case letters are used much more often than upper case letters.

Note: This is the same concept as International Morse Code - more common letters like "e" (dit) are shorter than less common letters like "h" (dit-dit-dit-dit).

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Hint: “S” = symbol Phase Shift Keying @31 Baud (symbol rate)

(A). The number 31 in the abbreviation for PSK31 is the approximate transmitted symbol rate.

PSK31 is a type of data mode of "Phase Shift Keying" at "31 baud." The actual baud rate used is 31.25 Hz. Similarly, the less common PSK10 and PSK 05 would correspond to baud rates of approximately 10 baud and 5 baud respectively.

For more info see Wikipedia: PSK31

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Hint: FEC forwards (TRANSMITS) redundant data

(C). Forward error correction allows the receiver to correct errors in received data packets by transmiting redundant information with the data.

This type of correction used in packet data transmissions is useful especially over "noisy" bands or in case of interference which may corrupt the signal.

For more info see Wikipedia: Forward error correction (FEC)

Hint: Look for the word “Data” in the answer.

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Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.[1] The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information.[2] With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

Hint: FSK is "identified" with a "mark".

https://en.wikipedia.org/wiki/Frequency-shift_keying

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Hint: since 31 is a baud CODE, remember VARICODE

Hint 2: Both the question and the answer have the word "code" in them.

Varicode is used sending characters in a PSK31 signal.

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Unlike coding systems such as Baudot (5 bits per symbol) and ASCII (7 bits per symbol) which use a fixed length sequence to represent characters, PSK31 uses a variable-length encoding (1-10 bits) which assigns more commonly used characters to shorter symbols.

Variable-length encoding is generally more time efficient, at least when using the English language, as it requires fewer total symbols to be transmitted for a given message. Improved efficiency is important because of PSK31's very low data rate of 31 symbols per second.

The name chosen for the encoding scheme is Varicode which can be remembered as a shortened form of "Variable Length Coding"

Some other digital modes have adopted varicode including PSK63, PSK128 (both essentially faster version of PSK31) and QPSK31, QPSK128, etc. which transmit two bits per symbol instead of one. The FreeDV digital voice mode uses Varicode to efficiently encode a text subchannel in its datastream.

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See also: http://en.wikipedia.org/wiki/Varicode

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(D) Overmodulation

Overmodulation is an often occurring "gotcha" for many new hams intending to work PSK31. The PSK mode requires modulation audio from your computer. However, if the audio volume/amplitude is too high, your radio has no choice but to distort the signal through the use of ALCs (Auto Levelling Circuits). If you're familiar with audio production, this is a similar process equivalent to Dynamic Range Compression.

The ALC prevents the signal from clipping, but introduces harmonics that manifests itself as copies of the original signal on the left and right of the signal.

SILLY HINT: Waterfalls are vertical. Too much water goes over the side.

https://3.bp.blogspot.com/-W14Qrvpj8gI/TsV1LrdpA8I/AAAAAAAAAG0/zwSQ3siYEs0/s1600/Splatter+2.JPG

The signal near 2050Hz shows this effect. Notice the copies of the signal.

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Check out WebSDR to see waterfall displays in action.

Notice that you can move the cursor horizontally along the x-axis in order to select the frequency, and that the display "falls" vertically over time. In other words: the x-axis is frequency, the y-axis is time.

You'll see a whole bunch of different signals. The CW signals will be very narrow, the data signals will tend to be quite wide, and phone signals somewhere in-between. If the signal is good, the display will show a bright line. A more attenuated line indicates that the signal isn't as strong. In other words: strength is intensity.

It is often advantageous to monitor the strength and frequency of multiple signals over a short duration of time. A waterfall display accomplishes just this.

If you've ever dealt with a spectrum analyzer, spectrogram, or a Fourier transform of a signal, you should notice that the horizontal axis of all the above plots is frequency. A waterfall plot adopts this convention.

(Silly tip: In a real waterfall, water "falls" over time, so time is the vertical axis. That may help discard incorrect answers.)

(Also, try this: A body of water typically runs horizontally, then vertically down a falls with greater intensity).

(Another tip: Frequency Horizontal+Signal Intensity+Time Vertical) = FH+SI+TV = Fuzzy TV)

(Sily tip: A waterfall is vertical. The last word in the correct answer is "vertical")

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