How is data transmitted?
It is broken down into very small units called packets.
Can you describe the structure of a data packet?
Packet Header
Paylod
Trailer
What does the payload contain?
Contains the actual data from the file that you are sending.
What does the trailer contain?
- Marker to indicate end of packet
- Data for any error detection systems
Can you describe the process of packet switching?
- Data is broken down into packets
- Each packet is individually sent from one device to another
- Each data packet could take a different route.
- A router controls the route a packet takes.
- Packets may arrive out of order
- After all packets arrive, packets are reorderd.
What is serial data transmission?
Data is sent one bit at a time down a single wire.
What is parallel data transmission?
Data is sent multiple bits at a time down multiple wires.
What is simplex tranmission?
data is transmitted in a single direction only
What is half duplex transmission?
data is transmitted in both directions, but only one direction at a time.
What is full-duplex transmission?
data is transmitted in both directions at the same time.
What are the advantages of serial data tranmission?
- Less chance of data being skewed as it is sent one bit at a time.
- As data is sent along 1 wire, there are less chances of interference, therefore less chances of error in data
- only 1 wire is need, therefore it is cheaper to buy and cheaper to manufacteur.
What is a USB and how does it work to transmit data?
Universal Serial Bus is an industry standard that is used to transmit data.
A USB connection uses serial data transmission to transmit data between the devices. However it is a special type of serial data transmission connection that is designed to transmit data at high speeds.
What are the advantages of the USB interface?
- simple interface. The USB cable to device can only fit into the USB one way. Therefore means less errors in connecting devices are likely to be made.
- The speed of a USB connection is relatively high, so data transfer is quick
- USB interface is a universal standard so a USB port is included in many different devices.
What are the disadvantages of the USB interface?
- The length of the USB cable is limited, normally to 5 metres.
- Tranmission speed is relatively high but slower than other types of connection such as ethernet.
What are the methods of error detection for detecting errors in data after transmission?
- Parity check
- checksum
- echocheck
What is a parity check?
- type of error detection method that adds an additional bit to each byte to create an odd or even sum.
What is a checksum?
Type of error detection method. A checksum uses a calculated value to check for errors. A value is calculated from the data that will be transmitted, before transmission takes place. Checksum values are compared after transmission to see if they match.
What is echo check?
A type of error detection method that sends a copy of the transmitted data back to the sender to be compared with the original data sent.
What is a check digit?
A type of error detection method that is used for data entry. A calculation is performed on the data entered to create a value. Check digits values are compared to see if data entered is correct.
What are some examples of when a check digit is used?
- ISBN (international standard book numbers)
- bar codes
What is automatic repeat query (ARQ)?
A type of error detection method that uses acknowledgement and timeout to see if data has arrived correctly after transmission.
What is acknowledgement?
A message that is sent from one device to another to indicate whether data is recieved correctly.
What is a timeout?
A period of time that is set and used to wait for an acknowledegment to be recieved.
What is encription?
a method of securing data for storage or transmission that scrambles it and makes it meaningless.
