The character-oriented protocol is depends the binary code of a character set. The code generally used is ASCII (American Standard Code for Information Interchange). It is a 7-bit code with an eighth bit used for parity. The code has 128 characters, of which 95 are graphic characters and 33 are control characters. The graphic characters involves the upper- and lowercase letters, the ten numerals, and multiple unique symbols.
The control characters are used to route information, organizing the test in the desired structure, and for the design of the printed page. The characters that control the transmission are known as communication control characters. These characters are displayed in the table. Each character has a 7-bit code and is defined by a three-letter symbol. The importance of each character in the control of data transmission is stated briefly in the function column of the table.
ASCII Communication Control Character
|0010110||SYN||Synchronous idle||Establishes synchronism|
|0000001||SOH||Start of heading||Heading of block message|
|0000010||STX||Start of text||Precedes block of text|
|0000011||ETX||End of text||Terminates block of text|
|0000100||EOT||End of transmission||Concludes transmission|
|0010101||NAK||Negative acknowledge||Negative acknowledgment|
|0000101||ENQ||Inquiry||Inquire if the terminal is on|
|0010111||ETB||End of transmission block||End of a block of data|
|0010000||DLE||Data link escape||Special control character|
The SYN character serves as a synchronizing agent between the sender and receiver. When the 7-bit ASCII code is used with an odd-parity bit in the most significant position, the assigned SYN character has the 8-bit code 00010110 which has the property that, upon circular shifting, it repeats itself only after a full 8-bit cycle.
When the sender starts sending 8-bit characters, it sends a few characters first and then transmits the actual message. The initial continuous string of bits accepted by the receiver is checked for an SYN character. In other words, with each clock pulse, the receiver tests the last eight bits received.
If they do not connect the bits of the SYN character, the receiver accepts the next bit, rejects the earlier high-order bit, and again checks the last eight bits received for an SYN character. This is repeated after each clock pulse and bit received until an SYN character is identified.
Once an SYN character is encountered, the receiver has framed a character. Therefore, the receiver counts each eight bits and accepts them as an individual character. Generally, the receiver checks two consecutive SYN characters to eliminate any difficulty that the first did not appear as a result of a noise signal on the line.