Broadband Packet Switching Technologies

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, and one of them is chosen by turning on one SOA Žposition D in the figure. and turning off the other 15 Žpositions E to S in the figure.. The finally selected wavelength is sent to the OSM through another AWG. Here, it is assumed that 1 from the first IOM is chosen for the OSM. The receiver at the OSM Žposition T in the figure. will receive not only the selected wavelength Ž 1 ., but also incoherent crosstalk and homowavelength crosstalk Žthose i’s with circles in the figure.. Homowavelength crosstalk channels come from different IOMs with the same selected wavelength, and incoherent crosstalk channels contain different wavelengths from the selected wavelength 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 and crosstalk ratio ŽCR, the reciprocal of the onroff ratio. of an SOA gate. To meet a BER requirement of 10y12 , one has to increase the input power by about 2 dB to compensate for the crosstalk with CR s y20 dB. Further-

330 OPTICAL PACKET SWITCHES

Fig. 11.43 Bit error rate versus received power with different crosstalk ratios ŽCR. 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

Fig. 11.45 Bit error rate vs. received power with different extinction ratios Žr . 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 effect of the SOA gate due to high input power, gain-clamped SOA gates w57x can be used to provide a constant optical gain Žf 22 dB. over a wider range of input power.

With the same system parameters as those used in w60, 61x, Figure 11.44 shows the BER performance using different signal extinction ratios wi.e., the ratio of the power of logic one Žmark. to the power of logic zero Žspace.x, 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 homowavelength 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.

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Broadband Packet Switching Technologies: A Practical Guide to ATM Switches and IP Routers

H. Jonathan Chao, Cheuk H. Lam, Eiji Oki

Copyright 2001 John Wiley & Sons, Inc. ISBNs: 0-471-00454-5 ŽHardback.; 0-471-22440-5 ŽElectronic.

CHAPTER 12

WIRELESS ATM SWITCHES

The goal of next-generation wireless systems is to enable mobile users to access ubiquitous multimedia information anytime anywhere. New mobile and wireless services include Internet access to interactive multimedia and video conferencing as well as traditional services such as voice, email, and Web access. 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 as laptop PCs and personal digital assistants w1, 2x.

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 research and development in many organizations worldwide w2, 3, 4, 5, 6x and is now under standardization within applicable bodies such as the ATM Forum w7x and ETSI.

Wireless ATM is intended to provide seamless support of qualitatively similar multimedia services on both fixed and mobile terminals. The realization of wireless ATM raises a number of technical issues that need to be resolved, however. First, there is a need for the allocation and standardization 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 layer, a medium access control ŽMAC., and a data link control ŽDLC. 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. Location management must be capable of tracking mobile users for delivery of an

337

338 WIRELESS ATM SWITCHES

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 lossfree 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 guarantee cell sequence during handoff. Performance of the switch is also discussed in that section.

12.1WIRELESS ATM STRUCTURE OVERVIEWS

12.1.1System Considerations

Within the Wireless ATM Working Group ŽWAG. of the ATM Forum, various reference configurations for a wireless ATM architecture are discussed w8, 9x. In a fixed wireless network scenario, the network components, switching elements, and end user devices Žterminals. 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 communicate 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.

The last scenario is ATM networks supporting personal communications ser®ice ŽPCS. access. In this scenario, the end user devices are PCS terminals. This scenario uses the mobility-supporting capabilities of the fixed ATM network to route traffic to the appropriate PCS base station. A PCS-to-ATM interworking function ŽIWF. is required to translate the data stream, which is delivered to the en users via a wireless link or delivered to the ATM network via a mobility-support ATM switch. The IWF resides in the PCS base station controller ŽBSC., which controls several base stations and manages wireless resources.

For the wireless access architecture, two approaches are under consideration: integrated access and modular access. An integrated access model incorporates all mobility and radio functions into ATM switches. This approach places more complexity in the switches. In a modular access model, ATM switches are alienated from the radio access mechanisms. ATM switches implement call and connection control and mobility management aspects of wireless ATM, whereas a separate physical entity, known as the access point ŽAP., implements the radio access functions and deals with all the radiospecific functionality, such as radio MAC and radio resource management functions. This approach reduces the effect of radio and mobility functions on the switches, but requires a new protocol, called access point control protocol ŽAPCP. w10x, to convey messages between the APs and the ATM switches.

Fig. 12.1 A wireless ATM system architecture.