Introduction
	Introduction
		Click here for subtitle 
		CSE 123A: Computer Networks
		
		George C. Polyzos
		
		Department of Computer Science and Engineering
		University of California, San Diego
	Motivation
		from Centralized
			to Distributed Systems
		Integration of 
			Computers & Communications
	Definitions
		Computer Network:
			an interconnection of autonomous computers
		Distributed Computer System:
			transparency is key
	Course Goals
		present principles of computer network design
		discuss trade-offs of various technologies
		familiarize with Internet (TCP/IP) protocols
		this is not a course on:
			operating systems
			distributed or network OS's
			distributed systems
			data communications
	Why Networks?
		Economics: Resource Sharing
			devices
			processing
			information
		Reliability
			decentralization
			replication
		Economic Reality
			micros vs. mainframes
		New Communication Modes
			remote collaboration
	Network Structure
		Hosts
		Communications Subnet
			interconnection characteristics
				point-to-point channels
				broadcast channels
					LANs (busses, rings), satellite channels
			transmission lines
				digital vs. analog transmission
			switching elements
				nodes - routers - gateways
	Switching Methodologies
		Circuit Switching
		Store-and-Forward
			Message Switching
			Packet Switching
			Cell Switching (ATM)
		Types of Traffic
			bursty, interactive
			bulk data transfers
			continuous media: audio, video
	Network Architectures
		Structured, Modular, Hierarchical designs
		Layers: abstractions
			virtual communication paths
			shielding from implementation details
			advantages
				reduce/control complexity/cost
				incorporation of different/new technologies
			disadvantages
				performance
	The ISO/OSI Reference Model
		Principles
			layer for different level of abstraction
			layer performs well defined function
			layer where standard protocols exist
			layer boundaries minimizing information flow across
			"reasonable" number of layers
	Design Issues for Layers
		connection establishment
		addressing
		direction of communication
			simplex
			half-duplex
			duplex
		error control
		flow control
	Physical Layer
		transmission of raw bits
		mechanical, electrical, and procedural interfaces
	Data Link Layer
		provide "error-free" transmission channel to network layer
		convert bits to frames
			frame boundaries
			error control
				ARQ
				duplicates
			flow control
	Network Layer
		controls the operation of the subnet
		routing
			static
			dynamic
		congestion control
		accounting (packets, bytes, etc.)
	Network Layer (cont.)
		internetworking
			segmentation
			addressing
			sequencing
			accounting
		broadcast subnets: thin network layer
	Transport Layer
		end-to-end communication path (usually reliable)
		isolation from "hardware"
		multiplexing/demultiplexing
		end-to-end flow control
	Transport Layer (cont.)
		types of service
			error-free, point-to-point, in sequence,
			flow controlled
			no correctness guarantees
			no sequencing
		establishing/terminating "connections"
			naming/addressing
			intra-host addressing (process)
	Session Layer
		establish sessions
			superset of connections
		dialogue control
		token management
			for critical operations
		synchronization
			checkpoints/restarts
	Presentation Layer
		syntax & semantics of messages
		data encoding (e.g., ASCII to EBCDIC)
		compression
		encryption/decryption
		authentication
	Application Layer
		applications ...
			file transfer, access & management
			e-mail
			virtual terminals
	Connection Oriented & Connectionless Services
		analogies
			connections: telephone system
			connectionless: postal system
		reliable services: ACKs
			file transfers
			digitized voice
		virtual circuits
		datagrams
	Layer Services Terminology
		entities
			software: processes
			hardware: I/O chips, etc.
		peer entities, layer N entity
		Service Access Point (SAP)
			unique address
			UNIX: SAP = socket
	Service Primitives
		request (for service)
		indication (of event)
		response (to an event)
		confirmation (of availability of service)
	Critique of the OSI Reference Model
		the only/obvious/best way?
		complexity of the model
		repetition of functions
		omissions
			data security
			network management
		originally ignored
			connectionless services (added later)
		communications vs. computer mentality
			Request: procedure
			Indication: interrupt
	Network Standardization
		Telephone Companies
			USA
			PTTs
		Standards Organizations
			by treaty (international)
				CCITT
			voluntary
				ISO - members: natl stds orgs (ANSI, DIN, etc.)
		de facto standards
		National Bureau of Standards (USA)
		IEEE
	Historical Notes
		The ARPAnet
			T's terminology (IMP, TIP, etc.)
			TCP/IP
			e-mail (SMTP), ftp, telnet, etc.
			the Internet
		"Public" Networks
			OSI model, X.25, etc.
		IBM's SNA, DECNET
	Historical Notes (cont.)
		LANs
			Ethernet (Xerox, DEC, Intel)
			Token Bus (GM)
			Token Ring (IBM)
		USENET, CSNET, BITNET, EARN