1. 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.

2. 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

3. 000-209 Intro to CS. 11/Comm. Systems 3 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?

4. 000-209 Intro to CS. 11/Comm. Systems 4 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

5. 000-209 Intro to CS. 11/Comm. Systems 5 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

6. 000-209 Intro to CS. 11/Comm. Systems 6 A bus topology provides a common or shared communications link used by cable TV companies continued

7. 000-209 Intro to CS. 11/Comm. Systems 7 A ring topology connects devices in a continuous loop used by older local area networks

8. 000-209 Intro to CS. 11/Comm. Systems 8 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 error detection 4. Protocols

9. 000-209 Intro to CS. 11/Comm. Systems 9 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

10. 000-209 Intro to CS. 11/Comm. Systems 10 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.

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

12. 000-209 Intro to CS. 11/Comm. Systems 12 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

13. 000-209 Intro to CS. 11/Comm. Systems 13 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.

14. 000-209 Intro to CS. 11/Comm. Systems 14 Telephone systems use miles and miles of twisted-pair cables. Twisted-pair cable terminates with a plastic RJ-45 connector. Twisted Pair Cable

15. 000-209 Intro to CS. 11/Comm. Systems 15 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

16. 000-209 Intro to CS. 11/Comm. Systems 16 Analog and Digital Signals

17. 000-209 Intro to CS. 11/Comm. Systems 17 The Conversion Process Transmission to neighborhood Switch box (within 1800 yards) Neighborhood switch May convert to digital. May remain analog Transmission to city Point of Presence central switch Central office PoP Definitely digital now National Backbone, All digital networks Central office PoP - incoming digital may be converted to analog Neighborhood switch If not analog already, converted at this point Modem in computer Converts analog to digital Transmission to neighborhood switch box (within 1800 yards)

18. 000-209 Intro to CS. 11/Comm. Systems 18 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

19. 000-209 Intro to CS. 11/Comm. Systems 19 ISDN a standard for digital transmission of voice and data uses circuit switching with ordinary telephone wire (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

20. 000-209 Intro to CS. 11/Comm. Systems 20 ADSL (Asymmetric Digital Subscriber Line) for transmitting digital information at a high bandwidth on existing 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

21. 000-209 Intro to CS. 11/Comm. Systems 21 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

22. 000-209 Intro to CS. 11/Comm. Systems 22 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

23. 000-209 Intro to CS. 11/Comm. Systems 23 How fiber-optic cable works: miniature lasers send pulses of light each fiber is a one-way communications channel light signals encounter little resistance

24. 000-209 Intro to CS. 11/Comm. Systems 24 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

25. 000-209 Intro to CS. 11/Comm. Systems 25 Coaxial cable - high-capacity communications cable consisting of a copper wire conductor common use is to carry television signals Coaxial cable contains shielding which increases bandwidth. Coaxial Cable continued

26. 000-209 Intro to CS. 11/Comm. Systems 26 Coaxial cable: has excellent bandwidth, but not as good as fibre-optic cable more expensive more difficult to work with than twisted-pair

27. 000-209 Intro to CS. 11/Comm. Systems 27 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

28. 000-209 Intro to CS. 11/Comm. Systems 28 Infrared transmissions occur below the visible light of spectrum. requires line-of-sight communication sender must be able to see the receiver Infrared Wireless

29. 000-209 Intro to CS. 11/Comm. Systems 29 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

30. 000-209 Intro to CS. 11/Comm. Systems 30 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

31. 000-209 Intro to CS. 11/Comm. Systems 31 801.11 802.11 is a family of specifications developed by the IEEE for wireless communications. IEEE = Institute of Electrical and Electronics Engineers 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

32. 000-209 Intro to CS. 11/Comm. Systems 32 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 devices