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1 All __ these ATC applications use the ACARS air-ground link, which was

neither designed __ nor initially approved as an ATC communication medium.

2 __ that purpose, ICAO developed standards and SARPs __ the ATN, which was

designed__ both air-ground and ground-ground communication.

3 __ the latter role, it is intended __ replace the AFTN, which has served the

industry well as a teletype-based, message-switching network __ many years.

4 __ the past 15 years, the Internet, which is based __ a different packet-switched

technology.

5 As ACARS became essential __ airline operations, the limitations __the initial

VHF link became intolerable, first because __coverage limitations and then

because __speed.

6 The former was solved __ two different ways.

7 Long-range data link was implemented first using Inmarsat satellites; this was

the basis __ initial FANS implementations__ the South Pacific.

8 The oceanic coverage provided __ satellites and data link was an improvement

__ HF voice services.

9 All __ the advantages __ data link __ voice communications were

highlighted__ the initial FANS trials and operational use.

10 High frequency data link (HFDL) provided another long-range ACARS

subnetwork that covered the north Polar Regions, which are not reached __

Inmarsat signals.

11 VHF Digital Link (VDL) Mode 2 provided a higher-speed subnetwork __

continental airspace.

12 All air-ground radios connect __ the MU or CMU __ send and receive

messages.

13 The CMU is connected __ all of the various radios that communicate __ the

ground.

14 All message blocks to or __ the aircraft, over any __ the air-ground

subnetworks, pass __ the MU.

15 Although the MU handles all ACARS message blocks, it does not perform a

message-switching function because it does not recombine message blocks __a

“message” prior __ passing it along.

16 It passes each message block __ accordance __ the “label” identifier, and it is

up__the receiving end system__recombine message blocks __ a complete

message.

17 The original OOOI messages were formatted and sent__the MU __ an avionics

unit that sensed various sensors placed around the aircraft and determined the

associated changes __ state. 18 __the modern transport aircraft, many other

avionics units send and receive routine ACARS messages.

19 The multifunction control and data unit (MCDU), along __the printer, is the

primary ACARS interface __ the flight crew.

20 Other units, such as the FMS or the ATSU, will also interact __ the crew for

FANS messages. 21 The vast majority __data link messages today are

downlinks automatically generated __ various systems __ the airplane.

22 The latest avionics __ each of the four sub- networks accepts an ACARS block

as a data message __ a data bus, typically ARINC 429.

23 The subnetwork avionics will then transform the message block __ the signals

needed __communicate __ the ground radio.

24 The original VHF subnetwork that was pioneered __ 1978 uses the same 25

kHz VHF channels used __ATC and AOC voice; the signal-in-space is

sometimes called plain-old-ACARS (POA) __reasons that will become clearer

when we discuss VDL Mode 2.

25 The VHF subnetwork uses a form __ FSK called MSK wherein the carrier is

modulated __ either a 1200 Hz or 2400Hz tone.

26 CSMA is nothing more than the automated version __ voice radio protocols

wherein the speaker first listens __ the channel __ initiating a call.

27 The VHF ACARS subnetwork is an example __ a connectionless link layer

protocol __ that the aircraft does not “log in” __ each ground station along its

route __flight.

28 The aircraft does initiate a contact __ the central message processor, and it does

transmit administrative message as it changes subnetworks.