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OCR for page 379
A.2.S Fault Tolerant Networks
Modern networks employ fault tolerant techniques to extend He availability of the network. Win
the advent of SONET technology in He mid 198Os, fault tolerant capabilities have become an
integral part of equipment designs, although it is typically a procurement option. This section
will discuss the counter-rotating ring fault tolerant technology Hat is widely employed in
available commercial products, and techniques that can be used in ITS implementation where
fault tolerant capabilities are not in the basic equipment design. Fault tolerant design offers many
benefits for ITS commun~cabon systems:
I. Has capacity to achieve system availability over 99%;
2. Permits maintenance staffing to be reduced as repair can usually be accomplished on a
scheduled, as opposed to emergency, basis. When integrated win modern network
management, the communication system detects and reports failures, often impending and
actual.
3. Reduces communication system operation costs. Although initial procurement costs are
often higher to provide for fault tolerant redundancy, life cycle costs are usually reduced.
The counter rotating ring and variations of it, are the predominant techniques employed in
modern communication systems, as depicted In Figure A.2.~-] web each terminal serving as a
repeater as well as adding or dropping data for local equipment. In this technique, each terminal
is connected to a ring of dual communication circuits, each in opposite directions. If a link fails,
two terminals still communicate via He alternate paw direction between terminals. Counter
rotating rings are most cost-effective when He geographic locations of terminal equipment
support cost-effective ring topology interconnection. llS systems can employ counter rotating
rings in both local loops or backbone loops.
L:\NCHRP\Phase2.rprX NCHRP3-S1 · Phase2FmalReport A2-35
OCR for page 380
OCR for page 382
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Counter rotating rings usually are cost-effective in backbone links, however, local links
sometimes have geographical terminal equipment placements that favor alternate fault tolerant
architectures. AddidonaBy, local links can employ terminal technology Mat does not cost-
effectively support counter rotating nngs. Figure A.2.~-2 presents an alternative fault tolerant
architecture for local links. Each terminal on a local link connects to two backbone subuets via
independent communication mediums. The exact configuration can vary depending on We
mediums and type of terminal. For example, modems often employ multidrop while fiber and
microwave wireless configurations usually require repeaters at each terminal.
The additional cost to provide a fault tolerant architecture is approximately twice We transceiver
cost which is $300.00 to $1000.00 per te~al for wire and fiber, plus We cost of the additional
wire and/or fiber in We cables. SONET and many fiber transceivers offer cost-effective fault
tolerant options. Wireless redundancy is often more expensive but frequently used for difficult
redundant links (refer back to Figure A.~.3.3-4~.
~;`NCHRP`Phase~p~\ NCHRP3-51 · Phase2F'nalReport A2-37
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