Chapter 6 Computer Networks OBJECTIVES After reading this

Chapter 6 Computer Networks

OBJECTIVES After reading this chapter, the reader should be able to: Understand the rationale for the existence of networks. Distinguish between the three types of networks: LANs, MANs, and WANs. Understand the OSI model and TCP/IP. List different connecting devices and the OSI layers in which each device operates. Understand client-server models.

6. 1 NETWORKS, LARGE AND SMALL

Network n Computer network – A combination of computers connected through transmission media. ¨ LAN (Local Area Network) ¨ MAN (Metropolitan Area Network) ¨ WAN (Wide Area Network) n Internetwork – (e. g. Internet) Networks can be connected using connecting device.

Model and Protocol Model – the specification set by a standards organization as a guideline for designing networks. n Protocol – a set of rules that controls the interaction of different devices in a network/internetwork. n

6. 2 OSI MODEL

Note: The OSI (Open Systems Interconnection) model is a theoretical model that shows how any two different systems can communicate with each other. n OSI Model – is a framework of 7 layers that gives network designers an idea of the functionality of each separate but related layer.

Figure 6 -1 The OSI model

n control information is added to the data in the form of headers or trailers. n the header or trailer is dropped in each layer. Flow of data in the OSI model Figure 6 -2

Functions of the Layers Physical 1. • transmit a bit stream over a physical medium. Data-Link 2. • • Organizes bits into logical units called frames. Node-to-node delivery Network 3. • Source-to-destination delivery of a packet. Transport 4. • Source-to-destination delivery of the entire message. Session 5. • Establishes, maintains, and synchronizes the dialog between communicating systems. Presentation 6. • Deals with the fact that different systems use different coding methods. Application 7. • Enables the user to access the network.

6. 3 CATEGORIES OF NETWORKS

Categories of networks Figure 6 -3

LAN (Local Area Network) n Allow resource sharing between computers. ¨ Computers ¨ Peripheral devices ¨ Transmission medium (e. g. cable) n 3 types of topology ¨ Bus topology ¨ Star topology ¨ Ring topology

LANs n Figure 6 -4 Hub – ¨a device that facilitates connection ¨ LAN acts logically like a bus. n Star – the dominant topology today.

MAN Figure 6 -5

MAN (Metropolitan Area Network) n Uses services provided by a network service provider. (Tel. Company) ¨ Individual users’ computers ¨ Organizations’ LANs n Many Tel. Company provide a popular MAN service called SMDS (Switched Multimegabit Data Services) ¨ Prior to SMDS's arrival in 1995, the only way to connect LANs was through a dedicated private line.

WAN Figure 6 -6

WAN (Wide Area Network) n The connection of individual computers or LANs over a large area (country, world). n User using a telephone line to connect to an ISP (Internet Service Provider) is using a WAN. ¨ Negotiates n n fee ISP Tel. company

6. 4 CONNECTING DEVICES

Figure 6 -7 Connecting devices

Repeater (L 1) n n Regenerates the signal. Extends the physical length of a network. Figure 6 -8

Note: Repeaters operate at the first layer of the OSI model.

Bridge/Switch n Bridge ¨A traffic controller ¨ Divide a long bus into smaller segments so that each segment is independent trafficwise. ¨ Regenerate data n Switch ¨A sophisticated bridge with multiple interfaces. ¨ A station that needs to send a frame sends it directly to the switch.

Bridge (L 1 -L 2) Figure 6 -9

Note: Bridges operate at the first two layers of the OSI model.

Figure 6 -10 Switch

Router Connect LANs, MANs, and WANs. n Routes a packet based on the logical address (network layer) of the packet. n Connect two independent networks: LAN-WAN, LAN-MAN, WAN-WAN… V. S. n Bridge n ¨ filters a frame based on the physical address (datalink layer) of the frame. ¨ Connect two segments of a LAN or two LANs belonging to the same organization.

Routers (L 1 -L 3) in an internet Figure 6 -11

Note: Routers operate at the first three layers of the OSI model.

Gateway (L 1 -L 7) A protocol converter. n Understands the protocols used by each connected network and is able to translate from one to another. n

Figure 6 -12 Connecting devices and the OSI model internetworking IP address Mac address packet frame networking

n ISP (Internet Service Provider) a company that provides access to the Internet. For a monthly fee, the service provider gives you a software package, username, password and access phone number. Equipped with a modem, you can then log on to the Internet and browse the World Wide Web, and send and receive e-mail. n ICP （Internet Content Provider） 網際網路內容提供者。指的是在網際網路上提供各種服務內容的廠商。 如Yahoo雅虎、AOL美國線上等，都是屬於ICP的一種。任何人只要 有網路的空間放置網頁，都可以成為ICP。而ICP的收入大都來自廣 告收入。 n Web portal A Web site or service that offers a broad array of resources and services, such as e-mail, forums, search engines, and on-line shopping malls. n ASP (Application Service Provider) ASP 業者向軟體公司購買應用程式，然後將此程式放在 ASP 業者的 機房內，然後供企業或個人以存取網路的方式連接到機房來使用應用 程式，並依據使用方式與規定向客戶收取費用。