Importance of Latency

Network capacity (`bandwidth') is increasing at a rapid pace with the deployment of fibre-to-the-home and mobile broadband. Latency is then becoming the main performance limitation for Internet services.

Latency is the time it takes to complete an operation:

  • Time between clicking a web link and the display of the page
  • Time between user input and resulting action in a game
  • Time to transfer a data packet through the Internet

The base round-trip latency (delay) for a network path is as follows:

  • nationally: 5-30 ms
  • continentally: 30-100 ms
  • intercontinentally: 100-300 ms

Transfer latency 10KBTransfer latency 100KBTransfer latency 1MB

This network round-trip latency, commonly called round-trip time, RTT, sets a limit to how quickly a data transfer can complete. The three graphs illustrate this for three different data transfer sizes: 10 KB, 100 KB and 1 MB. The graphs plot the total time it takes to complete a data transfer against the data rate of the network path for four RTT values.

From these graphs we can, for example, find out that:

  • for a 10 KB transfer with a 30 ms RTT, we don't gain much in completion time with an access data rate above 10 Mbit/s
  • an intercontinental transfer of 100 KB data will not complete in less than 0.8 seconds
  • we only benefit from a 1 Gb/s subscription if the RTT is low (<= 5 ms) and the data transfer size is large (>=1 MB)