1. Chapter 6 Computer Networks

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

3. NETWORKS, LARGE AND SMALL NETWORKS, 6.1

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

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

6. OSI MODEL 6.2

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

8. Figure 6-1 The OSI model

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

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

11. CATEGORIESOFNETWORKSCATEGORIESOFNETWORKS 6.3

12. Figure 6-3 Categories of networks

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

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

15. Figure 6-5 MAN

16. MAN (Metropolitan Area Network) Uses services provided by a network service provider. (Tel. Company)  Individual users’ computers  Organizations’ LANs 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.

17. Figure 6-6 WAN

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

19. CONNECTINGDEVICESCONNECTINGDEVICES 6.4

20. Figure 6-7 Connecting devices

21. Figure 6-8 Repeater (L1) Regenerates the signal. Extends the physical length of a network.

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

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

24. Figure 6-9 Bridge (L1-L2)

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

26. Figure 6-10 Switch

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

28. Figure 6-11 Routers (L1-L3) in an internet

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

30. Gateway (L1-L7) A protocol converter. Understands the protocols used by each connected network and is able to translate from one to another.

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

32. 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. 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。 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. 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。