Quality of Service or QoS is the quality of a call over a network. It also refers to the ability to prioritize certain types of traffic on an IP network. In the case of VoIP, this typically means prioritizing voice traffic at a higher level than other forms of traffic such as data so that voice traffic will not be delayed or dropped.  An MPLS network will allow you to prioritize this data ahead of all other data traffic in order to maintain quality.

Latency causes delays in packet delivery. Physical distance, the number of router hops, encryption, and voice/data conversion all impact latency. Users begin noticing latency as a service level issue when roundtrip latency is greater than 250 milliseconds (ms). The International Telecommunications Union recommends that latency never exceed 300 ms round-trip.  Over long distances, i.e. from the USA to Asia, the shortest path circuit and least number of hops can make the difference between satisfactory and unsatisfactory voice communications.

Jitter occurs when voice packets are sent and received with timing variations. Jitter is effectively a variation of packet delay where delays actually impact the quality of the conversation. Think of jitter as variable delays in packet delivery. Participants will notice delays in the conversations impacted by jitter. As a result, many service providers now account for maximum jitter levels.

Packet Loss takes place when packets are dropped.  This can be due to a variety of factors and is very common when using VoIP over the internet, with or without a VPN tunnel. It usually shows up as dropped conversations or “tinny” sounds. Packet loss should never exceed 1% and most service providers guarantee service levels with .5% or less packet loss. Packet loss of 1% translates into one voice clip or skip every three minutes, while packet loss of .25% translates into one error every 53 minutes.  When latency is high, packet loss is often high, as well.

Prioritizing VoIP traffic over the network at Layers 2 and 3 yields latency and jitter improvements. Policy based network management, bandwidth reservation, Type of Service, Class of Service, and Multi-Protocol Label Switching (MPLS) are all widely used techniques for prioritizing VoIP traffic at Layers 2 and 3.  VPLS networks will provide the highest level of quality available, where appropriate.