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📁 通讯类的标准。对要开发SS7的朋友有很大帮助的。（通讯协议）
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<BODY>
<DIV class=pg></DIV><!-- bitmap and vector images are written here -->
<DIV class=im0><IMG height=64 src="002_40.files/left.jpg" width=564 
border=0></DIV>
<DIV class=im1><IMG height=231 src="002_40.files/002_40_2.jpg" width=443 
border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>的延迟，但是协议开销要求的带宽直线上升。</NOBR></SPAN> <SPAN 
class=ps1><NOBR>简单地说，如果链路上允许速率适配，在任意给定时刻从用户接收的呼叫数目存在不确</NOBR></SPAN> <SPAN 
class=ps2><NOBR>定性。在速度下降情况下，如果用非压缩语音建立起大量呼叫，占用了上行链路中除了<SPAN 
class=em0>40K</SPAN>之</NOBR></SPAN> <SPAN 
class=ps3><NOBR>外的全部带宽，那么情况会变得混乱起来。这时，介质网关服务的另一用户拨打了一个号码。</NOBR></SPAN> <SPAN 
class=ps4><NOBR>在新呼叫的信令期间，合适的编码解码方案并没有和远端协商，或者该呼叫对于<SPAN 
class=em0>PSTN</SPAN>用户和</NOBR></SPAN> <SPAN class=ps5><NOBR><SPAN 
class=ft1>G.711<SPAN class=em1>缺省的呼叫是断开的。这时呼叫将不会被接受（<SPAN 
class=em0>MGCP</SPAN>很可能返回“资源不足”错误编</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps6><NOBR>码，<SPAN class=em0>H.323</SPAN>会报错<SPAN class=em0>ARJ<SPAN 
class=em1>，</SPAN></SPAN>而<SPAN class=em0>SIP</SPAN>实现会返回警告编码<SPAN 
class=em0>370<SPAN class=em1>）</SPAN></SPAN>。如果服务保证同时接受所有用户的</NOBR></SPAN> <SPAN 
class=ps7><NOBR>所有呼叫，可能会出现服务级别一致性问题。显然，仔细的网络设计和服务设计是很重要的。</NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft2>2.4 &nbsp;&nbsp;<SPAN 
class=em3>边缘带宽管理—关于</SPAN>AAL2</SPAN></NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN 
class=ft1>AAL2<SPAN class=em1>是最新的</SPAN>ATM<SPAN 
class=em1>适配层，在一个虚拟电路内提供可变长度的包有效载荷，用于有端到</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps10><NOBR>端要求的应用。<SPAN class=em0>AAL2</SPAN>的结构非常简单，包括两个底层栈（子层）。<SPAN 
class=em0>CPS</SPAN>层从特定信道发送和</NOBR></SPAN> <SPAN 
class=ps11><NOBR>接收包，该信道连接到特定服务会聚子层，或者直接连接到产生有效载荷的应用上。图</NOBR></SPAN> <SPAN 
class=ps12><NOBR><SPAN class=ft1>2-22</SPAN></NOBR></SPAN> <SPAN 
class=ps13><NOBR>中是<SPAN class=em0>ITU-T</SPAN>建议<SPAN 
class=em0>I.363.2</SPAN>中规定的<SPAN class=em0>AAL2</SPAN>两个子层的协议关系（参见<SPAN 
class=em0>[34]<SPAN class=em1>）</SPAN></SPAN>。</NOBR></SPAN> <SPAN 
class=ps14><NOBR><SPAN class=ft3>图<SPAN class=em4>2-22 
&nbsp;&nbsp;AAL2</SPAN>协议结构</SPAN></NOBR></SPAN> <SPAN 
class=ps15><NOBR>在相同物理介质中管理音频、音频带和非实时数据非常困难，但是并非到此为止。如果</NOBR></SPAN> <SPAN 
class=ps16><NOBR>使用了压缩，在使用干线协议时，带宽管理的任务会再次改变。例如，如果使用<SPAN 
class=em0>AAL</SPAN>作为多</NOBR></SPAN> <SPAN 
class=ps17><NOBR>媒体流的多路复用机制，当发送传真或使用调制解调器拨号接入时，对于音频带（传真、调</NOBR></SPAN> <SPAN 
class=ps18><NOBR>制解调器）的支持会造成包括<SPAN class=em0>G.711</SPAN>有效载荷扩展的问题。<SPAN 
class=em0>ATM VToA</SPAN>规范中建议为支持音频</NOBR></SPAN> <SPAN 
class=ps19><NOBR>带呼叫而进行带宽扩展（参见<SPAN class=em0>[36]<SPAN 
class=em1>）</SPAN></SPAN>。</NOBR></SPAN> <SPAN 
class=ps20><NOBR>当连接上同时使用静音抑制和回声消除的压缩时，呼叫处理硬件和软件对音频带信号的</NOBR></SPAN> <SPAN 
class=ps21><NOBR>反应必须足够快，如检测到的<SPAN 
class=em0>2100Hz</SPAN>的应答音。需要一种方法来立刻通知它所控制的端点关</NOBR></SPAN> <SPAN 
class=ps22><NOBR>闭回声消除和回声抑制。同时扩充连接占据的虚拟时隙（在<SPAN 
class=em0>AAL2</SPAN>术语中是信道<SPAN class=em0>ID<SPAN 
class=em1>）</SPAN></SPAN>。一些复</NOBR></SPAN> <SPAN 
class=ps23><NOBR>杂设备能对音频带音调做出反应，扩展有效载荷，但是需要系统元件的介入，以关闭回声消</NOBR></SPAN> <SPAN 
class=ps24><NOBR>除和静音抑制。新的调节可能在呼叫持续期间内一直保持。</NOBR></SPAN> <SPAN 
class=ps25><NOBR>由于两个原因，简单的基于<SPAN 
class=em0>IP</SPAN>的传输性能会大大提高。首先，有效载荷每比特的协议开</NOBR></SPAN> <SPAN 
class=ps26><NOBR>销大大减少。其次，这种方法允许链路层上的有效载荷多路复用，这在性能和容量方面都优</NOBR></SPAN> <SPAN 
class=ps27><NOBR><SPAN class=ft4>96&nbsp;&nbsp;<SPAN class=em2>IP </SPAN><SPAN 
class=em5>电话技术：稳定的</SPAN><SPAN class=em2>VoIP</SPAN><SPAN 
class=em5>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps28><NOBR><SPAN 
class=ft5>文档</SPAN></NOBR></SPAN> <SPAN class=ps29><NOBR><SPAN 
class=ft6>服务特定会聚子层</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft7>(SSCS)</SPAN></NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN 
class=ft6>通用部分子层</SPAN></NOBR></SPAN> <SPAN class=ps32><NOBR><SPAN 
class=ft7>(CPS)</SPAN></NOBR></SPAN> </BODY></HTML>

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