Frame Relay Applications: Voice over Frame Relay

The fourth frame relay application we are covering in our series from the Network Consultants Handbook is VoFr -- certain to be a major consideration in your future, if not present. This in-depth study explains all the technical data you need for reference in using this budding implementation, complete with tables and illustrations.

By Cisco Press | Posted Feb 5, 2002
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Network Consultants Handbook - Frame Relay
by Matthew Castelli

Network Consultants Handbook -- Click here to go to publisher's site

Voice over Frame Relay (VoFr)
Voice over Frame Relay (VoFr) has been recently enjoying the general acceptance of any efficient and cost-effective technology. In the traditional plain old telephone service (POTS) network, a conventional (with no compression) voice call is encoded, as defined by the ITU pulse code modulation (PCM) standard, and utilizes 64 kbps of bandwidth. Several compression methods have been developed and deployed that reduce the bandwidth required by a voice call down to as little as 4 kbps, thereby allowing more voice calls to be carried over a single Frame Relay serial interface (or subinterface PVC). Table 15-14 demonstrates these different compression algorithms and the bandwidth utilized per algorithm.

Table 15-14: Voice Compression Algorithms with Bandwidth Utilization

Encoding/Compression Bit Rate (Bandwidth Required)
G.711 PCM (A-Law/U-Law) 64 kbps (DS0)
G.726 ADPCM 16, 24, 32, 40 kbps
G.729 CS-ACELP 8 kbps
G.728 LD-CELP 16 kbps
G.723.1 CELP 6.3/5.3 kbps variable


NOTE:   A common concern regarding VoFR is keeping latency and jitter within acceptable limits. This can be a challenge in a network that is built around applications that can tolerate both; however, it is possible to pull a "trick or two" within the network.

One approach is to increase the CIR over each Frame Relay PVC that will carry voice traffic and not mark the voice traffic as DE.

Another approach is to implement separate PVCs for voice and data applications, segregating the traffic prior to transmission from the router.

A third approach is to work with a network service provider that offers PVC of different levels of delay/priority. Some providers offer as many as three PVC levels, or priorities:

  • Top priority for delay-sensitive traffic (such as voice and SDLC)
  • No (or middle) priority for traffic that can tolerate some level of delay (such as LAN traffic)
  • Low priority for applications that can tolerate significant levels of delay (such as Internet access and e-mail)


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