These solutions are not necessarily mutually exclusive, and in fact, it is not unusual
to have multiple, complementary solutions deployed in a single Service Provider network.
For example, a Service Provider offering E-LAN services in the MAN may use
a Bridging/Switching solution, but they may also use a WDM solution to extend the
distances covered. In some cases, of course, these individual solutions may be part of
the same commercial solution, for example, a bridge/switch using WDM cards.
5 Ethernet PONs only are discussed
128 Chapter 4
Service Providers can offer seamless Ethernet services to the end user across multiple
underlying solutions primarily because of the common Carrier Ethernet layer.
For example, a customer with an Ethernet Private Line (i.e., connecting two of its locations
via an Ethernet Link) may be connected over a copper pair to a Service Provider
Point-Of-Presence (POP) using a Ethernet over copper solution and beyond this, may
employ an Ethernet over WDM solution to traverse over fiber to another POP and then
terminate at the other location using an Ethernet over copper solution. Even though
this Ethernet Private Line is being delivered employing two solutions, these underlying
solutions (Ethernet over Copper and Ethernet over Fiber/WDM) would usually be
hidden from the end customer (who may just want a seamless service with an associated
Service Level Agreement).
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