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Quality of Service (QoS)

The days of crackly calls on a rainy day are long gone. With the wide spread use of ISDN and digital PBXs users are accustomed to picking up a handset and making calls that both parties can hear clearly and which do not suffer drop-out, background noise, echo or distorted speech. Business users today will not accept anything other than this level of quality from new IP based solutions.

With IP Telephony, conversations are sampled and transmitted as discrete data packets across the network, then re-assembled at the far end of the call. If the underlying IP network is not properly managed then problems can occur with:

Dropped Packets – switches and routers might fail to deliver (drop) some packets if they arrive on links that are heavily congested. With data, a level of packet loss is acceptable and quite normal. Any missing packets are simply resent a few milliseconds later. Voice on the other hand will not tolerate packet loss. The network must be engineered with mechanisms that ensure that voice packets get through and that only data packets are dropped when congestion occurs.

Delay – If links are heavily utilized it might take a long time for a packet to reach its destination, because it gets held up in long queues, or takes a less direct route to avoid congestion. Excessive delays in speech can make a conversation very awkward. Once again the network must be engineered to ensure that voice packets always get to travel in the fast lane.

Jitter – ideally packets in a voice stream will travel from source to destination in the same amount of time. However if there is other traffic sharing a link then some packets can be delayed more than others. This variation in delay is known as jitter and a certain amount causes no problem as buffers are used to compensate. However if the variation becomes excessive them it can seriously affect the quality of voice. Therefore the network must be engineered to ensure that voice always gets prioritized thus avoiding delays.

QoS – a network engineered with the mechanisms described above is said to be QoS enabled. It is important that QoS is enabled at all points in the speech path. SIPcom’s network is designed in just that way. The effectiveness of the QoS mechanism is constantly being monitored to ensure that everything is working correctly and statistical data on the quality of each individual call is logged on the system.