WO2010088830A1 - 无源光网络系统的时间同步方法及其同步系统 - Google Patents

无源光网络系统的时间同步方法及其同步系统 Download PDF

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WO2010088830A1
WO2010088830A1 PCT/CN2009/075787 CN2009075787W WO2010088830A1 WO 2010088830 A1 WO2010088830 A1 WO 2010088830A1 CN 2009075787 W CN2009075787 W CN 2009075787W WO 2010088830 A1 WO2010088830 A1 WO 2010088830A1
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Prior art keywords
time
onu
olt
information
time information
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PCT/CN2009/075787
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English (en)
French (fr)
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袁立权
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中兴通讯股份有限公司
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Priority to JP2011546570A priority Critical patent/JP5314768B2/ja
Priority to US13/141,330 priority patent/US8768169B2/en
Priority to ES09839547.8T priority patent/ES2561901T3/es
Priority to BRPI0920477-6A priority patent/BRPI0920477B1/pt
Priority to EP09839547.8A priority patent/EP2372932B1/en
Publication of WO2010088830A1 publication Critical patent/WO2010088830A1/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
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0667—Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/12—Arrangements providing for calling or supervisory signals
    • 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

Definitions

  • the present invention relates to the field of passive optical network technologies, and in particular to a time synchronization method for a passive optical network system and a synchronization system thereof.
  • Time synchronization in a communication system means that the time of the communication device is coordinated with the world time, also called phase synchronization.
  • Communication networks have requirements for synchronization between communication systems, especially for wireless devices.
  • Table 1 lists the specific requirements for synchronization of wireless devices:
  • Passive Optical Network technology is a point-to-multipoint optical access technology. It consists of an OLT (Optical Line Terminal) on the central office and an ONU (Optical Network Unit) on the user side. Optical network unit) and 00 ⁇ Optical Distribution Network, optical distribution network).
  • OLT Optical Line Terminal
  • ONU Optical Network Unit
  • Optical network unit Optical Network Unit
  • 00 ⁇ Optical Distribution Network, optical distribution network There are many types of passive optical networks, which can be roughly divided into a wavelength division PON, a power PON, and a hybrid PON that are combined with each other.
  • the power PON is divided into APON (ATM Passive Optical Network) due to different link layer protocols.
  • Source optical network GPON (Gigabit Passive Optical Network) and EPON (Ethernet Passive Optical Network).
  • Power ⁇ generally adopts TDM (Time Division Multiplex) broadcast mode for downlink, and adopts TDMA (Time Division Multiple Access) for uplink, and can flexibly form tree, star, and bus.
  • a topographic structure such as a type (a typical structure is a tree structure).
  • hybrid PON means that multiple power PONs exist simultaneously in one ODN network, and these power PONs use different wavelengths.
  • FIG. 1 is a schematic diagram of the networking and clock transmission of the PON in the mobile communication field.
  • Figure 1 shows the typical networking of the PON in mobile communication.
  • the PON needs to complete the transfer of two tasks: the delivery of traffic and the delivery of clock synchronization.
  • Figure 1 shows the way in which clock streams are transmitted and distributed.
  • the PON network protocol itself only implements frequency synchronization, that is, the ONU is synchronized with the OLT, but the phase difference of each ONU is arbitrary, that is, it cannot support time synchronization, and cannot satisfy the mobile communication or network application with time synchronization requirements.
  • IEEE1588 can achieve time synchronization between network devices.
  • Point-to-point links provide the highest accuracy, introducing a boundary clock, independent of delay jitter, and achieving a time accuracy of 10 ns per hop;
  • Fig. 2 shows a schematic diagram of the IEEE 1588 protocol implementation mechanism.
  • the master device sends a time-stamped synchronization packet to the slave device, Slave.
  • the device receives the synchronization packet at its local T2 time.
  • the following equation can be established.
  • the Delay is the delay when the synchronization packet is transmitted from the master device to the slave device.
  • T2 T1+Delay +Offset ( 1 )
  • the Slave device sends a Delay_Req (delay request) message to the master device at the local T3 time;
  • the Master device sends a Delay_Resp message to the Slave device at its local T4 time.
  • the following equation can be established:
  • T4 T3+Delay -Offset ( 2 )
  • the IEEE1588 protocol is based on the assumption that the network characteristics are symmetric, that is, the delay and jitter from the master device to the Salve device are the same as the delay and jitter from the slave device to the master device, and the link jitter size directly determines the time synchronization. Precision.
  • the GP0N network that is, the P0N network of the ITU G.984.1 ⁇ 4 specification, uses the protocol stack of the GP0N network carrying Ethernet protocol packet as shown in FIG.
  • the GEM (GPON Encapsulation Method) layer can perform packet segmentation and reassembly of the upper layer service data stream, so that one packet of the upper layer data stream may be divided into multiple copies. Each one is sent at a different time point that is not fixed.
  • the receiving end detects whether all the fragments of the data packet are aligned, and after assembly, assembles and recovers the complete upper layer data message.
  • the GP0N network is a network with an unfixed delay. That is, its jitter is very large, and the IEEE1588 protocol cannot be directly operated on the GPON network. Therefore, in the passive optical network, the ONU cannot be accurately synchronized with the OLT in time. Summary of the invention
  • the present invention aims to provide a time synchronization method for a passive optical network system to solve the problem that the ONU cannot be accurately synchronized to the OLT in time in a passive optical network.
  • a time synchronization method in a passive optical network configured to perform time synchronization on an optical line terminal (OLT) to an optical network unit (ONU), the method comprising:
  • the OLT and the ONU use a management channel to transmit time information.
  • the time information includes a transmission delay between the OLT and the ONU and a sending time identification information of the time information.
  • the ONU calculates a time offset from the OLT according to the time information, and performs time synchronization with the OLT.
  • the management channel is a physical layer operation management and maintenance (PLOAM) message channel; and the sending time identification information of the time information is a sending moment of the PLOAM message.
  • PLOAM physical layer operation management and maintenance
  • the management channel is an Optical Network Element Management Control Interface (OMCI) message channel;
  • OMCI Optical Network Element Management Control Interface
  • a time deviation between the ONU and the OLT is: a difference between a sum of a transmission time of the time information and the transmission delay and a local time of the ONU.
  • the information transmission delay is obtained by the ranging function of the passive optical network system; or the PLOAM message channel is used to simulate the IEEE1588 protocol for measurement.
  • the present invention also provides a time synchronization system in a passive optical network, the system comprising:
  • the optical line terminal time synchronization receiving processing module is configured to implement time synchronization with the upper device;
  • the optical line terminal time information sending module is configured to calculate the transmission between the OLT and the ONU And transmitting the time information to the ONU through the management channel;
  • the time information includes a transmission delay between the OLT and the ONU and a sending time identification information of the time information;
  • the time synchronization receiving and processing module of the optical network unit is configured to receive time information sent by the OLT, and calculate a time offset from the OLT according to the time information, and perform time synchronization with the OLT.
  • the optical line terminal time information sending module transmits the time information to the ONU by using a PLOAM message channel, and the sending time identification information of the time information is a sending time of the PLOAM message.
  • the optical line terminal time information sending module transmits the time information to the ONU by using an OMCI message channel, where the sending time identification information of the time information is a sending time of a frame that triggers time synchronization and a frame of the frame. number.
  • the time deviation is: a difference between a transmission time of the time information and a sum of the transmission delay and the local time of the ONU.
  • the information transmission delay is obtained by the ranging function of the passive optical network system; or the PLOAM message channel is used to simulate the IEEE1588 protocol for measurement.
  • the invention utilizes the GPON-specific POLAM message channel or the OMCI message channel to realize the transmission of time in the PON segment, and the transmission of the PLOAM message or the OMCI message in the PON segment is not fragmented, thereby avoiding the GPON network to the upper layer protocol datagram.
  • the technical flaws caused by the unfixed delay of the text ensure that each ONU can be accurately synchronized to the OLT.
  • FIG. 1 is a schematic diagram of networking and clock transmission of a PON in a mobile communication field
  • FIG. 2 is a schematic diagram of an IEEE 1588 protocol implementation mechanism
  • FIG. 3 is a protocol stack of a GPON network carrying Ethernet protocol packet;
  • FIG. 5 is a block diagram of a time synchronization system in a passive optical network according to an embodiment of the present invention; flow chart. BEST MODE FOR CARRYING OUT THE INVENTION
  • a method for synchronizing time in a passive optical network including: performing time synchronization on an optical line terminal to an optical network unit, and further comprising: transmitting:
  • the time information includes a transmission delay between the optical line terminal and the optical network unit, and the transmission delay can be obtained by using a ranging function of the PON system, or using a PLOAM (Physical Layer Operation Administration Maintenance) Message 7 carries the same information as the IEEE 1588 protocol, and the IEEE 1588 protocol is simulated for measurement.
  • PLOAM Physical Layer Operation Administration Maintenance
  • the management channel may be a PLOAM message channel or an ONU Management and Control Interface (OMCI) message channel.
  • OMCI ONU Management and Control Interface
  • Step S402 Obtain a time offset (Offset) between the optical line terminal and the optical network unit by calculation;
  • Step S403 Perform frequency and time synchronization with the optical line terminal by using the received time information on the optical network unit;
  • the time information further includes a sending moment of the PLOAM message, that is, a local time of the OLT sent by the PLOAM message; and the sending time of the PLOAM message included in the received PLOAM message by the ONU Transmission delay or information used to calculate the transmission delay, calculate and obtain the time offset value (Offset) between the OLT and the ONU, and synchronize the frequency and time with the calculated time offset value.
  • a sending moment of the PLOAM message that is, a local time of the OLT sent by the PLOAM message
  • the sending time of the PLOAM message included in the received PLOAM message by the ONU Transmission delay or information used to calculate the transmission delay calculate and obtain the time offset value (Offset) between the OLT and the ONU, and synchronize the frequency and time with the calculated time offset value.
  • Offset time offset value
  • the PLOAM message includes information of seconds and nanoseconds.
  • the PLOAM message containing time information needs to be sent periodically.
  • the time information further includes a sending moment of the frame in which the OLT transmits a certain trigger time synchronization and a frame number of the frame.
  • the ONU After receiving the frame that triggers the time synchronization, the ONU calculates the time offset value (Offset) between the OLT and the ONU according to the transmission time and the transmission delay of the frame, and synchronizes the frequency and time with the calculated time offset value.
  • Offset time offset value
  • the method before the optical line terminal sends the message in the form of PLOAM to the optical network unit, the method further includes: performing time synchronization between the optical line terminal and the upper level device of the optical line terminal by using the method of the IEEE1588 protocol.
  • time synchronization is performed between the optical network unit and the downlink device of the optical network unit by using the method of the IEEE 1588 protocol. In turn, it is guaranteed to be connected to the optical network unit.
  • the IEEE1588 clock protocol is run between the OLT and the uplink device and between the ONU and the downlink device, and the PLOAM message is periodically used between the optical line terminal and the optical network unit in the PON segment of the OLT and the ONU.
  • Time transfer this embodiment runs in the PON segment, and synchronizes the time between the uplink device and the OLT and the downlink device and the ONU.
  • the GPON-specific POLAM message channel or the OMCI message channel is used to realize the transmission of the time in the PON segment, and the transmission of the PLOAM message or the OMCI message in the PON segment is not fragmented, and the PLOAM message is sent as a whole.
  • the GPON network avoids the delay of the upper layer protocol data packet delay, and realizes the synchronous transmission of time on the PON network to ensure that each ONU has the same time, if the standard operation is performed between the ONU and its lower-level devices.
  • the ONU can also accurately pass the time to the lower-level devices, ensuring that the devices connected to each ONU have the same time.
  • the system comprises: an optical line terminal time synchronization receiving processing module, and an optical line terminal time information transmission Time-synchronized receiving and processing modules for sending modules, fiber-optic systems that transmit time information, and optical network units:
  • the OLT time synchronization receiving processing module is configured to implement time synchronization with the upper device, and adjust the local time according to the time information sent by the upper device, so that the time of the OLT is accurately synchronized with the upper device.
  • the OLT time information sending module is configured to calculate a transmission delay of the OLT to the ONU, and transmit time information through the management channel.
  • the OLT can also send information for calculating the transmission delay of the OLT to the ONU to the ONU, and the ONU calculates the transmission delay of the OLT to the ONU.
  • the management channel of the PON system can be used to manage and maintain the message bearer time information.
  • the time information in this mode includes the time when the PLOAM message is sent and the transmission delay between the OLT and the ONU or the information used to calculate the transmission delay.
  • the OMCI channel of the GPON system can also be used for further management of the channel.
  • the time information in this mode includes the transmission delay between the OLT and the ONU, the transmission time of the frame in which the OLT sends the trigger time synchronization, and the frame number of the frame.
  • the fiber optic system that transmits time information is a point-to-multipoint fiber optic system and is a physical medium that carries time information.
  • the time synchronization receiving and processing module of the optical network unit is configured to receive time information sent by the OLT, and calculate a time deviation between the OLT and the ONU according to the time information, thereby synchronizing the local ONU time.
  • the method of time synchronization message of information overcomes the problem that the ONU cannot accurately synchronize with the OLT in time in the passive optical network, thereby achieving the effect of time synchronization in the passive optical network.
  • the flow chart, the PON network time synchronization mechanism implementation method of the preferred embodiment specifically includes: running a standard IEEE 1588 protocol between the OLT and the device connected thereto, and the upper device is operated. In the Master mode, the OLT runs in the Slave mode, and the OLT is accurately synchronized with the superior device in time;
  • the "superior device” sends a time-stamped synchronization (Sync) message to the OLT.
  • the OLT device receives the synchronization message at its local T2 time, and establishes the following equation, where Delay is the synchronization message from the "superior device” to The OLT is delayed in transmission.
  • T2 T1+Delay +Offset ( 1 )
  • the "superior device” sends a Follow_up message to the OLT;
  • the OLT sends a Delay_Req message to the "upper device" at the local T3 time;
  • the "superior device” sends a Delay_Resp message to the OLT at its local T4 time. Establish the following equation:
  • T4 T3+Delay -Offset ( 2 )
  • the Sync, Follow_up, Delay_Req, and Delay_Resp messages are sent between the "superior device" and the 0LT, and the Offset value can be dynamically updated.
  • the clock source can be used to provide the network reliability.
  • the source and backup clock sources are selected according to the IEEE15888 protocol.
  • 0LT device works in master mode of clock, 0NU runs in slave mode, 0NU is synchronously synchronized to 0LT in time; if PL0AM channel is used, 0LT sends time-stamped synchronous PL0AM message to 0NU at time T1, 0NU is at its local T2 time
  • the trigger is synchronized with the local time according to T2
  • the time offset (Offset) between the OLT and the ONU is the difference between the ONU local clock and T2
  • the ONU adjusts the local clock according to the Offset value.
  • the Delay can be obtained by the GP0N inherent ranging method. It can also be obtained by simulating the 1588 protocol, that is, using the PL0AM message to simulate the delay measurement process of 1588. The specific method is the same as the 1588 method of measuring the point-to-point link.
  • Each 0NU is synchronized to 0LT, and precise synchronization of time is achieved between each 0NU.
  • the standard IEEE1588 protocol is run between 0NU and "downlink devices".
  • the “downlink device” can be accurately synchronized to 0NU in time.
  • the “downlink device” is the base station.
  • the protocol message of the delivery time is transmitted and received by hardware.
  • modules or steps of the present invention can be implemented by a general-purpose computing device, which can be concentrated on a single computing device or distributed over a network composed of multiple computing devices. Alternatively, they may be implemented by program code executable by the computing device, such that they may be stored in the storage device by the computing device, or they may be separately fabricated into individual integrated circuit modules, or they may be Multiple modules or steps are made into a single integrated circuit module. Thus, the invention is not limited to any specific combination of hardware and software.

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  • Computer Networks & Wireless Communication (AREA)
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Description

无源光网络系统的时间同步方法及其同步系统 技术领域
本发明涉及无源光网络技术领域， 具体而言， 涉及一种无源光网络系 统的时间同步方法及其同步系统。 背景技术
通信系统中的时间同步是指通信设备的时间和世界时协调一致， 也称 相位同步。
通信网络对通信系统间的同步都有要求， 特别是无线设备， 表 1 列出 了无线设备对同步的具体要求：
Figure imgf000003_0001
表 1
无源光网络（Passive Optical Network, PON )技术是一种点到多点的 光接入技术， 它由局侧的 OLT ( Optical Line Terminal, 光线路终端）、 用户 侧的 ONU( Optical Network Unit,光网络单元）以及00^ Optical Distribution Network, 光分配网络）组成。 无源光网络有多种类型， 大致可以分为波分 PON、 功率 PON和这两种相互结合的混合 PON。 功率 PON又因链路层的 协议不同， 分为 APON ( ATM Passive Optical Network, 异步传输模式的无 源光网络）， GPON ( Gigabit Passive Optical Network, 千兆无源光网络）和 EPON ( Ethernet Passive Optical Network , 以太无源光网给）。
功率 ΡΟΝ,—般其下行采用 TDM ( Time Division Multiplex,时分复用 ) 广播方式，上行采用 TDMA ( Time Division Multiple Access,时分多址接入 ) 方式， 而且可以灵活地组成树型、 星型、 总线型等拓朴结构 （典型结构为 树形结构）。
所谓混合 PON是指在一个 ODN网络中同时存在多个功率 PON, 这些 功率 PON采用不同的波长。
PON网络可用做接入段的承载网络，图 1为 PON在移动通信领域的组 网及时钟传递的示意图，如图 1所示为 PON在移动通信中的典型组网。 PON 需要完成两个任务的传递： 业务流的传递和时钟同步的传递， 附图 1 中给 出了时钟流的传递和分配方式。
PON网络协议本身只实现了频率同步，即 ONU同步于 OLT,但各 ONU 的相位差是任意的， 即无法支持时间同步， 无法满足移动通信或有时间同 步要求的网络运用。
IEEE1588可以实现网络设备间的时间同步。
1 )基于网络特性对称的假设， 采用主从方式， 在 Packet上进行时间戳 标记， 周期时钟发布， 接收方进行时钟偏移测量与延迟测量；
2 )点对点的链路可提供最高的精度， 引进边界时钟， 与延迟抖动无关， 可达到每跳 10ns级时间准确度；
3 )可实现频率与时间同步。
下面描述其详细实现机理， 图 2示出了 IEEE1588协议实现机理的示意 图。
1 )假定 Master (主）和 Slave (从）设备间的时间差是 Offset;
2 )在 T1时刻 Master设备向 Slave设备发送带时间戳的同步包， Slave 设备在其本地 T2时刻接收到同步报文， 可以建立如下方程式， 其中 Delay 是同步报文从 Master设备到 Slave设备的传输时延迟。
T2=T1+Delay +Offset ( 1 )
3 ) Master设备向 Slave设备发送 Follow_up (跟踪 ) 消息；
4 ) Slave设备在本地 T3时刻向 Master设备发送 Delay_Req (延迟请求 ) 消息；
5 ) Master设备在其本地 T4时刻向 Slave设备发送 Delay_Resp (延迟 响应） 消息。 可以建立以下方程：
T4=T3+Delay -Offset ( 2 )
6 )联合方程（ 1 )和（2 )可以求出 Offset值。
7 )周期性的在 Master和 Slave设备间发送， Sync(同步）消息、 Follow_up (跟踪）消息、 Delay_Req (延迟请求）消息和 Delay_Resp (延迟响应）消 息， 就能动态更新 Offset值， 从而保持 Slave设备在时间上同步于 Master 设备。
8 ) Master设备和 Slave设备间可以获得 10ns级的时间准确度。
从上面描述可知， IEEE1588 协议是基于网络特性对称的假设， 即从 Master设备到 Salve设备的延迟和抖动， 与从 Slave设备到 Master设备的 延迟和抖动相同， 且链路抖动大小直接决定了时间同步的精度。
GP0N网络， 即 ITU G.984.1~4规范的 P0N网络， 其对以太网业务的 承载采用如图 3示出的 GP0N网络承载以太网协议包的协议栈。 为提高承 载效率， GEM ( GPON Encapsulation Method, 千兆无源光网络封装方法 ) 层可以对上层业务数据流进行包的切分和重组， 这样上层数据流的一个报 文有可能被切分成多份， 每一份在不固定的不同的时间点被发送， 收端检 测数据包的所有分片是否到齐， 到齐后进行组装， 恢复出完整的上层数据 报文。 然而， 对上层数据报文来说， GP0N 网络是个延迟不固定的网络， 即其抖动很大， IEEE1588协议无法直接在 GPON网络上运行， 因此， 在无 源光网络中不能保证 ONU在时间上精确同步于 OLT。 发明内容
有鉴于此， 本发明目的在于提供一种无源光网络系统的时间同步方法， 以解决在无源光网络中 ONU在时间上不能精确同步于 OLT的问题。
一种无源光网络中时间的同步方法， 用于光线路终端（OLT )向光网络 单元（ ONU )执行时间同步， 该方法包括：
所述 OLT与所述 ONU之间使用管理通道进行时间信息的传递； 所述时间信息包括所述 OLT和所述 ONU间的传输时延及所述时间信 息的发送时刻标识信息；
所述 ONU根据所述时间信息计算与所述 OLT之间时间偏差， 进行与 所述 OLT的时间同步。
进一步地， 所述管理通道为物理层操作管理维护（PLOAM )消息通道； 所述时间信息的发送时刻标识信息为 PLOAM消息的发送时刻。
进一步地， 所述管理通道为光网络单元管理控制接口 （OMCI )消息通 道； 所述时间信息的发送时刻标识信息为触发时间同步的帧的发送时刻以 及该帧的帧号。
进一步地， 所述 ONU与所述 OLT之间的时间偏差为： 所述时间信息 的发送时刻与所述传输时延之和与所述 ONU本地时间的差。
进一步地， 所述信息传输时延通过所述无源光网络系统的测距功能得 到； 或使用 PLOAM消息通道模拟 IEEE1588协议进行测量获得。
基于上述方法， 本发明还提出一种无源光网络中的时间同步系统， 该 系统包括：
光线路终端时间同步接收处理模块， 用于实现与上级设备的时间同步； 光线路终端时间信息发送模块， 用于计算 OLT与 ONU之间的传输时 延， 并通过管理通道传递时间信息给 ONU; 所述时间信息包括所述 OLT和 所述 ONU间的传输时延及所述时间信息的发送时刻标识信息；
光网络单元的时间同步接收和处理模块， 用于接收 OLT发送的时间信 息， 并根据所述时间信息计算与所述 OLT之间时间偏差， 进行与所述 OLT 的时间同步。
进一步地， 所述光线路终端时间信息发送模块， 使用 PLOAM 消息通 道传递所述时间信息给所述 ONU, 所述时间信息的发送时刻标识信息为 PLOAM消息的发送时刻。
进一步地， 所述光线路终端时间信息发送模块， 使用 OMCI消息通道 传递所述时间信息给所述 ONU, 所述时间信息的发送时刻标识信息为触发 时间同步的帧的发送时刻以及该帧的帧号。
进一步地， 所述时间偏差为： 所述时间信息的发送时刻与所述传输时 延之和与所述 ONU本地时间的差。
进一步地， 所述信息传输时延通过所述无源光网络系统的测距功能得 到； 或使用 PLOAM消息通道模拟 IEEE1588协议进行测量获得。
本发明利用 GPON特有的 POLAM消息通道或 OMCI消息通道， 实现 时间在 PON段的传递， PLOAM消息或 OMCI消息在 PON段的传输是不被 分片的， 从而避开了 GPON网络对上层协议数据报文延迟不固定的特性所 照成的技术缺陷， 保证各 ONU都能精确同步到 OLT。 附图说明
图 1为 PON在移动通信领域的组网及时钟传递的示意图；
图 2为 IEEE1588协议实现机理的示意图；
图 3为 GPON网络 载以太网协议包的协议栈； 图 5为本发明实施例的无源光网络中时间同步系统的方框图； 流程图。 具体实施方式 下面将参考附图并结合实施例， 来详细说明本发明。 图， 在本发明的实施例中， 提供了一种无源光网络中时间的同步方法， 包 括： 在光线路终端向光网络单元执行时间同步， 还包括如下步骤： 的传递；
所述时间信息包括光线路终端和光网络单元间的传输时延（ Delay ) ,该 传输时延可通过 PON 系统的测距功能得到， 或者使用 PLOAM ( Physical Layer Operation administration Maintenance, 物理层操作管理维护 ) 消息 7 载和 IEEE1588协议相同信息， 模拟 IEEE1588协议进行测量获得。
所述的管理通道可以是 PLOAM 消息通道或者使用光网络单元管理控 制接口 （ONU Management and Control Interface, OMCI ) 消息通道。
步骤 S402 : 通过计算获得光线路终端和光网络单元间的时间偏差 ( Offset );
步骤 S403: 在光网络单元上使用接收的时间信息与光线路终端进行频 率和时间同步；
( 1 )若所述管理通道使用 PLOAM, 则所述时间信息还包括 PLOAM 消息的发送时刻， 即 PLOAM消息发送的 OLT的本地时间； ONU通过接收 到的 PLOAM消息中包含的 PLOAM消息的发送时刻和传输时延或用于计 算获得传输时延的信息， 计算并获得 OLT 与 ONU 之间的时间偏差值 ( Offset ) , 并以计算获得的时间偏差值进行频率和时间的同步。
其中 PLOAM消息包括秒和纳秒的信息。
所述的包含时间信息的 PLOAM消息需要定期发送。 ( 2 )若所述管理通道使用 OMCI, 则所述时间信息还包括 OLT发送某 个触发时间同步的帧的发送时刻以及该帧的帧号。 ONU接收到触发时间同 步的帧后， 根据该帧的发送时刻及传输时延计算出 OLT与 ONU之间的时 间偏差值（ Offset ) , 并以计算获得的时间偏差值进行频率和时间的同步。
根据本发明实施例的同步方法， 在光线路终端向光网络单元发送 PLOAM形式的消息之前还可以包括： 采用 IEEE1588协议的方法在光线路 终端和光线路终端的上级设备间执行时间同步。
根据本发明实施例的同步方法，采用 IEEE1588协议的方法在光网络单 元和光网络单元的下联设备间执行时间同步。 进而保证与光网络单元相连
Figure imgf000009_0001
本发明实施例在 OLT 和上联设备间及 ONU 和下联设备间运行 IEEE1588时钟协议， 而在 OLT和 ONU的 PON段， 采用光线路终端与光 网络单元之间周期性使用 PLOAM形式的消息， 实现时间的传递， 该实施 例在 PON段运行， 把上联设备和 OLT间及下联设备和 ONU间的时间同步 起来。
本发明实施例利用 GPON特有的 POLAM消息通道或 OMCI消息通道， 实现时间在 PON段的传递， PLOAM消息或 OMCI消息在 PON段的传输是 不被分片的， PLOAM消息被发送时是作为一个整体一次性发送完毕， 避开 GPON网络对上层协议数据报文延迟不固定的特性， 在 PON网络上实现时 间的同步传递， 保证各 ONU有相同的时间， 如果在 ONU和其下级设备间 运行标准的 IEEE1588协议， ONU还能把时间精确地传递给下级设备， 保 证各 ONU下接的设备间也有相同的时间。 该系统包括： 光线路终端时间同步接收处理模块， 光线路终端时间信息发 送模块、 传递时间信息的光纤系统和光网络单元的时间同步接收和处理模 块：
其中 OLT时间同步接收处理模块， 用于实现与上级设备的时间同步， 根据上级设备发送的时间信息调整本地的时间， 从而是 OLT的时间精确同 步于上级设备。
其中 OLT时间信息发送模块， 用于计算 OLT到 ONU的传输时延， 并 通过管理通道传递时间信息。 OLT也可以发送用于计算 OLT到 ONU的传 输时延的信息给 ONU, ONU计算 OLT到 ONU的传输时延。
进一步管理通道可以使用 PON系统物理层操作管理维护消息承载时间 信息， 该方式下的时间信息包括 PLOAM消息的发送时刻及 OLT与 ONU 之间的传输时延或用于计算获得传输时延的信息。
进一步管理通道也可以使用 GPON系统的 OMCI通道， 该方式下的时 间信息包括 OLT与 ONU之间的传输时延、 OLT发送触发时间同步的帧的 发送时刻以及该帧的帧号。
其中的传递时间信息的光纤系统是点到多点的光纤系统， 是承载时间 信息的物理媒质。
其中的光网络单元的时间同步接收和处理模块， 用于接收 OLT发送的 时间信息， 并根据时间信息计算出 OLT和 ONU之间的时间偏差， 进而同 步本地的 ONU时间。 信息的时间同步消息的方法，所以克服了在无源光网络中不能保证 ONU在 时间上精确同步于 OLT的问题，进而达到了无源光网络中时间同步的效果。 流程图， 该优选实施例的 PON网络时间同步机制实现方法， 具体包括： 在 OLT和与其相连的设备间运行标准的 IEEE1588协议， 上级设备运 行在 Master模式， OLT运行在 Slave模式， OLT在时间上精确同步于上级 设备；
假定"上级设备"和 OLT设备间的时间差是 Offset;
在 T1时刻"上级设备"向 OLT发送带时间戳的同步（ Sync )报文， OLT 设备在其本地 T2时刻接收到同步报文， 建立如下方程式， 其中 Delay是同 步报文从"上级设备"到 OLT的传输时延迟。
T2=T1+Delay +Offset ( 1 )
"上级设备"向 OLT发送 Follow_up消息；
OLT在本地 T3时刻向 "上级设备"发送 Delay_Req消息；
"上级设备"在其本地 T4时刻向 OLT发送 Delay_Resp消息。 建立如下 方程式：
T4=T3+Delay -Offset ( 2 )
联合方程（ 1 )和（2 )可以求出 Offset值。
周期性的在"上级设备"和 0LT间发送， Sync、 Follow_up、 Delay_Req 和 Delay_Resp消息， 就能动态更新 Offset值。
在 0LT进行时间偏差 （Offset ) 的动态、 实时补偿， 就能实现 0LT在 时间上同步于 "上级设备"。
如果 0LT同时连接两台 "上级设备" 就能实现时钟源的主备， 提供网 络可靠性， 主备时钟源头的选择根据 IEEE15888协议实现。
在 0LT和 0NU间不运行标准的 IEEE1588协议。 0LT设备工作在时钟 的 Master模式， 0NU运行在 Slave模式， 0NU在时间上精确同步于 0LT; 如果使用 PL0AM通道，在 T1时刻 0LT向 0NU发送带时间戳的同步 PL0AM消息， 0NU在其本地 T2时刻接收到同步 PLO AM消息， 则 0LT 向 0NU发送的消息中包括： T2, 其中 T2=T1+Delay , 而 0LT和 0NU之间 的时间偏差（ Offset )就是 0NU本地时钟和 T2之间的差， 0NU根据 Offset 值来调整本地时钟。
如果使用 OMCI通道， 在计算出将要发送时间同步帧的时间 Tl , ONU 在其本地 T2时刻接收到相应的帧， 则 OLT设置相应的管理实体为 T2 , 其 中 T2=T1+Delay, 在 ONU接收到这个帧号时， 就触发根据 T2与本地时间 同步， 而 OLT和 ONU之间的时间偏差（ Offset )就是 ONU本地时钟和 T2 之间的差， ONU根据 Offset值来调整本地时钟。
其中 Delay可以通过 GP0N固有的测距方式获得。也可以通过模拟 1588 协议的方式获得， 即使用 PL0AM消息模拟 1588的时延测量过程， 具体方 法和 1588测量点到点链路的方法相同。
各 0NU都同步于 0LT, 各 0NU间就实现了时间的精确同步。
在 0NU和 "下联设备" 间运行标准 IEEE1588协议， "下联设备" 能在 时间上精确同步于 0NU, 在移动通信领域"下联设备"就是基站。
根据本发明优选实施例的同步方法， 为了保证时间精度， 传递时间的 协议报文都用硬件来收发。
显然， 本领域的技术人员应该明白， 上述的本发明的各模块或各步骤 可以用通用的计算装置来实现， 它们可以集中在单个的计算装置上， 或者 分布在多个计算装置所组成的网络上， 可选地， 它们可以用计算装置可执 行的程序代码来实现， 从而， 可以将它们存储在存储装置中由计算装置来 执行， 或者将它们分别制作成各个集成电路模块， 或者将它们中的多个模 块或步骤制作成单个集成电路模块来实现。 这样， 本发明不限制于任何特 定的硬件和软件结合。
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。

Claims

权利要求书
1.一种无源光网络中时间的同步方法， 用于光线路终端（OLT )向光网 络单元（ ONU )执行时间同步， 其特征在于：
所述 OLT与所述 ONU之间使用管理通道进行时间信息的传递； 所述时间信息包括所述 OLT和所述 ONU间的传输时延及所述时间信 息的发送时刻标识信息；
所述 ONU根据所述时间信息计算与所述 OLT之间时间偏差， 进行与 所述 OLT的时间同步。
2.根据权利要求 1 所述的方法， 其特征在于， 所述管理通道为物理层 操作管理维护（PLOAM )消息通道； 所述时间信息的发送时刻标识信息为 PLOAM消息的发送时刻。
3.根据权利要求 1 所述的方法， 其特征在于， 所述管理通道为光网络 单元管理控制接口 （OMCI )消息通道； 所述时间信息的发送时刻标识信息 为触发时间同步的帧的发送时刻以及该帧的帧号。
4.根据权利要求 1所述的方法， 其特征在于， 所述 ONU与所述 OLT 之间的时间偏差为： 所述时间信息的发送时刻与所述传输时延之和与所述 ONU本地时间的差。
5.根据权利要求 1或 4所述的方法， 其特征在于， 所述信息传输时延 通过所述无源光网络系统的测距功能得到； 或使用 PLOAM 消息通道模拟 IEEE1588协议进行测量获得。
6.—种无源光网络中的时间同步系统， 其特征在于， 包括：
光线路终端时间同步接收处理模块， 用于实现与上级设备的时间同步； 光线路终端时间信息发送模块， 用于计算 OLT与 ONU之间的传输时 延， 并通过管理通道传递时间信息给 ONU; 所述时间信息包括所述 OLT和 所述 ONU间的传输时延及所述时间信息的发送时刻标识信息； 光网络单元的时间同步接收和处理模块， 用于接收 OLT发送的时间信 息， 并根据所述时间信息计算与所述 OLT之间时间偏差， 进行与所述 OLT 的时间同步。
7.根据权利要求 6所述的系统， 其特征在于， 所述光线路终端时间信 息发送模块， 使用 PLOAM消息通道传递所述时间信息给所述 ONU, 所述 时间信息的发送时刻标识信息为 PLOAM消息的发送时刻。
8.根据权利要求 6所述的系统， 其特征在于， 所述光线路终端时间信 息发送模块， 使用 OMCI消息通道传递所述时间信息给所述 ONU, 所述时 间信息的发送时刻标识信息为触发时间同步的帧的发送时刻以及该帧的帧 号。
9、 根据权利要求 6所述的系统， 其特征在于， 所述时间偏差为： 所述 时间信息的发送时刻与所述传输时延之和与所述 ONU本地时间的差。
10、 根据权利要求 6至 9任一项所述的系统， 其特征在于， 所述信息 传输时延通过所述无源光网络系统的测距功能得到； 或使用 PLOAM 消息 通道模拟 IEEE1588协议进行测量获得。
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