Script: Voice Over Frame Relay audio primer

By Tim Greene
Network World

Every month you write out a check to the phone company, but wouldn't it be nice if you could use the telephone for free?

That is exactly what some computer network managers are doing with a technology known as voice over frame relay. Frame relay is a transmission method designed to send data over wide area networks. It divides up the data bit stream into variable-sized segments called frames and sends them across switches in carrier networks to connect different corporate sites.

Frame relay service providers take advantage of the fact that not all customers will use the bandwidth they paid for at the same time. Carriers oversubscribe the switches - they take in more bandwidth than the switch is designed to handle. That keeps costs down for carriers and prices down for customers.

And for the most part, it works out fine. Customers' traffic gets through, although there may be some delay and some frames may be dropped when a switch gets clogged. Because of delay, though, frame relay for a long time was not considered as a way to send voice traffic. With delay, the voice would arrive at its destination with gaps and clipped syllables - unacceptable for carrying on a conversation.

So frame relay networks have been set up to swap data, but when users want to talk, they pick up the phone. If there is enough talk betweeen workers in the same company, the corporate network manager has probably installed a private telephone network linking corporate sites.

But now, in an attempt to save money and streamline their operations, some network managers are blending their private network voice traffic onto their frame relay networks.

To do so, they install a gateway between their telephone networks and their data networks. The gateway converts digital voice signals coming from the private telephone switch into frames and drops the frames onto the frame relay data network.

The voice frames can then travel anywhere the data network goes. When they arrive at the desired site, the voice frames pass through another gateway that turns them back into regular digitized voice traffic. That traffic is sent to the private voice switch at the second site. The switch directs the call to the proper telephone.

Since most frame relay service is billed by how much bandwidth is reserved - not by how many frames are actually sent - that voice traffic rides for free, provided there is excess or unused capacity on the data net.

It sounds neat, efficient and inepensive. But it is not perfect.

First, all that voice traffic does chew up bandwidth. If enough voice calls are made, the extra packets on the network may slow down the data traffic the network was designed to handle in the first place. That means everybody using the network for data will have to put up with slower performance or the network manager will have to buy more bandwith - an added expense. But it may be worth it if the cost of the extra bandwidth is less than the cost of a separate voice network.

And, as we have already noted, frame relay could not guarantee timely arrival of voice frames. As voice frames cross the frame relay network, they must wait in line for access to network devices, just like all the data frames.

Some of those other frames are enormous, and voice packets can get stuck waiting for them to pass through. For many data applications, it makes little difference if the frames are delayed. They can be reassembled at the receiving end in plenty of time to make sense.

Makers of voice over frame relay equipment are dealing with the problems. They are compressing voice traffic so it takes up less bandwidth on the frame relay network. They are also fragmenting large data packets into smaller ones so when a voice packet does have to wait in line, it doesn't have to wait so long.

And they are tagging voice frames so that intelligent network devices can recognize them. The devices can then give them priority over other traffic, reducing delay and improving voice quality.

With continuing improvements and word of mouth that the technology really does work, voice over frame relay is becoming more popular.

If you have a frame relay network and suspect you can squeeze on a little more traffic, you might want to give it a try. After all, who doesn't like to get something for nothing?