📄 002_8.htm

📁 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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<DIV class=im3><IMG height=2 src="002_8.files/002_8_4.jpg" width=134 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN class=ft0>&#8226; 
APP&nbsp;<SPAN class=em0>：</SPAN>Application-Specific packet<SPAN 
class=em0>，</SPAN><SPAN class=em0>特定应用的包</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps1><NOBR>这里定义了<SPAN 
class=em1>RTCP</SPAN>包的类型，并给出了一些在实时应用中使用该协议的讨论。</NOBR></SPAN> <SPAN 
class=ps2><NOBR><SPAN class=ft0>SR<SPAN class=em0>（</SPAN>Sender Report<SPAN 
class=em0>，</SPAN><SPAN class=em0>发送者报告）：<SPAN 
class=em1>SR</SPAN>包携带了来自连接中有效发送者的统计数据。</SPAN>SR</SPAN></NOBR></SPAN> <SPAN 
class=ps3><NOBR>应该根据针对特定网络设计的带宽分配算法进行发送，以允许统计信息流向网络中最活跃的</NOBR></SPAN> <SPAN 
class=ps4><NOBR>包产生者。<SPAN class=em1>SR</SPAN>和<SPAN 
class=em1>RR</SPAN>的带宽分配应该来源于算法，或者在会话建立时设置为连接参数的一部</NOBR></SPAN> <SPAN 
class=ps5><NOBR>分。</NOBR></SPAN> <SPAN class=ps6><NOBR><SPAN class=ft0>RR<SPAN 
class=em0>（</SPAN>Receiver Report<SPAN class=em0>，</SPAN><SPAN 
class=em0>接收者报告）：<SPAN 
class=em1>RR</SPAN>包携带了来自非有效发送者的参与者的统计数</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps7><NOBR>据。在确定<SPAN class=em1>RTCP</SPAN>包传输间隔时，相对于发送方而言，对<SPAN 
class=em1>RTCP</SPAN>可用带宽的切分会有意地偏向</NOBR></SPAN> <SPAN 
class=ps8><NOBR>接收方。一个新的扩展<SPAN class=em1>RTP</SPAN>的草案提议（<SPAN 
class=em1>Schulzrinne/Casner/Frederick/Jacobson</SPAN>，<SPAN 
class=em1>ietf-avt-rtp-</SPAN></NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN 
class=ft0>new-03.txt<SPAN class=em0>，</SPAN><SPAN 
class=em0>正在进行的工作）中，建议把</SPAN>5%<SPAN class=em0>的会话带宽分配给</SPAN>RTCP<SPAN 
class=em0>包，其中</SPAN>1.75%<SPAN class=em0>分给发</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps10><NOBR>送方，<SPAN 
class=em1>3.75%</SPAN>分给接收方&nbsp;。这一点并不显而易见。在一个会议呼叫中有多个参与者的大型拓</NOBR></SPAN> 
<SPAN class=ps11><NOBR>扑里，如何使算法的带宽分配能够实时地动态保持，这是一个相当难以确定的问题。但是，</NOBR></SPAN> 
<SPAN class=ps12><NOBR>对于中等大小的会议，所建议的算法可以迅速测量带宽，并保持统计信息的流动方式偏爱接</NOBR></SPAN> 
<SPAN class=ps13><NOBR>收包的用户胜于创建包的用户。静态分配的<SPAN 
class=em1>RTCP</SPAN>带宽最可能出现在简单情况下，例如两方的</NOBR></SPAN> <SPAN 
class=ps14><NOBR>基本电话。</NOBR></SPAN> <SPAN class=ps15><NOBR><SPAN 
class=ft0>SDES<SPAN class=em0>（</SPAN>Source DEScription<SPAN 
class=em0>，</SPAN><SPAN class=em0>信源说明）：<SPAN 
class=em1>SDES</SPAN>包允许在</SPAN>RTP<SPAN class=em0>包内把</SPAN>SSRC<SPAN 
class=em0>数值和发送数</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>据的用户的实际标识绑定起来。标识可以是用户名称、<SPAN 
class=em1>EMAIL</SPAN>地址、主系统的用户登录标识</NOBR></SPAN> <SPAN 
class=ps17><NOBR>中的一个或多个。其中也可以包含一些其他可选项，如电话号码、用户位置、用来产生介质</NOBR></SPAN> <SPAN 
class=ps18><NOBR>流的应用，可能还有呼叫参与者没有严格解释的一条注释。</NOBR></SPAN> <SPAN 
class=ps19><NOBR><SPAN class=ft0>IETF<SPAN class=em0>的音频视频传输（<SPAN 
class=em1>Audio</SPAN></SPAN></SPAN></NOBR></SPAN> <SPAN class=ps20><NOBR><SPAN 
class=ft0>Video Transport<SPAN class=em0>，</SPAN>AVT<SPAN 
class=em0>）</SPAN><SPAN class=em0>工作组正在进行的工作中也允许一种扩展</SPAN>SDES<SPAN 
class=em0>包的机制。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps21><NOBR>端点需要在会话开始时发送一个<SPAN 
class=em1>SDES</SPAN>包，以明确标识每个参与者。后面将要讨论的混</NOBR></SPAN> <SPAN 
class=ps22><NOBR>合器，把来自所有参与者的包合并起来，生成一个合成包，其中包含的字节块和原来的包一</NOBR></SPAN> <SPAN 
class=ps23><NOBR>样多。</NOBR></SPAN> <SPAN class=ps24><NOBR><SPAN 
class=ft0>BYE<SPAN class=em0>（</SPAN>Hang up from a session<SPAN 
class=em0>，</SPAN><SPAN class=em0>挂断会话）：<SPAN 
class=em1>BYE</SPAN>包告诉呼叫的所有参与者，发送者正在</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps25><NOBR>离开。<SPAN class=em1>BYE</SPAN>包不能被混合，因为正在离开的用户需要由<SPAN 
class=em1>SSRC</SPAN>明确标识。可选地，正在使用的</NOBR></SPAN> <SPAN 
class=ps26><NOBR>应用能够找出离开的原因。我们的看法是，由于在检测到环状信源或者</NOBR></SPAN> <SPAN 
class=ps27><NOBR><SPAN class=ft0>SSRC<SPAN 
class=em0>冲突时会使用</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps28><NOBR><SPAN 
class=ft0>BYE<SPAN class=em0>，</SPAN><SPAN class=em0>所以</SPAN>BYE<SPAN 
class=em0>应该包含离开的原因、已达到收集统计数据的目的。断开连接的另一个可能</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps29><NOBR>原因是，在<SPAN class=em1>SDES</SPAN>包中出现了重复的标识，但是<SPAN 
class=em1>AVT</SPAN>正在进行的工作中对此没有明确要求。如</NOBR></SPAN> <SPAN 
class=ps30><NOBR>果有持久的原因使得<SPAN class=em1>SDES</SPAN>包中存在冲突的<SPAN 
class=em1>SSRC</SPAN>或重复的用户标识，了解连接断开的原因对</NOBR></SPAN> <SPAN 
class=ps31><NOBR>于找出该问题会有所帮助。</NOBR></SPAN> <SPAN class=ps32><NOBR><SPAN 
class=ft0>APP<SPAN class=em0>（</SPAN>Application-specific RTCP packet<SPAN 
class=em0>，</SPAN><SPAN class=em0>特定应用的包）：<SPAN 
class=em1>APP</SPAN>包可以用来在端点间运行</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps33><NOBR>的应用之间传送信息，现在还只是试验性地提出。它可能的应用包括，当具有一定特权的用</NOBR></SPAN> <SPAN 
class=ps34><NOBR>户加入呼叫时，用它为远程的服务实例交换信息，但是企业界正在考虑用更复杂的方法实现</NOBR></SPAN> <SPAN 
class=ps35><NOBR>服务实例和用户接入授权，如<SPAN class=em1>CORBA</SPAN>环境。</NOBR></SPAN> 
<SPAN class=ps36><NOBR>如图<SPAN class=em1>2-4</SPAN>所示，<SPAN 
class=em1>SR</SPAN>报头中包含<SPAN class=em1>RTP</SPAN>版本（<SPAN 
class=em1>V=2</SPAN>），在包尾可能有填充（<SPAN class=em1>P=1</SPAN>），以及包中出</NOBR></SPAN> 
<SPAN class=ps37><NOBR>现的接收报告数值（<SPAN class=em1>rc</SPAN>）。<SPAN 
class=em1>SR</SPAN>的有效载荷类型是<SPAN class=em1>200<SPAN 
class=em0>，</SPAN></SPAN>包的长度在最后<SPAN class=em1>16</SPAN>比特中。如果设置了<SPAN 
class=em1>P</SPAN></NOBR></SPAN> <SPAN 
class=ps38><NOBR>比特位，包的最后一个字节说明了要从包中忽略的字节数目，包括它自身在内。在<SPAN 
class=em1>RTP</SPAN>包中，</NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft2>64&nbsp;&nbsp;<SPAN class=em2>IP </SPAN><SPAN 
class=em3>电话技术：稳定的</SPAN><SPAN class=em2>VoIP</SPAN><SPAN 
class=em3>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps40><NOBR><SPAN 
class=ft3>文档</SPAN></NOBR></SPAN> <SPAN class=ps41><NOBR><SPAN 
class=ft4>我们讨论这个提议中的算法是为了解释为<SPAN 
class=em4>RTCP</SPAN>包保持动态可变带宽分配中的问题。</SPAN></NOBR></SPAN> </BODY></HTML>
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