WO2010105475A1 - 一种无源光网络时间传递的方法及系统 - Google Patents

一种无源光网络时间传递的方法及系统 Download PDF

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Publication number
WO2010105475A1
WO2010105475A1 PCT/CN2009/074351 CN2009074351W WO2010105475A1 WO 2010105475 A1 WO2010105475 A1 WO 2010105475A1 CN 2009074351 W CN2009074351 W CN 2009074351W WO 2010105475 A1 WO2010105475 A1 WO 2010105475A1
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Prior art keywords
time
pulse
onu
olt
processing module
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PCT/CN2009/074351
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English (en)
French (fr)
Inventor
陆建鑫
王静璇
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中兴通讯股份有限公司
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Priority to EP09841758.7A priority Critical patent/EP2410672B1/en
Priority to US13/257,466 priority patent/US8725002B2/en
Priority to ES09841758.7T priority patent/ES2524600T3/es
Publication of WO2010105475A1 publication Critical patent/WO2010105475A1/zh

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0682—Clock or time synchronisation in a network by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
    • H04Q11/0062—Network aspects
    • H04Q11/0067—Provisions for optical access or distribution networks, e.g. Gigabit Ethernet Passive Optical Network (GE-PON), ATM-based Passive Optical Network (A-PON), PON-Ring
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0652—Synchronisation among time division multiple access [TDMA] nodes, e.g. time triggered protocol [TTP]
    • H04J3/0655—Synchronisation among time division multiple access [TDMA] nodes, e.g. time triggered protocol [TTP] using timestamps
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
    • H04Q11/0062—Network aspects
    • H04Q2011/0079—Operation or maintenance aspects
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
    • H04Q11/0062—Network aspects
    • H04Q2011/0088—Signalling aspects

Definitions

  • the present invention relates to the field of point-to-multipoint passive optical network (PON) communication, and more particularly to a method and system for accurately transmitting time on a passive optical network.
  • PON point-to-multipoint passive optical network
  • Optical access technology provides large bandwidth and high reliability, which is the development direction of access technology.
  • PON is one of the main technologies for optical access, among which A/BP0N, EP0N and GP0N have been scaled up.
  • PON technology has the full service access capability of data, voice and TDM (Time Division Multiplex) to meet the evolving wireless access requirements.
  • TDM Time Division Multiplex
  • the convergence characteristics of the PON can make the entire network layer clear, convenient to construct, and efficient to manage.
  • Mobile base stations such as CDMA2000, TD-SCDMA, and WiMAX have strict requirements for time synchronization.
  • Mobile networks provide some value-added services that also require strict time synchronization.
  • mobile radio base stations mainly use wireless timing methods such as GPS. In building a high-quality wireless network, the use of wired network timing is of great economic and stability significance.
  • the Precision Time Protocol ( ⁇ ) in IEEE1588 has been widely used in wireless base stations.
  • the ⁇ protocol works better when the network is delayed and symmetric. If it is an asymmetric network, the asymmetric node needs to be processed.
  • ⁇ is an uplink and downlink delay asymmetric network, which directly processes the ⁇ protocol, has a large workload, a complicated processing flow, and occupies more network bandwidth.
  • one of the main purposes of the present invention is to provide a method for time transmission of a passive optical network, which is used to solve the problem that the processing flow is complicated, the protocol workload is large, and the network is occupied when the time is transmitted using the ⁇ protocol.
  • Technical issues of bandwidth In order to achieve the above object, the technical solution of the present invention is achieved as follows:
  • a method for time transfer of a passive optical network comprising: generating a periodic second pulse time stamp based on a local reference counter and a time information TOD above a second trigger;
  • the OLT transmits the ranging information, the periodic second pulse time stamp, and the TOD to the ONU;
  • the ONU predicts the next second time based on the periodic second pulse time stamp, TOD and ranging information and outputs a corresponding second pulse.
  • the step of the ONU predicting the next second time and outputting the corresponding second pulse comprises: the ONU estimating the second pulse period corresponding to the local reference counter according to the periodic second pulse time stamp;
  • the ONU estimates the time corresponding to the next second pulse timestamp according to the second pulse period, the ranging information, and the currently received second pulse timestamp, and outputs a second pulse based on the local reference counter;
  • the time corresponding to the next second pulse timestamp is the sum of the currently received second pulse timestamp and the second pulse period minus one-half of the ranging information value.
  • the above method further includes:
  • the OLT periodically performs ranging on the attached ONU. If the ranging information of the ONU is changed, the new ranging information is transmitted to the ONU.
  • the ranging information is unicast and transmitted to the ONU corresponding to the ranging information through a management plane or a service plane, and the periodic second pulse timestamp and the TOD are multicast transmitted to all the downlinks through the management plane or the service plane. ONU.
  • the present invention also provides an optical line terminal supporting time transfer of a passive optical network, where the optical line terminal OLT includes an OLT time processing module.
  • the OLT time processing module is configured to generate a periodic second pulse time stamp and a time information TOD based on the local reference counter under the trigger of the periodic second pulse; performing ranging on the optical network unit ONU to generate ranging information; Time stamp of the second pulse, time information TOD or more And the measured ranging information is transmitted to the ONU.
  • the 0LT time processing module includes:
  • a time source selection processing module configured to select an external time source and convert the external time source into a unified system clock, a second pulse PPnS, and a time information TOD or more;
  • An OLT processor configured to perform ranging to the ONU according to the passive optical network protocol and obtain ranging information; and generate a second pulse time based on the local reference counter according to the system clock and the second pulse PPnS input by the time source selection processing module Poke
  • An OLT controller configured to form a data packet by using a second pulse time stamp generated by the OLT processor, a time information TOD output by the time source selection processing module, and ranging information measured by the OLT processor, and passing through the OLT processor Transfer to the ONU.
  • the OLT processor includes:
  • a timestamp generating module configured to generate a second pulse timestamp based on the local reference counter according to the second pulse PPnS transmitted by the time source selection processing module;
  • a ranging module configured to implement a ranging function of the ONU according to a passive optical network protocol to obtain ranging information
  • a protocol processing module configured to perform processing functions of a passive optical network protocol.
  • the timestamp generating module includes a reference counter
  • the timestamp generating module is configured to use a latch to generate a second pulse timestamp, and latch a value of the reference counter on a rising edge of the second pulse;
  • the timestamp generating module is further configured to use a multipoint control protocol MPCP counter as a reference counter in the EPON; in the GPON, a combination of a superframe counter and an intra counter in the Ident field in the GTC frame of the transport convergence layer is used as Base counter.
  • the OLT time processing module is configured to transmit the ranging information to the ONU corresponding to the ranging information through a management plane or a service plane in a unicast manner; and time-stamp and time-seconds in a broadcast manner.
  • the information TOD is transmitted to all ONUs through the management plane or service plane;
  • the OLT time processing module is further configured to periodically perform ranging on the ONU, if found When the value of the ranging information of the ONU changes, new ranging information is transmitted to the ONU.
  • the present invention also provides an optical network unit supporting time transfer of a passive optical network, where the optical network unit ONU includes an ONU time processing module.
  • the ONU time processing module is configured to predict a next second time and output a corresponding second pulse according to a periodic second pulse time stamp transmitted by the optical line terminal OLT, time information TOD above the second time and ranging information.
  • the ONU time processing module includes:
  • An ONU processor configured to receive a periodic second pulse time stamp transmitted by the OLT, time information TOD and ranging information, and provide a reference counter for the time synchronization module;
  • An ONU controller configured to estimate a second pulse period based on a local reference counter according to a periodic second pulse time stamp received by the ONU processor; according to the second pulse period, ranging information, and a currently received second pulse The timestamp estimates the time corresponding to the next second pulse timestamp;
  • the time synchronization processing module is configured to receive a time corresponding to a next second pulse time stamp transmitted by the ONU controller, compare it with a reference counter in the ONU processor, and synchronously output a second pulse and time information.
  • the working state of the time synchronization processing module is: a following state or a self-generated state
  • the time synchronization processing module is configured to compare the next second pulse time set by the ONU controller with the reference counter output by the ONU processor, and output the second pulse PPmS when the two are equal, and output the current TOD.
  • the time synchronization processing module is configured to output the second pulse PPmS and TOD information according to the previous second pulse period and the second pulse time.
  • the present invention also provides a system for time transfer of passive optical networks, the system comprising an optical line termination as described above and an optical network unit as described above.
  • the invention combines the ranging characteristics of the PON in the time transmission process, and the time transmission accuracy is high.
  • the hardware cost of the OLT and the ONU is low.
  • the PON itself uses a single-copy broadcast to deliver time information, and the occupied bandwidth is extremely small.
  • Figure 1 is a system structure diagram of a PON architecture
  • FIG. 2 is a structural diagram of an OLT time processing module system in the precise time transfer system of the present invention
  • FIG. 3 is a structural diagram of an ONU time processing module system in the precise time transfer system of the present invention.
  • FIG. 1 is a system structure diagram of a PON architecture.
  • the PON architecture is generally composed of an OLT (Optical Line Terminal), an Optical Distribution Network (ODN), and an Optical Network Unit (ONU).
  • OLT Optical Line Terminal
  • ODN Optical Distribution Network
  • ONU Optical Network Unit
  • the distance between the OLT and the ONU is relatively long.
  • the EPON Ethernet Passive Optical Network
  • the time provider is typically a mobile time server or GPS.
  • the dotted frame portion is part of the technical solution of the present invention.
  • the OLT time processing module in the present invention is located in the OLT, and the ONU time processing module is located in the ONU.
  • the purpose of the present invention is to accurately transmit the time information on the OLT side to the ONU side.
  • the time error is required to be less than 3 microseconds, and in the TDS-CDMA system, the error requirement is not more than 1.5 microseconds.
  • the technical solution of the invention can realize that the time transfer error from the OLT to the ONU does not exceed 100 nanoseconds.
  • the time information is divided into seconds pulse (PPnS) and time information above the second in the PON system.
  • TOD TOD
  • PON PON characteristics for delivery.
  • the core idea of the present invention is to efficiently, economically and accurately transmit time information using PON point-to-multipoint and ranging characteristics.
  • the OLT time processing module is configured to divide the current time into a second pulse PPnS and a time information TOD above a second, wherein the periodic second pulse generation is based on the local end.
  • the second pulse timestamp of the reference counter, and the second pulse timestamp and the second time time information TOD generation corresponding data packet is broadcasted to all the ONUs connected below; and the distance measurement information is obtained for each ONU connected to the next according to the relevant protocol. That is, the RTT (Round Trip Time), and the RTT value is sent to each ONU in unicast mode.
  • OLT time processing mode The block includes: an OLT time source selection processing module, an OLT processor, and an OLT controller.
  • the OLT time source selection processing module is configured to perform time source selection, wherein the time source may be a wireless timing or a wired network timing, and the submodule outputs a unified format system clock, time information TOD (Time Of Day) and The second pulse PPnS (Pulse Per n Second, n is a positive integer, generally 1, 2), the TOD and the second pulse constitute accurate time information, and the OLT time source selection processing module can also output time state information, which includes:
  • clock source type level, status
  • clock module lock indication tilt, free running, and lock
  • the TOD contains the year, month, day, hour, minute, and second corresponding to the current second pulse.
  • the OLT time source selection processing module outputs the system clock as a reference clock of the OLT device.
  • the system clock does not have to be synchronized with the time source. If it is synchronized, it will help the ONU time information output to be more accurate.
  • the OLT time source selection processing module outputs a second pulse PPnS, which is used by the trigger timestamp generation module to generate a second pulse timestamp corresponding to the second pulse, and the second pulse simultaneously triggers the OLT controller as a second pulse timestamp and a second time or longer. Trigger information for information TOD transmission.
  • the OLT processor is used to complete all functions of the OLT interface of the passive optical network.
  • the module provides the ranging function of the ONU for time transfer, and obtains the RTT value of the ranging information. Since the passive optical network uses the time division multiplexing communication method on the uplink, accurate ranging for each ONU is a prerequisite for the normal operation of the passive optical network.
  • the OLT processor module further includes: a timestamp generation module, a ranging module, and a protocol processing module.
  • the ranging module is configured to complete the ranging function according to the passive optical network protocol, and obtain the ranging information RTT; the protocol processing module is configured to complete the processing function of the passive optical network protocol; and the timestamp generating module is configured to select and process according to the time source.
  • the second pulse PPnS transmitted by the module generates a second pulse time stamp based on the local reference counter.
  • the timestamp generation module includes a reference counter, and the reference counter may use different forms in different PON systems.
  • the selection principle is that the reference counter of the OLT is associated with the reference counter of the ONU, and generally there is a relatively fixed difference. This difference is related to hardware processing time and physical link
  • the return time (RTT) is related, and the reference counter between the OLT and the ONU is about RTT/2.
  • the timestamp generation module typically uses a latch to generate a second pulse timestamp, which latches the value of the reference counter on the rising edge of the second pulse.
  • the present invention selects the MPCP (Multipoint Control Protocol) counter defined by IEEE 802.3 as the reference counter.
  • the EPON sends an MPCP data unit
  • the current MPCP counter value is passed to the ONU.
  • the ONU receives the MPCP data unit, compares the received MPCP value with the local counter value, and updates the local MPCP counter if there is a difference.
  • the difference between the ONU and the MPCP counter of the OLT is RTT/2 + Ac, where Ac is the difference caused by the uplink and downlink asymmetry during the whole backhaul processing, and the difference is generally small, which can be tested by the value. Partial compensation.
  • the reference counter implemented by the present invention is a combination of a super frame counter and an intra counter in the Ident field in the GPON Transmission Convergence (GTC) frame.
  • GTC GPON Transmission Convergence
  • the frame is transmitted at a frequency of 8K, and the frame header contains a 30-bit superframe counter, which is incremented by one for each frame transmitted.
  • the intra counter counts from the beginning of the GTC frame at a frequency of 155.52 MHz with a period of 19,400.
  • the ONU side reads the current superframe value from the GTC frame. From the time of receiving the GTC frame header, it also accumulates the intraframe counter by itself at a frequency of 155.52 MHz.
  • the GPON ranging mechanism is different from EPON.
  • the focus is on the total backhaul time to the OLT. Therefore, the RTT contains the ONU transmission processing time Ts, so the total delay between the ONU and the OLT reference counter is about (RTT- Ts)/2, the actual value can be compensated by the test.
  • the OLT controller is configured to obtain the RTT value of each ONU from the OLT processor, and send it to each ONU through the OLT processor in a unicast manner. When the RTT of an ONU changes, the ONU is notified again.
  • the OLT controller receives the second pulse trigger information of the time source selection processing module, reads the second pulse time stamp generated by the timestamp generation module, and broadcasts the OLT processor together with the TOD and the time state output by the time source selection processing module. Send to all ONUs. Of course, it can also be sent separately.
  • the time-related information sent by the OLT controller may be communicated through the PON data (service) plane or through the PON management plane.
  • a management plane is used.
  • an extended OAM frame is used in the EPON, and is transmitted through the OMCC (ONU Manage and Control Channel) in the implementation of the GPON.
  • the OLT controller can fine tune the timestamp and the RTT value of the ONU according to the specific system characteristics, so that the time information output by the ONU is more accurate.
  • the ONU time processing module is configured to receive time related information (second pulse timestamp, TOD and time state information, etc.) and RTT value transmitted by the OLT side, and pass the cycle. Estimate and next second time prediction, output the corresponding accurate second pulse.
  • time related information second pulse timestamp, TOD and time state information, etc.
  • the ONU time processing module includes an ONU processor, an ONU controller, and a time synchronization processing module.
  • the ONU processor is used to complete all functions of the ONU interface of the passive optical network.
  • the module cooperates with the OLT time processing module to complete the ranging function, receives the time related information sent by the OLT time processing module, and provides a reference for the time synchronization processing module. counter.
  • the ONU controller is used to perform the following functions:
  • the time corresponding to the next second pulse timestamp is estimated (the currently received second) Pulse time stamp + period ⁇ - RTT/2 ), and set the time value to the time synchronization processing module;
  • the time synchronization processing module is configured to receive a time corresponding to a next second pulse time stamp transmitted by the ONU controller, compare it with a reference counter in the ONU processor, and synchronously output a second pulse and time information.
  • the time synchronization processing module has two working states, a following state and a self-generating state.
  • the next second pulse time set by the ONU controller is compared with the reference counter output by the ONU processor. When they are equal, the second pulse PPmS is output, and the current TOD information is output.
  • Self-generating state The second pulse and TOD information are output by itself according to the previous second pulse period and second pulse time. This working state is caused by a loss or abnormal condition of the OLT time source. In the self-generated state, the OLT time source may be temporarily lost or anomalous. At this time, the ONU can continuously output time information and maintain the accuracy of the time information for a certain period of time.
  • the method for transmitting time of a passive optical network comprises: generating a periodic second pulse time stamp based on the local reference counter and a time information TOD above the second trigger under the trigger of the sexual second pulse;
  • the OLT transmits the ranging information, the periodic second pulse time stamp, and the TOD to the ONU;
  • the ONU predicts the next second time based on the periodic second pulse time stamp, TOD and ranging information and outputs a second pulse corresponding to the next second time.
  • the time information output by the ONU time processing module of the present invention has a relatively high precision.
  • the time ambiguity introduced by the whole system is related to the clock period of the reference counter.
  • the deviation between the ONU time and the OLT is less than 2 count clock cycles.
  • EPON/GPON reference counters are listed in the specific facilities, but do not constitute a limitation of the present invention, nor are they limited to EPON and GPON.
  • the above description is only for the preferred embodiment of the present invention, and is not intended to limit the present invention, and various changes and modifications may be made by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and scope of the present invention are intended to be included within the scope of the present invention.
  • the time information output by the ONU time processing module of the present invention has a relatively high precision.
  • the time ambiguity introduced by the whole system is related to the clock period of the reference counter.
  • the deviation between the ONU time and the OLT is less than 2 count clock cycles.

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Description

一种无源光网络时间传递的方法及系统 技术领域
本发明涉及点到多点无源光网络（ Passive Optical Network, PON )通信 领域， 尤其涉及一种在无源光网络上^"确传递时间的方法和系统。
背景技术
光接入技术提供大带宽， 高可靠性， 是接入技术的发展方向。 PON是光 接入的主要技术之一， 其中 A/BP0N、 EP0N和 GP0N都已得到了规模应用。
随着数据业务的飞速发展， 数据接入的可移动性要求不断突现， 有线、 无线融合将更好的满足用户体验。 PON技术具备数据、 语音和 TDM ( Time Division Multiplex )全业务接入能力， 能够满足不断演进中的无线接入需求。 特别是在微蜂窝、 家庭基站接入应用中， PON的收敛特性能使整个网络层次 清楚、 施工方便、 管理高效。
CDMA2000, TD-SCDMA以及 WiMAX等移动基站对时间同步有严格的 要求。 移动网络提供一些增值业务是也需要严格的时间同步。 目前移动基站 上主要使用无线授时方式如 GPS等技术。 在构建高质量的无线网络中， 使用 有线网络授时在经济、 稳定性上都有重要的意义。
在网络 IP化的进程中 , IEEE1588中的网络精确定时协议（ Precision Time Protocol , ΡΤΡ )在无线基站上得到了广泛应用。 但是 ΡΤΡ协议在网络上下 行延时对称情况下工作比较好， 如果是非对称网络需要对非对称节点进行处 理。 ΡΟΝ是一个上下行延迟非对称网络， 在 ΡΟΝ上直接处理 ΡΤΡ协议工作 量大、 处理流程复杂， 且占用较多的网络带宽。
发明内容
有鉴于此， 本发明的主要目的之一在于提供一种无源光网络时间传递的 方法，用于解决在 ΡΟΝ上使用 ΡΤΡ协议进行时间传递时处理流程复杂、协议 工作量大、 占用较多网络带宽的技术问题。 为达到上述目的， 本发明的技术方案是这样实现的：
一种无源光网络时间传递的方法， 包括： 性秒脉冲的触发下产生基于本端基准计数器的周期性的秒脉冲时间戳和秒以 上时间信息 TOD;
OLT将所述测距信息、 周期性的秒脉冲时间戳及 TOD传送给 ONU;
ONU根据所述周期性的秒脉冲时间戳、 TOD及测距信息预测下一秒时 间并输出对应秒脉冲。
进一步地， ONU预测下一秒时间并输出对应秒脉冲的步骤包括： ONU根据所述周期性的秒脉冲时间戳估算对应于本端基准计数器的秒 脉冲周期；
ONU根据所述秒脉冲周期、测距信息及当前接收到的秒脉冲时间戳估算 出下一个秒脉冲时间戳对应的时刻， 并输出基于本地基准计数器的秒脉冲； 其中，
所述下一个秒脉冲时间戳对应的时刻为当前接收到的秒脉冲时间戳与所 述秒脉冲周期之和减去二分之一的测距信息值。
进一步地， 上述方法还包括：
OLT周期性对下挂的 ONU进行测距，若发现有 ONU的测距信息发生变 化， 则将新的测距信息传送给该 ONU。
进一步地， 所述测距信息通过管理平面或业务平面单播传送给该测距信 息对应的 ONU , 所述周期性的秒脉冲时间戳及 TOD通过管理平面或业务平 面多播传送给所有下挂的 ONU。
本发明还提供一种支持无源光网络时间传递的光线路终端， 所述光线路 终端 OLT包括 OLT时间处理模块，
所述 OLT时间处理模块设置成在周期性秒脉冲的触发下产生基于本端基 准计数器的周期性的秒脉冲时间戳和秒以上时间信息 TOD; 对光网络单元 ONU进行测距生成测距信息； 将所述秒脉冲时间戳、 秒以上时间信息 TOD 及测得的测距信息传送给 ONU。
进一步地， 所述 0LT时间处理模块包括：
时间源选择处理模块， 其设置成选择外部时间源并将外部时间源转换成 统一的系统时钟、 秒脉冲 PPnS及秒以上时间信息 TOD；
OLT处理器， 其设置成按照无源光网络协议实现对 ONU测距并得到测 距信息； 以及根据所述时间源选择处理模块输入的系统时钟及秒脉冲 PPnS 产生基于本地基准计数器的秒脉冲时间戳；
OLT控制器， 其设置成将 OLT处理器产生的秒脉冲时间戳、 时间源选择 处理模块输出的秒以上时间信息 TOD 以及 OLT处理器测得的测距信息形成 数据包并通过所述 OLT处理器传送给 ONU。
进一步地， 所述 OLT处理器包括：
时间戳生成模块， 其设置成根据所述时间源选择处理模块传送的秒脉冲 PPnS产生基于本地基准计数器的秒脉冲时间戳；
测距模块，其设置成按照无源光网络协议实现对 ONU的测距功能，得到 测距信息；
协议处理模块， 其设置成完成无源光网络协议的处理功能。
进一步地， 所述时间戳生成模块包含有基准计数器，
所述时间戳生成模块是设置成釆用锁存器来产生秒脉冲时间戳， 在秒脉 冲的上升沿锁存基准计数器的值；
所述时间戳生成模块还设置成在 EPON中釆用多点控制协议 MPCP计数 器作为基准计数器； 在 GPON中釆用传输汇聚层 GTC帧中的 Ident字段中的 超帧计数器和帧内计数器的组合作为基准计数器。
进一步地，所述 OLT时间处理模块是设置成以单播方式将测距信息通过 管理平面或业务平面传送给该测距信息所对应的 ONU; 以及以广播方式将秒 脉冲时间戳及秒以上时间信息 TOD 通过管理平面或业务平面传送给所有 ONU;
所述 OLT 时间处理模块还设置成周期性对 ONU进行测距， 若发现有 ONU的测距信息值发生变化， 则将新的测距信息传送给该 ONU。 本发明还提供一种支持无源光网络时间传递的光网络单元， 所述光网络 单元 ONU包括 ONU时间处理模块，
所述 ONU时间处理模块设置成依据光线路终端 OLT传递的周期性的秒 脉冲时间戳、秒以上时间信息 TOD及测距信息预测下一秒时刻并输出对应秒 脉冲。
进一步地， 所述 ONU时间处理模块包括：
ONU处理器， 其设置成接收 OLT传送的周期性的秒脉冲时间戳、 秒以 上时间信息 TOD及测距信息， 并为时间同步模块提供基准计数器；
ONU控制器， 其设置成根据所述 ONU处理器接收的周期性的秒脉冲时 间戳估算基于本端基准计数器的秒脉冲周期； 根据所述秒脉冲周期、 测距信 息及当前接收到的秒脉冲时间戳估算下一个秒脉冲时间戳对应的时刻；
时间同步处理模块，其设置成接收所述 ONU控制器传送的下一个秒脉冲 时间戳对应的时刻，并将其与所述 ONU处理器中的基准计数器比较， 同步输 出秒脉冲及时间信息。
进一步地， 所述时间同步处理模块的工作状态为： 跟随状态或自行产生 状态；
跟随状态下，所述时间同步处理模块是设置成将所述 ONU控制器设定的 下一个秒脉冲时刻与 ONU处理器输出的基准计数器比较，两者相等则输出秒 脉冲 PPmS, 同时输出当前 TOD信息；
自行产生状态下， 所述时间同步处理模块是设置成依据之前的秒脉冲周 期和秒脉冲时刻 , 自行输出秒脉冲 PPmS和 TOD信息。
本发明还提供一种无源光网络时间传递的系统， 所述系统包括如上所述 的光线路终端和如上所述的光网络单元。
本发明在时间传递过程中结合了 PON的测距特性， 时间传递精确度高，
OLT、 ONU硬件成本低， 利用 PON本身单拷贝广播下发时间信息， 占用带 宽极少。 附图概述
图 1为 PON架构的系统结构图；
图 2为本发明精确时间传递系统中 OLT时间处理模块系统结构图； 图 3为本发明精确时间传递系统中 ONU时间处理模块系统结构图。
本发明的较佳实施方式
以下结合附图对本发明的优选实施例进行说明， 应当理解， 此处所描述 的优选实施例仅用于说明和解释本发明， 并不用于限定本发明。
图 1 为 PON架构的系统结构图， PON构架一般由 OLT ( Optical Line Terminal, 光线路终端） 、 ODN ( Optical Distribution Network, 光分配网络） 和 ONU ( Optical Network Unit, 光网络单元）三部分组成。 OLT与 ONU距 离一般比较远， EPON ( Ethernet Passive Optical Network , 以太无源光网络） 可达 20Km, GPON ( Gigabit-Capable PON )可达 60Km。 时间提供者一般为 移动时间服务器或者 GPS。 虚线框部分是本发明技术方案所涉部分， 本发明 中的 OLT时间处理模块位于 OLT中， ONU时间处理模块位于 ONU中。
本发明的目的在于将 OLT 侧的时间信息精确的传递到 ONU侧， 在 CDMA2000系统中， 要求时间误差不超过 3微秒， 在 TDS-CDMA系统中该 误差要求不超过 1.5微秒，而通过本发明的技术方案能够实现从 OLT到 ONU 的时间传递误差不超过 100纳秒。
PON 系统中将时间信息分为秒脉冲 （PPnS ) 以及秒以上的时间信息
( TOD ) , 结合 PON特点进行传递。 本发明的核心思想是： 利用 PON点到 多点以及测距的特性进行高效、 经济、 精确地传递时间信息。
图 2为本发明精确时间传递系统中的 OLT时间处理模块的结构图， OLT 时间处理模块用于将当前时间分为秒脉冲 PPnS和秒以上时间信息 TOD, 其 中周期性的秒脉冲产生基于本端基准计数器的秒脉冲时间戳， 并将秒脉冲时 间戳及秒以上时间信息 TOD生成相应数据包广播给所有下挂的 ONU; 以及 按照相关协议对下挂的每个 ONU测距，得到测距信息即环回距离 RTT( Round Trip Time ) , 并将 RTT值通过单播方式下发给每个 ONU。 OLT时间处理模 块包括： OLT时间源选择处理模块、 OLT处理器及 OLT控制器。
OLT时间源选择处理模块用于进行时间源选择， 其中时间源可以是无线 授时， 也可以是有线网络授时， 该子模块输出统一格式的系统时钟、 秒以上 的时间信息 TOD ( Time Of Day )和秒脉冲 PPnS ( Pulse Per n Second , n为 正整数， 一般为 1,2 ) , TOD和秒脉冲构成了精确的时间信息， 同时 OLT时 间源选择处理模块还可输出时间状态信息， 其包括：
{时钟源类型、 等级、 状态； 时钟模块锁定指示 （跟踪、 自由运行和锁 定） }
TOD包含当前秒脉冲所对应的年、 月、 日、 时、 分、 秒。
OLT时间源选择处理模块输出系统时钟作为 OLT设备的参考时钟。系统 时钟不是必须与时间源同步， 如果同步， 则有利于 ONU 时间信息输出更为 准确。
OLT时间源选择处理模块输出秒脉冲 PPnS,该秒脉冲用于触发时间戳生 成模块产生该秒脉冲对应的秒脉冲时间戳， 该秒脉冲同时触发 OLT控制器， 作为秒脉冲时间戳和秒以上时间信息 TOD传送的触发信息。
OLT处理器用于完成无源光网络 OLT接口所有功能，在本发明中该模块 为时间传递提供 ONU的测距功能， 得到测距信息 RTT值。 由于无源光网络 上行釆用时分复用通信方式，对每个 ONU准确的测距是无源光网络正常工作 的前提。
OLT处理器模块进一步包括： 时间戳生成模块、 测距模块、 协议处理模 块。
测距模块用于按照无源光网络协议完成测距功能， 得到测距信息 RTT; 协议处理模块用于完成无源光网络协议的处理功能； 时间戳生成模块用于根 据所述时间源选择处理模块传送的秒脉冲 PPnS产生基于本地基准计数器的 秒脉冲时间戳。
时间戳生成模块包含有基准计数器，基准计数器在不同的 PON系统中可 能会釆用不同的形式， 选择原则是 OLT的基准计数器与 ONU的基准计数器 相关联， 一般来说是存在相对固定差值， 该差值与硬件处理时间和物理链路 回程时间 （RTT )相关， OLT与 ONU的基准计数器差约 RTT/2。 时间戳生成模块一般釆用锁存器来产生秒脉冲时间戳， 在秒脉冲的上升 沿锁存基准计数器的值。
对于基准计数器在 EPON具体实现上， 本发明选择 IEEE802.3所定义的 MPCP (多点控制协议 )计数器作为基准计数器。 在 EPON发送 MPCP数据 单元时， 将把当前 MPCP计数器值传递给 ONU。 ONU接收到 MPCP数据单 元，将接收的 MPCP值与本地计数器值作比较，如果有差异则更新本地 MPCP 计数器。依据 EPON测距原理， ONU和 OLT的 MPCP计数器之间相差 RTT/2 + Ac, 其中 Ac为整个回程处理过程中上下行非对称引起的差异， 该差异一 般较小， 可通过测试对该值进行部分补偿。
对于基准计数器在 GPON具体实现上， 本发明所实施的基准计数器的是 GPON传输汇聚层（ GPON Transmission Convergence, GTC )帧中的 Ident字 段中的超帧 (Super frame)计数器和帧内计数器的组合。 在 GPON下行帧中， 帧以 8K的频率发送， 帧头中包含 30bit的超帧计数器， 每发送一帧增加 1。 帧内计数器按照 155.52MHz的频率， 从 GTC帧开始时计数， 周期为 19,400。 ONU侧从 GTC 帧内读取当前超帧值， 从接收到 GTC 帧头开始， 也按照 155.52MHz的频率， 自行累加帧内计数器。 GPON测距机制与 EPON有所不 同， 其关注的是到达 OLT的总的回程时间， 因此 RTT中包含了 ONU的发送 处理时间 Ts, 因此 ONU与 OLT基准计数器之间总的延迟约为 (RTT-Ts)/2, 实际值可以通过测试进行补偿。
OLT控制器用于从 OLT处理器中获取每个 ONU的 RTT值，用单播的方 式通过 OLT处理器发给每个 ONU。 当某个 ONU的 RTT发生改变时， 重新 通知 ONU。
OLT控制器接收到时间源选择处理模块的秒脉冲触发信息， 读取时间戳 生成模块生成的秒脉冲时间戳，并与时间源选择处理模块输出的 TOD和时间 状态一起用广播方式通过 OLT处理器发送给所有的 ONU。 当然也可以分开 发送。
OLT控制器发送的上述时间相关信息可以通过 PON数据（业务）平面通 信， 也可以通过 PON管理平面通信。 本发明的具体实施中釆用了管理平面， 如具体在 EPON中釆用了扩展的 OAM帧， 在 GPON的实施中通过 OMCC ( ONU Manage and Control Channel )传递。 OLT控制器可以根据具体系统特 性，对时间戳和 ONU的 RTT值进行微调， 以便使 ONU输出的时间信息更为 准确。
图 3为本发明精确时间传递系统中的 ONU时间处理模块， ONU时间处 理模块用于接收 OLT侧传递的时间相关信息（秒脉冲时间戳、 TOD及时间状 态信息等）和 RTT值， 并通过周期估算和下一秒时间预测， 输出对应的准确 秒脉冲。
ONU时间处理模块包括 ONU处理器、 ONU控制器、时间同步处理模块。 ONU处理器用于完成无源光网络 ONU接口所有功能， 在本发明中该模 块按照协议配合 OLT时间处理模块完成测距功能、 接收 OLT时间处理模块 发送的时间相关信息以及为时间同步处理模块提供基准计数器。
ONU控制器用于完成如下功能：
1 , 接收 ONU处理器从 OLT侧得到的秒脉冲时间戳、 测距信息 RTT、 TOD和时间状态信息；
2, 根据 OLT侧周期发送的秒脉冲时间戳估算基于本端基准计数器的秒 脉冲周期7 Ϊ ; 估算方法为： 将每两个相邻秒时间戳相减， 得到当前计数器的 周期， 根据历史的周期取平均值从而获得 ϊ。
3 , 根据本 ONU估算的秒脉冲周期 ϊ、 从 OLT侧接收到的本 ONU的测 距信息 RTT及当前接收到的秒脉冲时间戳估计下一个秒脉冲时间戳对应的时 刻（当前接收到的秒脉冲时间戳 +周期 Ϊ - RTT/2 ) , 并将该时刻值设置到时 间同步处理模块中；
4, 根据当前秒脉冲时间戳所对应的 TOD, 在其基础上增加 1秒， 得到 下一秒的 TOD值， 并设置到时间同步处理模块中。
5, 根据接收到的时间状态信息设置时间同步处理模块的工作状态。
时间同步处理模块用于接收所述 ONU控制器传送的下一个秒脉冲时间 戳对应的时刻，并将其与所述 ONU处理器中的基准计数器比较， 同步输出秒 脉冲及时间信息。 时间同步处理模块有两种工作状态， 跟随状态和自行产生 状态。 跟随状态： 将 ONU控制器设定的下一个秒脉冲时刻与 ONU处理器输出 的基准计数器比较， 两者相等则输出秒脉冲 PPmS, 同时输出当前 TOD信息。
自行产生状态： 依据之前的秒脉冲周期和秒脉冲时刻， 自行输出秒脉冲 和 TOD信息。 该工作状态在 OLT时间源丟失或异常情况产生。 在自行产生 状态下， OLT时间源可能临时丟失或产生异常， 这时 ONU仍能连续输出时 间信息， 并且在一定时间内维持时间信息的精度。
本发明提供的一种无源光网络时间传递的方法包括： 性秒脉冲的触发下产生基于本端基准计数器的周期性的秒脉冲时间戳和秒以 上时间信息 TOD;
OLT将所述测距信息、 周期性的秒脉冲时间戳及 TOD传送给 ONU;
ONU根据所述周期性的秒脉冲时间戳、 TOD及测距信息预测下一秒时 间并输出下一秒时间对应的秒脉冲。
本发明 ONU时间处理模块输出的时间信息具备了相当高的精度。整个系 统引入的时间模糊度与基准计数器的时钟周期相关， 在补偿较好的系统中， ONU时间与 OLT之间偏差小于 2个计数时钟周期。
在具体设施中列举了 EPON/GPON在基准计数器的选择和实现方式， 但 不构成对本发明的限制， 也不局限 EPON和 GPON。 以上所述仅为本发明的 优选实施例而已， 并不用于限制本发明， 对于本领域的技术人员来说， 本发 明可以有各种更改和变化。 凡在本发明的精神和原则之内， 所作的任何修改、 等同替换、 改进等， 均应包含在本发明的保护范围之内。
工业实用性
本发明 ONU时间处理模块输出的时间信息具备了相当高的精度。整个系 统引入的时间模糊度与基准计数器的时钟周期相关， 在补偿较好的系统中， ONU时间与 OLT之间偏差小于 2个计数时钟周期。

Claims

权 利 要 求 书
1、 一种无源光网络时间传递的方法， 其包括：
性秒脉冲的触发下产生基于本端基准计数器的周期性的秒脉冲时间戳和秒以 上时间信息 TOD;
OLT将所述测距信息、 周期性的秒脉冲时间戳及 TOD传送给 ONU;
ONU根据所述周期性的秒脉冲时间戳、 TOD及测距信息预测下一秒时 间并输出下一秒时间对应的秒脉冲。
2、 根据权利要求 1所述的方法， 其中， 预测下一秒时间并输出下一秒时 间对应的秒脉冲的步骤包括：
ONU根据所述周期性的秒脉冲时间戳估算对应于本端基准计数器的秒 脉冲周期；
ONU根据所述秒脉冲周期、测距信息及当前接收到的秒脉冲时间戳估算 出下一个秒脉冲时间戳对应的时刻， 并输出基于本地基准计数器的秒脉冲； 其中
所述下一个秒脉冲时间戳对应的时刻为当前接收到的秒脉冲时间戳与所 述秒脉冲的周期之和减去二分之一的测距信息值。
3、 根据权利要求 1或 2所述的方法， 其还包括， OLT周期性对下挂的 ONU进行测距， 若发现有 ONU的测距信息发生变化， 则将新的测距信息传 送给测距信息发生变化的 ONU。
4、 根据权利要求 3所述的方法， 其中， 所述测距信息通过管理平面或业 务平面单播传送给该测距信息对应的 ONU, 所述周期性的秒脉冲时间戳及 TOD通过管理平面或业务平面多播传送给所有下挂的 ONU。
5、 一种支持无源光网络时间传递的光线路终端， 所述光线路终端 OLT 包括 OLT时间处理模块，
所述 OLT时间处理模块设置成在周期性秒脉冲的触发下产生基于本端基 准计数器的周期性的秒脉冲时间戳和秒以上时间信息 TOD; 对光网络单元 ONU进行测距生成测距信息； 将所述秒脉冲时间戳、 TOD及测得的测距信 息传送给 ONU。
6、 根据权利要求 5所述的光线路终端， 其中， 所述 OLT时间处理模块 包括：
时间源选择处理模块， 其设置成选择外部时间源并将所选择的外部时间 源转换成统一的系统时钟、 秒脉冲 PPnS及 TOD；
OLT处理器， 其设置成按照无源光网络协议实现对 ONU测距并得到测 距信息； 以及根据所述时间源选择处理模块输入的系统时钟及秒脉冲 PPnS 产生基于本地基准计数器的秒脉冲时间戳；
OLT控制器， 其设置成将 OLT处理器产生的秒脉冲时间戳、 时间源选择 处理模块输出的 TOD 以及 OLT处理器测得的测距信息形成数据包并通过所 述 OLT处理器传送给 ONU。
7、 根据权利要求 6所述的光线路终端， 其中， 所述 OLT处理器包括： 时间戳生成模块， 其设置成根据所述时间源选择处理模块传送的秒脉冲 PPnS产生基于本地基准计数器的秒脉冲时间戳；
测距模块，其设置成按照无源光网络协议实现对 ONU的测距功能，得到 测距信息；
协议处理模块， 其设置成完成无源光网络协议的处理功能。
8、 根据权利要求 7所述的光线路终端， 其中， 所述时间戳生成模块包含 有基准计数器，
所述时间戳生成模块是设置成釆用锁存器来产生秒脉冲时间戳， 并在秒 脉冲的上升沿锁存所述基准计数器的值；
所述时间戳生成模块还设置成在以太无源光网络 EPON中釆用多点控制 协议 MPCP计数器作为基准计数器； 在千兆比无源光网络 GPON中釆用传输 汇聚层 GTC帧中的 Ident字段中的超帧计数器和帧内计数器的组合作为基准 计数器。
9、 根据权利要求 5所述的光线路终端， 其中， 所述 OLT时间处理模块是设置成以单播方式将测距信息通过管理平面或 业务平面传送给该测距信息所对应的 ONU; 以及以广播方式将秒脉冲时间戳 及 TOD通过管理平面或业务平面传送给所有下挂的 ONU;
所述 OLT 时间处理模块还设置成周期性对 ONU进行测距， 若发现有 ONU的测距信息值有变化，则将新的测距信息传送给测距信息值发生变化的 ONU。
10、 一种支持无源光网络时间传递的光网络单元， 所述光网络单元 ONU 包括 ONU时间处理模块，
所述 ONU时间处理模块设置成依据光线路终端 OLT传递的周期性的秒 脉冲时间戳、 TOD及测距信息预测下一秒时刻并输出下一秒时刻对应的秒脉 冲。
11、 根据权利要求 10所述的光网络单元， 其中， 所述 ONU时间处理模 块包括：
ONU处理器， 其设置成接收所述 OLT传送的周期性的秒脉冲时间戳、 秒以上时间信息 TOD及测距信息， 并为时间同步模块提供基准计数器；
ONU控制器， 其设置成根据所述 ONU处理器接收的周期性的秒脉冲时 间戳估算基于本端基准计数器的秒脉冲周期； 根据所述秒脉冲周期、 测距信 息及当前接收到的秒脉冲时间戳估算下一个秒脉冲时间戳对应的时刻；
时间同步处理模块，其设置成接收所述 ONU控制器传送的下一个秒脉冲 时间戳对应的时刻，并将其与所述 ONU处理器中的基准计数器比较， 同步输 出秒脉冲及时间信息。
12、 根据权利要求 11所述的光网络单元， 其中， 所述时间同步处理模块 的工作状态为： 跟随状态或自行产生状态；
跟随状态下，所述时间同步处理模块是设置成将所述 ONU控制器设定的 下一个秒脉冲时刻与 ONU处理器输出的基准计数器比较，两者相等则输出秒 脉冲， 同时输出当前 TOD信息；
自行产生状态下， 所述时间同步处理模块是设置成依据之前的秒脉冲周 期和秒脉冲时刻， 自行输出秒脉冲和 TOD信息。
13、 一种无源光网络时间传递的系统， 所述系统包括如权利要求 5-9任 一项所述的光线路终端 OLT、 以及如权利要求 10-12任一项所述的光网络单 元 ONU。
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