Shared-Risk Logical Span Groups in Span-Restorable Optical
Networks: Analysis and Capacity Planning Model**
TRLabs, 7th Floor, 9107-116th Street NW, Edmonton, Alberta, Canada T6G 2V4,
Wayne D. Grover
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta Canada T6G 2V4,
Received September 2002; Accepted August 2003
Abstract. In an optical transport network distinct logical groups of lightwave channels between neighboring OXC nodes (called spans)
may sometimes be realized over a common physical resource such as a duct or conduit, and hence share a common cause of failure.
This is closely related to the concept of shared risk on individual channels or links, called SRLGs, which is relevant to pre-planned path
protection schemes with shared capacity on backup paths. But when considering span-restorable networks, shared risk over logical
spans (not individual channels) is the corresponding issue of concern. This work considers several aspect of how such shared-risk span
groups (SRSG) aﬀect the protection capacity design and other aspects of span-restorable mesh networks. We provide a model for
capacity planning any span-restorable network in the presence of a known set of such shared-risk spans and study the relationship
between capacity requirements and the number and placement of such situations. This provides guidelines as to how many SRSGs can
be sustained before the capacity penalty becomes severe and methods to diagnose which of them are the most limiting to overall
protection eﬃciency. One ﬁnding of interest is that if a given percentage of all possible dual-failure combinations incident to a common
node are allowed for in the design, then nearly the same percentage of other dual-span failure combinations (any two spans in the
network) will also be restorable. We also show that designing a network to withstand even a small number of multi-span co-incident
spared-risk span groups will yield a signiﬁcant improvement in overall dual-failure restorability and hence also in network availability.
Keywords: SRLG, optical network design, transport networks, capacity planning, restoration and protection, restorability, avail-
ability, optimization, WDM
Today interruptions in communication services
can result in serious ﬁnancial and other conse-
quences. Despite considerable eﬀorts at physical
protection of cables, FCC statistics show that
metro networks annually experience 13 cuts for
every 1000 miles of ﬁber, and long haul networks
experience three cuts for 1000 miles ﬁber . Even
the lower rate for long haul networks implies a
cable cut every 4 days on average in a not untyp-
ical network with 30,000 route-miles of ﬁber.
Notably the Gartner Group even predicts that
‘‘Through 2004, large U.S. enterprises will have
lost more than $500 million in potential revenue
due to network failures that aﬀect critical business
functions’’ . Accordingly, network restoration
(or protection) has become an integral part of the
design of optical transport networks. Recent years
have seen great progress in techniques for design-
ing survivable networks, most often with the intent
of handling any one single span failure at a time.
In an optical network a span is the set of all single-
hop lightwave channels between adjacent optical
cross-connect (OXC) nodes. Ideally, any single
physical structure failure aﬀects only channels
** Presented at Optical Networking and Communications Conference (Opti Comm 2002), Boston, MA, USA, July–August 2002.
Photonic Network Communications, 9:1, 35–53, 2005
2005 Springer Science+Business Media, Inc. Manufactured in The Netherlands.