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Processing Delay in Computer Networks: Interactive Router Simulation

Processing delay is the time a router needs to examine the packet header, check for bit errors and look up the output link. On modern routers it is usually microseconds.

About this simulator

Processing delay is the time a router spends examining a packet before it can be forwarded: checking the header for bit errors, looking up the output link in the forwarding table and handing the packet to the output queue. On modern routers it is measured in microseconds or less. The simulation breaks it into stages so you can see where the time goes.

How it works

  1. The router receives the packet and verifies the header checksum.
  2. It looks the destination address up in its forwarding table (a larger table needs more comparisons).
  3. It hands the packet to the output queue. The total cycles divided by the CPU clock is the processing delay.

Formulas used

  • dproc=total CPU cyclesclock rated_{\text{proc}} = \dfrac{\text{total CPU cycles}}{\text{clock rate}}
  • lookup cycles≈⌈log⁡2(table entries)⌉×cycles per comparison\text{lookup cycles} \approx \lceil \log_2(\text{table entries}) \rceil \times \text{cycles per comparison}

Worked example

Suppose a router needs 500 CPU cycles per packet and runs at 2 GHz. The processing delay is 5002 000 000 000=250 ns\dfrac{500}{2\,000\,000\,000} = 250\ \text{ns}, so one core can process about 4,000,000 packets per second. Doubling the clock to 4 GHz halves the delay; growing the forwarding table from 1,024 to 1,000,000 entries adds only about 10 extra comparisons because lookup grows with log₂(entries).

Enter these numbers in the simulator above to see every step.

When to use it

  • Understanding the four parts of nodal delay: processing, queuing, transmission and propagation.
  • Seeing why a larger routing table barely slows a router that uses efficient lookup.
  • Answering exam questions about why processing delay is usually negligible.

Frequently asked questions

What is processing delay?

It is the time a node takes to inspect a packet's header, detect errors and decide where to forward it. It is the first of the four delays that make up nodal delay.

How do you calculate processing delay?

Divide the number of CPU cycles needed to process one packet by the processor's clock rate: dproc=cyclesclock frequencyd_{\text{proc}} = \dfrac{\text{cycles}}{\text{clock frequency}}. Real values are usually microseconds or less.

Is processing delay important?

On modern high-speed routers it is usually tiny compared with transmission, propagation and queuing delay, but it matters on slow devices and in software routers.