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QoS and VoIP

Updated on March 21, 2013
Implementing VoIP QoS
Implementing VoIP QoS

VoIP – Layering QoS over the Internet

One of the advantages of using the Internet for delivering services is that it is incredibly flexible. But this flexibility comes at a price. The traditional PSTN phone system is single-purpose, but it serves that purpose incredibly well. When we want to use VoIP, we have to ensure that the underlying network serves our needs and presents us with a certain Quality of Service or QoS that makes using VoIP a pleasant experience.

Most people use the Internet to view webpages. For this activity, certain delays are more than acceptable. Different parts of the pages can load at different times depending on what is immediately available. This asynchronous loading system works very well but when we try and transport voice data, we have to be more careful.

VoIP is a real-time protocol and is particularly sensitive to latency, jitter, lag, and packet loss. All of these can degrade the quality of the call. Let us look at them one by one and see what they are and what impact they have.


This is the time taken for IP packet to reach its destination. It is based on a variety of factors including the speed of your own network as well as congestion on the Internet itself. As a real-time P2P protocol, the overall latency should not be more than 150 ms. This means that the latency within your own network has to be far less. Anything more than this, and people begin to interrupt each other because they think the other is not talking. When the latency is too large, conversation becomes almost impossible. There are other mediums where latency is high and people have developed specific procedures for communicating with each other using them. You've seen military personnel talking on their radios saying "over" in order to indicate that they've finished. This is because high latency makes it difficult to actually detect an interruption in speech.

Packet Loss

Congestion on the network may also lead to IP packets being dropped. Below a certain threshold, this is not noticeable, but above that parts of speech begin to go missing and even though our brain compensates, it leads to a poor call experience.

There are several ways to enforce QoS for VoIP systems. One of these is to embed it directly into the router so that it gives a higher priority to VoIP traffic. This is particularly relevant in corporate networks where it has to compete with large amounts of regular traffic as well. Contact your local Boston hosted PBX service provider to find out how you can get started with the hosted VoIP phone system.


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