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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
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.