Micro-controller based Internet phone

Abstract

This work describes research towards the development of a micro-controller based, standalone Internet telephone to be used as an alternative to conventional line telephones. Our definition of 'stand-alone' refers to the unit's capability to perform its function wholly without the need for an attached computer. The unit should be low cost and capable of allowing two users to communicate using the units. Bandwidth usage should be kept low to allow the unit to be used over dial up connections which are prevalent in South Africa. The units should be easy to use as the anticipated users may be unskilled. A module containing a 16-bit micro-controller, an Ethernet controller, flash memory and RAM was chosen as the controller. The module came with a real-time operating system and a TCPlIP stack. The session initiation protocol (SIP) was selected to perform the signalling. SIP uses the session description protocol (SDP) to negotiate the attributes of the media session to be established. The real-time transport protocol (RTP) was implemented to transport encoded audio between the end points. The RTP control protocol (RTCP) was implemented to provide basic quality of service parameters. The ITU-T recommendation G.729 annex A was the voice codec selected. Codec ICs were used to encode and decode the audio. The implementations were designed specifically for a two user, direct communication environment. That is two phone units were developed that communicated directly with each other and not through intermediary servers.