1. IE 20303

2. 2 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

3. 3 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

4. 4 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

5. Analog and Digital Signaling 5

6. 6 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

7. Analog and Digital Data and Signals 7

8. Analog and Digital Treatment of Signals

9. 9 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

10. 10 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

11. 11 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

12. 12 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)

13. Modulation of Analog Signals for Digital Data 13

14. Business Data Communications, 5e 14 Amplitude Shift Keying (ASK) In radio transmission, known as amplitude modulation (AM) The amplitude (or height) of the sine wave varies to transmit the ones and zeros Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

15. 15 1 0 0 1 ASK Illustration

16. 16 Frequency Shift Keying (FSK) In radio transmission, known as frequency modulation (FM) Frequency of the carrier wave varies in accordance with the signal to be sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog bandwidth than ASK

17. 17 1 1 0 1 FSK Illustration

18. 18 Phase Shift Keying (PSK) Also known as phase modulation (PM) Frequency and amplitude of the carrier signal are kept constant The carrier signal is shifted in phase according to the input data stream Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

19. 19 0 0 1 1 PSK Illustration

20. Voice Grade Modems Designed for digital transmission over ordinary phone lines Uses 4-kHz bandwidth Adheres to ITU-T standards 20

21. 21 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

22. Cable Modems 22

23. 23 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.

24. ADSL Modem Application 24

25. 25 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

26. Pulse-Code Modulation Example 26

27. 27 Digital Encodingof Digital Data Most common, easiest method is different voltage levels for the two binary digits Typically, negative=1 and positive=0 Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level

28. 28 Differential NRZ Differential version is NRZI (NRZ, invert on ones) Change=1, no change=0 Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

29. 29 Problems With NRZ Difficult to determine where one bit ends and the next begins In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

30. 30 Biphase Alternatives to NRZ Require at least one transition per bit time, and may even have two Modulation rate is greater, so bandwidth requirements are higher; maximum modulation rate is twice NRZ Advantages Synchronization due to predictable transitions Error detection based on absence of a transition

31. 31 Manchester Code Transition in the middle of each bit period Transition provides clocking and data Low-to-high=1 , high-to-low=0 Used in Ethernet and other LANs

32. 32 Differential Manchester Midbit transition is only for clocking Transition at beginning of bit period=0 Transition absent at beginning=1 Has added advantage of differential encoding Used in token-ring

33. 33 Digital Signal Encoding Schemes

34. 34 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

35. Analog Sine-Wave Signals 35

36. 36 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

37. Asynchronous Transmission 37

38. 38 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

39. Synchronous Transmission More efficient than asynchronous transmission Preamble, postamble and control information are typically < 100 bits Introduces the need for error checking 39

40. 40 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

41. 41 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

42. 42 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%)

43. Error Detection Process 43