Introduction Communication Modes Transmission Modes Asynchronous Transmission

1. Introduction • Communication Modes • Transmission Modes • Asynchronous Transmission • Synchronous Transmission - Character-oriented and Bit-oriented • LSI devices

2. Communication Modes • 3 common communication modes are simplex, half-duplex and duplex. • Simplex: Transmit in one direction. For e.g. a radio station broadcasting and data logging system • Half-duplex: Two interconnected devices exchanges information (data) alternately For e.g. a “walkie-talkie” • Duplex: To interconnected devices transmit and receive simultaneously. A telephony system

3. Transmission modes • 2 Types: • Asynchronous • Synchronous • Data can be transmitted between two terminal equipment (DTE) in multiples of a fixed-length unit, • computer must be able to decode the messages of the fixed-length unit correctly. To do this it must know: • The bit rate being used (time duration of each bit), clock synchronization, • The start and end of each element, byte sync, • The start and stop of each complete message block or frame, frame sync.

4. Asynchronous transmission • data are generated at random intervals, e.g. typing on a keyboard of a computer. • Signal on the transmission line will be in the idle (off) state for long time intervals. • It is necessary for the receiver to resynchronize at the beginning of each character. • This is done by adding additional bits to the beginning and end ( start and stop bits respectively) to encapsulate the original data bits.

5. The polarity of start and stop bits are opposite to ensure at least one transition between each successive character. • Figure indicates that to transmit an 8 bit data, a total of 10 bits (for I start and I stop bit) or 11 bits (for I start and 2 stop bits) are utilized

6. Synchronous transmission • There is no need for a start bit and stop bit(s) for each character in synchronous transmission. • It can be a more efficient method of transmitting serial data • The receiver and the transmitter are synchronized • Large block of data characters one after the other are sent with no time intervals between characters. • The receiver automatically knows that every 8 bits received after synchronization represents a data character. • When data is not being sent through a synchronous data link the line is held in a marking condition.

7. The transmitter sends out one or more unique characters called SYNC characters or a unique bit pattern called a flag to mark the start of transmission • The receiver uses the SYNC/ flag characters to synchronize its internal clock with that of the transmitter. • The transmitter then shifts in the and converts them to parallel form for the computer. • Frequently used in high speed modems and digital communication channels. • There will be some control characters after the SYNC characters and before the END character. • Two ways of organizing a synchronous data link : Character (or byte) oriented and bit oriented.

8. Character oriented scheme • Made up of a variable number of 7- or 8-bit character • Transmitted as a contiguous string of binary bits with no delay between them. • The receiving device therefore having achieved clock synchronism must be able to: • 1. detect the start and end of each character (character synchronism) and • 2. detect the start and end of each complete frame (frame synchronism). • The main aim of which is to make synchronization process independent of the actual contents of a frame. • This type of synchronization scheme is said to be transparent / data transparent • Used in the binary synchronous control protocol (BSC) known as Basic Mode ( will be covered later).

9. Character synchronization - transmitter sending two/more SYNC characters at start each transmitted frame. • The receiver by scanning the received bit stream until it detects the know pattern of the SYNC character to achieve character synchronization. • Additional precaution must be taken to ensure that the end-of-frame termination character (ETX) is not present within the frame content • To achieve this a pair of characters is used both to signal the start and end of a frame as shown Figure • character (or byte) stuffing - To avoid abnormal termination due to the frame contents containing the end-of-frame character sequence, the transmitter inserts a second data link escape (DLE) character into the transmitted data streams whenever it detects a DLE character in the contents of the frame. • End of frame is a unique DLE-ETX sequence.

10. Bit oriented scheme • Each transmitted frame may contain an arbitrary number of bits • The opening and closing flags fields indicating the start and end of the frame are the same (0111111 0). • To achieve data transparency it is necessary to ensure that the flag sequence is not present in the frame contents. • Bit stuffing - The transmitter detects whenever there is a sequence of 5 contiguous binary 1 digits and automatically inserts an additional binary 0. • Flag sequence 0 1 1 1 1 1 1 0 can never be transmitted between opening and closing flags. • Commonly used in high data link control (HDLC).

13. LSI Device for Serial Communications • Universal Synchronous Asynchronous Receiver and Transmitter or USART is used to support asynchronous transmission. • It is a programmable device and a user can, by simply loading a predefined control word (bit pattern) into the device, specifying the required operating characteristics. • User initializes by loading with the required bit pattern to define the required operating characteristics. • Operating characteristics are the data units, 5,6,7, or 8 bits/characters, odd or even parity or non, one or more stop bits and the bit rates for transmission and reception.

14. Sending and Receiving characters asynchronously using the 8251A • Data characters can be sent to and read from the 8250 on an interrupt basis or on a polled basis. • Transmit a character on a polled basis, the 8250 status register is read and checked over and over again until the Tranmitter buffer empty bit (D0) is found to be 1. • A data character is then written to the 8250 data address. • Reading a character a polled basis is a similar process, except that the data ready bit (D0) of the status register is polled to determine when a character is ready to be read. Status register bit D1, D2, and D3 can be checked to see if a parity error, overrun or a framing error has occurred.