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Electronic Science Electronic Communication 33. Data Communication Data Communication

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Electronic Science Electronic Communication 33. Data Communication Data Communication 1 Module -30 Data Communication 1. Introduction 2. Basic Data Communication System 3. Various data codes and their representation 3.1 Baudot code 3.2 Morse code 3.3 ASCII code 3.4 EBCDIC code 4. Data Representation 5. Data Transmission Modes 5.1 The direction of the exchanges 5.2 The number of data bits sent simultaneously 5.3 Synchronization between the transmitter and receiver 6. Standard Organizations for Data Communication 7. Summary Learning outcome – After studying this module, you will be able to: 1. Understand the concept of data communication system 2. Learn the various data codes and their representation 3. Understand the various data transmission modes 4. Know about different types of data communication standard organizations 5. Apply the knowledge of encoders in different digital systems Electronic Communication Electronic Science 33. Data Communication 2 1. Introduction Data communication refers to the exchange of data between two or more devices through suitable transmission media either wired or wireless as shown in fig. 1. It permits the transfer of binary or digital information between remote computers. The data communication circuits comprise of electronic equipments that aids in the interconnection of digital computer equipments. Figure 1: Computer Network Almost any type of data can be digitized. The effectiveness, low cost, reliability and high speed of digital technology have made data communication more widespread and inevitable in today‟s world. One of the major applications of data communication is internet, linking billions of devices worldwide and through which we have access to extensive range of information and resources at our fingertips. One of the other applications is Electronic mail (e-mail) services that enable us to send data from our personal computer (PC) to anyone anywhere in this world and receive information from anyone across the globe. 2. Basic Data Communication System The Data Communication System has basic five components as shown in fig. 2: 1. Message: It is the information data that is intended to be communicated. Information data can be in the form of text, numbers, image, audio and video. Electronic Communication Electronic Science 33. Data Communication 3 2. Sender: The device that sends the data information is called the sender or transmitter. Sender can be a computer, workstation, telephone handset, video camera, mobile phone and other peripheral devices. Figure 2 : Block diagram of Data Communication System 3. Transmission media: The physical path through which data is transmitted or sent from the sender to the receiver. It is also known as communication channel. Transmission medium can be wired like twisted-pair wire, co-axial cable, fiber optic cable, telephone line etc. or it can be wireless over the free space using radio, microwave and infrared signals. 4. Receiver: The device that receives the data information from the sender is called the receiver. It is also called Sink. A receiver may be a computer, workstation, telephone handset, mobile phone, television set, printer, fax machine, and so on. 5. Protocol: Protocol is a set of rules that governs data communication. It refers to the agreement between the communicating devices. It defines the procedures the devices will use during the process of communication. Without a proper protocol, the devices may be connected but they cannot communicate with each other. Numerous protocols are being used to provide the networking capabilities specifying the data rate, flow control, data segmentation and assembly, sequence control, error detection and control. Electronic Communication Electronic Science 33. Data Communication 4 3. Various data codes and their representation Data is communicated between computers as sequences of binary digits or bits. A data code refers to the way in which bits are grouped together to represent different symbols. A sequence of bits is grouped to form a data character and an encoding scheme translates each group of bits into a character. Hence a unique binary code for every possible character to be communicated is generated and stored in the computer. The two communicating devices must use the same code in order to communicate properly. There are a number of different codes as shown in fig.3, but the most common code in use today is the ASCII code. Figure 3: Simple block diagram of digital system 3.1 Baudot code The Baudot code developed in the year 1875 was named after the pioneer in telegraph printing Emile Baudot. It was used extensively in early teletype machines which was like a typewriter and was used to send and receive coded signal over a communication link. Pressing a key on the typewriter keyboard generates a unique code which is further transmitted to the receiving machine that prints the corresponding character. It is recognized as the first fixed-length character code for machines. It is a five bit code representing 32 different characters. In order to accommodate 26 letters of the alphabet, 10 numbers, and various punctuation marks, it uses two shift codes: letter and figure shift codes. Two of the 32 combinations were used to select the shift codes. If the message is Electronic Communication Electronic Science 33. Data Communication 5 preceded by the letter shift code (11011), all of the following codes are interpreted as alphabet letters. When preceded by the figure shift code (11111), all the following characters are interpreted as numbers or punctuation marks. LETTER FIGURE BINARY LETTER FIGURE BINARY A - 11000 Q 1 11101 B ? 10011 R 4 01010 C : 01110 S BEL 10100 D $ 10010 T 5 00001 E 3 10000 U 7 11100 F ! 10110 V ; 01111 G & 01011 W 2 11001 H # 00101 X / 10111 I 8 01100 Y 6 10101 J ' 11010 Z " 10001 K ( 11110 LETTER SHIFT 11011 L ) 01001 FIGURE SHIFT 11111 M . 00111 SPACE 00100 N , 00110 Line feed (LF) 01000 O 9 00011 Blank(null) Blank 00000 P 0 01101 Table 1: Baud Dot Code For example, if we have 01001 preceded by letter shift code 11011, the character is interpreted as alphabet L and the same binary bit combination when preceded by figure shift code 11111, is interpreted as a right closing parenthesis. As seen from the table 1, letter shift and figure shift code are represented by 11011 and 11111 respectively. In most data communications, baudot has been replaced by codes that can represent more characters and symbols. 3.2 Morse code In 1844, Samuel F. B. Morse successfully demonstrated an electrical telegraph system. The transmitting end of the telegraph system sent text information in the form of electrical pulses Electronic Communication Electronic Science 33. Data Communication 6 along wires which controlled the electromagnet at the receiving end. Morse developed a special code called Morse code to transmit and receive messages at high speeds up to 80 words per minute. Morse code is a series of dots and dashes representing alphabets, numbers and punctuation marks. Dot and dash are usually represented as a short electrical pulse and a long electrical pulse respectively. The telegraph key when depressed causes current flow in an electromagnetic coil that attracts the armature and releases it quickly when the current stops, making clicks during both the instances. When the armature is closed for a short duration, a dot is produced and when closed for longer time, a dash is produced. The transmitter switches the carrier signal on and off to produce dot and dashes. Table 2: Morse code Table 2 displays the Morse telegraph code. As we can see from the table, alphabet A is represented by single dot and single dash, B is represented by single dash and three dots and so on. 3.3 American standard code for Information Interchange (ASCII) The ASCII code was developed by a committee of the American National Standards Institute (ANSI) for binary data coding. ASCII code is a 7-bit code representing 128 alphanumeric symbols with a distinctive code word. The least significant bit is designated bit 0 and the most significant bit is designated as bit 1. The first three bits from MSB onwards indicate whether a number, letter or character is being specified. Electronic Communication Electronic Science 33. Data Communication 7 As seen from table 3, the first 32 values (character code 0 to 31) are non-printing control characters, such as NUL- null character, STX- start of text, ETX- end of text, LF -Line Feed, DLE- Data link escape, US-Unit separator, CR- Carriage return, File separator(FS),Group Separator(GS) CAN-cancel, SUB-substitute ,ESC- escape etc . Mostly they are used to control peripherals such as printers.Codes 32-127 are common for all the different variations of the ASCII table, they are called printable characters, representing letters, digits, punctuation marks, and a few miscellaneous symbols. Almost every character can be found on the computer keyboard. ASCII BINARY ASCII BINARY ASCII BINARY ASCII BINARY NUL 00000000 SP 00100000 @ 01000000 ` 01100000 SOH 00000001 ! 00100001 A 01000001 a 01100001 STX 00000010 " 00100010 B 01000010 b 01100010 ETX 00000011 # 00100011 C 01000011 c 01100011 EOT 00000100 $ 00100100 D 01000100 d 01100100 ENQ 00000101 % 00100101 E 01000101 e 01100101 ACK 00000110 & 00100110 F 01000110 f 01100110 BEL 00000111 ' 00100111 G 01000111 g 01100111 BS 00001000 ( 00101000 H 01001000 h 01101000 HT 00001001 ) 00101001 I 01001001 i 01101001 LF 00001010 * 00101010 J 01001010 j 01101010 VT 00001011 + 00101011 K 01001011 k 01101011 FF 00001100 , 00101100 L 01001111 l 01101100 CR 00001101 - 00101101 M 01001101 m 01101110 SO 00001110 . 00101110 N 01001110 o 01101111
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