Basic Principles of Digital Communication-2

Data Communication System:

Every data communications system requires:
• A source of data (a transmitter or line driver), which converts the information into a form suitable for transmission over a link
• A receiver that accepts the signal and converts it back into the original data
• A communications link that transports the signals. This can be copper wire, optical fiber, and radio or satellite link.

In addition, the transmitter and receiver must be able to understand each other. This requires agreement on a number of factors. The most important are:
• The type of signaling used
• Defining a logical ‘1’ and a logical ‘0’
• The codes that represent the symbols
• Maintaining synchronization between transmitter and receiver
• How the flow of data is controlled, so that the receiver is not swamped
• How to detect and correct transmission errors

The physical factors are referred to as the ‘interface standard’; the other factors comprise the ‘protocols’.

The physical method of transferring data across a communication link varies according to the medium used. The binary values 0 and 1, for example, can be signaled by the presence or absence of a voltage on a copper wire, by a pair of audio tones generated and decoded by a modem in the case of the telephone system, or by the use of modulated light in the case of optical fiber.


Modes of Communication:

In any communications link connecting two devices, data can be sent in one of three
communication modes. These are:
• Simplex
• Half duplex
• Full duplex

A simplex system is one that is designed for sending messages in one direction only.
 
A duplex system is designed for sending messages in both directions.

Half duplex occurs when data can flow in both directions, but in only one direction at a time.

In a full-duplex system, the data can flow in both directions simultaneously.

Asynchronous System:
An asynchronous system is one in which each character or byte is sent within a frame.
The receiver does not start detection until it receives the first bit, known as the ‘start bit’.

The start bit is in the opposite voltage state to the idle voltage and allows the receiver to synchronize to the transmitter for the following data in the frame.
The receiver reads in the individual bits of the frame as they arrive, seeing either the logic 0 voltage or the logic 1 voltage at the appropriate time. The ‘clock’ rate at each end must be the same so that the receiver looks for each bit at the time the transmitter sends it.

However, as the clocks are synchronized at the start of each frame, some variation can be tolerated at lower transmission speeds. The allowable variation decreases as data transmission rates increase, and asynchronous communication can have problems at high speeds (above 100 kbps).
  
Asynchronous frame format
An asynchronous frame may have the following format:
Start bit: Signals the start of the frame
Data: Usually 7 or 8 bits of data, but can be 5 or 6 bits
Parity bit: Optional error detection bit
Stop bit(s): Usually 1, 1.5 or 2 bits. A value of 1.5 means that the level is held for 1.5 times as long as for a single bit.

The transmitter and receiver must be set to exactly the same configuration so that the data can be correctly extracted from the frame. As each character has its own frame, the actual data transmission speed is less than the bit rate. For example, with a start bit, seven data bits, one parity bit and one stop bit, there are ten bits needed to send seven bits of data. Thus the transmission of useful data is 70% of the overall bit rate.

Synchronous Systems:
In synchronous systems, the receiver initially synchronizes to the transmitter’s clock pulses, which are incorporated in the transmitted data stream. This enables the receiver to maintain its synchronization throughout large messages, which could typically be up to 4500 bytes (36 000 bits). This allows large frames to be transmitted efficiently at high data rates. The synchronous system packs many characters together and sends them as a continuous stream, called a packet or a frame.

Synchronous Frame Format:
Preamble: This comprises one or more bytes that allow the receiving unit to synchronize with the frame.
SFD: The start of frame delimiter signals the beginning of the frame.
Destination: The address to which the frame is sent.
Source: The address from which the frame originated.
Length: The number of bytes in the data field.
Data: The actual message.
FCS: The frame check sequence is for error detection.
Each of these is called a field.