Do you need to be an electrical engineer to make a phone call? Of course not.

— James Albright

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Updated:

2017-09-15

But understanding what a data packet is will help you troubleshoot your aircraft's phone system or make decisions on what you need and do not need to satisfy your connectivity needs on your next trip.

1 — Bits

2 — Bytes

3 — Bytes to data

4 — Packets

5 — The Internet

6 — The basics of an airplane data system

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1

Bits

You can't talk data without first considering a "bit" . . . A bit is a solitary item of information that is either true or false. You generate a bit electrically with a switch which is either connected or isn't.

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A simple switch in a simple circuit

A transistor is basically a switch. You sandwich one "semiconductor" between two others. Depending on what kind of voltage you give the semiconductor in the middle, the two semiconductors on the outside are connected or aren't.

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An example transistor diagram

2

Bytes

So our simple electronic switch can turn a current on or off, and therefore generate a "1" or a "0" as an output. If you put eight of these transistors together, you can increase the possible outcomes from the simple 0 or 1, to any nmber from 0 to 255. How is that possible?

One byte, as we have seen, can reprented a 0 or a 1.

Two bytes, on the other hand, can represent four numbers:

  • First byte 0, second byte 0 . . . Number 0
  • First byte 1, second byte 0 . . . Number 1
  • First byte 0, second byte 1 . . . Number 2
  • First byte 1, second byte 1 . . . Number 3

When you add a third bit, the count goes to 8. A shorthand for this effect is found by raising the number of options (2) by the power of the number of bits. So if you do this eight times to form a byte, the number of options is n = 28 = 256.

So what can you do with 256 choices in a single byte of information? Here is one solution, known as "ASCII" . . .

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ASCII-Code in Binary, Michael Goerz


3

Bytes to data

The beauty of each byte of information is you can string them together to make sentences, parapraphs, books, and so on. But it goes beyond that. You can convert a photo, for example into a string of bytes. (You break down the photo into little squares, called pixels, and you describe that pixel as black or white (0 or 1) or some other combination of colors. You agree on how the string of numbers are interpreted by the sender and receiver and you can very accurately reproduce the image.) Do that enough times and you have a video. The same holds true for audio.


4

Packets

Just like the analog system, sending these 0s and 1s over wires can introduce noise and when that happens you lose data. But if you package an agreed upon number of bytes into a packet, you have a way of making sure that what you sent is indeed what is received. What you do is add up all those individual bytes (each one a number from 0 to 255), and send that either as the first or last part of the packet. If the received packet doesn't add up to the expected value, you resend the packet until it does.


5

The Internet

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The Internet (the black box they are staring at), from https://www.youtube.com/watch?v=iDbyYGrswtg

If you put two computers together so they can exchange information, you have a network. The exchange of information usually happens using data packets. The U.S. Department of Defense came up with a way to do that over several networks in 1966, soemthing called the Advanced Research Projects Agency Network (ARPANET), also known as the Defense Advanced Research Projects Agency Network (DARPANET). This led eventually to the Internet as we know it today, a network that can connect any system to any other system. Most telecommunications systems use the Internet as the medium with which to exchange data packets. Your phone calls, for example, probably leave your local system bound for the Internet, where the intended receiver takes them for eventual rebroadcast to another phone.


6

The basics of an airplane data system

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Example aircraft Internet connectivity

  • Data within the airplane is sent in "packets" via a router to either a SATCOM transceiver and antenna or to an Air-to-Ground tranceiver and antenna. From there they are sent to the Internet and to their intended destinations. These destinations return data packets in the reverse order. All of this happens in milliseconds.
  • Satellite-based systems are divided into three frequency bands: L-band, Ku-band, and Ka-band.

Source: The Gulfstream Journal, September 28, 2018

References

(Source material)

Connectivity Solutions, Gulfstream, gulfstream.com/connectivity

The Gulfstream Journal, September 28, 2018, www.mygulfstream.com

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