on (LANE), multiprotocol over ATM (MPOA), and a collection of others. CRS is the most basic of the ATM services. It delivers precisely what its name implies: a "raw pipe" transport mechanism for cell-based data. As such it does not provide any ATM bells and whistles, such as quality of service discrimination; nevertheless, it is the most commonly imple- mented ATM offering because of its lack of implementation complexity. CES gives service providers the ability to offer a selection of band- width levels by varying both the number of cells transmitted per second and the number of bytes contained in each cell. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies Figure 7-12 Frame-relay bearer service (FRBS) in ATM ATM Switches Frame Relay Frame Relay VTOA is a service that has yet to be clearly defined. The ability to transport voice calls across an ATM network is a nonissue, given the availability of Class A service. What are not clearly defined, however, are corollary services such as 800/888 calls, 900 service, 911 call handling, enhanced services billing, SS7 signal interconnection, and so on. Until these issues are clearly resolved, ATM-based, feature-rich telephony will not become a mainstream service, but will instead be limited to simple voice-and there is a difference. FRBS refers to the ability of ATM to interwork with frame relay. Con- ceptually, the service implies that an interface standard allows an ATM switch to exchange date with a frame-relay switch, thus allowing for interoperability between frame- and cell-based services. Many manufac- turers are taking a slightly different tack, however: They are building switches with soft, chewy cell technology at the core, and surrounding the core with hard, crunchy interface cards to suit the needs of the cus- tomer. For example, an ATM switch mi