Reliability

Reliability is a quality of service characteristic; however, it is relatively important and warrants
separate consideration. As noted in the quality of service description, reliability is frequently a
factor in determining whether a backup link is required. Some designers will use multiple PVCs
to provide a greater level of reliability when problems are anticipated in the WAN cloud; this
differs from those situations when the designer is concerned with reliability in the local loop or
in the last portion of the circuit. In these situations, a separate connection is warranted.
The designer might also wish to use separate components in remote locations to further augment
reliability. This migrates the objective into the category of redundancy. It would require
disparate routers, circuits, data service unit/channel service unit (DSU/CSU) terminations, and
electrical systems to become fully fault tolerant, although it might also require placing the
equipment in two separate telephone closets with different building entrances to different service
providers’ offices. Different providers would further add to the redundancy of the design
and its ultimate survivability, which is synonymous to reliability. See Table 22.8 for a comparison
of various WAN technologies. Please note that this table refers to the technology’s inherent
capability to recover from data corruption, error, or topology change.