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  Switching On Frame Relay SVCs February 22, 1999
However, these new applications need high reliability and low latency, increasing WAN costs and forcing some managers to redesign their PVC (Permanent Virtual Circuit)-based frame relay networks to accommodate the extra traffic. Most hub-and-spoke frame relay configurations have failure points at the hub that could disrupt communications (see "SVCs Complement PVC-Based Frame Relay Networks" below). Furthermore, dial-up access to frame relay--both internal and between organizations--can make any-to-any connectivity desirable. SVCs are much more suited to these types of applications than are PVCs. Fortunately, not all network providers are ignoring SVCs. Both MCI WorldCom and Qwest Communications offer them domestically, with the Qwest SVC service being particularly aggressive about their use. Using Qwest's quoted rates of 4 cents per megabyte, a voice over frame relay call using a 16-Kbps codec (coder/ decoder) costs just over 1 cent per minute. All carriers use PVCs today, partly because they're a good technological match for routed internetworks and host-to-terminal cluster connections, and partly because they are simple to administer. Customers make relatively few changes to PVC-based networks, and everyone benefits from the simplicity of flat-rate billing. But PVCs simply aren't flexible enough for many users: Carriers can't respond quickly enough to change orders, and disaster recovery is not as automatic as it should be. Additionally, some applications and configurations are better served by the occasional any-to-any connection than predesignated paths with static bandwidth assignments. A-OK SVCs In our lab tests, we found that frame relay SVC performance is nearly indistinguishable from PVC performance (see "Good News About SVCs: What Our Lab Tests Reveal,"). But though the services work properly, there are network design and configuration issues that you should think about before implementing SVCs in your frame relay network. Properly implemented, SVCs can reduce overall WAN costs and improve service quality. Used incorrectly, SVCs costs can skyrocket--both in regard to real out-of-pocket expenses and in the administrative burden of auditing usage. Not everyone will welcome this complex challenge, but there is a definite payback for some.
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