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Chapter 2. Internet Protocol and Optical Networking

Chapter 2. Internet Protocol and Optical Networking

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2.1 Increasing Network Efficiency

One problem with any stacking of layers, is that the grooming of each layer's traffic into demands on the next lower layer inevitably fails to

yield 100% fill. The net utilization multiplies all these fractional fill levels and becomes poorer with increase in the number of layers. For

instance, the assignment of logical IP links between routers onto ATM VCs or VPs may typically result in 80% utilization (or less) of the

bandwidth actually allocated to those VPs. Similarly the STS-3c fill from the ATM VP cell flows may be only 80%. And the fill of the

OC-192s on wavelengths in the optical layer may itself be only another 80%. The net utilization of an OC-192 lightpath with customer

payload is the product of each individual trans-layer efficiency, or, in this example: 0.8 x 0.8 x 0.8 which is ~51%. Figure 2-1(a) illustrates

this cascading of fractional fill levels down the four-level stack. The overall inefficiency is greater still if we also have ring-based protection

at the SONET layer. At its most efficient, SONET ring protection adds another multiplier of 0.5 because working and protection channels

are exactly matched.



Figure 2-1. Evolution toward "IP over optics:" four layers down to about "2.5 layers."



Achieving higher overall efficiency is one of the central aims of the mesh-based approach to survivability. Despite what commentators

sometimes say, capacity is not free and capacity efficiency is important. Even if capacity was thought to be free (and/or a "glut" of dark fiber

abundant in the ground)—planning, installation, testing, and management to bring on additional "lit-fiber" capacity, and integrating it into

operational use is certainly not free—so efficiency in its use always pays dividends. An efficient network is inherently more flexible and

able to absorb more demand growth or churn before physical additions of new plant and capacity have to be installed. With the Internet

driving growth at up to 200% per annum, the problem has often been that capacity simply cannot be laid down fast enough and precisely

where it is needed to fully keep up with the growth. In these cases efficiency in the first place, by design, is as good as extra capacity on

the ground.

But in fact the simple cost of equipment for added capacity—in the quantities needed is by network operators—is also by no means free.

Since much of this book is aimed at the design of capacity-efficient mesh-survivable networks, it is important to address the often

[1]

remarked view that "capacity is free." If so, why would one worry about optimizing capacity efficiency? The basis for this is typically a

calculation that assumes a lightpath or fiber bearing, say, a fully-loaded OC-192 and takes the ratio of total cost to total Mb/s carried. The

numerical ratio is indeed small. Notwithstanding this, an inter-exchange carrier may have annual budgets for incremental transport

equipment of $300 to 600 million. It then depends on the point of view as to whether the cost of capacity, and hence the benefit of capacity

efficiency, is significant. If mesh efficiencies can take 20% or more off of the latter kind of annual budget, it should be well-worth the CEOs

time and attention.

[1]



The point about capacity cost is that it is similar to the notion that "MIPS are free" with computers. Price per MIP

may be asymptotically low, but applications always use more. RAM or disk storage is similar. Price per bit is

vanishingly small, but the quantity required is always rising. As a result neither is ever insignificant in practice.



But with the four layer stack it is also complex and labor intensive to provision new services. Each layer tends to have its own

management systems. And any one layer can act as a bottleneck to throughput and/or provisioning speed even if all three other layers are



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generously provisioned. Thus a simple two layer model for services and transport is attractive: the services layer is IP-based with MPLS

and the transport layer is based on a DWDM optical path switched network.

However, in getting to the two layer model, we do not want to abandon the important functions of ATM and SONET in the four layer model.

The role of ATM will be assumed by the IP layer, with MPLS and GMPLS to replace ATM. Similarly, many roles of SONET can be referred

down into the optical cross-connecting and transport layer. Certain functions of SONET cannot be eliminated—such as formatting bit

streams for physical transmission, framing, error monitoring and so on. They can, however, be realized by "lightweight" implementation of

the full SONET standards, or by digital wrapper that puts SONET-like overheads on optical channels (to follow). SONET itself is also being

extended in terms of flexibility and payload types by developments such as GFP, virtual concatenation, and LCAS. And there is every

expectation that—especially in long-haul networks—SONET ring protection can be replaced by a true mesh-based optical transport

network (OTN) using optical cross-connects (OXC) and mesh-survivability schemes. The outcome is expected to be a "two and a half"

layer model conceptualized in Figure 2-1(c), evolving through the intermediate step, (b). This brings the initially idealized view of "services

and transport" layers introduced in Chapter 1, much closer to reality. It also makes all of the capacity planning and design models that

follow in the book even more directly relevant to industry practice. Let us now look at some of the specific technologies by which this

almost-two-layer model of IP services over optical transport can be realized.



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2.2 DWDM and Optical Networking

In principle, wavelength division multiplexing (WDM) is the same as FDM except that it occurs at optical frequencies—wavelengths in the

1310 and 1550 nm ranges. Several optical carrier signals occupy the same fiber at non-overlapping center wavelengths (or ls). A different

payload can be simultaneously modulated onto each optical carrier as long as their laser center frequencies are suitably spaced apart. The

main drivers for the deployment of WDM multiplexing are the depletion of fiber capacity in long haul networks due to demand growth and

favorable economics of WDM systems relative to alternatives such as installing additional fiber or further upgrading single fiber TDM

systems to bit rates beyond 10 Gb/s (OC-192). Although the capacity of early point-to-point WDM systems was limited from 2 to 4 ls,

systems with over 1,000 ls have been demonstrated in the laboratory and optical ADMs (OADMs) and optical cross-connects (OXCs)

allow wavelengths to be routed and switched in much the same way as timeslots are in SONET networks.

This section is intended to give basic background on WDM concepts, issues, and network elements—to provide a self-contained "setting

of the stage" for our subsequent studies. To delve deeper, a range of books on DWDM and optical networking are available. Starting from

the most accessible, in the sense of the technical content, [Kart00] is a popular primer on optical transmission and technology. G

[ ora01]

covers WDM but also SONET, GbE, LAN and WAN networks in a practical high-level way. [Free02] covers optical fiber, lasers, detectors,

amplifiers, regenerators, and WDM multiplex devices somewhat more extensively than [Kart00] but at a similar level of technical depth.

[StBa99] and [RaSi98] are more comprehensive textbooks on optical networking including theoretical treatments of topics such as

wavelength routing. [Rama01] is a more specialist volume addressing transmission impairment analysis, optical amplifier placement and

wavelength conversion technologies. [SiSu00] and [Stav01] are edited compilations of chapters based on specialist research papers on

optical network performance issues. Finally, [Dixi03] is a volume that spans both IP and WDM layer considerations with chapters from a

variety of experts in both layers. These sources primarily address the challenging basic problem of "working" signal transmission. A few

include overviews of protection switching and signaling, or ring concepts for survivability, but generally all are complimentary to the present

effort in that they emphasize the technology and basic network planning, leaving the domain of mesh-based survivability strategies to be

explored in depth in this book.



2.2.1 Coarse and Dense WDM

In dense wave-division multiplexing (DWDM) numerous laser sources are operated in a closely spaced frequency plan, defining a bank of

precisely offset optical carrier frequencies. It is called dense WDM to contrast with the prior technology of coarse WDM. In coarse WDM

(CWDM) two to four lasers operate independently at widely separated frequency ranges in the 1550 nm and 1310 nm low-loss windows of

the optical fiber. CWDM was intended primarily to achieve pair-gain on a point-to-point basis, effectively doubling or tripling the fiber count

between two nodes. CWDM lasers could have a much broader spectral occupancy and looser center frequency stability and relative

power levels than DWDM laser sources that must be of extremely narrow linewidth and are usually under active feedback control for

absolute center frequency stabilization. In DWDM maintaining relatively equal power levels over all carriers can also be an important and

difficult challenge not present in CWDM. Figure 2-2 contrasts coarse and dense WDM wavelength plans and illustrates the concept of

DWDM based on tightly spaced and controlled optical carriers in the 1500 nm range where optical fiber attenuation is at its lowest.



Figure 2-2. (a) Coarse and (b) dense wave-division multiplexing.



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DWDM systems require precise standardization of the carrier frequencies to use. Current ITU standards define a grid of 81 wavelengths in

the "C-band" starting from 1528.77 nm incrementing in multiples of 50 GHz (0.39nm). Commercial systems with 16, 40, 80 and 128

wavelengths per fiber have been announced based on this frequency grid. ITU standards so far only define the 50 or 100 nm spacing grids

but laboratory work at 25 nm spacing is occurring and up to 1000 wavelength have been reported on a single fiber [Bell99]. The number

and selection of channels implemented on this grid depends on the application requirements such as reach, data rate on the carrier, fiber

type, optical filter technologies used, etc., so there is no one set number of wavelength channels associated with all DWDM systems per

se.



2.2.2 Optical Amplifiers

The single most important invention that enables DWDM networking is the optical amplifier (OA). An optical amplifier counteracts fiber

attenuation over a complete band of lightwave channels at once, without demodulating or in any other way individually processing each

optical carrier. In the most common type of OA, an erbium doped fiber amplifier (EDFA), an optical pump laser creates a photon population

inversion in a section of erbium doped fiber. The much weaker multi-channel optical input signal from the fiber causes a higher power

output of stimulated emission, as in a laser itself, which tracks the input signal thus directly amplifying it in the optical domain. Without this

key technology there would be little impetus for DWDM-based optical networking per se because each channel would have to be

individually detected and regenerated or amplified each time fiber loss accumulated above threshold. There would still be multiplexing gain

in the use of the fiber itself, but no corresponding relief in the amount of physical equipment needed at each regenerator or amplifier

station along the fiber route. The EDFA has been so successful that the term is often used as a synonym for OA itself. But there are other

OA technologies, such as the semiconductor optical amplifier (SOA) and erbium doped waveguide amplifier (EDWA).



2.2.3 Regenerators

The optical amplifier is an analog device. It functions as a broadband linear amplifier of incoming light power to offset fiber attenuation or

other insertion losses in the optical path. In practice, OAs are neither perfectly linear nor noise-free. They add noise as well as boost any



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incoming noise with the signal. Other fiber and amplifier related impairments accumulate in the optical path such as chromatic dispersion,

polarization mode dispersion, and types of crosstalk. The important point here is that these impairments add up to a point where

regeneration (as opposed to amplification) is required. In regeneration, more specifically in a "3R" regenerator, each channel is individually

demodulated from its optical carrier and converted to an electrical form where its bit-timing is extracted and used to sample the binary

state of each symbol. Jitter is removed from the recovered clock, new 1/0 symbols generated, and these are remodulated onto a new

outgoing carrier wavelength. "3R" thus refers to retiming, regenerating and retransmitting.

Regeneration is a per-channel process involving electro-optical conversion and high speed electrical processing of each channel

individually and is therefore more costly than optical amplification. Often the regeneration function needed along an optical path would be

provided by optical-electrical-optical (o-e-o) cross-connects along the route of the path. The need for regeneration is one of the reasons

that "transparency" (optical-only processing without wavelength conversion) is likely to be limited in optical networks. As long as there is a

need for regeneration, it makes sense for at least some of the cross-connects in the network to be electrical-core devices as opposed to

purely passive optical (o-o-o) space switches. Any time the signal has been demodulated to the electronic domain for regeneration or

switching, we can also change wavelengths as needed because the signal is applied to a new transmitting laser to return it to the optical

domain. Less often a bank of stand-alone regenerators may be needed on long-haul facility routes, contributing considerable extra cost to

the signal path. With present OA spacings of around 60 to 80 km, regenerator spacings are typically around 550 to 600 km. Note that in

the design of the DWDM link these spacings are not independent. If OAs are more frequent, optical signal to noise ratio will be better

preserved allowing for a longer distance between regenerators (and the converse).

For a variety of network planning purposes, the detailed design of a transmission span between nodes is usually summarized into a single

incremental cost for adding a channel to the span as a whole or a set of costs associated with modular capacity addition options along the

entire span. Such cost coefficients summarize the net effect of the detailed transmission design efforts, taking into account each span's

different length and exact equipment types and spacings and each of the technology options available to the planner. The per-channel

cost may or may not include a pro-rated allocation of all the "get started" costs associated with the acquisition of the route, installing ducts,

cables and equipment huts, etc., before the first channel is turned up.



2.2.4 Optical Add/drop Multiplexers (OADMs)

The structure and function of an OADM almost completely follows from its SONET ADM predecessors, although a variety of different

technical implementations exist for selecting the optical channels to add or drop. In principle an OADM can be based on electronic

detection of payloads, electronic add/drop, and remodulation of pass-through and add signals onto new outgoing wavelengths. Such an

OADM is capable of full wavelength conversion as well as add/drop and pass-through functions. But especially in an OADM it is desirable

to leave pass-through channels in the optical domain. Many OADM designs therefore tend to be based on passive, purely-optical

add/drop/through channel filters without any wavelength conversion. Only add/drop channels need to go through e/o and o/e conversion

respectively.

In addition, because of the technical challenges of selecting or inserting single wavelengths, OADMs may be based on a waveband

add/drop principle, where groups of 4 or 16 wavelengths are handled as a whole for add or drop purposes and all other wavelengths pass

through the OADM to the other fiber. Also, because of the technological challenges involved, an OADM may or may not be reconfigurable

under program control. A fully reconfigurable OADM (called a ROADM) provides complete wavelength routing functionality, selecting any

incoming channel from one fiber to either drop locally (on the same wavelength) or pass on through to the outgoing fiber. Similarly the

outgoing wavelength multiplex or can be fed on a per-channel basis either from the incoming line or the local add port. Figure 2-3

illustrates some of these key aspects of an OADM.



Figure 2-3. An OADM multiplexer where N of K channels can be added or dropped.



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The OADM handles one fiber pair (of which only one direction is shown) on which there may be K wavelength channels. Generally some

number N, less than K, channels can be selected for add/drop functions. If the incoming WDM demux is programmable in terms of the

wavelengths selectable, or if an optical switch is included (not shown), then the OADM is reconfigurable in that any N channels may be

added or dropped. Otherwise only a specific block of N channels is accessible for add/drop processing at the OADM, based on the optical

filter sets physically installed in the OADM. Optical interference filters based on thin film coatings and fiber Bragg gratings are examples of

technologies used to separate channel bands by reflectance outside the passband and transmission in the band. The same filter

techniques can be used to select the drop channels as well as insert the add channels and remultiplex all the outgoing channels. The

incoming optical channel demultiplexer can be based on diffraction gratings, waveguide interferometers or series connected

single-channel reflectance filters to separate each wavelength into a separate local fiber for processing in the OADM.



2.2.5 Transparent, Opaque and Translucent Optical Networks

In its purest form "all-optical" networking refers to transport networking in which the logical connections between node pairs are realized by

assigning them a specific DWDM wavelength path (of suitably low loss, noise and distortion) end-to-end. Each lightpath must be routed

from its source to destination without any electronic processing at intermediate nodes. Thus resulting path is said to be "transparent" not

(ironically) because it is optical all the way, but because any payload can be modulated on the respective optical carrier wavelength and

received at the far end. In other words, transparency refers to the fact that there is no dependency on the payload being in a specific

format in terms of framing, bit-rate, line-coding, power level, jitter and so on, as is usually the case when electrical circuits have to handle

the signal en route. In practice today, transparency requires a wavelength assignment that must be uniquely reserved for the path on each

fiber en route. But in principle a transparent network will allow a lightpath to occupy different wavelengths on its links when an all-optical

wavelength conversion technology is available. Such an all-optical path that does not change wavelength is also called a pure wavelength

path (WP). The main advantages of WP networking are that wavelength converters (WCs) are not needed and the path is transparent to

payload. In practice, a pure WP network would also employ only all-optical (o-o-o) cross-connects that generally take less power and

space than a corresponding o-e-o cross-connect. Disadvantages associated with pure WP networking are, however, as follows:



Wavelength assignment is much more complex because lightpaths must use the same wavelength end-to-end. To assign a

wavelength then requires that it is free on all spans en route. Logically this is like operating a SONET network in which

payloads cannot change their timeslot assignments in successive spans. The shortest path routing problem for working

demands is replaced by the routing and wavelength assignment (RWA) problem, which is NP-hard (a measure of

computational complexity covered in Chapter 4).

To avoid wavelength blocking, where a path cannot be routed because a single wavelength is not available on every span of

any route, more total capacity (lightwave channels) is needed on each span and the DWDM technology employed must also

support more wavelengths per fiber than a corresponding network where wavelengths can be re-assigned at each node.

The benefit of regeneration is not obtained to "reset" the accumulation of optical transmission impairments at each node. Each

optical path must therefore be individually transmission engineered or limits imposed on the length and routing of end-to-end

paths to contain transmission impairments and loss.

For the same reason, fault isolation becomes more difficult and there are no signal-associated overhead channels to support

numerous operations and maintenance functions that employ payload-associated adjacent node signaling.



Current technological limitations, and issues in network control and management, also make such all-optical networks difficult to

implement. Performance assurance is especially an issue if switched all-optical paths are to bear 10 to 40 Gb/s payloads reliably. At these

rates the point-to-point DWDM link design problem is so dependent on dispersion, noise, polarization, nonlinear and relative-power effects



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in fiber and OAs that detection and electronic regeneration at each OXC node is almost required to retain assurances of end-to-end BER

under arbitrary patterns of cross-connection of carriers between multi-channel DWDM links.

The opposite of a pure WP or "transparent" network, that consists only of transparent paths, is (not surprisingly) called an "opaque" or

virtual wavelength path (VWP) network. A VWP is defined as an end-to-end optical path that may use more than one optical wavelength

along its route. Wavelength assignments to VWPs can be changed as needed, through wavelength conversion (WC) at cross-connect

nodes. In the future, all-optical wavelength conversion may be possible, in which case a VWP could remain all-optical end-to-end, but at

present wavelength conversion technically implies optical to electrical conversion and remodulation onto a new laser (i.e., o-e-o

processing). There are practical advantages, however, to being "forced" to do o-e-o conversion in the OXC. Once the payload is returned

to the electrical domain it can be accessed for performance monitoring, regeneration, and overhead signaling functions as well.

Provisioning operations, capacity design, fault-location, transmission engineering and capacity requirements are all more favorable with

VWP networking. In particular wavelength blocking is eliminated so that problems of routing and capacity design are all logically similar to

that of a SONET environment. In addition, Digital Wrapper (DW) and Generalized Framing Protocol (GFP) developments (to follow) both

address the issue of retaining payload transparency without requiring optical transparency. In other words, payloads can have digital rate

and format transparency through o-e-o conversion and regeneration stages.

However, a fully opaque network implies O/E and E/O transponders at each node and large electronic switching cores, which are

expensive and power-consuming operating at 10 to 40 Gb/s. Thus, using large electronic core optical switches to form an entirely opaque

network may not be feasible. Studies have shown, however that—from a capacity and blocking standpoint—a small pool of WCs

associated with each optical cross-connect, or a small number of nodes that have full WC capabilities [StDo00], can effectively eliminate

lightpath blocking.

This leads to the concept of translucent optical networks that strike a practical balance between transparency and opaqueness in an

optical network in terms of either "transparent islands" or a "translucent" optical network. In the former, a large-scale optical network is

divided into several domains (i.e., islands) of optical transparency. The idea is that within a domain, transmission engineering factors are

suitably controlled or assured so that a lightpath can transparently reach any node without intermediate signal regeneration. But for

communications between different domains, electronic switches are used at the domain boundaries, acting as 3R regenerators and

wavelength converters while relaying the lightpaths crossing the domain boundaries.

A translucent optical network is a somewhat more general option in which the idea is to have a relatively small number of strategically

chosen opaque nodes (VWP-OXC) at which wavelength conversion and regeneration is possible, with all other nodes being transparent

WP-OXC. This is called sparse placement of the o-e-o switches. Rather than being dedicated to routing lightpaths only in and out of

transparent islands, these switches are shared by all paths of the network as a whole. Studies have shown that with about one in three

nodes being VWP-OXC nodes, wavelength mismatch blocking can be essentially eliminated from the network. Another highly pragmatic

approach is to use PVWP OXC nodes everywhere with the pool of WC resources dimensioned so as to essentially eliminate wavelength

blocking. The number of WC resources may often be no more than needed to provide for unavoidable signal regeneration needs in any

case. Typically, if there is a small pool of wavelength converters at each OXC node, lightpath blocking performance is close to that of a

network with full wavelength conversion on each port at every node. A translucent network may, therefore, either have subset of OXC

nodes that have a full WC capability (i.e., o-e-o nodes) or be comprised of all-optical OXC nodes that have a limited pool of

wavelength-changing transducers to avoid lightpath blocking situations or to apply regeneration for an optical signal reaching its limits of

noise accumulation. For further reference see [RaSa97], [YaLa96], [SuAz96], [SuAz98].

Industry terminology refers to either type of path (WP or VWP) generically as a lightpath when the WP or VWP distinction is not crucial.

Similarly, all three types of optical network, transparent, opaque, or translucent are generically called an optical transport network (OTN).



2.2.6 Routing and Wavelength Assignment (RWA) Problem

In an optical network where there is no wavelength conversion or limited conversion abilities (insufficient to make lightpath blocking

negligible) one encounters the routing and wavelength assignment (RWA) problem. In general, there are two ways that blocking can occur,

either through capacity blocking or wavelength mismatch blocking.

Capacity blocking is a simple insufficiency of available channels to support any routing of an additional lightpath between a desired pair of

nodes. This is logically identical to the blocking that arises in attempting to route telephone connections through a network of trunk groups.

In a data network, or a SONET DCS-based mesh network, for example, the routing of demands that minimizes cost is generally a shortest

path route. Total flow may be balanced over a few shortest routes, and if survivability is concerned the working route choices may change

somewhat, but the route is chosen independent of the individual channels that may be employed, because a SONET DCS can always

connect any input timeslot to any output timeslot. Blocking on the desired route would arise only from a shortage of total capacity on some

span.



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If, however, the OXC nodes in a DWDM network cannot change wavelength, or are limited in doing so, the choice of route and the

assignment of wavelength(s) must be a combined decision if resource requirements are to be minimized. This gives rise to the intensively

studied RWA problem and algorithms for RWA. In this context, blocking may arise even if there are available channels, because an

end-to-end assignment of a single wavelength that can use the available channel(s) is not possible.

RWA problems can be either on-line or off-line in nature. An off-line RWA problem assumes all lightpath requirements are known and we

seek a single overall solution for the assignment of routes and wavelengths to meet lightpath requirements. A typical objective is to serve

all requirements using the fewest number of wavelengths in total or if the number of wavelengths is given, to minimize the total number of

unserved demands. The wavelength assigned to each path must be free on at least one fiber pair on each hop of the route assigned to the

same demand. Thus off-line RWA problems usually take the form, or are a built-in part of, overall network design problems. Interestingly,

while routing as a standalone problem, and wavelength assignment as a separate problem (given a route) are individually simple

polynomial-time problems, their combined global solution for a set of demands is of exponential complexity for exact solution (more on

these concepts in Section 4.2).

On-line RWA algorithms are the operational counterpart intended for handling one demand at a time in a dynamic demand

arrival/departure environment. On-line RWA algorithms for mesh networks are much simpler than off-line RWA problems because they

necessarily handle each demand individually (in a "greedy" algorithmic sense). A useful basic concept underlying on-line RWA algorithms

is the layered graph approach. The layered graph is a copy of the actual topology instantiated in each wavelength that the network

employs. In a pure WP network all these graph layers are interconnected only at the access nodes where initial wavelength assignments

can be made. If certain other nodes have full or limited wavelength conversion capabilities, this is also represented by links connecting the

corresponding wavelength planes. Depending on the technology of the nodes, the inter-layer links can be either zero-cost or reflect the

cost of wavelength conversion. (Note that in the limit of a full conversion at each node, the layered graph simply becomes the multigraph

expansion of what we otherwise normally treat as a simple capacitated graph.) Any links currently used in any layer are marked with

infinite cost. Once the layered graph (plus links in use) construct has been built or updated, the on-line RWA problem can be solved as a

shortest (i.e., least-cost) path finding problem through the overall graph. The overall graph has many more links than the one physical

graph, but shortest-path problems can be solved quickly at large sizes, so this is not necessarily onerous. A lightpath request is blocked

only if no path then exists for it at all in the overall layered graph. Otherwise the path found specifies both the route and the wavelength (or

wavelengths) assigned based on the layer (or layers) traversed by the end-to-end shortest path.

Additional heuristic principles are usually also built into RWA algorithms to "hedge" on the exhaustion of capacity over edges through

strategies where the link routing cost in each layer inversely reflects how much capacity remains on the graph edge as a whole (e.g, the

distributed relative capacity loss algorithm (DRCL) [ZaJu00]). Another general principle that improves overall blocking (and speeds

execution) is similar to the concept of "link-packing" in multistage switching networks. That is, to systematically consider each wavelength

layer in sequence, always placing new paths into the first layer at which a route is feasible (e.g., the adaptive first fit (FF) algorithm—also in

[ZaJu00]). Over a network as a whole, link-packing or first-fit principles tend to keep the incoming and outgoing links on each wavelength

at a node either both in use (together) or both free, ready for a new path establishment. Without a link-packing bias, blocking is more likely

because the free links that exist at a node have no tendency to be available as pairs on the same wavelength, facilitating transparent

optical connections.

The RWA problem and RWA algorithms are already extensively covered in the literature. See for example [StBa99], [RaSi98], [Rama01],

[SiSu00], [Stav01], [Dixi03] and the surveys Z

[ aJu00], [AsSh01], [RaSi95], [ShBo00] . It is important here only that we understand the issue

and nature of RWA. Some of the design models that follow in later studies inherently involve solutions of (off-line) RWA as constituent

subproblems. But because the book is not primarily focused on the operational time-scale we do not consider on-line RWA algorithms

further.



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2.3 Optical Cross-Connects (OXC)

Optical cross-connects (OXC) play a role in DWDM-based networking that is analogous to that of the B-DCS in a SONET mesh network.

Both are non-blocking space switches that can connect from any input fiber to any output fiber and perform switching in a secondary

dimension as well. For the B-DCS this is time-slot position switching between the incoming/outgoing OC-n frames on each fiber. For the

OXC, it is switching from one wavelength to another, as well as from one fiber to another in space. Generally, a B-DCS would always be

capable of switching STS tributaries from any input fiber to any timeslot on any output fiber, whereas OXCs may not have a full

wavelength switching capability.



2.3.1 Wavelength Path Optical Cross-connect (WP-OXC)

In wavelength path (WP) optical network, the nodes cannot perform any wavelength con-version. As a consequence, every path must

have the same wavelength on each fiber. The respective OXC architecture is depicted in Figure 2-4. The wavelength channels on each

in-coming fiber are demultiplexed (but remain optical) to separate local jumper fibers. Then all copies of the same wavelength from each

incoming fiber are directed to an optical switch module where they can be routed to any outgoing fiber and multiplexed with other

wavelengths. Note that because wavelength channels are already separated when demultiplexed, the middle-stage optical switches do not

have to be frequency selective or frequency specific; they need only redirect the entire band of light off each incoming fiber to a specified

outgoing fiber. In other words, the central optical switches can be essentially a set of mirrors. Figure 2-5 shows a 2-D MEMS (micro

electro-mechanical system) implementation of an 8x8 switchable mirror array, a prime candidate for this type of pure optical space-switch.



Figure 2-4. Pure WP-OXC with no wavelength conversion or regeneration (except add/drop

transponders).



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Figure 2-5. An 8x8 optical layer switch based on 2-dimensional MEMS (source [Lin98]).



The overall structure of the OXC shown in Figure 2-4 makes use of the fact that each wavelength can be used only one time per fiber. For

M incoming and M outgoing fibers, each with K wavelengths, K separate (M+k)x(M+k) optical space switches are used wherek is an

allowance for the desired number of local add/drop channel opportunities, managed by the local access interface. Electro-optic conversion

is provided at the local add/drop interface so that drop signals may be demodulated from their DWDM transport wavelength (in the 1550

nm range), regenerated electrically by a corresponding interface card (for example, a SONET OC-n receiver or a Gigabit Ethernet (GbE)

receiver). The regenerated drop signals may then be delivered electrically to terminating equipment if close by, or, more commonly

delivered up to a few km over a "short reach" fiber link using less stringent lower cost single-fiber-per-carrier optics in the 1.3 µm range.

The latter short-reach fiber link is sometimes referred to as "uncolored" or "gray" optics because it is now outside of the DWDM domain

and there is no highly precise requirement for the single 1.3 µm laser wavelength used on such links (in contrast to when the signal was

borne on a precisely "colored" DWDM carrier in the 1550 nm range). In the "add" signal path at the local access interface, OC-n, GbE, or

other payload formats are received (also on short reach optics) and either further prepared for transmission (by addition of GFP or DW

overheads, for instance) or directly applied to modulate a laser source at the assigned DWDM wavelength. The circuit pack that interfaces

to a payload in its native format and/or further preparing it for transmission and applies it to an assigned DWDM wavelength (and

vice-versa) is often called a transponder.



2.3.2 Optical Cross-Connect for Virtual Wavelength Paths (VWP-OXC)

In VWP (or "opaque") optical networks, the OXC nodes are capable of full regeneration and wavelength conversion. Any incoming

lightpath can be transformed to any other wavelength on any other outgoing fiber link through optical-to-electrical conversion and

remodulation on a new laser wavelength. The VWP-OXC is therefore also often called an o-e-o switch. (To effect the VWP-OXC function

without electro-optic conversions will eventually require a pure optical wavelength translation technology.) Figure 2-6 shows one

architecture for a VWP-OXC using a single (M·K+k)x(M·K+k) optical space-switch core. Following the core space switch each lightpath can

change its wavelength before entering the next fiber. This is why the core switch must be able to access the whole set of fibers



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