IP-telephony or Voip (Voice over IP) is a large concept and we will not be able to go into all details in this small article. What we can do, however is to cover the most important prerequisite to got the telephony working in your existing network. It is proven to work put it puts high demands on your network equipment to handle real time traffic, something that it probably has never been doing before.

As the voice and other sounds is flowing in a stream without interruption the timing is crucial when transporting it in data packets. The analog sound stream is first digitalized in a decoder and then divided in packets that can be transported over an IP network. A packet that been delayed too long by queuing for an overloaded transmission line can as well be thrown away immediately instead of taking place in the network as it will be useless when it arrived to the destination as that part of the flow probably already is delivered. Because of this it is most common to instead of TCP use the UDP protocol without flow control and with a better timing. The voice traffic also has to have priority in the network queues.

To be able to handle certain different delays on different packet in the flow there is always a buffer at the receiver side that save the packets for a short while before delivering it to the decoder. This way slower packets can be up to the receiver before it is delivered. The delay caused by the buffer is critical for the quality of the sound and for the experience of the conversation. It should not exceed 100 mS to have a good quality and there should not be delay over 150 mS or delay variations over 25 mS in the network.

A way to achieve this is to in the packets add information about how delay sensitive the packet is. This is done in the IP packets TOS (Type of Service) field and all equipment in the path between sender an receiver should be able to handle this information. The LAN (local area networks) and the switches is your network managers responsibility but the WAN (Wide area Network) is probably hired from a telecom operator so the quality has to be defined in an SLA (Service Level Agreement) according to your needs. The service provided from operators is called QoS (Quality of service) and you need to reserve certain bandwidth to be used by real time traffic (mostly possible to be used by other traffic if not used by Voip)

In most of the cases where IP telephony don't worked in a good manner it is a problem in the local area network. It has been used for web and mail traffic that is not time sensitive at all and does not have the functions for Quality of Service. If the equipment is old there might be a lack of such functionality and you need to replace the equipment.