1. Data Communications: The Basics Chapter 4 The Management of Telecommunications Houston H. Carr and Charles A. Snyder

2. 2 Chapter 4 Introduction Data communications is changing the industrialized world. It connects machines together to increase the reach of the humans that rely on them to make decisions.

3. 3 Chapter 4 Introduction Telecommunications began with the telegraph on copper wire and has progressed to digital video via satellite and fiber optic.

4. 4 Chapter 4 Telegraph (circa 1837) The first electrical means of communicating over a distance. The telegraph was not user oriented because it depended on skilled intermediaries.

5. 5 Chapter 4 Timeline of Early Telecommunications Development Telegraph invented by S. Morse Western Union started A.G. Bell patents telephone Bell Company founded AT&T created Marconi wireless telegraphy Vacuum tube developed 1837185618761877188518861913

6. 6 Chapter 4 Morse and computer codes A code is the representation of a character or symbol by a number. In data communications this is a binary number, composed of 1’s and 0’s.

7. 7 Chapter 4 Morse and computer codes Letter Morse codeEquivalent digital bits 7-bit ASCII (computer) code A■ _0 11 0 0 0 0 0 1 B_ ■ ■ ■1 0 0 01 0 0 0 0 1 0 C_ ■ 1 0 1 0 0 0 0 1 1 D_ ■ ■1 0 01 0 0 0 1 0 0 E■01 0 0 0 1 0 1 F■ ■ _ ■0 0 1 01 0 0 0 1 1 0 G_ _ ■1 1 01 0 0 0 1 1 1 H■ ■ 0 0 1 0 0 1 0 0 0 I■ 1 1 0 0 1 0 0 1 J■ _ _ _0 1 1 11 0 0 1 0 1 0 K_ ■ _1 0 11 0 0 1 0 1 1 L■ _ ■ ■0 1 0 01 0 0 1 1 0 0 etc

8. 8 Chapter 4 Code length The more bits per character in a code, the longer the time it takes to transmit or the wider the required bandwidth (greater speed)

9. 9 Chapter 4 7-Bit and 8-Bit codes Seven-bit codes define 128 characters (2 7 = 128) Eight-bit codes define 256 characters.

10. 10 Chapter 4 8-bit ASCII The exact codes in a code set depend on the use of the code definition. Code sets used for graphics will differ from the same code when used for data communications. The Morse Code is a variable length code, unsuitable for modern computers.

11. 11 Chapter 4 Wireless telegraphy Wireless telegraphy was as amazing in its time as the first satellite communications were later.

12. 12 Chapter 4 Teletypewriter The teletypewriter ushered in the era of machine-based communications, as it provided storage, switching, and machine encoding. Additionally, it allowed the use of unskilled operators and off-line preparation.

13. 13 Chapter 4 Public Switched Network The public switched (voice) network telephone The simple instrument Bell invented spawned the network that now connects most home and businesses in the industrial world.

14. 14 Chapter 4 Communications channels Simplex (one-way)

15. 15 Chapter 4 Communications channels Half-duplex transmission (one way at a time)

16. 16 Chapter 4 Communications channels Full-duplex transmission (both directions simultaneously)

17. 17 Chapter 4 Digital data Digital data are literally data represented by combinations of numbers For example, 1s and 0s They can be data, text, voice, images, video, or facsimile Having all of these in a digital form means we can use the same network to carry them all, simultaneously.

18. 18 Chapter 4 Effect of noise on digital data transmission Noise is any unwanted signal; it is unwanted because it can change the desired signal

19. 19 Chapter 4 Serial verses parallel Telephone lines are serial channels  Used for most data communications  Bits of the character follow each other down a single wire or its equivalent PC printer cables are usually parallel channels  The number of wires from sender to receiver is equal to or greater than the number of bits for a character

20. 20 Chapter 4 Parallel transmission Message is the letter “K” = 1011100 in ASCII

21. 21 Chapter 4 Serial transmission Message is the letter “K” = 1011100 in ASCII

22. 22 Chapter 4 DTE and DCE Data terminating equipments (DTEs) are noncommunciations-oriented components of a data communications environment. Data communications devices (DCEs) are communications-oriented components of a networks, such as telephone switching equipment, media, modems, etc.

23. 23 Chapter 4 DTE and DTE

24. 24 Chapter 4 Modem A modem is a DCE device. It is a MOdulator-DEModulator device. Modems use amplitude, frequency, or phase shift to encode more that one bit per baud.

25. 25 Chapter 4 Bits per second The notation of speed for data communications is bits per second (bps). Indicates the rate we are sending the component parts of characters sender to the receiver. We refer to this as bandwidth.

26. 26 Chapter 4 Bits per second An asynchronous network uses 10-bit character at a rate of 300 bps it will transmit 30 characters per second.

27. 27 Chapter 4 Baud Sometimes used interchangeably with bps, but they are not necessarily the same. Baud refers to the number of times a change of analog signal occurs in the circuit. If one signal change carries per bit, then baud and bps are the same, e.g. baud = bps. As data rate increases above 600 bps over a 4 Khz circuit, the baud limit is reached. Thus, a 14,000 bps modem is a 600 baud modem.

28. 28 Chapter 4 Frequency shift keying (FSK) Frequency Shift Keying (FSK) is used by the simplest modems to modulate and demodulate the signal. FSK uses specific tones to represent 1’s and 0’s in each direction.

29. 29 Chapter 4 Baud rate and FSK For FSK to work, the circuit must have bandwidth of 1.5 times the baud rate. At 1200 baud, 1800 Hz would be needed for transmission in both directions, which would require a total analog bandwidth of 3600 Hz, which exceeds the voice spectrum of a POTS line.

30. 30 Chapter 4 Modulations techniques To achieve faster speeds, a modem has to be able to encode more than one bit per baud. Modulation techniques  Phase shift modulation  Quadrature amplitude modulation (QAM)  Trellis code modulation

31. 31 Chapter 4 Example of Phase Changes Possible by Use of QAM Phase Change (Degrees) Relative Amplitude Quadbit 03001 051001 45√20000 453√210000 9030010 9051010 135√20011 1353√21011 18030111 18051111 225√20110 2253√21110 27030100 27051100 315√20101 3153√21101

32. 32 Chapter 4 Phase Shift Modulation

33. 33 Chapter 4 Differential Phase Shift Modulation

34. 34 Chapter 4 Asynchronous Transmission In asynchronous transmission each character is considered by itself with no relation to the character sent before it or after it.  All timing and error checking are within the bits of the character.

35. 35 Chapter 4 Asynchronous data transmission

36. 36 Chapter 4 Synchronous transmission With synchronous communications, the unit of control is a block of data, also called a packet.  A group of text, numbers, or binary characters.  Header characters, which include timing and possible address information, begin the block and are added in front of the real data being sent. The portion after the data is the data is the trailer and contain error checking.

37. 37 Chapter 4 Synchronous data transmission

38. 38 Chapter 4 Attenuation Attenuation is characteristic of electrical and photonic signals. These signals diminish or weaken as they travel from away from their source.

39. 39 Chapter 4 Amplifiers Amplifiers (or repeaters) are used to increase the signal strength of analog signals.

40. 40 Chapter 4 Signal-to-noise ratio Signal-to-noise ration is the relationship between the level of desired signal strength and the level of undesirable noise. A high S-to-N ratio is desired.

41. 41 Chapter 4 Error detection Error detection is a means to determine if the data that was transmitted have been corrupted by noise. Simplest form is echo detection.

42. 42 Chapter 4 Parity checking Parity checking is the simplest error detection scheme for asynchronous data Has a low reliability of finding all errors

43. 43 Chapter 4 Parity checking Asynchronous data has one character for parity checking

44. 44 Chapter 4 Block check characters Block check characters (BCC) are bytes in the trailer that are used to determine that the block of data was received as sent.

45. 45 Chapter 4 Cyclic redundancy checking Cyclic redundancy checking (CRC) is the error detection scheme of choice. CRC-32 is better then CRC-16. It is the most complex, the most common, and the most reliable error checking mode used for synchronous communications.

46. End of Chapter 4 The Management of Telecommunications: Houston H. Carr and Charles A. Snyder