wdm and sdm in future optical networks

7/30/2019 WDM and SDM in Future Optical Networks

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Abstract

Increasing bandwidth demands, caused by growing

numbers of users, increasing popularity of the Internet,

multi-media services, and higher demands on quality, urge

the development of ever faster networks. To meet these

demands, optical techniques are being introduced in pub-

lic networks but on a link-to-link basis. This paper

addresses future networks that will be to a large extent all-

optical. A comparison is made of the strengths and weak-

nesses of two multiplexing techniques: wavelength divi-

sion multiplexing (WDM) and space division multiplexing

(SDM). It is prompted by the fact that both SDM and

WDM are viable options for the optical layer of all-optical

networks. Several issues are considered: switches, single-

and multi-hop operation, core and access network, and

scalability.

1. Introduction

In recent years an increasing amount of research has

been devoted to the development of multi-wavelength

optical networks [4,13,20,21]. The goal is to exploit the

capacity of optical fibres better, which is much greater

than single wavelength systems can utilise. Whereas cur-

rent systems provide several hundred Mb/s to a few Gb/s,

WDM systems promise capacities on the order of several

Tb/s without dramatic electronic speed improvements.

Thus, a new era of cheap, massive bandwidth is envi-

sioned.

Among the challenges is to keep signals in the photonic

domain. We then speak of all-optical networks. Thesehave several advantages. One is protocol transparency, to

the point that signals may be analogue or digital. WDM

networks can transport these over the same fibres, at dif-

ferent wavelengths. An all-optical infrastructure is bit-rate

independent, so it can be used at ever higher speeds, which

makes it future proof.

Yet, is all-optical multi-wavelength technology really

needed to provide Gb/s speeds to the user? Several factors

impede the deployment of large scale all-optical networks.

Also, SDM is often a good alternative for WDM, even if it

does not provide high utilisation of bandwidth in the fibre.

We shall compare WDM and SDM as they are the basic

options for the construction of all-optical networks. They

define the lightpaths in which other multiplexing tech-

niques: TDM, SCM, and CDMA, can be used. Those help

diminish the complexity of switching but we will not look

at them here. A comprehensive study of combining several

switching techniques is presented in [26]. Other work on

this topic can be found in [12] and [27].

To see how SDM can be competitive with WDM, con-

sider the following points:

1. All-optical networks have physical limitations due to

a.o. cross-talk, noise, wavelength alignment, non-line-

arities [5]. These make purely all-optical networks

infeasible.2. In the access network, fibres are usually laid in large

bundles. In hybrid fibre-coax (HFC) networks, e.g.,

typically 48 or 96 fibres. Also in other topologies fair

multiplicities may be expected. It is then more natural

to use separate fibres than wavelengths.

3. Wavelength reuse allows a modest number of wave-

lengths to serve large numbers of users: [6] shows that

multi-hop networks with 8 to 32 wavelengths can serve

up to hundreds of millions of users. [24] presents com-

parable results for smaller networks.

4. Fibre amplifiers work over a narrow wavelength range:

30 nm. However, certain WDM devices require wide

wavelength spacing (about 9 nm in acoustooptic fil-ters). This limits the number of wavelengths to 3 or 4

where amplification is needed.

The results of point 3 can also be applied to SDM net-

works if one exchanges wavelengths for parallel fibres.

One could conclude that where fibre multiplicity is suffi-

cient, SDM can do the same job as WDM. But if this is so,

why should we use WDM?

WDM and SDM in Future Optical Networks

H.J.H.N. Kenter, S.M. Heemstra de Groot

Tele-Informatics and Open Systems GroupDepartment of Computer Science, University of Twente

P.O. Box 217, 7500 AE Enschede

The Netherlands


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1. WDM provides more bandwidth when there are very

few fibres, e.g., in long-haul lines like the Transatlantic

line [13]. Bit rates can be increased only up to a limit

(about 40 Gb/s) because of dispersion. WDM is the

solution to make more bandwidth available.

2. WDM allows new users or sub-networks to be con-

nected to existing fibre networks without laying more

fibres, by adding wavelengths. This makes WDM bet-

ter scalable.

3. Signals must be amplified after certain distances or

splitting. A single amplifier can amplify all wave-

lengths in the erbium window. This reduces amplifier

costs for WDM.

4. As we shall see in section 2, WDM can lead to smaller

or cheaper switches.

We see that SDM is a strong alternative to WDM where

fibre multiplicity is sufficient. On the other hand, WDM

has some inherent advantages. So, we must compare them

further. For instance, what switching techniques can be

used? How well do they scale? Which technique leads to acheaper network? But also: can SDM and WDM be used

together?

2. Switching techniques

In this section we will have a look at some switch

types. Figure 1 shows a part of a network with a number of

switches. In space multiplexed networks, signals are trans-

ported over separate fibres, while in the wavelength multi-

plexed networks they can be carried over one fibre. For

simplicity, we will not look at mixtures, which could be

necessitated by limits in the number of wavelengths per

fibre, e.g., due to fibre amplifiers.

Obviously, in the space switched network space

switches must be used. For WDM networks there are more

options, which take advantage of the nature of WDM. We

shall look at the use of 12 and 22 space switches and theacoustooptic tunable filter (AOTF) as basic switching ele-

ments. We will not consider packet switches because we

do not expect them to become practical soon, largely

because optical buffering is so difficult.

2.1. 12 and 22 space switches

Figure 2.a shows two basic space switches: the 12switch and the 22 switch. Besides the suggested states,they may also be used to broadcast (at the penalty of a

power split). Various switching methods can be used in

these devices, e.g., directional couplers, or Mach-Zehnder

interferometers. For an introduction see [13,19,27].

The various possibilities for the realisation of these

switches give rise to different physical characteristics, e.g.,

wavelength dependency, polarisation dependency, switch-

ing speed, cross-talk, and signal loss. We will not take this

into account in our discussion.

2.2. The acoustooptic tunable filter

Acoustooptic filters, shown schematically in Figure

2.b, are four-port devices with an additional control port.

The normal ports are the two input and output fibres, the

control port is the RF signal driving the Surface AcousticWave (SAW) transducer, which defines a number of super-

imposed gratings in the optical path. The device can be

used as a tunable filter, wavelength selective splitter, com-

biner, or 22 switch. The tuning range is about 200 nm.Because of the wavelength spacing of about 9 nm, about

22 wavelengths can be resolved. The tuning time is on the

order of microseconds, so AOTFs are best used for fairly

long-lived connections. It is clearly too long for per-packet

switching.

AOTFs can route multiple wavelengths simultaneously.

The RF signal applied to the SAW transducer puts the

device in cross or bar state for each wavelength individu-

ally. For our discussion, this is all we need to know. Formore information, see [13]. Section 2.4 shows that these

routing properties can lead to smaller switches. The

number of switched wavelengths is programmed by the

SAW transducer input. We shall say that AOTFs are soft

with respect to the number of wavelengths. An AOTF-

based network can easily upgrade to a larger number of

wavelengths. In a space switched network this requires the

installation of new equipment.

n

inputs outputs

Figure 1. a. Part of a network, b. Switch with n = 4 channels per link

one fibrefour wavelengths,

or four fibresa. b.


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2.3. Compound switches

In the literature a great deal has been said on the con-

struction of switches from basic switching elements. For

circuit switches there are a few important issues: the

number of switching elements, signal degradation, thelevel of blocking, and ease of routing.

A number of well-known composition methods are: the

Clos method, the tree method, the Benes network, and the

shuffle configuration. Of these, the Clos and tree method

yield strictly non-blocking switches, the Benes network

essentially non-blocking switches, and in shuffle networks

it depends on the number of shuffle stages. Essentially

non-blocking switches are rarely if ever used for circuit

switching as existing connections may have to be re-

routed. Routing decisions are complicated in shuffle net-

works. So, for non-blocking switches the Clos and tree

methods are the best options. These can be further com-

pared with respect to the number of switching elements

and the number of elements in the signal path (signal deg-

radation). In large switches it is unlikely that all inputs and

outputs will be used simultaneously, so a non-zero level of

blocking is often acceptable.

The Clos method (Figure 3) divides an switch

into a column of switches and two columns of

N Nk

N

n----

N

n---- N

n----

switches. The condition must be met.

Recursive application with and leads to a

switch of switch ele-

ments or, in terms of , . This

assumes that the and switches are built as in

Figure 3.c. The number of elements in the signal path isthen bound by .

Figure 4 shows the tree method, which leads to a

shorter signal path length and, for small , to smaller

switch sizes. The switch size is given by

. With , this yields

. The signal path length is .

2.4. Wavelength switches

WDM switches can be built from space switches with

multiplexers and demultiplexers, or from devices that

exploit the wavelength multiplexed nature of the signals.

Figure 5 shows a four fibre, four wavelength switch using

a space switch. Clearly, a full-blown 1616 space switch isnot needed: it would allow superfluous paths that could

lead to conflicts. Instead, only one 44 space switch isneeded per wavelength, as in Figure 5.b.

Figure 5 shows how a switch for the same number of

fibres but variable number of wavelengths can be built

n k k 2n 1n 2= k 3=

2K

2K M

2K

32 3K 1

28 2K 1+=

N MN

32

3------ N

3 2 14N+=n k k n

2 4K+ LK

2 6K+

N

M2

K 22K 1+

2K 1+

= K Nlog2

=

MN

O N2

= LK 2K=

surface acoustic wave

SAW transducer

inputs outputs

polarisation splitters

Figure 2. a. Space switches and b. acoustooptic tunable filter, symbols and states

symbol up state down state

cross statebar statesymbol symbol

12 space switch, symbol and states

22 space switch, symbol and states

a. b.

N

nknk

nk

N

n----

N

n----

N

n----

N

n----

N

n----

N

n----

N

knkn

kn

Figure 3. Clos strictly non-blocking network

22

23 =

=

b.

a. c.


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using AOTFs. As long as the number of wavelengths can

be accommodated by single AOTFs, the number of

AOTFs depends only on the number of input and output

fibres. For an 88 switch operating at sixteen wavelengths,the space switch needs switch elements,

while an AOTF-based switch needs 20 AOTFs, and is able

to switch any number of wavelengths up to a certain maxi-

mum. This is true for all methods mentioned in Section2.3.

This comparison is a little unfair because AOTFs are

bigger than 12 switches. Also, it may prove difficult tointegrate several AOTFs on a chip because of their size

and the power needed to drive the SAW transducers. Onthe other hand, a reasonable number of 12 elements canbe integrated on chip. [14] reports a 44 switch using 24such elements in tree configuration. Moreover, the cross-

talk characteristics of AOTFs may be worse. Lastly, as we

saw before, the use of fibre amplifiers may limit the

number of wavelengths. Also, space switches can take

16 80 1280=

inputs outputs

Figure 5. Switch with four fibres, variable number ofwavelengths using AOTFs

advantage of WDM techniques by converting to wave-

length multiplexed format intermediately. An example

where space and wavelength multiplexing are combined

can be found in [22].

From the discussion of space switches and AOTF-

based switches, we find:

WDM enables switches of far fewer elements than cur-

rent SDM technology allows. WDM networks can be soft w.r.t. the number of

wavelengths.

However, AOTFs also introduce new problems:

They require fairly wide wavelength spacing.

They limit either the number of wavelengths to 3 or 4,

or the optical path length.

The tuning time is fairly long.

2.5. Wavelength conversion and wavelength reuse

In the switches discussed so far, signals stay at the same

wavelength and so they will along the whole path through

the network. A request for a connection over a path isblocked when no wavelength is free on all links along that

path. Wavelength conversion enables a different wave-

length to be used on each link, and so decreases the block-

ing probability. A simple way to convert wavelengths is to

detect the optical signal and use the resulting signal to

modulate a properly tuned laser.

Figure 4. Tree construction of strictly non-blocking switches and a 44 example

nn

nn

nn

nn

n

n

n

n

nn

n

n

n

n

n

n

n

n

n

n

1

1

conflict 1

2

3

4

Figure 6. Four fibre, four wavelength space switch. a. Full-blown, b. One switch for each wavelength

a. b.


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Wavelength conversion also enhances wavelength

reuse, that is, the use of the same wavelength in different

parts of a network. Wavelength reuse and wavelength con-

version together make possible the construction of net-

works of many more nodes than wavelengths, to the point

claimed in [6] where the network size becomes virtually

independent of the number of available wavelengths.

WDM networks without wavelength conversion can be

mimicked by SDM networks. A WDM signal is identified

by its wavelength, an SDM signal by the fibre it is on. Letus now tag each fibre in a bundle with a label. A WDM

switch without wavelength conversion corresponds with

an SDM switch in which signals enter and leave at fibres

with the same label. Wavelength converting WDM

switches are mimicked by SDM switches that can route

outgoing signals on any fibre in a bundle. We shall call

such SDM switches and networks label-free. Clearly,

wavelength conversion is not an inherent advantage of

WDM.

WDM switches with wavelength conversion and label-

free SDM switches must be able to route a signal from any

input to any output. A WDM switch for wavelengths

and input and output fibres can be built from a space switch and wavelength convertors that convertto a fixed wavelength, Figure 7. This configuration can be

found in, e.g., [11,24]. A corresponding label-free SDM

switch needs only the switch.

c

P cP cPcP

cP cP

WDM devices can lead to smaller designs: tunable

wavelength convertors allow a simplification of the space

switch. Figure 8.a shows a space switch partitioned into

44 switches. This is wasteful as it takes more switch ele-ments and results in a longer signal path. However, when

used in Figure 7.b, tunable wavelength convertors make

the third column unnecessary, Figure 8.b. AOTFs lead to

even smaller switches, but they need two tunable wave-

length conversion stages to ensure that signals that enter or

leave at the same fibre, use distinct wavelengths in theswitching stage (Figure 8.c).

It depends on the cost of tunable convertors whether

WDM switches with wavelength conversion can be

cheaper than label-free SDM switches. Section 3 shows

that wavelength conversion can also be achieved in the

access nodes of multi-hop networks. So, the switch size

and the mode of operation (single- or multi-hop) deter-

mine whether WDM or SDM leads to cheaper switches in

a network with wavelength or label conversion.

3. Single-hop and multi-hop routing

In contrast to single-hop systems, which route a con-nection directly from its source to its destination, in multi-

hop networks a connection may exit the optical network at

various intermediate nodes, where routing decisions are

made, and be re-inserted in the optical network. After a

fibrebundle

Figure 7. a. Label-free SDM switch, b. WDM switch with wavelength conversion

wavelengthconvertor

=

a. b.

b.a.

Figure 8. a. Label-free SDM switch built from 44 elementsb. WDM switch with tunable wavelength convertorsc. AOTF-based switch with wavelength conversionc.


6/10

number of hops, the connection reaches its destination.

Multi-hop networks provide packet-switching and allow

lightpaths, and therefore transmission capacity, to be

shared. The lightpaths form a network in its own right,

called the virtual topology. For more on multi-hop routing,

see [6,21]. A multi-hop network, discussed extensively in

[6], with AOTFs and access nodes with ATM switching

capability, is drawn in Figure 9.

The figure shows two connections, one of which is

routed in three hops. The multi-hop connection uses dif-

ferent wavelengths on its hops. The wavelength conver-

sion is achieved in the access nodes by electronic means.

The overall blocking probability between source and des-

tination can be much smaller than in a single-hop network.

Single-hop routing is often said to scale worse than

multi-hop, even with wavelength conversion. However,

the results of analysis and simulations in [24] for arbitrary

topology networks with fairly few nodes (up to one thou-

sand), are not dramatically different from those for multi-

hop networks [6], which are based on ShuffleNet. Theauthors of [6] expect different, but not radically different

results for other virtual topologies. They express the need

to develop of algorithms to construct virtual topologies

that are scalable and modular, can be made to fit geo-

graphic patterns, can be adapted to changing traffic condi-

tions, and can route around network faults.

Circuit-switched connections have the full bandwidth

of the channels they use. In optical networks, this is the

bandwidth of the transmission equipment on the fibre or

the wavelength. Multi-hop connections occupy as many

receiver-transmitter pairs as hops, and take capacity from

the switches at the access nodes. They may take a longer

overall route through the optical network and use up a

larger portion of the network capacity than strictly neces-

sary, depending on the efficiency of the virtual topology.

The aforementioned study [6] shows that a ShuffleNet

serving 100 million users requires 12 hops on average

when 8 wavelengths are used. Multi-hop access nodes

require routing capability and are therefore expensive

(Figure 9, due to [6]). Single-hop access nodes only need

as many pairs of transmitters and receivers as the number

of connections they must support.

Multi-hop networks fully regenerate signals at the

access nodes. If hops can be kept within a certain length

(not in long-haul links) less amplifiers are needed, and

AOTFs can be used without being limited to 3 or 4 wave-

lengths. A hierarchical routing approach can enable this.

In this approach, the network is divided in areas. The first

level routing decision is which areas to cross. The second

level routes connections within an area. In single-hop net-

works signals stay optical, so amplification and regenera-

tion take extra equipment.Packet-switching is easy to do in multi-hop systems by

putting packet switches in the access nodes. For single-

hop routing it is much more difficult because it requires

optical packet-switching. High-speed switching tech-

niques are under development [8,9,22], but buffering is

very difficult. Options include: routing packets through

big coils of fibre, deflection routing [21] (or hot potato

routing [28]), or routing on another wavelength. Without

buffering, performance will be unacceptably low. The

technology for multi-hop routing is rapidly maturing, but

that for all-optical packet and ATM-cell switching, is still

experimental. So, for the near term future multi-hop rout-

A A

A

AA

A

A = access node

Figure 9. A multi-hop network, with details of the optical cross-connects and the access nodes. This figure is due to [6].

2.5 Gb/s

Cross-connect

LocalAccess

OC-48 ADM

STS-12c STS-3c

8x8 STS-3c

ATM Switch

STS-12c

Access

Switch

1

8

1

8

RCVRArray

XMTRArray

STS-3cLocal Access

STS-12cLocal Access

1

2

1

1

all-optical cross-connectnetwork


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ing is the most attractive option for packet-switching.

Because the ITU-T has specified ATM as the transmission

mode for B-ISDN, multi-hop routing seems the best candi-

date for a future optical public network.

Although multi-hop operation is often discussed with

respect to WDM, it should be clear that multi-hop routing

can just as easily be implemented over SDM networks.

The cost implications and control complexities are diffi-

cult to compare. Therefore, the choice for single-hop or

multi-hop does not lead to a clear answer as to which of

WDM and SDM should be used. On the other hand,

regardless of the latter, we have seen some compelling rea-

sons to use multi-hop routing.

4. The core network versus the access

network

In this section we will examine where SDM and WDM

are best used in the core and the access network. Let us

first make clear what we mean by core and access net-work. Simply put, the core network is the network that

interconnects user areas, while the access network con-

nects the customers within a user area, which is a fairly

small geographical region. The access network is a collec-

tion of small and metropolitan area networks with high

connectivity and many terminals. It extends from the user

terminals to a point where high traffic aggregation is

reached. In telephone and ATM networks, it extends to the

Local Exchange (LEX) or Transit Exchange (TEX)

switch. Examples include: city neighbourhoods, business

parks, corporate networks, LANs. The core network con-

nects access networks. It is less dense and carries highly

aggregated traffic. This definition does not make a sharp

distinction but allows us to discuss these aspects of public

networks.

4.1. The core network

The core network crosses long distances, using a lot of

so-called long-haul lines. Because of attenuation in the

fibre, amplification and signal regeneration are needed.

Fibre is already employed in the core network, together

with other technologies such as copper, radio links, and

satellite. Fibre is commonly used with synchronous

digital transmission systems: PDH, SDH, SONET.Common topologies are ring and star.

The fibre multiplicity is usually small. The Transatlan-

tic cable, for instance, has only two fibres.

The topology of the core network changes as nodes and

trunks are added, but this happens at a modest rate: cit-

ies, business parks, and campuses do not pop up very

fast.

4.2. The access network

The access network covers relatively small areas, up to

50 or 100 kilometres across. Link lengths may be short

enough that no amplifiers and signal regenerators are

needed. As suggested earlier, the right virtual topolo-

gies may make this possible. However, certain topolo-

gies bring about splitting of fibres, which necessitates

amplification.

In the access network fibre is mostly used where aggre-

gation is high. This is because bringing fibre to the

home takes enormous investments. An attractive can-

didate for getting fibre nearer to the home is hybrid

fibre coax (HFC).

Fibre multiplicity is bigger in the access network than

in the core network because of the large number of ter-

minals to be connected.

The access network undergoes frequent topology

changes: new terminals are added daily. Individual

additions may not give rise to topology changes, butafter a number of additions the network must adopt a

new configuration to serve traffic flows better.

In the access network traffic is less aggregated than in

the core network. Therefore, there may be stronger

traffic fluctuations that the network has to cope with.

Currently, HFC networks [18,23] are mostly disjoint from

telecommunication networks, but this is rapidly changing.

Many activities are going on to provide new services over

HFC, like telephony [7] and data-oriented services such as

video on demand and Internet. These are pursued by

DAVIC and IEEE 802.14 [3,10]. HFC may play an impor-

tant role in future optical networks simply because they

are there. If their fibre can be included in the public net-work, big savings can be made on fibre installation [2].

The star, or sometimes ring of stars, topology of HFC net-

works reflects their aim, which is distribution of TV sig-

nals. There may be as many as 200 to 5,000 homes on a

fibre. HFC is not designed for two-way communication, so

that is a challenge to achieve. Work is being done to intro-

duce interactive services in HFC networks through the use

of WDM [15].

There are considerable differences between access net-

works. LANs and corporate networks are intended for

inter-user traffic, while in blocks of homes inter-user traf-

fic is much less. In the public network, therefore, the part

of the access network nearest to the user needs only multi-

plexing capability, and switching is postponed until multi-

ple multiplexed traffic streams come together at the LEX.

In particular, the access nodes of multi-hop networks

could assume the function of a LEX, so that customer

premises equipment (CPE) needs only be able to multiplex

and can be kept affordable.


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4.3. Where SDM, where WDM?

The introduction argued that in the access network we

may as well use SDM because fibres are laid in bundles of

ample multiplicity. SDM is good for static network topolo-

gies with ample multiplicity. In the core network, fibre

multiplicity is low while traffic is highly aggregated, so

WDM is needed for high connectivity. For scalability,WDM is the method of choice. At and near the user

premises, equipment must be inexpensive. CPE needs to

have no more than de/multiplexing capability; the cabling

serves as a distribution network to a node with switching

capability. Multiplexing nodes could be used near the cus-

tomer premises to take care of the de/multiplexing func-

tions, so that only distribution is required in the last mile

to the customer. Between multiplexing node and switching

nodes, WDM can be used to anticipate future network

growth. Given the current network infrastructure, we

arrive at the following, see Figure 10:

WDM in the core network because of low fibre multi-

plicity. WDM in the switching part of the access network for

reasons of scalability.

SDM in the distribution part of the access network

because of costs.

Optionally, multiplexing nodes to further reduce CPE

costs.

We find that WDM is the most appropriate multiplex-

ing method for the core network and the switching part of

the access network. Moreover, it is pursued by so many

research groups that it is bound to happen some day, so it

simply will enter the public network sooner or later. How

then, can we make SDM and WDM cooperate? Evolution

will be needed from SDM networks to WDM. If we do not

want to connect them in the electrical domain, we must

find ways to connect them optically. This raises a number

of questions: Can WDM signals be routed through SDM

networks? Do we have to convert between WDM andSDM? The latter seems obvious, but can we avoid it to

some extent? Is cooperation of SDM and WDM so diffi-

cult that we had better to forget about the whole exercise

and join networks electrically? Let us make an inventory

of what can and cannot be done:

At small ranges a transparent SDM network can trans-

port WDM signals as lump signals, Figure 11. At

longer ranges this causes problems, think of signal

regeneration.

Wavelength multiplexed signals need demultiplexing

and/or wavelength conversion to route signals individ-

ually. This could be done near the SDM/WDM border.

Wavelength conversion is not needed when we routeWDM signals through an SDM network. It is needed

for routing SDM signals through WDM networks.

The access nodes are a natural place to join SDM and

WDM networks.

Figure 12 illustrates the second bullet point. The last

possibility, joining SDM and WDM networks in multi-hop

access nodes in the multi-hop scenario was discussed ear-

lier.

A

futureconnections

1

new

multiplexingnode

accessnetwork

WDM WDM

SDM

Figure 10. WDM and SDM in the core and access network

corenetwork

WDM SDM WDM

1

2

3

1

2

3

Figure 11. SDM network passing WDM signals as lump signals.


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5. Conclusions

We have compared WDM and SDM for use in future

optical networks. WDM enables networks with less fibres,

less optical amplifiers, and less complex switches. On the

other hand, in small areas where fibres are already availa-

ble in bundles of high multiplicity, SDM can provide the

same connectivity. Also, space switches behave better

regarding signal degradation and seem to allow a higherdegree of on-chip integration. The promise of WDM of

enabling more bandwidth and simpler switches is there-

fore not decisive.

WDM offers better scalability, with respect to gradual

increase of bandwidth and continuous addition of users

and sub-networks. SDM is better for the distribution part

of the access network. Costs impede the introduction of

WDM in the customer premises at short term. Therefore,

SDM will continue to play an important role in the access

network.

Since the public network is expected to offer B-ISDN,

public optical networks must be able to perform packet

(ATM cell) switching. As this is difficult to realise in sin-

gle-hop mode, the best suited routing mode is multi-hop.

This also leads to better control of signal degradation.

Multi-hop routing is not an entirely all-optical solution,

but studies by other authors have shown that large scale

all-optical networks are not feasible anyway.

Last but not least, SDM and WDM can be combined in

the same network, which is important for network evolu-

tion.

6. References

[1] Barry R.A., Humblet P.A., Models of Blocking Proba-bility in All-Optical Networks with and Without Wave-

length Changers, IEEE Journal on Selected Areas in

Communications, vol. 14, no. 5, pp. 858-867, June 1996

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SDM WDM

1

2

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more

WDM

networks

Figure 12. De/multiplexing and wavelength conversion between SDM and WDM network

MUX


10/10

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wdm and sdm in future optical networks

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