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QoS Recommendations for VoIP

Updated on December 9, 2010

Qos and VoIP

Despite VoIP being an Internet application, the data it carries doesn't or isn't supposed to behave like regular Internet traffic. The inherent flexibility of the Internet allows whichever packets of data coming first to be received and displayed. This means that web pages for example can have their components load in any given order leading people to say that the Web is "stateless."

When talking over the Internet however, we can't afford to have packets coming in whenever they want. For people to understand each other, voice must come in sequentially and on time. In order to ensure this happens, it's critical to maintain "Quality of Service" or QoS standards for VoIP data. QoS standards take the form of certain measurements which must be met if VoIP is to be effective. Below, we take a look at some common parameters.


VoIP QoS Recommendations

Once we've determined that VoIP traffic needs to be managed differently, we need to be able to tell the network how this is to be accomplished. In addition, we will need to perform high level management to ensure that VoIP always has the minimum bandwidth necessary for it to work efficiently at all time.

So the first step in applying QoS for VoIP is to set up a virtual LAN or VLAN which has a separate channel for VoIP traffic with a certain amount of bandwidth dedicated for it. Once this has been set up, we can then instruct the network about what parameters are to be maintained for VoIP traffic. How exactly this is done depends a great deal on the hardware used. Some Cisco routers for example, already create automatic VLANs for VoIP traffic and implement default QoS rules. Small Business VoIP systems will greatly benefit from such automatic configuration.

For those doing it manually, here are a few guidelines as to what the values of the various parameters should be:

Latency is an important metric and it consists of the time required for a roundtrip of a packet. Experimentally we know that a delay of 250ms is noticeable by users of VoIP and it's recommended to have a QoS of 150ms for this. This is the total roundtrip including the path traversed by the packet over the Internet over which one has little control. This means that one has to try and minimize the latency of their own network as much as possible and it should be much lower than 150ms.

Jitter refers to the variance of the latency and is an important factor in determining how much buffer should be kept while sending and receiving VoIP data. This mustn't be more than 0.5ms no matter how the jitter is measured (there are several techniques.)

Finally, Packet Loss refers of course to the percentage of packets lost in transit. For VoIP, this should never exceed 1% and many providers specify no more than 0.1%.

These are a few metrics that must be taken into consideration when determining VoIP QoS. Read up some IP Business Phone Reviews to find out more about which systems offer what QoS levels.


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