g., consecutive Ethernet frames
may belong to different flows) unidirectional stream of Ethernet frames that share a common treatment for
the purpose of transfer steering across the MEN.
60 Chapter 2
same EVC. The attributes associated with an EVC are shown in Figure 2.24 (at the end
of the chapter) and are employed when specifying an Ethernet service.
NOTE There may be one or more subscriber flows mapped to a particular EVC. This
capability enables an EVC to provide data privacy and security.
There are two basic rules that govern the delivery of Ethernet frames over an EVC.
A service frame must never be delivered back to the UNI where it originated, and the
Ethernet frame contents (including MAC addresses) must remain unchanged. The MEF
has defined two types of EVCs: Point-to-Point or Multipoint-to-Multipoint. In a Point-to-
Point EVC, exactly two UNIs must be associated with one another whereas in a Multipointto-
Multipoint EVC, two or more UNIs must be associated with one another. Thus, an EVC
can be used to construct a Layer 2 Private Line or a Layer 2 VPN12 service.
Network to Network Interfaces (NNI) As noted in the reference Service Architecture,
one or more Service Providers can be used to deliver Carrier Ethernet services.
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