Optical packet switching Network - Engineering Seminar Report

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.

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 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.

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.

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:

Ø    Demux and mux: Separates and combines wavelength

Ø    Optical splitter: Sends copies of packet to control element and/or        label eraser

Ø    Label eraser: Removes the label header.

Ø    Label writer: Places a new label on the packet.

Ø    Wavelength converter: Places packets onto one of the several     wavelength.

Ø    Buffer: Holds until mux is ready to process it

Ø    Control element: Controls the operations of the label writer, the          wavelength converter, and the buffer.

ASYNCHRONOUS OPTICAL PACKET SWITCHING AND LABEL SWAPPING IMPLEMENTATIONS

         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:

Ø   Converting input packets at any wavelength to a shorter wavelength, which is chosen to optimize the SOA XGM extinction ratio.

Ø   Converting random input packet polarization state to a fixed state set by  a local InP distributed feedback (DFB) for all optical filter operation

Ø   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:

Ø   Enables the new label at new wavelength to be passed to outputs   using a fixed optical band reject filter

Ø   Reverts the bit polarity to its original state

Ø   Is optimized for wavelength up conversion

Ø   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:

Ø   Multiplex several low bit-rate DWDM channels into a single high bit rate OTDM channels

Ø   Demultiplex a single high bit-rate OTDM channels into several low bit-rate DWDM channel

Ø   Add and/or drop a time slot from an OTDM channel

Ø   Wavelength route OTDM signals.

ADVANTAGES

Ø   Does not require O-E-O conversion

Ø   Low cost

Ø   High bit rate

Ø   Delay is the order of nanoseconds

Ø   Semiconductor based all-optical wavelength converters are compact

Ø   They are readily lend them selves to integration and mass      production

CONCLUSION

         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.