Optical Transmission, Switching, and Subsystems IV, edited by Chang Soo Park, Shizhong Xie, Curtis R. Menyuk, Ken-ichi Kitayama, Proc. of SPIE Vol. 6353, 635305, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.691646 Proc. of SPIE Vol. 6353 635305-1

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Performance and Cost Model for Optical Packet Switching System with Shared Wavelength Converter Pool

Jin-Sung Im, JungYul Choi, Ji-Hwan Kim, Minho Kang, J.-K.Kevin Rhee

Optical Internet Research Center, Information and Communications University, 119 Munjiro, Yuseong-Gu, Daejeon 305-732, S. Korea, Email : [email protected]

ABSTRACT

We propose a cost effective optical packet switching system using shared wavelength converter pool. We also present its cost model and show the overall system cost for the target performance. The benefits of the proposed system is compared with the conventional approaches in terms of blocking performance and overall system cost

Keywords: optical packet switching system (OPSS), cost model, shared wavelength converter, electrical buffer

1. INTRODUCTION

In optical packet or burst switching networks, optical packet or burst loss due to contention is a fundamental limiting factor against practical applications. High implementation cost of an all-optical packet switching system is another challenging issue. To solve these problems, several contention resolution techniques have been proposed, such as deflection routing, wavelength converter, and fiber delay line (FDL) buffer. The existing schemes, however, do not sufficiently resolve the contention problem and require costly optical systems to achieve satisfactory performance [1].

Thus a novel optical switching system which can reduce high blocking probability with low cost of realization is anticipated. This paper proposes a novel optical switching system that achieves several orders of magnitude improvement in packet loss rate compared with existing schemes. By applying shared wavelength converters with electrical buffer, it can significantly reduce blocking probability and overall system cost. The following section presents the proposed switching system and cost model to show its benefits by comparing the conventional approaches [2].

2. THE PROPOSED OPTICAL PACKET SWITCHING SYSTEM

The proposed optical switching system with shared wavelength converters and electrical buffers is shown in Fig. 1.

In the figure, the number of input/output fiber is F, the number of wavelength per fiber is L, and the number of input/output channel of the pool is B. Therefore, the number of total input channels of an optical switch module is F × L + B. Optical packets from input channels are switched and forwarded to the available output channel at the destination output fiber, where switching and forwarding is based on the control packet information. In general, when two or more optical packets attempt to access one output channel, contention occurs and the optical packets are lost. In the proposed switching system, however, contended packets are forwarded to a buffer module which is the newly proposed device.

After a series of buffering processes, data packets are successively forwarded to an available output channel through the switching module of the optical packet switching system.

Fig 2 shows the blocking probability with the proposed switching system, where F is 8, W is 8, with Poisson arrival and negative exponentially distributed packet size. Legend SWC refers to the sharing ratio of wavelength conversion, i.e. the ratio of wavelength converters to the total input channels. From the figure, the proposed system outperforms the dedicated wavelength converter to output wavelength without electrical buffer (solied curve of DWC) [3].

Optical Transmission, Switching, and Subsystems IV, edited by Chang Soo Park, Shizhong Xie, Curtis R. Menyuk, Ken-ichi Kitayama, Proc. of SPIE Vol. 6353, 635305, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.691646

Proc. of SPIE Vol. 6353 635305-1

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Fig. 1 Proposed optical switching system Fig. 2 Blocking probability for a 8-wavelength system

3. COST MODEL

3.1 Estimated unit cost assumption

We assume the estimated unit cost for optical devices as shown in Table 1. The unit cost for MUX/DEMUX, optical switch fabric (OSF), and electrical switch fabric (ESF) are referenced to the cost per channel. For example, a unit cost of MUX capable of multiplexing 8 channels is 100*8 = 800US$. A MUX/DEMUX is assumed to be manufactured using arrayed waveguide grating or thin-film filter technology. Optical switch fabric is based on fast MEMS (microelectro- mechanical system) technology [4]. In order to compensate manufacturing difficulty in switch scalability, OSF is assumed be incremental cost per channel for larger port counts. An all-optical wavelength converter (WC) consists of input optical amplifier (typical a semiconductor optical amplifier – SOA), fixed-wavelength laser, cross-gain or cross- phase modulation (XGM or XPM) optical modulator, and output optical amplifier. A tunable OEO consists of a photo- detector, clock and data recovery, fixed or tunable laser, and Mach-Zehnder modulator, with very-short reach optical interfaces to the shared electrical buffers or electrical switch fabrics. An electrical buffer module consists of electrical buffer and electrical switch fabric to select available wavelength converter. An FDL is a fiber in a small spool and the fiber loss is assumed to be compensated somewhere else in the node. Under the assumption of 10Gbps data rate, 100KByte packet length, and the granularity set a half of the burst length, the unit fiber length is required to be 8 km (2*10⁸ m/s*100Kbyte/10Gbps/2) and the unit cost of FDL is 640US$, where fiber cost of 0.08US$ per meter is assumed.

Table 1. Estimated unit cost assumptions for optical devices (US$).

Component W:8 W:16 W:32 W:64 Price Spot MUX/DEMUX /ch 100 100 100 100 Current

OSF 800 900 1000 1100 Current

ESF 125 125 125 125 Mass production

Tunable OEO 4500 Current

OE,EO 1100 Current

Electrical Buffer 500 Current

WC 5600 Mass production

SOA 1500 Mass production

FDL 640 Current

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3.2 Overall system cost for target blocking probability

Let us compare overall system cost of the proposed optical packet switching system (OPSS) with dedicated wavelength converters with FDL buffer (DWCFDL) and all electrical switching system (EXC) for achieving the given target blocking performance. Figure 3 shows the structure of DWCFDL and EXC. The DWCFDL structure employs an all-optical approach which is equipped with dedicated WC at each wavelength and shared FDL buffers. Different FDL buffer depth and shared use of buffer result in different performance. The EXC structure has all electric switch and buffers as shown in Fig. 3 (b) [5][6].

(a) Dedicated wavelength converter with FDL buffer (b) Electrical cross connect Fig. 3 Alternative approaches with DWCFDL and EXC

Table 2 shows the required amount of SWCEBs and FDLs for target blocking probability 10^-6 at an offered load 0.3 and 0.5, respectively. Numbers for SWCEB and FDL implies the required input channel numbers at the shared wavelength converter pool and FDL buffers, respectively. Blocking probability is obtained from computer simulation. Basically, the required number of input channels for SWCEB is larger than that for DWCFDL. With 64 wavelengths per port, the OPSS with DWCFDL does not require any FDL buffer, below the offered load 0.5. This is thanks to the use of dedicated wavelength converters at each wavelength.

Table 2. Required amount of SWCEB and FDL for target blocking probability 10^-6, (The node degree of connectivity is 8).

OPSS DWCFDL Number of

λ channels W

Offered load

(ρ)

Number of SWCEB (sharing ratio)

Blocking Probability

(x 10^-6)

Number of FDL (sharing ratio)

Blocking Probability

(x 10^-6) 0.3 26 (40%) 0.6 16 (25%) 0.7 8

0.5 46 (72%) 1.5 40 (62%) 2 0.3 36 (28%) 0.9 6 (5%) 1.4 16

0.5 72 (56%) 1.3 18 (63%) 1.2 0.3 54 (21%) 1.4 0 (0%) 7*10-3 32

0.5 120 (47%) 1.1 10 (4%) 1.1 0.3 86 (17%) 1.9 0 (0%) 5*10-10 64

0.5 200(39%) 1.4 0(0%) 0.2

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Based on this result, we can directly compare overall system cost of OPSS with DWCFDL and EXC for the same performance as show in Fig. 4. The system cost of the OPSS with SWCEB costs lower than that of the DWCFDL. For offered load of 0.3 and 0.5, the proposed OPSS system may lower the system cost by approximately 64% and 46%, respectively, than those of EXCs. This means that our proposed optical packet switching system is a very practical and economical solution by virtue of employing opaque buffer elements.

(a) At offered load 0.3 (b) At offered load 0.5 Fig. 4 Overall system cost of OPSS with SWCEB and DWCFDL for target blocking probability 10^-6

4. CONCLUSIONS

Transparent all optical packet switching system is regarded as an ultimate solution for the next generation optical internet. However, the present optics technology still requires costly optical devices and systems to achieve satisfactory performance for practical applications. As an intermediate, but more economical solution, we proposed a wavelength converter pool which is shared at the switching system and has buffering capability using electrical memory.

Performance of the switching system was presented in terms of blocking probability. Benefit of the shared wavelength converter pool was observed through system cost analysis. Our system demonstrates a potential cost savings up to 64 %, compared with an all-electrical cross connect system. In conclusion, the proposed shared wavelength converter pool (SWCEB) system design can substantially overcome the performance and cost drawbacks of OPS and OBS technologies, which enable realization of the practical optical packet switching system.

Acknowledgment – This work was supported in part by the KOSEF-OIRC project.

REFERENCES

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S. Jensen, "Current readout of infrared detectors," Opt. Eng. 26(3), 241-248 (1987).

2. S. Verma, H. Chaskar, R. Ravikanth, Optical burst switching: a variable solution for terabit IP backbone, IEEE Network, vol. 14, issue 6, pp. 48-53, Nov./Dec. 2000.

3. T. Y. Chai, et al., Design and performance of optical cross- connect architectures with converter sharing, Optical Networks Magazine, vol. 3, no. 4, pp. 73-84, Jul./Aug., 2002.

4. P. D. Dobbelaere, K.Falta, L. Fan, S. Gloeckner, and S. Patra, Digital MEMS for Optical Switching, IEEE Communications Magazine, vol. 40, no.3, pp. 88-95, Mar. 2002.

5. C. M. Gauger, Optimized Combination of Converter Pools and FDL Buffers for Contention Resolution in Optical Burst Switching, Photonic Network Communications, vol. 8, no. 2, pp. 139-148, Sep. 2004., 2004

6. C. M. Gauger, Dimensioning of FDL Buffers for Optical Burst Switching Nodes, in Proc. Of Optical Network Design and Modeling Conference (ONDM’02), 2002.