Optical packet switching: A reality check

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Abstract

This paper presents an analysis of the energy consumption in a number of optical switch fabric architectures for optical packet-switched applications and compares them to electronic switch fabrics. Optical packet switching does not appear to offer any substantial power consumption advantages over electronic packet switching. Therefore, there is no compelling case for optical packet switching.

Introduction

In 1993, the present author published a paper [1] that included the following opening statement: “Self-routing photonic packet switches will be important components for future all-optical networks”. In the intervening years, other authors also introduced papers with similar assertions about the future need for photonic (i.e. optical) packet switching. Proponents of optical packet switching often draw attention to the limited bandwidth of electronic devices and the supposed high bandwidth and low power consumption of optical alternatives. In addition, it is often argued that optical packet switches do not need optical to electronic (O/E) and electronic to optical (E/O) conversions in the signal path. However, just as the present author did not justify the opening statement quoted above from [1], assertions about the benefits of optical packet switching are often not backed up by an analysis of the relative merit of competing electronic and optical technologies.

The purpose of the present paper is to undertake a reality check on the viability of optical packet switching. The focus of this paper is on the energy consumption of competing optical and electronic technologies. As the capacity of packet switches grows, energy consumption will become an increasingly important engineering consideration that will drive the choices between competing technologies. A key parameter in any comparison of optical and electronic switch technologies is the energy required to pass each bit of data through the switch. The energy per bit and, to a lesser extent, the size of each device is the most fundamentally important parameter for comparing technologies in future packet switches. Whichever technology dominates in the future (optics or electronics), it will be required to consume significantly less energy than the electronic devices in current-day packet switches, and it should enable its integration into very small packages. If optical technologies cannot reduce the power consumption of packet switching, they are unlikely to find application in future packet switches [2].

In general, the two key functions performed on packets as they pass through a packet switch are (a) buffering, and (b) switching. A recent paper by the present author [2] showed that buffering packets in optical delay lines presents major challenges, even when the buffer is very small. Delay lines consume more energy and occupy a larger footprint than electronic buffer technologies. The present paper focuses on switching technologies used to route packets from the input to the output of the switch.

The analysis presented here applies to a broad class of packet-based switching architectures, including the so-called photonic packet switches, optical packet switches, and optical label switches. The power consumption of a number of optical switch fabrics for optical packet switching is investigated and this is compared to estimates of power consumption in electronic switch fabrics based on CMOS technology. A key conclusion of this paper is that optical packet switching does not appear to offer any substantial advantages over electronic packet switching and there is no compelling case for optical packet switching. This paper is based on material presented at the Workshop on Optical packet switching at PS’2006.

The comparisons in this paper are based on aggressive but plausible projections of current optical technology, and projections of CMOS electronics based on the ITRS Semiconductor Roadmap [3]. The analysis presented here uses a bit rate of 40 Gb/s, but the conclusions will be similar when scaled to higher bit rates such as 100 Gb/s.

Section snippets

Packet switch architectures

Fig. 1(a) and (b) show the key components of the data plane of an electronic packet switch and an optical packet switch, respectively. The input/output (I/O) ports on the switches are connected to the incoming and outgoing fibers via WDM demultiplexers and multiplexers, respectively. There are F incoming fibers and F outgoing fibers in Fig. 1. Each fiber carries K wavelengths.

Buffers in a packet switch can be placed at the input ports, at the output ports, or shared between the inputs and

Power consumption in optical switch fabrics

In this section, we develop an energy dissipation model of an optical switch fabric and use this model to identify power dissipation “hot spots” in the switch fabric.

Fig. 5 is a schematic of the internal components of a wavelength-interchanging optical cross connect switch fabric for packet switching. Also shown in Fig. 5 are one of the input demultiplexers, one of the output multiplexers, and three of the output buffers. The shaded rectangle in Fig. 4 represents the FK × FK

Comparison of switch fabric technologies

This section compares the power dissipation of the switch technologies discussed in the previous sections. The results presented here are based on an analysis presented in [2].

Table 1 shows the total power consumption and the energy per bit, for a 400 Tb/s switch fabric operating at 40 Gb/s. Data are presented for AWG-based switch fabrics, for an SOA gate array, and for a CMOS switch fabric. The AWG-based switch fabrics are subdivided into fabrics using optical (O/O) wavelength converters, and

Conclusions

This paper has carried out a reality check on the potential for optical packet switching to replace today’s established technology that is heavily based on electronics. The most important consideration in this kind of comparison is energy consumption and the associated heat dissipation problems. The data presented here indicates that AWG-based switch fabrics using wavelength converters potentially offer advantages over electronic switch fabrics. However, it is possible that optoelectronic

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