optical packet switching nw seminar report

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

Dept. of ECE MESCE Kuttippuram1


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.



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



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.



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



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



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



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)



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.



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.



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



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



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



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.



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.



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.



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



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.



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.



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



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



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



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.



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



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
