UNIT 5 2 More on Computer Networks Feng

UNIT 5. 2 More on Computer Networks Feng Lin Sichuan Univ.

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

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 …

Transmission Media Two main categories: ◦ Guided ― Physical connection ◦ Unguided ― Wireless Connection,

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

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.

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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.

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.

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

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

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.

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

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)

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

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

Client/Server architecture Client Server ◦ A network architecture in which each computer or process on the network is either a client or a server. ◦ 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 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 ).

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)

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

P 2 P 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

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Hybrid of Client-Server and P 2 P Login, and register the IP Address Return the IP Addresses of friends and state of them… Client-server architecture P 2 P architecture 26

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 27

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. 28

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 29

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 30

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

Dial-up modem Dial-up connection – is relatively simple and inexpensive because the necessary equipment and software 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

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

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

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

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

High-speed telephone services ISDN (Integrated Services Digital Network) – 64 Kbps or 128 Kbps ◦ 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

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 x. DSL – 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

T 1 – high-speed 1. 544 Mbps digital network ◦ Consists of 24 individual channels (64 Kbps for each) ◦ Popular for businesses and ISPs T 3 ◦ ◦ 672 channels Supports data rates of about 43 Mbps Sometimes referred to as DS 3 (Digital Service-3) lines Provide many of the links on the Internet backbone

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

Wireless LAN ◦ IEEE 802. 11 b and IEEE 802. 11 g aka Wi - Fi Wireless Ethernet (LAN, aka WLAN) standards Backward-compatible standards, operating at 2. 4 GHz 802. 11 b – 11 Mbps (~6 Mbps, real) shared Ethernet max speed, original Wi-Fi standard (2001) 802. 11 g – 54 Mbps (~30 Mbps, real) shared Ethernet max speed, newer Wi-Fi standard (2003+) 802. 11 a – 54 Mbps but not backwards compatible with 802. 11 b or 802. 11 g (not recommended)

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

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

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

data travelling direction ◦ Simplex – signal travels in only one direction sender receiver ◦ 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

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.

Type of signal ◦ Analog: Those signals that vary with smooth continuous changes. A continuously changing signal similar to that found on the speaker wires of a high-fidelity stereo system. ◦ Digital: Those signals that vary in steps or jumps from value to value. They are usually in the form of pulses of electrical energy (represent 0 s or 1 s).

Number of channels ◦ Single channel - Capable of only sending/receiving one signal at a time. Phone line: Single line = single phone call at a time. ◦ Multichannel - Capable of more than one channel at a time. Fiber-optic cable, microwaves, Satellite transmissions.

The speed at which the signal is transmitted In digital systems: Speed is measured in. . . ◦ Bits per second (bps). The number of bits (0’s and 1’s) that travel down the channel per second. ◦ Baud rate The number of bits that travel down the channel in a given interval. The number is given in signal changes per second, not necessarily bits per second.

The capacity of communications links Bandwidth: Digital ◦ Number of bits per second (bps) that can be sent over a link. ◦ The wider the bandwidth, the more diverse kinds of information can be sent. ◦ Simplest is voice, most sophisticated is moving videos. Bandwidth: Analog ◦ The difference between the highest and lowest frequencies that can be sent over an analog link (like phone lines). ◦ Measurement is given in hertz (Hz). For both: The wider the bandwidth, the more information can flow