LIGHTPATH SURVIVABILITY WITH QOT GUARANTEES

Key Issues in Wavelength-Routed Photonic Networks

Luiz Gustavo Zuliani, Stefano Giordano, Michele Pagano

Dept. of Information Engineering, University of Pisa, Pisa, Italy

Davide Adami

CNIT Research Unit - Dept. of Information Engineering, University of Pisa, Pisa, Italy

Keywords: Photonic networks, Survivability, RWA, Physical impairments, QoT, Residual link capacity, Power

constraint.

Abstract: The Routing and Wavelength Assignment (RWA) problem has been extensively studied over the last

decade, but only in recent years algorithms were specifically developed (or adapted) to consider

transmission layer physical impairments in photonic (all-optical) networks, while providing lightpath

survivability. This work discuss the last contributions in the field, and presents a new impairment-aware

RWA algorithm for survivable photonic networks. As a work in progress, the new algorithm is presented

with open design decisions.

1 INTRODUCTION

Nowadays, the Internet backbone traffic is mainly

carried by Wavelength Routed Networks (WRN)

based on WDM lines due to the high capacity, low

Bit Error Rate (BER) and low per-bit monetary

transmission cost of these systems. In order to setup

a lightpath (LP), i.e., an optical circuit between two

nodes, a route must be calculated and available

wavelengths must be allocated in all links that

compose the path. This problem is NP-complete, and

is known as Route and Wavelength Assignment

(RWA). It has been extensively studied in the last

decade considering different aspects, mainly

minimization of resource allocation, online

operation and LP survivability. For online operation,

usually the RWA problem is divided in two sub-

problems that are solved separately: first, a shortest

path algorithm is used to compute the route, then an

heuristic is used for wavelength allocation.

With the advance of DWDM systems (carrying

hundreds of channels per fiber, each one operating at

rates up to 40 Gb/s), the electronic processing at the

nodes has became a bottleneck. To overcome this

problem, all-optical (or photonic) crossconnects

(PXCs) that do not realize optical-electrical-optical

(OEO) conversion were introduced, and meshed

WRNs based on PXCs without wavelength

conversion capabilities (all-optical wavelength

conversion is still an immature technology) are

slowly being deployed. As a side effect of OEO

absence, signals are not regenerated at each hop

anymore, and accumulate transmission impairments

that affect the QoT and consequently the BER at the

end node. Impairments-aware RWA (IA-RWA)

algorithms have being envisaged, using different

network impairment models, resulting in different

performance (Azodolmolky, et al., 2008). However,

specifically regarding network survivability, the IA-

RWA problem is even more complex, and a reduced

number of works is available (Zhai, et al., 2007).

In the first part of this work, the most recent IA-

RWA solutions for Survivable Photonic WRNs are

reviewed. Aspects such as key issues, design

decisions and performance evaluation metrics are

discussed. The second part presents the IA-RWA for

survivable networks being developed by the authors.

Comparisons between the algorithm under study and

the previously presented works are also depicted. At

the end, conclusions are shown.

165

Zuliani L., Giordano S., Pagano M. and Adami D. (2010).

LIGHTPATH SURVIVABILITY WITH QOT GUARANTEES - Key Issues in Wavelength-Routed Photonic Networks.

In Proceedings of the International Conference on Data Communication Networking and Optical Communication Systems, pages 165-168

DOI: 10.5220/0003033201650168

 SciTePress

2 SURVIVABLE IA-RWA

Generally speaking, the main goal of all survivable

IA-RWA algorithms is to provide LP resilience.

Despite that fact, they can be designed in very

different ways, depending on the constraints

considered for the WRN itself but also for the LPs.

Survivable IA-algorithms can be classified in

function of the network impairment model used, the

type of the combined IA-RWA process, the type of

resilience and the quality levels offered (evaluated

using ad-hoc performance metrics). In the following

subsections the most recent IA-RWA solutions

(Zhai, et al., 2007; Askarian, et al., 2008; Kim, et al.,

2008; Markidis & Tzanakaki, 2008; Jirattigalachote,

et al., 2009) are reviewed under the perspective of

each of these characteristics.

2.1 Network Impairment Models

Transmission in optical fibers is affected by a

number of physical impairments. The most relevant

are intersymbol interference (ISI), amplified

spontaneous emission (ASE), polarization mode

dispersion (PMD) and node and interchannel

crosstalking (Zhai, et al., 2007). The predominant

impairment depends on many factors, like the

quality of fibers and node components, the LP

optical signal power and bandwidth, and the

wavelength spacing between channels.

All of the cited works consider ISI, ASE and

both crosstalking forms as noise-like terms, and the

sum of their variances is accounted for the Q factor

calculation, which is a signal-to-noise ratio. The LP

BER is estimated in function of the Q factor with a

simple equation.

PMD was ignored in all works, because it is

relevant only at data rates of 40 Gb/s and beyond.

2.2 RWA Combined Process

As stated by Azodolmolky, et al. (2008), the routing,

wavelength assignment and QoT evaluation

processes can be combined in many ways, with

different levels of complexity and performance. The

best (and most complex) IA-RWA algorithms

consider the physical impairments during the RWA

phase, and also estimate the BER of the candidate

LP.

Three of the reference works divide the IA-RWA

problem in two sub-problems. To calculate the work

and backup paths, it was used fixed-alternate routing

with Yen's algorithm (offline) and Dijkstra

algorithm (online). Non IA-routing used link length

as link cost metric, and IA-routing used the Q-

penalty metric (Markidis & Tzanakaki, 2008), that is

also calculated as noise-like terms. The wavelength

Assignment was realized using the following

algorithms: First Fit (FF), Last Fit (LF), Best Fit

(BF), Random Pick (RP) and Most Used (MU). It is

important to note that these heuristics present

different behavior in ideal networks and physical

impaired networks (He, et al., 2009). Zhai, et al.

(2007) and Markidis & Tzanakaki (2008) presented

single-phase RWA process, where the shortest path

for each wavelength plane is calculated.

All proposals evaluate the BER of candidate

LPs. If the BER is under a predefined value (usually

Q factor equal to 6 or 7), the request is blocked.

2.3 Protection and Restoration

LP resilience can be pre-configured or just pre-

planned. In both cases the backup LP is already

computed, but only in the former case the resources

are already allocated to the backup LP. If the backup

LP carries the same traffic as the working LP even

before failure, this kind of resilience is called 1+1

dedicated protection. If the backup LP is used for

Best Effort traffic or not used at all, it is called 1:1

dedicated protection. Protection is very efficient

(service disruption is inferior to 50 ms), but is also

the most expensive kind of resilience.

Pre-planned resilience is also called restoration,

and can be dedicated or shared. In both cases the

wavelength remains unused in the fiber links until

the restoration mechanisms are activated. Therefore,

the fiber remains “dark”, at least for that particular

channel. In the case of shared restoration, a

wavelength reserved for shared backup remains free

to be used in other shared backup path computations,

i.e., it can (and possibly will) be used to protect

more than one LP. Restoration is better for the

overall network QoT, because the backup LPs

remain dark and do not interfere with the QoT of the

working LPs. Also, shared restoration improves the

network resources utilization. On the other hand,

when a LP must be restored through a pre-planned

computation, there is no guarantee that a) it will

satisfy the required BER and b) it will not

compromise the QoT of other established LPs. That

situation is even worse in the case of shared

restoration. That happens because when a new LP

must be setup, the IA-RWA engine does not take

into account the physical impairments of dark

wavelengths used to restore LPs.

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All kind of resilience described in this section

are investigated in the referred works, with

interesting results.

2.4 Performance Evaluation Metrics

Usually, IA-RWA algorithms are evaluated through

simulations using either real-world topologies (like

the classic 14 nodes NSF topology) or mesh toroid

networks due to the high degree of symmetry and

connectivity. Results are in the form of the blocking

probability in function of the traffic load. Other

interesting metrics found in the cited works are:

 Vulnerability Ratio or QoT-Vulnerability: the

probability that, in the case of a link failure, a

pre-planned backup LP cannot be restored due to

unacceptable QoT;

 Cascading Failure Vulnerability: the probability

that a given LP become unusable due to physical

impairments induced by the activation of pre-

planned backup LPs;

 Failure Ratio: it is defined as the ratio between

the number of connections that are not recovered

due to unacceptable QoT to the number of

working LPs affected by a link failure. It is

averaged over all single link failures;

 Running time: The time needed to compute the

LP from the instance of request arrival.

Interesting values are the average and worst case

scenario.

2.5 LP Application

LPs are used by the clients of the optical layer to

transport two types of traffic, basically: mission

critical and non-critical applications traffic. The

former requires strict QoT guarantees and protection

mechanisms (downtime can be as low as 5 min per

year), usually remains active for very long periods

but do not require a short setup time. On the other

hand, non-critical applications require a short setup

time, but have loose requirements regarding QoT

and resilience. An IA-RWA must be designed to

satisfy one or another type of LPs, due to the

tradeoffs involved. Most of the IA-RWA algorithms

presented in the referred works have different

versions with protection and restoration.

3 PROPOSED ALGORITHM

The objective of the proposed algorithm is to

compute LPs with dedicated 1+1 or 1:1 protection,

with assured QoT and survivability. The main goal

is to satisfy these conditions while minimizing the

resource allocation.

Different from the referred works, the network

impairment model used in the proposed IA-RWA is

not based on the sum of noise-like terms (one for

each dominant impairment), but instead is based on

the minimum and maximum power constraints. The

minimum power constraint, which is best known as

sensitivity level, assures that optical signals can be

properly detected by all optical devices. Thus, the

survivable IA-RWA algorithm not only gives a path

and a wavelength for each of the LPs (work and

backup), but also the optical power that must be

injected at the ingress PXC in order to guarantee the

requested BER at the egress PXC. The maximum

power constraint imposes a limit to the optical

power on fiber links. This way, the fiber

nonlinearities (that are completely power-dependent,

such as channel crosstalking) can be indirectly

managed. The network impairment model and the

impairment validation process of the proposed

algorithm are based on the ASE noise and on the

desired Q factor, and use the analytical model

discussed by Pavani, et al. (2008).

The first part of the algorithm solves the RWA

problem in a combined way. For each wavelength

available to use at the source node (i.e., the

wavelengths that the available transponders at

ingress PXC can tune), a couple a disjointed shortest

paths is calculated using the Suurballe algorithm.

Physical impairments are considered during the

RWA phase as the link cost metric. The cost of a

given fiber link is a function of the residual

wavelength and optical power capacity. Thus, the

lower is the number of used wavelengths and the

total optical power that traverses a link, the lower

will be its cost. Another strategy introduced by the

proposed algorithm in order to minimize the

blocking probability is the Critical Link Avoidance

(CLA). The “altruist” idea of avoiding using

particular links to save them for future requests was

introduced by the Asynchronous Criticality

Avoidance (ACA) protocol. Except for sharing this

concept, CLA technique is completely different from

ACA by any perspective. All links that are labeled

as critical are initially pruned from the physical

topology. If, after the first attempt, no path is found

in any of the wavelength planes, the process is

repeated again, but this time considering all critical

links. To define a link as critical, upper bounds for

the wavelength use and total optical power are being

considered.

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After the IA-RWA phase, the BER is verified in

all work and backup LP candidates. All work-

backup LP pairs which the BER is too high or whose

links can not accommodate their share of optical

power (due to the maximum power constraint) are

removed from the list of candidates. It is still to be

decided if a work-backup LP pair is to be removed if

only the backup LP do not met with the BER

requirements. After this step, all candidate LPs are

known to satisfy the BER requirements. At this

point, the final LP pair selection must take place. To

effectively choose the best candidates, a number of

simple heuristics are being considered. Simulations

must be performed to determine which ones are

relevant, and in which order. When more than one

candidate matches the criterion, the matched ones

are compared on the basis of the next criterion.

When only a pair of LP candidates are left (not

necessarily with the same wavelength), these are the

work and backup LP to be established. The

heuristics to be considered (so far) are:

 lowest number of critical links;

 LP pair that uses the same wavelength, if the cost

difference between the work and backup LPs (in

terms of number of hops and/or transmission

power) is bellow a given upper bound;

 lowest transmission power;

 lowest number of hops;

 LP whose wavelength is the FF, MU or other

wavelength assignment heuristics.

4 CONCLUSIONS

The RWA problem is a well studied area, but only in

the last years works were conducted considering

fully transparent (photonic) WRNs, physical

impairments and LP survivability. In this work,

some of the last contributions in the survivable IA-

RWA field were reviewed. It was discussed the

network impairment model used, the mechanisms

for routing, wavelength assignment and physical

impairments validation, and the proposed

performance evaluation metrics. It was also

presented a new survivable IA-RWA algorithm to

compute LPs with dedicated 1+1 or 1:1 protection,

with assured QoT and survivability. The proposed

algorithm uses a different impairment model based

on sensitivity and maximum power constraints, and

uses the wavelength load and total optical power as

metrics to define the link costs and the criticality

levels. The introduced CLA saves critical links in

order to minimize the blocking probability.

Simulations are needed to be carried out to verify

if the added complexity minimizes the blocking

probability of setup requests. The performance

evaluation metrics and topologies introduced by the

referred works must be considered.

REFERENCES

Azodolmolky, S., et al., 2008. A survey on physical layer

impairments aware routing and wavelength

assignment algorithms in optical networks. Computer

Networks, 53, pp. 926-944.

Zhai, Y., et al., 2007. Performance of dedicated path

protection in transmission-impaired DWDM networks.

In: IEEE ICC 2007.

Askarian, A., et al., 2008. Protection and restoration from

link failures in DWDM networks: a cross-layer study.

In: IEEE ICC 2008.

Kim, S., et al., 2008. QoT-guaranteed protection:

survivability under physical layer impairments. In:

ICST BROADNETS 2008.

Markidis, G., Tzanakaki, A., 2008. Routing and

wavelength assignment algorithms in survivable

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ICST BROADNETS 2008.

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networks. Journal of Lightwave Technology, 27 (5),

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Pavani, G. S., et al., 2008. Distributed approaches for

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