面向未来的接入技术――GEPON

dragonchina

<p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体;"><p>&nbsp;</p></span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><b><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><span lang="EN-US"><p></p></span></span></b></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><b><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p>&nbsp;</p></span></b></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><b><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">一、引言</span></b><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">进入<span lang="EN-US">21</span>世纪以来，全球宽带接入网进入了大发展阶段。我国宽带接入网在近两年发展也十分迅速。据初步统计，到<span lang="EN-US">2004</span>年底我国宽带用户达到<span lang="EN-US">2 500</span>万，成为仅次于美国的第二大宽带接入市场<span lang="EN-US">,</span>其中主导技术是<span lang="EN-US">ADSL</span>，大约占<span lang="EN-US">70</span>％左右，其次是以太网技术，还有少量宽带无线接入和电缆调制解调器接入。在国内，宽带接入对固网运营商的业务发展起着举足轻重的作用，已成为固网运营商主要的投资方向和业务收入来源之一。<span lang="EN-US"><p></p></span></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">随着<span lang="EN-US">Internet</span>的高速发展，用户在不断地增加，出现了<span lang="EN-US">IPTV</span>业务、各类互联网互动业务如（视频会议、视频聊天、网络游戏、大文件传输业务）等应用，对网络带宽提出了越来越高的要求（如图<span lang="EN-US">1</span>所示）。为了满足用户对带宽增长的要求，实现接入网的高速化、宽带化和智能化，各种接入技术层出不穷，这些接入技术中目前被认为最有前途的是光接入技术（<span lang="EN-US">FTTx</span>）。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="center" style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">图<span lang="EN-US">1. </span>用户带宽需求不断增长</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">无源光网络<span lang="EN-US">(PON)</span>技术是为了支持点到多点应用发展起来的光接入技术，<span lang="EN-US">ON</span>由光线路终端<span lang="EN-US">(OLT)</span>、光网络单元（<span lang="EN-US">ONU</span>）和光分配网络（<span lang="EN-US">ODN</span>）组成。其本质特征为<span lang="EN-US">ODN</span>全部由无源器件组成，无源的特性使得网络布放更加灵活，无需机房和电源等；共享光纤的特性能够节省大量的光纤资源，使得接入网线路成本更低。正是</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">由于易维护、高带宽、低成本等优点，<span lang="EN-US">ON</span>成为光接入中的佼佼者，被认为是通过单一平台综合接入语音、数据、视频等多种业务的理想物理平台。</span><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><br/></span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">　　<span lang="EN-US">ON</span>技术自诞生以来，根据数据链路层协议的不同，分为<span lang="EN-US">APON</span>（基于<span lang="EN-US">ATM</span>）、<span lang="EN-US">BPON</span>（基于<span lang="EN-US">ATM</span>）、<span lang="EN-US">GPON</span>（基于<span lang="EN-US">GFP</span>）、<span lang="EN-US">GEPON</span>（基于以太网），几种<span lang="EN-US">ON</span>技术的比较如表<span lang="EN-US">(1)</span>。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">表<span lang="EN-US">1. </span>不同<span lang="EN-US">ON</span>技术间的比较</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan; mso-margin-top-alt: auto;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><shapetype id="_x0000_t75" coordsize="21600,21600" stroked="f" filled="f" path="m@4@5l@4@11@9@11@9@5xe" ospt="75" opreferrelative="t"><stroke joinstyle="miter"></stroke><formulas><f eqn="if lineDrawn pixelLineWidth 0"></f><f eqn="sum @0 1 0"></f><f eqn="sum 0 0 @1"></f><f eqn="prod @2 1 2"></f><f eqn="prod @3 21600 pixelWidth"></f><f eqn="prod @3 21600 pixelHeight"></f><f eqn="sum @0 0 1"></f><f eqn="prod @6 1 2"></f><f eqn="prod @7 21600 pixelWidth"></f><f eqn="sum @8 21600 0"></f><f eqn="prod @7 21600 pixelHeight"></f><f eqn="sum @10 21600 0"></f></formulas><path oconnecttype="rect" gradientshapeok="t" oextrusionok="f"></path><lock aspectratio="t" vext="edit"></lock></shapetype><shape id="_x0000_i1025" type="#_x0000_t75" alt="" style="WIDTH: 24pt; HEIGHT: 24pt;"></shape></span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">以</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">前人们认为将<span lang="EN-US">ATM</span>（异步转移模式）技术和<span lang="EN-US">PON</span>技术相结合的<span lang="EN-US">APON</span>技术是实现综合接入的理想模式。然而，由于数据业务的爆炸式增长，<span lang="EN-US">ATM</span>技术暴露出效率不高、协议复杂等弱点，而<span lang="EN-US">IP</span>技术则日渐兴起。由于以太网在传输<span lang="EN-US">IP</span>业务时具有效率高、协议简单等优点，</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">此外目前<span lang="EN-US">IP</span>网络普遍建设，基于以太网的技术的元器件结构比较简单，性能高且价格便宜，</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">所以越来越多的人认为将千兆以太网技术和<span lang="EN-US">PON</span>技术相结合的<span lang="EN-US">GEPON</span>（<span lang="EN-US">Gigabit Ethernet Passive Optical Network</span>）技术是取代<span lang="EN-US">APON</span>（<span lang="EN-US">ATM Passive Optical Network</span>），实现高速、宽带、综合接入的理想途径</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">，成为最重要的<span lang="EN-US">FTTH</span>技术</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><b><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">二、<span lang="EN-US">GEPON</span>技术原理</span></b><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">GEPON</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">的系统结构如图<span lang="EN-US">2</span>所示。<span lang="EN-US">GEPON</span>系统主要由中心局的光线路终端（<span lang="EN-US">OLT</span>）、包含无源光器件的光分配网（<span lang="EN-US">ODN</span>）、用户端的光网络单元<span lang="EN-US">/</span>光网络终端（<span lang="EN-US">ONU</span>）以及网元管理系统（<span lang="EN-US">EMS</span>）组成，通常采用点到多点的树型拓扑结构。在下行方向，<span lang="EN-US">IP</span>数据、语音、视频等多种业务由位于中心局的<span lang="EN-US">OLT</span>，采用广播方式，通过<span lang="EN-US">ODN</span>中的<span lang="EN-US">1</span>：<span lang="EN-US">N</span>无源光分路器分配到<span lang="EN-US">PON</span>上的所有<span lang="EN-US">ONU</span>单元。在上行方向，来自各个<span lang="EN-US">ONU</span>的多种业务信息互不干扰地通过<span lang="EN-US">ODN</span>中的<span lang="EN-US">1</span>：<span lang="EN-US">N</span>无源光分路器耦合到同一根光纤，最终送到位于局端<span lang="EN-US">OLT</span>的接收端。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体;"><shape id="_x0000_i1026" type="#_x0000_t75" alt="" style="WIDTH: 24pt; HEIGHT: 24pt;"></shape></span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">&nbsp;<p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">&nbsp;<p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体;">图<span lang="EN-US">2. GEPON</span>系统结构</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">&nbsp;<p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">GEPON</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">采用单纤波分复用技术（下行<span lang="EN-US">1490nm</span>，上行<span lang="EN-US">1310nm</span>），仅需一根主干光纤和一个<span lang="EN-US">OLT</span>，传输距离可达<span lang="EN-US">20</span>公里。在<span lang="EN-US">ONU</span>侧通过光分路器分送给多达<span lang="EN-US">32</span>个用户，因此可降低<span lang="EN-US">OLT</span>和主干光纤的成本压力。<span lang="EN-US"><br/></span>　　<span lang="EN-US">GEPON</span>具有同时传输<span lang="EN-US">TDM</span>、<span lang="EN-US">IP</span>数据和视频广播的能力，其中<span lang="EN-US">TDM</span>和<span lang="EN-US">IP</span>数据采用<span lang="EN-US">IEEE 802.3</span>以太网的格式进行传输，辅以电信级的网管系统，足以保证传输质量。通过扩展第三个波长（通常为<span lang="EN-US">1550nm</span>）即可实现视频业务广播传输。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; tab-stops: list 36.0pt; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">为了确保与<span lang="EN-US">IEEE 802</span>的结构兼容，<span lang="EN-US">GEPON</span>采用了点到点仿真技术，使得<span lang="EN-US">GEPON</span>介质成为一系列点到点链接的组合。根据<span lang="EN-US">IEEE 802.3ah</span>规定，<span lang="EN-US">GEPON</span>系统物理层传输的是标准的以太网帧，对此，<span lang="EN-US">802.3ah</span>标准中采用逻辑链路标识方式（<span lang="EN-US">LLID</span>）为每个不同的<span lang="EN-US">ONU</span>分配一个不同的<span lang="EN-US">LLID</span>。这样每个<span lang="EN-US">ONU</span>只能接收带有自己的<span lang="EN-US">LLID</span>的数据报，其余的数据报丢弃不再转发。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">GEPON</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">从<span lang="EN-US">OLT</span>到多个<span lang="EN-US">ONU</span>下行传输数据采用<span lang="EN-US">TDM</span>技术，如图<span lang="EN-US">3</span>所示：<span lang="EN-US"><shape id="_x0000_i1027" type="#_x0000_t75" alt="" style="WIDTH: 24pt; HEIGHT: 24pt;"></shape></span></span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="center" style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: Arial; mso-font-kerning: 0pt;">图<span lang="EN-US">3. GEPON</span>下行<span lang="EN-US">TDM</span>技术</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">数据从<span lang="EN-US">OLT</span>到多个<span lang="EN-US">ONU</span>广播式下行，根据<span lang="EN-US">IEEE802.3</span>协议，每一个包的包头表明是给<span lang="EN-US">ONU</span>（<span lang="EN-US">ONU1</span>、<span lang="EN-US">ONU2</span>、<span lang="EN-US">ONU3......ONUN</span>）中的唯一一个。另外，部分包可以是给所有的<span lang="EN-US">ONU</span>（广播式）或者特殊的一组<span lang="EN-US">ONU</span>（组播），在光分路器处，流量分成独立的三组信号，每一组载有所有指定<span lang="EN-US">ONU</span>的信号。当数据信号到达该<span lang="EN-US">ONU</span>时，它接收给它的包，摒弃那些给其它<span lang="EN-US">ONU</span>的包。举例，图<span lang="EN-US">3</span>中，<span lang="EN-US">ONU1</span>收到包<span lang="EN-US">1</span>、<span lang="EN-US">2</span>、<span lang="EN-US">3</span>，但是它仅仅发送包<span lang="EN-US">1</span>给终端用户<span lang="EN-US">1</span>，摒弃包<span lang="EN-US">2</span>和包<span lang="EN-US">3</span>。<span lang="EN-US"><shape id="_x0000_i1028" type="#_x0000_t75" alt="" style="WIDTH: 24pt; HEIGHT: 24pt;"></shape></span></span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="center" style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">图<span lang="EN-US">4. </span>上行<span lang="EN-US">TDMA</span>技术</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">图<span lang="EN-US">4</span>中所示，<span lang="EN-US">GEPON</span>上行采用时分多址复用技术（<span lang="EN-US">TDMA</span>），分时隙给<span lang="EN-US">ONU</span>管理上行流量，时隙是同步的，以便当数据信号耦合到一根光纤时各个<span lang="EN-US">ONU</span>的上行包不会互相干扰。<span lang="EN-US">ONU</span>在<span lang="EN-US">ONU</span>指定的时隙上行数据给<span lang="EN-US">OLT</span>，采用时分多址复用避免数据传输冲突，即上行采用争用方式，下行采用广播方式。 </span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">EPON</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">上行传输是多个<span lang="EN-US">ONU</span>时分复用上行带宽，因此恰当的带宽分配机制必不可少。<span lang="EN-US">EPON</span>上行系统的带宽分配可考虑两种机制：静态带宽分配和动态带宽分配。静态带宽分配对带宽采取固定配置的方式，系统按照各<span lang="EN-US">ONU</span>预定的带宽进行初始配置，运行期间其值保持不变。静态分配时若为保证传输性能以各<span lang="EN-US">ONU</span>的峰值速率为基准分配带宽，因各<span lang="EN-US">ONU</span>的数据流往往不是同时处于峰值速率而导致整个系统带宽没有充分利用，系统资源利用率降低。若以数据的平均速率为基准分配带宽，常常会出现当某些<span lang="EN-US">ONU</span>有大的突发数据分组到来时，数据不能及时发送出去，从而导致数据的丢包率和时延增加；而另外一些<span lang="EN-US">ONU</span>的数据流量却小于平均速率，系统资源没有被充分利用。由此可见当<span lang="EN-US">EPON</span>承载突发性很强的数据业务时，静态带宽分配的效率是比较低的。动态带宽分配（<span lang="EN-US">DBA</span>，<span lang="EN-US">Dynamic Bandwidth Assignment</span>）对带宽采取实时调度方式，系统按照<span lang="EN-US">ONU</span>实时上报的请求统筹安排，动态调整授权给<span lang="EN-US">ONU</span>的带宽，从而充分利用系统资源，同时改善时延等性能。<span lang="EN-US"><p></p></span></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">由于光纤信道时延较大的特点，<span lang="EN-US">ONU</span>与<span lang="EN-US">OLT</span>之间的距离将会影响到上行信道的复用，如果准确测量各个<span lang="EN-US">ONU</span>到<span lang="EN-US">OLT</span>的距离并能精确的调整<span lang="EN-US">ONU</span>的发送时延，则可以减小<span lang="EN-US">ONU</span>发送窗口间的间隔，从而提高上行信道的利用率并减小时延。<span lang="EN-US"><p></p></span></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">GEPON</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">系统是使用<span lang="EN-US">TDMA</span>方式支持点到多点拓扑，因此所有的<span lang="EN-US">ONU</span>都必须和<span lang="EN-US">OLT</span>的全网时间保持同步，由于不同的<span lang="EN-US">ONU</span>与<span lang="EN-US">OLT</span>的距离各有不同，因此为了避免冲突，对此作一定的补偿，通过计算不同<span lang="EN-US">ONU</span>到<span lang="EN-US">OLT</span>之间的往返时间<span lang="EN-US">(RTT)</span>来完成这一功能。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">OLT</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">和<span lang="EN-US">ONU</span>都有一个本地计数器提供本地时间戳。<span lang="EN-US">OLT</span>发送<span lang="EN-US">MPCP</span>协议帧时，把计数器的值复制到报文的时间戳字段，在<span lang="EN-US">ONU</span>接收到<span lang="EN-US">MPCP</span>协议帧时，设置它本地计数器的值为接收到的<span lang="EN-US">MPCP</span>协议帧的时间戳字段的值。通过这个过程，所有的<span lang="EN-US">ONU</span>都同步到<span lang="EN-US">OLT</span>的时钟。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">同样，<span lang="EN-US">ONU</span>发送<span lang="EN-US">MPCP</span>协议帧时，也要把它本地计数器的值复制到<span lang="EN-US">MPCP</span>协议帧的时间戳字段，<span lang="EN-US">OLT</span>接收到<span lang="EN-US">MPCP</span>协议帧后，用所接收到的时间戳来计算和验证<span lang="EN-US">OLT</span>和<span lang="EN-US">ONU</span>之间的<span lang="EN-US">RTT</span>，图<span lang="EN-US">5</span>演示了<span lang="EN-US">RTT</span>的计算方法。<span lang="EN-US"><shape id="_x0000_i1029" type="#_x0000_t75" alt="" style="WIDTH: 24pt; HEIGHT: 24pt;"></shape></span></span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" align="left" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 24.1pt; TEXT-ALIGN: left; mso-pagination: widow-orphan;"><span lang="EN-US" style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">&nbsp;</span><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">图<span lang="EN-US">5. RTT</span>的计算方法</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;"><p></p></span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-SIZE: 9pt; COLOR: black; FONT-FAMILY: 新宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt;">总之，<span lang="EN-US">GEPON</span>是几个最佳的技术和网络结构的结合。<span lang="EN-US">EPON</span>以以太网为载体，采用点到多点结构、无源光纤传输方式，下行速率目前可达到<span lang="EN-US">10Gb/s</span>，上行以突发的以太网包方式发送数据流。另外，<span lang="EN-US">GEPON</span>也提供一定的运行维护和管理<span lang="EN-US">(OAM)</span>功能。<span lang="EN-US">GEPON</span>技术和现有的设备具有很好的兼容性，而且<span lang="EN-US">GEPON</span>还可以轻松实现带宽<span lang="EN-US">10Gb/s</span>的平滑升级。新发展的服务质量<span lang="EN-US">(QoS)</span>技术使以太网对语音、数据和图像业务的支持成为可能，这些技术包括全双工支持、优先级<span lang="EN-US">(p802.lp)</span>和虚拟局域网<span lang="EN-US">(VLAN)</span>。由于以太网的广泛应用，<span lang="EN-US">EPON</span>一经出现就受到众多设备厂商的青睐。</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: 新宋体;"><p></p></span></p><p></p>