CHAPTER 2: METHODS

Which of the following is found in the payload of a data packet?

• Destination address

• The actual data being sent

• Error detection code

• Sequence number

Answer explanation

Payload contains the main body of the packet. It contains the actual data or message being transmitted e.g., part of a file, an email text, a video frame.

Destination address belongs to the header, which holds addressing and control information.

Error detection code usually found in the trailer e.g., checksum to check for corruption.

Sequence number also part of the header, used to reassemble packets in the correct order..

Which of the following correctly describes the three main parts of a packet?

• Bit, byte, frame

• Source, destination, data

• Address, error code, checksum

• Header, payload, trailer

A packet trailer contains error detection information. What is its purpose?

• To check whether data has been corrupted

• To recover lost packets

• To store the destination IP address

• To encrypt the data

Answer explanation

The trailer often contains an error detection code such as a checksum or cyclic redundancy check.

When the packet arrives, the receiving device recalculates this code based on the received data.

If the calculated code matches the one in the trailer, the data is likely correct.

If it does not match this indicates corruption during transmission.

Recover lost packets that is handled by retransmission requests.

Store destination address that’s in the header, not trailer.

Encrypt the data, encryption is a separate process, not part of the trailer.

Why is a sequence number included in the structure of a packet?

• To identify the start and end of a file

• To prevent data from being corrupted during transmission

• To ensure packets can be reassembled in the correct order

• To encrypt the packet for security

Answer explanation

When data is transmitted over a network, it is broken into many smaller packets. These packets may take different routes and arrive at the destination out of order.

• The sequence number in each packet header tells the receiving computer the correct position of the packet in the original data stream.

• This allows the receiving computer to reassemble packets in the right order, even if they arrive mixed up.

• Error detection is handled by checksums in the trailer (not sequence numbers), and encryption is provided by other protocols.

Why is data split into packets before being transmitted?

• To prevent viruses spreading across the network

• To remove the need for error checking

• To allow multiple users to share the same communication line efficiently

• To make transmission slower but more reliable

Answer explanation

Allowing multiple users to share the same communication line efficiently, this is the main reason data is packetised. Packet switching allows pieces of different users’ data to travel across the same network at the same time, making efficient use of bandwidth. Data is split into packets to improve efficiency of transmission, so many users can share network resources. Each packet can take its own route to the destination, and the receiving computer reassembles them in order.

Which of the following statements best describes the structure of a data packet?

• A packet contains a header, payload, and trailer, each with a specific role.

• A packet is a continuous stream of binary digits with no divisions.

• A packet is a single unit of storage used to hold data inside the CPU.

• A packet consists only of the message data being transmitted.

Answer explanation

A data packet is divided into three main sections:

• Header that control information such as source and destination address, sequence number

• Payload holds the actual data being transmitted

• Trailer is for error detection code, e.g., checksum

Packet switching is used to transmit data packets across the network.

Identify the device that controls which path is taken by each data packet.

• Switch

• Hub

• Gateway

• Router

Answer explanation

In packet-switched networks, the router examines the destination address in each packet header and decides the most efficient path for the packet to take across the network.

In packet switching, what happens if a network link is busy or unavailable?

• The router chooses an alternative path

• The switch retransmits the packet

• The router stores the packet until the link is free.

• The hub delays the packet until the network is clear

Answer explanation

Routers dynamically reroute packets to find the best available path, which is why packet switching is efficient.

What is the main purpose of error detection information in a packet trailer?

• To correct corrupted data automatically

• To speed up the transmission process

• To identify the sender of the packet

• To check if the packet data has been altered during transmission

Answer explanation

The purpose of error detection information is to check if the data has been corrupted during transmission and, if necessary, request retransmission so that the message can be reassembled correctly.

Purposes of Error Detection Information in a Packet Trailer

1. Detect transmission errors

   • When data is sent across a network, it may get corrupted due to noise, interference, or collisions.

   • Error detection codes (e.g., checksum, parity bit, cyclic redundancy check) allow the receiver to check if the packet has been altered.

2. Ensure data integrity

   • The receiver can compare the recalculated checksum with the one in the trailer.

   • If they match, the data is assumed to be intact.

3. Request retransmission if errors are found

   • If the error detection check fails, the receiver can request that the sender retransmits the packet.

   • This helps maintain reliable communication.

4. Support efficient packet switching

   • Instead of forwarding corrupted data, routers/switches can drop faulty packets, reducing wasted network traffic.
     
     

Which data transmission method is described below?
“Data is transmitted down a single wire, one bit at a time, in one direction only.”

• Serial Simplex

• Parallel Simplex

• Serial Half-Duplex

• Parallel Full-Duplex

Answer explanation

• Serial refers to one bit at a time, single wire.

• Simplex refers to one-way communication only.