中国铁通集团有限公司技术部杨涛YangTao,TechnologyDepartment, China Railcom
我国互联网宽带接入用户在2006年初就已达到3800万户，而且保持着良好的发展态势。这给光接入网络的发展奠定了良好的基础。据预测，2008年将成为推广FTTH技术应用的关键时期。
FTTH技术的发展与国外应用趋势
PON作为FTTH方式的一种实现技术，从1987年提出概念开始至今，已逐步形成了APON、BPON、EPON和GPON等4种类型。PON具有的可灵活地组成各种拓扑结构、下行数据采用TDM广播方式、上行数据采用TDMA方式以及技术架构在运用过程中易维护且不占用机房等优势，使其成为FTTH方式的首选实现技术之一。目前PON技术中EPON和GPON是应用的主流趋势。
EPON即以太网PON，是2000年底由IEEE802．3工作组提出的系列标准。EPON采用的基于以太网技术的封装方式，使其非常适合承载IP类业务。EPON采用单纤波分复用技术，可提供上下行对称1.25Gbit/s的传输速率，并逐步实现了优先级、VLAN。EPON是一种点到多点的物理和逻辑拓扑结构，运营商在采用该技术来实现FTTH方式时必须对MAC加以改造，使其更适于发展FTTN+DSL方式。
目前，我国在EPON技术的研究上已位居世界领先水平。作为国家“863”计划重大项目的EPON，在关键技术上已取得突破，并降低了成本，从而使其在随后的商业化运作中取得了主动权，使国产EPON设备在应用中获得了很大的发展。与此相关，我国也在制定EPON设备的互通及网络管理方面的规范标准。
GPON即GigabitPON，是2001年由FSAN组织继APON/BPON后推动形成的ITU-TG.984系列标准，其技术实现要达到下行最大传输速率2.488Gbit/s，上行最大传输速率1.244Gbit/s，即工作速率高于1Gbit/s。GPON比EPON更注重对多业务的支持能力。GPON在TC层设计了ATM和GEM两种封装模式，改进了各种不同应用方式下的灵活调动性，并可实现全程的QoS保证，使业务达到TDM级的抖动和延迟性能。GPON在设计上具备点对点方式的传输和管理能力，可支持各种业务类型、速率模式和传输网络，因此由GPON技术来实现FTTH的方式已成为一种发展趋势。该方式虽然具有适应未来发展多业务的融合适配能力，但是它目前还存在的成熟度不够和价格偏高等不利因素，也导致它在目前还无法获得大规模的普及应用。
从国外的发展来看，日本、美国、欧洲和韩国的经验具有借鉴意义。其中，日本运营商在推动FTTH发展方面取得的成功最为显著。日本自己制定了PON的技术标准，而且日本政府早在2003年就采用FTTH技术开展E-Japan计划，并以此来加强宽带基础设施建设。2年后，日本FTTH的应用保有量超过了ADSL方式，其主要推广的模式是用PON网络方式建设FTTH。
美国也为家庭用户提供FTTH方式。虽然美国政府在管制和税收方面制定了促进FTTH发展的有利政策，但是其进展缓慢，目前在广大的农村地区仍多采用FTTP的方式，其每个用户的接入带宽可达30Mbit/s左右。在欧洲，新开展宽带业务的荷兰、瑞典正在积极发展FTTH的方式，而技术上一直采用DSL方式的英国、德国却没有跟上FTTH发展的步伐。在韩国，开展FTTH主要靠内容提供商来推动，包括视频点播、互动游戏等节目。
从总体来看，政府在FTTH的发展中起了很大的作用，特别是在提供保障其内容运营合法的政策以及为内容共享提供有效监管等方面，表现得尤其突出。
FTTH技术在我国发展的考虑
随着通信技术的发展，目前仍存在以ADSL为代表的电域接入方式和以PON为代表的光域接入方式两种方式。以PON为代表的光接入方式实现FTTH，网络结点少而且施工简单，其性能和安全可靠性都相对较高。然而，目前FTTH在我国还处于起步阶段，采用PON设备及铺设成本都很高。另外，随着ADSL技术的成熟，使FTTH中直接使用PON做为终端接入设备在应用上还远远不如ADSL应用的普及。因此FTTH方式还需要长时间的市场培育和发展。而FTTN中光纤是到达电缆交接箱所在处，一般可覆盖多个楼群，其包容性强、优势明显、发展空间巨大。FTTN方式可以涵盖FTTP(光纤到驻地)、FTTB(光纤到大楼)等方式，其与DSL一起使用便于小区的宽带改造，目前有PON实现FTTH、FTTN+LAN和FTTN+DSL等3种方式可供选择。
具体采用什么样的组合方式取决于FTTH部署的具体要求。在我国的具体实施中，应考虑两方面因素：其一，光网络接入应用的发展取决于用户承受的成本。光纤用户终端设备(ONU/ONT)所处的位置关系到应用中的组网结构与成本。光纤越靠近用户桌面，其提供的带宽越大，每户分摊的成本也越高。其二，光网络接入应用发展取决于用户的消费带宽的情况。据预测，在今后10年中家庭用户带宽消耗将达到20Mbit/s左右，而ADSL2+正好能满足该需求。
结合网络的改造投入和具体技术应用来看，采用FTTN＋DSL方式作为宽带接入已成为大多数用户的普遍选择。ADSL2/2+技术的成熟和设备价格下降，使ADSL2+成为宽带接入“最后一公里”的主流发展趋势。对已有铜线的小区可以大力发展FTTN+DSL方式的宽带接入，而对不具备电缆线的新小区，可以尝试直接采用FTTH的方式。目前，虽然房地产商也介入到FTTH的技术推广中来，但是其实际要达到的目的仅是借此来提升小区的档次，并未能为用户提供多种业务。因此，FTTH方式的核心推动力在于网络的有效运营，尤其是丰富内容的提供上。
展望
从国际发展趋势来看，虽然以GPON为代表的FTTH技术最具发展潜力，但是目前GPON设备价格较高，不易在国内大规模推广。而基于光纤接入以太网技术和铜线传输技术相结合的手段，即EPON技术实现FTTN的方式与DSL接入的结合，因其相对成熟且廉价，并能提供稳定高速的宽带业务接入和强大的业务管理能力，将成为我国宽带接入手段中的主要方式。
Preliminary Investigation of FTTH Technology and Its Industry
The number of China’s Internet broadband access subscribers had surged up to 38 million by the beginning of 2006, and has been maintaining a good development momentum. This lays down a good foundation for the development of optical access networks. It is predicted that 2008 will become a key year to popularize FTTH applications.
Advances in FTTHTechnology and the Trend of Its Application Abroad
PONasaway of realizing FTTH technology has gradually formed 4 types of APON, BPON, EPON and GPON since its concept was put forth in 1987. PON’s advantages of allowing flexibly forming all types of topology, downlink data adopting TDM broadcasting mode, uplink data adopting TDWA mode and its technical architecture easy to maintain and occupying no machine room during its use makes it one of first choices in realizing FTTH. Presently EPON and GPON represent the mainstream trend of application of PON technology.
EPON,whichstandsfor Ethernet PON, is a series standard the IEEE802.3 Working Group put forward late 2000. The Ethernet-based encapsulation mode EPON adopts makes it very good for bearing IP services. EPON adopts single-fiber WDM technology, can provide uplink and downlink symmetrical 1.25Gbit/s transmission rate, and gradually realizes priority level, VLAN. EPON is a kind of point-to-multipoint logical topology. In adopting this technology to realize FTTH, operators must transform MAC to make it better suit FTTN+DSL.
Presently,Chinahasrank among the advanced in the world in the studies of EPON technology. EPON as a major project in the national 863 Program has made breakthroughs in key technologies and reduced cost, thus acquiring the initiative in follow-up commercial operation, and enabling domestic EPON equipment to make much headway in application. China is also formulating standards for EPON equipment interworking and network management.
GPON,shortforGigabit PON, is ITU-T G.984 series standard formed following APON/BPON in 2001. Its technological realization reaches downlink maximum transmission rate of 2.488Gbit/s, and uplink maximum transmission rate of 1.244Gbit/s, namely, operating rate higher than 1Gbit/s. GPON pays greater attention to multi-service support ability than EPON. GPON designs two encapsulation modes of ATM and GEM at the TC layer, improves flexible dispatching in different application modes, and can realize all-distance QoS assurance, enables services to achieve TDM-level jitter and delay performance. GPON has point-to-point transmission and management capability in design, can support various types of service, rate modes and transmission networks. Therefore, realizing FTTH with GPON technology has become a development trend. Despite its adaptability to future development of multi-service, this mode still has unfavorable factors including insufficient maturity and higher price, which results in its failure to be applied in a big way.
Asregardsinternationaldevelopments, experience in Japan, the U.S., Europe and South Korea that merits attention. The success Japanese operators have achieved in promoting FTTH development is most prominent.
Japan has formulated its own PON standard. And the Japanese Government adopted FTTH technology to launch E-Japan project as early as 2003, and used it to strengthen broadband infrastructure construction. Two years later, FTTH applications exceeded ADSL in Japan. Its main way of popularization is to construct FTTH with PON.
The U.S. also provides FTTH for home subscribers. The U.S. government formulates favorable policies for FTTH development in regard of regulation and taxation. But progress has been slow. In the vast rural areas, FTTP remains the main mode, whose access bandwidth per subscriber can reach about 30Mbit/s. In Europe, Holland and Sweden where broadband service is newly launched are actively developing FTTH, and the U.K. and Germany that used to adopt DSL do not keep pace with FTTH development. In South Korea, FTTH is mainly promoted by CPs, including VOD and interactive games.
Over all,governmentsplaya big role in the development of FTTH, especially in providing policies to ensure legal content operation and providing effective supervision and control for content sharing.
Considerations On FTTH Development in China
With the development of telecom technologies, there still exist two access methods of electric access represented by ADSL and optical access represented by PON. Realizing FTTH by using optical access method represented by PON features less network nodes and simple construction. Its performance and security and reliability are all relatively high. However, FTTH remains at its starting stage in China, and the cost of both PON equipment and installation is very high. In addition, with the maturity of ADSL technology, direct use of PON as terminal access equipment in FTTH is not so popular as ASDL applications. Therefore, FTTH mode still requires long-time market cultivation and development. And the fiber in FTTH reaches where the cable cabinet is, and generally can cover a number of clusters of buildings. It features strong containing ability, marked advantages and huge scope for development. FTTN may contain FTTP (fiber to the premises), FTTB (fiber to the building) and other modes. Its use together with DSL will facilitates broadband transformation in residential districts. At present there are three options of FTTH realized with PON, FTTN+LAN and FTTN+DSL.
The specific combination depends on specific requirements of FTTH deployment. In specific implementation in China, two factors should be considered: first, the development of optical access application depends on the cost subscribers afford. The location of fiber user terminal equipment (ONU/ONI) has a bearing on the networking architecture and cost in applications. The more closer to the user desktop the fiber is, the larger the bandwidth provided will be, and the higher the cost shared by per user will be. Second, the development of optical network access application depends on the consumption of bandwidth by users. It is predicted that bandwidth consumption by home subscriber in ten years to come will reach some 20Mbit/s, and ADSL2+ is precisely able to meet this demand.
In light of network renovation investment and specific technological applications, adopting FTTN+DSL as broadband access has become general choice by most subscribers. The maturity of ADSL2/2+ technology and the drop of equipment price make ADSL2+ mainstream development trend of broadband access “last mile”. For residential districts that have had copper wires, FTTN+DSL mode can be vigorously developed, and for new residential districts without cable lines, we may try to adopt FTTH mode. Presently, though real estate developers are involved in FTTH promotion, their real purpose is only to upgrade the residential district, it fails to provide multiple services for users. Therefore, the core driving force of FTTH mode lies in effective operation of the network, particularly in the provision of rich content.
Prospect
According to the international development trend, FTTH technology represented by GPON has the greatest development potential, but GPON equipment price is currently high, large-scale popularization is not easy in China. But the means combining fiber access-based Ethernet technology with copper wire transmission technology, namely, combination of realizing FTTN with EPON technology with DSL access, thanks to its being relatively mature and cheap, and its ability to provide stable, high-speed broadband service access and strong service manageability, will become the chief mode in China’s broadband access means.
