Angela Chiu
John Strand
AT&T
Internet Draft
Document: draft-chiu-strand-unique-olcp-00.txt July, 2000
Unique Features and Requirements for The Optical Layer Control Plane
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Abstract
Advances in the Optical Layer control plane is critical to ensure
tremendous amount of bandwidth generated by the 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
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; cross-connect technologies, both electrical and optical,
promise the ability to establish very high bandwidth connections
within milliseconds; and the insatiable appetite of the Internet for
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high capacity "pipes" 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 are widely acknowledged to be incapable of
scaling to meet exploding demand or to establish connections as
rapidly as needed. Consequently in the last year much attention has
been paid to new control plane architectures based on data
networking protocols from IETF such as RSVP-TE and CR-LDP from MPLS,
IGPs including OSPF and IS-IS. These architectures feature
distributed routing and control logic, auto discovery and self
inventorying, and many other advantages.
The potential of these new architectures for optical networking are
enormous; however, to be successful they need to be adapted to the
specific technological, service, and business context characteristic
of optical networking. 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.
We most definitely are not claiming that these differences are fatal
to these new architectures, only that the new 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
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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.
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
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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".
- 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 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 between the DWDMÆs and OADMÆs 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.
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, etc. have been deployed. Thus the state information that
would need to be disseminated is likely to be voluminous, possibly
vendor specific, and likely to be hard to pin down. However it is
very desirable to solve these issues, possibly by advertising only
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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 from other links. 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.
The routing of lightpaths through an all-optical network has
received extensive attention. (For recent reviews, see [Yates99],
[Ramaswami98], [Mukherjee97]). One aspect of this problem that is
still troublesome is the impact of transmission impairments on
signal quality. 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. 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:
Maximum private line bit rates have rapidly increased from DS3 (45
Mb/sec) through OC-3 (155 Mb/sec), OC-12 (622 Mb/s) and OC-48 (2.5
Gb/sec) to OC-192 (10 Gb/sec). OC-768 (40 Gb/sec) is now under
discussion. As bit rates increase it is necessary to increase power.
This makes impairments and nonlinearities more troublesome. Thus a
contained domain of transparency sized so all routes can support an
OC-768 would necessarily be quite small (perhaps <100 km in
diameter). A domain sized for OC-192 very likely be significantly
smaller than one sized for OC-48. This suggests that more aggressive
domain sizing might have some benefits.
Optical technology is advancing very rapidly and is making ever-
larger domains possible. Of particular importance in this respect
are advances in optical cross-connects (OXCÆs) and ultra-long OTSÆs
employing Raman amplification and other techniques. The use of all-
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optical networking in metro and access applications is under very
rapid evolution also.
Optical layer control plane architectures are under intense
discussion in the ITU, IETF, OIF, and other standards bodies. The
general approach being considered for routing is to adapt the Open
Shortest Path First (OSPF) protocol from IP ([Moy98]). A number of
the specifics of this adaptation depend strongly on whether
transmission impairments need to be explicitly considered in the
routing process. To give one example, the link state information
advertised might need to contain information about the specific
impairments on the link.
In this document we assume that these considerations have led to the
deployment of a domain of transparency that is too large to ensure
that all potential routes have adequate signal quality. Our goal is
to understand the impacts of the various types of impairments in
this environment and to recommend a practical method for doing
routing in this situation.
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.
2.2.2 Impairment Constraints ([Tkach98])
Impairment constraints can be classified into two categories, linear
and nonlinear. Linear effects are independent of signal power and
affect wavelengths individually. Amplifier spontaneous emission
(ASE) and Polarization Mode Dispersion (PMD) are examples. On the
other hand, fiber nonlinearities are significantly more complex:
they generate not only dispersion on individual channel, but also
crosstalk between channels which causes dependency across channels.
Examples include four-photon mixing, cross-phase modulation, self-
phase modulation, stimulated Brillouin scattering, and stimulated
Raman scattering. Here, we assume that proper system design will
compensate for those effects (for example, Raman amplification and
FEC mechanisms both serve to allow lower power to be used and thus
move systems towards the linear regime) and/or simulation studies
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can be used to rule out certain fiber type(s). We consider only the
linear regime in this document. We assume that chromatic dispersion
is compensated in fiber lines and can be neglected. Hence PMD and
signal to noise ratio (SNR) are the only impairment constraints that
need to be considered in determining the path of a lightpath through
a transparent optical subnetwork. We examine the role of each in
this regard.
- PMD: 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 10% of the bit duration [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 constraint on the maximum length of an M-fiber-span
transparent segment where a fiber span in a transparent network
refers to a segment between two optical amplifiers. The
constraint depends on a set of parameters including the length
and the fiber PMD parameter of each of the M fiber spans. (The
detailed equation is omitted due to the format constraint.) For
typical fibers with PMD parameter of 1 picosecond per square
root of km, based on the constraint, the maximum length of the
transparent segment should not exceed 100km and 6.75km 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 of the transparent segment should not
exceed 10000km and 675km 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.
- SNR: Based on the bit rate and type of transmitter-receiver
technology (e.g., FEC), an acceptable optical SNR level (SNRmin)
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 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. A constraint
on the maximum number of spans can be obtained [Kaminow97] which
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depends on a set of parameters including SNRmin, P, optical
bandwidth B, amplifier gain G and spontaneous emission factor n
for each 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 without FEC where the
requirement on SNRmin becomes 25dB, the maximum number of spans
drops down to 3.
2.2.3 Implications For Routing and Control Plane Design
Here, we describe the main implications that these two main
impairment constraints have on routing algorithm and control plane
design.
The optimal routing problem with the two constraints is in general
more computational intensive. However, relatively simple heuristics
can be used in practice. If the ingress node of a lightpath does
path selection, in order to check whether the two constraints are
satisfied or not for a given path, it needs to obtain all the
relevant parameters for each span on the path if they vary from one
span to another. These parameters typically do not change
dynamically, and are often stored in some database. So the ingress
node (or some other node that makes the path selection decision) can
retrieve the information from the database when needed, or the
information can be advertised by the node attached to the span at
the topology discovery stage.
Note that in some circumstances, it may be useful to consider
nonlinear effects also. Nevertheless, the two constraints described
here are enough to illustrate the impact of the impairment
constraints on the routing algorithm and control plane design in
transparent subnetworks.
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
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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 the routing
logic normally used in 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.
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:
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- 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 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.)
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C C
| / \
| / \
| / \
A------X------B A---------------B
(a) Fiber Cable Topology (b) Fiber Topology
Figure 2-3. 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
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)
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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 Natural 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.
o 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.
[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
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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
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
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[Doverspike00] for more detailed discussions on various
protection/restoration schemes.
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
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. 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
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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
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
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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.
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,
Mark Shtaif, Bob Tkach, and our other AT&T colleagues.
References:
[Ashwood00] Ashwood-Smith, P. et al., "MPLS Optical/Switching
Signaling Functional Description", Work in Progress, draft-ashwood-
generalized-mpls-signaling-00.txt.
[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.
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[Chaudhuri00] Chaudhuri, S., Hjalmtysson, G., and Yates, J.,
"Control of Lightpaths in an Optical Network", Work in Progress,
draft-chaudhuri-ip-olxc-control-00.txt.
[Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS
Protocols in the Optical Network Control Plane", submitted for
journal publication, June, 2000 (online at
http://www.research.att.com/~jyates/IPoverWDMpublications.html).
[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.
[Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol,
Addison-Wesley, 1998.
[Mukherjee97] Mukherjee, B., Optical Communication Networks, McGraw
Hill, 1997.
[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.
[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).
Authors' Addresses:
Angela Chiu
AT&T Labs
100 Schulz Dr., Rm 4-204
Red Bank, NJ 07701, USA
Phone: +1 (732) 345-3441
Email: alchiu@att.com
John Strand
AT&T Labs
100 Schulz Dr., Rm 4-212
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Red Bank, NJ 07701, USA
Phone: +1 (732) 345-3255
Email: jls@att.com
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