optical packet switching nw seminar report
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Optical Packet Switching N/W Seminar Report ‘03
INTRODUCTION
With in today's Internet data is transported using wavelength
division multiplexed (WDM) optical fiber transmission system that carry
32-80 wavelengths modulated at 2.5gb/s and 10gb/s per wavelength.
Today’s largest routers and electronic switching systems need to handle
close to 1tb/s to redirect incoming data from deployed WDM links. Mean
while next generation commercial systems will be capable of single fiber
transmission supporting hundreds of wavelength at 10Gb/s and world
experiments have demonstrated 10Tb/shutdown transmission.
The ability to direct packets through the network when single fiber
transmission capacities approach this magnitude may require electronics to
run at rates that outstrip Moor’s law. The bandwidth mismatch between
fiber transmission systems and electronics router will becomes more
complex when we consider that future routers and switches will potentially
terminate hundreds of wavelength, and increase in bit rate per wavelength
will head out of beyond 40gb/s to 160gb/s. even with significance
advances in electronic processor speed, electronics memory access time
only improve at the rate of approximately 5% per year, an important data
point since memory plays a key role in how packets are buffered and
directed through a router. Additionally opto-electronic interfaces dominate
the power dissipations, footprint and cost of these systems, and do not
scale well as the port count and bit rate increase. Hence it is not difficult to
see that the process of moving a massive number of packets through the
multiple layers of electronics in a router can lead to congestion and exceed
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the performance of electronics and the ability to efficiently handle the
dissipated power.
In this article we review the state of art in optical packet switching
and more specifically the role optical signal processing plays in
performing key functions. It describe how all-optical wavelength
converters can be implemented as optical signal processors for packet
switching, in terms of their processing functions, wavelength agile
steering capabilities, and signal regeneration capabilities. Examples of
how wavelength converters based processors can be used to implement
asynchronous packet switching functions are reviewed. Two classes of
wavelength converters will be touched on: monolithically integrated
semiconductor optical amplifiers (SOA) based and nonlinear fiber based.
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PACKET SWITCHING IN TODAY'S
OPTICAL NETWORKS
Routing and transmission are the basic functions required to move
packets through a networks In today’s internet protocol (IP) networks, the
packet routing and transmission problems are designed to handle
separately. A core packet network will typically interface to smaller
networks and/or other high capacity networks.
A router moves randomly arriving packets through a network by
directing them from its multiple inputs to outputs and transmitting them
on a link to next router. The router uses information carried with arriving
packets (e.g. IP headers, packet type and priority) to forward them from its
input to output ports as efficiently as possible with minimal packet loss
and disruption to packet flow. This processing of merging multiple
random input packet streams onto common output is called statistical
multiplexing. In smaller networks, the links between routers can be made
directly using Ethernet; however in high capacity metropolitan enterprise
transmission systems between routers employ synchronous framing
techniques like synchronous optical network (SONET), packet over
SONET (pos), or Gigabit Ethernet (Gig E). This added layer of framing is
designed to simplify transmission between routers and decouple it from
packet routing and forwarding process. Figure 1 illustrate that the
transport that connects routers can designed to handle the packets
asynchronously or synchronously. The most commonly used approaches
(SONET, pos, Gig.E) maintain the random nature of packet flow by only
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loosely aligning them with in synchronous transmission frames. Although
not widely used in today’s networks, packets may also be transmitted
using a fixed time slotted approach, similar to older token ring and fiber
distributed data interface (FDDI) networks, where they are placed with in
an assigned time slot is frame, as illustrated in lower portion of Fig. 1
Fig.1. The function of a router is to take randomly arriving packets
on its input and statistically multiplex onto its outputs. Packets may be
transmitted between routers using a variety of asynchronous or
synchronous network access and transmission techniques.
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ALL OPTICAL PACKET SWITCHING
NETWORKS
In all optical packet switching network the data is maintained in
optical format throughout the routing and transmission processes. One
approach that has been widely used is all-optical label swapping (AOLS).
AOLS is intended to solve potential mismatch between dense WDM
(DWDM) fiber capacity and router packet forwarding capacity, especially
as packet data rate increase beyond that easily handled by electronics
(>40gb/s). Packets can be routed independent of the payload bit rate,
coding format or length. In this approach a lower bit rate label is attached
to front end of the packet. The packet bit rate is then independent of the
label bit rate, coding format or length. AOLS is not limited to handle only
IP packets but can also handle asynchronous transfers mode ATM) cells,
optical bursts, data file transfer, and other data structures without SONET
framing. Migrating from POS to packet routed networks can improve
efficiency but reduce latency.
In this approach a lower bit rate label is attached to the front end of
the packet. The packet bit rate is then independent of the label bit rate and
the label can be detected and processed using lower-cost electronics in
order to make routing decisions. However, actual removal and
replacement of label with respect to packet is done with optics. While the
packet contains original electronic IP network data and routing
information specifically used in the optical routing layer. The label may
also contain bits for error checking and correction as well as source and
destination information and framing and timing information
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Fig. 2. An all optical label swapping network for transparent all-
optical packet switching
An example AOLS network is illustrated in FIG.2. IP packets enter
the network through an ingress node where they are encapsulated with an
optical label and retransmitted on a new wavelength. once inside the
AOLS network, only the label is used to make the routing decisions, and
the packet wavelength is used to dynamically redirect them to next node.
At internal core nodes label is optically erased, the packet is optically
regenerated, a new label is attached, and the packet is converted to a new
wavelength. Packets and their labels may also be replicated at an optical
router realizing the important multicast function. Throughout the networks
the contents that first enter the core network (eg. The IP packet header and
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the payload) are not passed through electronics and are kept intact until
the packet exist the core optical network. These functions-label
replacement, packet regeneration, and wavelength conversion –are
handled in optical domain using optical signal processing techniques and
may be implemented using optical wavelength conversion technology.
The overall function of an optical labeled packet switch is shown in
FIG.3a.The switch can be separated into two planes, data and control. The
data plane is the physical medium over which packets are switched. This
part of the switch is bit-rate-transparent and able to handle packet with any
format out to very high bit rate. The control plane has two levels of
functionality. The decisions and control level executes the packet handling
process including switch control, packet buffering, and scheduling. This
control section operates not at packet bit rate but instead at the slower
label bit rate and does not need to be bit rate transparent. The other level
of control plane supplies routing information to the decision level. This
information is more slowly varying and may be updated throughout the
network on a less dynamic basis than the packet control.
The optical label swapping technique is shown more detail in
FIG.3b.Optically labeled packets at the input have a majority of the input
optical power directed to upper photonic packet processing plane and a
small portion of the optical packets directed to the lower electronics label
processing plane. The photonic plane handles optical data regeneration,
optical label removal, optical label rewriting, and the packet rate
wavelength switching. The lower electronic plane recovers into an
electronic memory and uses lookup tables and other digital logic to
determine the new optical label and wavelength in the upper photonic
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plane. A static fiber delay line is used at photonic plane input to match the
processing delay differences between the two planes. In future, certain
portions of the label swapping functions may be handled using optical
techniques
An alternative approach to the random access technique described above
is to use time division multiple access (TDMA) technique where packet
bits are synchronously located within time slot dedicated to that packet.
For example, randomly arriving each on different wavelength, are bit rate
interleaved using all-optical orthogonal time division multiplexer
(OTDM). For example if a 4:1 OTDM is used, every fourth bit at the
output belongs to first incoming packet and so on. A TDM frame is
defined as the duration of the one of all of the time slots. Once packets
have been assembled into frames at the network edge, packets can be
removed from or added to a frame using optical add/drop multiplexers
(OADM)
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OPTICAL SIGNAL PROCESSING AND OPTICAL
WAVELENGTH CONVERSION
Packet routing and forwarding functions are performed today using
digital electronics, while transport between routers is supported using
high-capacity DWDM transmission and optical circuit-switched systems.
Optical signal (OSP) is currently used to support transport functions
optical dispersion compensation and optical wavelength multiplexing and
demultiplexing.
Today’s routers relay on dynamic buffering and scheduling to
efficiently move IP packets. However, optical dynamic buffering
techniques do not currently exist. To realize optical packet switching new
techniques must be developed for scheduling and routing. The optical
wavelength domain can be used to forward packets on different
wavelength with the potential to reduce the need for optical buffering and
decreased collision probability.
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FUNCTIONS OF AN OPTICAL ROUTER
The major functions of a router are:
Demux and mux: Separates and combines wavelength
Optical splitter: Sends copies of packet to control element and/or
label eraser
Label eraser: Removes the label header.
Label writer: Places a new label on the packet.
Wavelength converter: Places packets onto one of the several
wavelength.
Buffer: Holds until mux is ready to process it
Control element: Controls the operations of the label writer, the
wavelength converter, and the buffer.
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ASYNCHRONOUS OPTICAL PACKET
SWITCHING AND LABEL SWAPPING
IMPLEMENTATIONS
Figure 4
The AOLS functions described in Fig.3 can be implemented using
monolithically integrated indium phosphide (InP) SOA wavelength
converter technology (SOA_IWC) technology. An example that employs
a two-stage wavelength converter is shown in Fig.4 and is designed to
operate with NRZ coded packets and labels. In general this type of
converter works for 10Gb/s and can be extended to 40Gb/s and possibly
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beyond. In Fig. Functions are indicated the top layer and photonic and
electronic plane implementations are shown in middle and lower layers. A
burst- mode photo receiver is used to recover the digital information
residing in the label. A gating signal is then generated by post receiver
electronics, in order to shut down the output of first stage, an InP SOA
cross-gain modulation (XGM) wavelength converter. This effectively
blanks the input label. The SOA converter turns on after the label passes
and input NRZ packet is converted to an out-of-band internal wavelength.
The lower electronic control circuitry is synchronized with well timed the
well-timed optical time-of-flight delays in the photonic plane. The first
stage WC is used to optically preprocess input packet by:
Converting input packets at any wavelength to a shorter wavelength,
which is chosen to optimize the SOA XGM extinction ratio.
Converting random input packet polarization state to a fixed state set
by a local InP distributed feedback (DFB) for all optical filter
operation
Setting the optical power bias point for the second wavelength
converter
The recovered label is also sent to a fast lookup table that generates
the new label and outgoing wavelength based on prestored routing
information. The new wavelength is translated to currents that set a
rapidly tunable laser to the new output wavelength. The wavelength is pre
modulated with the new label using an InP electro-absorption modulator
(EAM) and input to an InP interferometric SOA-WC (SOA-IWC). The
SOA-IWC is set in its maximum transmission mode to allow the new label
to pass through. A short time after the label is transmitted (determined by
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guard band), the WC is biased for inverting operation, and the packet
enters the SOA-IWC from the first stage and drives one arm of the WC,
imprinting the information onto the new wavelength. The second stage
wavelength converter:
Enables the new label at new wavelength to be passed to outputs
using a fixed optical band reject filter
Reverts the bit polarity to its original state
Is optimized for wavelength up conversion
Enhances the extinction ratio due to its nonlinear transfer function
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The label swapping functions may also implemented at higher 40
and 80Gb/s using RZ coded packets and NRZ coded labels. This approach
has been demonstrated using the configuration in Fig.5. The silicon-based
label processing electronics layer is basically the same as in Fig. 4. In this
implementation nonlinear fiber cross phase modulation (XPM) is used to
erase the label, convert the label and regenerate the signal. An optically
amplified input RZ packet efficiently modulate sidebands through fiber
cross phase modulation onto a new continuous wave (CW) wavelength
converter, while the NRZ –label XPM induced sideband modulation very
in efficient and the label is erased or suppressed. The RZ modulated
sideband is recovered using a two-stage filter that passes a single side
band. The converted packet with erased label is passed to the converter
output where it is reassembled with a new label. The fiber XPM converter
also various signal conditioning and digital regeneration functions also
including extinction ratio enhancement of RZ signals and polarization
mode dispersion compensation.
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ALL-OPTICAL WAVELENGTH CONVERSION
USING SOA
Research into wavelength conversion using SOAs is well developed
in many of the major optoelectronics research laboratories worldwide.
Three main methods of wavelength conversion have been explored: cross-
gain modulation (XGM), four-wave mixing (FWM), and cross-phase
modulation (XPM). These will be discussed in the following sections of
this article, and results from work on all-optical wavelength conversion
performed at BT Laboratories will be shown to illustrate the current status.
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Cross-Gain Modulation
The rate of stimulated emission in an SOA is dependent on the
optical input power. At high optical injection, the carrier concentration in
the active region is depleted through stimulated emission to such an extent
that the gain of the SOA is reduced. This effect is known as gain
saturation and typically occurs for input powers of the order of 100 µW or
more.
Gain saturation can be used to convert data from one wavelength to
another. Two optical signals enter a single SOA with one carrying
amplitude modulated data and the other being of constant power (CW). If
the peak optical power in the modulated signal is near the saturation
power of the SOA, the gain will be modulated in synchronism with the
power excursions. When the data signal is at a high level (a binary 1), the
gain is depleted, and vice versa. This gain modulation is imposed on the
unmodulated input beam. Thus, an inverted replica of the input data is
created at the target wavelength.
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Until recently, the speed of wavelength conversion using SOA gain
saturation was thought to be limited by the intrinsic carrier lifetime of
around 0.5 ns. However, recent work has shown that the speed of such
devices is greater than the limit of a few gigabits per second this lifetime
would imply. This is because the effective carrier lifetime, which can be
decreased by the use of high optical injection, and longitudinal
propagation effects, which can shape pulses as they traverse the SOA,
must be considered. Under high optical injection the rate of stimulated
emission in the SOA increases, and this can reduce the effective lifetime
to as low as 10ps
Interferometric Devices
It was noted in the discussion of XGM wavelength converters that
accompanying the gain modulation with carrier density changes is a
modulation of the refractive index of the SOA. This cross-phase
modulation (XPM) can be utilized to good effect in interferometric
arrangements to obtain wavelength conversion devices with significant
advantages over those relying on XGM alone. In such devices the light to
be switched is split into two paths containing SOAs, and a relative phase
shift is induced by the optical switching signal entering one of the SOAs,
which saturates the gain. When the light is recombined, constructive or
destructive interference will occur depending on the phase difference
between the two paths. The unperturbed state of the interferometer can be
set up for constructive or destructive interference so that injection of a
switching signal causes either a decrease or increase, respectively, in the
wavelength-converted signal. The state of the interferometer is typically
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set by adjusting the injection current in the two SOAs or by a separate
phase tuning element in a passive waveguide. Thus, the first advantage to
note for interferometric wavelength converters over XGM is the choice
between inverting or noninverting operation.
Structures analogous to Michelson and Mach-Zehnder
interferometers can be made by hybrid integration of SOAs and couplers
or by monolithic integration. Hybrid devices consisting of discrete SOAs
and fiber couplers were used in early experiments; however, the need to
maintain relative path lengths to within a fraction of a wavelength makes
these devices vulnerable to environmental changes such as temperature or
vibration. Monolithic integration offers considerable stability advantages
in addition to compactness.
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SYNCHRONOUS OTDM
Synchronous switching system have been used extensively for
packet routing: however their implementations using ultra fast optical
signal processing techniques technique is fairly new. In the remainder of
the article we summarize optical time domain functions for synchronous
packet networks. These include the ability to:
Multiplex several low bit-rate DWDM channels
into a single high bit rate OTDM channels
Demultiplex a single high bit-rate OTDM
channels into several low bit-rate DWDM channel
Add and/or drop a time slot from an OTDM
channel
Wavelength route OTDM signals.
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ADVANTAGES
Does not require O-E-O conversion
Low cost
High bit rate
Delay is the order of nanoseconds
Semiconductor based all-optical wavelength converters are compact
They are readily lend them selves to integration and mass
production
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SUMMARY
In this article we review optical signal processing and wavelength
converter technologies that can bring transparency to optical packet
switching with bit rate extending beyond that currently available with
electronic router technologies. The application of optical signal processing
technique to all optical label swapping and synchronous network functions
is presented. Optical wavelength converter technologies show promise to
implement packet-processing functions. Non-linear fiber wavelength
converters and indium phosphide optical wavelength converters are
described
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REFERENCE
IEEE communication (Feb.-2003,march-2000)
IEEE lightwave tech. (dec.1998, june-1996)
IEEE photonic tech. (Dec-2000)
Scientific American (Jan.-2001)
Optical Networks by Rajiv Ramaswami
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ABSTRACT
Optical packet switching promises to bring the flexibility and
efficiency of Internet to transparent optical networking with bit rate
extending beyond that currently available with electronic router
technologies. New optical signal processing have been demonstrated that
enable routing at bit rates from 10gb/s to beyond 40gb/s.in this article we
review these signal processing techniques and how all optical wavelength
converters technology can be used to implement packet switching
functions. Specific approaches that utilize ultra fast all-optical nonlinear
fiber wavelength converters and monolithically integrated wavelength
converters are discussed.
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CONTENTS
INTRODUCTION
PACKET SWITCHING IN TODAY’S OPTICAL
NETWORKS
ALL OPTICAL PACKET SWITCHING
FUNCTIONS OF AN OPTICAL ROUTER
OPTICAL SIGNAL PROCESSING AND OPTICAL
WAVELENGTH CONVERSION
ASYNCHRONOUS OPTICAL PACKET SWITCHING AND
LABEL SWAPPING IMPLEMENTATIONS
ALL OPTICAL WC USING SOA
SYNCHRONOUS OTDM
ADVANTAGES
SUMMARY
REFERENCES
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ACKNOWLEDGEMENT
I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of the
Department, Electronics and Communication Engineering, for providing me
his invaluable guidance for the Seminar.
I express my sincere gratitude to my Seminar Coordinator and Staff in
Charge Mr. Manoj K, for his cooperation and guidance in the preparation
and presentation of this seminar.
I also extend my sincere thanks to all the faculty members of
Electronics and Communication Department for their support and
encouragement.
Shajil P
Dept. of ECE MESCE Kuttippuram25
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