- Technologies that allow users to transmit data from one point to another
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 data transmission
- 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
Asynchronous data transmission
- 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.
Synchronous data transmission
- A synchronous data transmission sends data as a stream of bits.
- The data also contains timing signals that are generated by an internal clock.
Universal Serial Bus
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A USB device has four pins
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Two pins are responsible for connecting wires, which carry the power and ground connection
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The two other pins connect the wires that transfer data
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Hence the cable used to connect two USB ports is a four-wire shielded cable.
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USB devices are detected by a computer due to a small change in voltage from the device being connected.
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Computer determines the specifications of the USB device
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Computer loads the appropriate driver to communicate with the device
Error-checking methods
- 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
Automatic Repeat Request (ARQ)
- 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
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Parity checking uses a parity bit in each byte of data. This bit is allocated before transmission.
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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.
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Systems may use even or odd parity.
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The sender and receiver make an agreement prior to the transmission about the type of parity.
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To find the exact location of the bit changed, a block of data is sent. The parity is checked both horizontally and vertically.
Checksum
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A value called a checksum is attached to the end of the block of data. This value can indicate transmission errors.
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Checksum = the sum of all the digits, if the sum of all digits is less than or equal to 255
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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
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The sender transmits the block of data with checksum
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When received at the receiver end, the checksum of the block of data is calculated.
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If the calculated checksum is equal to the checksum transmitted by sender, then there are no transmission errors
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If both the checksums are not equal, the sender is requested to resend the data.
Echo check
- 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.
Check digits
- Check digits is another method that is used to detect errors.
- In this method, an additional digit is added to the end of a string of numbers
- This bit is calculated using a standard algorithm and is called a check digit