Transcript UNIT 5. Instruction to Computer Networks

Feng Lin
Sichuan Univ.
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Classification of Computer Networks
Transmission Media
LAN,MAN,WAN
Client/Sever vs Peer to Peer
How can we connect to Internet
More on Communication
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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 …
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Two main categories:
◦ Guided ― Physical connection
◦ Unguided ― Wireless Connection,
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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
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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.
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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.
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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
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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
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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.
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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 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
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Client/Server architecture
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Client
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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 ).
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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)
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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
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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
A
D
B
C
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A
D
B
C
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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
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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
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Dial-up via modem
Cable television line
Personal satellite link
Wireless or cell phone service
High-speed telephone services
◦ ISDN, DSL
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LAN
◦ Wired/wireless
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Home Networks
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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
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Lowest-capacity coaxial cable has far greater capacity
than POTS lines
Cable’s bandwidth is divided among 3 activities
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Need an Ethernet
NIC
Need a cable modem,
which converts your
computer’s signal
into one that can
travel over the CATV
network
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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
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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
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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
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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
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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
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T1 – high-speed 1.544 Mbps digital network
◦ Consists of 24 individual channels (64 Kbps for each)
◦ Popular for businesses and ISPs
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T3
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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
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company/univ local area network
(LAN) connects end system to
edge router
Ethernet:
◦ 10 Mbs, 100Mbps, 1Gbps,
10Gbps Ethernet
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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
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Classification of Communication
The speed at which the signal is transmitted
The capacity of communication link
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Classification of Communication
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By data travelling direction
By coordination of communication devices
By the type of signal
By the number of channels used in communication
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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
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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.
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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).
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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.
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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.
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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.
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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).
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For both: The wider the bandwidth, the more information can flow