The originator's initial SETUP message specifies the minimum acceptable bandwidth needed for the call, including committed burst and excess burst size. You'll need a detailed understanding of how your carrier calculates these values before you can configure your equipment, or calls may not set up properly. For example: Recall that committed burst size is the maximum number of bits the network agrees to transfer, measured over an interval T. Most would expect a CIR setting of 32 Kbps to translate to a committed burst size of 32 kilobits, but they could well be wrong if the carrier doesn't define T as one second. In the MCI WorldCom case, T is assigned the value 1.5 if the port speed runs at sub-T1 speeds. Therefore, a 32-Kbps CIR uses a committed burst value of 48,000 bps (32 Kbps = 48 Kbps/1.5), and connections won't get established if you configure the SVC for 32,000 bps.

Customer equipment maps network-specific addresses (phone numbers or IP addresses, for example) into these E.164 addresses, and when necessary using subaddresses passed in the Q.931 call setup to route a call (for example, to a specific voice station port on a remote frame relay access device). This call detail then gets logged by the network equipment, either locally or to a network-based logging device.

The Most Complex Task Finally, analyzing usage data is the most complex aspect of implementing SVCs. While Qwest and MCI WorldCom bill SVCs on total traffic in megabytes, other carriers are considering time-based billing--a more natural match for both voice and video applications. This billing alternative not only affects how equipment should be configured (for example, time-based billing places more importance on shutting down calls quickly than in selecting the most-efficient codec), but it also affects how it needs to be reported.

To verify call detail from the carrier equipment, logs must be collected and summarized. Matching detail against carrier-supplied data--as is commonly done in switched call accounting--is difficult, and in the short term may well be impossible. Note that neither MCI WorldCom nor Qwest is offering electronic call detail at this time. Moreover, building a reporting system for frame relay SVC data is a do-it-yourself process, since common reporting platforms and frame relay management systems do not automatically handle this information.

Send your comments on this article to David Willis at dwillis@nwc.com.