Although this is a techhnology dedicated blog, I decided to start a series of tutorial posts about a general technology which is not directly related to Cisco but it is a field in which Cisco is again a major player. This is IP Telephony and Voice over IP (VoIP). The two terms, IP Telephony and VoIP, are related around the same concept but in my opinion they are not exactly the same thing. Many people refer to these two terms interchangeably but they are not exactly the same. So, before moving on lets clarify the difference between IP Telephony and VoIP.
The truth is that despite the long presence of the VoIP technology, its penetration is not very widespread. According to In-Stat/MDR company, in 2004 VoIP was used only by 12% of U.S. businesses, a figure which is estimated to reach 70% this year. At international level the percentages are even lower. It is worth noting that large companies have the highest penetration. Factors related to delay the penetration of IP telephony are issues related to attitude (commitment to traditional technology), while clearly related with the technological immaturity of the technology in some places and the high cost of transition to IP platform. The technology is now mature enough and affordable, so it will also have higher rates of market growth compared to previous years.
VoIP can also be used by legacy TDM based PBX systems to transport voice calls over an IP WAN network or even over the Internet. Special voice gateways are used to connect to the legacy PBX telephone system on one end and to the IP network on the other end in order to translate the TDM voice stream into IP voice packets. So to summarize, IP Telephony is the overall concept of the modern form of voice communication which harnesses the power and features of VoIP technology in order to offer the overall experience of communicating effectively and with lots of extra features.
Bandwidth requirements are critical and are determined not only by the transmission rate of the codec used (from 3-4 to 64 Kbit / sec), but the extra load on the network, called IP headers, and other factors. Due to the presence of pauses during conversations, a technology was developed for detecting voice activity (Voice Activity Detection, VAD). With VAD, bandwidth requirements are reduced roughly in half. Thus, for example, for the G.711 codec with bandwidth of 64 kbit / sec, with the use of VAD technology the total bandwidth for a voice channel will be about 40 kbit / sec.
IP Telephony systems are those using entirely IP packets for voice communication, as explained before. In contrast to packet switched telephone systems (those based on IP protocol), conventional telephone systems apply the logic of direct connection between the two communicating voice parties through a dedicated circuit reserved exclusively for each contact. Thus the term Circuit switched telephone systems. In packet switched systems, however, the same communication line can be used to simultaneously pass different kinds of packets. Thus, the voice packets of one or more conversations may travel through the same route as other packets transferring data, video etc. This is the main difference between traditional telephony which is implemented to the public switched telephone network (PSTN) and telephony implementation on IP networks (or more generally to packet switched networks).
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