IntroductionPart 1 Chapter 1
Before we start to examine the technical issues of computer networks in detail, it is worth devoting some time to pointing out why people are interested in computer networks and what they can be used for.
Uses of Computer Networks • Business Applications • Home Applications • Mobile Users • Social Issues
Business Applications of Networks • Usually companies have a substantial number of computers: • To monitor production • To keep track of inventories • To do the payroll • etc • Initially, each of these computers may have worked in isolation from the others • At some point, management may have decided to connect them to be able to extract and correlate information about the entire company Computer Networks
Business Applications of Networks (2)Computer communication goals for companies • 1) Resource sharing • Physical resources: printers, scanners, and CD burners • Information: customer records, inventories, accounts receivable, financial statements, tax information • Information resource sharing in a company usually represented as clients connected to a server • Server: powerful computers storing data and programs • Client: simpler computer used to access remote data Clients and server are usually connected through a network
Business Applications of Networks (3) • A network with two clients and one server.
Business Applications of Networks (4) • Communication between client and server • The client process sends a message over the network to the server process. • The client process then waits for a reply message. • The server process gets the request, it performs the requested work or looks up the requested data and sends back a reply. • The client-server model involves requests and replies.
Business Applications of Networks (5)Computer communication goals for companies (Cont.) • 2) Provide a powerful communication medium among employees: email, write reports together, videoconferencing. • 3) Doing business electronically with other companies: . • 4) Doing business with consumers over the Internet.
Home Network Applications • Access to remote information: Surfing the World Wide Web for information, fun, newspapers, digital libraries, … etc • Person-to-person communication: Emails, instant messaging, peer-to-peer communication (Napster) • Interactive entertainment: video on-demand, live television • Electronic commerce: home shopping, access to financial institutions
Home Network Applications (2) • In peer-to-peer system there are no fixed clients and servers.
Home Network Applications (3) • Some forms of e-commerce.
Mobile Network Users Wireless networking and mobile computing are often related, they are not identical. E.g. • Combinations of wireless networks and mobile computing.
Social Issues • Some of the major social issues resulted from computer network uses • Newsgroups or chat rooms are set up on topics like politics, religion, or sex. People usually misuse such topics to attack or offend others. • Networks should be like newspapers and magazines or like telephone company and post offices? Filtering its contents? • Identity theft is becoming a serious problem • Combinations of wireless networks and mobile computing.
Network Hardware • Types of transmission technology • Broadcast links • Point-to-point links
Network Hardware Broadcast Networks • Have a single communication channel that is shared by all the machines on the network. • Short messages (packets) sent by any machine are received by all the others. The intended receiving (address field) machine processes the packet. Otherwise it is ignored. • Broadcast systems generally also allow the possibility of addressing a packet to all destinations (broadcasting) by using a special code in the address field. • Some broadcast systems also support transmission to a subset of the machines, something known as multicasting.
Network Hardware Broadcast Networks • Analogy: • Somebody at the end of a corridor with many rooms off it and shouting ‘‘Watson, come here. I want you.’’ Although the packet may actually be received (heard) by many people, only Watson responds. The others just ignore it. • An airport announcement asking all flight 644 passengers to report to gate 12 for immediate boarding.
Network HardwarePoint-to-point Networks • point-to-point networks consist of many connections between individual pairs of machines. • To go from the source to the destination, a packet may have to first visit one or more intermediate machines. • Multiple routes, of different lengths, are possible. • As a general rule (although there are many exceptions), smaller, geographically localized networks tend to use broadcasting, whereas larger networks usually are point-to-point. • Point-to-point transmission with one sender and one receiver is sometimes called unicasting.
Network Hardware • Classifying networks based on their scale: • Local Area Networks • Metropolitan Area Networks • Wide Area Networks • Wireless Networks • Home Networks • Internetworks
Network Hardware • Classification of interconnected processors by scale.
Network Hardware Local Area Networks • LANs are distinguished from other kinds of networks by three characteristics: • (1) their size: single building or campus of up to a few kilometers in size (which means that the worst-case transmission time is bounded and known in advance). • (2) their transmission technology: cable to which all the machines are attached. • (3) their topology: various topologies exists: Bus and Ring
Network Hardware Local Area Networks • Two broadcast networks • (a) Bus • (b) Ring
Network Hardware Local Area Networks • Speed: traditional LANs10 Mbps to 100 Mbps, Newer LANs operate at up to 10 Gbps • IEEE 802.3 (Ethernet) :Computers on an Ethernet can transmit whenever they want to; if two or more packets collide, each computer just waits a random time and tries again later. • IEEE 802.5 (IBM token ring): having the machines take turns, is a ring-based LAN operating at 4 and 16 Mbps. • FDDI is another example of a ring network. • Broadcast networks can be further divided into static and dynamic, depending on how the channel is allocated.
Network Hardware Metropolitan Area Networks • A metropolitan area network, or MAN, covers a city. • The best-known example of a MAN is the cable television network available in many cities. • The cable TV network operators expanded to provide internet to users with some changes to their original systems.
Network Hardware Metropolitan Area Networks • A metropolitan area network based on cable TV.
Network Hardware Wide Area Networks • A wide area network, or WAN, spans a large geographical area, often a country or continent. • Hosts (customers PCs) and Subnets (ISP or telephone company owned and operated) • Subnet: • Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or even radio links. • Switching elements (routers) are specialized computers that connect three or more transmission lines. Junction point!
Network Hardware Wide Area Networks • Relation between hosts on LANs and the subnet.
Network Hardware Wide Area Networks • store-and-forward or packet-switched networks • when a process on some host has a message to be sent to a process on some other host, the sending host first cuts the message into packets, each one bearing its number in the sequence. • These packets are then injected into the network one at a time in quick succession. • The packets are transported individually over the network and deposited at the receiving host, where they are reassembled into the original message and delivered to the receiving process. • A stream of packets resulting from some initial message is illustrated in Fig. 1-10 (next slide).
Network Hardware Wide Area Networks • A stream of packets from sender to receiver.
Network Hardware Wide Area Networks • In some networks all packets from a given message must follow the same route; in others each packet is routed separately. • Routing decisions are made locally. • When a packet arrives at router A, it is up to A to decide if this packet should be sent on the line to B or the line to C. • How A makes that decision is called the routing algorithm. • Many of them exist. We will study some of them in detail in Chap. 5. • Not all WANs are packet switched. A second possibility for a WAN is a satellite system. Each router has an antenna through which it can send and receive.
Network Hardware Wireless Networks • Categories of wireless networks: • System interconnection • Wireless LANs • Wireless WANs
Network Hardware Wireless Networks • System interconnection • Bluetooth a short-range wireless network. Allows system components together, digital cameras, headsets, scanners, and other devices to connect to a computer by merely being brought within range. • Wireless LANs • Every computer has a radio modem and antenna with which it can communicate with other systems. • Standard for wireless LANs: IEEE 802.11, which most systems implement and which is becoming very widespread.
Network Hardware Wireless Networks • Wireless WANs • The radio network used for cellular telephones is an example of a low-bandwidth wireless system. • The first generation was analog and for voice only. • The second generation was digital and for voice only. • The third generation is digital and is for both voice and data.
Network Hardware Wireless Networks • Wireless WANs (cont.) • Wireless LANs an operate at rates up to about 50 Mbps over distances of tens of meters. While cellular systems (Wireless WANs) operate below 1 Mbps, but the distance between the base station and the computer or telephone is measured in kilometers rather than in meters. • High-bandwidth wide area wireless networks are also being developed (IEEE 802.16).
Network Hardware Wireless Networks • Almost all wireless networks hook up to the wired network at some point to provide access to files, databases, and the Internet.
Network Hardware Wireless Networks • (a) Bluetooth configuration • (b) Wireless LAN
Network Hardware Wireless Networks • (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).
Network Software • Protocol Hierarchies • Design Issues for the Layers • Connection-Oriented and Connectionless Services • Service Primitives • The Relationship of Services to Protocols
Network SoftwareProtocol Hierarchies • To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. • In a sense, each layer is a kind of virtual machine, offering certain services to the layer above it. • The fundamental idea is that a particular piece of software (or hardware) provides a service to its users but keeps the details of its internal state and algorithms hidden from them. • Layer n on one machine carries on a conversation with layer n on another machine.
Network SoftwareProtocol Hierarchies • The rules and conventions used in this conversation are collectively known as the layer n protocol. • Protocol is an agreement between the communicating parties on how communication is to proceed. • A five-layer network is illustrated in the following Figure.
Network SoftwareProtocol Hierarchies • Layers, protocols, and interfaces.
Network SoftwareProtocol Hierarchies • No data are directly transferred from layer n on one machine to layer n on another machine. • Each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. • Below layer 1 is the physical medium through which actual communication occurs. • In the previous figure, virtual communication is shown by dotted lines and physical communication by solid lines. • Between each pair of adjacent layers is an interface. • The interface defines which primitive operations and services the lower layer makes available to the upper one.
Network SoftwareProtocol Hierarchies • A set of layers and protocols is called a network architecture. • The specification of an architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol. • Neither the details of the implementation nor the specification of the interfaces is part of the architecture because these are hidden away inside the machines and not visible from the outside.
Network SoftwareProtocol Hierarchies • It is not even necessary that the interfaces on all machines in a network be the same, provided that each machine can correctly use all the protocols. • A list of protocols used by a certain system, one protocol per layer, is called a protocol stack. • The subjects of network architectures, protocol stacks, and the protocols themselves are the principal topics of this book.
Network SoftwareProtocol Hierarchies • Analogy • Imagine two philosophers (peer processes in layer 3), one of whom speaks Urdu and English and one of whom speaks Chinese and French. • Since they have no common language, they each engage a translator (peer processes at layer 2), each of whom in turn contacts a secretary (peer processes in layer 1).
Design Issues for the Layers • Addressing: because we have multiple destinations • Error Control: physical communication circuits are not perfect • Flow Control: is how to keep a fast sender from swamping (flooding) a slow receiver with data. • Multiplexing: use the same connection for multiple, unrelated conversations. • Routing: multiple paths between source and destination, where to go?