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RSVP reestablishes the state. In both cases the new request may fail if there is not enough capacity after the failure.

18. The Wireless ATM Working Group of the ATM Forum (an industry group) is addressing the problems of end user mobility. This effort may be the only avenue for extending ATM to the end user.

19. Latencies in the wireless channel are not only high but also variable over time because of fluctuations in retransmission. Forward error correction can mitigate this problem somewhat but imposes a penalty even when the channel quality is good.

20. Because of the error characteristics of wireless links, some of the QoS issues need to be addressed locally at the link layer rather than from an end-to-end perspective. The DARPA PRNet had a strategy of accomplishing enough at the link level that TCP could handle the remaining reliability issues. However, this approach requires interaction between the link layer and higher layers (e.g., if the link layer needs to implement a stronger channel code, then its transmission rate may be reduced or its delay increased). In addition, the wireless channel may be so degraded that little can be done at the link level to improve matters. There needs to be a way to cope with this situation through higher-layer protocols.

21. Software security is another category but it is not unique to wireless communications and therefore is not addressed here.

22. Some security concerns are being alleviated in the transition from analog to digital systems, which offer an inherent advantage because the meaning of a pattern of 1s and 0s cannot be casually discerned.

23. For example, systems based on the GSM standard keep the key in a separate smart card, not in the telephone.

24. For example, most contemporary software radios use commercial filters by Graychip, Inc., or Harris Corp. for highly programable channel access to FDMA, TDMA, and CDMA systems with the low size, weight, and power of ASICs.

25. A CCD detector turns light into an electric charge, which is then transformed into the binary code recognized by computers. Some commercial cameras use this technology, but they remain expensive.

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