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1
On Shared Risk Link Group
Optimization (Invited)
Guangzhi Li, Dongmei Wang, Timothy Gallivan, and Robert Doverspike
AT&T Labs, New Jersey, USA,
{guangzhi.li,dongmei.wang,timothy.gallivan,rdoverspike}@att.com
Abstract— Shared Risk Link Groups (SRLGs) have
been investigated during the past 10 years due to
optical network innovation and deployment. Dense
Wavelength
Division
Multiplexing
(DWDM)
technology has significantly increased the data
transmission capability per fiber. Service providers
build their overlay networks, such as IP networks, on
top of optical networks and all optical networks are
built over some combination of DWDM equipment
and fibers. If there is a single DWDM system outage
or fiber outage, the set of overlay network links
dependent on the failed resource would all fail at the
same time. The set of links failed by a common
resource outage is called a shared risk link group
(SRLG). SRLGs have been designed and implemented
in many network planning tools and some network
routing protocols. A single SRLG represents one
potential outage (or failure mode) and a large service
provider’s network could easily contain tens of
thousands of failure modes. The greater the number
of SRLGs, the more difficult it is to attain good
performance from planning tools (like routers) whose
computations are dependent on the number of
SRLGs. For some routing protocols using SRLG
information, the situation becomes even worse
because a routing protocol may have space
constraints to hold a limited number of SRLGs. These
issues create a challenge to optimize the SRLG
calculations such that the SRLG-related functions
are not impacted or the impacts to the SRLG-related
functions are limited. This paper takes a closer look
at the SRLG optimization issue and proposes
algorithms for how to reduce the number of SRLGs
for different applications.
Index
Terms—Shared
Risk
Link
Group,
Optimization, Algorithm, Case Study.
I. INTRODUCTION
service provider overlay networks are built on
Most
top of optical networks and all optical networks
are built over some combination of DWDM equipment
and/or fibers. If there is a single DWDM system
outage or fiber cut, the set of overlay links routed over
the failure risk (the DWDM system or fiber) would fail
simultaneously. This set of simultaneously failed
links is called a shared risk link group (SRLG).
SRLGs are typically represented by identifiers that
are associated with all of the links in the set. These
IDs are used by network planning tools or network
routing protocols to represent the different ways that
the network can fail, i.e., the failure modes. The
SRLG IDs can also be viewed as a representation of a
network’s diversity, since two links that are not
simultaneously failed by an SRLG ID can be
considered to be diverse under that particular mode of
failure.
For example, an IP link (part of an overlay
network) between two routers may have multiple
SRLG IDs associated with it and a single SRLG ID
could be shared by multiple IP links. Thus the SRLG
information for each IP link describes a list of SRLG
IDs to which the link belongs. An SRLG can also
represent a potential node outage, such as a total or
partial router outage or outages due to router
maintenance or software upgrade procedures in the IP
network.
To manage the total number of SRLG IDs, the
lower-layer topology information is often consolidated.
For example, for the purpose of restoration planning,
a long path of fiber cables that do not bifurcate at
intermediate locations can be aggregated into a single
SRLG ID (i.e., the path does not encounter a splice
location where some fibers are spliced into a different
cable or end at an fiber patch panel in a central
office). Conversely, for a given SRLG ID, we can list
all the IP links that route over that SRLG. Each
SRLG ID is unique within a network routing domain.
For diverse routing and protection purposes, IGP
(interior gateway protocol) routing protocols in both
the standardized specification and in commercial
products would support the assignment of SRLG
information to the network links. IETF RFC 4203 [1]
specifies an SRLG as a sub-TLV (type, length, value)
of the link TLV. The value is an unordered list of
integer SRLG IDs that the link belongs to.
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Low Rate Private
Line Services
Mid Rate Private
Line Services
IP Services
Wideband
DCS Layer
High Rate Private Line
Services
IP Layer
IOS Layer
SONET /SDH
Ring Layer
OTN Layer
Layer with automatic restoration
Layer without automatic restoration
DWDM = Dense Wavelength Division Multiplexing
ROADM = Reconfigurable Optical Add/Drop
Multiplexer
Coherent/
ROADM /Pt-to-pt
DWDM Layer
DCS = Digital Cross-Connect System
IOS = Intelligent Optical Switch
Fiber Layer
OTN = Optical Transport Network
Coherent: More advanced optical network
Figure 1: A Sample of Overlay Networks
Figure 1 shows a typical service provider’s overly
networks. The bottom of the network layers is the
fiber network, different DWDM sub-networks,
including early point-to-point DWDM sub-networks,
large reconfigurable optical add/drop multiplexing
(ROADM)
sub-networks
and
recent
colorless/directionless coherent optical sub-networks.
In order to better use optical network capacity and
support relatively lower-speed private line services,
service providers may build an electronically-switched
optical network layer, including SONET/SDH ring
networks, intelligent optical switching networks, or
optical transport networks. As more and more
services and applications are IP based, the IP network
is necessary for a service provider and it can be built
on top of the electronically-switched optical network
layer and/or on top of the DWDM optical network
layer. A link in the IP network is a circuit in the
optical networks. Of course there are digital crossconnect networks on top of the electronically-switched
optical network to support very low rate services.
Thus for all wired services, the data transmission
must ultimately go down to the fibers. If a fiber is cut,
or an optical device fails, a large number of upperlayer services may go down simultaneously. In order
to support protection/restoration at the upper layer
overlay networks, we need to know which overlay
network links share a common failure risk and which
overlay network links are unlikely to fail at the same
time. This information is provided by associating
SRLG IDs (called bundle IDs in some products) with
the overlay network links.
In the AT&T Intelligent Optical Switch (IOS)
network, the equipment node is the Ciena Core
Director. The nodes are connected by “lines” (SONET
OC-48s or OC-192s) and multiple lines between the
same switch pair are aggregated into “links”. The
Ciena OSRP (Optical Switching and Routing Protocol)
2
protocol defines a list of SRLG bundle IDs for each
OSRP link [2,3]. From the point of view of the Core
Director and its Element Management System, these
bundle IDs are simply integers and constraints
associated with each link, i.e., they have no actual
topological graph model for lower layer networks.
Each bundle ID may represent a portion of the
underlying fiber path or it may represent some other
failure risk (like an intra-office tie cable or some piece
of equipment).
IEFT RFC 4203 does not specify a maximum length
for the list of SRLGs per link, but some commercial
products implement maxima. For example, the Ciena
OSRP routing protocol enforces a maximum list
length for the number of bundle IDs per link [3].
However, in reality, there are links whose SRLGs
exceed the maximum number. As mentioned above, if
each SRLG represents the smallest unit of an
individual fiber span (i.e., cable between two cable
splice locations1), the number of SRLG IDs could
easily exceed 40,000 IDs in a large carrier’s fiber
network. One solution is to combine multiple fiber
spans (without bifurcation) into one super fiber span
to reduce the number of SRLGs.
B
C
D
A
E
G
F
(a) Overlay network
3 B
A
G
1
11
12
16
4
C
2
15
18
10
13
14
19
9
F
5
6
8
24
20
21
7
23
22
D
17
E
(a) Fiber network
Figure 2: Overlay Network with Fiber Spans
Figure 2 shows an example of one simple overlay
1This
fiber span definition is over-simplified to make it clear. A fiber
span can encompass multiple fiber cables and may not have splice
points at its ends. Two cables travelling diversely could converge in
a man-hole cover (no splice) and travel together for some distance
before separating again. In such a case, a single fiber span would
contain both cables for the distance that they travel together. That is,
fiber spans are really defined in terms of physical proximity of
multiple cables.
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network with associated fiber spans. In this example,
link AB routes over span 1, span2, span3; link BC
over span4; link CD routes over span5, span6; link AD
routes over span1, span12, span11, span10, span9,
span8, and span7; link AG routes over span1, span12,
span11, span10, span13, span14, span15, and span16;
link DE routes over span17; link GE routes over
span16, span15, span14, span13, span9, span24,
span23; link EF routes over span20, span21, span22;
link GF routes over span16, span18, span19. There
are a total of 24 spans. If each span is one SRLG,
there are total 24 SRLGs. Both link AD and link GE
have 7 SRLGs and link AG has 8 SRLGs. If we
combine multiple spans into one super span, we could
combine span2, span3 into span2-3; span5, span6 into
span5-6; span10, span11, span12 into span10-11-12;
span 7, span8 into span7-8; span13, span14, span15
into span13-14-15; span18, span19 into span18-19;
and span20, span21, span22 into span20-21-22,
span23, span24 into span23-24. After combination, we
can reduce SRLG number from 24 into 13 without
impacting the information carried by the SRLGs. The
number of SRLGs per link is also reduced, for
example, link AD has only 4 SRLGs, and link AG only
has 4 SRLGs.
SRLG has been proposed as a fundamental concept
for failure management in networks [4]. In general,
SRLG information is maintained manually by the
network operator with the knowledge of the physical
fiber routes of the network. In some cases, SRLG auto
discovery schemes can also be used [5]. SRLG is an
important component in survivable network design.
In order to provide the failure-independent protection
to a customer demand from any single SRLG failure,
a pair of SRLG-disjoint paths should be determined.
That is, the links along service path must not share
any common SRLGs with the links along the
restoration path. According to research work [6],
finding a pair of SRLG disjoint paths is an NP-complete
problem. Thus some network management tools may need to
rely on integer linear programing to find SRLG diverse paths.
During the past decade, almost all of research papers related
on SRLGs are focusing on either restoration capacity planning
[7,8,9], or distributed signaling [10,11], or SRLG diverse path
algorithms [12,13,14], or trap avoidance algorithm [15]. To
the best of our knowledge, we have not seen any research
work focusing on how to optimize SRLG size in a network,
which is our objective of this paper [16].
The paper is organized as follows: In section II, we
describe several SRLG-related functions. Our
objective is to reduce the number of SRLGs without
impacting those SRLG-related functions. Then we
present SRLG optimization algorithms in section III.
Section IV proves the correctness of our algorithm and
discusses why our optimization does not impact the
SRLG-related functions. In section V, we present a
few case studies of implementations
optimization algorithms in network
applications. Section VI is our conclusion.
3
of SRLG
operation
II. SRLG FUNCTIONS
SRLGs were invented for diverse routing, including
protection and restoration. If two paths do not share a
common SRLG, these two paths won’t fail
simultaneously by a single SRLG failure. In practical
network management, we classify SRLG functions
into two classes—completely diverse routing and
maximally diverse routing.
Completely Diverse Routing is useful in a number of
different situations. Following are a few examples. In
a network with IGP supporting SRLG information,
each node has the view of the entire network,
including the list of SRLG IDs in each link. Then each
node or the element management system is able to
provide a Fast Reroute Computation. In link-based
MPLS FRR (multi-protocol label switching fast
reroute), each backup LSP (label switched path) is a
list of links that are diversely routed from a given
link. In node-based FRR, a backup LSP is also a list of
links, but further depends on the next hop of the
primary LSP at each node along its path (the backup
LSP skips the next node). Each node in the LSP is
required to create an SRLG diverse backup LSP to its
next-next hop node except the last two nodes. The
second last node is required to create a SRLG diverse
backup LSP to the last node. During any outage, the
right upstream node detects the outage and it
switches the LSP traffic to the SRLG-diverse backup
LSP immediately. In any single-SRLG outage, such a
scheme provides the fastest recovery to the failed
LSPs. Fully-diverse routing is also used in end-to-end
Protection/Restoration Path Computations. In end-toend protection/restoration schemes, each source node
or the element management system needs to find one
service path and one protection/restoration path for
each demand, and the two end-to-end paths have to be
SRLG diverse. A Diverse Routes Computation may be
necessary when a customer wants to create his own
overlay network by provisioning several mutual
diverse LSPs. Since finding two SRLG diverse paths
is NP-complete problem, the network will use either a
heuristic algorithm or integer linear programming to
find the diverse routing paths. Either way, we do not
want any two paths to share any SRLGs. Fullydiverse routing plays a useful role in Maximum
Restoration Capacity Calculations. To enable rapid
restoration, some intelligent networks pre-calculate
the restoration path for each service path and store
the restoration path in the source node of the service
path [3]. Once an outage occurs, the source node
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detects (or is notified of) the outage and starts the
restoration process using the pre-calculated SRLGdiverse restoration path. To reduce required
restoration capacity, the network is usually designed
to consider only single SRLG outages and thus
restoration paths could share restoration capacity
over non-simultaneous SRLG outages. This is called
shared
mesh
restoration
[10].
Network
management can use the fully-diverse restoration
paths to calculate the maximum restoration capacity
required on each link for any single SRLG failure.
Maximal Diverse Routing is an option when it is
impossible to find completely diverse routing paths for
a demand set in the network. In maximally-diverse
routing, a shared SRLG penalty is minimized, where
each SRLG is associated with some penalty number.
For example, each fiber span has distance and the
distance could be the penalty number when routes
overlap on that span. Each overlapping node may also
be associated with some positive penalty number.
III. OPTIMIZATION ALGORITHMS
One may define any potential outage as one SRLG,
such as a city, a building, a switch or cross-connect
component, a conduit, a fiber span, etc. Although
there are SRLGs in upper layers, such as router
common equipment outages, the most common SRLG
represents the potential outage of some sort of fiber
spans, or spans for simplicity. As mentioned
previously, in this case we need to explore methods to
consolidate the size of the SRLG set for large carriers,
but without affecting the network restoration metrics
or network SRLG functions. In this paper, we mainly
consider how to optimize SRLGs from spans only and
the optimization algorithms provided in this paper
can be easily extended to other type of SRLGs. Our
span-based SRLGs have following properties: (1) each
span is included in only one SRLG; (2) two links
sharing no SRLG are span diverse (not necessarily
node diverse); (3) two links sharing a SRLG are not
span diverse, and they share all of the spans in the
shared SRLG; (4) SRLGs are the smallest possible set
of quantities that fully represent the diversity of
overlay network; (5) spans comprising a given SRLG
are not required to be geographically adjacent, which
is more general than the situation in Figure 2.
The diagram in Figure 3 illustrates the general
relationship among links of an overlay network (such
as the IP layer or intelligent network layer), SRLGs,
and spans. Each circle represents an overlay link and
contains the spans over which it routes. By examining
the areas of overlap, for the purposes of restoration
calculation of this overlay network, we could group
the 16 fiber spans into 7 SRLGs. For example, the
link associated with the red bubble route (the top
4
circle) over SRLGs 1, 2, 3 and 4, the blue route (the
right circle) over SRLGs, 3, 4, 6, and 7, the green
route (the left circle) over SRLGs, 2, 3, 5, and 6.
Figure 3: Example of Grouping Spans into SRLGs
Thus, for network capacity design, we
independently consider the failure of each SRLG and
how rerouting is accomplished. For example, the
outage of SRLG-2 represents an outage of either fiberspan 6 or 10. Next, we will formally describe how to
combine spans2 into SRLGs for a specific network
G(V,E), where V is the set of overlay network nodes
and E is the set of overlay network links (which we
will refer to simply as links). Assume we know the
specific fiber routes of each link. Then each link l has
a list of fiber spans that it routes over, denoted as Fl ,
and each fiber span f has a list of links that route over
it (called the dependent set of links), denoted as Lf.
Now we take a close look at following cases:

The dependent set of fiber span x equals the
dependent set of another fiber span y, Lx = Ly: in
this case, we can combine these two fiber spans
into one single SRLG because if two upper-layer
links are diverse from one fiber span x, the same
two upper-layer links must also be diverse from
another fiber span y. Actually we can combine all
fiber spans with the same dependent set into one
single SRLG. This is exactly what we showed in
Figure 3.

The dependent set of fiber span x is a subset of the
dependent set of fiber span y: if two upper-layer
links do not share span y, the same two upperlayer links must not share span x. For the first
three SRLG related functions in section II of
complete diverse routing, we can drop fiber span x
and keep span y as one SRLG. However for
maximal diverse routing, we need to keep both
2 We use span consolidation as one example. Other failure modes
can be consolidated similarly.
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span x and span y and cannot drop the SRLG
associated with span x. For example, in Figure 3,
for complete diverse routing, we may only need to
maintain SRLG 3 per link and drop all other
SRLGs. However for maximal diverse routing, we
need to maintain all SRLGs for a single link,
shared by two links, and shared by all three links,
i.e., all the 7 SRLGs as well as the SRLG penalty
associated with each SRLG overlap, say the total
fiber span distance of each SRLG.
The above observations can be easily cast into two
algorithms to quickly compute a reduced set of SLRGs
for each network link that are adequate to accurately
perform the complete diverse routing and maximal
diverse routing functions.
Input: network G(V,E) and span route of each link
Output: SRLG info for each link
Algorithm 1: for complete diverse routing
[1] read network
[2] find the list of spans for each link Fl
[3] for each span f, find the list of links over it Lf and
set span[f].remove = 0
[4] for i=0 to n-1, where n is the number of spans
[5]
for j=i+1 to n
[6]
if Li = Lj , mark span[j].removed = 1
[7]
else if Li  Lj , mark span[i].removed = 1
[8]
else if Lj  Li , mark span[j].removed = 1
[9]
end j
[10] end i
[11] for each span f, associate SRLG[sf] with span f if
span[f].remove = 0
[12] for each link l, define reduced SRLG set Sl = 
[13] for each span f in Fl
[14]
if span[f].remove = 0, add SRLG[sf] to Sl
[15] end for each span f
[16] end for each link l
[17] for each link l, report SRLG info of Sl.
Algorithm 2: for maximal diverse routing
[1] read network
[2] find the list of spans for each link Fl
[3] for each span f, find the list of links over it Lf,
record span[f].length, and set span[f].remove = 0
[4] for i=0 to n-1, where n is the number of spans
[5] for j=i+1 to n
if Li = Lj , mark span[j].removed = 1, and
record span[i].length += span[j].length
[6] end j
[7] end i
[8] for each span f, associate SRLG[sf] with span f if
span[f].remove = 0 and set SRLG[sf].penalty =
span[f].length
[9] for each link l, define reduced SRLG set Sl = 
[10] for each span f in Fl
5
[11]
if span[f].remove = 0, add SRLG[sf] to Sl
[12] end for each span f
[13] end for each link l
[14] for each link l, report SRLG info of Sl.
IV. ALGORITHM CORRECTNESS
In above SRLG optimization algorithm 1, we mark
following two types of fiber spans as removed: (1) if
two spans have exactly the same set of dependent
links, we combine them together and leave only one
span to represent them. Basically the network
separates the set of spans into different groups based
on their dependent links. Each group is assigned one
SRLG ID. This idea is illustrated in Figure 2 and
Figure 3; (2) if the dependent links of one group is a
subset of the dependent links of another group, we
can drop the first group and only keep the second
group. The reason is that when two upper-layer links
are diverse, they must be diverse on all SRLG groups,
i.e., they do not share any common SRLG. If two
upper-layer links are diverse on the second group,
they must be diverse on the first group. So we are safe
to drop the first group for completely-diverse routing.
In Figure 3, SRLG1 only supports link1, SRLG2
supports both link1 and link2, while SRLG3 supports
link1, link2, and link3. In this case, we can drop
SRLG1 and SRLG2, and only keep SRLG3. Similarly
we can also drop SRLG4, SRLG5, SRLG6, and SRLG7
without losing essential information about link
diversity. Thus it is easy to verify the algorithm
correctness for completely-diverse routing functions
listed in section II.
Next we look at the maximal restoration capacity
calculation function. In a shared mesh restoration
scheme [4], we define a matrix called failneed[s][l],
where s is the SRLG index and l is the link index.
Matrix failneed[s][l] maintains the restoration
capacity needed in link l if SRLG s fails. The maximal
restoration capacity is defined as: R[l] = maxs
failneed[s][l] over all SRLGs. For any two SRLG s1
and s2, assume Ls1  Ls2. If we can prove
failneed[s1][l]  failneed[s2][l] for any link l, then we
are safe to drop SRLG s1 without impacting the
calculation of R[l].
For any outage s and a set of paths P, we define Ps
=  {pP, p∩Ls ≠}, i.e., the subset of paths routed
over SRLG s. For any path pP, we define Kp as the
set of links of p, Cp as the capacity of p, and p* as the
pre-calculated completely-diverse restoration path of
p. We further define Vs = {l: lp*, pPs}. Then we
have failneed[s][l]= p Ps, l p* Cp, i.e, failneed[s][l] is
the sum of the bandwidth of all demands failed by s
whose restoration paths use link l. Assume Ls1  Ls2,
then for any p Ps1, p∩Ls1 ≠, we have p∩Ls2 ≠. So we
have Ps1  Ps2. Thus for any l Vs1, we have
failneed[s1][l] failneed[s2][l]. For any lVs1,
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failneed[s1]l]=0. So for any link l, we have
failneed[s1][l]  failneed[s2][l] when Ls1  Ls2.
According to the definition of maximal restoration
capacity calculation formula, we can drop SRLG s1
without impacting the maximal restoration capacity
calculation for any link.
Algorithm 1 can be used for completely-diverse
routing, but cannot be used for maximally-diverse
routing. For example, if we apply algorithm 1 to the
network of Figure 2, we have link AB with SRLG1;
link BC with SRLG4; link CD with SRLG5; link AD
with SRLG1, SRLG9; link AG with SRLG1, SRLG16;
link GE with SRLG16, SRLG9; link DE with SRLG17;
link GF with SRLG16; and link EF with SRLG20. If
we need to find two maximally-diverse routes between
A and G, the solution could be AG, and AD-DE-GE,
which only share common SRLG1 and SRLG16. In
fact, these two routes also share SRLG10-11-12 and
SRLG13-14-15. These two routes are not maximal
diverse routes. On the other hand, if we apply
algorithm 2 to the same network, we only combine
spans with same set of dependent set and accumulate
the combined distance. In this way, we do not lose any
failure related information. The maximal diverse
routes between A and G will be route AG and route
AB-BC-CD-DE-EF-FG, which only share common
SRLG1 and SRLG16.
V. CASE STUDIES
We have used the SRLG optimization processes
described above in many of AT&T’s internal
management tools. In this case study section, we
describe a DWDM network planning tool with
completely-diverse routing and maximally-diverse
routing using SRLG optimization. We also describe a
process based on SRLG optimization to drop extra
SRLGs due to protocol limitations for the intelligent
optical network.
AT&T’s DWDM layer is a highly heterogeneous
network, including DWDM systems from early pointto-point DWDM systems to recent reconfigurable
optical add/drop multiplexing (ROADM) systems;
from 2.5Gbps per wavelength systems to 40Gbps per
wavelength systems; from optical switching to
electronic switching; from wavelength routing to demultiplexing sub-channel routing; et al. In order to
help planners make cost-effective service provisioning
decisions, we built a web-based interactive tool with
integrated optimization algorithms to route high
speed circuits over heterogeneous networks through a
graphic user interface. The tool provides visualization
of the heterogeneous DWDM network and the status
of its network links (sub-network type, distance,
maximum and available wavelengths number,
available 10G and 40G channels, deployment date,
6
and other related information). It can show the whole
network or any particular set of sub-networks filtered
with the sub-network types and/or the supported
maximum speeds. It can also predict and highlight
hot-spot links, i.e., DWDM links that will be
exhausted by a specified future date based on
historical circuit requests and wavelength usage
information.
The tool also has the capability to generate fullydiverse and maximally-diverse routes for groups of
circuits. Since finding two SRLG-diverse routes is NPhard [6], we developed a built-in integer linear
programming (ILP) model to find multiple diverse
routes, including complete diverse routing and
maximal diverse routing. In order to speed up the ILP
processing time, we implemented algorithm 1 for
complete diverse routing and algorithm 2 for maximal
diverse routing. Working experience shows that our
optimized ILP can complete most task computations
within 1 minute and provide visualization of several
candidate paths for interactive navigation. The tool
has been deployed since 2009 and is actively used by
network planners for their daily circuit planning
tasks.
AT&T has a large intelligent optical switched
network [3]. The Ciena Core Director defines a list of
SRLG IDs (known as bundles in Ciena’s terminology)
for each link with a limited maximal number of
bundle IDs. This is typically less than the number of
fiber spans needed to describe the link's diversity. If
the number of real bundle IDs is larger than the
maximal number, the list of bundle IDs must be
truncated. In this case, which bundle ID should be
dropped becomes a critical question and bundle ID
optimization is required. Bundles are re-computed
periodically to keep current with ongoing changes in
the network link and fiber span data. When changes
are required, an attempt is made to minimize changes
to existing bundle IDs.
In the intelligent optical switched network, we are
required to provide both complete diverse routing and
maximal diverse routing if complete diverse routing is
unavailable. Thus we cannot completely drop bundles
consisting of subset link groups (group i if Li  Lj).
Instead we use a numeric number to measure the
importance of each bundle related to the nature of the
link overlaps and its total mileage. We consider three
factors for each bundle: (1) the mileage associated
with the bundle; (2) the number of simultaneous
outages that the bundle represents; (3) whether the
links failed by the bundle are a subset of those failed
by another bundle. When the number of bundles
exceeds the maximum allowed, bundles having lesser
importance are dropped until the desired number is
achieved. The following formula is used to rank the
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importance of every bundle based on the above three
criteria:
Importance(bundle) =
1000000*isSuperSet(bundle) +
10000*linkCount(bundle) + mileage(bundle)
where isSuperSet(bundle) = 1 if the links failed by the
bundle are not a sub-set of any other bundle failure; it
is zero otherwise. linkCount(bundle) = number of
links simultaneously failed by the bundle,
mileage(bundle) = the sum of the mileages of the fiber
spans in the bundle. This solution basically combines
algorithm 1 and algorithm 2 together and provides
similar results to complete diverse routing and
maximal diverse routing.
VI. CONCLUSION
In this paper, we studied the SRLG optimization
issue in detail. After considering the relationship and
importance of individual SRLGs, we proposed
algorithms on how to reduce the number of SRLGs for
completely-diverse routing and maximally-diverse
routing. These SRLG-reduction strategies are useful
for improving the performance of network planning
tools that depend on SRLG information and in
selecting which SRLGs are most important when
network management systems impose a limit.
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2012, WH2.
Guangzhi Li (M’00) is a
principle member of technical
staff researcher at AT&T labsresearch. He got his PhD and
MS in computer science from the
College of William and Mary.
His research interests include
IP-based control plane for
optical networks, restoration
schemes
and
algorithms,
network
simulation
and
performance evaluation as well
as network related applications.
He holds about 30 patents and has published more than 90
research papers in journals and conferences.
Dongmei
Wang
(M’00)
received her PhD in physics
from the college of William and
Mary
and
joined
AT&T
research lab at 2000. During
the past 12 years, she has
worked on areas of network
design
and
optimization.
Recently she is leading an
important
IPAG
network
design for mobility access. She
holds about 20 patents and has
published about 60 journal and conference papers.
Timothy Gallivan is a Senior
Network Engineer at AT&T. He
obtained a PhD in physics from the
University of Texas, Austin TX
USA (1989) and an MS in
Electrical Engineering from the
Georgia Institute of Technology,
Atlanta GA USA (1984).
Robert
Doverspike
received
his
undergraduate
degree from the University of
Colorado and Masters and Ph.D.
degrees
from
Rensselaer
Polytechnic Institute (RPI). He
began his career with Bell Labs
and, upon divestiture of the Bell
System, went to Bellcore (now
Telcordia). Later, he returned to
AT&T Labs (Research) where he
is now Executive Director of
Network Evolution Research. Dr. Doverspike has made extensive
contributions to the field of optimization of multi-layered
transmission and switching networks and pioneered the concept of
packet transport in metro and long distance networks. He also
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pioneered work in spearheading the deployment of new
architectures for transport and IP networks, network restoration,
and integrated network management of IP-over-optical-layer
networks. He has over 1500 citations to his books and articles over
diverse areas/publications such as Telecommunications, Optical
Networking, Mathematical Programming, IEEE Magazine, IEEE
Communications Society, Operations Research, Applied Probability,
and Network Management. Dr. Doverspike holds many professional
leadership positions and awards, such as INFORMS Fellow, IEEE
Fellow, member of Optical Society of America (OSA), co-founder of
the INFORMS Technical Section on Telecommunications, OFC
(Optical Fiber Communications) Network Technologies and
Applications
Committee,
DRCN
(Design
of
Reliable
Communications Networks) Steering Committee, and Associate
Editor of the Journal of Heuristics.
8