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 the transmission path. The state of
polarization fluctuates with time and its distribution is very
important also. It is generally required that the total PDL on
the path be maintained 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
Dispersion Compensation Fiber (DCF). 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 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 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 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.
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) Unless a certain mechanism is built in the OTS to take
advantage of shorter spans, spans shorter than a typical span
length D need to be treated as a span of length D instead of with
its real length. 2) Spans that are longer than D would have a
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
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 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
[Hjalmtysson00], 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:
- 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 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) OLXCs 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.)
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 OLXCs 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
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? Traditionally, 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