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Long Distance Communiction

Long Distance Communiction. Encoding used by RS-232 cannot work in all situations Over long distances Using existing systems like telephone Different encoding strategies needed . Long Distance Communiction.

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Long Distance Communiction

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  1. Long Distance Communiction • Encoding used by RS-232 cannot work in all situations • Over long distances • Using existing systems like telephone • Different encoding strategies needed

  2. Long Distance Communiction • It becomes difficult to send square waves through a bandwidth limited channel, such as a phone line, because the higher frequencies are filtered out. • Without the higher frequencies, the wave no longer looks like a square wave. This makes it difficult for the receiver to determine the actual bit value. • To avoid this problem, the data is send only as sine waves.

  3. Oscillating signals • Continuous, oscillating signal will propagate farther than electric current • For long-distance communication • Send a sine wave (called a carrier wave) • Change (modulate) the carrier to encode date

  4. Illustration of a Carrier • Carrier • Usually a sine wave • Oscillates continuously • Frequency of carrier fixed

  5. Encoding data with a carrier • Modifications to basic carrier encode data for transmission • Technique called modulation • Same idea as in radio, television transmission • Carrier modulation used with all types of media - copper, fiber, radio, infrared, laser

  6. Types of Modulation • Amplitude modulation (AM radio) • Frequency modulation (FM radio) • Phase shift modulation (used for data)

  7. Amplitude modulation Carrier frequency • The amplitude of the sine wave is changed. • A 0 is represented by a wave that has half the energy or height in the graph. • A 1 is represented by a full sized wave 2 3 4 5 1 o Actual wave to be sent

  8. Amplitude Modulation • Strength of signal encodes 0 or 1 • One cycle of wave needed for each bit • The wave reduce to 2/3 to encode a 1 bit, 1/3 to encode a 0 bit. • Data rate limited by carrier bandwidth

  9. Frequency modulation • The frequency of the sine wave is changed. • In the diagram, 01100 is transmitted in blue. • According to the blue diagram, 0 is sent using a low frequency and 1 is sent using a higher frequency. 1 2 3 4 5 0

  10. Phase-Shift Modulation • The phase of the sine wave is changed. • The value 01100 is sent by shifting the wave 180 to represent a 1 bit while sending the wave unshifted to represent a 0.

  11. Modem • A device that encodes digital information in a carrier wave for transmission across copper wires or a dialup telephone connection. • A pair of modems permits two-way communication.

  12. Modem • Modem contains separate circuitry for… • Modulation of outgoing signal • Demodulation of incoming signal • MODEM= MOdulator / DEMOdulator

  13. Illustration of ModemsUsed Over a Long Distance • Separate wires carry signals in each direction • Modulator on one modem connects to demodulator on other

  14. Types of Modems • Conventional modem • Use four wires • Transmit modulated electrical wave • Optical modem = Fiber modem • Use glass fibers • Transmit modulated light • Wireless modem = RF Modem • Use air / space • Transmit modulated RF wave

  15. Types of Modems(continued) • Dialup modem • Use voice telephone system • Transmit modulated audio tone Note: in practice, a dialup modem uses multiple tones simultaneously • Cable modem • send digital information over the coaxial cables used for cable television.

  16. Dialup modem • A dialup modem contains circuitry that mimics a telephone. • A dialup modem uses a carrier that is an audible tone. • A pair of dialup modems offers full duplex communication. • In practice, the modems must use different carrier tones to avoid having both modems transmit at the same tone.

  17. Illustration of Dialup Modem • Modem can • Dial • Answer • Carrier is audio tone

  18. Cable Modem • A cable modem connects a home computer (or network of home computers) to residential cable TV service. • Like the television "set top" box, cable modems are usually supplied by the service provider and not a piece of equipment individuals need to shop for on their own.

  19. Cable Modem • Most cable modems supply a 10 Mbps Ethernet connection for the home LAN, although cable modem service rarely if ever performs at those speeds. • The performance of a cable modem Internet connection can vary depending on the utilization of the shared cable line in that neighborhood, but typical data rates range from 300 Kbps to 1500 Kbps.

  20. Modem Terminology • Full-duplex modem • Provides 2-way communication • Allows simultaneous transmission • Uses four wires • Half-duplex modem • Does provide 2-way communication • Transmits in one direction at any time • Uses two wires

  21. Recall • Propagation delay • Time required for signal to travel across medium • Determined by physics • Bandwidth • Maximum times per second signal can change • Electrical property of physical transmission system

  22. Multiplexing • Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. 

  23. The General Concept of Multiplexing • Separate pairs of communications travel across shared channel • Multiplexing prevents interference • Each destination receives only data sent by corresponding source

  24. Multiplexing Terminology • Multiplexor • Device or mechanism • Accepts data from multiple sources • Sends data across shared channel • Demultiplexor • Device or mechanism • Extracts data from shared channel • Sends to correct destination

  25. Multiplexing • As long as each carrier operate at a different frequency than the others, it remains independent. • In practice, two carrier operating at almost the same frequency or at exact multiples of a frequency can interfere with one another.

  26. Basic Types of Multiplexing

  27. Frequency Division Multiplexing (FDM) • Signals generated by each sending device modulate different carrier frequencies.  These modulated signals are then combined into a single composite signal that can be transported by the link. 

  28. FDM Multiplexing

  29. FDM Demultiplexing

  30. Time Division Multiplexing (TDM) • Multiple transmissions can occupy a single link by subdividing them and interleaving the portions.  • We say that TDM is a round robin use of a frequency.

  31. Time Division Multiplexing (TDM) • TDM can be implemented in two ways: • synchronous TDM • asynchronous TDM.

  32. Synchronous TDM • The multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit.  • A frame consists of one complete cycle of time slots.  Thus the number of slots in frame is equal to the number of inputs.

  33. Synchronous TDM

  34. Synchronous TDM

  35. Synchronous TDM

  36. Asynchronous TDM • Each slot in a frame is not dedicated to the fix device.  • Each slot contains an index of the device to be sent to and a message.  • Thus, the number of slots in a frame is not necessary to be equal to the number of input devices.

  37. Asynchronous TDM • More than one slots in a frame can be allocated for an input device.  • Asynchronous TDM allows maximization the link.  • It allows a number of lower speed input lines to be multiplexed to a single higher speed line.

  38. Asynchronous TDM • A frame contains a fix number of time slots.  Each slot has an index of which device to receive.

  39. Asynchronous TDM

  40. Asynchronous TDM

  41. Recall Multiplexing • Frequency Division Multiplexing (FDM) • Multiple items transmitted simultaneously • Uses multiple “channels” • Time Division Multiplexing (TDM) • Only one item at a time on shared channel • Item marked to identify source • Demultiplexor uses identifying mark to know where to deliver

  42. Transmission Schemes • Baseband transmission • Uses only low frequencies • Encodes data directly • Broadband transmission • Uses multiple carriers • Can use higher frequencies • Achieves higher throughput • Hardware more complex and expensive

  43. Scientific Principle BehindFrequency Division Multiplexing • Note: this is the same principle that allows a cable TV company to send multiple television signals across a single cable Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference

  44. Wave Division Multiplexing • Facts • FDM can be used with any electromagnetic radiation • Light is electromagnetic radiation • When applied to light, FDM is called wave division multiplexing • Informally called color division multiplexing

  45. Summary • Various transmission schemes and media available • Electric current over copper • Light over glass • Electromagnetic waves • Digital encoding used for data • Asynchronous communication • Used for keyboards and serial ports • RS-232 is standard • Sender and receiver agree on baud rate

  46. Summary (continued) • Modems • Used for long-distance communication • Available for copper, optical fiber, dialup • Transmit modulated carrier • Phase-shift modulation popular • Classified as full- or half-duplex • Two measures of digital communication system • Delay • Throughput

  47. Summary (continued) • Multiplexing • Fundamental concept • Used at many levels • Applied in both hardware and software • Two basic types • Time-division multiplexing (TDM) • Frequency-division multiplexing (FDM) • When applied to light, FDM is called wave-division multiplexing

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