The Metro Ethernet Forum (MEF) defined and standardized three different topologies available for Metro and Wide Area Networks based on the Ethernet networking standard. The initial scenario was limited to a single carrier providing all access and network services. Without a standardized interface to connect the Ethernet networks of carriers and local access providers, the scope of Carrier Ethernet was too limited. Ethernet Network-to-Network Interface (ENNI) standardizes the interconnectivity between different networks.
The three-layer model shows the environment which an encryptor should be able to support. IEEE 802.1 (Ethernet) is only just one of many different transport options and each of the upper network protocols (MPLS and IP) of the Application Services Layer can in turn serve as transport layer. It is actually quite complex to fully support and secure Carrier Ethernet-based networks. .
Each of the MEF-topologies requires a transport network, that meets the require-ments of the respective Ethernet service. Transport networks are not necessarily na-tive Ethernet, as Ethernet can be transmitted over other transport networks, such as OTN, Sonet/SDH, IP and MPLS. The term Carrier Ethernet describes any transport network that is used for the transport of the Ethernet frames. The transport networks used within a MAN or WAN are not necessarily homogeneous. Depending on the placement of the encryptors and transport networks used, the original Ethernet frame can be encapsulated or tunneled. The delivered frame will be identical to the frame sent.
Originally Ethernet is a standard for local area networks (LAN), that is defined, main-tained and extended by IEEE. Carrier Ethernet builds on that standard, but is adapted to the needs of metro area networks (MAN) and wide area networks (WAN). Standards body for Metro and Carrier Ethernet is the Metro Ethernet Forum (MEF). Regional and wide area networks differ quite substantially from local area networks and this has implications on the requirements for an encryptor. What is relatively easy to accomplish within a LAN or for a single point-to-point connection, becomes much more complicated due to the different scenarios that have to be supported.
The model used by the Metro Ethernet Forum is based on three layers. The MEF concentrates its efforts on the middle layer, the Ethernet services.
Carrier Ethernet: Three-Layer Model and Transport Networks