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In the Beginning ...

What does it take to send a message?

Try an experiment: the party game of telephone

• An initial message is whispered to the first person in the chain
• He or she repeats it to the next person in the sequence
• At the end, the message usually come out quite differently than when it started!

Sending a message reliably is not as easy it seems.

Thought experiment #1: how to send a message via smoke signals

• Need to encode the message in series of smoke puffs
• Smoke puffs can be long or short: these are the symbols in the smoke alphabet
• Certain patterns of long and short puffs could represent a pre-arranged signal ("the enemy is coming!")
• Or map certain sequences in the smoke alphabet to letters (a short puff = "a", a long puff = "b", two short puffs = "c", and so on).
• This leads to a series of questions:
  • How many sequences do we need to encode messages in English?
  • How do we indicate the start and end of a message?
  • How do we know that the intended receiver got the message?
    • "Ready to send" "Clear to send" handshake

Thought experiment #2: how to send a message via runners

• Write down the message and give it to a runner to deliver it to the intended receiver
  • Many receivers can see the smoke signals at the same time. This is broadcast communications.
  • Runner can deliver the message to only one receipient at a time. This is point-to-point communications.
• How do we know that the message was received?
  • We can expect an acknowledgement return message, sent from the recipient back to the sender via the runner.
  • We can wait for a certain period of time to receive the acknowledgement
  • . If the time expires (times out), we send the message again.
• What happens if an enemy intercepts the message?
  • The message could be written in code (cryptography = secret writing)
    • A very simple transposition code:
    • A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
    • B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
    • this is a secret message
    • uijt jt b tfdsfu nfttbhf

Key elements of building a communications systems:

1. Messages are constructed from symbols in a basic alphabet.
2. Each symbol can represent a complete message or symbols can be composed to construct arbitrary messages.
3. A signaling convention is needed to ensure that the receiver has seen the symbol being sent before the next symbol can be sent.
4. It is useful if the receiver can signal to the sender to "send faster" or "slow down." This is called flow control.
5. A signaling convention is needed to ensure that the receiver has received the entire message before the next message can be sent. This is called error control.
6. It is useful if the receiver has some method for determining if the message is correct, before sending it on. This is called error checking.
7. If messages are to be kept secret, they can be encoded using cryptography. This process is called encryption.

Historical Discussion

Human Messengers

490 BC: Phidippides runs from Marathon to Athens (36.2 km, 22.5 miles) to warn the Athenians of an approaching Persian army (he supposedly died from the strain!).

Human messagers on foot or horseback in Egypt (Second Millennium BC) or China (First Millennium BC). Relay stations on major roads.

approx. 650 BC: Babylonian kings place guard stations along major roads. Messages travel by relay courier from one station to the next. Fire beacons introduced to permit signaling from one station to the next without the need for human runners.

"We have observed still another device of Cyrus' (King of Persia, circa 530 BC) for coping with the magnitude of his empire; by means of this institution he would speedily discover the condition of affairs, no matter how far distant they might be from him: he experimented to find out how great a distance a horse could cover in a day when ridden hard, but not so hard as to break down, and then he erected post-stations at just such distances and equipped them with horses, and men to take care of them; at each one of the stations he had the proper official appointed to receive the horses and riders and send on fresh ones. They say, moreover, that sometimes this express does not stop all night, but the night-messengers succeed the day messengers in relays, and when this is the case, this express, some say, gets over ground faster than the cranes." Xenophon (430-355 BC) in Cyropaedia.

14 AD: Roman relay system

• 80 km (50 miles) per day for regular mail
• 160 km (100 miles) per day for express mail

1280 AD: Kublai Khan

"Let us turn now to the system of post-horses by which the Great Khan sends his dispatches. You must know that the city of Khan-balik is a centre from which many roads radiate to many provinces, one to each, and every road bears the name of the province to which it runs. The whole system is admirably contrived. When one of the Khan's messengers sets out along any of these roads, he has only to go 25 miles and there he finds a posting station. Here the messengers find no less than 400 horses, stationed here by the Great Khan's orders and always kept in readiness for his messengers when they are sent on any mission. And you must understand that posts such as these, at distances of 25 or 30 miles, are to be found along all of the main highways leading to the provinces. The whole organization is so stupendous and so costly that it baffles speech and writing." Marco Polo

 

• 200-250 miles per day; 300 miles per day in extreme circumstances
• "poste haste": riders signal their coming via horns, change horses in relay fashion

1861 AD: US Pony Express.

• Covers 150 km per day, rider changes horse every 15 km.
• Covers 3200 km from Missouri to California in 10 days.
• Note that the Pony Express only lasted from March 1860 to October 1861!

Pidgeons

776 BC: Homing pidgeons used to announce the outcome of the Olympic Games to the Athenians.

1918 AD: British Air Force used over 20,000 homing pidgeons during WW I

1981 AD: Pidgeons used to transmit negatives of drawings from Sunnyvale to a test station 40 km (25 mi) away.

"The pidgeon took just half the time and less than 1% of the dollar amount of the car. Over a 16-month period the pidgeons transmitted hundreds of rolls of film and lost only two." Jon Bentley

Mirrors and Flags

How do you transmit messages faster than is possible with (human, horse, or bird) muscle power?

Heliographs

Using mirrors to signal messages: helio = sun, graph = to write, that is, write with the sun.

37 AD: Used by the Roman Emperor Tiberius

"For nearly ten years during the reign of the wise but unpopular emperor Tiberius, Rome was ruled from the island of Capri. Each day he sent orders to the mainland by a type of "heliograph" which transmitted the sun's rays by means of a mirror of polished metal."

There is some dispute about whether Tiberius used mirrors or some form of signal torches.

1810 AD:

"Prof. Gauss ... invented a device to direct a controlled beam of sunlight to a distant station. It included "silvered and unsilvered mirrors" fixed at right angles to each other. The operator looked in the unsilvered mirror at the distant station. Then he turned both mirror's so the sun's image (reflected faintly from the plain surface of the unsilvered mirror) was superimposed over the distant station, automatically directing the beam from the silvered mirror in the same direction."

This device was extensively used by the British and American armies as a "wireless" field telegraph. In 1869, Henry Mance added movable mirror so device could be used to signal Morse code. Used extensively in British India up to 1890.

Used by U.S. General Miles in battles with Native Americans in Arizona in the late 1880s. Set up 27 signaling stations, 40-50 km apart. Averaged about 16 words per minute.

1851 AD: Charles Babbage invents a light-flashing machine, the "occulting telegraph:"

"I then, by means of a small clock-work and an argand lamp, made a numerical system of occultation, by which any number might be transmitted to all those within sight of a source of light."

What happens if the day is overcast? Or if the sun is behind your back?

Flags

Signaling with coded flags

Early references in Plutarch (46-120 AD) about Greek naval battles circa 410 BC.

900 AD: Byzantine Navy:

"During naval operations, the captains of the ships were expected to observe the "pamphylus" of the admiral, who gave orders by signaling from different sides and heights of the central flagship with banners of various colors, or with firs and smoke. A whole code of signals existed with which the commanders and their crews had to be acquainted."

1738 AD: de la Bourdonnais introduces numerical code for flags. 10 colored flags indicating the numerials 0-9. 3 sets of flag yield 1000 different signals.

Question: to represent the 26 letters of the alphabet and the ten numerials, what is the minimum number of flags you would need using a system along the lines of de la Bourdonnais?

• 36 flags maximum, one for each symbol.
• or 2 x 2 x 2 x 2 x 2 x 2, 12 flags in a binary code
• or 4 x 4 x 3, 11 flags in a quartary code
• or 8 x 5, 13 flags in an octal code
• In each of the cases, it is just as easy to represent 64 unique configurations of the flags, so add a variety of special, frequently encountered messages that receive their own unique flag code.

1763 AD: de Bigot, founder of the French Marine Academy in Brest.

• 336 distinct flag signals for predefined events or ship-to-ship commands
• Protocol rules: e.g., preparatory signal flag for synchronization, receiver must repeat message to indicate he received it, repeater vessels to "broadcast" a signal to the fleet
• Alphabetical list of all signals used; number associated with each signal
• Associated standard manuevering diagrams

Cannons, flares, lights for transmitting coded messages during night or fog.

1790 AD: Lord Richard Howe, British Royal Navy, The Howe Code

• Ten colored flags
• Six special flags for control codes: ack, termination, etc.
• Dictionary of 260 numbered entries, extended in 340 in 1799

1803 AD: Admiral Sir Home Popham, British Royal Navy, "Telegraphic Signals of Marine Vocabulary," known as the Popham Code, and later as the .

• 10 colored flags = A - K (I and J share same flag!) or numerials 0 - 9
• 15 combinations of two flags = rest of alphabet
• 3000 numbered sentences and phrases in 3 series (3 different indicator flags plus 3 flags)

Examples of the Popham Code Book

#	Word	#	Word	#	Phrase
26	Able	1026	Aback	2026	Shall I leave off action?
27	Above	1027	Abate-d-ing-ment	2027	I have been in action
28	About	1028	Abrupt-ly	2028	I have not seen any action
29	Abreast	1029	Abundance-t-ly	2029	I have heard of an action
30	Absence-t-ed-ing-tee	1030	Accomodate-d-ing	2030
31	Absolutely	1031	Accomplish-ed-ing-ment	2031	I am clear for action
32	Accept-ed-ing-ance	1032	Account-ed-ing	2032	Shall I commence action?
33	Accidently	1033	Accurate-ly	2033
34	Accompany-i-ed	1034	Accuse-d	2034	In coming into action, do you lead?
35	According-ly	1035	Acknowledge-d-ing-ment	2035	Go to the Admiral's office

1805 AD: The Battle of Trafalgar, Admiral Horatio Nelson

"England expects every man will do his duty": seven words are in the code book (single hoist of 3 flags), "duty" must be spelled out with single and double flag hoists

Wig Wag

1860 AD: Albert James Myer, US Army Doctor: signaling scheme with flags or torches based on two basic motions: move to the left or right.

Fire Beacons

1184 BC: Siege of Troy: systematic description of fire signals to transmit messages (Homer, Aeschylus, Vergil).

500-400 BC: Herodotus, Thucydides describe use of beacon signals by the Greeks and Persians.

113 AD: Emperor Trajan: torch signaling stations.

No descriptions of permanent installations, two-way communications, or a system of encoding messages.

Watchmen and Stentors

315 BC: Persia: first recorded use of stentors, that is, shouters, who passed messages between calling posts. Used for short, high priority messages. Fire signals used at night. Other messages by mounted couriers.

52 BC: Julius Caesar: via load calls across fields and plains, a message could be sent 150 miles between sunrise and sunset.

The First Telegraphs

150 BC: Polybius Histories.

"Now in former times, as fire signals were simple beacons, they were for the most part of little use to those who used them. ... it is chiefly unexpected occurences which require instant consideration and help -- all such matters defied communications by [prearranged] fire signal."

Describes Aeneas' scheme for signaling messages (350 BC) using a pre-calibrated water vessel to synchronize sender and receivers. Raising a torch when the unstoppered vessel reaches a particular level indicates a particular prearranged message.

Still requires prearrangement of message meanings. This is the first recorded description of the need for a method to encode the alphabet into signals in order to transmit arbitrary messages.

Scheme he used: 5 groups of 5 letters (Greek has 24 letters), encoded by 10 torches. First set of five indicate which group (raised on the left), then second set of five indicate letter within group (raised on the right).

Initial synchronization by raising and lower two torches. Out of band signaling by raising torches from both right and left at the same time. Viewing tubes to focus the field of view.

Modern reconstructions have been able to achieve eight letters per minute.

Not much progress over the next 2000 years! However, the development of the telescope in the 16th Century made it possible to separate the signaling stations by a greater distance. Clocks improved the synchronization of the communications.

Long Distance Communications Methods

Method	First Recorded Use		Last Recorded Use
Pidgeons	Egypt	2900 BC	California	AD 1981
Runner/Couriers	Egypt	1928 BC	Pony Express	AD 1860
Beacons/Torches	Troy	1184 BC	England	AD 1588
Calling Posts	Persia	400 BC	Germany	AD 1796
Heliographs	Greece	400 BC	Arizona	AD 1886
Flags	Greece	400 BC	Maritime Use	today