When a user downloads a large file over a packet-switched network, the file is first divided into packets. Each packet is then sent as an independent unit across the network. Routers play a crucial role in directing these packets towards the destination (the user's device). Each router examines the destination address in the packet header and forwards the packet to the next hop along the path to the destination.

Network Congestion: If the network becomes congested (i.e., there is too much traffic), packets may experience delays or be dropped. This can lead to a slower download speed and potentially require retransmissions of lost packets. Techniques like Quality of Service (QoS) can be used to prioritize certain types of traffic and mitigate the effects of congestion. The receiving device then reassembles the packets in the correct order to reconstruct the original file.

Diagram:

When a large file is transferred over a network using TCP, the file is divided into smaller, manageable packets. TCP ensures reliable and ordered delivery. Here's how the process works:

1. Segmentation: The sending device (sender) divides the file into segments (packets). Each segment is assigned a sequence number.
2. Header Addition: A TCP header is added to each segment, containing information like the source and destination port numbers, sequence number, and checksum.
3. Transmission: The segments are transmitted over the network.
4. Reassembly: The receiving device (receiver) receives the segments. It uses the sequence numbers in the TCP headers to reassemble the segments in the correct order.
5. Error Checking: The receiver uses the checksum in the TCP header to check for errors during transmission. If a segment is corrupted, the receiver requests a retransmission of that segment from the sender.
6. Flow Control: TCP also implements flow control mechanisms to prevent the sender from overwhelming the receiver.

Essentially, the sequence numbers act as markers, allowing the receiver to piece the fragmented data back together in the original order, even if the packets arrive out of sequence or are lost and need to be retransmitted. The TCP protocol guarantees that the data is delivered completely and in the correct order.

Advantages of Packet Switching over Circuit Switching:

• Bandwidth Utilization: Packet switching is more efficient in using network bandwidth. Resources are only used when packets are being transmitted, unlike circuit switching where a dedicated connection is reserved regardless of whether data is being sent.
• Reliability: Packet switching is more robust. If a network link fails, packets can be rerouted along alternative paths. Circuit switching relies on a single, dedicated path, so a failure can disrupt the entire connection.
• Setup Time: Packet switching requires minimal setup time. Packets can be sent immediately without establishing a dedicated connection. Circuit switching requires a dedicated connection to be established before data transmission begins.

Disadvantages of Packet Switching over Circuit Switching:

• Variable Delay: Packets may experience variable delays due to congestion or differing routes. This can be problematic for real-time applications like voice or video.
• Overhead: The packet headers add overhead to the data, reducing the effective data throughput.
• Complexity: Packet switching networks are more complex to manage than circuit switching networks.