wireless nirvana.htm

无线通信中的加解密,系统安全的研究与分析研究

      <P class=medium-normal>Unlike Ethernet <STRONG><EM>network</EM></STRONG> 
      nodes, which can monitor <STRONG><EM>network</EM></STRONG> activity 
      directly and instantly detect collisions, cellular 
      <STRONG><EM>networks'</EM></STRONG> mobile terminals listen to and 
      communicate only with the base station, not with one another. This 
      increases latency relative to wireline <STRONG><EM>networks</EM></STRONG> 
      as the <STRONG><EM>network</EM></STRONG> must coordinate all 
      communications. For packet communications, the mechanism to transmit data 
      packets occurs in two stages. First, the mobile terminal uses a control 
      channel to request a traffic channel. The control channel is a 
      random-access channel and collisions can occur from other terminals, in 
      which case the terminal must repeat the request. The 
      <STRONG><EM>network</EM></STRONG> responds by assigning a traffic channel 
      (specified time slots in the case of GPRS and a specified code in the case 
      of 1XRTT) for data transmission. The complexity of managing radio access 
      results in wireless-<STRONG><EM>network</EM></STRONG> latencies higher 
      than those of a wireline <STRONG><EM>network</EM></STRONG>. Round-trip 
      times of .5 seconds to 1.5 seconds are typical. Latency can slow down some 
      applications more than throughput, especially if the application shuttles 
      a lot of messages. Delays also can restrict what applications are 
      feasible. Some applications, such as packetized voice, are highly 
      sensitive to delay. On the other hand, streaming applications that are not 
      interactive should not have a latency problem.</P>
      <P class=medium-normal>With both GPRS and 1XRTT, the 
      <STRONG><EM>network</EM></STRONG> separates packet data from circuit data 
      at the base-station controller and connects it to a separate packet 
      infrastructure. The core packet nodes in GPRS are SGSN (Serving GPRS 
      Support Node), which tracks user locations, and GGSN (Gateway GPRS Support 
      Node), which handles IP-address management and gateways to external 
      <STRONG><EM>networks</EM></STRONG>, such as the Internet. In the case of 
      1XRTT, the core packet node is PDSN (Packet Data Serving Node); it 
      performs much of the same functions the SGSN and GGSN do. This internal 
      architecture is of little consequence to users, other than the fact that 
      server-side connections are via packet networks, and not telephone 
      <STRONG><EM>networks</EM></STRONG>.</P><A name=AN0007584743-5>
      <CENTER>
      <H3><A id=hd_toc title="IP Packet Architecture  " 
      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">IP 
      Packet Architecture </A></H3></CENTER>
      <P class=medium-normal>To control data sessions, modem vendors (or 
      operators) provide a software utility that reports connection status and 
      handles management. When the user invokes a data session, the 
      <STRONG><EM>network</EM></STRONG> dynamically assigns an IP address to a 
      mobile terminal-by the GGSN in GPRS and by the PDSN in 1XRTT. From a usage 
      point of view, the cellular <STRONG><EM>network</EM></STRONG> looks like 
      an ISP, with IP packets routing to and from the mobile user. Like an ISP, 
      the operator may also provide an e-mail address but is less likely to 
      provide other services such as Web hosting.</P>
      <P class=medium-normal>GPRS and 1XRTT are steps in an evolution of 
      capability. The next step for GPRS will be EDGE (Enhanced Data Rates for 
      GSM Evolution), a radio upgrade that promises to triple throughputs. EDGE 
      retains the same core infrastructure and uses the same spectrum and same 
      time-division approach, but adds sophisticated radio mechanisms to alter 
      modulation and error correction dynamically based on the instantaneous 
      radio environment, thus increasing spectral efficiency and user 
      throughputs. EDGE <STRONG><EM>networks</EM></STRONG> will become available 
      in 2003, with nearly all North American carriers committed to that 
      technology's deployment. Beyond that, GSM carriers will deploy wideband 
      <STRONG><EM>CDMA</EM></STRONG> (WCDMA)-also referred to as UMTS (Universal 
      Mobile Telecommunications System)-a version of 
      <STRONG><EM>CDMA</EM></STRONG> different from 
      <STRONG><EM>CDMA</EM></STRONG>2000 that initially will boost peak data 
      throughputs to 2 Mbps. That throughput eventually will jump to 10 Mbps, 
      with a technology called High Speed Downlink Packet Access.</P>
      <P class=medium-normal>For <STRONG><EM>CDMA</EM></STRONG>2000, the 
      evolution path includes a technology called 1XEVDO (1X Evolution Data 
      Only), which boasts peak throughputs of 2.4 Mbps and 1XEVDV (1X Evolution 
      Data and Voice), which will have peak downlink speeds of 5 Mbps.</P>
      <P class=medium-normal>Keep in mind these factors when you consider the 
      future of wireless data:</P>
      <P class=medium-normal><STRONG>• Peak speeds are not the same as average 
      speeds. </STRONG>For instance, though WCDMA tops out at 2 Mbps, this 
      represents the total cell capacity; typical users will likely get 200 Kbps 
      to 300 Kbps throughput on a loaded <STRONG><EM>network</EM></STRONG>.</P>
      <P class=medium-normal></P>
      <P class=medium-normal><STRONG>• These deployments will take time. 
      </STRONG>Though EDGE, essentially a 
      <STRONG><EM>network</EM></STRONG>-software upgrade, is almost here, WCDMA 
      and 1XEV deployments could take several years. These technologies will be 
      more expensive and complicated and will require a new radio-access 
      <STRONG><EM>network</EM></STRONG>.</P>
      <P class=medium-normal></P>
      <P class=medium-normal><STRONG>• Business plans for 3G 
      <STRONG><EM>networks</EM></STRONG> are up in the air, </STRONG>with demand 
      for wireless data services uncertain and the rapid deployment of public 
      WLANs possibly channeling user-data subscriptions away from cellular 
      <STRONG><EM>networks</EM></STRONG>. Following the overall telecom 
      meltdown, many operators are delaying their 3G deployment plans.</P>
      <P class=medium-normal></P>
      <P class=medium-normal><STRONG>• Spectrum is an issue. </STRONG>Operators 
      can deploy EDGE and <STRONG><EM>CDMA</EM></STRONG>2000 easily in existing 
      spectrum, but WCDMA uses 5-MHz radio channels-it's not called wideband for 
      nothing. Finding room for these channels won't be easy. Most countries in 
      Europe and Asia have auctioned new 3G spectrum for 3G service. The United 
      States is a little further behind, having just recently identified which 
      bands might be feasible.</P>
      <P class=medium-normal>Meantime, GPRS and 1XRTT are real. Which is the 
      better service? 1XRTT has a throughput advantage for the moment, but GPRS 
      is available in more countries. And the upgrade to EDGE should more than 
      match 1XRTT, though users will need new equipment to take advantage of the 
      service. From all other perspectives, the offerings are largely 
      equivalent. The table on page 78, "Wireless Technology Time Line," 
      compares capabilities and deployments of the different cellular 
      technologies.</P><A name=AN0007584743-6>
      <CENTER>
      <H3><A id=hd_toc title="Using the Networks  " 
      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">Using 
      the Networks </A></H3></CENTER>
      <P class=medium-normal>Getting started with these 
      <STRONG><EM>networks</EM></STRONG> is easy, but getting full satisfaction 
      requires attention to detail. Establishing connections is straightforward, 
      IP-based applications work immediately, and you can almost instantly 
      realize the convenience of anywhere, anytime communications. But you must 
      consider where service is available, which platforms to use, performance 
      variations, <STRONG><EM>security</EM></STRONG>, usage costs and networking 
      idiosyncrasies. Fortunately, these are manageable once you understand 
      them.</P>
      <P class=medium-normal>Service availability is the best place to start 
      because if you don't have service, the rest of the points are moot. All 
      the operators are planning nationwide service, and if you can get voice 
      service, you will be able to have higher-speed data service, a significant 
      improvement over <STRONG><EM>networks</EM></STRONG> such as CDPD, in which 
      service coverage does not match the voice footprint. But this won't happen 
      overnight. T-Mobile offers GPRS everywhere it offers GSM voice service, 
      but AT&amp;T Wireless and Cingular Wireless use TIA/EIA-136 TDMA 
      technology for their voice <STRONG><EM>networks</EM></STRONG> and are 
      rolling out new GSM/GPRS <STRONG><EM>networks</EM></STRONG>. AT&amp;T