Data transmission types and methods

►   Data Packets

Q1. a)  Explain what is meant by  Data Packet .

• Data packet is a small segment of a large data file which is to be sent over computer network (also called 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 its destination.

b)  Describe the  structure of 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 packet.

Figure : Data Packet Structure

c)  Data packets contain a header. Name three  components of header  that you would expect to find in a packet.

The packet header consists of -

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)  Data packets also have a  payload  and a  trailer . Explain these two terms.

(1)     Payload : 

⇒  Payload refers to the actual data being transported by the packet. (usually about 64KiB).

⇒  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.

Q2.  Describe what is meant by Packet switching.

• Packet switching is a method of data transmission in which a large file is broken up into a number of data packets.
• Each packet is then sent independently in different path to its destination, moving from one router to another. So, the journey time of each packet will be different.
• Once all the data packets reaches its destination, they are reassembled in correct order.
• If the data is received without error, then a confirmation message is sent to the sender, otherwise the receiver requests to re-transmit the data.

Q3.  Describe different  stages of packet switching  while transmitting data over a network.

1. The data file to be sent is split into chunks that are packaged in specially formatted units called packets.
2. Each packet contains header (with IP-address of sender and receiver, sequence number), payload (actual data) and trailer (end mark of packet).
3. Packets sent through network, moves from one router to another taking different paths.
4. If the data has been received without any error, then the receiver sends a confirmation message to the sender.
5. If no confirmation message, sender retransmits data again.

Q4.  The diagram below shows how a message is split up into a number of packets before it is sent from computer ‘A’ to computer ‘B’ through several possible paths.

a)  Using the above diagram, explain  how packet switching works  when sending a 500KiB file from computer 'A' to computer 'B'.

1. The file of size 500KiB would be broken up into 8 chunks (of about 64KiB size) called packets.
   Packet switching allows packets of variable length (7 x 64KiB + 1 x 52KiB).
2. Each packet from computer 'A' will take different path moving from one router to another (like, 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)  Using the diagram, explain  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 its destination computer 'B', they have to be re-ordered in 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.

Q5.  Packet switching is used to send packets of data over a network.

a)  Give three  benefits  of using packet switching.

1. Packets can be sent along different routes; no need to relay 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)  Give three  disadvantages  of using 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)  Data packets can get lost due to 'bouncing' around from router to router and never reaching their destination.

(i)  Describe the problems caused by  data packet 'bouncing' .

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

(ii)  Explain how  Hopping  is used to prevent data packets being lost due to bouncing.

⇒  Hopping is a method to overcome 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

Q6. a)  The trailer in a packet will use one form of error checking.

Explain what is meant by a  Cycle Redundancy Check (CRC) .

⇒  CRC is an error checking method, where the sender adds up all the 1-bits in the payload and store 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 compare it with the received hex value in the trailer.

⇒  If it matches, then the data received is considered as error free, otherwise a requests for re-transmission is made.

b)  The payload contains the following data :

11110000   10000011   00110011   00111111   11111110   11100011

Use this data to show how the receiving computer can verify that the received payload was error-free.

⇒  Count the number of 1-bits in 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.

Q7.  Suppose a video conference is taking place between delegates in two different countries. Packet switching is being used to send video and sound data between the delegates :

a)  Describe any potential problems with sound and video quality.

⇒  Packet switching allows equal sharing of bandwidth but cannot give guarantee about 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.

Note : Packet switching is useful for transferring data that doesn’t require real-time responsiveness.

b)  Describe how these problems could be caused.

⇒  In packet switching, the data packet could move from router to router and not 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)  Describe how it is possible for a system to deal lost packets and prevent them from slowing down the transmission process.

⇒  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 request to retransmission of the packet is made.

d)  Explain why you think packet switching might improve data security.

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

►   Modes of data transmission : Simplex, Half-duplex & Full-duplex transmission.

Mode of data transmission refers to the direction of the flow of data, whether in one direction only or in both direction.

Figure : Transmission modes

Q1.  Give difference between Simplex, Half-duplex and Duplex ways of transmission.

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.
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.
Duplex :	Full-duplex data transmission allows data to flow in both direction simultaneously at the same time. Example : Telephony (VoIP Call) and Video chatting.

►   Types of data transmission : Serial & Parallel Transmission

Types of data transmission refers to the method of in which bits are transmitted over the communicational channel, whether in one bit or multiple bits are sent at a time.

Figure : Types of data transmission

Q1.  Describe  Serial data transmission .

⇒  Transfer of data bits, one bit at a time, sequentially over a single communication line or wire.

⇒  Data arrives at its destination fully synchronised in the correct order.

⇒  Serial data transmission works well over long distances.

⇒  Data is transmitted at a slower rate than parallel because it uses only one wire or channel.

Q2.  Describe  Parallel data transmission .

⇒  Transfer of data bits, several bits at a time (usually 8-bits), simultaneously over separate communication line or wire.

⇒  Over longer distances, data can become skewed or unsynchronised.

⇒  Parallel data transmission works well over short distances.

⇒  Internal circuits of computer uses parallel transmission because it is faster than serial as it uses multiple wires or channels.

Q3.  Give difference between  Serial  and  Parallel transmission .

1. Serial communication transfers one bit at a time.
   Parallel communication transfers multiple bits at a time (usually 8-bits).
2. Serial transmits bits of data sequentially.
   Parallel transmits bits of data simultaneously.
3. Serial transmission is slower than Parallel.
4. Serial communication port requires single data wire.
   Parallel communication port requires multiple wires.

Q4.  Give two advantages and disadvantages of  Serial transmission .

Advantages	Disadvantages
Reduces the cost as it uses single wire.	Serial transmission is slower than parallel.
Perfect for transmitting over longer distances as there are no synchronisation issues.	Use of conversion devices at source and distination may increase the cost. (To convert parallel to serial vise versa)
It has less chance of data being distorted or skewed.	It occupies 20% of overhead additional data than useful information leading to wastage of bandwidth.

Q5.  Give two advantages and disadvantages of  Parallel transmission .

Advantages	Disadvantages
Parallel transmission is faster than serial.	It is a costly as it require multiple wires.
It is ideal for transferring data over short distances.	Data could be out of synchronisation, it could be skewed or distorted when transmitted over a long distances. Cross-talk between parallel lines of wires is possible.

Q6.  Give three application of Serial transmission.

• Data transmission between computer and peripheral device using USB (Universal Serial Bus) Cable.
• Connecting Hard disk to the mother board using SATA (Serial Advanced Technology Attachment) cable.
• Connecting computer to the modem using Serial COM port.
• Wireless WiFi or Bluetooth connection uses serial transmission.

Q7.  Give three application of Parallel transmission.

• Data transmission within CPU between registers using data, control or address bus.
• Data transmission between CPU (Microprocessor) and RAM chip.
• Connecting Hard disk to the mother board using PATA (Parallel Advanced Technology Attachment) cable.

Q8.  Give three reasons why serial transmission, rather than parallel transmission is used to connect devices to a computer.

1. Single wire reduces cost.
2. A single wire means that there is less chance of Interference or Cross talk.
3. In single wire data bits will not get skewed or out of synchronization.

►   Universal Serial Bus (USB)

Q1. a)  Describe  USB (Universal Serial Bus) .

⇒  USB is an external bus with plug-and-play interface.

⇒  It is used to connect external peripheral devices with computer.

⇒  It uses asynchronous serial data transmission. Allows both hulf-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.

b)  Give three reasons of using USB ports over traditional Serial ports.

1. Data transfer rate of USB port is much higher (between 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.

Q2.  Give three benefits and drawbacks of using the USB System.

 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 Gpbs).
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 equipments.

 Drawbacks : 

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

Q3.  Explain  what happens when a device is plugged into a computer using one of the USB ports.

⇒  The USB port provides power supply to the device, to activate and communicate with computer.

⇒  The OS (operating system) sends a request to the USB port to identify the device.

⇒  The unique vendor-id, serial number and type of the device stored in device descriptor is send to the OS.

⇒  The OS compares the information of device descriptor with the descriptor of the vendor specific device driver installed with OS.

⇒  If it matches, then it will be loaded the device driver automatically.

⇒  If it doesn’t match, then OS will ask the user to install the driver software for that device.

Q4.  State different types of USB ports found in desktop and laptops.

The three default or standard format of USB port or connectors found in desktop or laptops are -

USB - A  :	The most common rectangular connector that only fits in only one way. Most of the Portable Storage devices uses USB-A connector. It is primarily be 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.
USB - B  :	This is an older connector, almost square at one end, designed for USB peripherals, such as printer, upstream port on 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.
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 & Synchronous transmission

Asynchronous & Synchronous transmission refers to the method of transmitting and collecting data back, and make sure that the received data is in the correct order.

Q1.  Give difference between Asynchronous and Synchronous data transmission with example.

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.

Example :	Data transmitted from satellite to the earth station.
	Discussion forums and email where sender and receiver need not to 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 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.

Example :	VoIP (Telephony) and Video calls.
	Network communication.
	Playing online Games.

Q2.  Describe how it is possible to ensure data arrives correctly identified at its destination when using Asynchronous data transmission.

⇒  The receiver detects the start and end bit of the data to know when the data starts and when it ends.

⇒  Without these control bits, it would be impossible to separate groups of data as they arrives.

⇒  This prevents data becoming mixed up.

Q3. a)  Describe how it is possible to ensure data arrives correctly identified at its destination when using Synchronous data 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)  Give one advantage and one disadvantage of using Synchronous data transmission.

Advantage :	Much faster data transmission rate than asynchronous.
Disadvantage :	If the timing is not accurate, data will be out of synch.

Q4.  Give difference between Asynchronous transmission and Synchronous transmission.