Introduction to Voice over IP (VoIP)

Before you start

Objectives: Learn what is VoIP, why should we use it, and which protocols are associated with VoIP.

Prerequisites: no prerequisites.

Key terms: network, data, phone, VoIP, voice, digital, analog, IP, calls, protocols

How Does VoIP Work

Voice over IP is a protocol which can be used to send voice data over data network (packet switched network). This way we can have a single network which is used for both voice and data traffic, and we don’t have to have a separate phone network for our phones.

Data on IP networks are digital data, so if we use ordinary analog phones in our environment, we first have to convert analog signal do to digital signal. For that we have to connect our analog phone to a special converter that converts the analog into digital signal. That digital signal can then be transported on the IP network. We can also replace our analog phones with digital ones. That way we can connect our digital IP phone directly to the data network. VoIP phone has to be connected to a special switches with Power over Ethernet (PoE) features. PoE is used to supply power to the VoIP phone trough an Ethernet cable. We can also use a special application on our computer which acts as a VoIP phone (often called softphone). This way we can simply use our microphone and the speakers for making phone calls on our computer (computer is connected to the data network).

Our data network can be connected to the public switch telephone network (PSTN). The digital voice data generated on the VoIP network has to be converted to analog signal for transmission on the public switch telephone network. This way we can communicate with devices which are still located on analog phone network. But the great advantage of VoIP is that we can also use Internet to carry our voice data. In many cases, organizations are charged a flat fee for Internet access which means that VoIP phone bills will be considerably less when compared to PSTN calls.

Another advantage is that we have to maintain only one network and that is our data network. When using a separate network for phone calls, we have to maintain a separate connection to the PSTN, which adds a bit of complexity of our network environment. But don’t be fooled, implementing VoIP can be a very complex task. VoIP requires new hardware devices which are not cheap, and it also requires that we learn to work with that hardware.


There are several protocols which can participate in the voice data transmission over IP network. The first protocol is the Real-Time Transport Protocol (RTP). RTP holds the data stream which contains the actual voice samples.

We use other protocols to set up, maintain and terminate the phone calls. These protocols might be:

  • H.323
  • Session Initiation Protocol (SIP)
  • Media Gateway Control Protocol (MGCP)

VoIP Quality

When using VoIP we have a challenge to maintaining the quality of the phone call. For example, one thing that is often present is the delay (latency) in the phone call. Delays cause long pauses in the conversation which might cause participants to constantly interrupt each other. Delay should be maximum 150 ms. Another thing is the jitter. Jitter is the variation of delay within the phone call, which causes unusual sound effects as the delay of packets varies. During the transport of voice data, some packets will be lost and this often happens because of the delay. In VoIP it is better to lose some packet then to receive rearranged information which happened because of the delay. Another effect which can happen is the echo (when we hear our own voice while we are talking). Some echo is acceptable, but too much echo will cause a distraction.

Another thing that we have to keep in mind is that if we lose power, our VoIP phones will not be available since they rely on our data network. Phones on PSTN would typically still work since they use power which is supplied through the phone on a separate circuit, trough the phone lines.

The term which is often used with VoIP to describe all the mechanisms that are used to ensure the quality on IP networks is the Quality of Service (QoS). Quality of Service often implies that we separate our voice data over IP traffic from the normal data traffic. This way we can give higher priority to VoIP data to decrease delay and other problems that we have mentioned.