B.A. (Mahayana Studies)000-209 Introduction to Computer Science
November 2005 - March 2006
11. Communications Systems
We look at the basic elements of a communications systems, and the various services available.
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Overview
1. What is a Communications System? 2. Bandwidth 3. Network Topology 4. Protocols 5. Packet Switching 6. Circuit Switching 7. Analog <-> Digital Conversion 8. Telephone Services 9. Cable-based Services 10. Wireless
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A communications system is a combination of hardware, software, and connecting links that transport data between a sender and a receiver.
A sender and receiver are linked by a communications channel e.g. telephone lines,
fiber-optic cable
1. What is a Communications System?
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The amount of data that can be sent over a network in a certain period of time.
Usually measured in bits per second (bps), kilobits per second (kbps), or megabits per second (mps).
Two main types: broadband (high capacity)
e.g. by using fibre-optic cable
narrowband (less capacity) e.g when using the telephone system
2. Bandwidth
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The shape of the interconnections in a communications system is its topology.
Star topology - the communications lines fan out from a central location every connection is dedicated to one user
3. Network Topology
continued
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A bus topology provides a common or shared communications link used by cable TV companies
continued
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A ring topology connects devices in a continuous loop used by older local area networks
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A communications protocol is a set of rules about how a sender and receiver should communicate.
The rules specify such things as data representation, signalling, authentication, and e
rror detection
4. Protocols
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5. Packet Switching
Data is separated into small packets.
Each packet is sent through the network using the best route available at that time.
At the receiving end, the packets are reassembled into the original data.
Packet switching is used by the Internet.
continued
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1. A message is divided into packets.
2. Each packet is addressed to its destination.
3. A packet might travel the shortest path to its destination.
4. If a route is congested or
inoperable, packets can be rerouted to
other links.
5. When the packets arrive at their destination, they are reassembled.
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Advantages of Packet Switching
Packet switching uses network bandwidth efficently.
It minimizes transmission latency the time it takes for data to pass across the network
Packet switching can deal with network failure.
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A dedicated communications path is established between two devices through one or more switching nodes.
Unlike packet switching, digital data is sent as a continuous stream of bits.
The telephone system uses circuit switching.
6. Circuit Switching
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Advantages of Circuit Switching
Bandwidth is guaranteed.
Any communication delay is only due to propagation time.
Primary advantage of the telephone system for computer communications is that it's cheap for users.
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Telephone systems use miles and miles of twisted-pair cables.
Twisted-pair cableterminates with a plastic RJ-45connector.
Twisted Pair Cable
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Old phone lines use analog sound signals. Modern systems use digital signals:
less susceptible to noise require simpler circuitry
It's possible to convert analog signals into digital signals (and vice versa).
7. Analog <-> Digital Conversion
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Analog and Digital Signals
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The Conversion Process
Transmission to neighborhoodSwitch box (within 1800 yards)
Neighborhood switchMay convert to digital. May remain analog Transmission to city
Point of Presence central switch
Central office PoPDefinitely digital now
National Backbone,All digital networks
Central office PoP - incoming digital may be converted to analog
Neighborhood switchIf not analog already, converted at this point
Modem in computerConverts analog to digital
Transmission to neighborhoodswitch box(within 1800 yards)
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POTS (plain old telephone service) is an analog service.
Voiceband modem - converts digital pulses into analog tones to send digital computer data over a POTS line.
8. Telephone-based Services
continued
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ISDN a standard for digital transmission of voice and
data uses circuit switching with ordinary telephone w
ire (and other media, such as fibre-optic cable) it requires hardware adapters at both ends of the
transmission basic (64 kbps); enhanced (128 kbps)
continued
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ADSL (Asymmetric Digital Subscriber Line) for transmitting digital information at a high bandwidth on existin
g phone lines
ADSL offers asymmetric data rates 1.5 to 9 Mbps when receiving (the downstream rate) 16 to 640 Kbps when sending (the upstream rate)
Speeds depends on distance from the telephone company office at most ~3 miles away
continued
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T1 Line - provides 1.544 Mbps send and receive capacity over a dedicated line
T3 Line - uses fiber-optic cables to provide service with a capacity of 44.736 Mbps the equivalent of 28 T1 lines enough to show full-screen, full-motion video
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Fiber-optic cable is a bundle of extremely small tubes of glass called optical fibers. thinner than human hair
Fiber-optic cable is replacing twisted-pair cable where high bandwidth is required
Fiber-Optic Cable
continued
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How fiber-optic cable works: miniature lasers send pulses of light each fiber is a one-way communications
channel light signals encounter little resistance
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Cable TV companies in the US have installed miles of high-bandwidth coaxial cables. carrying capacity far in excess of POTS lines
A cable modem is a device designed to demodulate a signal from the cable and translate it back into Internet data.
9. Cable TV-based Services
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Coaxial cable - high-capacity communications cable consisting of a copper wire conductor common use is to carry television signals
Coaxial cablecontains shieldingwhich increasesbandwidth.
Coaxial Cable
continued
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Coaxial cable: has excellent bandwidth, but not as good as
fibre-optic cable more expensive more difficult to work with than twisted-pair
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Radio-based systems that allow transmission of information without a physical connection, no need for copper wire or fiber-optic
Cellular, infrared, microwave, and satellite broadcasting are forms of wireless communication.
Common wireless standards include 802.11b, and Bluetooth.
10. Wireless Communications
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Infrared transmissions occur below the visible light of spectrum. requires line-of-sight communication
sender must be able to see the receiver
Infrared Wireless
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High-frequency radio waves used for point-to-point and one-directional communication of audio and data.
Requires line-of-sight, and ground stations must be within 30 miles of each other.
Many communications systems transmit microwave signals between a land-based ground station and a satellite.
Microwaves
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Direct satellite service (DSS) uses a geosynchronous or low-earth orbit satellite to send television, voice or computer data directly to a satellite dish. transmits in one direction – downstream requires a standard modem and phone line for
upstream transmission
Satellite
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801.11
802.11 is a family of specifications developed by the IEEE for wireless communications. IEEE = Institute of Electrical and Electronics Enginee
rs
802.11b: provide data rates of up to 11 Mbps per second at distances up to approximately 300 feet. also called Wi-Fi line-of-sight not required
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Bluetooth
An open standard for short-range (30 feet) and low speed (up to 1Mbps) wireless transmission of digital voice and data. line-of-sight not required low-power (unlike 802.11b), so suitable for portable d
evices