Introduction
Chapter 1
COMPUTER NETWORKS
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History
•
Definitions
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Computer networks
•
Distributed systems
COMPUTER NETWORKS
History
- Key technology = Information gathering, processing, distribution - Computer systems - highly centralized
- Merging of Computers and Communications - computer networks
Definition
A computer network consists of computers, printers and other equipments that are connected together so they can communicate with each other.
COMPUTER NETWORKS
Computer networks – a collection of autonomous computers that can exchange information
The connection: - copper wire - fiber optics - microwaves - infrared - communication satellites
DISTRIBUTED SYSTEMS
In a distributed system, a collection of independent computers appears to its users as a single coherent system
Has a single model or paradigm that it presents to its users.
Expl: the World Wide Web
Conclusion: A distributed system is a software system
built on the top of a network.
Uses of Computer Networks
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Business Applications
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Home Applications
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Mobile Users
Business Applications
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Initial applications
• To monitor production • To keep track of inventories • To do payroll
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Resources sharing
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Sharing information
Business Applications of Networks
Business Applications of Networks (2)
The client-server model involves requests and replies.
Home Network Applications
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Access to remote information
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Person-to-person communication
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Interactive entertainment
Access to remote information
• Surfing the WEB for remote information (arts, business, government, history, hobbies, recreation, science, sports, travel,
– or just for fun: comes in too many ways to mention, plus some ways
that are better to left unmentioned
• Many newspapers (magazines and scientific journals) have gone on line and can be personalized ( tell a newspaper that you want everything about corrupt politicians)
• The on-line digital library: many professional organizations (ACM, IEEE Society) have journals and conference proceedings on-line, the book sized notebook computer is a reality!!!!!!!!!!!
• NOTE: All of these applications involve interactions between a person and a remote data base full of information.
Person-to-person communication
• Basically the21 century answer to the 19th centuries' telephone
• E-mail is used by millions of people. It contains audio, video, as well as text and pictures.
• Instant messaging: allows two people to type messages at each other in real time
• Chat room: a multiperson version
• Worldwide news groups: with discussions on conceivable topic
• Peer-to- peer communication
– in this form individuals who form a loose group can communicate with others in the group.
– Every person can communicate with one or more other people
Home Network Applications (2)
In peer-to-peer system there are no fixed clients and servers.
Person-to-person communication
• The next generation peer-to- peer systems eliminates the central database by having each user maintain his own database locally as well as providing a list of other nearby people who are members of the systems. The lookup process can be repeated indefinitely.
• Other communication-oriented application include using the Internet to carry telephone-calls, video phone and internet radio. Telelearning
• Legal applications for peer-to-peer communications:
– fans sharing public domain music, families sharing photos, e-mail is also a peer-to peer application.
Interactive entertainment
• A huge and growing industry
• Important application:
– video on demand, live television
– Game playing; if the games are played with goggles and three-dimensional real-time we have a kind of worldwide shared virtual reality
– ……….
•
Electronic commerce
• Home shopping is already popular and enables users to inspect the on-line catalogs of thousand of companies. It provides a lot of abilities:
– Instant video of any product
– On-line technical support
• Access to financial institutions:
– Manny people already pay their bills, manage their bank accounts and handle their investments electronically
• Electronic flea markets
– On line auctions of second-hand goods have become a massive industry. They are more of a peer-to-peer system, sort of consumer-to-consumer.
– Some of these forms e-commerce have acquired cute little tags based on the fact that “to” and “2” are pronounced the same.
Home Network Applications (3)
Some forms of e-commerce.
Mobile Network Users
Network Hardware
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Local Area Networks
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Metropolitan Area Networks
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Wide Area Networks
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Wireless Networks
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Home Networks
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Internetworks
Network hardware
Types of transmission technology
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Broadcast links –single comm channel
shared by all the machines
•
Point-to-point links – many
connections between individual pairs
of machines- multiple routes
Broadcast Networks (2)
Classification of interconnected processors by scale.
Broadcast Networks (2)
Classification of interconnected processors by scale OBS
Distance is important as a classification metric because different techniques are used at different scales.
Local Area Networks
Privately owned networks within a single building or campus of up to a few km in size.
Classification criterion (1) their size
(2) the transmission technology (3) their topology
(1) The worst case transmission time is bounded
(2) A cable to witch all machines are attached
(3) Bus Ring
Local Area Networks
Two broadcast networks
(a) Bus
Local Area Networks
(a) Bus
Expl: IEEE 802.3, called Ethernet is a bus based broadcast network with decentralized control operating at
10Mbps to 10Gbps
(b) Ring
Expl: IEEE 802.5 (the IBM taken-ring) is a ring based LAN operating at 4 and 16 Mbps.
Broadcast Networks
• Static allocation:
- time division into discrete intervals (slots) - use a round robin algorithm
- wastes channel capacity
• Dynamic allocation:
– Centralized:
- a single entity (bus administration unit)
– Decentralized:
- each machine decides for itself to transmit - many algorithms exists to bring order
Metropolitan Area Networks
A metropolitan area network based on cable TV.
Wide Area Networks
Wide Area Networks
Relation between hosts on LANs and the subnet
Separation of the pure communication aspects of the network from the applications aspects ( the hosts) greatly simplifies the network design
The subnet elements:
- transmission lines ( copper wire, optical fiber, radiotrans. lines)
- switching elements – specialized computers
Wide Area Networks
Packet – switched network
- all packets from a given message must follow the same route - each packet is routed separately – routing decisions are made
locally
A subnet organized according to this principle is called a store-and – forward or packet switched network
A satellite system
- each router has an antenna
Wide Area Networks (2)
The principle of PS WANs
A stream of packets from sender to receiver.
Wireless Networks
Categories of wireless networks:
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System interconnection networks
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Wireless LANs
Wireless Networks (2)
(a)System interconnection-Bluetooth configuration (b)Wireless LAN
Wireless Networks (3)
1) System interconnection networks
- Ex: Bluetooth, a short range wireless network - Uses the master-slave model
- the master impose: what address to use - when they can broadcast
- how long they transmit
Wireless Networks (4)
2) Wireless LANs
- Every computer has a radio modem and antenna - Uses:
- in small offices - in older buildings - conference rooms
Standard for wireless LANs: IEEE 802.11
Wireless Networks (5)
3)Wireless WANs:
• Low-bandwidth wireless system Ex: The radio network used for cellular tel.
- Operate at rate up to 50Mbps - Over distances of tens of meters
• Cellular systems operate below 1Mbps
• The distance between the BS and the computer or telephone is measured in km
• High bandwidth wide area wireless networks:
-
high-speed Internet- local multipoint distribution service -Standard: IEEE 802.16
Wireless Networks (6)
(a)Individual mobile computers
(b) A flying LAN
Home Network Categories
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Computers (desktop PC, PDA, shared peripherals
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Entertainment (TV, DVD, VCR, camera, stereo, MP3)
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Telecomm (telephone, cell phone, intercom, fax)
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Appliances (microwave, fridge, clock, furnace, airco)
Internetworks
• Def: A collection of interconnected networks is called an internetwork or internet
– Expl: a collection of LANs connected by a WAN
Subnet WAN
Internetworks (2)
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Subnet – networks – internetworks : differences
– Subnet make sense in the context of wide area network = collection of routers and communication lines
– Subnet + hosts = network
– Internetwork = Connecting a LAN and a WAN
Network Software
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Protocol Hierarchies
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Design Issues for the Layers
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Connection-Oriented and Connectionless Services
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Service Primitives
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The Relationship of Services to Protocols
Protocol Hierarchies
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Stack of layers or levels
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The name, the no., the content, the function – differ
from network to network
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Each layer – services – higher layers
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Each layer = virtual machine
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Layer n (M1) ↔ Layer n (M2) = the rules and
conventions = PROTOCOL
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A protocol is an agreement between the
communicating parties on how communications
is to proceed
Protocol Hierarchies (2)
A five layers network
Layers, protocols, and interfaces.
Protocol Hierarchies (3)
•
Peers:
» Processes» Hardware devices » Human beings
– Communicate using the protocol
– No data are transferred from layer n (M1) to layer n (M2)
– Data and information control are passed to the layer below
– Interfaces – between adjacent layers
• defines which primitive operations and services the lower layer makes available to the upper one
Protocol Hierarchies (4)
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Defining clean interfaces between layer
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Each layer perform a specific collection of well
understood functions
•
Well designed interfaces make simple to replace the
implementation of one layer
Def: A set of layers and protocols is called a network
architecture
Specifications of an architecture:
- to write the program
- to build the hardware for each layer
A list of protocols used by a certain system, one protocol
per layer is called a protocol stack
Protocol Hierarchies (5)
Design Issues for the Layers
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Addressing
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Data transfer
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Error Control
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Flow Control
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Multiplexing
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Routing
Connection-Oriented and Connectionless
Services
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Connection oriented (co)
– Modeled after the telephone system
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Connectionless service (cs)
– Modeled after postal system
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Quality of service: (co), (cs)
Connection-Oriented and Connectionless
Services
Six different types of service.
Service Primitives (2)
•
A service is specified by a set of primitives
(operations) available to a user to accesses the service
– Modeled after the telephone system
•
Protocol stack in the operating system – systems calls
Service Primitives
Five service primitives for implementing a simple connection-oriented service.
Service Primitives (3)
Packets sent in a simple client-server interaction on a connection-oriented network.
Services to Protocols Relationship
The relationship between a service and a protocol.
Reference Models
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The OSI Reference Model
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The TCP/IP Reference Model
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A Comparison of OSI and TCP/IP
•
A Critique of the OSI Model and Protocols
Reference Models (2)
The OSI reference
model.
Reference Models (3)
Network layer – Controls the operation of the subnet
- Packets routes:
- based on static tables
- determined at the start of each conversation highly dynamic – determined for each packet -reflect the network load
- Controls congestion
- Provides Qos (delay, transit time, jitter,…)
- Allows heterogeneous networks to be interconnected
Note In broadcast networks routing problem is simple → network layer is thin or nonexistent
Reference Models (4)
Transport layer – Functions
- Accepts data from above (session layer) - Split it into smaller units
- Pass these to the network layer and
- Ensure that the pieces arrive correctly at the other end - Determines what type of service to provide to the session layer and to the users
- error-free channel that delivers messages or bytes
in order in which they are sent - transports isolated messages
- broadcasts messages to multiple destinations Note End-to-end layer
Reference Models (5)
• Arpanet – DoD (USA), Internet
• TCP/IP Reference Model – 1974 – Cerf and Kahn – 1985 Leiner & al. – 1988 Clark
• Connections to remain intact as long as source and destination machines are functioning
• A flexible architecture – Transferring files
Reference Models (6)
• Solution: Packet-switching network based on a connectionless internetwork layer called INTERNET LAYER
• Internet (generic sense) layer (TCP/IP) – function:
– Permits hosts to inject packets into any network and have them travel independently to the destination (potentially) on a different network
• Analogy with the mail system
• Defines an official packet format and protocol, called IP (Internet Protocol) – Packet routing is the major issue here
• Similar in functionality to the network layer in OSI model
Reference Models (7)
Reference Models (8)
• Transport layer (TCP/IP) – function: allow peers entities on
the source and destination hosts to carry on a conversation
• End-to end protocols:
– TCP (Transmission Control Protocol)
• Reliable connection oriented • Handles flow control
– UDP ( User Datagram Protocol)
• Unreliable connectionless protocol
• For applications without TCP/IP sequencing and TCP/IP flow control
• Applications in which prompt delivery is much important than accurate delivery
Reference Models (9)
Reference Models (10)
• Application layer (TCP/IP) – contains all higher level
protocols
– Early included protocols:
• TELNET – virtual terminal
• FTP - file transfer
• SMTP – electronic mail
– Added later protocols:
• DNS – Domain name system
• NNTP
• HTTP
• Host – to – Network layer
– Host has to connect to the network using some protocol
– Protocol used is not defined, varies from host to host and network to network
Comparing OSI and TCP/IP Models
In common:
• Stack of independent protocols
• Functionality of layers roughly similar
– Transport layer (transport providers) provide an end-to-end
network independent transport service
– The layers above transport are application-oriented users of
the transport services
Many differences between the two models and not between the protocols
Comparing OSI and TCP/IP Models(2)
Concepts central to the OSI model
• Services
• Interfaces
• Protocols
OSI model contribution:
Makes distinction between these three concepts explicit
Comparing OSI and TCP/IP Models (3)
TCP/IP did not originally distinguish between
• Services
• Interfaces
• Protocols – came first and the model was just a description of the existing protocols
More specific and obvious differences
• The number of layers
– OSI - 7 layers – TCP/IP - 4 layers
– Network, transport, application layers included in both models
• The area of connectionless versus conn. - oriented
– OSI
• supports both c-l and c-o communication in the network layer
• Only c – o communication in the transports layer – TCP/IP
• Only c-l communication in the network layer
• C-l and c-o communication in the transport layer- giving the users a chance – important for simple request-response protocols
A Critique of the OSI Model and Protocols
Why OSI did not take over the world
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Bad timing
•
Bad technology
•
Bad implementations
A Critique of the OSI Model and Protocols(2)
The time at which a standard is established is critical to its success.
• The amount of activity surrounding a new subject ( burst of research activity)
• Standards elaboration
• Corporations discover the subject and make investments Note
– The OSI standards protocols got crushed
– The TCP/IP protocols were already in widespread use by research universities by the time the OSI protocols appeared
A Critique of the OSI Model and Protocols(3)
A Critique of the OSI Model and Protocols (4)
Bad technology
• The choice of 7 layers was more political then technical
• Extraordinarily complex
• Some functions reappear in each layer
Recommandation
– Error control must be done in highest layer
A Critique of the OSI Model and Protocols (5)
Bad implementations
• The initial implementations were huge, unwieldy and slow
• People associate OSI with poor quality .
• TCP/IP was part of Berkeley UNIX and was quite good ( not to mention free). People began using it quickly. Bad politics
• Many people, especially in academia thought TCP/IP as part of UNIX.
• OSI was widely thought to be creature of the European telecommunication ministries, European Community and later U.S. Government
A Critique of the TCP/IP Reference Model
Problems:
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Service, interface, and protocol not distinguished
•
Not a general model
•
Host-to-network “layer” not really a layer
•
No mention of physical and data link layers
•
Many protocols were ad hoc produced – now they
are hard to replace
Conclusions
• The OSI model (minus the session and presentation layers) has proven to be exceptionally useful for for discussing computer networks
• OSI protocols have not become popular
• The TCP/IP model is practically nonexistent
• Protocols are widely used
• In this course
– We will use a modified OSI model
Hybrid Model
The hybrid reference model to be used in this course.
Example Networks
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The Internet
•
Connection-Oriented Networks:
X.25, Frame Relay, and ATM
•
Ethernet
The ARPANET
(a) Structure of the telephone system.
(b) Baran’s proposed distributed switching system.
The ARPANET (2)
The ARPANET (2)
• Subnet
– IMPs (Interface Message Processors) – minicomputers
• Connected by 56kbps transm. lines • Reliability – at least 2 connections/IMP
• Datagram subnet
• Network nodes
– IMP + host in the same room
• Messages (host → IMP) at up to 8063 bits • Software
– Subnet software
• The IMP-end of the HOST-IMP connection
• The IMP-IMP protocol
• A source IMP to destination IMP ( for reliability)
– Outside the subnet software
• The Host -end of the HOST-IMP connection
• The HOST-HOST protocol
• The application software
ARPANET, NSFNET
• TCP/IP was created to handle communications over
internetworks
• A convenient program interface to the network – sockets
• DNS (Domain Name System) was created to organize
machines into domains and map hosts names into addresses. It became a generalized distributed database system for storing information related to naming
• NSFNET:
– Consists of a backbone network and the regional networks.
– Was connected to the ARPANET
• ANS (Advance Network and Services)
NSFNET
The NSFNET backbone in 1988.
Internet Usage
The number of networks, machines and users connected to the ARPANET grew rapidly after TCP/IP became the only
official protocol on 1983. When NSFNET and ARPANET were interconnected the growth became exponential.
the glue that holds the Internet together is the TCP/IP reference model and the TCP/IP protocol stack.
Traditional applications (1970 – 1990) • E-mail • News • Remote login • File transfer • New one: www
