1. Introduction to Communications Digital Signals & Binary Codes
CT1037N
Introduction to Communications
Digital Signals & Binary Codes
Er. Saroj Sharan Regmi
Lecture 06

2. Signal Representation & Spectral Analysis
Last Lecture: 05
Signal Representation & Spectral Analysis
Signals and Systems,
Continuous- and Discrete- Time Systems,
Continuous- and Discrete- Time Signals,
Fourier Series,
One-Sided Amplitude Frequency Spectrum,
Two- Sided Amplitude Frequency Spectrum.

3. Digital Signals & Binary Codes
Today’s Lecture: 06
Digital Signals & Binary Codes
Review Signals & Analogue Signals.
Digital Signals.
Advantages of Digital Signals.
Binary Digital Signals.
Binary Signal Ratios.
Data Codes.
Data Coding Efficiency.
Data Coding Noise Immunity.
Bit Error Rate.
Encoding Schemes.

4. Signals Analogue Signals
Signal: An electrical voltage or current which varies with time and is used to carry messages or information from one point to another.
Analogue Signals
Analogue signals represent information by varying continuously with time.
Noise added to the analogue signal can greatly affect the accuracy of the information.
Limiting the level of noise is not an easy or inexpensive process.

5. Digital Signals
Digital signals vary abruptly and change between distinct voltage or current levels.
The most widely used form of a digital signal is binary, (two states).
Usually represented as 0 Volts (0V) and 5 Volts (+5V),
Also as: 0 & 1, Low & High, OFF & ON, False & True.
Information is represented as a train of pulses arranged in specific combinations.
A digital waveform can decrease the effect of noise on the information to a very great effect.

6. Advantages of Digital Signals
A digital waveform is less sensitive to noise than an analogue signal:
Decreases the effect of noise on the information to a very great effect.
Less cross-talk (co-channel interference).
Lower distortion levels.
Faded signals are more easily recreated.
Greater transmission efficiency.

7. Binary Digital Signals
Signals arising from computer type equipment designed to transmit information in coded form.
Bits: The individual 1's and 0's.
Bit Slot Duration, : The time required by each bit to be transmitted.

8. Bandwidth in Digital Signals
Binary Digital Systems:
Bit Rate: The number of bits transmitted per second.
Other (Non-Binary) Digital Systems:
Baud Rate: The number of symbols transmitted per second.
Each symbol is composed of more than 1 bit.

9. Binary Signal Ratios
Ratios in Binary Signals enable us to see the relationship between a bit set at 1 and another set at 0. T
: the 1 bit slot duration,
T : the periodic time,
T - :the 0 bit slot duration.

10. Mark Space Ratio Duty-Cycle Ratio
The ratio between the 1's and the 0's.
used in Morse Code.
Duty-Cycle Ratio
The ratio between the 1's and the periodic time of one cycle.
Expressed as a percentage of period.

11. Binary Signal Ratios: Example
What is the mark-space ratio and the duty-cycle of a binary transmission with periodic time T = 1 ms when = 200 µs ?
T = 1 ms
= 200 µs = 0.2 ms

12. Data Codes
Data codes have always been in widespread use even since mankind’s early history:
From the use of hand signals to mirror flashing signals across the land, to smoke signals of the American Indians, information has been coded and sent by a variety of means.
Data codes are the way in which bits are grouped together to represent different symbols.
The sender and receiver must use the same code in order to communicate properly.
ASCII is the most widespread data code in use today:
7-bit code that can represent a total of 128 symbols (27 = 128).
Limited by the number of symbols it can represent.

13. Number of bits in a Data Code
The number of bits in a code will dictate the maximum number of symbols which can be represented.
Alternatively, the maximum number of symbols required will dictate the number of bits that must be included in a code.
n = number of bits in a code
M = number of symbols represented

14. Number of bits in a Data Code (…2) Maximum Number of Symbols
bits in Code
Maximum Number of Symbols
BCD 4
24 = 16
BAUDOT 5
25 = 32 ? 6
26 = 64
ASCII 7
27 = 128
EBCDIC 8
28 = 256 10
210 = 1024
UNICODE 16
216 = 65536

15. Data Coding Efficiency
The efficiency with which a code can be used to represent a group of symbols.
Based on:
the number of symbols requiring representation, and
the number of bits that a code must have to enable the representation of all symbols.

16. Data Coding Efficiency: Example 1
How many bits would be required to distinguish the 88 keys of a piano and what would the coding efficiency be?

17. Data Coding Efficiency: Example 2
Compare the efficiency of the binary versus the decimal coding systems when representing the 26 letters of the alphabet:
The binary code:
The decimal code:

18. Data Coding Noise Immunity
Binary coding is more immune to noise than any other form of coding.
Consider the example below:
The Binary Code:
The Decimal Code:
10V
Threshold = 5V
Threshold = 0.5V 0V

19. Bit Error Rate (BER)
BER relates to the number of possible erroneous bits received.
Shows the quality of a particular communications link.
BER is an error probability defined as:
BER values for a digital transmission system are normally specified and depend on the Signal-to-Noise Ratio (SNR) of the detection system.
A typical BER requirements is in the region of 10-9.

20. Bit Error Rate (BER): Example
Example: What is the BER of a transmission consisting of 5Gb if the erroneous bits received are 3?

21. Morse Code One of the first character codes developed.
A crude but effective way of transmitting characters over a telegraph circuit.
Developed with a telegraph operator in mind who:
sent combinations of dots (short beep) and dashes (long beep),
paused for a short time between letters.
Unsuitable for computer communications:
due to the extra time required between the transmission of each character,
limiting number of characters that could be represented.

22. Morse Code (…2)

23. Baudot Code
One of the first character codes developed with machines in mind.
Uses 1s and 0s to represent characters.
Uses 5 bits of information per character.
Case sensitive:
lower case characters represent letters,
upper case characters represent figures.
Results in 32 different characters per case.
Used for many years on telex equipment and is still used on some teletype machines.
Unsuitable for high speed data communications due to the time required to switch between cases.

24. Baudot Code (…2)

25. BCD EBCDIC (Binary Coded Decimal)
4-bit code which allows for up to 16 characters / symbols (24 = 16).
Often used to represent digits 0 to 9 but insufficient number of characters for anything else.
6-bit version allows for up to 64 characters.
EBCDIC
(Extended Binary Coded Decimal Interchange Code)
8-bit standard which allows for up to 256 characters (28 = 256).
Not all codes are used hence gaps exist.
Common on IBM mainframes and related products.

26. (American Standard Code for Information Interchange)
ASCII
(American Standard Code for Information Interchange)
The ASCII code is the most popular code for serial data communications today.
It is a 7-bit code allowing up to 128 combinations (27 = 128), and thus supports upper and lowercase characters, numeric digits, punctuation symbols, and special codes.
ASCII is also used as the data code for keyboards in computers.
Control codes are used and are represented as symbols.
Used in binary synchronous communication, and device control codes in communicating with devices such as printers or terminals.

27. © C i s c o S y s t e m s , I n c .
The 7-bit ASCII Code

28. Data (Line) Encoding
Data encoding puts the coded information into a form which will enable its transfer through a certain medium.
The simplest data encodings have undesirable timing and electrical (dc) characteristics.
Line codes have been designed to have desirable transmission properties.
Bi-phase encoding transition, such as the Manchester encoding, has no dc component whatever the word transmitted thus offering desirable electrical characteristics.
Both digital and analogue data can be encoded into various forms.

29. Data (Line) Encoding (…2)
Data encoding describes how the bits are actually signalled on the wire.
Different signal elements are used to represent binary 1 and binary 0.
Encoding scheme is the mapping from binary digits to signal elements.
minimizes errors in determining the start and end of each bit,
minimizes errors in determining whether each bit is 1 or 0.

30. Data Encoding Schemes There are various techniques for data encoding:
TTL: Transistor-Transistor Logic,
NRZ-L: Non-Return to Zero-Level,
NRZI: Non-Return to Zero-Inverted,
Manchester Tx (+),
Manchester Tx (-),
Differential Manchester,
MLT3: Multi-Level Threshold-3,
etc.

31. Data Encoding Schemes (…2)

32. Data Encoding Schemes (…3)

33. Data Encoding Schemes (…4)

34. Summary Defined Digital Signals.
Characteristics of Binary Digital Signals.
Reviewed the Mark Space Ratio and the Duty Cycle mechanisms.
Data encoding efficiency.
Data codes are the way in which bits are grouped together to represent different symbols. Examples are Morse, Baudot, BCD, EBCDIC, ASCII, etc.
Binary coding is more immune to noise than any other form of coding.
Data Encoding puts the coded information into a form which will enable its transfer through a certain medium.
Examples of Encoding Schemes are TTL, NRZ-L, NRZI, Manchester Tx (+), Manchester Tx (-), Differential Manchester, MLT3, etc.