What is delay, the performance metric of the network layer?

The performance of a network can be measured in terms of Delay, Throughput, and Packet Loss. Network delay is a critical performance metric that affects user experience and application performance.

Let us understand the concept of delay and its various types in network communication.

What is Network Delay?

Network delay is the total time taken for a packet to travel from its source to its destination. As a packet traverses through the network, it encounters multiple types of delays at different stages of its journey.

Types of Network Delay

Processing Delay

Processing delay is the time required by a router to examine the packet header, determine the destination, and decide the appropriate outgoing route. This includes extracting the destination IP address, performing routing table lookups, and making forwarding decisions.

Consider the analogy of posting a letter: when a postman receives mail, he examines the destination address (state, city) and decides which direction to forward it based on postal routing rules. Similarly, routers process packet headers to make forwarding decisions.

Processing Delay = Time to examine packet + Routing table lookup + Forwarding decision

Queuing Delay

Queuing delay is the time a packet waits in the router's buffer before being processed for transmission. When multiple packets arrive simultaneously or the router is busy processing other packets, incoming packets must wait in a queue.

This delay varies from 0 to infinite depending on network congestion. If the router is idle when a packet arrives, queuing delay is zero. However, during heavy traffic, packets may experience significant queuing delays.

Factors affecting queuing delay:

• Buffer size of the router

• Processing speed of the router

• Network traffic intensity

Transmission Delay

Transmission delay is the time required to push all packet bits onto the outgoing link. It depends on the packet length and the link's bandwidth capacity.

Transmission Delay = Packet Length (L) / Bandwidth (B)
                  = L (bits) / B (bits/sec)

For example, transmitting a 1000-bit packet over a 100 Mbps link takes 10 microseconds.

Propagation Delay

Propagation delay is the time taken for a signal to physically travel from the source to the destination through the transmission medium. This depends on the physical distance and the speed of signal propagation in the medium.

Propagation Delay = Distance (D) / Propagation Speed (V)

Where V is typically close to the speed of light in the transmission medium (fiber optic, copper wire, or wireless).

Comparison of Delay Types

Conclusion

Network delay consists of four components: processing, queuing, transmission, and propagation delays. Understanding these delay types helps network engineers optimize network performance and troubleshoot latency issues effectively.