Internet DRAFT - draft-chiu-strand-unique-olcp
draft-chiu-strand-unique-olcp
Angela Chiu
Celion Networks
John Strand
AT&T
Internet Draft
Document: draft-chiu-strand-unique-olcp-02.txt Robert Tkach
Expiration Date: August 2001 Celion Networks
James Luciani
Tollbridge
Features and Requirements for The Optical Layer Control Plane
Status of this Memo
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Abstract
Advances in the Optical Layer control plane are critical to ensure
that the tremendous amount of bandwidth generated by DWDM technology
be provided to upper layer services in a timely, reliable, and cost
effective fashion. This document describes some unique features and
requirements for the Optical Layer control plane that protocol
designers need to take into consideration.
1. Introduction
Chiu/Strand [Page 1]
Features and Requirements February 2001
For The Optical Layer Control Plane
The confluence of technical advances and service needs has focused
intense interest on optical networking. Dense Wave Division
Multiplexing (DWDM) is allowing unprecedented growth in raw optical
bandwidth; new cross-connect technologies promise the ability to
establish very high bandwidth connections within milliseconds; and
the insatiable appetite of the Internet for high capacity ``pipesÆÆ
has caused transport network operators to tear up their forecasts
and add optical capacity as fast as they can.
Critical to these advances are improvements to the "Optical Layer
Control Plane" -the software used to determine routings and
establish and maintain connections. Traditional centralized
transport operations systems (OSÆs) are widely acknowledged to be
incapable of scaling to meet exploding demand or establishing
connections as rapidly as needed. Consequently much attention has
been paid recently to new control plane architectures based on data
networking protocols such as MPLS and OSPF/IS-IS), under the
Generalized MPLS (GMPLS) umbrella. These architectures feature
distributed routing and control logic, auto discovery and self
inventorying, and many other advantages. OSPF/IS-IS provides a
constraint-based routing capability that takes bandwidth
availability into account.
The potential of these new architectures for optical networking is
enormous; however, to be successful they need to be adapted to the
specific technological, service, and business context characteristic
of optical networking. In order to identify some of the enhancements
necessary in GMPLS to make it applicable to optical networks, this
document attempts to describe several aspects of optical networking
which differ from those in the data networking environment inspiring
these new architectures:
- Section 2 describes some distinctive technological and
networking aspects of optical networking that will constrain
routing in an optical network, and
- Section 3 gives a transport network operatorÆs perspective on
business and operational realities that optical networks are
likely to face which are unlike those in data networking.
Particular emphasis is placed on the multihop optical network
problem. We most definitely are not claiming that these differences
are fatal to these new architectures, only that the new
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architectures must be built upon a detailed appreciation of the
unique characteristics of the optical world.
2. Constraints On Routing
Optical Layer routing is less insulated from details of physical
implementation than routing in higher layers. In this section we
give examples of constraints arising from the design of network
elements, from the accumulation of signal impairments, and from the
need to guarantee the physical diversity of some circuits.
2.1 Reconfigurable Network Elements
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 (OADMÆs). 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 2-1: An OTS With OADM's - Functional Architecture
In Fig.2-1, 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.
Chiu/Strand et al [page 3]
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In Fig. 2-1, D and E are DWDMÆs and X and Y are OADMÆs. 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". Another term frequently used is "wave
group".
- 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. Note that tunable
receivers are becoming a reality and for certain applications.
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 OADMÆs 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 OADMÆs 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 OADMÆs 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.
Chiu/Strand et al [page 4]
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This capability 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 operatorÆs
OSÆs. However in principle the EMS could participate in the routing
process. The constraints on reconfiguration are likely to be quite
complex, dependent on the vendor design and also on exactly what
line cards have been deployed. Thus the state information needed for
routing is likely to be voluminous and possibly vendor specific.
However it is very desirable to solve these issues, possibly by
advertising only an abstraction of the complex configuration options
to the external world via the control plane.
2.2 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 eliminated transparency to bit
rates and frame format. These transponders are quite expensive and
they also constrain the rapid evolution to new services - for
example, they tend to be bit rate and format specific. 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 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
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understand the impacts of the various types of impairments in this
environment.
2.2.1 Problem Formulation
We consider a single domain of transparency. 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 applies 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 dispersion 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.
2.2.2 Polarization Mode Dispersion
For a transparent fiber segment, the general rule for the PMD
requirement is that the time-average differential time delay between
two orthogonal state of polarizations should be less than a% of the
bit duration. (A typical value for a is 10 [ITU]. More aggressive
designs to compensate for PMD may allow higher than 10%. This would
be a system parameter known to the routing process.) This results in
a upper bound on the maximum length of an M-fiber-span transparent
segment, which is inverse proportion to the square of bit rate and
fiber PMD parameter where a fiber span in a transparent network
refers to a segment between two optical amplifiers (The detailed
equation is omitted due to the format constraint). For typical
fibers with PMD parameter of 0.5 picosecond 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. With newer fibers assuming PMD parameter
equals to 0.1 picosecond per square root of km, the maximum length
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of the transparent segment should not exceed 10000km and 625km for
bit rates of 10Gb/s and 40Gb/, respectively. 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.
2.2.3 Amplifier Spontaneous Emission
ASE degrades the signal to noise ratio. An acceptable optical SNR
level (SNRmin) which depends on the bit rate and transmitter-
receiver technology (e.g., FEC) needs to be maintained at the
receiver. It also includes all the margins for those impairments
that are not treated explicitly as described in the following
subsection. 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 with 80km
long in each. Startups have announced ultra long haul systems that
are claimed to be able to support up to thousands of km. Although
these general rules are helpful in network planning, more detailed
information on the SNR reduction in each component should be used to
determine whether the SNR level through a given transparent segment
is within the required value. This would provide flexibility in
provisioning or restoring a lightpath through a transparent
subnetwork. Here, we assume that the average optical power launched
at the transmitter is known as 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 nonlineararity. A constraint
on the maximum number of spans can be obtained [Kaminow97] which is
proportional to P and inverse proportional to SNRmin, optical
bandwidth B, amplifier gain G-1 and spontaneous emission factor n of
the optical amplifier. (Again, the detailed equation is omitted due
to the format constraint.) LetÆs 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.
2.2.4 Other Impairments
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 it is generally required to maintain the total PDL on the
path to be within some acceptable limit, typically 1dB margin in
OSNR.
Chiu/Strand et al [page 7]
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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.
Crosstalk Since crosstalk in the system affects Q which is a measure
of the electrical signal-to-noise ratio assuming Gaussian noise
statistics, it can be factored in with some margin in Q. As a
result, one can increase the OSNR requirement by some modified
margin.
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. 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.
Further work is required to determine the validity of this approach.
However, it is possible that there could be an advantage in
designing systems which 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.
2.2.5 Implications For Routing and Control Plane Design
- Additional state information will be required by the routing
algorithm for each type of impairment that has the potential of
being limiting for some routes.
Chiu/Strand et al [page 8]
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- 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 important to know whether
chromatic dispersion has been dealt with on per-link basis, and
whether the domain is operating in a linear or nonlinear regime.
- In situations where only PMD and/or ASE impairments are
potentially binding the optimal routing problem as two
constraints OSPF algorithm enhancements will be needed. However,
it is likely that relatively simple heuristics could be used in
practice.
Additionally, 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. This is discussed in [Chaudhuri00],
where it is concluded that advertising detailed wavelength
availabilities on each link is not likely to scale. Instead they
propose 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.
- 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. This approach
would however also have a significant affect on OSPF.
2.3 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.
Chiu/Strand et al [page 9]
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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, ROWÆs 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 carriersÆ fiber cables in close
proximity to each other. Similarly, in a metropolitan network,
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 OLXCÆs. 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:
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A B
A-------------B \ /
\ /
X-----Y
/ \
C-------------D / \
C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
Figure 2-2: 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. 2-3(a), where A-X-B is the major route, X
the manhole, and C the smaller office. The actual fiber topology
would then look like Fig. 2-3(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 2-3. Fiber Cable vs Fiber Topologies
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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
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.
Chiu/Strand et al [page 12]
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- 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.
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.
[Chaudhuri00] introduced the term "Shared Risk Link Group" (SRLG) to
describe the relationship between two non-diverse links. The above
discussion 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.)
- Extent of Compromise: For compromised outside plant, this would
be the length of the sharing.
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 (3000 km)
OTS). The mapping between links and different types of SRLGÆs 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. On
the other hand, in order to be able to perform distribute path
selection at each node that satisfies certain diverse routing
Chiu/Strand et al [page 13]
Features and Requirements February 2001
For The Optical Layer Control Plane
criterion, each network element may need to propagate the
information of number of channels available for each channel type
(e.g., OC48, OC192) on each channel group, where channel group is
defined as a set of channels that are routed identically and should
be given unique identification. Each channel group can be mapped
into a sequence of fiber cables while each fiber cable can belong to
multiple SRLGÆs based on their definitions.
2.4 Other Unique Features of Optical Networks
There are other major differences between optical networks and IP
networks that have significant impacts on the design of the Optical
Layer control plane. They include the following two areas.
- Bi-directionality: In an IP network, Label Switched Paths (LSPs)
are inherently unidirectional. However, current transport
networks are bi-directional oriented, mostly due to the
evolution of two-way transmission in Public Switched Telephone
Network and by SONET/SDH line protection schemes [Doverspike00].
This often requires the bi-directional connections provided by
the optical layer to use the same numbered channel in each
direction. As a result, a channel contention problem may occur
between two bi-directional request traveling in opposite
directions. Signaling mechanisms have been proposed to resolve
this type of contention [Ashwood00].
- Protection and restoration: In an IP network, when a backup LSP
is pre-established to protect against failure(s) on a working
LSP, the backup LSP does not occupy any physical resources
before a failure occurs. However, in an optical network, a pre-
established optical connection for backup does occupy the ports
and channels on the path of the connection. This can be used for
the 1+1 protection, but not for shared mesh protection. Instead
with shared mesh protection, the backup path can be pre-selected
with or without the associated channels being chosen prior to
any failure, then cross-connect ports/channels physically after
a failure on the working path has been detected. See
[Doverspike00] for more detailed discussions on various
protection/restoration schemes.
2.4 Discussion and Summary
Dealing with diversity seems to be an unavoidable requirement on
optical layer routing. It requires dealing with additional
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Features and Requirements February 2001
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constraints in the routing process but most importantly requires
additional state information to be available to the routing process.
The physical constraints of optical technology apply inside an all-
optical ``domains of transparencyÆÆ. TodayÆs OTS is a simple
``domain of transparencyÆÆ consisting of WDM Mux/Demuxers and
Optical Amplifiers. Because an OTS is not easily reconfigurable
these constraints are dealt with at the time of installation and
donÆt complicate routing and the control plane.
As domains of transparency become both larger and software
reconfigurable as discussed earlier, these physical constraints on
connectivity and transmission quality become increasingly of concern
to the control plane. It is important to note that at present this
evolution is largely technology driven: vendors pushing the
technology envelope are competing fiercely to provide solutions
which have higher capacity, can go further all-optically, are more
reconfigurable, and are more cost-effective. Routing constraints,
which are essentially a by-product of this competitive dynamic, may
well become more complex. As vendors pursue their diverse visions it
is quite plausible that the optical layer of the future will be made
up of heterogeneous technologies which differ significantly in their
routing implications.
What are the control plane architecture choices in such an
eventuality? Alternative approaches that deserve consideration are:
- Per-Domain Routing: In this approach each domain could have its
own tuned approach to routing. Inter-domain routing would be
handled by a multi-domain or hierarchical protocol that allowed
the hiding of local complexity. Single vendor domains might
have proprietary intra-domain routing strategies. This approach
has the advantage of providing carriers a flexible way to digest
technologies and subnetworks that for whatever reason are not
using a standard intra-domain routing protocol.
- Enforced Homogeneity: The capabilities of the control plane
would impose constraints on system design and network
engineering. As examples: If control plane protocols did not
deal with non-linear impairments carriers would require their
vendors to provide transport systems where these constraints
were never binding. Transmission engineers could be required to
only deploy domains where every possible route met all
constraints not handled explicitly by the control plane even if
the cost penalties were severe.
- Additional Regeneration: At (selected) OLXCÆs within a domain of
transparency, the control plane could insert O/E/O regeneration
into routes with transmission problems. This might make all
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routes feasible again, but at the cost of additional cost and
complexity and with some loss of rate and format transparency.
- Standardized Intra-Domain Routing Protocol: The examples
discussed in Section 2 suggest that a single standardized
protocol which tries to deal with the full range of possible
topological and transmission constraints will be extremely
complex and will require a lot of state information. However
when combined with limited application of the two previous
approaches it might be more plausible.
Given the complexity of physical and connectivity impairments and
diversity requirements, a valid question to ask is whether a
centralized routing model, where routing is done centrally using a
centralized database with a global network view would be better than
the distributed model favored in the Internet. Here, we provide some
pros and cons on each model.
To the extent that the per-domain routing approach just discussed is
used, the choice of model might be different depending on the
characteristics of the domain. For example, in a domain like Fig.
2-1 it seems likely that a centralized model is more appropriate
because network elements like tunable lasers and reconfigurable
OADM's seem on the surface to be unlikely peers to much more complex
devices like OXC's or routers. On the other hand, a purely "opaque"
domain where impairment constraints play no role in routing would
appear to be an excellent candidate for the distributed model.
In the context of the complexities discussed in this paper, a
centralized model has some advantages:
- Information such as SRLGÆs and performance parameters which
change infrequently and are unlikely to be amenable to self-
discovery could reside in a central database and would not need
to be advertised.
- Routing dependencies among circuits (to ensure diversity, for
example) is more easily handled centrally when the circuits do
not share terminals since the necessary state information should
be more easily accessible in a centralized model.
- Pre-computation of restoration paths and other computations that
can benefit from the use of global state information may also
benefit from centralization.
There are, of course, significant disadvantages to the centralized
model when compared to a distributed model:
- If rapid restoration is required, it is not possible to rely on
a centralized routing system to compute a recovery path for each
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Features and Requirements February 2001
For The Optical Layer Control Plane
failed lightpath on demand after a failure has been detected.
The distributed model arguably will not have this problem.
- The centralized approach is not consistent with the distributed
routing philosophy prevalent in the Internet. The reasons which
drove the InternetÆs architecture-scalability, the inherent
problems with hard state information, etc.-are largely relevant
to optical networking. In addition there is the major
disadvantage that a centralized approach would seem to preclude
integrated routing across the IP and optical boundary.
A related issue is whether routes should be pre-computed. It has
been suggested, for example, that all routes (or at least a large
number) be pre-computed and stored in a central database. This
potentially might allow more sophisticated algorithms to be used to
filter out the routes violating transmission constraints. There are
however serious disadvantages (in addition to the disadvantages of
the centralized model given above):
- In a large national network there are just too many routes that
might be needed, by orders of magnitude. This is particularly
true when diversity constraints and restoration routing may
force weird routings.
- Every time any parameter changes anywhere in the network all
routes using the impacted resource will need to be reexamined.
3. Business and Operational Realities
The Internet technologies being applied to define the new Optical
Layer control plane evolved in a very different business and
operational environment than that of today's transport network
provider. The differences need to be clearly understood and dealt
with if the new control plane is going to be a success. The Optical
Interworking Forum, one of the principal standards groups in this
area, has recently formed a Carrier Subgroup to provide guidance
from this perspective for their standards activities.
In this section we touch on two aspects of this problem: Business
Models and the management of the introduction of new technology.
3.1 Business Models
The cost of providing gigabit connections is expected to drop
rapidly, but will still require dedicated use of expensive and
periodically scarce capacity and equipments. Therefore the ability
to control network access, and to measure and bill for usage, will
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be critical. Also, lightpath connections are expected to have quite
long holding times (weeks-months) compared to LSPs in an IP network.
Therefore the collection of usage data and the nature of the
connection establishment process have very different characteristics
in the Optical Network than in an IP network.
In addition, industry revenues from legacy services (voice and
private line) are expected to dwarf those from IP transport for the
next few years. ?? John: could you find a reference from RHK?
Meeting the needs of these services and migrating them to the
operatorÆs newer service platforms will also be a critical need for
operators with extensive embedded revenues. Thus the needs of
services based on SONET/SDH, Ethernet, ATM, etc. will need to be
given attention. In addition most operators hope that they will
have many different ISP's and Intranets as customers. Thus the
customer base for most operators will be quite diverse.
Another area of prime concern is Operations Systems (OSÆs). The
opportunity to create a thinner and more nimble network management
plane by off-loading many provisioning and data-basing functions
onto a vendor-provided control plane and/or Element Management
System (EMS) holds the promise of large and immediate benefits to
operators in the form of reduced software development and more rapid
deployment of new functionality. This is a critical area to achieve
scalability.
In the short term the principal benefits of the proposed control
plane are two: rapid provisioning and a reduction in the cost and
complexity of OSÆs and operations. Both of these benefits require
that circuits be controlled end-to-end by the new control plane, for
otherwise the provisioning times will be determined by those of the
older, much slower segments and OS costs and OS and operations
complexity may actually go up because of the need to interwork the
old and the new worlds. To avoid this the capabilities of the new
control plane need to be available end-to-end as soon as possible.
This will put a premium on the rapid development of standards for
interworking across trust boundaries, for example between Local
Exchange Carrier's and national networks.
3.2 Managing The Introduction Of New Technology
We expect optical layer hardware technology to continue to evolve
very rapidly, with a very real possibility of additional
"disruptive" advances. The analog nature of optical technology
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compounds this problem for the control planes because these advances
are likely to be accompanied by complex technology-specific
constraints on routing and functionality. (Sections 2.1 and 2.2
above provide examples of this.) An architecture which allows the
gradual and seamless introduction of new technologies into the
network without time-consuming and costly changes to embedded
technologies and especially control planes is highly desirable.
When compared to the IP experience several distinctions stand out:
- The optical layer control plane seems more likely to be buffeted
by hardware changes than is the IP control plane.
- Optical layer innovations are currently being driven by start-up
companies, with product innovation well ahead of the standards
process. Efforts at control plane standardization are much less
mature than comparable IP efforts. This is a matter of
considerable concern because neither rapid provisioning nor the
operational improvements desired are likely if each vendor has a
proprietary control plane, with interworking between vendors
(and hence between networks, in most cases) left as a problem
for operators' OS's to solve.
3.3 Service Framework Suggestions
For the reasons given above and others, we expect that the best
model for an optical layer control plane within a trust domain is
one that pays heavy attention to the management of heterogeneous
technologies and associated service capabilities. This might be done
by hiding complexities in subnetworks. These subnetworks would then
advertise only a standardized abstraction of their connectivity,
capacity, and functionality capabilities. Hopefully this would allow
even disruptive technologies such as all-optical subnetworks to be
introduced with a minimum of impact on preexisting parts of the
trust domain.
Each network operator will have a need to define "branded" services
- bundles of service functionality and SLA's with a specific price
structure. In a heterogeneous network it will be necessary to map a
customer request for such a "branded" service onto the specific
capabilities of each subnetwork. This suggests a hierarchical model,
decisions about these mappings, and also about policies for peering
with other networks and overall management of the service offerings
available to specific customers managed centrally but application of
these policies handled at the local or subnetwork level.
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4. Security Considerations
The solution developed to address the requirements defined in this
document must address security aspects.
5. Acknowledgments
This document has benefited from discussions with Michael Eiselt and
Mark Shtaif.
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Features and Requirements February 2001
For The Optical Layer Control Plane
[Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R.,
"Wavelength Converters in Dynamically-Reconfigurable WDM Networks",
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Authors' Addresses:
Angela Chiu
AT&T Labs
200 Laurel Ave., Rm A5-1F06
Middletown, NJ 07748
Phone:(732) 420-9057
Email: alchiu@att.com
John Strand
AT&T Labs
200 Laurel Ave., Rm A5-1D06
Middletown, NJ 07748
Phone:(732) 420-9036
Email: jls@att.com
Robert Tkach
Celion Networks
1 Shiela Dr., Suite 2
Tinton Falls, NJ 07724
Phone:(732) 747-9909
Email: bob.tkach@celion.com
James Luciani
PO Box 1010
Concord, MA 01742
Phone:(978) 266-2854
james_luciani@mindspring.com
Chiu/Strand et al [page 21]
