1 for details). Both active
components also have other functions. The OLT connects the optical access
network to the metropolitan area network (MAN) or wide area network (WAN),
178 Chapter 7
typically termed backbone while ONUs typically aggregate traffic streams from individual
subscribers and prepare them for transmission toward the OLT. ONUs additionally
employ packet-prioritization mechanisms or scheduling, enabling full QoS
support and enforcement of service-level agreements (SLAs) between the Internet
service provider (ISP) and the end subscribers. The OLT typically employs complex
mechanisms responsible for bandwidth allocation in the shared upstream channel as
well as a number of agents dealing with registration of new subscriber units in the
network, ranging, link control, and so on.
Several multipoint topologies have been suggested for the access network, including
tree, tree-and-branch, ring, or bus (see Figure 7.2 ). Use of 1 ?— 2 optical tap couplers and
1 ?— N optical splitters allows for virtually any deployment architecture, thus making
EPONs a very flexible architecture capable of meeting any requirements in terms of
providing connectivity for end subscribers.
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