Implementation and evaluation of a mobile tetherless VoIP/PSTN gateway

A voice-over-IP (VoIP) gateway bridges IP-based packet-switched networks (i.e. Internet) with public circuit-switched telephone networks (i.e. PSTN). The key building block of a VoIP gateway is a telephony card that interfaces with the PSTN and converts signals from the PSTN to bits that can be manipulated by a computer and vice versa. Because commercially available telephony cards only work with wired PSTN lines, almost all existing VoIP gateways are tethered and therefore do not support the kind of mobility enabled by modern wireless communications technology. This paper describes the implementation and evaluation of a mobile VoIP gateway called WGate that is designed specifically to bridge wireless VoIP clients and cellular phones, and can thus be easily deployed on demand in particular geographical locations. The key innovation of WGate is the ability to use a Bluetooth link as a wireless backplane by exploiting the Hands-Free profile of the Bluetooth protocol stack and eventually turning a set of commodity bluetooth-capable cell phones into a multi-port telephony card. Empirical measurements on a working prototype show that this approach can scale a VoIP gateway up to 8 cell phones because state-of-the-art Bluetooth adapters can only support up to 8 simultaneous Synchronous Connection-Oriented (SCO) connections when they operate in physically close proximity.