Support for Robbed Bit Signaling (RBS), Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI) lines are provided by Digium’s series of digital telephony cards. The digital cards are provided in one (1), two (2), and four (4) port varieties for both PCI and PCI-Express slot types. An optional DSP module provides hardware-based echo cancellation for Digium’s digital PRI cards; digital BRI cards include on-board DSP-based echo cancellation [9]. When a telephone call is placed across a digital network, including VoIP calls, the human voice is transformed from its analog form into a digital form. This rendering of speech sound into digital form utilizes a computer algorithm to encode the signal - compression. The decoding of the digital signal is decompression. The word codec is a combination of these two functions compression – decompression [10]. All VoIP calls use some kind of codec, the most universal is a codec called G.711, available in two forms: ulaw (used in North America) and a-law (used everywhere else). G.711 renders spoken voice into a data stream that utilizes 64kbit/s of bandwidth and is not computationally intensive. G.711 is also the codec used to transmit data across digital PSTN links – ISDN BRI and PRI lines