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The first approach adapts all services to ATM cells, which are then
transported over an optical network using a concatenated SONET
frame. Several standards address this approach, including Telcordia
GR-2837, which defines ATM virtual path support on SONET rings,
and Telcordia GR-2842, which presents requirements for ATM-based
access multiplexers. The second approach adapts services to IP
frames for transport over an optical network using either a channel-
ized or a concatenated SONET frame. The IETF multiprotocol label
switching (MPLS), differentiated services (Diffserv), and resource
reservation services protocol (RSVP) standards address a number of
issues associated with this approach. The third approach again
adapts all services to IP frames for transport directly over an optical
network. However, this approach eliminates the intermediate
SONET layer. Each of these approaches has the potential to increase
bandwidth utilization on fiber access networks.
Many network operators plan to use ATM over SONET for inte-
grated delivery of services because of its support for a wide range of
service types and its robust and mature quality-of-service (QoS) fea-
tures. Telcordia’s GR-2837 standard defines several approaches to
support of ATM traffic on fiberoptic rings. The most efficient of the
approaches uses a concatenated SONET frame, eliminating the
SONET virtual tributary (VT) structure while retaining the section
and line overhead bytes. Using ATM technology increases the gran-
ularity of transport bandwidth to minimize the wasted bandwidth
resulting from using the traditional SONET hierarchy. ATM over
SONET technology also supports fast automatic protection switch-
ing at the ATM virtual path level or the SONET layer. This
addresses one drawback of earlier approaches to protection switch-
ing using ATM virtual channels or IP routing table recalculations,
which were much slower. Finally, current ATM over SONET tech-
nology accommodates ring network topologies rather than assuming
point-to-point or mesh topologies. This enables the technology to be
used on fiber access rings that are prevalent in many parts of the
United States. Figure 8-13 is a representation of what can be gained
or lost by eliminating any of the protocols or layers in the OSI.
Services may be established on an on-demand basis using ATM
switched virtual circuit (SVC) signaling. QoS features also include
more efficient allocation of bandwidth on access networks by provid-