The MEF has defined two service attributes regarding CE-VLAN tag support: CE-VLAN
ID preservation and CE-VLAN CoS preservation. The CE-VLAN tag consists of both the
CE-VLAN ID and CE-VLAN CoS, so a service may preserve one, both, or neither.
The CE-VLAN ID preservation is an EVC service attribute that defines whether it
is preserved across the EVC or not (if not, it is mapped to another value). This is useful
for services such as LAN extension.
The CE-VLAN CoS preservation is an EVC service attribute that indicates whether the
802.1p bits are preserved across the EVC or not (if not, it is mapped to another value).
CE-VLAN IDs must be mapped when one UNI of a pair supports tagging whereas
the other does not.
Service Multiplexing Service multiplexing provides the ability for a UNI (a physical interface)
to support multiple EVCs and precludes the need for a separate physical interface
to support each EVC. As illustrated in Figure 2.9, there are multiple EVCs between
UNI A and other UNIs in a network (assume that UNI A is at a higher bandwidth physical
interface than the other UNIs). By service multiplexing at UNI A, multiple EVCs can
be accommodated without needing multiple physical interfaces at UNI A.
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