Support for circuits is practically unavailable.
However, for large service providers to build cost effective packet-based networks,
support for circuits is required and the MEF??™s statement on multiple standardized services
and TDM support indicates this as a requirement of Carrier Ethernet.
330 Chapter 12
Quality of Service Every switch or router can offer some form of Quality of Service.
However, for the effective transport of voice and video the QoS performance must be
quite rigorous. Video and voice lose significant quality and become almost impossible to
transport unless the network can guarantee tight controls on delay and delay variation.
Networks of Ethernet switched introduce queuing delay that makes accurate delivery
of video and voice unpredictable.
Reliability A legacy of the voice network is a standard restoration time of < 50ms.
This evolved to ensure that a failure of a voice circuit could be correct in a time rapid
enough to be undetectable to the human ear during a voice call. Ethernet switching and
routing have restoration mechanisms including Spanning Tree and Rapid Spanning
tree that restore in under a minute for large networks, which is fine for many data
applications but not within the tolerance required for voice and video.
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