CPSC 3600 - DAY 2
JANUARY 13, 2017

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Computer Network - an interconnected collection of autonomous computers 
able to exchange information.  A computer network usually requires users
to explicity login onto one machine, explicitly submit jobs remotely,
move files/data around the network.

Distributed Systems - The existence of multiple autonomous computers in a 
computer network is transparent to the user.  The operating system automatically
 allocates jobs to processors, move files among various computers without 
explicit user intervention.

Motivation for Computer Network Applications
--------------------------------------------

Business Applications:

	.Resource Sharing: Data, programs, equipment are avaliable to users
	regardless of their physical location.

	.High reliability: Files and databases could be duplicated on multiple
	machines.  Multiple CPUs prevent total system loss.

	.Economically sound: Networked micro computers using the client-server
	model offer better price/performance ratio than mainframes.

Personal Applications:

	.Access to remote information: Financial information, database access
	using peer-to-peer model, the Web, newsgroups.

	.Person to person communication: Email, voice, video-conferencing.

	.Interactive entertainment: Video on demand, interactive TV, networked
	games.

Classification of Computer Networks
-----------------------------------

Based on transmission mode:
	
Broadcast networks:
	
	.Use a single communication channel shared by all computers in the
	network.

	.Short messages(packets) are sent by any machine and recieved by all
	other computers on the network.

	.An address is used in the message to select the target machine.

	.Most localized networks are broadcast networks.

Point to point networks:

	.Consist of many connections between individual pairs of machines.

	.A message (packet) may have to visit one or more intermediate 
	machines before reaching its intended target.

	.Routing algorithms play an important role.

	.Most large are networks are point-to-point networks.

Based on network scale:
	
	.Personal area networks (PANs): a person

		.Ex: Bluetooth that connects a computer with it's peripherals
	
	.Local area networks (LANs): room, building, campus
		.Broadcast using a bus (Ethernet) or ring topology (Token ring)
		
		.At any instant only one machine can transmit successfully
		unless switches are used to segment the network

		.Operate at 10 Mbps (Ethernet), 100 Mbps (Fast Ethernet),
		1000 Mbps (Gigabit Ethernet).

	.Metropolitan area networks (MANs): city
		.Best known example of MANs are cable television networks

	.Wide area networks (WANs): large geographical networks
		.consists of hosts (machines intended to run applications)

		.Hosts and their LANs are connected by a communication subnet
		which carries messages from host to host.

	.Global networks (Internet)
		.Internetworks are networks with different software and
		hardware interconnected as one network.

		.Gateways are computers used to translate between the different
		hardware and software components of the internetwork.

		.The internet is the largest example of internetworks.

		.The internet started in the late sixties as ARPANET, a gov't
		sponsored network between a small number of universities
		and gov't centers.

		.The National Science Foundation expanded ARPANET into NSFNET
		several thousand hosts in 1988.

		.TCP/IP emerged as its standard network software.