Capacity Design Studies of Span-Restorable
Mesh Transport Networks With Shared-Risk
Link Group (SRLG) Effects
John Doucette, Wayne D. Grover
[email protected], [email protected]
TRLabs and University of Alberta
Edmonton, AB, Canada
OptiComm 2002
Boston, MA, USA
30/July/2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Outline
• What is a Shared-Risk Link Group (SRLG)?
• Research Questions
• Experimental Approach
• Design Formulation
• Results
–
–
–
–
Cost of Protecting Against SLRGs
Co-Incident SRLG Effects on Restorability
Effects of Co-Incident SRLG Location
Identifying Troublesome SRLGs
• Concluding Remarks
2
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
What is a Shared-Risk Link Group (SRLG)?
• Here a Shared-Risk Link Group (SRLG) is a set of spans with
a common cause of failure.
• A Co-Incident SRLG is an SRLG involving multiple (nominally
disjoint) spans, incident upon a common node.
– Most common type of multi-span SRLG.
Span A-B
Span A-Z
B
A
Z
A
Co-Incident
SRLG
Example
C
Span A-C
3
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Research Questions
• What impacts do SRLGs have on survivable network capacity
requirements?
– How costly is it to protect against SRLGs?
• How do SRLGs affect a network’s restorability?
• Which types of SRLGs are most costly to protect against?
• How many SRLGs can be sustained before the capacity penalty
becomes too severe?
• Can we identify which SRLGs are most troublesome?
– When is it worthwhile to take physical measures to eliminate an
SRLG?
4
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Experimental Approach
• Define new design model:
– Spare capacity design for 100% restorability in the face of all
single span failures and specified SRLGs.
• Test with varying numbers and position of random
co-incident SRLGs.
– What is the cost of spare capacity for full restorability?
– Test network has 40 nodes, 70 spans, 780 O-D demands of
1-10 wavelength paths each.
• Analyze and rank individual co-incident SRLG impact
based on nodal degree and location.
– Which individual SRLGs cost more to make fully restorable?
5
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
SRLG-Tolerant Design Formulation
Minimize:
Total Spare Capacity
Subject To:
(1) Restoration flow for all single-span failure
scenarios
(2) Restoration flows for all specified SRLG scenarios
(3) No restoration flow on co-failed spans in SRLG
scenarios
(4) Spare capacity to support (1) and (2)
6
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Cost of Protecting Against SLRGs
250%
Maxim um
Co-Incident SRLG
Test Case
Spare Capacity Cost
(relative to single failure only)
200%
Average of
Dual Failure
Test Cases
It costs nearly as
much to make a
given percentage of
co-incident SRLGs
restorable as it does
to make the same
percentage of all
possible dual span
failures restorable.
150%
100%
Minim um
Co-Incident SRLG
Test Case
50%
Average of
Co-Incident SRLG
Test Cases
0%
0
10
20
30
40
50
60
70
80
90
100
Percentage of Possible SRLGs in Existence
7
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Co-Incident SRLG Effects on Restorability
Making a given percentage of all co-incident SRLGs
restorable provides restorability/availability nearly
as good as making the same percentage of all dual
span pairs restorable.
1
0.95
0.95
0.9
0.9
Co-Incident SRLGs
Average Dual Failure Restorability if
Netw ork Designed for Specified % of
Co-Incident SRLGs (use left axis)
0.85
0.85
Random Dual
Failure SRLGs
0.8
Average Dual Failure Restorability if
Netw ork Designed for Specified % of
Dual Failures (use left axis)
0.75
0.8
0.75
Percentage of Dual Failures Restorable
if Netw ork Designed for Specified %
of Co-Incident SRLGs (use right axis)
% Fully restorable
dual failures
0.7
0.7
0.65
0
10
20
30
40
50
60
70
80
90
0.65
100
Percentage of Possible SRLGs or Dual Failures in Existence
8
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Percentage of Restorable Dual Failures .
Making even a
small number of
co-incident
SRLGs restorable
will make a much
greater number
of dual failures
fully restorable.
Network-Average R 2 (a,b )
1
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Classifying Node Locations
“edge”
“near-edge”
9
“interior”
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Effects of Co-Incident SRLG Location
50.0%
Edge
45.0%
Edge Average
40.0%
Near-Edge
Co-incident SRLGs at
nodes in the interior of
the network are more
costly than those
closer to the edge.
Spare Capacity Cost
(relative to single failure only)
Near-Edge Average
35.0%
Interior
Interior Average
30.0%
25.0%
20.0%
15.0%
10.0%
5.0%
0.0%
0
1
2
3
4
5
Num ber of Co-Incident SRLGs
10
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Effects of Co-Incident SRLG Location (2)
Co-incident SRLGs at
degree-3 nodes are
more costly than those
at degree-4 nodes.
50.0%
45.0%
D3
Spare Capacity Cost
(relative to single failure only)
40.0%
D3 Average
D4
35.0%
D4 Average
30.0%
25.0%
Degree-3
20.0%
15.0%
Degree-4
10.0%
5.0%
0.0%
0
1
2
3
4
5
Num ber of Co-Incident SRLGs
11
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Identifying The Most Troublesome SRLGs
Test
Normalized
SRLGs
Cost
Case # SRLGs
Cost
Rank Removed Improvement
A random set of 18
co-incident SRLGs
and evaluated
their costs as a set
and individually.
Remove the most
costly SRLGs and
re-solve the
capacity design
with remaining
SRLGs.
0
1
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
2
3
5
5
0
18
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
17
16
15
14
0.00%
45.93%
0.00%
0.64%
0.82%
8.82%
0.04%
2.26%
5.46%
1.88%
1.88%
1.81%
2.65%
13.36%
0.68%
0.35%
2.77%
4.05%
11.09%
0.00%
35.87%
32.45%
25.59%
20.45%
12
/
/
17
14
12
3
16
8
4
10
9
11
7
1
13
15
6
5
2
18
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
L
L, Q
L, Q, D
L, Q, D, G
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
21.9%
29.3%
44.3%
55.5%
Removing the
4 worst ranked
SRLGs gives a
55% reduction
in capacity
penalty.
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002
Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects
Concluding Remarks
• Co-incident SRLGs are more troublesome than typical dual span
failures.
– Believed to be most common type of multi-span SRLG.
– Requires more spare capacity on average than other SRLGs or dual
failures.
– Dual failure restorability (hence availability) responds best to
investment for coverage of the co-incident SRLGs.
• Co-incident SRLG effects are generally more troublesome
(require more capacity) if located at degree-3 nodes in the
network interior.
• Identification and removal of the individually most troublesome
co-incident SRLGs gets most of the benefit of removal of all
SRLGs.
13
John Doucette and Wayne D. Grover
OptiComm 2002 - Boston, MA, July 2002