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An overview of modern data communications, focusing on analog and digital signals, compression techniques, data integrity methods, and powerline communications. various historical codes such as Morse and Baudot, the concept of bit rate, and the importance of error detection and correction. The document also discusses different types of noise and the significance of powerline communications.
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Nevil Brownlee, Ulrich Speidel and Clark Thomborson for stimulating discussions and critical comments.
(^1) B. A. Forouzan. Data Communications and Networking, McGraw Hill, 4th edition, New York, 2007. (^2) W. A. Shay. Understanding Data Communications and Networks, 3rd edition, Brooks/Cole, Pacific Grove, CA, 2004.
Connected devices have to “understand” each other to be able to communicate. Communication standards assure that communicating devices represent and send information in a “compatible way”. There are two types of ways to transmit data: via digital signals, which can be represented either electronically (by sequences of specified voltage levels) or optically, via analog signals, which are formed by continuously varying voltage levels.
Digital signals are graphically represented as a square wave: the horizontal axis represents time and the vertical axis represents the voltage level.
PCs often communicate via modems over telephone lines using analog signals which are formed by continuously varying voltage levels:
There are three types of transmission media, each with many variations: conductive metal, like copper or iron, that carries both digital and analog signals; coaxial cable and twisted wire pairs are examples, transparent glass strand or optical fibre that transmits data using light waves, no physical connection that transmits data using electromagnetic waves (as those used in TV or radio broadcast).
Regardless of implementation, all switches are in one of two states: open or closed, symbolically, 0 and 1. Bits can store only two distinct pieces of information. Grouping them, allows for many combinations: two bits allow 2^2 = 4 unique combinations: 00, 01 , 10 , 11 three bits allow for 2^3 = 8 combinations, ten bits allow for 2^10 = 1, 024 combinations, fifty bits allow for 2^50 = 1, 125 , 899 , 906 , 842 , 624 combinations, n bits allow for 2n^ combinations.
Grouping bits allows one to associate certain combinations with specific items such as characters, numbers, pictures. Loosely speaking, this association is called a code. Not every association is a code as we shall soon learn. A difficult problem in communications is to establish communications between devices that operate with different codes. There are standards, but not all standards are compatible! The nice thing about standards is that you have so many to choose from.
Morse code is a variable-length code: letter codes have different lengths; the letter E code is a single dot (1000), the letter H code has four dots (1010101000); the code (0000000) for an inter-word gap (the ‘space’ character) is of length 7; Reason: more frequent letters are assigned shorter codes, so messages can be sent quickly.
Do you think we have got a problem? More precisely, how can we tell a digit from a letter? Answer: using the same principle that allows a keyboard key to represent two different characters. On the keyboard we use the Shift key; the Baudot code uses the extra information 11111 (shift down) and 11011 (shift up) to determine how to interpret a 5-bit code. Upon receiving a shift down, the receiver decodes all codes as letters till a shift up is received, and so on.
