The Cisco T1/E1 circuit emulation service modules (CESMs) are designed for use with the MGX 8230, MGX 8250 series wide-area edge switches. The CESM-8T1 and CESM-8E1 offer eight T1s or E1s for structured and unstructured circuit services over an ATM backbone for the new world of data networking.The CESM provides constant-bit-rate (CBR) services over an ATM network. It allows circuit-based equipment, such as PBXs, to be interconnected over an ATM backbone via CBR connections. The current CESM-8 supports both channelized (n x 64 Kbps) and unchannelized (T1/E1) circuit-based equipment. In ATM Forum terminology, the terms structured data transfer (SDT) and unstructured data transfer (UDT) are used for channelized and unchannelized circuit emulation, respectively.