Data Transmission

Serial Data Transmission

Well suited for long distance communication, but the speed of the transmission is lower as there is only one channel used.
There is no need for data synchronisation as bits are sent in one channel.
USB is a common example

Parallel transmission is faster than serial transmission as multiple channels are used.
Used internally within a computer
An internal timer synchronises the data on different channels
8-64 bit data buses are used

In async data transmissions, the data is sent in groups in an agreed bit pattern. Bits are combined with a start and a stop bit.

A USB device has four pins
Two pins are responsible for connecting wires, which carry the power and ground connection
The two other pins connect the wires that transfer data
Hence the cable used to connect two USB ports is a four-wire shielded cable.
USB devices are detected by a computer due to a small change in voltage from the device being connected.
Computer determines the specifications of the USB device
Computer loads the appropriate driver to communicate with the device

When data is transmitted through a channel, it could get corrupted or changed by electric fields or dirty equipment
Hence, it is very important to implement error checking method sin computer technology.
These methods are used to detect and correct errors:
- Automatic Repeat Request (ARQ)
- Parity Checking
- Majority Voting
- Checksum
- Echo check
- Check digits

In Automatic Repeat Request, ACKNOWLEDGEMENT and TIMEOUT ensure that error-free messages are received at the destination.
ACK is a message sent by the receiver to confirm that they have received the data correctly.
TIMEOUT is the time allowed to elapse before an ACK is received
If the ACK is not received within the TIMEOUT, the message is automatically resent.

Parity checking uses a parity bit in each byte of data. This bit is allocated before transmission.
An even parity has an even number of bits 1s in a byte. An odd parity has an odd number of bit 1s in a byte.
Systems may use even or odd parity.
The sender and receiver make an agreement prior to the transmission about the type of parity.
To find the exact location of the bit changed, a block of data is sent. The parity is checked both horizontally and vertically.

A value called a checksum is attached to the end of the block of data. This value can indicate transmission errors.
Checksum = the sum of all the digits, if the sum of all digits is less than or equal to 255
Else, the following algorithm is used:
- Divide the sum X by 256
- Round down to the nearest whole number, Y
- Multiply Y by 256 and denote this number as Z
- Checksum = X-Z
The sender transmits the block of data with checksum
When received at the receiver end, the checksum of the block of data is calculated.
If the calculated checksum is equal to the checksum transmitted by sender, then there are no transmission errors
If both the checksums are not equal, the sender is requested to resend the data.

In this the data received is sent back to the sender
The sender compares the two sets of data for transmission errors
This method is not very reliable because it cannot be decided in which transmission the data was corrupted.
An error-free echo check would prove that no data has changed or been corrupted.

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