VoFR (Voice over Frame Relay)

Referring to techniques for transmitting real-time voice over a

frame relay network. The standards for VoFR were set forth in the Frame Relay Forum’s FRF11.1,Voice over Frame Relay Implementation Agreement (December 1998). As frame relay is a packet data network intended for LAN-to-LAN internetworking rather than isochronous traffic, levels of latency, loss, and error are variable and unpredictable in nature, which creates issues for real-time voice communications. However, a number of major domestic and international carriers offer, and even promote,VoFR as part of an integrated network solution and a managed service offering.The design may include separate VoFR permanent virtual circuits (PVCs) and oversized committed information rates (CIRs).

Still,VoFR must contend with issues of latency, jitter, loss and error, and does so through the use of various low bit-rate compression algorithms, the most popular of which are in the CELP (Code-Excited Linear Prediction) family. CELP and other compression algorithms support very reasonable business quality voice, at bit rates as low as 8 kbps, under conditions of low network congestion or where the voice traffic remains within its CIR. In order to mitigate the inherent difficulties of VoFR, some manufacturers and carriers offer various priority management techniques. Some service providers also offer PVCs of varying levels of delay/priority, usually by mapping the frame relay connection to an ATM connection with these properties. Priority levels generally are defined as follows:

• Real-Time Variable Frame Rate: Top priority; suited to delay-sensitive, mission-critical applications such as voice and SNA.

• Non Real-Time Variable Frame Rate: No-priority designation; suited to LAN and business class Internet and intranet IP traffic.

• Available/Unspecified Frame Rate: Low-priority designation; suited to Internet access, e-mail, file transfer, monitoring, and other low-priority applications.

See also CELP, CIR, compression, congestion, frame relay, Frame Relay Forum, isochronous, LAN, latency, managed service provider, packet, PVC, real time, voice, and VoIP.

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