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.
Why might packets need to be re-ordered ?
Packets may have arrived in the wrong order
Each packet travelled using a different route
Some packets take longer to transmit than others
Some packets may not arrive (or may be corrupted) and may be requested to be resent.
Data Transmission
Method:
Serial: data is transmitted1 bit at a time,singlewire is used
Parallel:several bitsof 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 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
Disadvantages of Serial
Slower data transmission compared to parallel (1 bit at a time)
Advantages of Parallel
Works well for short distances
Faster method of transmission (several bits at a time)
Used for integrated circuits
Disadvantages of Parallel
Data can be skewed over long distances
Data arrives out of order
**Computer connected to router in different room --- Serial or
Parallel?**
Serial:Data arrives in order sent // does not need reordering. Less likely to experience interference. Can transmit over a longer distance. Data transmission speed is adequate.
Parallel:Faster transmission speed than serial. Very long connection not needed. Next room is (likely) within distance for parallel.
**Describe how photographs would be transmitted using serial data
transmission**
Data is sentone bit at a time
Asinglewire 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