Chapter 1. Introduction COMPUTER NETWORKS. History Definitions Computer networks Distributed systems

Transcription

1 Chapter 1 Introduction COMPUTER NETWORKS History Definitions Computer networks Distributed systems

2 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 Networks come in many sizes, shapes and forms.

3 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 Business Applications Home Applications Mobile Users Social Issues

4 Business Applications Initial applications To monitor production To keep track of inventories To do payroll Resources sharing Sharing information Business Applications of Networks A network with two clients and one server.

5 Business Applications of Networks (2) The client-server model involves requests and replies. Home Network Applications Access to remote information Person-to-person communication Interactive entertainment Electronic commerce

6 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 19 th centuries' telephone 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 There is no fixed division into clients and servers

7 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, is also a peer-to peer application.

8 Interactive entertainment A huge and growing industry Important application: video on demand, live television Game playing; if the games are played with goggles and threedimensional 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-toconsumer. Some of these forms e-commerce have acquired cute little tags based on the fact that to and 2 are pronounced the same.

9 Home Network Applications (3) Some forms of e-commerce. Mobile Network Users Combinations of wireless networks and mobile computing.

10 Network Hardware Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks Network hardware Types of transmission technology Broadcast links single comm channel shared by all the machines Point-to-point links many connections between individual pairs of machines- multiple routes Scale

11 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.

12 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 (b) Ring

13 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 (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

14 Metropolitan Area Networks A metropolitan area network based on cable TV. Wide Area Networks Relation between hosts on LANs and the subnet.

15 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 - satellite networks are inherently broadcast

16 Wide Area Networks (2) The principle of PS WANs A stream of packets from sender to receiver. Wireless Networks Categories of wireless networks: System interconnection networks Wireless LANs Wireless WANs

17 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 - what frequencies they can use

18 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 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

19 Wireless Networks (6) (a) Individual mobile computers (b) A flying LAN Home Network Categories Computers (desktop PC, PDA, shared peripherals Entertainment (TV, DVD, VCR, camera, stereo, MP3) Telecomm (telephone, cell phone, intercom, fax) Appliances (microwave, fridge, clock, furnace, airco) Telemetry (utility meter, burglar alarm, babycam).

20 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) 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» = Connecting 2 LANs

21 Network Software Protocol Hierarchies Design Issues for the Layers Connection-Oriented and Connectionless Services Service Primitives The Relationship of Services to Protocols Protocol Hierarchies Stack of layers or levels The name, the no., the content, the function differ from network to network Each layer services higher layers Each layer = virtual machine Layer n (M1) Layer n (M2) = the rules and conventions = PROTOCOL A protocol is an agreement between the communicating parties on how communications is to proceed

22 Protocol Hierarchies (2) A five layers network Layers, protocols, and interfaces. Peers:» Processes Protocol Hierarchies (3)» 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

23 Protocol Hierarchies (4) Defining clean interfaces between layer 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) Example information flow supporting virtual communication in layer 5.

24 Design Issues for the Layers Addressing Data transfer Error Control Flow Control Multiplexing Routing Connection-Oriented and Connectionless Services Connection oriented (co) Modeled after the telephone system Connectionless service (cs) Modeled after postal system Quality of service: (co), (cs) Reliable services

25 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 Primitives depends on the service being provided

26 Service Primitives Five service primitives for implementing a simple connectionoriented service. Service Primitives (3) Packets sent in a simple client-server interaction on a connection-oriented network.

27 Services to Protocols Relationship The relationship between a service and a protocol. Reference Models The OSI Reference Model The TCP/IP Reference Model A Comparison of OSI and TCP/IP A Critique of the OSI Model and Protocols A Critique of the TCP/IP Reference Model

28 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

29 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 Clark Connections to remain intact as long as source and destination machines are functioning A flexible architecture Transferring files Real-time speech transmission

30 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) The TCP/IP reference model.

31 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) Protocols and networks in the TCP/IP model initially.

32 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 In common: Comparing OSI and TCP/IP Models 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

33 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 The model did not fit any other protocol stacks

34 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 Bad timing Bad technology Bad implementations Bad politics

35 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) D. Clark s theory of standards: the apocalypse of the two elephants.

36 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

37 A Critique of the TCP/IP Reference Model Problems: 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 Concentrate on TCP/IP and related protocols

38 Hybrid Model The hybrid reference model to be used in this course. The Internet Example Networks Connection-Oriented Networks: X.25, Frame Relay, and ATM Ethernet Wireless LANs: 802:11

39 The ARPANET (a) Structure of the telephone system. (b) Baran s proposed distributed switching system. The ARPANET (2) The original ARPANET design.

40 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) ANSNET-1990 NAP ( Network Access Point)

41 NSFNET The NSFNET backbone in Internet Usage The number of networks, machines and users connected to the ARPANET grew rapidly after TCP/IP became the only official protocol on 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 ( ) News Remote login File transfer New one: www

42 Architecture of the Internet Overview of the Internet.