wireless nirvana.htm

无线通信中的加解密,系统安全的研究与分析研究

      Wireless expects to have most of its <STRONG><EM>network</EM></STRONG> 
      deployed by the end of this year; Cingular Wireless, by the end of 
      2003.</P>
      <P class=medium-normal>On the <STRONG><EM>CDMA</EM></STRONG> side, the two 
      largest operators are upgrading their existing 
      <STRONG><EM>networks</EM></STRONG> to 1XRTT and both will offer broad 
      coverage by the end of this year. Sprint PCS has upgraded most of its 
      <STRONG><EM>network</EM></STRONG>, and Verizon Wireless says it expects to 
      cover 90 percent of its 30 million subscribers by year's end. Check with 
      the operators. Most show coverage maps on their Web sites for these 
      services.</P>
      <P class=medium-normal>Increasingly, customers are asking about global 
      coverage, and here GPRS has the advantage of availability in 64 countries. 
      However, make sure your provider has a roaming agreement with a foreign 
      operator in the country of interest. In addition, GSM operates in 
      different radio bands in different locations, so your device must be able 
      to tune to the appropriate frequencies. Fortunately, multiple-band GPRS 
      phones and modems are available.</P><A name=AN0007584743-7>
      <CENTER>
      <H3><A id=hd_toc title="Throughput Versus Latency  " 
      href="http://web11.epnet.com/citation.asp?tb=1&amp;_ug=sid+55725B2C%2D1A6E%2D4291%2D99B0%2DA6A8441EF92B%40sessionmgr6+dbs+a3h+cp+1+F22C&amp;_us=frn+1+hd+False+hs+True+cst+0%3B2+or+Date+fh+False+ss+SO+sm+ES+sl+0+dstb+ES+mh+1+ri+KAAACBTB00061445+22DE&amp;_uso=tg%5B2+%2D+tg%5B1+%2DAB+tg%5B0+%2DAB+db%5B0+%2Da3h+hd+False+clv%5B0+%2DY+op%5B2+%2DAnd+op%5B1+%2DAnd+op%5B0+%2D+cli%5B0+%2DFT+st%5B2+%2D+st%5B1+%2Dcdma+st%5B0+%2Dnetwork++security+mdb%5B0+%2Dimh+DD39&amp;cf=1&amp;fn=1&amp;rn=1#toc">Throughput 
      Versus Latency </A></H3></CENTER>
      <P class=medium-normal>Because Packet-Switched data 
      <STRONG><EM>networks</EM></STRONG> don't deliver a dedicated circuit, they 
      can't promise consistent throughputs either. This is the new services' 
      most significant caveat. With traditional modems, users have a dedicated 
      circuit and can expect a consistent performance level. This is also true 
      with circuit-switched data for cellular. In the case of the new packet 
      <STRONG><EM>networks</EM></STRONG>, the medium is shared, just like 
      Ethernet, but the total available bandwidth per cell site is much lower 
      than Ethernet's. Although <STRONG><EM>CDMA</EM></STRONG>2000 carriers 
      quote their <STRONG><EM>networks</EM></STRONG> as having peak speeds of 
      144 Kbps, and say users can expect 40 Kbps to 60 Kbps, they fail to 
      mention that at peak times under heavy voice loading, data throughput may 
      drop to 10 Kbps. GPRS can suffer from this effect too. Also, once the 
      number of data users increases, the available data channels will be 
      shared, and average throughput will go down. How much? It depends to what 
      extent the operators manage QoS (quality of service) for data. As with 
      voice capacity, they can increase data capacity by allocating more radio 
      channels (assuming they have spectrum available) or by adding base 
      stations (a costly endeavor). So take the figures quoted by operators as a 
      starting point, but do your own testing to determine how well your 
      applications operate. Furthermore, keep in mind that any testing you do 
      today may not capture how the <STRONG><EM>networks</EM></STRONG> will 
      perform once popularity increases. Given the historical low data usage on 
      cellular <STRONG><EM>networks</EM></STRONG>, this is a problem operators 
      would love to have. Furthermore, operators have had similar voice-capacity 
      challenges in some markets, but have managed to keep up with demand. In 
      the case of Web access, most operators provide optional Web acceleration 
      servers that work quite well. The accelerators increase Web throughput 
      significantly by reducing the file size of images, compressing text and 
      reducing the number of TCP connections used to download pages. Some 
      operators include specialized e-mail gateways that let smart phones access 
      conventional e-mail services.</P>
      <P class=medium-normal>Latency-the amount of time it takes packets to 
      traverse the <STRONG><EM>network</EM></STRONG>-is as important as 
      throughput. Wireless <STRONG><EM>networks</EM></STRONG> have higher 
      latency than wireline <STRONG><EM>networks</EM></STRONG>. In 
      <STRONG><EM>CDMA</EM></STRONG>2000 1XRTT 
      <STRONG><EM>networks</EM></STRONG>, round-trip times for small packets can 
      be 500 milliseconds to 600 ms, and with GPRS can be closer to 800 ms. High 
      latency will take a toll on applications that require many small data 
      transactions. Keep in mind, however, that operators are still tuning their 
      <STRONG><EM>networks</EM></STRONG> for optimal performance. Do the 
      available data rates and latencies support existing enterprise 
      applications, such as VPNs, Microsoft Outlook, Lotus Notes, Web-based 
      applications and database access? Yes, but you still will want to be 
      careful how much data you download, for performance and cost 
      reasons.</P><A name=AN0007584743-8>
      <CENTER>
      <H3><A id=hd_toc title="Pricing It Out  " 
      href="http://web11.epnet.com/citation.asp?tb=1&amp;_ug=sid+55725B2C%2D1A6E%2D4291%2D99B0%2DA6A8441EF92B%40sessionmgr6+dbs+a3h+cp+1+F22C&amp;_us=frn+1+hd+False+hs+True+cst+0%3B2+or+Date+fh+False+ss+SO+sm+ES+sl+0+dstb+ES+mh+1+ri+KAAACBTB00061445+22DE&amp;_uso=tg%5B2+%2D+tg%5B1+%2DAB+tg%5B0+%2DAB+db%5B0+%2Da3h+hd+False+clv%5B0+%2DY+op%5B2+%2DAnd+op%5B1+%2DAnd+op%5B0+%2D+cli%5B0+%2DFT+st%5B2+%2D+st%5B1+%2Dcdma+st%5B0+%2Dnetwork++security+mdb%5B0+%2Dimh+DD39&amp;cf=1&amp;fn=1&amp;rn=1#toc">Pricing 
      It Out </A></H3></CENTER>
      <P class=medium-normal>Although Verizon Wireless offers a flat-rate 
      business plan at $99 per month, most plans are usage-based, and charge 
      between $1 and $10 per megabyte, depending on the size of the monthly 
      commitment, with $4 per megabyte being typical. It is quite easy in an 
      interactive data session over an hour to transfer more than a megabyte of 
      data. So costs can add up quickly. Moreover, most users, including 
      experienced ones, have no idea what volume of data different transactions 
      consume. You will want to characterize your applications for data volume 
      before you commit to a pricing plan. Pricing may also influence the 
      application architecture you choose-a thin-client approach transmits less 
      data than a fat-client setup. See "Wireless Data Service Pricing" on page 
      75 for costs for different data plans. Note that these are changing on a 
      regular basis, and the trend will be downward as competition 
increases.</P>
      <P class=medium-normal>Platforms raise another interesting question. In 
      the past, laptop users would attach a data-capable cell phone or plug in a 
      PC Card modem, but the number of options is increasing quickly. First are 
      phones with ever-increasing data capability. Although for some time phones 
      have had microbrowsers for viewing specially formatted Web content, the 
      newest ones can actually execute programs, enabled by both Sun 
      Microsystems' Java 2 Micro Edition and Qualcomm's Binary Runtime 
      Environment for Wireless. New smart phones also incorporate PDA 
      capabilities, using either proprietary platforms or platforms supplied by 
      Microsoft, Palm or Symbian. These slightly heavier and larger phones make 
      sense for users who want voice and data on one device, but who favor 
      voice.</P>
      <P class=medium-normal>For those who favor data, the platform of interest 
      may be the phone-enabled PDA, which looks like a PDA rather than a phone. 
      Palm OS, Pocket PC and RIM Blackberry represent the leading platforms, and 
      devices are available for both GPRS and 1XRTT 
      <STRONG><EM>networks</EM></STRONG> (see <A 
      href="http://www.nwc.com/">www.nwc.com/</A> 1321/1321sp2.html for a recent 
      review of Sprint's <STRONG><EM>CDMA</EM></STRONG>2000 service on a 
      Handspring Treo PDA/phone). Wireless-enabled PDAs might just become the 
      platform of choice for GPRS and 1XRTT, while notebooks may gravitate to 
      wireless hotspots based on 802.11 technology that have the higher 
      bandwidth preferred by communications-intensive applications.</P><A 
      name=AN0007584743-9>
      <CENTER>
      <H3><A id=hd_toc title="But Is It Secure?  " 
      href="http://web11.epnet.com/citation.asp?tb=1&amp;_ug=sid+55725B2C%2D1A6E%2D4291%2D99B0%2DA6A8441EF92B%40sessionmgr6+dbs+a3h+cp+1+F22C&amp;_us=frn+1+hd+False+hs+True+cst+0%3B2+or+Date+fh+False+ss+SO+sm+ES+sl+0+dstb+ES+mh+1+ri+KAAACBTB00061445+22DE&amp;_uso=tg%5B2+%2D+tg%5B1+%2DAB+tg%5B0+%2DAB+db%5B0+%2Da3h+hd+False+clv%5B0+%2DY+op%5B2+%2DAnd+op%5B1+%2DAnd+op%5B0+%2D+cli%5B0+%2DFT+st%5B2+%2D+st%5B1+%2Dcdma+st%5B0+%2Dnetwork++security+mdb%5B0+%2Dimh+DD39&amp;cf=1&amp;fn=1&amp;rn=1#toc">But 
      Is It Secure? </A></H3></CENTER>
      <P class=medium-normal>WLAN <STRONG><EM>security</EM></STRONG>, or lack 
      thereof, has received a huge amount of attention, but cellular 
      <STRONG><EM>networks</EM></STRONG> are a different story. First, operators 
      employ rigorous authentication mechanisms to protect against fraud. As for 
      encryption, <STRONG><EM>CDMA</EM></STRONG> 
      <STRONG><EM>networks</EM></STRONG> do not encrypt data traffic, but it is 
      inherently difficult to eavesdrop because <STRONG><EM>CDMA's</EM></STRONG> 
      spread-spectrum signal is intended to look like l