Circuit emulation service
Previously, in spite of the popularity of the new generation of public fast packet switching networks such as frame relay and ATM, many, if not most, customers still used simple circuits to build private networks. Whether they were called point-to-point private lines, leased lines, or dedicated circuits, their key characteristics were always the same. Circuits supply customers with pure bandwidth and nothing else. They form passive “bit pipes” from point to point and do not examine, process, store, or otherwise massage any of the user’s bits as they make their way through the circuit-based network.
With very few exceptions, most circuit services are based on the T-carrier (North America) and E-carrier (almost everywhere else) hierarchies. Large private networks might use anywhere from 10 to 100 of the lowest multiplexed rate circuits, and each circuit could have up to 24 (T-carrier) or 30 (E-carrier) channels running at 64 kbps inside them. This is not only a huge investment in equipment, but also it constitutes an enormous inertia against sweeping changes.
These large networks could not possibly migrate to ATM overnight. A company with ten sites in five states could not even begin to contemplate such a step. So the question becomes, How can ATM allow customers to gracefully migrate from their private line environment to a public network environment using ATM cells? Circuit emulation service (CES) is an answer. CES makes the public ATM network “look like” a collection of private circuits. Customers need not make any changes to end-user equipment to put it on the ATM network. It works just like before.
CES plays two key roles in ATM networks. First, CES allows graceful migration from T-carrier or E-carrier. Second, CES provides the foundation for voice and telephony over ATM (VTOA). VTOA allows customers’ (or carriers’) current voice services to be carried over the ATM network between PBXs or central office switches.
ATM CES is defined by both the ATM Forum and an ITU-T recommendation. The definitions are virtually identical.