History of Voice Over IP

Voice communication, as we know it in the modern age, was invented by Alexander Graham Bell in 1876- the Telephone.

The architecture of today's Public Switched Telephone Network (PSTN) is a direct descendant of the original manned switchboards of Bell's days. Voice is transmitted in one way: sampled in 8-bit bytes, 8000 times a second, for an aggregate rate of 64 Kbps. The entire telephone network is designed around this rate and for this one type of traffic - voice.

PSTN is a circuit-switched architecture: a direct connection is closed between two users. The users have exclusive and full use of the circuit until the connection is released. The connection is bi-directional with very low delay between the two end points.

PSTN was designed for circuit switching of voice calls. This fact makes it very difficult to add new services to the network, or change traffic handling methods. Another major disadvantage of circuit switching is that once a circuit is established, it consumes the full-predefined bandwidth, no matter if the call contains many silent periods.

The increasing difficulties in maintaining the existing networks and upgrading them to many new demands together with the inefficient bandwidth usage, has driven the market to seek after a new architecture. The demand was for a technology that will enable easy and rapid growth and updating, together with efficient use of bandwidth that will enable more simultaneous connections with larger volumes on the same bandwidth.

During the early 90's the Internet was beginning its commerical spread. The Internet Protocol (IP), part of the TCP/IP suite (developed by the U.S. Department of Defense to link dissimilar computers across many kinds of data networks) seemed to have the necessary qualities to become the successor of the PSTN. IP works in a packet-switching method: the data is sliced and bundled into packets (pieces of information). These packets are transmitted from the source end-point to the destination end-point by moving from router to router until they reach their destination. No circuit is closed between the two end points as in the circuit-switched PSTN. IP as a connectionless mode is very efficient because no bandwidth is spent when there is nothing to transmit. When transmission is needed, IP enables using all the free bandwidth.

Another big advantage of IP - it enables easy and rapid network growth and updating, because of the IP design. It is possible to add features to a part of the network without changing other parts.

The great efficiency in the use of bandwidth causes problems when dealing with voice - real-time transmission needs constant rates with small (known) delays, otherwise the conversation sounds abnormal. Other problems that might occur in IP: packet loss due to routers overflow, packets arrive out of order due to different routes, causing unacceptable delays. These problems do not appear in PSTN because real-time transmission is inherent in circuit-switched networks.

The first VoIP application was introduced in 1995 - an "Internet Phone". An Israeli company by the name of "VocalTec" was the one developing this application. The application was designed to run on a basic PC. The idea was to compress the voice signal and translate it into IP packets for transmission over the Internet. This "first generation" VoIP application suffered from delays (due to congestion), disconnections, low quality (both due to lost and out of order packets) and incompatibility.

VocalTec's Internet phone was a significant breakthrough, although the application's many problems prevented it from becoming a popular product.

Since this step IP telephony has developed rapidly. The most significant development is gateways that act as an interface between IP and PSTN networks.

Later developments are gatekeepers and Multi-point Control Units that have more advanced control tools for VoIP communications.

How is Voice Over IP implemented today? find out more in our next chapter - Voice Over IP Today