Request for Comments: 4054 A. Chiu, Ed.
Category: Informational AT&T
May 2005
Impairments and Other Constraints on Optical Layer Routing
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Optical networking poses a number challenges for Generalized Multi-
Protocol Label Switching (GMPLS). Fundamentally, optical technology
is an analog rather than digital technology whereby 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 that
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.
Table of Contents
1. Introduction ................................................. 2
2. Sub-IP Area Summary and Justification of Work ................ 3
3. Reconfigurable Network Elements .............................. 3
3.1. Technology Background .................................. 3
3.2. Implications for Routing ............................... 6
4. Wavelength Routed All-Optical Networks ....................... 6
4.1. Problem Formulation .................................... 7
4.2. Polarization Mode Dispersion (PMD) ..................... 8
4.3. Amplifier Spontaneous Emission ......................... 9
4.4. Approximating the Effects of Some Other
Impairments Constraints ................................ 10
4.5. Other Impairment Considerations ........................ 13
4.6. An Alternative Approach - Using Maximum
Distance as the Only Constraint ........................ 13
4.7. Other Considerations ................................... 15
4.8. Implications for Routing and Control Plane Design ...... 15
5. More Complex Networks ........................................ 17
6. Diversity .................................................... 19
6.1. Background on Diversity ................................ 19
6.2. Implications for Routing ............................... 23
7. Security Considerations ...................................... 23
8. Acknowledgements ............................................. 24
9. References ................................................... 25
9.1. Normative References ................................... 25
9.2. Informative References ................................. 26
10. Contributing Authors ......................................... 26
1. Introduction
Generalized Multi-Protocol Label Switching (GMPLS) [Mannie04] aims to
extend MPLS to encompass a number of transport architectures,
including optical networks that incorporate 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 of challenges
for GMPLS. Fundamentally, optical technology is an analog rather
than digital technology whereby 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 that 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 the need to guarantee the
physical diversity of some circuits are discussed.
Since the purpose of this document 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 [Strand02].
The organization of the contribution is as follows:
- Section 2 is a section requested by the sub-IP Area management for
all new documents. 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 and
incorporates 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.
2. Sub-IP Area Summary and Justification of Work
This document merges and extends two previous expired Internet-Drafts
that were made IPO working group documents to form a basis for a
design team at the Minneapolis IETF 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
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. The material
covered is directly aimed at establishing a framework and
requirements for routing in an optical network.
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). 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 OADMs - 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:
- 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 DWDMs and X and Y are OADMs. 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 that will pass
transparently to a distant adaptation function. Adaptation
functionality that 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 may 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 OADMs 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.
For a fixed configuration of the OADMs 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 OADMs 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 OSes. 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 Shared Risk Link Group (SRLG - see Section 6.2) 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 deployed until recently 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]). 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 that 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 that 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 that 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
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 that 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] or [Kaminow02] 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 some 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
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
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.