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222 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 2, JANUARY 15, 2012

A Novel Contention Resolution Scheme ofHybrid Shared Wavelength Conversion for

Optical Packet SwitchingHuan-Lin Liu, Bin Zhang, and Song-Lei Shi

AbstractWavelength conversion is an efficient way to improvethe optical packet blocking performance for all-optical packet

switching networks. The synchronous OPS node architecture,with a hybrid shared wavelength conversion (HSWC) schemebased on shared per outputfiber and shared per node wavelengthconversion, is put forward to reduce the packet loss probability inthis paper. A heuristics is developed into assigning the wavelengthchannels for packets in the proposed optical nodes. The limited

range wavelength converters first available (LFA) algorithm and

parametric wavelength converters first available (PFA) algorithmare used to schedule contending optical packets to available wave-length channels. Simulation results show that the hybrid shared

wavelength conversion scheme has significant performance im-provement in terms of packet loss probability. And the utilization

of wavelength converter is greatly improved for OPS node withhybrid shared wavelength conversion scheme. Furthermore,the PFA scheduling algorithm compared with LFA algorithm isvalidated to improve the wavelength assignment efficiency and

reduce the required total wavelength converters for OPS nodes.

Index TermsContention resolution, limited range wavelengthconverter, optical packet switching, packet loss probability, para-

metric wavelength converter.

I. INTRODUCTION

A S the demand for link speed and bandwidth keeps in-creasing, wavelength division multiplexing (WDM)optical network that supports wide bandwidth are believed

to be future backbone transport networks. Optical switching

technologies are motivated to rapidly exchange information

by the enormous bandwidth carried out on any optical fiber.

Among all of the switching schemes, optical packet switch

appears to be a strong candidate with the high speed, data

rate, format transparency and configurability for high-speed

networks [1][4].

Manuscript received February 18, 2011; revised July 24, 2011, November 06,2011; accepted December 20, 2011. Date of publication December 28, 2011;date of current version January 26, 2012. This research was funded by the sci-entific research fund of Chongqing municipal commission (KJ110527), and byChongqing Natural Science Foundation (CSTC 2010BB2413), and by nationalnature science foundation of China (NSFC 60972069, 61071117).

H.-L. Liuand S.-L. Shiare with theKey Laboratory of Optical Fiber Commu-nication Technology, Chongqing University of Posts and Telecommunications,Chongqing 400065, China (e-mail: [email protected]).

B. Zhang is with School of Economics and Management, Beijing Uni-versity of Posts and Telecommunications, Beijing 100876, China (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2011.2182036

In OPS network, contention arises when thereare twoor more

incoming optical packets at the same wavelength contending for

the same output wavelength channel. How to resolve the optical

packets contention, therefore, has become a critical technology

for OPS nodes. The first candidate for contending resolution

is to use wavelength conversion that exploits the wavelength

domain, although other resolution mechanisms also exist, such

as fiber delay lines and deflection routing [5][8].

Thus, wavelength converters become the most impor-tant components for contention resolution in wavelength

domain [8][10]. An ideal full range wavelength con-

verter (FRWC), however, has to be realized by cascading

a given number of realistic limited range wavelength con-

verters (LRWCs). Wavelength converters are costly devices

and therefore, a particular effort has been devoted to de-

signing cost-effective optical node architectures by minimizing

the number of wavelength converters constrained to a pre-

scribed packet loss probability [11][16]. Several OPS node

architectures, such as single per link (SPL), share per output

fiber (SPOF), and share per node (SPN), sharing limited range

wavelength converters, was proposed in [11]. Also, it wasvalidated by simulation that the SPN architecture employing

with LRWCs (SPN-LRWC), was the greatest cost-saving of the

three architectures.

Recently, the shared per node OPS architecture with para-

metric wavelength converters (PWC) is proved that it can

reduce the required number of wavelength converters [15],

[16]. A PWC allows multiple wavelengths to be simultaneously

converted from one to another. The PWC is considered to be

becoming feasible. A number of simultaneous multi-channel

wavelength conversions of over 30 channels have been reported

using fiber [17] and a LiNbO waveguide [18]. A high-capacity

field transmission experiment using multi-channel wavelength

conversion has also been demonstrated [19]. Moreover, the

parametric process is fully transparent to various types of

advanced modulation formats such as differential phase shift

keying (DPSK) [20] and quadrature phase shift keying (QPSK)

[21]. These studies show that guard bands can be provided with

suitable channel spacing.

In order to further reduce the packet loss probability and

improve the utilization of wavelength converters, this paper

presents a hybrid shared wavelength conversion scheme. The

proposed scheme mixes shared per node with LRWCs and

shared per output fiber with PWCs. Thus, the affection of guard

bands in PWCs can be eliminated by the use of LRWCs. There

are two different scheduling algorithms to assign wavelength

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packet loss probability of OPS would be further decreased by

using the hybrid shared wavelength conversion scheme.

III. SCHEDULINGALGORITHM

Since HSWC is a bufferless contention resolution for the

OPS node, the packet waiting time in the OPS node would be

smaller compared with the delay of the access network andcan be ignored. A major concern in the switch is packet loss,

which is owed to be lack of output wavelength channel, be

lack of wavelength converters or conversion limited. Because

two different wavelength converters are employed, as shown

in Fig. 1, there are two different scheduling algorithms on

which kind of wavelength converters firstly selected: LRWC

first available (LFA) algorithm and PWC first available (PFA)

algorithm, respectively. Actually, not all ports of a PWC are oc-

cupied simultaneously, so that PWCs are preferred to improve

the utilization of PWC. In this part, we mainly introduce the

PFA algorithm because the two algorithms are similar except

the priority of wavelength converters in the last phase.The PFA algorithm assigns the LRWCs and PWCs for the

scheduled packets to available output wavelength channels,

and it would be referred to the allocation of LRWCs, PWCs and

wavelength channels. As shown in Fig. 4, the PFA algorithm

includes three different phases that are performed sequentially:

initialization (INI) phase, direct output wavelength channel

assignment (DOWCA) phase, and LRWCs and PWCs as-

signment (LPA) phase. Before describing the PFA algorithm

adopted in the HSWC architecture, the following notations are

introduced:

1) , similar to ,

represents an arrival optical packet carried by wavelength

and directed to the output fiber . The packet isdenoted as the -th packet arriving on wavelength

and directed to output fiber .

2) denotes output wave-

length channel at output fiber with wavelength .

3) is the set containing packets that are waiting for being

scheduled wavelength channels. It is consist of packets

, and maybe there are two or more same packets

coming from different input fibers, simultaneously.

4) is the set containing idle wavelength channels. It is ini-

tialized as , containing the all output wavelength chan-

nels . is denoted that

the number of idle channels currently in is .5) is the set of all possible packets that

can be scheduled to the output wavelength channels

by wavelength conversion. The set is obtained

by using only PWCs, is obtained by using only

LRWCs, and is obtained by using both PWCs

and LRWCs.

In the INI phase, the main variables used by the algorithm

are initialized. The input wavelength channels of each input in-

terface are scanned and the incoming packets carried by wave-

length are collected in the set . The set is initialized as

, here . Finally, the number of LRWCs is initialized

as , and the number of PWCs at each outputfiber is .

In the DOWCA phase, the wavelength channels of the output

fiber are sequentially scanned. When the wavelength channel

Fig. 4. PWCfirst available (PFA) scheduling algorithm flow chart.

is considered, the packets in the set are scanned. If

there exist such packets as from different input wavelength

channel in the set , one of the packets randomly selected, is

scheduled to be on the output wavelength channel . Then

one packet and the channel are removed from and

, respectively. If there is no packet in the set , the

wavelength channel remains available and it may serve

contending packets in the LPA phase. The computational com-

plexity of DOWCA phase is for each output, because

each output wavelength channels have to be scanned.

In the LPA phase, the remaining packets direct to the outputfiber are gathered in the set . Notice the LPA phase contains

three parts: PWCs assignment, LRWCs assignment and both

PWCs and LRWCs combined assignment, which are denoted

by the variable in Fig. 4, respectively. As

long as and re not empty and at least one wavelength

converter is available, the set is scanned from to

. Firstly, PWCs are selected for converting the packets to

idle output wavelength channels by canned from to

. When the channel is scanned, if there exist a set

belonging to without the intersection of and empty

and the PWCs on the output fiber are available, a packet

selected randomly in , is scheduled on the output wavelength

channel . Then the packet and the channel are re-

moved from and , respectively. And the state of the PWC

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LIUet al.: A NOVEL CONTENTION RESOLUTION SCHEME OF HYBRID SHARED WAVELENGTH CONVERSION FOR OPS 225

Fig. 5. An example of packets schedule for an output fiber carrying 4 wavelengths. (a) Five arriving packets. (b) S cheduled packets at the end of the DOWCAphase. (c) Scheduled packets at the e nd of the LPA phase.

used is updated. Secondly, LRWCs are selected by scanned

from to . When the channel is scanned, if

there exist a set belonging to without the intersection of

and empty and available LRWC, a packet se-

lected randomly in , is scheduled to the output wavelength

channel . Then and are updated. And set .

Finally, both LRWCs and PWCs are used by canned fromto When the channel is scanned, if there

exist a set belonging to without the intersection of and

emptyand there are LRWCs and PWCs available, a

packet selected randomly in , is scheduled to the output

wavelength channel . Then and are updated. Both the

state of the PWC used and the variable are updated, too. In

the end, the packets still in the set are discarded. Given that

in the worst case, the computational complexity of LPA phase

is for each outputfiber because of scanning

the output wavelength channels for three conversion methods.

Therefore we can confirm that the PFA algorithm computa-

tional complexity is obtained by the sum of the complexity in

the DOWCA phase and LPA phase as

for each output fiber. Because all of the output fibers

have to be scanned, the computational complexity of PFA algo-

rithm in the HSWC architecture is .

The LFA scheduling algorithm, different from PFA algorithm

in the LPA phase, tries to find firstly whether available LRWCs

can solve the contention, then whether available PWCs, and last

whether combination of a LRWC and a PWC. If one of the three

methods could solve the contention, the next channel will beconsidered like this until the circle is end.

Fig. 5 shows an example of packets scheduling in the OPS

node architecture with HSWC. The architecture includes

input fibers and output fibers and each fiber supporting

wavelengths.

In Fig. 5(a), four packets are arriving on wavelength and

one packet is arriving on wavelength . Given the available

number of LRWCs , conversion range and output

fiber . And there are one PWC shared for each output fiber

without guard bands. The ratio of coupler before PWCs is 2:1,

meaning that the parametric is 2. In particular, the conversion

process of PWC includes and .

The actions performed in DOWCA phase are described by

Fig. 5(b). Packets and are scheduled for the channel

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and , respectively. The rest idle wavelength channels

will be scheduled for packets in LPA phase.

The results of packets scheduled at the end of LPA phase are

described in Fig. 5(c). For the channel , there is no packet

can be scheduled just by PWC or just by LRWC. So LRWC and

PWC have to be combined for a packet. Firstly, the wavelength

carrying the packet is converted to wavelength by

a LRWC. Then the PWC can be used to change the wavelength

from to . Thus the packet has been scheduled for the

channel . As for , there are no packets can be scheduled

directly. But wavelength carrying packets can be converted

to by the PWC. So a packet can be selected randomly from

the rest packets, suchas . Intheend,both ofthe PWC input

ports are occupied and one of the LRWCs is used. The packet

has to be dropped because of no idle output wavelength

channel.

IV. SIMULATION ANALYSIS ANDDISCUSSION

Firstly we discuss optical packet loss probability (PLP) per-formance among HSWC, SPN-PWC and SPN-LRWC architec-

tures. Provided the OPS nodes are synchronous, meaning that

all arriving packets have a fixed size and these arrived on each

wavelength are synchronized in a time-slot. It is assumed that

thepackets arrived on theNWinput wavelength channels at each

time slot are independent, and a packet on each input wave-

length channels occurs with probability that is equal to traffic

load. Also, each arriving packet has the same probability

direct to one of any output fibers. In the following simulation,

we assume that the output or input fiber number equals 4, and

wavelength channel number equals 32 for the three architec-

tures. Especially, PWCs number equals 1 for each outputfiber

in HSWC architecture.In order to contrast the performance of SPN-PWC,

SPN-LRWC, and HSWC, we keep equal number of wave-

length converters in the three architectures. The sum of PWCs

number and the LRWCs number in HSWC architecture is equal

to LRWC number in SPN-LRWC, and also equal to PWCs

number in SPN-PWC. The parameters of LRWCs in HSWC

are same with these in SPN-LRWC, such as conversion range

. And the parameters of PWCs in HSWC are same with these

in SPN-PWC, such as coupler ratio . For example, when the

number of wavelength converters , it is meaning

that there are 4 PWCs and no LRWCs in HSWC, 4 PWCs in

SPN-PWC, and 4 LRWC in SPN-LRWC. Actually, we com-pare the performance among the three architectures when their

implementation costs are different. Therefore, when the same

number of wavelength converters is used, the cost of HSWC

is lower than that of SPN-LRWC, and it is higher than that of

SPN-PWC.

Figs. 6 and 7 show the differences of packet loss prob-

ability in SPN-PWC, SPN-LRWC and HSWC when traffic

load and , respectively. It is obvious that the

packet loss probability cannot be further cut down in the three

architectures when reaching certain level. This is because that

conversion is limited or there are no idle wavelength channels

for contending optical packets. Also, we can find from the

comparison among the three architectures that HSWC can ob-

tain the lowest packet loss probability value when wavelength

Fig. 6. PLP of SPN-PWC, SPN-LRWC and HSWC when wavelength con-verters number is changed .

Fig. 7. PLP of SPN-PWC, SPN-LRWC and HSWC when wavelength con-verters number is changed .

converters are enough. Actually, the HSWC architecture can

get lower packet loss probability than the SPN-LRWC with

equivalent wavelength converters number. On the other hand,

HSWC architecture can greatly reduce the cost for reaching cer-

tain packet loss probability when compared with SPN-LRWC.

Although the cost of SPN-PWC is lower than that of HSWCwhen the number of wavelength converters is same, the PLP of

SPN-PWC is far lower than that of HSWC.

Fig. 8 shows thepacket loss probability against the traffic load

for SPN-PWC, SPN-LRWC and HSWC. We assume that there

are sufficient wavelength converters ( is enough

here) for gaining the lowest PLP value at the traffic load from

to . From the results we can see that the PLP of

three architectures would go up when increasing the traffic load.

The reason is that the resources of output wavelength channels

for contending optical packets become scarce when increasing

the traffic load. Furthermore, we can find that the PLP of the

HSWC is obviously lowest of the three at any traffic load.

From Figs. 79, we can conclude that the performance of

OPS node is obviously improved by the PWCs together with

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LIUet al.: A NOVEL CONTENTION RESOLUTION SCHEME OF HYBRID SHARED WAVELENGTH CONVERSION FOR OPS 227

Fig. 8. PLP of SPN-PWC, SPN-LRWC and HSWC when traffic load ischanged .

Fig. 9. PLP of PFA and LFA algorithm in HSWC .

LRWCs. The PWC can convert multiple wavelengths simulta-

neously; so it can avoid the waste of LRWCs. Above all, a con-

tending packet has a third better way to be converted by succes-

sively using LRWCs and PWCs. This way can further reduce

PLP because of using PWCs together LRWCs.

Then we compare the performance of PFA and LFA sched-

uling algorithm for the HSWC architecture in the Figs. 9 and 10.

When increasing wavelength converters number, PFA and LFAalgorithm can be gradually close to the lowest packet loss prob-

ability. Before reaching the lowest packet loss probability, we

can obviously see the difference. The PFA algorithm can reach

the lowest value earlier than LFA algorithm, and the PLP of

PFA is always lower than that of LFA algorithm. That is to say,

the PFA scheduling algorithm can maximize the utilization of

wavelength converters. Therefore the PFA algorithm can take

advantage of the benefit of PWCs that allow multiple wave-

lengths to be simultaneously converted from one to another.

We further study the influence of the LRWC conversion range

on the HSWC architecture. As is shown in Fig. 11, the packet

loss probability is a function of the LRWCs number when

and . We can find that

the lowest PLP of different conversion ranges are obtained by

Fig. 10. PLP of PFA and LFA algorithm in HSWC .

Fig. 11. PLP of HSWC when the conversion range of LRWC is changed.

increasing conversion range. But we must note that the decre-

ment is died out after because of no idle wavelength

channels. And the decrements become smaller and smaller with

the increasing conversion range. For example, the lowest PLP

is , and corresponding to the

conversion range , and , respectively. We can easily

get that the decrements are and . However,

the cost of LRWC is more expensive when increasing conver-

sion degree. Therefore we should make compromise between

the performance and the cost.

V. CONCLUSION

This paper presents an optical packet contention resolution

scheme with hybrid shared wavelength conversion (HSWC) for

all-optical packet switching nodes. And the parametric wave-

length converterfirst available (PFA) algorithm is proposed for

the HSWC architecture to achieve a maximum of wavelength

utilization. The HSWC architecture can largely decrease the op-

tical packet loss probability largely compared with SPN-PWC

and SPN-LRWC, respectively. Therefore the HSWC architec-

ture can improve the utilization of wavelength converters and

reduce required wavelength converters number. It is helpful to

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design and implement the all-optical packet switching. More-

over, we have to note that some factors would influence the PLP

in HSWC, such as the ratio of LRWC number to PWC number,

input port number of PWC, input fibers number and wavelength

channels number.

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Huan-Lin Liu received the Ph.D. degree from Chongqing University,Chongqing, China, in 2008 and the M.S. degree from Chongqing University ofPosts and Telecommunications, Chongqing, China, in 2000.

Her research interests include all-optical network research, all opticalswitch structure and scheduling algorithm research, information acquiring, andprocessing.

Bin Zhangreceived the M.E. degree from the Chongqing University of Postsand Telecommunication, Chongqing, China, in 2011.

His research focused on optical switching and scheduling algorithms. He iscurrently working toward the Ph.D. degree in the Beijing University of Postsand Telecommunications, Beijing, China.

Song-Lei Shireceived the B.S. degree from the PLA Information EngineeringUniversity, Henan, China, in 2009. He is currently working toward the M.E. de-gree at the Chongqing University of Posts and Telecommunication, Chongqing,China.

His research interests include all-optical wavelength conversions, opticalpacket switching systems, and optical multicast.

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