Packet Switching and Delays in Computer Network

Packet Switching in computer networks is a method of transferring data to a network in the form of packets. In order to transfer the file fast and efficiently over the network and minimize the transmission latency, the data is broken into small pieces of variable length, called packet. At the destination, all these small parts (packets) have to be reassembled, belonging to the same file. A packet is composed of a payload and various control information. No pre-setup or reservation of resources is needed. 

Packet Switching

Packet Switching uses the Store and Forward technique while switching the packets; while forwarding the packet each hop first stores that packet then forwards.

• This technique is very beneficial because packets may get discarded at any hop for some reason. More than one path is possible between a pair of sources and destinations.
• Each packet contains the Source and destination address using which they independently travel through the network. In other words, packets belonging to the same file may or may not travel through the same path. If there is congestion at some path, packets are allowed to choose different paths possible over an existing network.

In packet switching the data is divided into small packets which allow faster movement of data. Each packet contains two parts that is Header and Payload, the header on each packet contains information. Below is the diagram of packet switching working.

Types of Delays in Packet Switching

Packet switching involves several types of delays that can affect the overall transmission time of data from source to destination. These delays occur at different stages during packet processing and transmission. The main types of delays are:

• Transmission Delay: Time required by the spent station to transmit data to the link.

• Propagation Delay: It is the time of data propagation through the link. It depends on the distance and the propagation speed of the medium (e.g., fiber optic, copper).

• Queueing Delay: It is the time spent by the packet at the destination's queue. It varies depending on network congestion and traffic load.

• Processing Delay: Processing time for data at the destination. It includes error checking and protocol handling.

• End-to-End Delay : Total time it takes for a packet to travel from the source to the destination. It is the sum of all the above delays: End-to-End Delay= Processing + Queuing + Transmission + Propagation

For more information on delays you can refer to the article Delays in Computer network.

Advantages of Packet Switching

• More efficient bandwidth usage: There is no need to reserve a dedicated path, which leads to better utilization of available bandwidth.

• Minimal transmission latency: Data can be transmitted as soon as it's ready, without waiting for a complete path to be established.

• Higher reliability: The destination can detect missing packets and request retransmission.

• Greater fault tolerance: If one link fails, packets can be rerouted through alternative paths.

• Cost-effective: Packet switching networks are generally cheaper and easier to implement and maintain.

Disadvantage of Packet Switching

• Unordered delivery: Packets may arrive out of sequence, while circuit switching ensures ordered delivery since all data follows the same path.

• Need for sequence numbers: To reassemble data correctly, each packet must carry a sequence number.

• Increased complexity at nodes: Each node must be capable of routing packets dynamically, which adds to the complexity.

• Potential transmission delays: Rerouting and queuing can introduce delays, especially under high network load.

• Less suitable for large, continuous data streams: While ideal for small or burst data, packet switching can be less efficient for large, constant data transmissions compared to circuit switching.

Types of Packet Switching

Connection-oriented Packet Switching (Virtual Circuit)

In connection-oriented packet switching, also known as Virtual Circuit Switching, a logical path or virtual connection is established between the sender and receiver before data transmission begins. This path is set up using a signaling protocol, and all packets belonging to that communication flow follow this predefined route.

Each virtual connection is identified by a Virtual Circuit ID, which is assigned by the switches or routers along the path. The data is divided into small packets, and each packet is given a sequence number to ensure they arrive in order at the destination.

This method involves three main phases:

Setup Phase : A path is established between sender and receiver. Address information is exchanged and recorded.

Data Transfer Phase : Packets are transmitted along the established route, with headers containing local information like length, timestamp, and sequence number.

Tear Down Phase : After the transmission is complete, the virtual circuit is released.

Some popular protocols which use the Virtual Circuit Switching approach are X.25, Frame-Relay, ATM, and MPLS(Multi-Protocol Label Switching).

Connectionless Packet Switching (Datagram)

In connectionless packet switching, each packet is treated independently and contains all the necessary addressing information, such as source and destination addresses, port numbers, and other control data. Unlike connection-oriented packet switching, there is no need for a connection setup or teardown phase.

Routing decisions are made dynamically for each packet, which means packets belonging to the same flow may take different paths and arrive at the destination out of order. Since the network does not guarantee delivery, reliability must be managed by end systems using higher layer protocols like TCP. This method offers flexibility and speed but requires additional mechanisms for ensuring data integrity and order.

Difference Between Packet Switching and Circuit Switching

Let us discuss how the Packet switching is different from the circuit switching.