Feng LinSichuan Univ.

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

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 …

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

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

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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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.

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.

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

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

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.

Microwave◦ Often used to communicate

with distant locations.◦ Must be line of sight.◦ Satellite communications use

microwaves.

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)

Networks can be classified by organizational structures◦Client/Server 客户 /服务器模式◦peer-to-peer 对等模式

• Network ClientsClients• Computers that request network resources

or services

• Network ServersServers• 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 ◦ 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 ).

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

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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Login, and register the IP Address

Return the IP Addresses of friends and state of them…

Client-server architecture

P2P architecture

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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

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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.

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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

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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

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 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

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

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

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

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

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

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

company/univ local area network (LAN) connects end system to edge router

Ethernet: ◦ 10 Mbs, 100Mbps, 1Gbps,

10Gbps Ethernet

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)

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

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

◦ 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

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 0s or 1s).

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.

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.

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