COMPARISON OF ALTERNATIVE SERVICE PROTECTION MECHANISMS
FOR WIDE AREA ATM NETWORKS
Satoru Ohta and Kazutoshi Nunokawa
NTT Optical Network Systems Laboratories
1-2356 Take, Yokosuka-shi 238-03, Japan
This paper compares service protection methods that are applicable to ATM networks.
The considered methods include one digital section level protection scheme and two
virtual-path level protection schemes. These methods are evaluated based on
network cost and virtual path unavailability. The computational procedures of theseparameters are presented. The evaluation results confirm that the virtual path levelprotection schemes are more advantageous than the digital section level protection
scheme in network cost and unavailability.
clarify, through numerical analysis, which
1 . INTRODUCTION
protection method achieves the mosteconomical and reliable ATM network. The
Some ten megabytes of user information may
employed measures are virtual pathbe transmitted within one second through a
unavailability and network cost for 100%
broadband telecommunication network using
restoration against single cable duct failure.
Asynchronous Transfer Mode (ATM). Thus, a
The results confirm the advantages of thesignificant amount of information will be lost if
virtual path level protection schemes.
even one failure occurs. This fact explains
why network survivability has become an
This paper is organized as follows. Section 2
important subject of study for ATM [1]-[3].
explores the protection methods to becompared. In Section 3, the assumption and
Several protection mechanisms have been
methods of computing network cost arestudied to establish survivable ATM networks
described. The method for deriving virtual[1], [4]. These studies proposed protection
path unavailability is described in Section 4.
schemes in which failed connections are
Finally in Section 5, the evaluation results arechanged over at the ATM virtual path level by
presented.
utilizing the independence of virtual pathbandwidth and routing. These virtual path
2 . PROTECTION METHODS
level protection mechanisms are further
categorized depending on whether or not
As specified in international standards [8],standby capacity is shared among multiple
ATM networks are configured in a hierarchicalvirtual paths [1].
manner and are composed of several logicalresource levels. Namely, the virtual channel
Although virtual path level protection is now
level is supported by the virtual path level, andavailable, failures can also be restored in a
the virtual path level is supported by the digital
lower level such as in the digital section layer.
section level. Furthermore, a digital section,
For example, the prototype cross-connect
the SDH transmission path, is set up in a fiber
system reported in Ref. [5] has a digital section
network, and the fiber is set up in a cable ductlevel change-over function. This function can
network.
be used to restore services when a failure
occurs in a duct or cable if a standby section isIf a failure occurs in a physical resource such
set in a different route from the working
as a fiber or a transmission line interface board
section route. Several conventional SDHof a cross-connect system, the affected(Synchronous Digital Hierarchy) level selfconnections must be changed over to standby
healing techniques [6] are also applicable toroutes to restore the services.
For the
ATM networks.
hierarchical configuration described above, thisprotection can be executed in any of the virtual
The problem is finding the optimal protection
channels, virtual paths, and digital section
method for ATM networks. Although related
levels. Since the reliability and cost of thestudies have been conducted for STM
network depends on which level is selected for
networks [7], there has been no investigation
protection, it is necessary to compare theinto the most recent ATM-based restoration
performance of alternative protection levels andschemes. The purpose of this paper is toselect the most appropriate method. This paper

compares one digital section level protection
method and two virtual path level protection
methods, which are considered to beestablished in present ATM technology.
The digital section level protection [5] isdenoted DS-APS (Digital Section AutomaticProtection Switch) in this paper. With DSAPS, every working digital section is protectedby a standby digital section as shown in
Fig. 1 (a).
The standby section route is
disjoint from the working section route. Thus,
communication is restored by changing over
the affected section due to a failure in a fiber
cable or cable duct.
Virtual path level protection is further
categorized depending on whether or not thespare capacity in a digital section is shared
among several virtual paths. According to theterminology in Ref. [1], the method withwhich spare capacity is shared is denoted as
VP-SH (Virtual Path Self-Healing) while themethod with which spare capacity is not shared
is denoted as VP-APS (Virtual Path AutomaticProtection Switch).
Transmission Link Interface
Working Digital Section

Standby Digital SectionNode (Crossconnect System)

Node (Crossconnect System)(a) DS-APS
Working Virtual Path Route

End Node

End Node

Transit Node

Transit NodeStandby Virtual Path Route(b) VP-APS and VP-SH
Figure 1. Considered protection methods.

With both VP-APS and VP-SH, two disjointroutes are provided for a virtual path: one is aworking route used for transmitting
information in a normal state and the other is astandby route used when a failure occurs in theworking route. This pre-defined standby route

scheme is explored in Fig. 1 (b). With VPAPS, the capacity of a digital section must belarger than the sum of the total working route
bandwidths and the total standby route
bandwidths in order to restore services from a
failure. Meanwhile, less capacity is needed for
VP-SH because the capacity reserved for astandby route is shared by virtual paths.
These three protection schemes are evaluatedthrough network cost and virtual pathreliability. These measures are computed for a
network that can restore any single failure in a
cable duct. The following sections describe
how these measures are obtained for eachmethod as well as the results of the evaluation.
3 . COST COMPUTATION
Network cost is evaluated by assessing theamount of physical resources such as thenumber of fiber systems or cross-connect
systems for each protection method. This
amount of resources is determined through a
network design, which determines thefollowing items so as to achieve a lessexpensive network for a given virtual pathdemand.
1. Appropriate virtual path routes in thetransmission path network;
2. Appropriate transmission path routes infiber systems; and3. Appropriate fiber system routes in the cableduct network.
For a fair comparison of protection methods,
the network should be optimized for eachmethod. However, the optimization of such ahierarchical network is a very difficult
problem, and the problem itself is a subject ofcomprehensive study.
Thus, this paper
assumes a trivial and primary design scheme
for each protection method.
Despite its
simplicity, this scheme is useful in clarifying
the characteristics of the protection methods.
This paper supposes that a fixed cable duct
network is given first. A cross-connect system
is placed at every node in the cable duct
network. For DS-APS, a working fiber isrouted between two adjacent nodes in the cableduct network directly through the cable duct
between them. The standby fiber between
these nodes is set through an alternative route,which is composed of several cable ducts
different from the duct supporting the working
fiber.
This configuration is shown in
Fig. 2 (a).
For VP-APS and VP-SH, aworking fiber is set in the same way as for DS-

APS. However, standby fibers are not usedbecause high reliability is supported by virtual
path level mechanisms.
The fiber
configuration for these methods is illustrated inFig. 2 (b).
Working Fiber Route

Working Fiber RouteStandby Fiber Route(a) DS-APS(b) VP-APS and VP-SH

Figure 2. Fiber configuration.

For simplicity, it is assumed that everytransmission path (or digital section) is
supported by one fiber system, that is, a digital
section is not constructed by multiple fiber
sections nor is it cross-connected at any node.
The route of a virtual path is determined byapplying either the shortest path algorithm or
the shortest disjoint path algorithm to eachsource-sink pair specified in a given virtual
path demand. Since no standby virtual pathroute is necessary for DS-APS, the route iscomputed by the shortest path algorithm for agiven source-sink pair. The distance between
adjacent nodes is set to the physical length of
the working fiber. For VP-APS and VP-SH,
it is necessary to find a standby virtual pathroute for a given sink and source. The standby
route must be node-disjoint from the working
virtual path route. Such a route is found by theshortest disjoint path algorithm [9].
Through these route assignment rules, virtual
path bandwidths are determined for each digital
section. The bandwidths then determine therequired number of SDH transmission paths
and thus determine the number of fibers with agiven capacity for the digital section. For DSAPS, the number of fibers is set sufficiently
large to support the total bandwidth of virtual
paths routed in the section. The number ofstandby route fibers is set equal to that ofworking route fibers.
For VP-APS, thenumber of required fibers is determined by the
total bandwidth of virtual paths, working or
standby routes which pass through the section.

The bandwidth reserved for a section cannot bederived as a simple sum of the virtual pathbandwidths for VP-SH. This is because thebandwidth portion reserved for standby isshared among multiple virtual paths. The
standby bandwidth for each digital section isdetermined such that a virtual path would berestored against any single cable duct failure
and any single transit node failure. The
procedure for deriving the standby bandwidthfor digital section i , denoted by Bstdby,i , is:[Procedure for VP-SH standby bandwidth]
1. for each section i do Bstdby,i := 0;
2. for each virtual path VPj do
3. for each resource x on the working route ofVPj do4.
for each section k on the standby routeof VPj do
begin
5.
Bk := the bandwidth needed for
section k to restore all paths passingx;
6.
if Bstdby, k < Bk then Bstdby, k := Bk ;end
Using these assumptions on a design scheme,
the amount of required physical resources isdetermined for a given virtual path demand anda given cable duct network. Then, network
cost is derived from the amount and unit priceof each physical resource.
In this paper, the network cost depends on thefollowing physical network resources.
• fiber• repeater• transmission line interface of a cross-connect
system• common part of a cross-connect system
The cost of a fiber is proportional to its total
length. The repeater cost is determined by the
number of used repeaters. Since a repeater
must be inserted at every specified distance,
the number of repeaters is derived from thesection distance and the number of fibers. The
transmission line interface cost of a crossconnect system is proportional to the numberof terminated fibers, while the cost of thecommon part is fixed and does not depend on
the number of fibers. The network cost iscomputed by giving unit prices for the above
resources.

4 . RELIABILITY COMPUTATIONThe unavailability of a specified virtual path isemployed as a measure to evaluate network
reliability achieved by each protection method.
The virtual path unavailability is defined as theprobability that the path is not useable forservice. Its value is determined by those of thephysical resources associated with its working
and standby routes. The physical resource
unavailability is associated with the following
failures.
1) cable duct distraction2) failure of a transmission line interface of across-connect system
3) failure of the common part of a crossconnect system
The cable duct distraction cuts off all fibers andthus virtual path routes included in the duct. A
line interface failure occurs independently in across-connect system and affects only thevirtual paths included in the failed interface.
Meanwhile, a common-part failure damages all
the virtual paths passing through and that are
terminated at a cross-connect system. It is
assumed that unavailability values of atransmission line interface, the cross-connect
common part, and a cable duct are given.
When DS-APS is applied, a digital section isprotected by the standby route. Thus, theunavailability of digital section i , Usctn,i , isdescribed as
Usctn,i = Usctn _ wkng,i ⋅ Usctn _ stdby,i ,where Usctn _ wkng,i and Usctn _ stdby,i are theunavailability of the working route and standby
route. The unavailability of each section routeis determined by those of the cable duct and theline interfaces.
Then, the unavailability of a virtual path that
employs DS-APS, denoted by U DS − APS , is
U DS − APS =∑ Usctn,i + ∑ Ucmmn, j ,sec tion i on VP

node j on VP

where Ucmmn, j is the unavailability of thecommon part of a cross-connect system.
Meanwhile, for VP-APS, the virtual path
unavailability, UVP − APS is expressed asfollows, considering that a virtual path isprotected on the end-to-end basis.
UVP − APS = Uwkng _ rt ⋅ Ustdby _ rt + ∑ Ucmmn, j ,end node jwhere Uwkng _ rt and Ustdby _ rt are theunavailability of the working and standby
virtual path routes. The former is defined as,
Uwkng _ rt = ∑ Usctn,i +∑ Ucmmn, j .
sec tion ion routetransit node jon route

Similarly, Ustdby _ rt is defined.
It is more complex to derive the virtual path
unavailability for VP-SH.
There are thefollowing cases for which a virtual path
becomes unavailable with VP-SH.
Case 1) A failure occurs at one of the crossconnect systems that terminates the virtual
path.
Case 2) Resources included in the working
and standby routes fail simultaneously.
Case 3) Failures occur simultaneously in theworking route and in a resource not included in
the working nor standby routes, and there isinsufficient capacity to restore both failures.
The unavailability brought about by Case 1 andCase 2 is calculated in the same way for VPAPS. This means that the unavailability is no
better for VP-SH than for VP-APS. When
Case 3 occurs, an affected virtual path may or
may not be restored depending on the sparecapacity on a link of the standby route and theorder in which the paths are restored.
Therefore, it is difficult to compute the exact
unavailability for Case 3. Nevertheless, it ispossible to obtain the upper bound for theunavailability by assuming that a virtual pathalways becomes unavailable for Case 3. This
upper bound will give a safe-side evaluation
for the reliability achieved by VP-SH. The
algorithm that computes the upper bound is:[Procedure for VP-SH unavailability]
1. UVP − SH := Uwkng _ rt ⋅ Ustdby _ rt + ∑ Ucmmn, j ;end node j

2. for each resource x included in theworking virtual path route do
3. for each resource y not included in theworking route nor in the standby route do
4.
if there is not enough spare capacity in alink on the standby route forsimultaneous failures in x and y thenbegin
5.
U x := the unavailability of x;
6.
U y := the unavailability of y;7.
UVP − SH := UVP − SH + U x ⋅ U y ;end5 . EVALUATIONThis section shows the results of network costand virtual path unavailability. Figure 3 shows
the cable duct network model used in theevaluation.
This model is a roughapproximation of the long distance network inJapan.
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A

B

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C

D

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E

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F





G

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



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DS-APS


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

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Figures 4, 5, and 6 show the network cost for
demand patterns A, B, and C. In thesefigures, the portions of link and node cost arealso explored. The figures show that DS-APS
yields the largest network cost for any demand
pattern. Partcularly, link cost is much higher
for DS-APS than VP-APS. Since link cost is adominant factor in the total network cost, VPAPS is a more economical alternative to DSAPS.








J

K


L


M


N

O

P



Q



VP-APS

Link



!

"

Figure 3. Network model.

VP-SH
0

For the cable duct network, the following
types of virtual path demand were given.
Pattern A: The same number of virtual paths
with the same bandwidth were set between
every source and destination pair. That is, thetraffic is uniform for any node pair.
Pattern B: Traffic is concentrated on two nodes
in the network. Let Ni , j be the number ofvirtual paths set between node i and node j .
Then,
A Wi W j
Ni, j =∑ Wk Wlk, l,

Ni, j =

A 1/ L i, j

∑ 1/ Lk, l α
k, l

20
30
10
Network Cost (Relative Value)40#

Figure 4. Network cost for demand pattern A.
DS-APS
VP-APS

Link
!

"

VP-SH
0

Node
10
20
Network Cost (Relative Value)30

Figure 5. Network cost for demand pattern B.

where Wi is the weight assigned for node i( Wi > 0 ) , A is a constant, and • is thelargest integer not greater than •.
The
bandwidth is set constant for every virtual
path.
Pattern C: Traffic is larger for closer nodes.
For this pattern, the number of virtual paths isα

NodeDS-APS
VP-APS

Link
"

Node
VP-SH
0

10
20
Network Cost (Relative Value)30

Figure 6. Network cost for demand pattern C.

,

where Li , j is a distance between node i andnode j . The bandwidth is set constant foreach virtual path.
For all demand patterns, the bandwidth of avirtual path was set at 10 Mb/s and the total
virtual path bandwidth is set to nearly 10 Gb/s.
The fiber capacity was selected to be 600
Mb/s. For demand pattern B, Wi was set at 7
for nodes K and N, and 1 for the other nodes.

It is expected that VP-SH has a cost advantage
because spare capacity is shared by multiple
standby virtual path routes. However, thefigures show that the advantage of VP-SH
depends on the demand patterns. For pattern
C, the cost for VP-SH is 12.3% lower thanthat for VP-APS.
Thus, network costdecreases considerably with VP-SH for thisdemand pattern. Meanwhile, the cost is only
5.5% lower for VP-SH than for VP-APS forpattern B. This means that VP-SH is not very
advantageous for this demand pattern.
Figure 7 compares the virtual path
unavailability between nodes A and N in the
network model for each protection method.
The employed demand pattern is C, which
affects the performance by VP-SH. The figure
shows that DS-APS is less reliable than theothers. This is because DS-APS is noteffective for a transit node failure. Thus, theunavailability of transit nodes are dominant for
the total virtual path unavailability. Therefore,
the reliability achieved by DS-APS becomesworse than those achieved by VP-APS and
VP-SH which are effective for transit node
failures. For VP-APS and VP-SH, the path
unavailability almost equals the total
unavailability of both end nodes because
duplicated routes bring high reliability to otherportions of a path. Thus, there is only a slightdifference between these.

operation at a repair of a failure are needed for
VP-SH.
Considering all the above points, it isconcluded that VP-APS is the best choice forreal ATM networks if the demand is uncertain.
Only if the demand is found to be suited to VPSH, and only if all the technical difficulties are
removed in the viewpoint of implementation,
network design, and operations, then VP-SH
should be introduced into ATM networks.
6 . CONCLUSION
This paper compared three protection methods
DS-APS, VP-APS, and VP-SH from theviewpoints of network cost and virtual path
unavailability. The results show that VP-APS
and VP-SH are more advantageous than DSAPS. The results also support the conclusionthat VP-APS is more practical because theadvantage of VP-SH is uncertain.
ACKNOWLEDGMENTS
The authors wish to thank Dr. Ikuo Tokizawa
and Dr. Toshinori Tsuboi.

DS-APS
VP-APS
VP-SH
%

0

1
$

2

Unavailability (Relative Value)&

Figure 7. Virtual path unavailability for eachprotection method.

From the above results, it is concluded that
DS-APS is not as beneficial as the others interms of cost and reliability. VP-SH yields alower network cost than does VP-APS whileproviding as high a level of reliability as VPAPS. However, the superiority of VP-SH is
still inconclusive. The reasons for this are:1) The cost advantage of VP-SH depends onthe demand pattern. The above results showthat VP-SH is less effective for a concentrated
demand pattern. Unfortunately, the demand in
the real world is expected to be similar to thispattern: traffic will be centered on a few large
cities. Thus, the cost advantage of VP-SH is
uncertain.
2) It is more difficult to implement and operate
VP-SH than VP-APS. For example, VP-SH
requires a mechanism that checks the sparecapacity at change over. This mechanism is
unnecessary in VP-APS. In addition, more
complicated network design and change back

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