Chapter 15 & 16:

1. Chapter 15 & 16:

2. Electromagnetic Signals • Analog Signal • signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal • Digital Signal • signal intensity maintains a constant level for some period of time and then changes to another constant level

3. Analog and Digital Waveforms

4. Periodic Signal Characteristics • Peak Amplitude (A) • Maximum signal value (strength), measured in volts • Frequency (f) • Repetition rate • Measured in cycles per second or Hertz (Hz) • Period (T) • Amount of time it takes for one repetition, T=1/f • Phase () • Relative position in time, measured in degrees

5. s(t) = (4/)  (sin (2ft) + (1/3) sin (2(3f)t)) Frequency Domain Concepts

6. Frequency Domain Concepts • Spectrum of a signal is the range of frequencies that it contains • Absolute bandwidth of a signal is the width of the spectrum • Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found • The greater the bandwidth, the higher the information-carrying capacity of the signal

7. Bandwidth • Width of the spectrum of frequencies that can be transmitted • if spectrum=300 to 3400Hz, bandwidth=3100Hz • Greater bandwidth leads to greater costs • Limited bandwidth leads to distortion

8. Analog Signaling

9. Voice/Audio Analog Signals • Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage • Human voice has frequency components ranging from 20Hz to 20kHz • For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

10. Voice Signals

11. Image/Video: Analog Data to Analog Signals • Image is scanned in lines; each line is displayed with varying levels of intensity • Requires approximately 4Mhz of analog bandwidth • Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

12. Digital Signals

13. Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate • bandwidth • physical environment • number of receivers • impairments

14. Impairments and Capacity • Impairments exist in all forms of data transmission • Analog signal impairments result in random modifications that impair signal quality • Digital signal impairments result in bit errors (1s and 0s transposed)

15. Transmission Impairments:Guided Media • Attenuation • loss of signal strength over distance • Attenuation Distortion • different losses at different frequencies • Delay Distortion • different speeds for different frequencies • Noise • distortions of signal caused by interference

16. Transmission Impairments:Unguided (Wireless) Media • Free-Space Loss • Signals disperse with distance • Atmospheric Absorption • Water vapor and oxygen contribute to signal loss • Multipath • Obstacles reflect signal creating multiple copies • Refraction - Change in signal speed due to atmospheric conditions • Thermal Noise- White noise, arises from thermal activity of devices

17. Types of Noise • Thermal (aka “white noise”) • Uniformly distributed, cannot be eliminated • Intermodulation • When different frequencies collide (creating “harmonics”) • Crosstalk • Overlap of signals • Impulse noise • Irregular spikes, less predictable Business Data Communications, 5e

18. Channel Capacity • The rate at which data can be transmitted over a given path, under given conditions • Four concepts • Data rate • Bandwidth • Noise • Error rate

19. Data Communication Components • Data • Analog: Continuous value data (sound, light, temperature) • Digital: Discrete value (text, integers, symbols) • Signal • Analog: Continuously varying electromagnetic wave • Digital: Series of voltage pulses (square wave) • Transmission • Analog: Works the same for analog or digital signals • Digital: Used only with digital signals

20. Analog DataSignal Options • Analog data to analog signal • Inexpensive, easy conversion (e.g., telephone) • Data may be shifted to a different part of the available spectrum (multiplexing) • Used in traditional analog telephony • Analog data to digital signal • Requires a codec (encoder/decoder) • Allows use of digital telephony, voice mail

21. Digital DataSignal Options • Digital data to analog signal • Requires modem (modulator/demodulator) • Allows use of PSTN to send data • Necessary when analog transmission is used • Digital data to digital signal • Requires CSU/DSU (channel service unit/data service unit) • Less expensive when large amounts of data are involved • More reliable because no conversion is involved

22. Analog and Digital Signaling

23. Transmission Choices • Analog transmission • only transmits analog signals, without regard for data content • attenuation overcome with amplifiers • signal is not evaluated or regenerated • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers • switching equipment evaluates and regenerates signal

24. Analog and Digital Data and Signals

25. Analog and Digital Treatment of Signals

26. Advantages of Digital Transmission • Cost – large scale and very large scale integration has caused continuing drop in cost • Data Integrity – effect of noise and other impairments is reduced • Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division • Security & Privacy – Encryption possible • Integration – All signals (Voice. Video, image, data) treated the same

27. Analog Encoding of Digital Data • Data encoding and decoding technique to represent data using the properties of analog waves • Modulation: the conversion of digital signals to analog form • Demodulation: the conversion of analog data signals back to digital form

28. Modem • An acronym for modulator-demodulator • Uses a constant-frequency signal known as a carrier signal • Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal • The receiving modem translates the analog signal back into digital data

29. Methods of Modulation • Amplitude modulation (AM) or amplitude shift keying (ASK) • Frequency modulation (FM) or frequency shift keying (FSK) • Phase modulation or phase shift keying (PSK)

30. Voice Grade Modems • Designed for digital transmission over ordinary phone lines • Uses 4-kHz bandwidth • Adheres to ITU-T standards

31. Cable Modems • Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable office • Cables used for television delivery can also be used to deliver data between subscriber and central location • Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique • Splitter is used to direct TV signals to a TV and the data channel to a cable modem

32. Cable Modems

33. Asymmetric DigitalSubscriber Line (ADSL) • New modem technology for high-speed digital transmission over ordinary telephone wire. • At central office, a combined data/voice signal is transmitted over a subscriber line • At subscriber’s site, twisted pair is split and routed to both a PC and a telephone • At the PC, an ADSL modem demodulates the data signal for the PC. • At the telephone, a microfilter passes the 4-kHz voice signal. • The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques.

34. ADSL Modem Application

35. Digital Encoding of Analog Data • Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form • Best-known technique for voice digitization is pulse-code modulation (PCM) • The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal. • Good-quality voice transmission can be achieved with a data rate of 8 kbps • Some videoconference products support data rates as low as 64 kbps

36. Pulse-Code Modulation Example

37. Analog Encoding of Analog Information • Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line. • Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band • A higher frequency may be needed for effective transmission • Analog-to-analog modulation permits frequency-division multiplexing

38. Analog Sine-Wave Signals

39. Asynchronous Transmission • Avoids timing problem by not sending long, uninterrupted streams of bits • Data transmitted one character at a time, where each character is 5 to 8 bits in length. • Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character. • Simple and cheap but requires an overhead of 2 to 3 bits per character

40. Asynchronous Transmission

41. Synchronous Transmission • Block of bits transmitted in a steady stream without start and stop codes. • Clocks of transmitter and receiver must somehow be synchronized • Provide a separate clock line between transmitter and receiver; works well over short distances, • Embed the clocking information in the data signal. • Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern • The data plus preamble, postamble, and control information are called a frame

42. Synchronous Transmission • More efficient than asynchronous transmission • Preamble, postamble and control information are typically < 100 bits • Introduces the need for error checking

43. Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide method for controlling errors • Error control process has two components • Error detection: redundancy introduced so that the occurrence of an error will be detected • Error correction: receiver and transmitter cooperate to retransmit frames that were in error

44. Error Detection: Parity Bits • Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) • Good for detecting single-bit errors only • High overhead (one extra bit per 7-bit character=12.5%) • Noise impulses are often long enough to destroy more than one bit

45. Error Detection: Cyclic Redundancy Check (CRC) • Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame • 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder • For a CRC of length N, errors undetected are 2-N • Overhead is low (1-3%)

46. Error Detection Process