Connectivity

Communications

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This is a work in progress . . .

Everything here is from the references shown below, with a few comments in an alternate color.

Last revision:

20170915

The Basics: Analog Communications

To understand analog communications, first consider the humble microphone. It is basically a flexible diaphragm that moves in and out in response to sound waves. The diaphragm is connected to a spiral of wires in a cylinder that moves up and down a magnet. That creates an elecrical signal that changes in voltage depending on how far in the diaphragm is deflected. So what you end up with is a wire that holds varying amounts of electricity that reflect the tone of the sound.

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Photo: An analog microphone
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At the end of the wire you construct a speaker, which is basically the opposite of a microphone. The wires are wrapped around another magnet and the voltage in the wire. The coil of wires are attached to another diaphragm which moves up and down as the voltage in the wire varies. That pulsating diaphragm generates sound.

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Photo: An analog speaker
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There are a lot of problems with this system:

  1. The amplitude of the sound generated diminishes with distance, so the longer the wire the weaker it becomes unless you boost it.
  2. The quality of the sound reproduction depends greatly not only the quality of the microphone and speaker set up, but also on the wires and whatever amplifiers you use to maintain the voltage over distance.
  3. You need wires, lots and lots of wires. And the more wires you have, the more you need to amplify the signal.
  4. To share the signal, you need more wires and more amplifiers.
  5. Storing the analog signal use to require other analog devices, such as tape or vinyl. Each of these increased the "noise" and decreased the quality.
  6. Routing the signal from where it is generated to where it was needed required vast arrays of switches.
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Photo: Lily Tomlin, phone company operator
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In the earliest days of "switched" phone systems, around 1880, you had an operator who simply pulled the line from one user and pushed it into the socket of another user. Each of these connections introduced more opportunities for noise from dirty connections and worn wires. If the operator was busy (or absent), the connection wasn't made.

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Photo: Early phone relays
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Automatic phone switchboards replaced operators with electronic relays. The systems required a dial from the sender which interrupted the signal in a series of pulses. Relays at the swichboard would essentially count the interuptions and turn those into numbers. The numbers added up to a destination for the call and a series of relays would connect until the caller was connected to the receiver. This normally worked for a limited number of phones and anything beyond that series of relays required a human operator to connect the call to another system where automatic relays could make the final connection. Each of these relays presented yet another opportunity for noise and dropped calls.

All of this started to change in the 1960s.

The Basics: Digital Communications

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

Bits

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

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Photo: A simple switch in a simple circuit
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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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Photo: An example transistor diagram
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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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Photo: ASCII-Code in Binary, Michael Goerz
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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.

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.

The Internet

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

The Basics of an Airplane Data System

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Photo:
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[The Gulfstream Journal, September 28, 2018

  • 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.

Air-to-Ground Network

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Photo: Air-to-Ground Network, Gulfstream Connectivity Brochure
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[The Gulfstream Journal, September 28, 2018

  • Air-to-Ground connectivity systems receive their signals from cell phone towers throughout the Continental U.S. and parts of Canada.
  • Air-to-Gorund aircraft antennas are typically two antennas on the underside of the aircraft. Some aircraft with 4G systems may have a single blade antenna.
  • The most advanced air-to-ground systems are capable of speeds up to 9 MBPS.
  • Aircraft will need to be above 10,000 feet to have good air-to-ground connectivity with ground transmitters.

In general, air-to-ground networks are good for:

  • Continental U.S. and portions of Alaska and Canada
  • Email with attachments
  • Voice calls
  • Texting
  • VPN
  • Internet browsing
  • Up to 3.1 Mbps

L-Band

The "L-Band" is what you probably use for your Inmarsat phones and datalink (ADS-C and CPDLC). If you also use it for Internet connectivity it probably has a branded name, such as "Swiftbroadband." As of 2018 we found this gets you about just over 100 KBPS but it could be worse depeneding on where in the world you are. The cost looks to be about $8 per MB but that adds up pretty quickly.

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Photo: L-Band Network, Gulfstream Connectivity Brochure
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[The Gulfstream Journal, September 28, 2018

  • The L-band signal is at the lower end of the microwave spectrum can provide voice and low-speed data connections. Examples are Iridium and Inmarsat I-3 and I-4.
  • Iridium has 66 low-earth satellites which create a global mesh which allows connectivity worldwide, including polar regions.
  • Inmarsat I-4 satellites operate in the L-band and provide global coverage except in the polar regions and various satellite gap transition areas. Swift Broadband is available on the I-4 network and provides speeds up to 432 KBPS.
  • Signals sent over the L-band have a lower through-put but are less susceptible to attenuation, making these systems reliable even in adverse weather conditions. This makes these systems a good choice for safety services.

In general, L-band networks are good for:

  • Worldwide operations (just about)
  • Email
  • Voice calls
  • Texting
  • Up to 0.432 Mbps

Ku-Band Network

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Photo: Ku-Band Network, Gulfstream Connectivity Brochure
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[The Gulfstream Journal, September 28, 2018

  • The Ku-band (12-18 GHz) and Ka-band (27-40 GHz) provide higher bandwidth on a higher frequency signal, but are more susceptible to signal attenuation.
  • The Ku-band will allow users to surf the web and send and receive emails. Streaming is possible but not reliable. ViaSat 3.0 provives up to 6 MBPS.
  • Ku-band covers much of the globe, but not China, Rusia, India, parts of Africa, and the polar regions.
  • Some T.V. systems use the Ku-band on antennas that are receive only.

In general, Ku-Band networks are good for:

  • Worldwide operations (just about)
  • Email with attachments
  • Voice calls
  • Texting
  • VPN
  • Internet browsing
  • Up to 4 Mbps

Ka-Band Network

Ka-Band is offered by Satcom Direct under the name "Jet Coonex" and in 2018 the pricing starts at $10,000 per month for 25GB. Yes, it is very expensive but the speeds are quite good. It is almost like working at your office with a good cable to your local Internet provider.

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Photo: Ka-Band Network, Gulfstream Connectivity Brochure
Click photo for a larger image

[The Gulfstream Journal, September 28, 2018

  • Ka-band coverage is global.
  • Ka-band systmes (like JetConnex) offer the highest available throughput, making video streaming and teleconferencing possible.

In general, Ku-Band networks are good for:

  • Worldwide operations (just about)
  • Email with attachments
  • Voice calls
  • Texting
  • VPN
  • Internet browsing
  • Video streaming
  • Up to 15 Mbps

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