B.A. (Mahayana Studies) 000-209 Introduction to Computer Science November 2005 - March 2006 11. Communications Systems We look at the basic elements of.

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.

000-209 Intro to CS. 11/Comm. Systems 2

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


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?


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


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


A bus topology provides a common or shared communications link used by cable TV companies

continued


A ring topology connects devices in a continuous loop used by older local area networks


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


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


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.


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.


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


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.


Telephone systems use miles and miles of twisted-pair cables.

Twisted-pair cableterminates with a plastic RJ-45connector.

Twisted Pair Cable


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


Analog and Digital Signals


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)


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


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


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


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


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


How fiber-optic cable works: miniature lasers send pulses of light each fiber is a one-way communications

channel light signals encounter little resistance


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


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


Coaxial cable: has excellent bandwidth, but not as good as

fibre-optic cable more expensive more difficult to work with than twisted-pair


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


Infrared transmissions occur below the visible light of spectrum. requires line-of-sight communication

sender must be able to see the receiver

Infrared Wireless


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


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


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


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

B.A. (Mahayana Studies) 000-209 Introduction to Computer Science November 2005 - March 2006 11. Communications Systems We look at the basic elements of.

Documents

communications systems

telephone systems

digital signals slide

digital data

modern systems

bandwidth slide

yards slide

network packet switching