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Optical Networking:


Email: deron_at_cc.ncue.edu.tw or deron_at_ms45.hinet.net. ???:????? 104?. ??: ... as that of NE, except that an additional phrase 'forecasted to be' is appended. ... – PowerPoint PPT presentation

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Title: Optical Networking:

Chapter 1
  • Optical Networking
  • Principles and Challenges

Course Information
  • Email deron_at_cc.ncue.edu.tw or
  • ???????? 104?
  • ??7232105-7047
  • Course Web Site http//
  • http//deron.csie.ncue.edu.tw

  • ??? 30
  • ??? 30
  • ????2? 20
  • ????1? 20

  • 1.1 Introduction
  • 1.2 Telecom Network Overview
  • 1.3 Telecom Business Models
  • 1.4 Roles of Three Fields in Optical Networking
  • 1.5 Cross-Layer Design
  • 1.6 TE vs . NE . vs . NP
  • 1.7 What is an Optical Network?
  • 1.8 Optical Networking Need Promise
  • 1.9 xDM vs . xDMA
  • 1.10 Wavelength-Division Multiplexing (WDM)
  • 1.11 WDM Networking Evolution
  • 1.12 WDM Network Constructions
  • 1.13 WDM Economics
  • 1.14 Sample Research Problems
  • 1.15 Road Map - Organization of the Book

1.1 Introduction
  • We need to be ready with the appropriate
    technologies and engineering solutions to meet
    the growing bandwidth needs of our information
  • Optical networking using wavelength-division
    multiplexing (WDM)

1.2 Telecom Network Overview
Access network Metropolitan-area network Backbone

The access network
  • Enable the end-users to get connected to the rest
    of the network infrastructure.
  • spans a distance of a few kilometers (lt20 km)
  • Current solution for access are dial-up modems,
    higher-speed lines (such as T1/E1), digital
    subscriber line (DSL), and cable modem.
  • However, the access network continues to be a
    bottleneck, and users require (and are demanding)
    higher bandwidth to be delivered to their
  • Passive optical networks (PONS) based on
    inexpensive, proven, and ubiquitous Ethernet
    technology (and referred to as EPON) seem an
    attractive proposition for this market segment.
  • PON technology in general, and EPON in
    particular, will be studied in Chapter 5.

metro-area network
  • The metro-area network typically spans a
    metropolitan region,
  • covering distances 20200 km.
  • Given the deep-rooted legacy of SONET/SDH ring
  • SONET Synchronous Optical Network
  • SDH Synchronous Digital Hierarchy
  • multi-wavelength versions of these rings are
    being deployed for our metro networks.
  • Important characteristics of optical metro
    networks will be discussed in Chapter 6.

Backbone network
  • The backbone network spans long distances, e.g.,
    each link could be a few hundreds to a few
    thousands of kilometers in length (gt200km).
  • set up to provide nationwide or global coverage.
  • Most telecom backbone networks are deployed today
    as an interconnection of "stacked" SONET/SDH
  • the fibers support multiple wavelengths using WDM
    transmission equipment however, by "tying
    together several wavelengths on different fiber
    segments, one can create logical rings, and these
    rings can "meet" one another at some junction
  • Backbone network will be discussed in Chapters 17
    and 18.

1.3 Telecom Business Models
  • Skip

1.4 skip
1.5 Cross-Layer Design
  • The need for tight coupling between network
    architectures and device capabilities.
  • Without a sound knowledge of device capabilities
    and limitations, one can produce architectures
    which may be unrealizable conversely, research
    on new optical devices, conducted without the
    concept of a useful system, can lead to
    sophisticated technology with limited or no

(No Transcript)
1.6 (TE) vs. (NE) vs. (NP)
  • Traffic Engineering (TE) vs. Network Engineering
    (NE) vs. Network Planning (NP)
  • TE "Put the traffic where the bandwidth is."
  • NE "Put the bandwidth where the traffic is."
  • NP "Put the bandwidth where the traffic is
    forecasted to be.

Traffic Engineering (TE)
  • Since the goal of TE is to "put the traffic where
    the bandwidth is," TE is essentially a "routing
    problem," where the traffic to be routed could be
    packets, packet flows, or bandwidth chunks (i.e.,
  • Routing and assigning appropriate bandwidth to
    packet flows and circuits is also referred to as
    bandwidth provisioning, or provisioning for
  • Thus, TE is an "online, dynamic problem whose
    decision-making time is very quick, perhaps on
    the order of milliseconds.
  • The typical performance metric used to evaluate a
    TE algorithm is
  • "blocking probability" (by (implicitly) assuming
    that the network is operating at "steady-state")
  • volume of control overhead,
  • convergence time (to reduce routing instability),

Network Engineering (NE)
  • As a network continues its operation, and as
    traffic on it builds up (perhaps asymmetrically),
    certain parts of the network may become more
    congested due to increasing traffic, and these
    parts may need "help" in the form of additional
    capacity to relieve the congestion.
  • The decision-making time is perhaps on the order
    of weeks or months.
  • Thus, a typical performance metric for a NE
    problem could be "exhaustion probability which
    determines when a current network, given a
    traffic-growth pattern, will run into capacity
  • This (NE) is a very realistic problem in our
    operational networks and, unfortunately, it has
    been underestimated in the academic research

Network Planning NP
  • The NP description is almost the same as that of
    NE, except that an additional phrase "forecasted
    to be" is appended.
  • NP corresponds to the planning (i.e., design) of
    a network from scratch, with a decision-making
    timescale of perhaps a few years.
  • A sample NP problem is the following
  • Given a set of traffic demands between various
    pairs of nodes (which is also called a "traffic
    matrix"), design the network for minimum cost,
    i.e., determine how much capacity to put on each
    link of the network, as well as routing of
    traffic through the network links. (Note that the
    typical performance/optimization metric for a NP
    problem is "cost.")

Network Planning NP
  • As an example, the cost could be the sum of the
    (bandwidth) cost of all the links. In the brief
    NP problem description (above), no statement was
    made about the connectivity between network nodes
    (i.e., "network topology").
  • By default, the network topology (or graph) may
    be given. But an additional dimension to the NP
    problem would be to also determine the topology
    (while achieving minimum cost).
  • This "dual" problem can be stated as follows
    Given the traffic demands, and the maximum cost
    (including perhaps the topology and capacity of
    each link), determine how to establish the
    demands so that the network throughput (in terms
    of carried demands) is maximized

Summary of TW, NW, NP
1.7 What is an Optical Network?
Optical network
  • The links require transmission equipment, while
    the nodes require switching equipment.
  • An optical amplifier can simultaneously amplify
    all of the signals on multiple wavelength
    channels (perhaps as high as 160) on a single
    fiber link, independent of how many of these
    wavelengths are currently carrying live traffic.
  • However, many attempts at developing all-optical
    switches have indicated that optical switching is
    still in its infancy.

Optical network
  • Thus, an optical network is not necessarily
    all-optical the transmission is certainly
    optical, but the switching could be optical, or
    electrical, or hybrid
  • Also, an optical is not necessarily
  • It could switch circuits (Chapters 7-12, 15-16),
  • sub-wavelength-granularity bandwidth pipes
    (Chapters 13-14), or
  • "bursts," where a burst is a collection of
    packets (Chapter 18).

  • As an example, consider that two users located at
    the two coasts of the USA, need to exchange some
    large files.
  • Under present mode of operation (PMO) of today's
    data networks, a simlple "traceroute" from Davis
    to Boston indicates that the file transfers may
    encounter 20 router hops, at each of which there
    exists the possibility of buffering (and hence
    delay), as well as loss (due to buffer overflow).
  • In future mode of operation (FMO) of data
    networks, by exploiting the underlying support
    from optical-networking technologies, one should
    be able to "dial up" a fat bandwidth pipe (of
    appropriate capacity and duration) to complete
    the task. It is not necessary that all such
    applications need to accomplished in "one hop.
  • 20 hops down to 3 or 1 (not necessary)

Separated control network plane
  • Note that signaling in IP networks is "in-band,"
    so (short) control packets may have to contend
    with (long) data packets for transmission
  • As data entities that need to get transferred
    over our networks get longer, control packets are
    expected to encounter more contention for
  • Thus, one can create a separated control network
    by setting aside a wavelength (or a
    sub-wavelength granularity bandwidth chunk using
    a traffic-grooming principle) on each link for
    this purpose, so that control packets have their
    own dedicated network.

1.8 Optical Networking Need Promise
  • Life in our increasingly information-dependent
    society requires that we have access to
    information at our finger tips when we need it,
    where we need it, and in whatever format we need
  • Internet and ATM networks - unfortunately, don't
    have the capacity to support the foreseeable
    bandwidth demands.

Fiber optic technology
  • huge bandwidth (nearly 50 terabits per second
  • low signal attenuation(??) (as low as 0.2 dB/km),
  • immunity to electromagnetic interference,
  • high security of signal because of no
    electromagnetic radiation, so difficult to
  • no crosstalk and interferences between fibers in
    the same cable,
  • low signal distortion(??),
  • low power requirement,
  • low material usage, small space requirement, and
    low cost.
  • high electrical resistance, so safe to use near
    high-voltage equipment or between areas with
    different earth potentials.

Solving Problem
  • Our challenge now is to turn the promise of
    optical fiber technology to reality to meet our
    information networking demands of the foreseeable
  • Solving Problem
  • Network lag.
  • Not enough bandwidth today
  • Exponential Growth in user traffic.

opto-electronic bandwidth mismatch
  • Given that a single-mode fiber's potential
    bandwidth is nearly 50 Tbps, which is nearly 3-4
    orders of magnitude higher than electronic data
    rates of a few gigabits per second (Gbps), every
    effort should be made to tap into this huge
    opto-electronic bandwidth mismatch.

Solution in Optical Network
  • In an optical communication network, this
    concurrency may be provided according to either
  • wavelength or frequency wavelength-division
    multiplexing (WDM),
  • time slots time-division multiplexing (TDM),
  • wave shape spread spectrum, code-division
    multiplexing (CDM).

Why not TDM or CDM?
  • Optical TDM and CDM are somewhat futuristic
    technologies today.
  • Under (optical) TDM, each end-user should be able
    to synchronize to within one time slot.
  • The optical TDM bit rate is the aggregate rate
    over all TDM channels in the system, while the
    optical CDM chip rate may be much each higher
    than user's data rate.

Why not TDM or CDM?
  • Both the TDM bit rate and the CDM chip rate may
    be much higher than electronic processing speed,
    i.e., some part of an end user's network
    interface must operate at a rate higher than
    electronic speed.
  • Thus, TDM and CDM are relatively less attractive
    than WDM, since WDM unlike TDM or CDM has no
    such requirement.

1.9 xDM vs. xDMA
  • We have introduced the term xDM where x W, T,
    C for wavelength, time, and code, respectively.
  • Sometimes, any one of these techniques may be
    employed for multiuser communication in a
    multiple access environment, e.g., for broadcast
    communication in a local-area network (LAN)
  • Thus, a local optical network that employs
    wavelength-division multiplexing is referred to
    as a wavelength-division multiple access (WDMA)
    network and TDMA and CDMA networks are defined

Basic Concept
  • WDM is the ability to combine
  • Multiple sources of data using
  • Multiple wavelengths (colors) of light on
  • One strand of fiber cable

Its Analog Transmission
Fiber Types ...
1.10 WDM
  • Wavelength-division multiplexing (WDM) is an
    approach that can exploit the huge
    opto-electronic bandwidth mismatch by requiring
    that each end-user's equipment operate only at
    electronic rate, but multiple WDM channels from
    different end-users may be multiplexed on the
    same fiber.
  • Under WDM, the optical transmission spectrum is
    carved up into a number of non-overlapping
    wavelength (or frequency) bands, with each
    wavelength supporting a single communication
    channel operating at whatever rate one desires,
    e.g., peak electronic speed.
  • WDM devices are easier to implement since,
    generally, all components in a WDM device need to
    operate only at electronic speed as a result,
    several WDM devices are available in the
    marketplace today, and more are emerging.

(No Transcript)
ITU recommended Bands
  • E 1360-1460 nm
  • S 1440-1530 nm
  • C 1530-1565 nm
  • L 1565-1625 nm
  • U 1625-1675 nm

Light Spectrum
1.10.1 ITU Wavelength Grid
  • There is a strong need for the standardization of
    WDM systems so that WDM components and equipments
    from difference vendors can inter-operate with
    one another.
  • Thus, industry standards for wavelengths have
    been developed under the leadership of the
    International Telecommunications Union (ITU)
  • A standard set of wavelengths, called the ITU
    grid, has been defined to coincide with the
    1550-nm low-loss region of the fiber.
  • Specifically, this grid is anchored at a
    frequency of 193.1 THz (which corresponds to a
    wavelength of 1552.52 nm).
  • There is a 100-GHz grid, which means that spacing
    between adjacent channels is 100 GHz, which
    corresponds approximately to 0.8-nm wavelength
    channel spacing around the anchor frequency.

WDM-routed networks
  • Optical signal and wavelength

TDM - Time Division Multiplexing
WDM - Wavelength Division Multiplexing
ITU wavelength grid
  • For denser packing of channels, a 50-GHz grid has
    also been defined around the same reference
    frequency of 193.1 THz ITU02b.
  • The 50-GHz grid is obtained by adding a channel
    exactly half way between two adjacent channels of
    the 100-GHz grid. Continuing this process, a
    25-GHz grid can also be defined, and it can
    support 600 wavelengths ITUOZb.

  • Thus, by allowing multiple WDM channels to
    coexist on a single fiber, one can tap into the
    huge fiber bandwidth, with the corresponding
    challenges being the design and development of
    appropriate network architectures, protocols,
    and algorithms.
  • WDM devices are easier to implement since,
    generally, all components in a WDM device need to
    operate only at electronic speed as a result,
    several WDM devices are available in the
    marketplace today, and more are emerging.

Development of WDM
  • Since 1990
  • Several Conference
  • ICC IEEE International Conference on
  • OFC Optical Fiber Communications
  • Country
  • U.S., Japan, Europe
  • WDM backbone, global coverage.

1.10.2 A sample WDM Networking Problem
  • End-users in a fiber-based WDM backbone network
    may communicate with one another via all-optical
    (WDM) channels, which are referred to as
  • A lightpath may span multiple fiber links, e.g.,
    to provide a "circuit-switched" interconnection
    between two nodes which may have a heavy traffic
    flow between them and which may be located "far"
    from each other in the physical fiber network
  • Each intermediate node in the lightpath
    essentially provides an all-optical bypass
    facility to support the lightpath.

WDM network
  • Complete graph, N nodes, N(N-1)links.
  • The number of links is increased with the number
    of nodes.
  • Technological constraints dictate that the number
    of WDM channels that can be supported in a fiber
    be limited to W.
  • RWA Problem (routing and wavelength assignment)
  • given a set of lightpaths that need to be
    established on the network, and given a
    constraint on the number of wavelengths,
    determine the routes over which these lightpaths
    should be set up and also determine the
    wavelengths that should be assigned to these
    lightpaths so that the maximum number of
    lightpaths may be established. .
  • Lightpaths that cannot be set up due to
    constraints on routes and wavelengths are said to
    be blocked, so the corresponding network
    optimization problem is to minimize this blocking

wavelength-continuity constraint
  • In this regard, note that, normally, a lightpath
    operates on the same wavelength across all fiber
    links that it traverses, in which case the
    lightpath is said to satisfy the
    wavelength-continuity constraint.
  • Thus, two lightpaths that share a common fiber
    link should not be assigned the same wavelength.

wavelength converter facility
  • However, if a switching/routing node is also
    equipped with a wavelength converter facility,
    then the wavelength-continuity constraints
    disappear, and a lightpath may switch between
    different wavelengths on its route from its
    origin to its termination.
  • RWA problem Routing and Wavelength Assignment
    (RWA) problem

1.11 WDM Networking Evolution
  • Point-to-Point WDM Systems
  • When the demand exceeds the capacity in existing
    fibers, WDM is turning out to be a more
    cost-effective alternative compared to laying
    more fibers.
  • installation/burial of additional fibers and
    terminating equipment (the "multifiber"
  • a four-channel "WDM solution" where a WDM
    multiplexer (mux) combines four independent data
    streams, each on a unique wavelength, and sends
    them on a fiber and a demultiplexer (demux) at
    the fiber's receiving end separates out these
    data streams and
  • OC-192, a "higher-electronic-speed" solution.

Four channels of point-to-point WDM
  • The analysis in MePD95 shows that, for
    distances lower than 50 km for the transmission
    link, the "multi-fiber" solution is the least
    expensive but for distances longer than 50 km,
    the "WDM" solution's cost is the least with the
    cost of the "higher-electronic-speed" solution
    not that far behind.
  • WDM mux/demux in point-to-point links is now
    available in product form from several vendors
    such as IBM, Pirelli, and ATT Gree96. Among
    these products, the maximum number of channels is
    160 today, but this number is expected to
    increase soon.

1.11.2 Wavelength Add/Drop Multiplexer (WADM) or
OpticalAdd/Drop Multiplexer (OADM
  • Architecture
  • A set of 2x2 switches (one switch per wavelength)
  • MUX
  • States
  • Bar state If all of the 2 x 2 switches are in
    the "bar" state, then all of the wavelengths flow
    through the WADM "undisturbed."
  • Cross state electronic control (not shown in
    Fig. 1.3), then the signal on the corresponding
    wavelength is "dropped" locally, and a new data
    stream can be "added" on to the same wavelength
    at this WADM location.
  • More than one wavelength can be "dropped and
    added" if the WADM interface has the necessary
    hardware and processing capability.

1.11.3 Fiber interconnection Device
  • passive star (see Fig. 1.11),
  • passive router (see Fig. 1.12), and
  • active switch (see Fig. 1.13).

passive star (see Fig. 1.11),
  • The passive star is a "broadcast" device, so a
    signal that is inserted on a given wavelength
    from an input fiber port will have its power
    equally divided among (and appear on the same
    wavelength on) all output ports.
  • "collision" will occur when two or more signals
    from the input fibers are simultaneously launched
    into the star on the same wavelength.
  • Assuming as many wavelengths as there are fiber
    ports, an N x N passive star can route N
    simultaneous connections through itself.

Passive Star
passive router (see Fig. 1.12),
  • A passive router can separately route each of
    several wavelengths incident on an input fiber to
    the same wavelength on separate output fibers
  • this device allows wavelength reuse, i.e., the
    same wavelength may be spatially reused to carry
    multiple connections through the router.
  • The routing matrix is "fixed" and cannot be
    changed. Such routers are commercially available,
    and are also known as Latin routers, waveguide
    grating routers (WGRs), wavelength routers (WRs),
  • Again, assuming as many wavelengths as there are
    fiber ports, a N x N passive router can route N2
    simultaneous connections through itself (compared
    to only N for the passive star) however, it
    lacks the broadcast capability of the star.

Passive Router
active switch (see Fig. 1.13).
  • The active switch also allows wavelength reuse,
    and it can support N2 simultaneous connections
    through itself (like the passive router).
  • But the active star has a further enhancement
    over the passive router in that its "routing
    matrix" can be reconfigured on demand, under
    electronic control.
  • However the "active switch" needs to be powered
    and is not as fault-tolerant as the passive star
    and the passive router which don't need to be
  • The active switch is also referred to as a
    wavelength-routing switch (WRS), wavelength
    selective crossconnect (WSXC), or just
    crossconnect (XC) for short. (We will refer to
    it as a WRS in this book.)

Active Switch
Wavelength Convertible Switch
  • The active switch can be enhanced with an
    additional capability, viz., a wavelength may be
    converted to another wavelength just before it
    enters the mux stage before the output fiber (see
    Fig. 1.6).
  • A switch equipped with such a wavelength-conversi
    on facility is more capable than a WRS, and it is
    referred to as a wavelength-convertible switch,
    wavelength interchanging crossconnect (WIXC), etc

1.11.4 Development of WDM networks
  • The first generation of WDM point-to-point
    physical links include design and development of
    WDM lasers and optical amplifiers (OAMP) Liu02.
  • The second generation of WDM is capable of
    establishing connection-oriented end-to-end
    lightpaths in the optical layer by introducing
    optical add/drop elements (WADM or OADM) and
    optical cross-connects (OXC).
  • The ring and mesh topologies can be implemented
    using these OADMs and OXCs.
  • The lightpaths are operated and managed based on
    a virtual topology over the physical fiber
    topology, and the virtual topology can be
    reconfigured dynamically in response to traffic
  • The technical issues of second-generation WDM
    include the development of OADM and OXC,
    wavelength conversion, routing and wavelength
    assignment (RWA), interoperability among WDM
    networks, network control and management.

3rd generation
  • The third generation of WDM is expected to
    support a connectionless optical network. The key
    issues include the development of optical access
    network (such as passive optical network (PON)),
    and optical switching technologies, generically
    referred to as Optical "X" Switching (OXS), where
    X P (for packet), B (for burst), L (for label),
    F (for flow), C (for cluster or circuit), etc.
    Sorne of these techniques, namely, OPS and OBS
    will be discussed in Chapters 17 and 18.

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1.12 WDM Network Construction
  • 1.12.1 Broadcast-and-Select (Local) Optical WDM
  • A local WDM optical network may be constructed by
    connecting network nodes via two-way fibers to a
    passive star,
  • The information streams from multiple sources are
    optically combined by the star and the signal
    power of each stream is equally split and
    forwarded to all of the nodes on their receive
    fibers. A node's receiver, using an optical
    filter, is tuned to only one of the wavelengths
    hence it can receive the information stream.
  • the passive-star can support multicast

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Passive-Star-Based Optical WDM LAN vs.
Centralized, nonblocking-Switch-Based LAN
  • Passive Star WDM has following advantages
  • In the space-division-switch solution, the
    "switching intelligence" is centralized.
    However, the passive star relegates the switching
    functions to the end nodes If a node is down,
    the rest of the network can still function.
    Hence, the passive-star solution enjoys the
    fault-tolerance ad-vantage of any distributed
    switching solution, relative to the
    centralized-switch architecture, where the entire
    network goes down if the switch is down.

Passive Star WDM has following advantages
  • it allows multicasting "for free." There are some
    processing requirements with respect to
    appropriately coordinating the nodal transmitters
    and receivers. Centralized coordination for
    supporting multicasting in a switch (also
    referred to as a "copy" facility) is expected to
    require more processing.
  • can be potentially much cheaper since it is
    purely glass with very little electronics.

1.12.2 Wavelength-Routed (Wide-Area) Optical
  • The network consists of a photonic switching
    fabric, comprising "active switches" connected by
    fiber links to form an arbitrary physical
  • Each end-user is connected to an active switch
    via a fiber link. The combination of an end-user
    and its corresponding switch is referred to as a
    network node.
  • Each node (at its access station) is equipped
    with a set of transmitters and receivers, both of
    which may be wavelength tunable. A transmitter at
    a node sends data into the network and a receiver
    receives data from the network.

(No Transcript)
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