A cost evaluation of service protection strategies
in ATM on SDH transport networks
Michaël Gryseels*(1), Satoru Ohta (2), Roberto Clemente (3) and Piet Demeester (1)(1) University of Gent, Dept. of Information Technology (INTEC), BELGIUM.
(2) NTT, Optical Network Systems Laboratories, JAPAN.
(3) CSELT, Dept. of Network Planning and Management, ITALY.
Abstract. Future multi-layer transport networks will involve recovery mechanisms in
different layers in the overall protection of services. In this paper, we study several multilayer recovery approaches to assure the resilience of ATM VP traffic in SDH on ATM
transport networks against a set of important failures. Our study is supported by the resultsof a network planning case study, where we compared the investment cost of the multi-layer
survivability strategies, in diverse network and traffic scenarios.

1. Introduction
Because of the ever-increasing amount of traffic carried by the network elements in transport networks,
the impact on society of communication services lost by a failure can be tremendous; survivability has thus
become a very important aspect of telecommunications network design. During the past years, several recovery
schemes [8] have been devised for ATM and SDH technologies, allowing fast protection against malfunctionsof links and nodes. Since future multi-layer transport networks will be based on SDH and ATM technologies,
operators will have to choose an appropriate combination of ATM and SDH recovery schemes, in order toprovide adequate resilience to different service types. This decision will be based on the recovery performanceand the implementation complexity, but most of all, the cost of a particular protection strategy.
The principal objective of this paper is to evaluate different multi-layer recovery approaches in ATM on
SDH transport networks in terms of investment cost. More in particular, we compare the following two options
to provide resilience to ATM VP traffic against a set of important failures: (1) ensuring the resilience solely
by ATM layer recovery or (2) leaving the resilience against cable cuts and SDH layer failures to an SDH
recovery scheme, and using ATM layer recovery only for ATM-specific failures.

2. Multi-layer recovery approaches
2.1.
Network scenario
In this paper, we consider a network scenario related to the transport of semi-permanent ATM Virtual Paths(VP) over SDH backbone networks, such as the core networks of public telecom operators. Over the past few
years, many operators have carried out major investments related to the deployment of SDH transport networks.
For instance, most national networks in Europe consist of a meshed transport network based upon SDH digitalcross-connect (DXC) systems operating at the VC-4 (150 Mbit/s) level. One can expect that public operators
will be tempted to reuse their SDH-based facilities to transport emerging ATM services over their core as well.
In the initial phase of ATM deployment, operators will rather use SDH VC-4 paths (supported by the SDH
DXC system) to interconnect their ATM backbone DXCs. Such configuration enables to share fiber systems
between ATM and SDH native traffic. As a matter of fact, the amount of ATM backbone traffic will in thenear-term not be sufficient to justify the installation of dedicated fiber systems between the ATM points ofpresence. Furthermore, ATM equipment may initially not be present in every backbone office, and the SDH
VC-4 layer can provide flexibility to VP traffic in offices without ATM (e.g., to allow a better line system
usage). Such ATM on SDH transport networks thus have a multi-layer structure (see also [4]), since the ATM
VP demands are subject to routing flexibility at the VC-4 and the VP level.
In order to assure high availability of both ATM VP services and SDH native services, an operator has to*

Research Assistant of the Fund for Scientific Research (F.W.O.-V.) - Flanders (Belgium)
E-mail: mgrys@intec.rug.ac.be, WWW: http://allserv.rug.ac.be/~mgryseel/attain that his traffic survives the most frequently occurring failures in the network. According to previous
studies [1], the most important failures in ATM on SDH backbone networks include single failures of cable
cuts, SDH DXC breakdowns and ATM DXC breakdowns.
It is important to understand that both SDH and ATM recovery schemes have to be deployed in ATM on
SDH networks, in order to ensure complete survivability of this failure set. This can be understood as follows.
On one hand, the survivability of SDH native services can only be guaranteed by SDH recovery schemes. On
the other hand, the SDH layer can not restore ATM VPs affected by the breakdown of ATM equipment. In
fact, a recovery scheme operating at the ATM network layer is needed to resolve ATM equipment failures.
Both recovery schemes are thus necessary for achieving the overall survivability goal. The issue relevant tothis study is that an operator may opt for a SDH recovery scheme (instead of ATM layer survivability) toprovide resilience to ATM VPs against burdensome failures at lower layers (such as cable cuts). This ispossible because the SDH network layer acts as a serving path layer for the ATM network layer.
2.2. Multi-layer recovery approaches
initial VP
routing

initial VC-4
routingalternate
VP routing

alternate
VC-4 routing

d

d

D

a

A

B

F

E

D

C

c

A

D

C

E

e

SDH VC-4 network layer

a

A

B

F

E

d

C

c

a

c

e

e

ATM VP network layer

Figure 1.a - Recovery at the highest layer

SDH VC-4 network layer

ATM VP network layer

Figure 1.b - Recovery at the lowest layer

Since ATM VP connections can make use of different (re-) routing levels (VP and VC-4), several recovery
approaches can be considered for them. A first possible approach is to resolve any expected failure at thehighest possible network layer. In our network scenario, this comes down to the following: SDH native traffic
is recovered from cable cuts and SDH DXC breakdowns by the SDH recovery scheme. On the other hand,
SDH paths supporting ATM VP logical links, which are affected by SDH failures, are not recovered by the
SDH scheme. The responsibility for recovering ATM traffic is left to the ATM recovery scheme, irrespective
whether the failure originates from an ATM or SDH equipment breakdown or a cable cut. As a result, only asingle recovery scheme is responsible for the resilience of a certain traffic type. This appears favorable from a
network management point of view. Another motivation for adopting this recovery at the highest layer
approach is that often transport networks carry various classes of services, each with different survivability
requirements. It seems easier to provide multiple reliability degrees when the survivability is realized by arecovery scheme residing in the higher network layers.
In spite of these advantages, the finer switching granularity of the higher network layers may involve a lotof burden in the event of physical failures, which may affect many connections at once. For instance, it isimportant to be aware of the failure multiplication occurring in multi-layer networks. This is because a single
failure in a lower network layer leads to multiple failing entities in a higher network layer. This is illustrated
in figure 1.a, where we show the recovery actions in case of a cable cut on a sample network. The failure
scenario at the ATM network layer clearly has a more complex nature than the original failure (e.g., a cable
cut may simultaneously affect many thousand VPs). Consequently, several precautions may have to be taken
to ensure a satisfying recovery performance (we refer to [2] for details).
Although physical failures (e.g., cable cuts) and SDH failures (e.g., SDH DXC breakdowns) can berecovered at the ATM network layer, it seems more sensible to provide ‘bulk recovery’ of these failures at theSDH network layer. The failure scenario at the SDH network layer is simpler in the number of recovery trigger
signals and paths to be changed over, because SDH recovery operates at a coarser granularity, and hence
enables a simpler (and probably faster) recovery.

From an overall cost point of view, SDH recovery can also be more efficient to resolve physical failures.
Although the finer granularity of ATM VP routing allows a better utilization of the spare resources on a single
layer basis, one must not forget that all the capacity (including the spare) of the ATM layer has to be supportedby the SDH network layer. Deploying a recovery scheme and its subsequent spare resources in a certain layer
implies an additional investment for all layers below. This observation may counterbalance the single-layer
cost-efficiency of ATM recovery in some cases, and favor to provide the resilience at a lower network layer.
The lowest layer in our network scenario is the SDH Multiplex Section layer, where it is not always possible
to deploy cost-effective recovery [6] (unless rings are used), and protection or restoration in the intermediate
VC-4 network layer may thus provide the best alternative.
Accordingly, we have considered a second recovery approach, called recovery at the lowest layer, whichcomes down to resolve SDH equipment failures and cable cuts at the SDH VC-4 network layer, irrespectiveof the source of the affected traffic. ATM VPs affected by these failures are recovered in aggregates, because
we recover the SDH paths supporting many of them. In figure 1.b, the lowest layer recovery actions in case of
the cut of link A-B are represented. The SDH VC-4 paths carrying ATM VPs are recovered by the SDH
recovery protocol, (the figure assumes SDH link restoration at the VC-4 level), whereas the routes in the ATM
network layer remain unchanged. From an ATM layer point of view, the situation (after SDH recovery) is as
if the ‘broken ATM links’ were repaired.
Remark that we still need the ATM recovery scheme in order to resolve ATM DXC breakdowns. However,the SDH layer survivability already provides adequate resilience to VPs routed directly in the ATM network
layer, and thus additional ATM protection is only necessary for multi-hop1 VPs (i.e., transiting at least one
ATM DXC). As a result, we may expect a much lower spare capacity requirement in the ATM network layer.
In other terms, since the protection of the heavier failures is left to the SDH layer, this recovery approachenables to get more value out of the ATM equipment.
Nevertheless, a successful deployment of this recovery approach still entails two challenges. A firstchallenge is related to the interworking functionality, which is needed in the network to coordinate the ATM
and SDH recovery actions (together responsible for the overall resilience of ATM VPs). Indeed, in the eventof physical failures, subsequent alarm messages will be passed through each layer after they are generated.
More specifically, the detection of SDH path alarm messages (AU-AIS) in the SDH VC-4 to ATM VP layer
adaptation function of the ATM equipment will cause the generation of VP-AIS messages. Without any interlayer recovery coordination of this alarm generation, the recovery scheme at the ATM layer may be triggered(for a failure that is planned to be resolved at the SDH layer). This may create a situation for ATM layer
resource competition causing network congestion and even disruption of ATM services.
The second challenge concerns the allocation of spare capacity. Because two recovery schemes are
responsible for the resilience of the ATM VPs, spare capacity has to be foreseen in both SDH and ATM
network layer. Care should be paid to avoid duplication of the protection. For instance, redundant protection
may occur when the SDH recovery would also protect the ATM spare resources. Redundant protection can
be avoided when the working and spare capacity of the ATM layer are carried in distinct SDH paths and when
protection selectivity is considered in the SDH network layer. Nevertheless, although redundant protection can
be avoided, we will still have to foresee some resources in the SDH network layer for the ATM spare capacity.
Eventually, this second drawback can also be avoided by adopting a survivability strategy based upon thecommon pool concept [3]. The essence of common pool consists in supporting the ATM spare resources over
the SDH spare capacity as preemptible traffic (also known as extra traffic), which provides the means to share
spare capacity across network layers. We refer to reference [3] for an in-depth study of the application of thissurvivability concept to meshed ATM on SDH networks, and concentrate in this paper on the comparison
between the above mentioned multi-layer recovery approaches.

3. Cost evaluation
3.1. Introduction
In order to compare the multi-layer survivability strategies in a quantitative way, we have performed a set ofplanning case studies, using different traffic and network models. We had several realistic data sets, generated

1

Remark that when a node fails, recovery of traffic originating or arriving at that node is impossible.

for the ACTS PANEL research project [1], at our disposal. These data sets entail two network topologies, a
16-node network with 36 physical links and a larger 32-node backbone network with 70 physical links. We
considered two different degrees of ATM service adoption: a medium and a high degree. Each of themcorresponds to a different placement of ATM equipment in the network (i.e., different ATM-node overlay
scenarios). In total, we considered four different ATM on SDH network models:Table 1 - Considered sample networks
Medium degree of ATM adoption
High degree of ATM adoption

Small network (16 offices)
6 ATM-equipped offices
8 ATM-equipped offices

Large network (32 offices)
10 ATM- equipped offices
16 ATM- equipped offices

With each ATM on SDH network is also associated a certain SDH native demand and an ATM VP demand
distribution. In the remaining of this section, the resulting network/traffic models will be denoted as small6,
small8, large10 and large16.
A cost comparison of different multi-layer recovery approaches on a network is of course very dependent
on the single-layer recovery options that are followed. In our study, we concentrated on path protection
schemes in both layers (that is, VP-APS in the ATM layer and end-to-end (e.g., SNCP-based) protection in the
SDH VC-4 layer). Such path protection schemes (i.e., end-to-end recovery using dedicated backup facilities)
will often be used for the resilience of connections of premium service level. In practice, an operator may
choose recovery schemes with different characteristics in both layers. However, our aim was to evaluate themulti-layer aspect of the recovery approach, and we assumed that similar recovery schemes are deployed inboth layers. For instance, it can be anticipated that using a “greedy” SDH protection scheme will be more
expensive than an ATM restoration scheme, which uses its spare resources in a more efficient way.
3.2. Experimental results
The network cost in the different cases was evaluated by assessing the amount of physical resources such as
the number of equipment and fiber systems required by each survivability approach. The amount of resources
was determined through a network dimensioning process, using network design tools developed in the past by
the authors. Because the recovery approaches involve different design constraints, we had to use different
design procedures. The cases using a recovery at the highest layer approach were dimensioned with the
network design tool described in [7]. For the lowest layer approach, we used a generalization of the multi-layer
planning approach proposed in [4]. When a self-healing backup VP scheme is used at the ATM layer, we used
a combination of our VP grouping heuristic [4] and the algorithm for spare capacity allocation of reference [5].
The costs taken into account during our evaluation origin from ATM equipment (backbone ATM DXC VP
and its STM-1 interface cards), SDH equipment (DXC 4/4, STM-1 interfaces, STM-16 line systems), and thecarrier cost. This carrier cost includes a cost term for the fiber and a cost term for the regenerators. We used
actual equipment costs, obtained from a survey of different ATM and SDH equipment manufactures. All costsin this paper are expressed in a normalized unit, corresponding to the cost of one STM-1 port on an SDH(STM-16 type) line system.
Table 2 - Investment costs for recovery at the highest layer
ATM equipment cost
SDH equipment cost
Carrier cost
ATM service costsmall6 small8 large10
2357.3 6332.2 5817.2
2074.9 5690.3 5767.5
1384.8 3455.5 4713.0
5817.0 15478.0 16297.7

large16
17552.6
16487.913031.4
47071.8

Table 3 - Investment costs for recovery at the lowest layer
ATM equipment cost
SDH equipment cost
Carrier cost
ATM service costsmall6 small8 large10 large16
1411.3 3294.4 3044.3 7999.5
2193.0 5749.0 6373.4 17792.2
1303.5 3569.9 4672.2 13301.2
4907.8 12613.3 14089.9 39093.0

Instead of comparing the overall cost in different cases, we decided to just compare the costs related to the(survivable) transport of the ATM VP traffic. In fact, the survivability strategies considered in this paper only
differ in the way they recover the ATM VPs, and the SDH native traffic is always treated the same way. In
order not to let the SDH native traffic obscure the cost comparison, we decided to evaluate the share of theoverall investment costs related to the ATM VP traffic. This so-called ATM service cost comprises the ATM
equipment cost, but also the cost of the SDH facilities occupied by ATM VP traffic. Since the SDH facilities
are shared between SDH paths carrying SDH native demand and SDH paths carrying ATM traffic, we
attributed each service type a share of the SDH installation cost that was proportional to its usage of the total
SDH network capacity. In Table 2 (and Table 3), the investment costs related to the ATM demands are listed
for the recovery at the lowest layer (and highest layer respectively), for each of the considered scenarios.
It is also interesting to compare the SDH and ATM link capacity required by each approach. These areshown in table 4 for both SDH and ATM network layers. Again, we only consider the link capacity related to
the ATM demands (that is, we exclude the SDH layer capacity occupied by the SDH native traffic). We haveexpressed the capacity (of both network layers) in a normalized unit, which represents 150 Mbit/s.
Table 4 - Link capacity
Recovery at the highest layer
ATM VPSDH VC4

small6154
280

small8439
759

large10
392
785

Recovery at the lowest layer

large16
1287
2199

small654
246

small8143658

large10129743

large16360
2090

It seems that in the network and traffic scenarios considered by us, the recovery at the highest layer
approach requires a higher investment budget (up to 20% more) than the recovery at the lowest layer approach.
There are several explanations for this result. First of all, with recovery at the lowest layer, only multi-hop VPs
require additional ATM layer protection (and subsequent ATM layer spare capacity). This results in a lower
ATM spare capacity requirement than the recovery at the highest layer approach. Secondly, the approaches
differ in the constraints that must be satisfied in finding working and backup VP routes. With the recovery at
the highest layer approach, the routes of a VP must be disjoint in both physical links and nodes. For the lowest
layer approach, the only requirement is to have no transit ATM DXC in common. Hence, the recovery at thelowest layer approach leads to looser constraints (i.e., more room for optimization), since the VP routing
possibilities are less limited.
Furthermore, all the ATM link capacity (including spare) has to be supported in the SDH network layer aswell. Although no SDH spare capacity has to be provisioned for the ATM VPs with recovery at the highest
layer, the extra deployment of the ATM spares involves more SDH paths to support the ATM network layer.
This will increase the amount of SDH facilities ‘occupied by ATM layer capacity’. In fact, the SDH capacity
required for the ATM VP demands is about the same for both recovery approaches (see table 4). With therecovery at the lowest layer, part of the spare resources for VP resilience are only put in the SDH network
layer, and only have an impact on the needed facilities of one network layer instead of two.
Concluding, it seems better to resolve burdensome physical and SDH failures using bulk recovery at theSDH VC-4 network layer than using ATM layer recovery schemes. This makes it possible to get more value
out of the ATM equipment rather than wasting some of the (more expensive) ATM layer capacity on the rather“mundane” task of protecting against lower layer failures.

...

Figure 2.a - Original node configuration

SDH
DXC

STM-1

SDH
DXC

...

SDH
DXC

...

L
T

...

L
T

DF-1
(STM-1)

L
STM-1
T

L
T

ATM
DXC

...

STM-1

SDH
DXC

...

L
T

...

fiber

...

...

L
T

fiber

...

STM-1
STM-1

SDH
DXC

L
T

...

STM-1

STM-1

ATM
DXC

...

ATM
DXC

...

ATM
DXC

SDH
DXC

L
T

Figure 2.b - Configuration using an STM-1DF

However, other motivations (see for instance section 2.2) may still convince a network operator to adopt
the recovery at the highest layer approach. In that case, it is meaningful to decrease in some way the cost of
the SDH layer capacity necessary to support the ATM network layer. A possible manner to obtain these SDH
resources at a lower price is to use STM-1 distribution frames (i.e., hard-wired interconnections of STM-1
interface ports) instead of SDH digital cross-connects for the VC-4 flexibility of the ATM VP traffic. The

resulting configuration is illustrated in figure 2.b. This equipment configuration provides the means to saveinterface ports on the SDH DXCs, which are now only accessed by SDH native traffic.
With recovery at the lowest layer, we can of course not replace the SDH DXC functionality by an STM-1
distribution frame (DF), since the SDH VC-4 layer has to provide fast recovery of physical and SDH layer
failures. The manually operated STM-1 DF can not be seen as a device able to ensure fast rerouting in the SDH
layer, and does also not include any monitoring or management capabilities. In the recovery at highest layer
approach, the VP resilience is provided solely by the ATM VP layer, and it is more acceptable to use STM-1
DFs to obtain the SDH VC-4 level flexibility for the VP traffic.
Table 5 - Results for STM-1 DF configuration
ATM equipment cost
SDH equipment cost
Carrier cost
ATM service costsmall6 small8 large10 large16
2357.3 6332.2 5817.2 17552.6
1206.9 3294.3 3413.5 9515.9
1384.8 3455.5 4713.0 13031.4
4949.0 13082.0 13943.7 40099.8

Table 5 presents the resulting investment costs of the networks with STM-1 DF configuration (recovery at
the highest layer). The results prove that the STM-1 DF configuration lowers the price to be paid for the SDH
layer capacity (compare with table 3). In fact, using the STM-1 DF enables to implement the recovery at thehighest layer approach at an investment cost comparable to the investment cost of the lowest layer approach.

4. Conclusions
In this paper, we studied different multi-layer recovery approaches to ensure the resilience of ATM VP and
SDH native services against a set expected failures. In order to evaluate the various multi-layer survivability
strategies in terms of investment cost, we carried out a network planning case study. Based on our results, weconclude that it seems to be cheaper to resolve burdensome failures like cable cuts in the SDH network layer.
In this recovery approach, a large part of the spare capacity has to be foreseen in the SDH layer only, whichenables to get more value out of the ATM equipment. When other motivations would still convince an operator
to employ the recovery at highest layer approach, an alternative equipment configuration using a cheaper
STM-1 distribution frame to obtain the VC-4 level flexibility for ATM traffic may be appropriate.

Acknowledgment Part of this work has been supported by the European Commission through the ACTS
project AC205/PANEL, and by the Flemish Government through IWT project ITA/950214/INTEC.

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