AS Computing

Data transmission

Basic data transmissionBaud

The rate that the voltage changes is called the Baud. If the voltage changes 10 times every second the baud is said to be 10. This does not necessarily mean that a 10-baud and a 600-baud modem transmit 10 and 600 bits per second.

Bit rate

Bits per second is a measure of the number of data bits (digital 0's and 1's) transmitted each second in a communications channel.

Communications channels

Simplex Data transmission Simplex communication is a mode in which data only flows in one direction.

One current usage of simplex communications in business involves certain point-of-sale terminals in which sales data is entered without a corresponding reply.

screen

keyboard

cpu

outputport

inputport

echo

Serial Link

Half Duplex Transmission Half-duplex communication adds an ability for a two-way flow of data between computer terminals.

In this directional mode, data travels in two directions, but not simultaneously.

Data can only move in one direction when data is not being received from the other direction.

This mode is commonly used for linking computers together over telephone lines.

Half Duplex Transmission

screen

keyboard

cpu

outputport

inputport

Serial Link

Duplex data transmission The fastest directional mode of communication is full-duplex communication.

Data is transmitted in both directions simultaneously on the same channel.

Full-duplex communication is made possible by devices called multiplexers.

Full-duplex communication is primarily limited to mainframe computers because of the expensive hardware required to support this directional mode.

Serial data transmission

Serial data transmission is the simplest,  most  economical, and easiest method of transferring digital information from one point to another;

The digital data is sent one bit at a time, this means only one pair of transmission wires is required.

Serial Data Transmission

Two types of transmission:

•asynchronous: transmitter and receiver clocks are independent

•synchronous: transmitter and receiver are synchronized

Parallel data transmission A data transmission method in which all the bits in a character are transmitted simultaneously on separate channels

If there is eight bits sent at a time, this will require 8 wires, one for each data bit.

Parallel transmission is obviously faster than serial, because more than one bit is sent at a time.

Parallel transmission is good only for short distances.

Asynchronous transmissionAsynchronous systems send data bytes between the sender and receiver by packaging the data in a frameOne start bit – one or more stop bits The transmitter and receiver must be preset in advance to an agreed-upon baud rateCheap but inefficient – large overhead (20% or more)Relatively low data rates Uses:

•suitable for data transmitted at random intervals (e.g. keyboard to computer)•used in the great majority of dial-up connections

For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message information.

When the channel is idle, the signal voltage corresponds to a continuous logic '1'. A data packet always begins with a logic '0' (the start bit) to signal the receiver that a transmission is starting.

Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity bit and stop bit.

Asynchronous transmission

Synchronous transmission

Arrival time of each bit is predictableTo prevent timing drift the receiver and transmitter clock are synchronized Clock signal transmitted either:

•over a separate line •or encoded into the data (Manchester, differential Manchester encoding) to allow a single line for both data and clock

Exercise