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
s(t) = (4/)  (sin (2ft) + (1/3) sin (2(3f)t))
･ The second frequency is an integer multiple of the first frequency. When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is referred to as the fundamental frequency.
･ The period of the total signal is equal to the period of the fundamental frequency. The period of the component sin (2ft) is T = 1/f, and the period of s(t) is also T, as can be seen from the figure.

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 6

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. Business Data Communications, 5e
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
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 Digital 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
Analog Signal
Digital Signal
Analog Data
Two alternatives: (1) signal occupies the same spectrum as the analog data (2) Analog data are encoded to occupy a different spectrum.
Analog data are encoded using a codec to produce a digital bit stream.
Digital Data
Digital data are encoded using a modem to produce analog signal.
Two alternatives: (1) signal consists of two voltage levels to represent two binary values (2) digital data are encoded to produce a digital signal with desired properties.

25. Analog and Digital Treatment of Signals
Analog Transmission
Digital Transmission
Analog Signal
Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data.
Assumes that the analog signal represents digital data. Signal is propagated through repeaters; at each repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal.
Digital Signal
Not used.
Digital signal represents a stream of 1s and 0s which may represent digital data or may be an encoding of analog data. Signal is propagated though repeaters; at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.

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 Digital Subscriber 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