Voice over Frame Relay (VoFR) uses Frame Relay networks to carry digitized voice packets. Building a private network to carry voice and data might not be financially feasible for many companies. VoFR does not use IP as voice traffic is actually addressed and encapsulated in Frame Relay frames. VoFR is typically used for site-to-site voice communications. As Frame Relay is a Layer 2 protocol, it is actually faster than VoIP as there is one less layer in the mix.
Frame Relay has been around for several years, and its standards are mature, with stable interoperability. VoFR allows voice to be compressed and transferred across a Frame Relay permanent virtual circuit (PVC).
A gateway router can connect directly to the frame circuit through either a multiflex trunk (MFT) interface or through a digital signal unit (DSU) attached to its serial interface. Using the compression algorithm G729A, you can squeeze voice calls down to about 8Kbps (total with overhead would be about 10.8Kbps). With a 768Kbps circuit and G729A encoding, you could theoretically support 70 concurrent calls.
Most installations require that voice traffic share a PVC with data traffic. Frame relay works on the principle of having a committed information rate that is guaranteed and a port speed that you can burst to, which differs by provider. Any frames that stray into the burst area are tagged and could be dropped.
What happens if the data traffic starts pushing the bandwidth into the burst area? Voice traffic will mix with data traffic, and it could be tagged for discard. Imagine how a conversation might sound if every other second was dropped. Quality of service (QoS) issues are critically important when it comes to packetizing voice. Implementing QoS on VoFR is easier than with other VoIP implementations. If configuration of QoS is not done correctly, you could end up with a very expensive project as everyone starts calling long distance just to have a coherent conversation.
Frame Relay Benefits
Frame Relay offers several benefits:
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Costs are relatively low compared with other types of circuits.
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Once a port is purchased, adding PVCs between two ports is simple and usually inexpensive.
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Can be oversubscribed which allows more bandwidth to be mapped to a port than the port would theoretically allow, though this could have a severe impact on voice traffic.
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It is available just about everywhere.
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It can support a wide range of bandwidth, from 16Kbps up.
When Does Frame Relay Make More Sense?
The single most useful aspect of VoFR lies in taking advantage of existing or low-cost circuits to provide tie-line functionality between PBXs. The header for frame relay takes up only a little more bandwidth. A call that has been compressed using G.729A uses about 8Kbps, and with frame relay overhead, that increases to 10.8Kbps.
With a typical tie-line replacement, this level of compression and bandwidth savings provide much more room for data to travel over the same circuit (Figure 1).
One of the most important things to avoid when packetizing voice is a multiple compressing and decompressing occurrence during a call. Every time the call goes through a CODEC, there is a delay as the call is encoded or decoded which degrades quality. It is important to note that the CODEC used at each end must match. Once the router has converted the bits into an analog waveform, it sends the call out of the appropriate phone port. (Figure 2)
When end-to-end calls are needed across multiple locations, it becomes an opportunity for other Layer 2 solutions such as ATM or point-to-point circuits. VoFR has the edge from an ease-of-implementation and management standpoint as well as for long-term cost savings.
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