Design of an Internet Fax & Voice Gateway

There has been a growing interest in recent years in developing real-time voice and fax communication software for use over the Internet. This paper discusses the design of an Internet Telephony Gateway (ITG) that is used to support the delivery of fax and/or voice services through the Internet to conventional telephones and fax machines on the Public Switched Telephone Network (PSTN). The interface board can offer multiple ports of voice and fax services in a single slot using the EISA bus. The hardware system comprises two microprocessors and one digital signal processor (DSP). One of the microprocessors is used for the telephone trunk line interface management and system control while the other one is used solely for the fax modem control. The DSP is used to compress and decompress the voice signal for real-time voice communication. Two algorithms to compress the digitized voice signal, namely, Global System for Mobile Communication (GSM) and Code Excited Linear Prediction (CELP), are used in the design. GSM is used to compress primary voice data packets while CELP is used to compress redundancy voice data packets to cater for potential lost data packets during network transmission. The software on the server gateway provides the functionality between two communicating gateways and manages the exchange of fax and voice between senders and recipients by using Internet as the transport medium.