 not you're using it may not be very cost-effective. Frame Relay provides the capability to burst above what you've committed to financially. (This is discussed in the next section "Frame Relay Networks.") Frame Relay is also useful in a multiprotocol environment. Although IP seems to rule the world, it is not the only protocol in use. There are SNA networks in place, still making use of IBM's Synchronous Data Link Control (SDLC). The larg- est legacy networks today are some of the billing systems run by the world's telco operators. Frame Relay is used by more than 60,000 enterprises worldwide, and those that are highly focused on multimedia applications use ATM. Few customers use only one protocol. They have multiple protocols in their networks, and Frame Relay can handle them all because it simply encapsulates another protocol into a Frame Relay envelope and carries it through the network-it doesn't care what's inside the envelope. Closed user groups-where you want to know who has access in and out of the network-can be achieved with Frame Relay, unlike with the public Internet, where you have no idea who's on there at any point in time. Frame Relay also allows you to predict the level of the network's performance, SO it enables you to set metrics. This makes it an especially attractive solution if you are operating with countries where there are good carrier infrastructures. Frame Relay Networks Frame Relay is an interface specification that defines how information must be packaged in order for a Frame Relay network to act on it and deliver it to its destina- tion. Therefore, it is not necessarily associated with a specific piece of equipment. The Frame Relay interface could reside on multiple platforms. In Figure 7.13, for Packet-Switched Networks example, the Frame Relay interface resides on the DTE, which is most likely a router but could also be a Frame Relay access device (FRAD), used to provide access for Voice over Frame Relay (VoFR). It could be a T-1 or an E-1 multiplexer with a Frame Relay interface.