CPSC 3600 - DAY 6
JANUARY 25, 2017

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LAYERS OF OSI
-------------

	.Physical Layer
		.This layers is concerned with the transmission of bits, making
		sure that when a sender sends a 1 bit, it is recieved as a 1
		bit, not as a 0 bit.
		.Some of the issues handled in this layer are:
			.How many volts for 0, how many for 1.
			.Number of bits per second to be transmitted.
			.Whether it is a one-way or two-way communication.
			.Many standards have been developed for serial
			communication lines

	.Data Link Layer
		.This layer groups bits into frames and ensures their correct
		delivery, free of errors.
		.It adds some bits at the beginning and the end of each
		frame plus the checksum.  Receiver of the frame verifies the
		checksum and requests retransmission if the checksum is not
		correct.
			.Checksum are extra bits that are used to determine
			whether the data is correct.
		
	.Logical Link Control and Medium Access Control Sublayers
		.The Logical Link Control (LLC) defines how data is transferred
		over the network medium (cable, wireless) and provides data
		link service to higher layers.
		.The Medium Access Control (MAC) layer defines who can use the
		network medium when multiple computers are trying to access it
		simultaneously.
		.For example: token passing or Carrier Sense Multiple Access
		with Collision Avoidance for Ethernet.
			.MAC layer is used to avoid collisons between machines
			trying to communicate at the same time.

	.Network Layer
		.This layer is concerned with the transmission of packets.  How
		packets are routed from source to destination.  It takes
		routing decisions, eg. the best path to send a packet.
		.Congestion occurs when too many packets are present at the same
		time.
		.In broadcast networks, the routing problem is simple, so the
		network layer is often thin or even nonexistent
		.The network layer may be quite complex in a large network.
	
	.Transport Layer
		.This layer ensures reliable service and deals with lost
		messages.
		.It breaks the messages into smaller packets, assigns sequence
		number and sends them.
		.Examples are the Transmission Control Protocol (TCP) and User
		datagram Protocol (UDP).
	
	.Session Layer
		.Very few applications use this layer.  It is mostly an enhanced
		version of the transport layer.  
		.It facilitates dialog control (keeping track of whose turn
		to transmit), token management (preventing two parties from
		attempting the same critical operation simultaneously) and
		synchronization (checkpointing long transmissions).
		.Many network implementations do not support it. For example,
		Internet suite does not support this layer.
	
	.Presentation Layer
		.Very few applications use this layer.  This layer is concerned
		with the syntax and semantics of the information transmitted.
		.It defines records and fields in them.  The sender can tell
		the receiver about the format.
		.It makes machines with different internal representations to
		communicate.

	.Application Layer
		.This is a collection of miscellaneous protocosl for high level
		applications.
		.Typical services are electronic mail, file transfer, web
		browsing, connecting to remote terminals, etc.
		.Protocols including HTTP, FTP, telnet, etc.
		.The layers of the OSI Reference Model were never fully 
		adopted by a real network architecture.


TCP/IP REFERENCE MODEL
----------------------

	+----------------------------------------+
	|	APPLICATION			 |	LAYER 5
	+----------------------------------------+
	|	TRANSPORT			 |	LAYER 4
	+----------------------------------------+
	|	INTERNET			 |	LAYER 3
	+----------------------------------------+
	|	NETWORK INTERFACE		 |	LAYER 2
	+----------------------------------------+
	|	PHYSICAL			 |	LAYER 1
	+----------------------------------------+


	.Physical Layer: specifies details about the underlying transmission
	medium, the associated hardware, electrical properties, radio frequency
	and signals.
	.Network Interface Layer/ MAC layer: specifies details about the
	communication over a single network, network addresses, and the max
	packet size.
	.Internet Layer: specifies communication