2.1 Types and Methods of Data Transmission

Transmission of Data

• Before data is sent it is broken up into equal size packets
• Data packets of a file can be sent across different routes (reduces congestion)
• Packets need to be reassembled when they reach their destination

Composition of a data packet:

• Packet header Contains: IP address of sender, IP address of receiver, number of packets, the sequence of packets
• Payload Contains the data of the packet
• Trailer Contains error checking method, and a method of identifying the end of the packet

Cyclic Redundancy Check

• Used to check data packets in the Packet Trailer
• Involves sending computer adding up all the 1-bits in the payload and stores it as a hex value in the trailer before it is sent.
• Once the packet arrives, the receiving computer re-calculates the number of 1-bits in the payload.
• If the two values match, then no transmission errors have occurred, otherwise the packet needs to be re-sent.

Explain how a file is transmitted using packet switching

• Data is broken into packets
• Each packet can take a different route
• A router controls which path the packet will take
• …selecting the fastest available path
• Packets may arrive out of order
• Once the last packet has arrived, all packets are reordered
• If a packet is missing, it is requested again

Advantages of Packet Switching

• There is no need to tie up a single communications line.
• It is possible to overcome failed, busy or faulty lines by re-routing packets.
• It is relatively easy to expand package usage.
• A high data transmission rate is possible

Disadvantages of Packet Switching

• Packets can be lost and need to be re-sent.
• The method does not work well with real-time streaming
• There is a delay at the destination while waiting for all of the packets to arrive and then to reassemble them in the correct order.

Data Transmission

Method:

• Serial: data is transmitted 1 bit at a time, single wire is used
• Parallel: several bits of data can be sent at the same time, several wires are used

Direction:

• Simplex: transmission in 1 direction
• Half duplex: transmission in both directions, not at the same time
• Full duplex: transmission in both directions simultaneously

Advantages & Disadvantages of Serial

• Works well for long distances
• Cheaper as it uses less wires
• Less likely for data to be skewed
• Data arrives in the correct order
• Slower data transmission compared to parallel (1 bit at a time)

Advantages & Disadvantages of Parallel

• Works well for short distances
• Faster method of transmission (several bits at a time)
• Used for integrated circuits
• Data can be skewed over long distances
• Data arrives out of order

Describe how photographs would be transmitted using serial data transmission

• Data is sent one bit at a time
• A single wire is used

USB

USB Features:

• Universal Serial Bus
• Serial transmission
• Universal standard
• Data transmission method
• Consists of 4 wires (2 for power, 2 for transmission)

USB Advantages/Disadvantages

• Device drivers are automatically detected
• …no need to manually search for them online/install them manually
• Can charge the device
• …so no additional source of power is needed
• Industry standard
• …so the device is more likely to be compatible with the computer
• Can only fit in one way
• …so there is less chance of connecting the device incorrectly
• It is backwards compatible
• …so no additional technology is needed / will work with older computers or devices
• Transfer rates are not as fast as other types of connections (eg Ethernet)
• Maximum length supported is 5m

What happens when a USB device is plugged into a computer?

• The computer automatically detects a device is being connected
• Appropriate device driver is loaded
• If it is the first time the device is being connected the computer will search for appropriate drivers so that the device will operate correctly
• Device will begin charging or power on