Network delay

Last Updated : Jun 19, 2018 |

In IP networks, packet delay (latency) is the length of time for a packet to traverse the network. Each element of the network, such as switches, routers, WAN circuits, firewalls, and jitter buffers, adds to packet delay.

Delay can have a noticeable effect on voice quality, but can be controlled in a private environment, such as a LAN or a WAN. Enterprises can reduce packet delays by managing the network infrastructure or by agreeing on a Service Level Agreement (SLA) with their network provider. An enterprise has less control over the delay when using the public Internet for VoIP.

Previously, ITU-T suggested 150 ms one-way delay as a limit for conversations. However, this value was largely misinterpreted as the limit to calculate a network delay budget for connections. Depending on the desired voice quality, network designers can choose to increase or decrease this number for their network.

Customers must consider the following issues when designing a VoIP network:

  • One-way delays of more than 250 ms can cause the well-known problem of talk-over. Talk-over occurs when both parties talk at the same time as the delay prevents them from realizing that the other person has already started talking.

  • In some applications, delays less than 150 ms can impact the voice quality, particularly when the voice is accompanied with an echo.

  • Long WAN networks is a major contributor to the network delay budget, averaging approximately 10-20 ms per 1000 miles. Some transport mechanisms, such as Frame Relay, can add additional delay. Additionally, staying within 150 ms, end to end, cannot be possible for all types of connections.

  • One-way delays of over 400 ms on signaling links between port networks and the S8300E server can cause port network instability.

Again, there is a trade-off between voice quality and the technical and monetary constraints which businesses must consider. For this reason, the following guidelines assist customers for configuring one-way LAN/WAN delay between endpoints, not including IP telephones:

  • 80 ms delay or less provides the best quality.

  • 80 ms to 180 ms delay provides Business Communication quality. This delay range is better than cell phone quality if echo is properly controlled and well suited for a majority of businesses.

  • Delays exceeding 180 ms can be acceptable depending on customer expectations, analog trunks used, codec type, and the presence of echo control feature in endpoints or network equipment.