The Cisco T1/E1 circuit emulation service modules (CESMs) are designed for use with the MGX 8220 edge concentrators, MGX 8230, MGX 8250 and the MGX 8800 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 MGX T1/E1 CESM service modules provide eight T1 or E1 circuit interfaces, with full support for structured (channelized) and unstructured (unchannelized) data transfer modes for circuit emulation services over an ATM backbone.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. In addition, flexible clocking mechanisms are provided to meet different application requirements. Synchronous clocking and asynchronous clocking, using either SRTS or Adaptive clock recovery, are both supported. As an enhancement, dynamic bandwidth allocation is supported via on-hook/off-hook detection to reduce backbone bandwidth consumed when it is not required by the applications. This allows other traffic streams, such as VBR and ABR traffic, to take advantage of the bandwidth normally reserved for the circuit traffic.