CPSC 3600 - DAY 14
MARCH 13, 2017

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				MEDIA ACCESS MECHANISMS
				-----------------------

MAC Protocols
		
		Control access to shared medium
		Two types of channel allocation:
			Static
			Dynamic

		Static channel allocation suffices when the set of
		communicating entities is known in advance and does not change;
		most networks require a form of dynamic channel allocation.

		For example: cellular telephone in a city.  As users move, turn
		on/off their phones, the number of phones operating in a range
		of a cell tower varies.

Channelization Protocols

		Employ and extend basic multiplexing techniques.
		May be static or dynamic
		Three basic types:
			FDMA - Frequency Division Multi-Access
			TDMA - Time Division Multi-Access
			CDMA - Code Division Multi-Access

FDMA
		Central controller provides a dynamic channel assignment
		A station uses a reserved control channel to communicate with 
		the controller.
		The station makes a request, the controller chooses a frequency
		that is currently unused and informs the station.
		The station uses the assigned carrier frequency for all comms.

TDMA
		Each active station is assigned a sequence number from 1 to N
		Each station transmit in order 1,2,3,...,N
		TDMA offers dynamic allocation, where a station is assigned a
		time slot when the station first appears on the network.

CDMA
		Allows multiple stations to transmit at the same time by 
		encoding each transmission mathematically.
		Refers to the calculation of Code Division Multiplexing.

Control Access Protocols
		
		Polling - centralized controller repeatedly polls stations and 
		allows each to transmit one packet
	
		Reservation - stations submit a request for the next round of
		data transmission

		Token passing - stations circulate a token; each time it
		receives the token, a station transmits one packet.

Polling
		A centralized controller is used.
		Each station is given an opportunity to transmit a packet, 
		using:
			Round robin order - each station has an equal
			opportunity
			Priority order - some stations have more opportunity

Reservation
		Often used with satellite systems.
		Each round of packet transmission is planned in advance.
		Stations inform a controller if they have data to send.
		Controller transmits a list of the stations that will be
		transmitting.
		Stations use the list to know when they should transmit.

Token Passing
		Special packet known as a token passed among senders (as in 
		ring topoligies)
		If a station has packets, it sends one packet each time token
		arrives.

Random access protocols

	+-------------------+------------------------------------------------+
	| Type		    |	Description				     |
	+-------------------+------------------------------------------------+
	| ALOHA		    | Historic protocol used in early radio network  |
	| 		    | in Hawaii; popular in textbooks and easy to    |
	|		    | analyze, but it is not used in real networks   |
	+-------------------+------------------------------------------------+
	| CSMA/CD           | Carrier Sense Multi-Access with Collision      |
	| 		    | Detection.  The basis for original Ethernet    |
	|		    | and most used random access protocol	     |
	+-------------------+------------------------------------------------+
	| CSMA/CA	    | Carrier Sense Multi-Access with Collision      |
	| 		    | Avoidance.  Basis of Wifi networks             |
	+-------------------+------------------------------------------------+

ALOHA
	Used in early network in Hawaii (ALOHAnet)
	Single, powerful transmitter in a central geographic location
	surrounded by a set of stations.
	Each station had a transmitter capable of reaching the central
	transmitter, but not powerful enough to reach all the other stations.

	Two carrier frequencies: inbound and outbound

	When a station has a packet to send, it transmits the packet on inbound
	frequency.

	The central transmitter repeats the transmission on the outbound freq.
	which all stations receive.

	The sending station knows that the transmission is successful if it 
	receives the copy of it's packet

	Interference is caused by two stations attempting to transmit 
	silmultaneously.  If inbound packets collide, each sender waits a
	random time and retransmits

CSMA/CD
	Used in original Ethernet (1973)
	Provides access to shared medium
	Principle features to handle collisions:
		Carrier Sense: monitor the cable to detect whether another 
		transmission was alreach in progress.
		Collision detection: if the signal on the cable differs from 
		the signal that the station is sending.
		Uses binary exponential backoff: to avoid the second collison,
		each computer doubles the range from which a delay is chosen.

	ALGORITHM:

	When a packet is ready, perform CS (wait for access)
	Delay for the interpacket gap
	Set variable x to the standard backoff range, d
	Attempt to transmit the packet and perform CD
	while (collision occurred during transmision) {
		choose q to be a random delay between 0 and x
		delay q microseconds
		double x in case needed for next round
		attempt to transmit the packet and perform CD
	}

CSMA/CA
	Alternative to CSMA/CD
	Used in wiereless networks (Wi-Fi)
	Needed because signals have limited distance, (delta)
	The hidden station problem: computer 1 cannot receive transmission when
	computers 2 and 3 communicate.

	Communicating pair exchanges Ready to Send (RTS) and Clear to Send (CTS)
	before transmitting packets.
	Any computer less than (delta) away will hear at least one of the RTS/
	CTS messages