g the standards with corporate objectives and defining network architectures, deployment scenarios, and test suites. The biggest single aspect that differentiates MEF is that it defines networks from the point of view of delivering transport as a service. MEF defines service interfaces to enable easy in- terconnection between two main entities: Subscriber: The organization that purchases CE services Service provider (SP): The organization that provides CE services Carrier is a term that is used to describe large telecommunications service providers. Carrier Eth- ernet (CE) describes extensions to base Ethernet standards that allow SPs to provide and market transport services to their customers using Ethernet technology. A good example of such a feature would be Q-in-Q (originally specified in IEEE 802. lad, now incorporated in IEEE 802.1Q), which allows double VLAN tags and allows much better separation of individual customer traffic in the SP network. MEF has published over 70 technical specifications and implementation agreements that have con- tributed to technology interoperability, operational efficiency, and innovation. MEF standards are public and can be freely downloaded without charge. MEF also has a certification program that provides conformity testing against the MEF specifications. New MEF standards are numbered sequentially as they are published-for example, MEF 1, MEF 2, MEF 3, and SO on. Complete revision of an existing standard retains the original number, with dot and numeral indicating the revision number. For example, MEF 22 is replaced by MEF 22.1, and MEF 22.1 is replaced by MEF 22.2. Any amendment to an existing standard uses a third level of numbering; for example, MEF 22.2.1 is an amendment to MEF 22.2. For the mobile and synchronization markets, MEF produced MEF 22.1, "Mobile back-haul Imple- mentation Agreement - Phase 2," which provides a definition of Ethernet services for use in mobile back-haul networks. There are subsequent updates: MEF 22.1 includes support for physical layer sync