common functionality, but the major difference is that the latter also covers aspects related to performance management. The 802.1ag OAM model provides a hierarchy of up to eight levels (maintenance levels) that allow end users, service providers, and operators to run their own OAM traffic Chapter 14 Next Generation Ethernet Customer Operator A Bridges Operator B Bridges Customer Equipment Equipment Customer Level Provider Level MIP MIP MIP Operator Level Fig. 14.3 Example of Ethernet OAM entities. independently of what is run at other levels. The number of levels required depends on the scenario under consideration. OAM traffic at a higher level is transported transparently by equipment at a lower level (e.g., the customer's OAM is transported transparently by the provider's switches). The entity to be managed is called a Maintenance Entity (ME) in Y.1731 language. An can be defined between appropriate points such as administrative domains, between switches, or even a single link. An example of the scope of ME and how they can organized into levels is illustrated in Figure 14.3. The ME construct can be used in a multipoint environment by grouping MEs into Maintenance Entity Groups (MEGs) SO that a multipoint construct with N endpoints has N(N - 1)/2 MEs associated with it. Each MEG has a unique ID, and this is used in OAM messages to confirm that the messages are for their intended recipients. At the ends of the ME are functions called MEG End Points (MEPs) that generate, receive, and process OAM frames that are used to manage the ME. Within the ME there can also be some intermediate points, MEG Intermediate Points (MIPs), which can respond to OAM messages from a MEP but do not originate them. OAM messages can be sent to a MIP to allow activities such as loopbacks. The OAM standards define not only the Operations, Administration, and Maintenance architecture but also the OAM messages and the format of OAM frames. The most important OAM message is the Continuity Check (CC) message, which is sent out periodically 