2.1 Types and Methods of Data Transmission

(a) Data Packet:

• A data packet is a small segment of a large data file which is to be sent over a computer network (also called a datagram).
• Each packet can be sent along different routes to its destination, if a particular route is very busy or fails.
• All data packets need to be reassembled in the correct order once they reach their destination.

(b) Structure of a data packet:

• Each packet is made up of three parts: header, payload and trailer.
• Header: contains IP address of origin, destination and other information like sequence number and total number of packets.
• Payload: carries the actual data which is to be transported.
• Trailer: contains information that marks the end of the packet.

Figure: Data Packet Structure — Header | Payload | Trailer.

• Header: IP address of the source device (sender), IP address of the destination device (receiver), sequence number of the packet (allowing correct reassembly), size of the packet (in bytes).
• Payload: The actual data in the packets (~64 KiB).
• Trailer: Method of identifying the end of the packet; some form of error checking to ensure packet arrives error-free.

(c) Three components of the header:

1. The IP Address of the sending device.
2. The IP Address of the receiving device.
3. The sequence number and total number of the packets — this is to ensure that all the packets can be reassembled into the correct order once they reach the destination.
4. The size of the packet (in bytes).

(d) Payload and Trailer:

(1) Payload:

• Payload refers to the actual data being transported by the packet (usually about 64 KiB).
• The payload is the only data collected for processing, after removing the header and trailer from the packet when it reaches the destination.

(2) Trailer:

• Trailer contains few bits of information that marks the end of packet. It allows each packet to be separated from each other.
• It also contains an error checking protocol called Cyclic Redundancy Check (CRC) which enables the computer to check if all the packets were received without error.

(a) How packet switching works for a 500 KiB file:

1. The file of size 500 KiB would be broken up into 8 chunks (of about 64 KiB size) called packets. Packet switching allows packets of variable length (7 × 64 KiB + 1 × 52 KiB).
2. Each packet from computer 'A' will take a different path, moving from one router to another (e.g. Router-A, R2, R5, R8, R7, R4, Router-B) to reach its destination computer 'B'.
3. Each router will determine the shortest possible route for each packet, depending on the number of packets waiting to be processed at each node.
4. Packets can reach the destination in a different order to that in which they were sent.
5. At the destination, the packets will need to be reassembled into their correct order using the information sent in the header.

(b) Why data packets need to be reassembled at computer 'B':

1. Packets would reach the destination computer 'B' in a different order than the order in which they are sent from computer 'A'.
2. So, once all the packets arrive at their destination computer 'B', they have to be re-ordered in the correct sequence.
3. All packets have to be assembled into one unit equivalent to the original data file by removing the header and trailer from each packet.

(a) Benefits of packet switching:

1. Packets can be sent along different routes; no need to rely on a single communication line.
2. It is possible to overcome busy or faulty lines by simply re-routing packets.
3. It allows simultaneous usage of the same channel by multiple users.
4. A high data transmission rate is possible.

(b) Disadvantages of packet switching:

1. Packets can be lost and need to be re-sent.
2. There is a delay at the destination while the packets are being re-ordered.
3. The method is more prone to errors with real-time streaming (like high quality voice calls).
4. They require complex protocols for delivery.

(c) Bouncing and Hopping:

(i) Problems caused by data packet 'bouncing':

• Bouncing refers to the transfer of a data packet from router to router and not being able to locate its final destination.
• Data packets that bounce around too many times may get lost.

(ii) How Hopping prevents data packets being lost due to bouncing:

• Hopping is a method to overcome the bouncing problem, where a hop number is added to the header of each packet, and this number is reduced by 1 every time it leaves a router.
• Each packet has a maximum hop number to start with. Once a hop number reaches zero and the packet hasn't reached its destination, then the packet is deleted when it reaches the next router.
• The missing packets will then be flagged by the receiving computer and a request to re-send these packets will be made.

Figure: Hop numbers between routers — Router 3 → Router 5 → Router 8 → Router 11 → Router 15 → Router 21 (with hop numbers decreasing 10, 9, 8, 7, 6, 5).

(a) Cyclic Redundancy Check (CRC):

• CRC is an error-checking method, where the sender adds up all the 1-bits in the payload and stores it as a hex value in the trailer before transmission.
• Once the packet arrives, the receiver recalculates the number of 1-bits in the payload and compares it with the received hex value in the trailer.
• If it matches, then the data received is considered error-free; otherwise, a request for re-transmission is made.

(b) Verifying the payload is error-free:

The payload contains:

• Count the number of 1-bits in the payload. The sum of all the 1s is 29.
• Convert denary 29 into hexadecimal, which is 1D.
• If the trailer of the received packet contains the hex value 1D, then the data received is considered error-free.

(a) Potential problems with sound and video quality:

• Packet switching allows equal sharing of bandwidth but cannot guarantee the quality or latency (delay in communication) of the data for real-time streaming.
• Packet loss occurs when one or more transmitted data packets fail to arrive at their destination.
• If packet loss rate is high or there is high latency, it can cause delays to be worse for real-time streaming applications like voice and video.
• The user may experience disruptions, such as slow service or loss of network connectivity that affects the sound and video quality.

(b) How these problems could be caused:

• In packet switching, the data packet could move from router to router and not be able to locate its final destination.
• Data packets that bounce around too many times may get lost.
• Packet loss is typically caused either by errors in data transmission or network congestion.

(c) Dealing with lost packets:

• A hop number is added to the header of each packet, and this number is reduced by 1 every time it leaves a router.
• Once a hop number reaches zero and the packet hasn't reached its destination, then the packet is deleted.
• And a request for retransmission of the packet is made.

(d) How packet switching might improve data security:

1. Packet switching uses Cyclic Redundancy Check (CRC) to detect errors in transmitted data packets and requests to resend the faulty packets.
2. It allows resending of lost data packets.
3. Packet switching is more reliable because it ensures that packets reach their destination.

Simplex:

Simplex data transmission allows data to flow in one direction only, that is, from sender to receiver only.

Example: Sending data from a computer to a printer.

Diagram: Mainframe → Direction of data → Monitor.

Half-duplex:

Half-duplex data transmission allows data to flow in both directions but not at the same time.

Example: Walkie-talkie conversation where only one person speaks at a time.

Diagram: Two workstations; at time 1 data goes right, at time 2 data goes left.

Duplex (Full-duplex):

Full-duplex data transmission allows data to flow in both directions simultaneously at the same time.

Example: Telephony (VoIP Call) and Video chatting.

Diagram: Two workstations with bidirectional arrows labeled "Direction of data all the time".

(a) Describe USB:

• USB is an external bus with a plug-and-play interface.
• It is used to connect external peripheral devices with a computer.
• It uses asynchronous serial data transmission. Allows both half-duplex and full-duplex.
• It provides power supply to the peripheral devices.

The USB cable consists of a four-wired shielded cable, with two wires for power (red and black). The other two wires (white and green) are for data transmission.

Diagram (USB cable internal wires): Red (+5V) · White (Data+) · Green (Data−) · Black (Ground) · Shield (no connection at USB device).

(b) Three reasons of using USB ports over traditional Serial ports:

1. Data transfer rate of USB port is much higher (12 Mbps to 480 Mbps) than traditional Serial port (1 Mbps to 10 Mbps).
2. USB port is capable of supplying power to the peripheral devices.
3. USB port automatically detects the device and installs the correct driver.
4. USB ports are much more durable than serial ports because pins in serial ports are very easy to bend or break, whereas USB pins are more robust and much more difficult to damage.

Benefits:

1. Automatically detects and installs the appropriate device drivers.
2. USB cables can only fit in one way that prevents incorrect connections.
3. USB supports different data transmission rates (from 1.5 Mbps to 5 Gbps).
4. No need for external power supply since cable supplies +5V power.
5. USB protocol re-transmits data if any errors are detected; this leads to error-free data transmission.
6. USB connections can transfer data using half-duplex or full-duplex.
7. It is relatively easy to add more USB ports if necessary, by using USB hubs.
8. USB is backward compatible (that is, older versions are still supported).
9. USB has become an industry standard, which has replaced most serial ports in consumer-grade computing equipment.

Drawbacks:

1. Standard USB only supports a maximum cable length of 5 m, beyond that, USB hubs are needed to extend the cable length.
2. USB ports can transmit data only over short distances, beyond which you will need to use USB hubs.
3. Data transmission rate of even the latest version USB V3 and V4 is much lower than FireWire and Ethernet Interfaces.

USB-A:

• The most common rectangular connector that only fits in one way.
• Most portable storage devices use the USB-A connector.
• It is primarily used on host computers and hubs. You won't find cables with USB-A on either end. In fact, connecting two computers with a USB-A cable could damage them both.

Diagram (USB-A connector pins): Power · Data− · Data D+ · Power +5V.

USB-B:

• This is an older connector, almost square at one end, designed for USB peripherals such as a printer, upstream port on a hub, or other larger peripheral devices.
• It is designed to allow the connection of peripheral devices without running the risk of connecting two host computers to one another.

Diagram (USB-B connector pins): Power +5V · Data D− · Data D+ · Power (Ground).

USB-C:

• It is the latest USB standard, which is a 24-pin connector that can fit into a USB-C port in either way round.
• It is much smaller and thinner than older USB connectors.
• It offers 100 watt (20 volt) power connectivity, which means full-sized devices can now be charged.
• It can carry data at 10 gigabits per second and can support DisplayPort and HDMI mode to transfer video and audio signal.
• USB-C is backward compatible (to USB 2.0 and 3.0) and is expected to become the new industry standard (universal) format.

Asynchronous transmission:

• Asynchronous data transmission refers to the data being transmitted in an agreed bit pattern, grouped together with the start and end control bits.
• The receiver detects the start and end bit of the data to know when the data starts and when it ends.

Diagram (Asynchronous): Sender → [Stop Bit | Data (e.g. 1011001) | Start Bit] → Receiver, with control bits around each data group.

Example:

• Data transmitted from satellite to the earth station.
• Discussion forums and email where sender and receiver need not be present at both ends at the same time.
• MIDI music file uses asynchronous protocol to play musical instruments.

Synchronous transmission:

• Synchronous data transmission is a continuous streaming of data included with a timing signal generated by an internal clock to ensure that the sender and receiver are synchronized with each other.
• Synchronous data transmission is faster than asynchronous and is used where speed is an important issue.

Diagram (Synchronous): Sender → [Frame] [Frame] [Frame] … → Receiver, with clock time signals between each frame.

Example:

• VoIP (Telephony) and Video calls.
• Network communication.
• Playing online games.

(a) Ensure data arrives correctly using Synchronous transmission:

• Transmits data in a continuous stream included with timing signals generated by an internal clock.
• The receiver counts how many bits were sent and then reassembles them into bytes of data with accurate timing.
• This ensures that the sender and receiver are synchronized with each other.

(b) Advantage and disadvantage of Synchronous transmission:

Advantage: Much faster data transmission rate than asynchronous.

Disadvantage: If the timing is not accurate, data will be out of synchronisation.