1. DATA AND PULSE COMMUNICATION
UNIT-III
DATA AND PULSE COMMUNICATION

2. History of Data Communication
1837- Telegraph was invented by Samuel
Morse
1849- First slow speed telegraph printer
1876- Telephone was invented
1949- All electronic diode based computer
1969- Internet began to evolve

3. Network Standards

4. Why Standards?
Standards provide a fixed way for hardware and/or software systems to communicate.
For example, USB enables two pieces of equipment to interface even though they are manufactured by different companies.
By allowing hardware and software from different companies to interconnect, standards help promote competition.

5. Types of Standards There are two main types of standards:
Formal: a standard developed by an industry or government standards-making body
De facto: standards that emerge in the marketplace and are widely used, but lack official backing by a standards-making body

6. The Standardization Processes Three Steps
Specification: developing the nomenclature and identifying the problems to be addressed.
Identification of choices: identify solutions to the problems and choose the “optimum” solution.
Acceptance: defining the solution, getting it recognized by industry so that a uniform solution is accepted.

7. Some Major Standards Making Bodies
ISO: International Organization for Standardization (
ITU-T: International Telecommunications Union –Telecom Group (
ANSI: American National Standards Institute (
IEEE: Institute of Electrical and Electronic Engineers (see standards.ieee.org)
IETF: Internet Engineering Task Force (

8. Digital Data Transmission
The transmission of binary data across a link can be accomplished either in parallel mode or serial mode
In parallel mode, multiple bits are sent with each clock pulse
In serial mode, one bit is sent with each clock pulse
There are 2 subclasses of serial transmission: synchronous and asynchronous

9. Parallel Transmission
Binary data may be organized into groups of n bits each
By grouping, we can send data n bits at a time instead of one. This is called parallel transmission
The advantage of parallel transmission is speed but its advantage is cost

10. FIGURE 3-8: PARALLEL DATA TRANSMISSION
Used most often for communication with local devices

11. Serial Transmission
In serial transmission, one bit follows another, so we need only one communicating channel to transmit data between 2 communicating devices
The advantage of serial transmission is the reduction of the cost of transmission over the parallel transmission
Serial transmission occurs in one of 2 ways: asynchronous or synchronous

12. FIGURE 3-9: SERIAL DATA TRANSMISSION
Used most often for data communication

13. Asynchronous Transmission
In asynchronous transmission, the timing of a signal is unimportant
Information is received and translated by agreed-upon patterns
Patterns are based on grouping the bit stream into bytes
The sending system handles each group independently, relaying it to the link whenever ready, without regard to a timer

14. Asynchronous Transmission (cont.)
To alert the receiver to the arrival of a new group, an extra bit called start bit is added to the beginning of each byte
To let the receiver know that the byte is finished, one or more additional bits called stop bits are appended to the end of the byte
This mechanism is called asynchronous because at the byte level, sender and receiver do not have to be synchronized
But within each byte, the receiver must still be synchronized with the incoming bit stream

15. Asynchronous Transmission (cont.)
When the receiver detects a start bit, it sets a timer and begins counting bits as they come in
After n bits, the receiver looks for a stop bit and after the stop bit is detected, it ignores any received pulses until the next start bit

16. Synchronous Transmission
In synchronous transmission, the bit stream is combined into longer frames which may contains multiple bytes
Each byte is introduced onto the transmission link without a gap between it and the next one
It is the responsibility of the receiver to reconstruct the information

17. Synchronous Transmission (cont.)
Without gaps and start/stop bits, timing becomes very important therefore the accuracy of the received information is completely dependent on the ability of the receiver to keep an accurate count of the bits as they come in

18. Simplex, Half-Duplex, and Full-Duplex Transmission
A communications channel is classified as one of three types:
(depending on the direction of transfer)
Simplex
Full-Duplex
Half-Duplex
Simplex: a simplex mechanism can only transfer data in a single direction
It is analogous to broadcast radio or television
Figure 9.8a illustrates simplex communication
Full-Duplex: allows transmission in two directions simultaneously
It is analogous to a voice telephone conversation
in which a participant can speak even if they are able to hear background music at the other end
Figure 9.8b illustrates the concept

20. DTE-DCE Interface
There are usually four basic functional units involved in the communication of data: a DTE and DCE on both end of transmission
The DTE generates the data and pass them to a DCE. The DCE converts the signal to a format appropriate to the transmission medium
When the signal arrives at the receiving end, this process is reversed

21. Data Terminal Equipment (DTE)
DTE includes any unit that functions either as a source of or as a destination for binary digital data
It can be a terminal, microcomputer, printer, fax machine and etc.

22. Data Circuit-Terminating Equipment (DCE)
DCE includes any functional unit that transmits or receives data in the form of an analog or digital signal through a network
Commonly used DCEs include modems

23. DTE-DCE Interface Standard
Many standards have been developed to define the connection between a DTE and a DCE
Each standard provides a model for the mechanical, electrical, and functional characteristics of the connection
The most active organizations defining the interface standard are the Electronic Industries Association (EIA) and the International Telecommunication Union-Telecommunication Standards Committee (ITU-T)

24. DTE-DCE Interface Standard (cont.)
The EIA standards are called EIA-232, EIA-422, EIA-449, and so on
The ITU-T standards are called the V series and the X series

25. EIA-232 Interface
Originally issued in 1962 as the RS-232 standard (recommended standard)
The most recent version, EIA-232-D, defines not only the type of connectors to be used but also the specific cable and plugs and the functionality of each pin

26. EIA-232 Mechanical specification
The EIA-232 defines the interfaces as a 25-wire cable with a male and a female DB-25 pin connection attached to either end. The length of the cable may not exceed 15 meters
A DB-25 connector is a plug with 25 pins, each of which is attached to a single wire with a specific function. However, fewer are actually used in current practice
Another implementation of EIA-232 uses a 9-wire cable with a mail and a female DB-9 pin connector attached to either end
9-pin connector is more commonly found in PCs but it covers signals for asynchronous serial communication only
Male connector is used on DTE and female connector is used on DCE

27. Electrical Specification
EIA-232 states that all data must be transmitted as logical 1s and 0s (called mark and space) using NRZ-L encoding, with 0 defined as a positive voltage and 1 defined as a negative voltage
EIA-232 defines 2 distinct ranges, one for positive voltages and one for negative
To be recognized as data, the amplitude of a signal must fall between 3 and 15 volts or between -3 and -15 volts
EIA-232 allows for a maximum bit rate of 20 kbps, although in practice this often is exceeded

28. Functional Specification
DB-25 Implementation
DB-9 Implementation

29. Functioning of EIA-232 in Synchronous Full-Duplex Transmission

30. Flow Control
Means to ask the transmitter to stop/resume sending in data
Required when:
DTE to DCE speed > DCE to DCE speed
(e.g. terminal speed = 115.2kbps and line speed = 33.6kbps, in order to benefit from modem’s data compression protocol)
without flow control, the buffer within modem will overflow – sooner or later
the receiving end takes time to process the data and thus cannot be always ready to receive

31. Modem The most familiar type of DCE is a modem
Modem is derived from the words Modulator and Demodulator
It is an electronics device used to transmit information of a computer to the destination through the telephone channel

32. Telephone Bandwidth
Traditional telephone line can carry frequencies between 300 Hz and 3300 Hz (note: some say that 3400 Hz)
All of this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility
However, the edges of this range cannot support the transmission of base-band digital data

33. Bell Modems
The first commercial modems were produced by the Bell Telephone Company in the early 1970
ITU-T
Bell
Baud Rate
Bit Rate
Modulation
V.21
V.22
V.23
V.26
V.27
V.29
103
212
202
201
208
209
300
600
1200
1600
2400
4800
9600
FSK
4-PSK
8-PSK
16-QAM

34. DATA COMMUNICATION CODES

35. MORSE CODE First character code developed
For transmitting data over telegraph wires
telegrams (remember Western Union)
Used dots (short beep) and dashes (long beeps) instead of 1’s and 0’s
More frequent the character, the fewer the beeps
Problems:
variable “length” character representation
required pauses between letters
no lower case, few punctuation or special characters
no error detection mechanism

36. FIGURE 3-2: MORSE CODE

37. BAUDOT CODE
One of first codes developed for machine to machine communication
Uses 1’s and 0’s instead of dots and dashes
For transmitting telex messages (punch tape)
Fixed character length (5-bits)
32 different codes
increased capacity by using two codes for shifting
11111 (32) Shift to Lower (letters)
11011 (27) Shift to Upper (digits, punctuation)
4 special codes for SP, CR, LF & blank
Total = = 56 different characters

38. BAUDOT CODE (cont.) Problems: International Baudot
required shift code to switch between character sets
no lower case, few special characters
no error detection mechanism
characters not ordered by binary value
designed for transmitting data, not for data processing
International Baudot
Added a 6th bit for parity
Used to detect errors within a single character

39. FIGURE 3-3: BAUDOT CODE

40. EBCDIC Extended Binary Coded Decimal Interchange Code
8-bit character code developed by IBM
used for data communication, processing and storage
extended earlier proprietary 6-bit BCD code
designed for backward compatibility or marketing?
still in use today on some mainframes and legacy systems.
Allows for 256 different character representations (28)
includes upper and lower case
lots of special characters (non-printable)
lots of blank (non-used codes)
assigned to international characters in various versions
used with/without parity (block transmissions)

41. ASCII CODE American Standard Code for Information Interchange
7-bit code developed by the American National Standards Institute (ANSI)
most popular data communication character code today
Allows for 128 different character representations (27)
includes upper and lower case
lots of special characters (non-printable)
generally used with an added parity bit
better binary ordering of characters than EBCDIC
Extended ASCII uses 8 data bits and no parity
Used for processing and storage of data
Allows for international characters
8th bit stripped of for transmission of standard character set

42. FIGURE 3-5: 7-BIT ASCII CODE

43. Error detection
Error detection means to decide whether the received data is correct or not without having a copy of the original message.
Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.
Make sense of message.

44. Types of Errors

45. Single-bit error

46. Burst error

47. Redundancy

48. Four types of redundancy checks are used in data communications

49. Vertical Redundancy Check
VRC

50. Longitudinal Redundancy Check
LRC

51. VRC and LRC

52. Checksum

53. Cyclic Redundancy Check
CRC

54. Cyclic Redundancy Check
Given a k-bit frame or message, the transmitter generates an n-bit sequence, known as a frame check sequence (FCS), so that the resulting frame, consisting of (k+n) bits, is exactly divisible by some predetermined number.
The receiver then divides the incoming frame by the same number and, if there is no remainder, assumes that there was no error.

55. Binary Division

56. Error Correction It can be handled in two ways:
receiver can have the sender retransmit the entire data unit.
The receiver can use an error-correcting code, which automatically corrects certain errors.

57. Single-bit error correction
To correct an error, the receiver reverses the value of the altered bit. To do so, it must know which bit is in error.
Number of redundancy bits needed
Let data bits = m
Redundancy bits = r
Total message sent = m+r
The value of r must satisfy the following relation:
2r ≥ m+r+1

58. Error Correction

59. Hamming Code

60. Hamming Code