Impairments and Other Constraints on Optical Layer Routing
draft-ietf-ipo-impairments-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4054.
|
|
|---|---|---|---|
| Authors | Angela Chiu , John Strand | ||
| Last updated | 2015-10-14 (Latest revision 2003-05-08) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4054 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Scott O. Bradner | ||
| Send notices to | <awduche@isocore.com> |
draft-ietf-ipo-impairments-05
Internet Draft John Strand (Editor)
Document: draft-ietf-ipo-impairments-05.txt Angela Chiu (Editor)
Informational Track AT&T
Expiration Date: November 2003
May 2003
Impairments And Other Constraints On Optical Layer Routing
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are
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Abstract
Optical networking poses a number challenges for GMPLS. Optical
technology is fundamentally an analog rather than digital technology;
and the optical layer is lowest in the transport hierarchy and hence
has an intimate relationship with the physical geography of the
network. This contribution surveys some of the aspects of optical
networks which impact routing and identifies possible GMPLS responses
for each: (1) Constraints arising from the design of new software
controllable network elements, (2) Constraints in a single all-optical
domain without wavelength conversion, (3) Complications arising in more
complex networks incorporating both all-optical and opaque
architectures, and (4) Impacts of diversity constraints.
1. Introduction
GMPLS [GMPLS] aims to extend MPLS to encompass a number of transport
architectures. Included are optical networks incorporating a number
of all-optical and opto-electronic elements such as optical cross-
connects with both optical and electrical fabrics, transponders, and
optical add-drop multiplexers. Optical networking poses a number
Chiu, Strand, Eds. Informational [Page 1]
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challenges for GMPLS. Optical technology is fundamentally an analog
rather than digital technology; and the optical layer is lowest in
the transport hierarchy and hence has an intimate relationship with
the physical geography of the network.
GMPLS already has incorporated extensions to deal with some of the
unique aspects of the optical layer. This contribution surveys some
of the aspects of optical networks which impact routing and
identifies possible GMPLS responses for each. Routing constraints
and/or complications arising from the design of network elements,
the accumulation of signal impairments, and from the need to
guarantee the physical diversity of some circuits are discussed.
Since the purpose of this draft is to further the specification of
GMPLS, alternative approaches to controlling an optical network are
not discussed. For discussions of some broader issues, see
[Gerstel2000] and [Strand2001].
The organization of the contribution is as follows:
- Section 2 is a section requested by the sub-IP Area management
for all new drafts. It explains how this document fits into the
Area and into the IPO WG, and why it is appropriate for these
groups.
- Section 3 describes constraints arising from the design of new
software controllable network elements.
- Section 4 addresses the constraints in a single all-optical
domain without wavelength conversion.
- Section 5 extends the discussion to more complex networks
incorporating both all-optical and opaque architectures.
- Section 6 discusses the impacts of diversity constraints.
- Section 7 deals with security requirements.
- Section 8 contains acknowledgments.
- Section 9 contains references.
- Section 10 contains contributing authors' addresses.
- Section 11 contains editors' addresses.
2. Sub-IP Area Summary And Justification Of Work
This draft merges and extends two previous drafts, draft-chiu-
strand-unique-olcp-02.txt and draft-banerjee-routing-impairments-
00.txt. These two drafts were made IPO working group documents to
form a basis for a design team at the Minneapolis meeting, where it
was also requested that they be merged to create a requirements
document for the WG.
In the larger sub-IP Area structure, this merged document describes
specific characteristics of optical technology and the requirements
Chiu, Strand, Eds. Informational [page 2]
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they place on routing and path selection. It is appropriate for the
IPO working group because the material is specific to optical
networks. It identifies and documents the characteristics of the
optical transport network that are important for selecting paths for
optical channels, which is a work area for the IPO WG. It is
appropriate work for this WG because the material covered is
directly aimed at establishing a framework and requirements for
routing in an optical network.
Related documents are:
draft-banerjee-routing-impairments-00.txt
draft-parent-obgp-01.txt
draft-bernstein-optical-bgp-00.txt
draft-hayata-ipo-carrier-needs-00.txt
draft-many-carrier-framework-uni-01.txt
draft-papadimitriou-ipo-non-linear-routing-impairm-01.txt
3. Reconfigurable Network Elements
3.1 Technology Background
Control plane architectural discussions (e.g., [Awduche99]) usually
assume that the only software reconfigurable network element is an
optical layer cross-connect (OLXC). There are however other software
reconfigurable elements on the horizon, specifically tunable lasers and
receivers and reconfigurable optical add-drop multiplexers (OADM's).
These elements are illustrated in the following simple example, which
is modeled on announced Optical Transport System (OTS) products:
+ +
---+---+ |\ /| +---+---
---| A |----|D| X Y |D|----| A |---
---+---+ |W| +--------+ +--------+ |W| +---+---
: |D|-----| OADM |-----| OADM |-----|D| :
---+---+ |M| +--------+ +--------+ |M| +---+---
---| A |----| | | | | | | |----| A |---
---+---+ |/ | | | | \| +---+---
+ +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E
+---+ +---+ +---+ +---+
| | | | | | | |
Figure 3-1: An OTS With OADM's - Functional Architecture
In Fig.3-1, the part that is on the inner side of all boxes labeled
"A" defines an all-optical subnetwork. From a routing perspective
two aspects are critical:
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- Adaptation: These are the functions done at the edges of the
subnetwork that transform the incoming optical channel into the
physical wavelength to be transported through the subnetwork.
- Connectivity: This defines which pairs of edge Adaptation
functions can be interconnected through the subnetwork.
In Fig. 3-1, D and E are DWDM's and X and Y are OADM's. The boxes
labeled "A" are adaptation functions. They map one or more input
optical channels assumed to be standard short reach signals into a
long reach (LR) wavelength or wavelength group which will pass
transparently to a distant adaptation function. Adaptation
functionality which affects routing includes:
- Multiplexing: Either electrical or optical TDM may be used to
combine the input channels into a single wavelength. This is
done to increase effective capacity: A typical DWDM might be
able to handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50
10 Gb/sec (500 Gb/sec total); combining the 2.5 Gb/sec signals
together thus effectively doubles capacity. After multiplexing
the combined signal must be routed as a group to the distant
adaptation function.
- Adaptation Grouping: In this technique, groups of k (e.g., 4)
wavelengths are managed as a group within the system and must be
added/dropped as a group. We will call such a group an
"adaptation grouping". Examples include so called "wave group"
and "waveband" [Passmore01]. Groupings on the same system may
differ in basics such as wavelength spacing, which constrain the
type of channels that can be accommodated.
- Laser Tunability: The lasers producing the LR wavelengths may
have a fixed frequency, may be tunable over a limited range, or
be tunable over the entire range of wavelengths supported by the
DWDM. Tunability speeds may also vary.
Connectivity between adaptation functions may also be limited:
- As pointed out above, TDM multiplexing and/or adaptation
grouping by the adaptation function forces groups of input
channels to be delivered together to the same distant adaptation
function.
- Only adaptation functions whose lasers/receivers are tunable to
compatible frequencies can be connected.
- The switching capability of the OADM's may also be constrained.
For example:
o There may be some wavelengths that can not be dropped at
all.
o There may be a fixed relationship between the frequency
dropped and the physical port on the OADM to which it is
dropped.
o OADM physical design may put an upper bound on the number
of adaptation groupings dropped at any single OADM.
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For a fixed configuration of the OADM's and adaptation functions
connectivity will be fixed: Each input port will essentially be
hard-wired to some specific distant port. However this connectivity
can be changed by changing the configurations of the OADM's and
adaptation functions. For example, an additional adaptation grouping
might be dropped at an OADM or a tunable laser retuned. In each case
the port-to-port connectivity is changed.
These capabilities can be expected to be under software control.
Today the control would rest in the vendor-supplied Element
Management system (EMS), which in turn would be controlled by the
operator's OS's. However in principle the EMS could participate in
the GMPLS routing process.
3.2 Implications For Routing
An OTS of the sort discussed in Sec. 3.1 is essentially a
geographically distributed but blocking cross-connect system. The
specific port connectivity is dependent on the vendor design and
also on exactly what line cards have been deployed.
One way for GMPLS to deal with this architecture would be to view
the port connectivity as externally determined. In this case the
links known to GMPLS would be groups of identically routed
wavebands. If these were reconfigured by the external EMS the
resulting connectivity changes would need to be detected and
advertised within GMPLS. If the topology shown in Fig. 3-1 became a
tree or a mesh instead of the linear topology shown, the
connectivity changes could result in SRLG changes.
Alternatively, GMPLS could attempt to directly control this port
connectivity. The state information needed to do this is likely to
be voluminous and vendor specific.
4. Wavelength Routed All-Optical Networks
The optical networks presently being deployed may be called "opaque"
([Tkach98]): each link is optically isolated by transponders doing
O/E/O conversions. They provide regeneration with retiming and
reshaping, also called 3R, which eliminates transparency to bit
rates and frame format. These transponders are quite expensive and
their lack of transparency also constrains the rapid introduction of
new services. Thus there are strong motivators to introduce
"domains of transparency" - all-optical subnetworks - larger than an
OTS.
The routing of lightpaths through an all-optical network has
received extensive attention. (See [Yates99] or [Ramaswami98]).
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When discussing routing in an all-optical network it is usually
assumed that all routes have adequate signal quality. This may be
ensured by limiting all-optical networks to subnetworks of limited
geographic size which are optically isolated from other parts of the
optical layer by transponders. This approach is very practical and
has been applied to date, e.g. when determining the maximum length
of an Optical Transport System (OTS). Furthermore operational
considerations like fault isolation also make limiting the size of
domains of transparency attractive.
There are however reasons to consider contained domains of
transparency in which not all routes have adequate signal quality.
From a demand perspective, maximum bit rates have rapidly increased
from DS3 to OC-192 and soon OC-768 (40 Gb/sec). As bit rates
increase it is necessary to increase power. This makes impairments
and nonlinearities more troublesome. From a supply perspective,
optical technology is advancing very rapidly, making ever-larger
domains possible. In this section we assume that these
considerations will lead to the deployment of a domain of
transparency that is too large to ensure that all potential routes
have adequate signal quality for all circuits. Our goal is to
understand the impacts of the various types of impairments in this
environment.
Note that as we describe later in the section there are many types
of physical impairments. Which of these needs to be dealt with
explicitly when performing on-line distributed routing will vary
considerably and will depend on many variables, including:
- Equipment vendor design choices,
- Fiber characteristics,
- Service characteristics (e.g., circuit speeds),
- Network size,
- Network operator engineering and deployment strategies.
For example, a metropolitan network which does not intend to support
bit rates above 2.5 Gb/sec may not be constrained by any of these
impairments, while a continental or international network which
wished to minimize O/E/O regeneration investment and support 40
Gb/sec connections might have to explicitly consider many of them.
Also, a network operator may reduce or even eliminate their
constraint set by building a relatively small domain of transparency
to ensure that all the paths are feasible, or by using some
proprietary tools based on rules from the OTS vendor to pre-qualify
paths between node pairs and put them in a table that can be
accessed each time a routing decision has to be made through that
domain.
4.1 Problem Formulation
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We consider a single domain of transparency without wavelength
translation. Additionally due to the proprietary nature of DWDM
transmission technology, we assume that the domain is either single
vendor or architected using a single coherent design, particularly
with regard to the management of impairments.
We wish to route a unidirectional circuit from ingress client node X
to egress client node Y. At both X and Y, the circuit goes through
an O/E/O conversion which optically isolates the portion within our
domain. We assume that we know the bit rate of the circuit. Also,
we assume that the adaptation function at X may apply some Forward
Error Correction (FEC) method to the circuit. We also assume we know
the launch power of the laser at X.
Impairments can be classified into two categories, linear and
nonlinear. (See [Tkach98] for more on impairment constraints).
Linear effects are independent of signal power and affect
wavelengths individually. Amplifier spontaneous emission (ASE),
polarization mode dispersion (PMD), and chromatic dispersion are
examples. Nonlinearities are significantly more complex: they
generate not only impairments on each channel, but also crosstalk
between channels.
In the remainder of this section we first outline how two key linear
impairments (PMD and ASE) might be handled by a set of analytical
formulae as additional constraints on routing. We next discuss how
the remaining constraints might be approached. Finally we take a
broader perspective and discuss the implications of such constraints
on control plane architecture and also on broader constrained domain
of transparency architecture issues.
4.2 Polarization Mode Dispersion (PMD)
For a transparent fiber segment, the general PMD requirement is that
the time-average differential group delay (DGD) between two
orthogonal state of polarizations should be less than fraction a of
the bit duration, T=1/B, where B is the bit rate. The value of the
parameter a depends on three major factors: 1) margin allocated to
PMD, e.g. 1dB; 2) targeted outage probability, e.g. 4x10-5, and 3)
sensitivity of the receiver to DGD. A typical value for a is 10%
[ITU]. More aggressive designs to compensate for PMD may allow
values higher than 10%. (This would be a system parameter dependent
on the system design. It would need to be known to the routing
process.)
The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
sqrt(km). The square of the PMD in a fiber span, denoted as span-
PMD-square is then given by the product of Dpmd**2 and the span
length. (A fiber span in a transparent network refers to a segment
between two optical amplifiers.) If Dpmd is constant, this results
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in a upper bound on the maximum length of an M-fiber-span
transparent segment, which is inversely proportional to the square
of the product of bit rate and Dpmd (the detailed equation is
omitted due to the format constraint - see [Strand01] for details).
For older fibers with a typical PMD parameter of 0.5 picoseconds per
square root of km, based on the constraint, the maximum length of
the transparent segment should not exceed 400km and 25km for bit
rates of 10Gb/s and 40Gb/s, respectively. Due to recent advances in
fiber technology, the PMD-limited distance has increased
dramatically. For newer fibers with a PMD parameter of 0.1
picosecond per square root of km, the maximum length of the
transparent segment (without PMD compensation) is limited to 10000km
and 625km for bit rates of 10Gb/s and 40Gb/, respectively. Still
lower values of PMD are attainable in commercially available fiber
today, and the PMD limit can be further extended if a larger value
of the parameter a (ratio of DGD to the bit period) can be
tolerated. In general, the PMD requirement is not an issue for most
types of fibers at 10Gb/s or lower bit rate. But it will become an
issue at bit rates of 40Gb/s and higher.
If the PMD parameter varies between spans, a slightly more
complicated equation results (see [Strand01]), but in any event the
only link dependent information needed by the routing algorithm is
the square of the link PMD, denoted as link-PMD-square. It is the
sum of the span-PMD-square of all spans on the link.
Note that when one has some viable PMD compensation devices and
deploy them ubiquitously on all routes with potential PMD issues in
the network, then the PMD constraint disappears from the routing
perspective.
4.3 Amplifier Spontaneous Emission
ASE degrades the optical signal to noise ratio (OSNR). An acceptable
optical SNR level (SNRmin) which depends on the bit rate,
transmitter-receiver technology (e.g., FEC), and margins allocated
for the impairments, needs to be maintained at the receiver. In
order to satisfy this requirement, vendors often provide some
general engineering rule in terms of maximum length of the
transparent segment and number of spans. For example, current
transmission systems are often limited to up to 6 spans each 80km
long. For larger transparent domains, more detailed OSNR
computations will be needed to determine whether the OSNR level
through a domain of transparency is acceptable. This would provide
flexibility in provisioning or restoring a lightpath through a
transparent subnetwork.
Assume that the average optical power launched at the transmitter is
P. The lightpath from the transmitter to the receiver goes through M
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optical amplifiers, with each introducing some noise power. Unity
gain can be used at all amplifier sites to maintain constant signal
power at the input of each span to minimize noise power and
nonlinearity. A constraint on the maximum number of spans can be
obtained [Kaminow97] which is proportional to P and inversely
proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
spontaneous emission factor n of the optical amplifier, assuming all
spans have identical gain and noise figure. (Again, the detailed
equation is omitted due to the format constraint - see [Strand01]
for details.) Let's take a typical example. Assuming P=4dBm,
SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
constraint, the maximum number of spans is at most 10. However, if
FEC is not used and the requirement on SNRmin becomes 25dB, the
maximum number of spans drops down to 3.
For ASE the only link-dependent information needed by the routing
algorithm is the noise of the link, denoted as link-noise, which is
the sum of the noise of all spans on the link. Hence the constraint
on ASE becomes that the aggregate noise of the transparent segment
which is the sum of the link-noise of all links can not exceed
P/SNRmin.
4.4 Approximating The Effects Of Some Other Impairment Constraints
There are a number of other impairment constraints that we believe
could be approximated with a domain-wide margin on the OSNR, plus in
some cases a constraint on the total number of networking elements
(OXC or OADM) along the path. Most impairments generated at OXCs or
OADMs, including polarization dependent loss, coherent crosstalk,
and effective passband width, could be dealt with using this
approach. In principle, impairments generated at the nodes can be
bounded by system engineering rules because the node elements can be
designed and specified in a uniform manner. This approach is not
feasible with PMD and noise because neither can be uniformly
specified. Instead, they depend on node spacing and the
characteristics of the installed fiber plant, neither of which are
likely to be under the system designer's control.
Examples of the constraints we propose to approximate with a domain-
wide margin are given in the remaining paragraphs in this section.
It should be kept in mind that as optical transport technology
evolves it may become necessary to include some of these impairments
explicitly in the routing process. Other impairments not mentioned
here at all may also become sufficiently important to require
incorporation either explicitly or via a domain-wide margin.
Other Polarization Dependent Impairments Other polarization-
dependent effects besides PMD influence system performance. For
example, many components have polarization-dependent loss (PDL)
[Ramaswami98], which accumulates in a system with many components on
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the transmission path. The state of polarization fluctuates with
time and its distribution is very important also. It is generally
required to maintain the total PDL on the path to be within some
acceptable limit, potentially by using some compensation technology
for relatively long transmission systems, plus a small built-in
margin in OSNR. Since the total PDL increases with the number of
components in the data path, it must be taken into account by the
system vendor when determining the maximum allowable number of
spans.
Chromatic Dispersion In general this impairment can be adequately
(but not optimally) compensated for on a per-link basis, and/or at
system initial setup time. Today most deployed compensation devices
are based on DCF (Dispersion Compensation Fiber). DCF provides per
fiber compensation by means of a spool of fiber with a CD coefficient
opposite to the fiber. Due to the imperfect matching between the CD
slope of the fiber and the DCF some lambdas can be over compensated
while others can be under compensated. Moreover DCF modules may only
be available in fixed lengths of compensating fiber; this means that
sometimes it is impossible to find a DCF module that exactly
compensates the CD introduced by the fiber. These effects introduce
what is known as residual CD. Residual CD varies with the frequency
of the wavelength. Knowing the characteristics of both of the fiber
and the DCF modules along the path, this can be calculated with a
sufficient degree of precision. However this is a very challenging
task. In fact the per-wavelength residual dispersion needs to be
combined with other information in the system (e.g. types fibers to
figure out the amount of nonlinearities) to obtain the net effect of
CD either by simulation or by some analytical approximation. It
appears that the routing/control plane should not be burdened by such
a large set of information while it can be handled at the system
design level. Therefore it will be assumed until proven otherwise
that residual dispersion should not be reported. For high bit rates,
dynamic dispersion compensation may be required at the receiver to
clean up any residual dispersion.
Crosstalk Optical crosstalk refers to the effect of other signals on
the desired signal. It includes both coherent (i.e. intrachannel)
crosstalk and incoherent (i.e. interchannel) crosstalk. Main
contributors of crosstalk are the OADM and OXC sites that use a DWDM
multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively sparse
network where the number of OADM/OXC nodes on a path is low,
crosstalk can be treated with a low margin in OSNR without being a
binding constraint. But for some relatively dense networks where
crosstalk might become a binding constraint, one needs to propagate
the per-link crosstalk information to make sure that the end-to-end
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path crosstalk which is the sum of the crosstalks on all the
corresponding links to be within some limit, e.g. -25dB threshold
with 1dB penalty ([Goldstein94]). Another way to treat it without
having to propagate per-link crosstalk information is to have the
system evaluate what the maximum number of OADM/OXC nodes that has a
MUX/DEMUX pair for the worst route in the transparent domain for a
low built-in margin. The latter one should work well where all the
OXC/OADM nodes have similar level of crosstalk.
Effective Passband As more and more DWDM components are cascaded,
the effective passband narrows. The number of filters along the
link, their passband width and their shape will determine the end-
to-end effective passband. In general, this is a system design
issue, i.e., the system is designed with certain maximum bit rate
using the proper modulation format and filter spacing. For linear
systems, the filter effect can be turned into a constraint on the
maximum number of narrow filters with the condition that filters in
the systems are at least as wide as the one in the receiver.
Because traffic at lower bit rates can tolerate a narrower passband,
the maximum allowable number of narrow filters will increase as the
bit rate decreases.
Nonlinear Impairments It seems unlikely that these can be dealt with
explicitly in a routing algorithm because they lead to constraints
that can couple routes together and lead to complex dependencies,
e.g. on the order in which specific fiber types are traversed
[Kaminow97]. Note that different fiber types (standard single mode
fiber, dispersion shifted fiber, dispersion compensated fiber, etc.)
have very different effects from nonlinear impairments. A full
treatment of the nonlinear constraints would likely require very
detailed knowledge of the physical infrastructure, including
measured dispersion values for each span, fiber core area and
composition, as well as knowledge of subsystem details such as
dispersion compensation technology. This information would need to
be combined with knowledge of the current loading of optical signals
on the links of interest to determine the level of nonlinear
impairment. Alternatively, one could assume that nonlinear
impairments are bounded and result in X dB margin in the required
OSNR level for a given bit rate, where X for performance reasons
would be limited to 1 or 2 dB, consequently setting a limit on the
maximum number of spans. For the approach described here to be
useful, it is desirable for this span length limit to be longer than
that imposed by the constraints which can be treated explicitly.
When designing a DWDM transport system, there are tradeoffs between
signal power launched at the transmitter, span length, and nonlinear
effects on BER that need to be considered jointly. Here, we assume
that an X dB margin is obtained after the transport system has been
designed with a fixed signal power and maximum span length for a
given bit rate. Note that OTSs can be designed in very different
ways, in linear, pseudo-linear, or nonlinear environments. The X-dB
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margin approach may be valid for some but not for others. However,
it is likely that there is an advantage in designing systems that
are less aggressive with respect to nonlinearities, and therefore
somewhat sub-optimal, in exchange for improved scalability,
simplicity and flexibility in routing and control plane design.
4.5 Other Impairment Considerations
There are many other types of impairments that can degrade
performance. In this section we briefly mention one other type of
impairment, which we propose be dealt with by either by the system
designer or by the transmission engineers at the time the system is
installed. If dealt with successfully in this manner they should not
need to be considered in the dynamic routing process.
Gain Nonuniformity and Gain Transients For simple noise estimates to
be of use, the amplifiers must be gain-flattened and must have
automatic gain control (AGC). Furthermore, each link should have
dynamic gain equalization (DGE) to optimize power levels each time
wavelengths are added or dropped. Variable optical attenuators on
the output ports of an OXC or OADM can be used for this purpose, and
in-line devices are starting to become commercially available.
Optical channel monitors are also required to provide feedback to
the DGEs. AGC must be done rapidly if signal degradation after a
protection switch or link failure is to be avoided.
Note that the impairments considered here are treated more or less
independently. By considering them jointly and varying the tradeoffs
between the effects from different components may allow more routes
to be feasible. If that is desirable or the system is designed such
that certain impairments (e.g. nonlinearities) need to be considered
by a centralized process, then distributed routing is not the one to
use.
4.6 An Alternative Approach - Using Maximum Distance As The only
Constraint
Today, carriers often use maximum distance to engineer point-to-
point OTS systems given a fixed per-span length based on the OSNR
constraint for a given bit rate. They may desire to keep the same
engineering rule when they move to all-optical networks. Here, we
discuss the assumptions that need to be satisfied to keep this
approach viable and how to treat the network elements between two
adjacent links.
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In order to use the maximum distance for a given bit rate to meet an
OSNR constraint as the only binding constraint, the operators need
to satisfy the following constraints in their all-optical networks:
- All the other non-OSNR constraints described in the previous
subsections are not binding factors as long as the maximum
distance constraint is met.
- Specifically for PMD, this means that the whole all-optical
network is built on top of sufficiently low-PMD fiber such that
the upper bound on the mean aggregate path DGD is always
satisfied for any path that does not exceed the maximum
distance, or PMD compensation devices might be used for routes
with high-PMD fibers.
- In terms of the ASE/OSNR constraint, in order to convert the ASE
constraint into a distance constraint directly, the network
needs to have a fixed fiber distance D for each span (so that
ASE can be directly mapped by the gain of the amplifier which
equals to the loss of the previous fiber span), e.g., 80km
spacing which is commonly chosen by carriers. However, when
spans have variable lengths, certain adjustment and compromise
need to be made in order to avoid treating ASE explicitly as in
section 4.3. These include: 1) If a span is shorter than a
typical span length D, unless certain mechanism is built in the
OTS to take advantages of shorter spans, it needs to be treated
as a span of length D instead of with its real length. 2) When
there are spans that are longer than D, it means that paths with
these longer spans would have higher average span loss. In
general, the maximum system reach decreases when the average
span loss increases. Thus, in order to accommodate longer spans
in the network, the maximum distance upper bound has to be set
with respect to the average span loss of the worst path in the
network. This sub-optimality may be acceptable for some networks
if the variance is not too large, but may be too conservative
for others.
If these assumptions are satisfied, the second issue we need to
address is how to treat a transparent network element (e.g., MEMS-
based switch) between two adjacent links in terms of a distance
constraint since it also introduces an insertion loss. If the
network element cannot somehow compensate for this OSNR degradation,
one approach is to convert each network element into an equivalent
length of fiber based on its loss/ASE contribution. Hence, in
general, introducing a set of transparent network elements would
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effectively result in reducing the overall actual transmission
distance between the OEO edges.
With this approach, the link-specific state information is link-
distance, the length of a link. It equals to the distance sum of all
fiber spans on the link and the equivalent length of fiber for the
network element(s) on the link. The constraint is that the sum of
all the link-distance over all links of a path should be less than
the maximum-path-distance, the upper bound of all paths.
4.7 Other Considerations
Routing in an all-optical network without wavelength conversion
raises several additional issues:
- Since the route selected must have the chosen wavelength
available on all links, this information needs to be considered
in the routing process. One approach is to propagate
information throughout the network about the state of every
wavelength on every link in the network. However, the state
required and the overhead involved in processing and
maintaining this information is proportional to the total
number of links (thus, number of nodes squared), maximum number
of wavelengths which keeps doubling every couple of years), and
the frequency of wavelength availability changes, which can be
very high. Instead [Hjßlmt²sson00] proposes an alternative
method which probes along a chosen path to determine which
wavelengths (if any) are available. This would require a
significant addition to the routing logic normally used in
OSPF. Others have proposed simultaneously probing along
multiple paths.
- Choosing a path first and then a wavelength along the path is
known to give adequate results in simple topologies such as
rings and trees ([Yates99]). This does not appear to be true in
large mesh networks under realistic provisioning scenarios,
however. Instead significantly better results are achieved if
wavelength and route are chosen simultaneously ([Strand01b]).
This approach would however also have a significant effect on
OSPF.
4.8 Implications For Routing and Control Plane Design
If distributed routing is desired, additional state information will
be required by the routing to deal with the impairments described in
Sections 4.2 - 4.4:
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- As mentioned earlier, an operator who wants to avoid having to
provide impairment-related parameters to the control plane may
elect not to deal with them at the routing level, instead
treating them at the system design and planning level if that is
a viable approach for their network. In this approach the
operator can pre-qualify all or a set of feasible end-to-end
optical paths through the domain of transparency for each bit
rate. This approach may work well with relatively small and
sparse networks, but it may not be scalable for large and dense
networks where the number of feasible paths can be very large.
- If the optical paths are not pre-qualified, additional link-
specific state information will be required by the routing
algorithm for each type of impairment that has the potential of
being limiting for some routes. Note that for one operator, PMD
might be the only limiting constraint while for another, ASE
might be the only one, or it could be both plus some other
constraints considered in this document. Some networks might not
be limited by any of these constraints.
- For an operator needing to deal explicitly with these
constraints, the link-dependent information identified above for
PMD is link-PMD-square which is the square of the total PMD on a
link. For ASE the link-dependent information identified is link-
noise which is the total noise on a link. Other link-dependent
information includes link-span-length which is the total number
of spans on a link, link-crosstalk or OADM-OXC-number which is
the total crosstalk or the number of OADM/OXC nodes on a link,
respectively, and filter-number which is the number of narrow
filters on a link. When the alternative distance-only approach
is chosen, the link-specific information is link-distance.
- In addition to the link-specific information, bounds on each of
the impairments need to be quantified. Since these bounds are
determined by the system designer's impairment allocations,
these will be system dependent. For PMD, the constraint is that
the sum of the link-PMD-square of all links on the transparent
segment is less than the square of (a/B) where B is the bit
rate. Hence, the required information is the parameter "a". For
ASE, the constraint is that the sum of the link-noise of all
links is no larger than P/SNRmin. Thus, the information needed
include the launch power P and OSNR requirement SNRmin. The
minimum acceptable OSNR, in turn, depends on the strength of the
FEC being used and the margins reserved for other types of
impairments. Other bounds include the maximum span length of the
transmission system, the maximum path crosstalk or the maximum
number of OADM/OXC nodes, and the maximum number of narrow
filters, all are bit rate dependent. With the alternative
distance-only approach, the upper bound is the maximum-path-
distance. In single-vendor "islands" some of these parameters
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may be available in a local or EMS database and would not need
to be advertised
- It is likely that the physical layer parameters do not change
value rapidly and could be stored in some database; however
these are physical layer parameters that today are frequently
not known at the granularity required. If the ingress node of a
lightpath does path selection these parameters would need to be
available at this node.
- The specific constraints required in a given situation will
depend on the design and engineering of the domain of
transparency; for example it will be essential to know whether
chromatic dispersion has been dealt with on a per-link basis,
and whether the domain is operating in a linear or nonlinear
regime.
- As optical transport technology evolves, the set of constraints
that will need to be considered either explicitly or via a
domain-wide margin may change. The routing and control plane
design should therefore be as open as possible, allowing
parameters to be included as necessary.
- In the absence of wavelength conversion, the necessity of
finding a single wavelength that is available on all links
introduces the need to either advertise detailed information on
wavelength availability, which probably doesn't scale, or have
some mechanism for probing potential routes with or without
crankback to determine wavelength availability. Choosing the
route first, and then the wavelength, may not yield acceptable
utilization levels in mesh-type networks.
5. More Complex Networks
Mixing optical equipment in a single domain of transparency that has
not been explicitly designed to interwork is beyond the scope of
this document. This includes most multi-vendor all-optical networks.
An optical network composed of multiple domains of transparency
optically isolated from each other by O/E/O devices (transponders)
is more plausible. A network composed of both "opaque" (optically
isolated) OLXC's and one or more all-optical "islands" isolated by
transponders is of particular interest because this is most likely
how all-optical technologies (such as that described in Sec. 2) are
going to be introduced. (We use the term "island" in this discussion
rather than a term like "domain" or "area" because these terms are
associated with specific approaches like BGP or OSPF.)
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We consider the complexities raised by these alternatives now.
The first requirement for routing in a multi-island network is that
the routing process needs to know the extent of each island. There
are several reasons for this:
- When entering or leaving an all-optical island, the regeneration
process cleans up the optical impairments discussed in Sec. 3.
- Each all-optical island may have its own bounds on each
impairment.
- The routing process needs to be sensitive to the costs
associated with "island-hopping".
This last point needs elaboration. It is extremely important to
realize that, at least in the short to intermediate term, the
resources committed by a single routing decision can be very
significant: The equipment tied up by a single coast-to-coast OC-192
can easily have a first cost of $10**6, and the holding times on a
circuit once established is likely to be measured in months.
Carriers will expect the routing algorithms used to be sensitive to
these costs. Simplistic measures of cost such as the number of
"hops" are not likely to be acceptable.
Taking the case of an all-optical island consisting of an "ultra
long-haul" system like that in Fig. 3-1 embedded in an OEO network
of electrical fabric OLXC's as an example: It is likely that the ULH
system will be relatively expensive for short hops but relatively
economical for longer distances. It is therefore likely to be
deployed as a sort of "express backbone". In this scenario a carrier
is likely to expect the routing algorithm to balance OEO costs
against the additional costs associated with ULH technology and
route circuitously to make maximum use of the backbone where
appropriate. Note that the metrics used to do this must be
consistent throughout the routing domain if this expectation is to
be met.
The first-order implications for GMPLS seem to be:
- Information about island boundaries needs to be advertised.
- The routing algorithm needs to be sensitive to island
transitions and to the connectivity limitations and impairment
constraints particular to each island.
- The cost function used in routing must allow the balancing of
transponder costs, OXC and OADM costs, and line haul costs
across the entire routing domain.
Several distributed approaches to multi-island routing seem worth
investigating:
- Advertise the internal topology and constraints of each island
globally; let the ingress node compute an end-to-end strict
explicit route sensitive to all constraints and wavelength
availabilities. In this approach the routing algorithm used by
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the ingress node must be able to deal with the details of
routing within each island.
- Have the EMS or control plane of each island determine and
advertise the connectivity between its boundary nodes together
with additional information such as costs and the bit rates and
formats supported. As the spare capacity situation changes,
updates would be advertised. In this approach impairment
constraints are handled within each island and impairment-
related parameters need not be advertised outside of the island.
The ingress node would then do a loose explicit route and leave
the routing and wavelength selection within each island to the
island.
- Have the ingress node send out probes or queries to nearby
gateway nodes or to an NMS to get routing guidance.
6. Diversity
6.1 Background On Diversity
"Diversity" is a relationship between lightpaths. Two lightpaths are
said to be diverse if they have no single point of failure. In
traditional telephony the dominant transport failure mode is a
failure in the interoffice plant, such as a fiber cut inflicted by a
backhoe.
Why is diversity a unique problem that needs to be considered for
optical networks? So far, data network operators have relied on
their private line providers to ensure diversity and so have not had
to deal directly with the problem. GMPLS makes the complexities
handled by the private line provisioning process, including
diversity, part of the common control plane and so visible to all.
To determine whether two lightpath routings are diverse it is
necessary to identify single points of failure in the interoffice
plant. To do so we will use the following terms: A fiber cable is a
uniform group of fibers contained in a sheath. An Optical Transport
System will occupy fibers in a sequence of fiber cables. Each fiber
cable will be placed in a sequence of conduits - buried honeycomb
structures through which fiber cables may be pulled - or buried in a
right of way (ROW). A ROW is land in which the network operator has
the right to install his conduit or fiber cable. It is worth noting
that for economic reasons, ROW's are frequently obtained from
railroads, pipeline companies, or thruways. It is frequently the
case that several carriers may lease ROW from the same source; this
makes it common to have a number of carriers' fiber cables in close
proximity to each other. Similarly, in a metropolitan network,
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several carriers might be leasing duct space in the same RBOC
conduit. There are also "carrier's carriers" - optical networks
which provide fibers to multiple carriers, all of whom could be
affected by a single failure in the "carrier's carrier" network.
In a typical intercity facility network there might be on the order
of 100 offices that are candidates for OLXC's. To represent the
inter-office fiber network accurately a network with an order of
magnitude more nodes is required. In addition to Optical Amplifier
(OA) sites, these additional nodes include:
- Places where fiber cables enter/leave a conduit or right of way;
- Locations where fiber cables cross;
Locations where fiber splices are used to interchange fibers between
fiber cables.
An example of the first might be:
A B
A-------------B \ /
\ /
X-----Y
/ \
C-------------D / \
C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
Figure 6-1: Fiber Cable vs. ROW Topologies
Here the A-B fiber cable would be physically routed A-X-Y-B and the
C-D cable would be physically routed C-X-Y-D. This topology might
arise because of some physical bottleneck: X-Y might be the Lincoln
Tunnel, for example, or the Bay Bridge.
Fiber route crossing (the second case) is really a special case of
this, where X and Y coincide. In this case the crossing point may
not even be a manhole; the fiber routes might just be buried at
different depths.
Fiber splicing (the third case) often occurs when a major fiber
route passes near to a small office. To avoid the expense and
additional transmission loss only a small number of fibers are
spliced out of the major route into a smaller route going to the
small office. This might well occur in a manhole or hut. An
example is shown in Fig. 6-2(a), where A-X-B is the major route, X
the manhole, and C the smaller office. The actual fiber topology
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would then look like Fig. 6-2(b), where there would typically be
many more A-B fibers than A-C or C-B fibers, and where A-C and C-B
might have different numbers of fibers. (One of the latter might
even be missing.)
C C
| / \
| / \
| / \
A------X------B A---------------B
(a) Fiber Cable Topology (b) Fiber Topology
Figure 6-2. Fiber Cable vs Fiber Topologies
The imminent deployment of ultra-long (>1000 km) Optical Transport
Systems introduces a further complexity: Two OTS's could interact a
number of times. To make up a hypothetical example: A New York -
Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same
right of way for x miles in Maryland and then again for y miles in
Georgia. They might also cross at Raleigh or some other intermediate
node without sharing right of way.
Diversity is often equated to routing two lightpaths between a
single pair of points, or different pairs of points so that no
single route failure will disrupt them both. This is too simplistic,
for a number of reasons:
- A sophisticated client of an optical network will want to derive
diversity needs from his/her end customers' availability
requirements. These often lead to more complex diversity
requirements than simply providing diversity between two
lightpaths. For example, a common requirement is that no single
failure should isolate a node or nodes. If a node A has single
lightpaths to nodes B and C, this requires A-B and A-C to be
diverse. In real applications, a large data network with N
lightpaths between its routers might describe their needs in an
NxN matrix, where (i,j) defines whether lightpaths i and j must
be diverse.
- Two circuits that might be considered diverse for one
application might not be considered diverse for in another
situation. Diversity is usually thought of as a reaction to
interoffice route failures. High reliability applications may
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require other types of failures to be taken into account. Some
examples:
o Office Outages: Although less frequent than route failures,
fires, power outages, and floods do occur. Many network
managers require that diverse routes have no (intermediate)
nodes in common. In other cases an intermediate node might
be acceptable as long as there is power diversity within
the office.
o Shared Rings: Many applications are willing to allow
"diverse" circuits to share a SONET ring-protected link;
presumably they would allow the same for optical layer
rings.
o Disasters: Earthquakes and floods can cause failures over
an extended area. Defense Department circuits might need
to be routed with nuclear damage radii taken into account.
- Conversely, some networks may be willing to take somewhat larger
risks. Taking route failures as an example: Such a network
might be willing to consider two fiber cables in heavy duty
concrete conduit as having a low enough chance of simultaneous
failure to be considered "diverse". They might also be willing
to view two fiber cables buried on opposite sides of a railroad
track as being diverse because there is minimal danger of a
single backhoe disrupting them both even though a bad train
wreck might jeopardize them both. A network seeking N mutually
diverse paths from an office with less than N diverse ROW's will
need to live with some level of compromise in the immediate
vicinity of the office.
These considerations strongly suggest that the routing algorithm
should be sensitive to the types of threat considered unacceptable
by the requester. Note that the impairment constraints described in
the previous section may eliminate some of the long circuitous
routes sometimes needed to provide diversity. This would make it
harder to find many diverse paths through an all-optical network
than an opaque one.
[Hjßlmt²sson00] introduced the term "Shared Risk Link Group" (SRLG)
to describe the relationship between two non-diverse links. The
above examples and discussion given at the start of this section
suggests that an SRLG should be characterized by 2 parameters:
- Type of Compromise: Examples would be shared fiber cable, shared
conduit, shared ROW, shared optical ring, shared office without
power sharing, etc.)
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- Extent of Compromise: For compromised outside plant, this would
be the length of the sharing.
A CSPF algorithm could then penalize a diversity compromise by an
amount dependent on these two parameters.
Two links could be related by many SRLG's (AT&T's experience
indicates that a link may belong to over 100 SRLG's, each
corresponding to a separate fiber group. Each SRLG might relate a
single link to many other links. For the optical layer, similar
situations can be expected where a link is an ultra-long OTS).
The mapping between links and different types of SRLG's is in
general defined by network operators based on the definition of each
SRLG type. Since SRLG information is not yet ready to be
discoverable by a network element and does not change dynamically,
it need not be advertised with other resource availability
information by network elements. It could be configured in some
central database and be distributed to or retrieved by the nodes, or
advertised by network elements at the topology discovery stage.
6.2 Implications For Routing
Dealing with diversity is an unavoidable requirement for routing in
the optical layer. It requires dealing with constraints in the
routing process but most importantly requires additional state
information û the SRLG relationships and also the routings of any
existing circuits from the new circuit is to be diverse û to be
available to the routing process.
At present SRLG information cannot be self-discovered. Indeed, in a
large network it is very difficult to maintain accurate SRLG
information. The problem becomes particularly daunting whenever
multiple administrative domains are involved, for instance after the
acquisition of one network by another, because there normally is a
likelihood that there are diversity violations between the domains.
It is very unlikely that diversity relationships between carriers
will be known any time in the near future.
Considerable variation in what different customers will mean by
acceptable diversity should be anticipated. Consequently we suggest
that an SRLG should be defined as follows: (i) It is a relationship
between two or more links, and (ii) it is characterized by two
parameters, the type of compromise (shared conduit, shared ROW,
shared optical ring, etc.) and the extent of the compromise (e.g.,
the number of miles over which the compromise persisted). This will
allow the SRLG's appropriate to a particular routing request to be
easily identified.
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7. Security Considerations
We are assuming OEO interfaces to the domain(s) covered by our
discussion (see, e.g., Sec. 4.1 above). If this assumption were to
be relaxed and externally generated optical signals allowed into the
domain, network security issues would arise. Specifically,
unauthorized usage in the form of signals at improper wavelengths or
with power levels or impairments inconsistent with those assumed by
the domain would be possible. With OEO interfaces, these types of
layer one threats should be controllable.
A key layer one security issue is resilience in the face of physical
attack. Diversity, as describe in Sec. 6, is a part of the
solution. However, it is ineffective if there is not sufficient
spare capacity available to make the network whole after an attack.
Several major related issues are:
- Defining the threat: If, for example, an electro-magnetic
interference (EMI) burst is an in-scope threat, then (in the
terminology of Sec. 6) all of the links sufficiently close
together to be disrupted by such a burst must be included in a
single SRLG. Similarly for other threats: For each in-scope
threat, SRLG's must be defined so that all links vulnerable to a
single incident of the threat must be grouped together in a
single SRLG.
- Allocating responsibility for responding to a layer one failure
between the various layers (especially the optical and IP
layers): This must be clearly specified to avoid churning and
unnecessary service interruptions.
The whole proposed process depends on the integrity of the
impairment characterization information (PMD parameters, etc.) and
also the SRLG definitions. Security of this information, both when
stored and when distributed, is essential.
This document does not address control plane issues, and so control-
plane security is out of scope. IPO control plane security
considerations are discussed in [Rajagopalam02]. Security
considerations for GMPLS, a likely control plane candidate, are
discussed in [Mannie02].
8. Acknowledgments
This document has benefited from discussions with Michael Eiselt,
Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
Faure, L. Noirie, and with our OIF colleagues.
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9. References
9.1 Normative References
[Goldstein94] Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F.,
Performance Implications of Component Crosstalk in Transparent
Lightwave Networks", IEEE Photonics Technology Letters, Vol.6, No.5,
May 1994.
[Hjßlmt²sson00] Gsli Hjßlmt²sson, Jennifer Yates, Sid Chaudhuri and
Albert
Greenberg, "Smart Routers - Simple Optics: An Architecture for the
Optical Internet, IEEE/OSA Journal of Lightwave Technology, December
2000,, Vo 18, Issue 12 , Dec. 2000 , pp. 1880 -1891.
[ITU] ITU-T Doc. G.663, Optical Fibers and Amplifiers, Section
II.4.1.2.
[Kaminow97] Kaminow, I. P. and Koch, T. L., editors, Optical Fiber
Telecommunications IIIA, Academic Press, 1997.
[Mannie02] Mannie, E. (ed.), "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", Interned Draft, draft-ietf-ccamp-
gmpls-architecture-03.txt, August, 2002.
[Rajagopalam02] Rajagopalam, B., et. al., "IP over Optical Networks:
A Framework", Internet Draft, draft-ietf-ipo-framework-02.txt June,
2002.
[Strand01] J. Strand, A. Chiu, and R. Tkach, "Issues for Routing in
the Optical Layer", IEEE Communications Magazine, Feb. 2001, vol. 39
No. 2, pp. 81-88.
[Strand01b] J. Strand, R. Doverspike, and G. Li, "Importance of
Wavelength Conversion In An Optical Network", Optical Networks
Magazine, May/June 2001, pp. 33-44.
[Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R.,
"Wavelength Converters in Dynamically-Reconfigurable WDM Networks",
IEEE Communications Surveys, 2Q1999 (online at
www.comsoc.org/pubs/surveys/2q99issue/yates.html).
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9.2 Informative References
[Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., and Coltun, R.,
"Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering
Control With Optical Crossconnects", Work in Progress, draft-
awduche-mpls-te-optical-01.txt.
[Bra96] Bradner, S., "The Internet Standards Process -- Revision 3,"
BCP 9, RFC 2026, October 1996.
[CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J.,
Edwards, W., "Performance Monitoring in Photonic Networks in Support
of MPL(ambda)S", Internet draft, work in progress, March 2000.
[Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in
Optical Network Restoration", IEEE Communication Magazine, February,
2001.
[Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is
Really Needed?" IEEE Communications Magazine, vol. 38 no. 10, Oct.
2000, pp. 154-160
[KRB01a] Kompella, K., et.al., "IS-IS extensions in support of
Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions-
01.txt, work in progress, 2001.
[KRB01b] Kompella, K., et. al., "OSPF extensions in support of
Generalized MPLS," Internet draft, draft-ospf-generalized- mpls-
00.txt, work in progress, March 2001.
[Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol,
Addison-Wesley, 1998.
[Passmore01] Passmore, D., "Managing Fatter Pipes," Business
Communications Review, August 2001, pp. 20-21.
[Ramaswami98] Ramaswami, R. and Sivarajan, K. N., Optical Networks:
A Practical Perspective, Morgan Kaufmann Publishers, 1998.
[Tkach98] Tkach, R., Goldstein, E., Nagel, J., and Strand, J.,
"Fundamental Limits of Optical Transparency", Optical Fiber
Communication Conf., Feb. 1998, pp. 161-162.
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10. Contributing Authors
This document was a collective work of a number of people. The text
and content of this document was contributed by the editors and the
co-authors listed below.
Ayan Banerjee
Calient Networks
5853 Rue Ferrari
San Jose, CA 95138
Email: abanerjee@calient.net
Dan Blumenthal
Calient Networks
5853 Rue Ferrari
San Jose, CA 95138
Email: dblumenthal@calient.net
John Drake
Calient Networks
5853 Rue Ferrari
San Jose, CA 95138
Email: jdrake@calient.net
Andre Fredette
Hatteras Networks
PO Box 110025
Research Triangle Park, NC 27709
Email: afredette@hatterasnetworks.com
Nan Froberg
PhotonEx Corporation
200 Metrowest Technology Dr.
Maynard, MA 01754
Email: nfroberg@photonex.com
Taha Landolsi
WorldCom, Inc.
2400 North Glenville Drive
Richardson, TX 75082
Email: taha.landolsi@wcom.com
James V. Luciani
900 Chelmsford St.
Lowell, MA 01851
Email: james_luciani@mindspring.com
Robert Tkach
Chiu, Strand, Eds. Informational [page 26]
Impairments And Other Constraints May 2003
On Optical Layer Routing
Celion Networks
1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724
Email: bob.tkach@celion.com
Yong Xue
WorldCom, Inc.
22001 Loudoun County Parkway
Ashburn, VA 20147
Email: yxue@cox.com
11. Editors' Addresses
Angela Chiu
AT&T Labs
200 Laurel Ave., Rm A5-1F13
Middletown, NJ 07748
Phone:(732) 420-9061
Email: chiu@research.att.com
John Strand
AT&T Labs
200 Laurel Ave., Rm A5-1D33
Middletown, NJ 07748
Phone:(732) 420-9036
Email: jls@research.att.com
Chiu, Strand, Eds. Informational [page 27]