UNIT 5.2 More on Computer Networks

1. UNIT 5.2 More on Computer Networks Feng Lin Sichuan Univ.

2. Agenda • Classification of Computer Networks • Transmission Media • LAN,MAN,WAN • Client/Sever vs Peer to Peer • How can we connect to Internet • More on Communication

3. Classification of Computer Networks • Depending on one’s perspective, we can classify networks in different ways • Based on transmission media: Wired (UTP, coaxial cables, fiber-optic cables) and Wireless • Based on network size: LAN and WAN (and MAN) • Based on management method: Peer-to-peer and Client/Server • Based on topology (connectivity): Bus, Star, Ring …

4. Transmission Media • Two main categories: • Guided ― Physical connection • Unguided ― Wireless Connection,

5. Transmission Media (cont.) • The Physical Connection. Physically connect computers together. • Use of wires or optical cables. • The connections are called guided/wired links. • Guided transmission media • Twisted pair • Coaxial cable • Fiber-optic cable

6. Transmission Media (cont.) • Twisted pair • Two wires twisted together. • Makes them less susceptible to acting like an antenna and picking up radio frequency information or appliance noise. • Telephone company uses twisted-pair copper wires to link telephones.

9. Transmission Media (cont.) • Coaxial cable • Also two wires: • One of the wires is woven of fine strands of copper forming a tube. • The wire mesh surrounds a solid copper wire that runs down the center. • Space between has a non-conducting material. • Makes them more impervious to outside noise.

10. Transmission Media (cont.) • Fiber-optic cable • Light is electromagnetic. • Can transmit more information down a single strand. • It can send a wider set of frequencies. • Each cable can send several thousand phone conversations or computer communications.

11. Transmission Media (cont.) • Wireless connections • The link is made using electromagnetic energy that goes through space instead of along wires or cables. • Unguided media: • Infrared • Radio frequency • Microwave

12. Transmission Media (cont.) • Infrared • Commonly used in TV and VCR remote controls. • Use infrared frequencies of electromagnetic radiation that behave much like visible light. • Must be in the line of sight. • Often used to connect keyboards, mouse

13. Transmission Media (cont.) • Radio frequency • Uses radio frequencies. • Function even though line of sight is interrupted. • Not commonly used because of the possible interference from other sources of electromagnetic radiation such as old electric drills and furnace motors.

14. Transmission Media (cont.) • Microwave • Often used to communicate with distant locations. • Must be line of sight. • Satellite communications use microwaves.

15. LAN, MAN & WAN • Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network) • Network in a City is call MAN (Metropolitan Area Network) • Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

17. Client/Sever vs Peer to Peer • Networks can be classified by organizational structures • Client/Server 客户/服务器模式 • peer-to-peer 对等模式

18. Client-Sever • Network Clients • Computers that request network resources or services • Network Servers • Computers that manage and provide network resources and services to clients • Usually have more processing power, memory and hard disk space than clients • Run Network Operating System that can manage not only data, but also users, groups, security, and applications on the network • Servers often have a more stringent requirement on its performance and reliability

19. Client/Server architecture • A network architecture in which each computer or process on the network is either a client or a server. • Client • Client interacts with both user and server • Clients are PCs or workstations on which users run applications. Clients rely on servers for resources, such as files,devices, and even processing power. • Server • Server accepts requests from clients and performs some service and returns results • Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers ).

21. Steps • Browser uses URL to locate server • Browser opens connection with server and requests a page • Server sends the page (HTML) • Browser renders the page on the display • Examples • Brower: Internet Explorer, Netscape, Mozilla • Server: Apache, products from Microsoft(IIS)

22. Two terms used in client-server framework are thin client and thick client • Thin client 瘦客户端does relatively little work (processing) typically providing little more than a user interface • Thick client 胖客户端carries out a substantial portion of the overall work of the system

23. P2P Network • Instead of having a central server that all clients communicate with, every member of a peer-to-peer network can communicate with any other member • Since peer-to-peer distribute processing, there are less potential communication bottlenecks

24. A D C B P2P

25. A D C B P2P

26. Hybrid of Client-Server and P2P Login, and register the IP Address Return the IP Addresses of friends and state of them… Client-server architecture P2P architecture

27. Communication System Topologies • Topology (拓扑) • The pattern or path of the interconnections in a communication system • Networks can use a single topology or a mixture of topologies • There are three principal topologies: • Bus topology • Ring topology • Star topology

28. Bus Topology • Bus topology 总线型- provides a common or shared communications pathway • All devices are connected to a central cable, called the bus or backbone. • Bus networks are relatively inexpensive and easy to install for small networks. Ethernet systems use a bus topology.

29. Ring Topology • Ring topology 环形- connects devices in a continuous loop • All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. • Ring topologies are relatively expensive and difficult to install, but they offer high bandwidth and can span large distances. • Used in local area networks

30. Star Topology • Star topology 星型- communications lines fan out from a central location • All devices are connected to a central hub. • Star networks are relatively easy to install and manage, but bottlenecks can occur because all data must pass through the hub. • every connection is dedicated to one user • expensive

31. How can we connect to Internet • Dial-up via modem • Cable television line • Personal satellite link • Wireless or cell phone service • High-speed telephone services • ISDN, DSL • LAN • Wired/wireless • Home Networks

32. Dial-up modem • Dial-up connection – is relatively simple and inexpensive because the necessary equipment and software are preinstalled on most new computers • Voice band modem – converts digital signals into wave format to go over telephone lines and then at destination waves are converted back into digital • Dial-up top speed is 56 Kbps

33. Cable television line • Lowest-capacity coaxial cable has far greater capacity than POTS lines • Cable’s bandwidth is divided among 3 activities

35. Need an Ethernet NIC • Need a cable modem, which converts your computer’s signal into one that can travel over the CATV network

36. Personal satellite link • Direct satellite service (DSS) uses a geosynchronous or low-earth satellite to send television, voice, or computer data directly to a satellite dish • Two-way satellite service ships both upstream and downstream data through the satellite • Offers 500 Kbps downstream • 40-60 Kbps upstream

38. Wireless or cell phone service • Can I access Internet services simply by using my cell phone? • Currently most cell phone access Internet by 3G • Cellular-ready PC card modem • Cellular telephone cable between telephone and computer

39. High-speed telephone services • ISDN (Integrated Services Digital Network) – 64Kbps or 128 Kbps • bps • A device called an ISDN adapter connects a computer to a telephone wall jack and translates the computer’s signal into signal that can travel over the ISDN connection

40. DSL (Digital Subscriber Line) – anywhere from twice as fast to approx. 125 times faster than 56 Kbps • Comes from phone company, requires proximity to a switching station • Several versions exist – ADSL, SDSL, HDSL • xDSL – refers to entire group of DSL technologies • Telephone line connected to DSL switch at your house • Line connected to DSL modem which connects to your computer’s Ethernet card

41. T1 – high-speed 1.544 Mbps digital network • Consists of 24 individual channels (64 Kbps for each) • Popular for businesses and ISPs • T3 • 672 channels • Supports data rates of about 43 Mbps • Sometimes referred to as DS3 (Digital Service-3) lines • Provide many of the links on the Internet backbone

43. LAN • company/univ local area network (LAN) connects end system to edge router • Ethernet: • 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet

44. Wireless LAN • IEEE 802.11b and IEEE 802.11g aka Wi - Fi • Wireless Ethernet (LAN, aka WLAN) standards • Backward-compatible standards, operating at 2.4GHz • 802.11b – 11Mbps (~6Mbps, real) shared Ethernet max speed, original Wi-Fi standard (2001) • 802.11g – 54Mbps (~30Mbps, real) shared Ethernet max speed, newer Wi-Fi standard (2003+) • 802.11a – 54Mbps but not backwards compatible with 802.11b or 802.11g (not recommended)

46. Home Networks Typical home network components: • DSL or cable modem • router/firewall/NAT • Ethernet • wireless access point

47. More on Communication • Classification of Communication • The speed at which the signal is transmitted • The capacity of communication link

48. Classification of Communication • By data travelling direction • By coordination of communication devices • By the type of signal • By the number of channels used in communication

49. data travelling direction • Simplex – signal travels in only one direction • Half duplex – signal travels in both directions, but only one direction at a time • Full duplex – signal travels in both directions at the same time sender receiver

50. coordination of communication devices • Synchronous communication • synchronized by a signal called a clock when sending and receiving data • expensive but very fast • Asynchronous communication • Data is sent byte by byte. • Cheaper and more commonly used.