Crosstalk Analysis OPTICAL INTERCONNECTION NETWORK FOR

OPTICAL PACKET SWITCHES 328 short-length fibers in the OIN. Finally, all the light reflections in the system are ignored and all the passive components are assumed to be polarization- insensitive. Ž . Figure 11.41 a shows that, without power compensation, the received power is only y30.5 dBm, much less than required sensitivity of about y20 Ž . y 12 dBm at the bit error rate BER of 10 for signals at 10 Gbitrs. To increase the received optical power, optical amplifiers are necessary to provide sufficient gain to compensate for the power loss in the OIN. In the proposed OIN, there are two kinds of optical amplifiers to perform the power compensation function. One is the SOA and the other is the EDFA. Ž . As shown in Figure 11,41 b , an EDFA with 10.5-dB gain at each IOM is used to amplify 16 wavelengths simultaneously, so that the sensitivity at the receiver is increased to y20 dBm. Note that the gain provided by the EDFA needs to be increased to 20 dB if the AWG, which is near the output link of type-II tunable filter, is replaced by a 16 = 1 combiner.

11.4.8 Crosstalk Analysis

Ž In the OIN, crosstalk is caused by the finite onroff ratio i.e., the ratio of . gain on and gain off, or G rG of an SOA gate. More specifically, ON OFF P s P = G , where P is the output power of an SOA gate in x, gate in, gate OFF x, gate the off state, and P is the input power of the SOA gate. Basically, in, gate crosstalk components are categorized into two types. One, referred to as incoherent crosstalk, can be taken into account by power addition. The second type, homowa®elength crosstalk, occurs when the signal channels and their beats fall within the receiver electrical bandwidth. The on and off states of SOAs, as switching gates, are controlled by their Ž . injection currents. An SOA in the off state without injection current absorbs incident optical power and blocks the signal channel. However, some optical power leaks from the SOA even when it is in the off state. The power leakage becomes crosstalk when it is combined with the signal at an output port of OIN. It is assumed that the OIN is fully loaded and packets arriving at all of its input ports are sent to their destined output ports. Only one output port of the OIN, as shown in Figure 11.42, is considered. Figure 11.42 shows the crosstalk path in an OOM, where up to 16 WDM Ž signals from 16 different IOMs are sent to the 16 = 16 switching fabric see . position A in the figure . Each WDM signal can carry up to 16 different wavelengths at each output port in the switching fabric. For each output of the OOM, only one of these 16 WDM signals will be selected and delivered to the tunable filter. That is, for each output of the 16 = 16 switching fabric only one SOA will be turned on and the other 15 SOA gates will be turned Ž . off see position B in the figure . Thus, there is only one dominant WDM signal, and up to 15 WDM crosstalk channels are combined with it to be sent Ž . Ž to the tunable filter at each output switch module OSM position C in the . figure . OPTICAL INTERCONNECTION NETWORK FOR TERABIT IP ROUTERS 329 Fig. 11.42 Crosstalk path of the 256 = 256 optical interconnection network. The tunable filter at each OSM selects the wavelength of the selected WDM signal, which is demultiplexed by an AWG to 16 different wavelengths, Ž . and one of them is chosen by turning on one SOA position D in the figure Ž . and turning off the other 15 positions E to S in the figure . The finally selected wavelength is sent to the OSM through another AWG. Here, it is assumed that ␭ from the first IOM is chosen for the OSM. The receiver at 1 Ž . the OSM position T in the figure will receive not only the selected Ž . wavelength ␭ , but also incoherent crosstalk and homowavelength crosstalk 1 Ž . those ␭ ’s with circles in the figure . Homowavelength crosstalk channels i come from different IOMs with the same selected wavelength, and incoher- ent crosstalk channels contain different wavelengths from the selected wave- length at the receiver. There are two sources that contribute to the incoherent crosstalk channels in the proposed OIN. One is from wavelengths other than the selected one in the same IOM, and the other is from wavelengths other than the selected one in the other 15 IOMs. Of these two sources of incoherent crosstalk, the former is dominant, and the latter is negligible because it passes through two SOA gates in the off state, one in the switching fabric and the other in the tunable filter. Figure 11.43 shows the BER as a function of the received optical power Ž . and crosstalk ratio CR, the reciprocal of the onroff ratio of an SOA gate. To meet a BER requirement of 10 y 12 , one has to increase the input power by about 2 dB to compensate for the crosstalk with CR s y20 dB. Further- OPTICAL PACKET SWITCHES 330 Ž . Fig. 11.43 Bit error rate versus received power with different crosstalk ratios CR of the SOA gate; the signal extinction ratio is 20 dB. Fig. 11.44 Bit error rate vs. received power with different extinction ratios and for CR s y20 and y14 dB. REFERENCES 331 Ž . Fig. 11.45 Bit error rate vs. received power with different extinction ratios r and with CR s y20 and y16 dB. more, to compensate for the power penalty for the CR of y16 dB, the input power needs to be increased by 1 dB. In order to reduce the gain saturation w x effect of the SOA gate due to high input power, gain-clamped SOA gates 57 Ž . can be used to provide a constant optical gain f 22 dB over a wider range of input power. w x With the same system parameters as those used in 60, 61 , Figure 11.44 w shows the BER performance using different signal extinction ratios i.e., the Ž . Ž .x ratio of the power of logic one mark to the power of logic zero space , r, with crosstalk ratios of y20 and y14 dB. As shown in Figures 11.44 and 11.45, the power penalty for these three signals with r s 20, 15, and 10 dB strongly depends on the BER. That is, the signals with lower extinction ratios have a larger space power, so that crosstalk channels can be significant. To alleviate the performance degradation due to the incoherent or homowave- length crosstalk, one should make the signal extinction ratio sufficiently high so that the beating between the signal data spaces and the crosstalk is negligible. A signal extinction ratio of 15 dB will be appropriate with a power penalty of about 0.1 dB and a CR of y20 to y16 dB. REFERENCES 1. P. E. Green, Fiber Optic Communication Networks, Prentice Hall, 1992. 2. N. V. Srinivasan, ‘‘Add-drop multiplexers and cross-connects for multiwavelength optical networking,’’ Tech. Dig., OFC ’98, San Jose, CA, pp. 57᎐58, 1998. OPTICAL PACKET SWITCHES 332 3. C. K. Chan, F. Tong, L. K. Chen, and K. W.Cheung, ‘‘Demonstration of an add-drop network node with time slot access for high-speed WDMA dual busrring packet networks,’’ Tech. Dig., OFC ’98, San Jose, CA, pp. 62᎐64, 1998. 4. G. Chang, G. 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The extension of broadband services to the wireless environment is being driven mainly by the increasing demand for mobile multimedia services coupled with the advent of high-performance portable devices such w x as laptop PCs and personal digital assistants 1, 2 . In recent years, wireless ATM has drawn much attention as a solution for QoS-based mobile multimedia services. It has been an active topic of re- w x search and development in many organizations worldwide 2, 3, 4, 5, 6 and is now under standardization within applicable bodies such as the ATM Forum w x 7 and ETSI. Wireless ATM is intended to provide seamless support of qualitatively similar multimedia services on both fixed and mobile terminals. The realiza- tion of wireless ATM raises a number of technical issues that need to be resolved, however. First, there is a need for the allocation and standardiza- tion of appropriate radio frequency bands for broadband communications. Second, new radio access and other wireless-channel-specific functions are required to operate at high speed. For example, a high-speed radio physical Ž . Ž . layer, a medium access control MAC , and a data link control DLC layer are necessary for implementing wireless ATM. Next, mobility management is required to support personal and terminal mobility. Mobility management involves two aspects: location management and handoff management. Loca- tion management must be capable of tracking mobile users for delivery of an 337 WIRELESS ATM SWITCHES 338 incoming call as they move around the network. Handoff management must be capable of dynamically reestablishing virtual circuits to new access points without disconnecting communication between a mobile terminal and its peer. A mobility-support ATM switch must guarantee in-sequence and loss- free delivery of ATM cells when it is involved in handoff. Finally, wireless ATM should provide uniformity of end-to-end QoS guarantees. However, providing such guarantees is not easy, due to limited wireless bandwidth, time-varying channel characteristics and terminal mobility. The remainder of this chapter is organized as follows. Section 12.1 outlines various reference configurations for a wireless ATM architecture and a wireless ATM protocol architecture. The wireless ATM protocol architecture is based on incorporation of wireless access and mobility-related functions into the existing ATM protocol stack. Section 12.2 reviews some recent proposals to build wireless ATM systems and related research work. Section 12.3 describes wireless-specific protocol layers. A radio physical layer, a MAC layer, and a DLC layer are summarized in that section. Section 12.4 discusses handoff and rerouting schemes. It also discusses cell routing and cell loss in a crossover switch during handoff. Section 12.5 introduces a mobility-support ATM switch architecture that can avoid cell loss and guar- antee cell sequence during handoff. Performance of the switch is also discussed in that section.

12.1 WIRELESS ATM STRUCTURE OVERVIEWS

12.1.1 System Considerations

Ž . Within the Wireless ATM Working Group WAG of the ATM Forum, various reference configurations for a wireless ATM architecture are dis- w x cussed 8, 9 . In a fixed wireless network scenario, the network components, Ž . switching elements, and end user devices terminals are fixed. The fixed wireless users are not mobile but connect over wireless media. This is the simplest case of wireless access provided to an ATM network, without any mobility support. Examples of this kind of service are fixed wireless LANs and network access or network interconnection via satellite or microwave links. In a mobile ATM network scenario, mobile end user devices communi- cate directly with the fixed network switching elements. The communication channel between the mobile end user devices and the switching elements can be wired or wireless. Mobile wired users change their points of attachment into the network over time, though connections are not maintained during times of movements. In contrast, mobile wireless users can maintain their connections while changing their points of attachment into the network. The next scenario is wireless ad hoc networks where there is no access node available. For example, such a network may consist of several wireless mobile terminals gathered together in a business conferencing environment.