CPSC 3600 - DAY 10
FEBRUARY 15, 2017

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RELIABILITY AND CHANNEL CODING
------------------------------

	Reliability is very important to computer networking as errors occur
	during transmission.

	Interference: electromagnetic radiation causes noise that disturbs
	radio transmissions and signals travelling across wires.
	Distortion: metal objects can block signals.
	Attenuation: signals become weaker with distance.

	Reliability and Transmission Errors:
		
		Errors cannot be eliminated completely.
		Errors can be detected and corrected.
		Error detection adds overhead.

		Example:
			One bit error in a bank transfer is significant.
			00001010 ($10) -> 10001010 ($138)
		
			One bit error in an image is less important
	
	+---+---+---+---+---+---+---+---+---+---+---+---+---+
Sent	| 1 | 0 | 1 | 1 | 0 | 0 | 0 | 1 | 0 | 1 | 1 |   |   |
	+---+---+---+---+---+---+---+---+---+---+---+---+---+
			
			Burst 7 bits
		| <-----------------------> |
	+---+---+---+---+---+---+---+---+---+---+---+---+---+
Rec'd	| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 |   |   |
	+---+---+---+---+---+---+---+---+---+---+---+---+---+


	Channel coding techniques can be divided into two broad categories:
		Forward Error Correcting
			Add additional information to data that allows a 
			reciever to verify that data arrives correctly and to
			correct errors, if possible
		Automatic Repeat reQuest (ARQ)
			A sender and reciever exchange messages to ensure that
			all data arrives correctly.

	Single Parity Checking

	On the sending side, an encoder adds and extra bit, called a parity bit
	A reciever removes the parity bit and checks it
	The sender and reciever must agree to use even or odd parity

	If using even parity, the sender adds a parity bit of 0 if there is an
	even number of 1 bits, and 1 if the number of 1 bits is odd.

	If using odd parity, the sender adds a parity bit of 1 if there is an
	even number of 1 bits, and 0 if the number of 1 bits is odd.

	Single parity checking can detect errors, but cannot correct them.

	Parity mechanisms can only handle errors where an odd number of bits
	are changed.

	When a burst error occurs in which two, four, or six bit change value,
	the reciever will incorrectly detect the error.

	Dataword: original message

	Codeword: encoded version of the message i.e. the message + add'l bits

	16-Bit Checksum

	This algorithm allows a message to be arbitrarily long, and computes a
	checksum over the entire message

	The message is divided into 16-Bit units, appending zeroes if it does
	not have an exact multiple of 16 bits

	A sender adds the numberic values and transmits the result.

	A reciever performs the same computation, but includes the checksum. If
	the result is zero, the transmission has no error