nt-to-point circuits such as T1 and DS3 facili- ties. With SONET came the ability to hub the traffic, a process that com- bines the best of cross-connection and multiplexing to perform a capability known as "groom and fill." This means that aggregated traffic from multiple sources can be transported to a hub, managed as individ- ual components and redirected out any of several outbound paths with- out having to completely disassemble the aggregate payload. Prior to SONET, this process required a pair of back-to-back multiplexers, some- times called an M13 (for "multiplexer that interfaces between DS1 and DS3"). This capability, combined with SONET's discreet and highly capa- ble management features, results in a wonderfully manageable system of network bandwidth control. Unintrusive Monitoring SONET overhead bytes are embedded in the frame structure, meaning that they are universally transported alongside the customer's payload. Thus, tight and granular control over the entire network can be realized, leading to more efficient network management and the ability to deploy services on an as-needed basis. 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. Telephony Telephony New Services SONET bandwidth is imminently scalable, meaning that the ability to provision additional bandwidth for customers that require it on an as-needed basis becomes real. As applications evolve to incorporate more and more multimedia content and, therefore, to require greater volumes of bandwidth, SONET offers it by the bucket load. Already, interfaces between SONET and Gigabit Ethernet are being written; interfaces to ATM and other high-speed switching architectures have been in exis- tence for some time. SONET Evolution SONET was initially designed to provide multiplexed point-to-point transport. However, as its capabilities became better understood and net- works bec