One-way delay (OWD) refers to the time it takes for a packet to travel from the source to the destination across a network. It is an important metric in many real-time applications, such as VoIP (Voice over Internet Protocol), video conferencing, and online gaming, where timely delivery of data packets is crucial to maintain quality of service.
Components of One-way delay:
Propagation Delay:
- The time taken for a packet to travel from the sender to the receiver through the medium. It’s determined mainly by the distance between the two ends and the speed of the medium (e.g., speed of light in fiber optic cables).
Transmission Delay:
- The time it takes to push the packet’s bits onto the link. This delay depends on the packet size and the bandwidth of the link.
Processing Delay:
- Time taken by devices (like routers or switches) to process the packet, check for errors, determine the outgoing interface, etc.
Queuing Delay:
- Time a packet waits in a queue before it can be transmitted, especially when there’s congestion or when the outgoing interface is busy.
When measuring OWD, it’s important to ensure synchronized clocks between the source and destination, especially if accurate measurements are crucial. Asymmetries in routing paths (i.e., the path from source to destination is different from the path from destination back to source) can also influence OWD.
Two-way delay or Round Trip Time (RTT) is another related metric. It measures the time taken for a packet to travel from the source to the destination and back again. RTT is essentially the sum of two OWDs (from source to destination and then from destination back to source).
For many real-time services, keeping the one-way delay low is essential. For example, in VoIP services, a one-way delay of more than 150 milliseconds can start to degrade call quality significantly.