Abstract:
A system and method for synchronizing clocks related to telecommunications throughout s point-to-multipoint optical network utilizes downstream data timed using a high frequency transmission clock to distribute timing information of a central telecom-based clock to remote terminals. In an exemplary embodiment, the point-to-multipoint optical network system is an Ethernet-based passive optical network (PON) system that operates in accordance with a Gigabit Ethernet standard. The timing information of the central telecom-based clock is extracted from the downstream data at each remote terminal by recovering the high frequency transmission clock and then, deriving a reference clock, which is synchronized with the central telecom-based clock, from the recovered transmission clock. The reference clock is then used to generate one or more telecom-related clocks that are needed by the remote terminal. The system and method allows telecom-related clocks throughout the system to be synchronized in an efficient and cost-effective manner.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to optical access networks, and more particularly to a system and method for synchronizing telecom clocks throughout a passive optical access network. 
     BACKGROUND OF THE INVENTION 
     The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. Although the bandwidth of backbone networks has experienced a substantial growth in recent years, the bandwidth provided by access networks has remained relatively unchanged. Thus, the “last mile” still remains a bottleneck between a high capacity LAN or Home network and the backbone network infrastructure. 
     Digital Subscriber Line (DSL) and Cable Modem (CM) technologies offer some improvements over more conventional last mile solutions. However, these technologies still do not provide enough bandwidth to support emerging services such as Video-On-Demand (VoD) or two-way video conferencing. In addition, not all customers can be covered by DSL and CM technologies due to distance limitations. 
     One broadband access network architecture that offers a solution to the “last mile” problem is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splifters and combiners to passively distribute optical signals between the OLT and the remote ONUs. 
     In the past, much of the PON development has been focused on ATM-based PONs. However, in recent years, there has been a growing interest in Ethernet-based PONs. This growing interest is partly due to the fact that about ninety-five percent (95%) of LANs currently use the Ethernet protocol. Therefore, Ethernet-based PONs are much more preferable than ATM-based PONs to interconnect Ethernet networks. Another contributing factor is that Ethernet is more compatible with the IP protocol, which is the protocol for the Internet. 
     However, unlike ATM, Ethernet was not originally designed to provide synchronization of telecom clocks to facilitate proper voice transmission through an Ethernet-based network. Therefore, in an Ethernet-based PON, synchronized telecom clocks may have to be independently extracted by the OLT and the ONUs from one or more external sources, such as central offices. Alternatively, a telecom clock may have to be distributed from a single source, such as the OLT, to the rest of the network, e.g., the ONUs, over a different transmission medium than the optical fibers that interconnect the OLT and the ONUs. However, these solutions significantly increase the overall cost of the PON components, as well as increase the complexity of the Ethernet-based PON. 
     In view of the above concern, there is a need for a system and method for economically and efficiently synchronizing telecom clocks throughout an Ethernet-based PON. 
     SUMMARY OF THE INVENTION 
     A system and method for synchronizing clocks related to telecommunications throughout s point-to-multipoint optical network utilizes downstream data timed using a high frequency transmission clock to distribute timing information of a central telecom-based clock to remote terminals. In an exemplary embodiment, the point-to-multipoint optical network system is an Ethernet-based passive optical network (PON) system that operates in accordance with a Gigabit Ethernet standard. The timing information of the central telecom-based clock is extracted from the downstream data at each remote terminal by recovering the high frequency transmission clock and then, deriving a reference clock, which is synchronized with the central telecom-based clock, from the recovered transmission clock. The reference clock is then used to generate one or more telecom-related clocks that are needed by the remote terminal. The system and method allows telecom-related clocks throughout the system to be synchronized in an efficient and cost-effective manner. 
     A method of synchronizing clocks related to telecommunications in a point-to-multipoint optical network in accordance with the present invention includes the steps of deriving a telecom-based clock at a first network terminal of the optical network from an external source, generating a data transmission clock from the telecom-based clock, transmitting data in variable-length packets from the first network terminal using the data transmission clock to embed a timing information of the telecom-based clock into the data, deriving a reference clock by extracting the timing information of the telecom-based clock from the data, and generating a remote telecom-related clock from the reference clock. The data transmission clock, the reference clock and the remote telecom-related clock are substantially synchronized with the telecom-based clock. The variable-length packets may be substantially compliant to an Ethernet-based protocol, such as a Gigabit Ethernet-based protocol. 
     The method may further include the step of generating a transmission-based clock using the transmission rate of the data. The transmission-based clock is substantially synchronized with the data transmission clock that defined the transmission rate. In an embodiment, two phase shifted transmission-based clocks are generated using the transmission rate. 
     In an exemplary embodiment, the clocks that are used by the method may be as follows: the data transmission clock may be a 125 MHz clock; the telecom-based clock and the reference clock may be 8 kHz clocks; the two phase shifted transmission-based clocks may be 62.5 MHz clocks that are phase shifted by 180 degrees to each other; and the telecom-related clock may be a 1.544 MHz clock, a 2.048 MHz clock, a 51.84 MHz clock, or any multiples thereof. 
     A system in accordance with the present invention includes a central access module coupled to an external telecommunications network, and a number of remote terminals optically coupled to the central access module. The central access module includes a network interface that is configured to obtain a telecom-based clock from the external telecommunications network, a transmission clock generator configured to generate a data transmission clock using the telecom-based clock, and a transmitting sub-system that transmits said data in variable-length packets at a prescribed data rate, which is defined by the data transmission clock to carry timing information of the telecom-based clock with the data. The data transmission clock is substantially synchronized with the telecom-based clock. The transmitting sub-system may be configured to transmit data in variable-length packets that are substantially compliant to an Ethernet-based protocol, such as a Gigabit Ethernet-based protocol. 
     Each remote terminal of the system includes a receiving sub-system that extracts the timing information of the telecom-based clock from the data and generates a reference clock, and a remote clock generator configured to generate a remote telecom-related clock from the reference clock. The remote telecom-related clock is substantially synchronized with the telecom-based clock at the central access module. 
     The receiving sub-system of a remote terminal may include a physical layer module that generates one or more transmission-based clock from the data transmitted from the central access module, and a frequency divider operatively coupled to the physical layer module that generates the reference clock from the transmission-based clock, which is substantially synchronized with said data transmission clock. In an embodiment, the physical layer module may be configured to generate two phase shifted transmission-based clocks. 
     In an exemplary embodiment, the clocks that are used by the system may be as follows: the data transmission clock may be a 125 MHz clock; the telecom-based clock and the reference clock may be 8 kHz clocks; the two phase shifted transmission-based clocks may be 62.5 MHz clocks that are phase shifted by 180 degrees to each other; and the telecom-related clock may be a 1.544 MHz clock, a 2.048 MHz clock, a 51.84 MHz clock, or any multiples thereof. 
    
    
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an Ethernet-based passive optical network (PON) system in accordance with the present invention. 
     FIG. 2 illustrates the problem of telecom clock synchronization across an Ethernet connection. 
     FIG. 3 illustrates the use of a data transmission timing clock to resolve the problem of telecom clock synchronization across an Ethernet connection. 
     FIG. 4 is a block diagram of the components of an OLT included in the Ethernet-based PON system. 
     FIG. 5 is a block diagram of the components of an ONU included in the Ethernet-based PON system. 
     FIG. 6 is a process flow diagram of a method of synchronizing telecom clocks in an Ethernet-based PON system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, an Ethernet-based passive optical network (PON) system  100  in accordance with the present invention is shown. The PON system provides telecommunications between a central office  102  and a number of telephones  104  supported by the PON system. The central office and the supported telephones are connected to the PON system through conventional telecommunication lines, such as DS 3  lines or T- 1  lines. The PON system maintains synchronization of clocks related to telecommunications throughout the system in an efficient and economic manner. 
     The Ethernet-based PON system  100  includes a central access module  106  that functions as a central hub for the system. The central access module contains a DS 3  terminal (DS 3 T)  108 , a switch  110 , and a number of optical line terminals (OLTs)  112 . In an exemplary embodiment, the central access module is a chassis-based system that contains eight (8) OLTs. Each OLT of the system is connected to a number of optical network units (ONUs)  114  by optical fibers  116  and one or more splitter/combiners  118 . As an example, each OLT may be connected to sixteen (16) ONUs. Thus, the PON system may support a total of one hundred twenty-eight (128) ONUs. In an embodiment, the PON system uses the IEEE 802.3 z protocol (commonly referred to as Gigabit Ethernet) to transmit variable-length packets of data between the OLTs and the ONUs. The ONUs of the system are connected, either directly or indirectly, to the telephones  104 , which are supported by the PON system. Although only a single telephone is shown to be connected to each ONU in FIG. 1, additional telephones may be connected to each ONU. The number of telephones that can be connected to an ONU is dependent on the bandwidth and the number of the telecommunications lines provided by that ONU. For example, if an ONU is connected to a single T 1  line, the ONU may be connected to and support up to thirty-two (32) telephones. In an embodiment, the telephones may be connected to a central phone switch, such as a PBX, which is connected to the ONU. 
     In order to provide acceptable quality voice communications between the telephones  104  and the central office  102 , the telecom clocks used by the OLTs  112  for telecommunications should be synchronized with the corresponding telecom clocks used by the ONUs  114 . A problem with providing synchronization between the OLTs and the ONUs is that the connections between the OLTs and the ONUs are based on Ethernet, and conventional Ethernet architectures are not designed to provide synchronization of telecom clocks across Ethernet connections. Therefore, the PON system  100  must resolve this problem of telecom clock synchronization in order to provide acceptable quality voice communications. 
     The problem of telecom clock synchronization is illustrated in FIG. 2, which includes a simplified version  200  of the PON system  100  of FIG.  1 . In FIG. 2, only a single OLT  212  and a single ONU  214  are shown. The OLT and the ONU are coupled to each other by an Ethernet connection  220 . For telecommunications between the OLT  212  and the central office  102 , the OLT uses a telecom clock  216 . For telecommunications between the ONU  214  and the telephone  104 , the ONU uses a telecom clock  218 . Thus, the telecom clocks  216  and  218  must be synchronized to provide proper telecommunications between the central office  102  and the telephone  104 . Any solution to the problem should efficiently and economically synchronize the telecom clock  216  at the OLT with the telecom clock  218  at the ONU. One prior art solution is for the OLT and the ONU to independently extract synchronized telecom clocks from one or more external sources. As an example, the telecom clock  216  may be derived from the central office  102 , while the telecom clock  218  may be derived from a different central office (not shown). Alternatively, the telecom clock  216  may be derived from the central office  102 , while the telecom clock  218  may be derived from a wireless clock source, which is synchronized with the central office. Another solution is for the OLT to transmit the telecom clock  216 , which may be derived from the central office  102 , to the ONU through a different medium than the Ethernet connection  220 . However, both of these solutions require components that will significantly increase the cost for each ONU of the Ethernet-based PON system  100 . Since the PON system may include up to one hundred twenty-eight (128) ONUs, even a modest increase in cost for each ONU will drastically increase the overall cost of the PON system. 
     The Ethernet-based PON system  100  economically solves the problem of telecom clock synchronization by utilizing a data transmission clock  222  that is used to time the transmission of data between the OLT  212  and the ONU  214 . In conventional Ethernet architectures, a data transmission clock is independent from other clocks of the network, such as clocks related to telecommunications. Consequently, the data transmission clock  222  would not typically be synchronized with the telecom clocks  216  and  218 , as illustrated in FIG.  2 . Thus, any data transmitted using the data transmission clock does not provide information to synchronize the telecom clock  218  at the ONU  214  with the telecom clock  216  at the OLT  212 . In contrast, the Ethernet-based PON system synchronizes the data transmission clock  222  with the telecom clock  216  used by the OLT to transmit the timing information of the telecom clock  216  to the ONU. The ONU then extracts the timing information from the transmitted data to generate the telecom clock  218 , which is synchronized with the telecom clock  216  of the OLT. In other words, the telecom clock  218  of the ONU is synchronized with the data transmission clock  222 , which in turn is synchronized with the telecom clock  216  of the OLT. FIG. 3 provides an illustration of a telecom clock  318  of the ONU that is synchronized with a data transmission clock  322 , which in turn is synchronized with a telecom clock  316  of an ONU. As shown in FIG. 3, the telecom clock  318  of the ONU is thus synchronized with the telecom clock  316  of the OLT. 
     Turning to FIG. 4, the components of an exemplary OLT  112  of the Ethernet-based PON system  100  are shown. The OLT includes a media access control (MAC) module  402 , a physical layer module  404 , a Gigabit Ethernet transceiver  406 , and a phase locked loop (PLL) module  408 . The functions of these OLT components are described with respect to data transmission, since the functions are merely reversed for data reception. The MAC module  402  performs various data transfer functions in accordance with the Gigabit Ethernet protocol, including generating variable-length packets of data and encoding the outgoing data using 8B/10B coding, i.e., encoding 8 bits of data into 10 code bits e physical layer module  404  receives the encoded data from the MAC module and serializes the data for transmission. The Gigabit Ethernet transceiver  406  converts the serialized bits of data, which are electrical signals, into optical signals. The PLL module  408  provides a data transmission clock of 125 MHz to the MAC module and the physical layer module. The 125 MHz data transmission clock is generated from an 8 kHz telecom-based clock, which is synchronized with the clock at the central office  102 . The 125 MHz data transmission clock is used by the physical layer module to transmit the serialized bits of data at 1.25 Gbps in accordance with the Gigabit Ethernet protocol. 
     The 8 kHz telecom-based clock used by the PLL module  408  is derived from the central office  102  through the DS 3 T  108  of the Ethernet-based PON system  100  shown in FIG.  1 . The DS 3 T receives a high frequency reference clock from the central office. As an example, the high frequency reference clock may be a 43.232 MHz clock (T 3 /DS 3  clock). The DS 3 T then divides the high frequency reference clock to produce the 8 kHz telecom-based clock, which is then distributed to the OLTs  112  by the switch  110 . At each OLT, the PLL module  408  receives the 8 kHz telecom-based clock and generates the 125 MHz data transmission clock, which is synchronized with the 8 kHz telecom-based clock. The 125 MHz data transmission clock is then transmitted to the MAC module  402  and the physical layer module  404 . At the physical layer module, the 125 MHz data transmission clock is used to time the transmission rate of data. The physical layer module multiplies the 125 MHz data transmission clock by a factor of ten, and then transmits the bits of data at a rate of 1.25 Gbps. Therefore, the transmitted data, which has been timed using the 125 MHz data transmission clock, is synchronized to the timing information of the 8 kHz telecom-based clock. 
     The components of an exemplary ONU  114  are shown in FIG.  5 . The ONU includes a Gigabit Ethernet transceiver  502 , a physical layer module  504 , a MAC module  506 , a frequency divider  508 , and a synchronizer  510 . Similar to the OLT components, the functions of the ONU components are described with respect to data reception, since the functions are merely reversed for data transmission. The Gigabit Ethernet transceiver  502  receives incoming bits of data in the form of optical signals and converts the optical signals to electrical signals. The physical layer module  504  then deserializes the converted bits of data and transmits the data bits to the MAC module  420 , where the data bits are processed in accordance with the Ethernet protocol. The physical layer module  504  also generates two 180-degree phase-shifted 62.5 MHz transmission-based clocks, which are synchronized with the 125 MHz data transmission clock used by the OLT  112  for data transmission, from the incoming bits of data. That is, the 125 MHz data transmission clock used by the OLT is recovered from the incoming data by the physical layer module  504  in the form of two phase-shifted 62.5 MHz clocks. These phase-shifted 62.5 MHz clocks are then transmitted to the MAC module  506 , where the two 62.5 MHz clocks are used to generate a 125 MHz clock, which is synchronized with the 125 MHz data transmission clock of the OLT, to process the received data. 
     The phase-shifted 62.5 MHz clocks are also transmitted to the frequency divider  508  of the ONU  114 . The frequency divider generates a reference 8 kHz telecom clock from the two phase-shifted 62.5 MHz clocks by dividing the phase-shifted clocks, in this case, by a non-integer, i.e., 7812.5. Consequently, the reference 8 kHz telecom clock is synchronized with the phase-shifted 62.5 MHz clocks. Thus, the frequency divider  508  generates a low frequency clock, i.e., the reference 8 kHz telecom clock, from high frequency clocks, i.e., the two 62.5 MHz clocks. The division of high frequency clocks to generate a low frequency clock reduces errors that may have been introduced into the high frequency clocks. Therefore, the reference 8 kHz clock, which is derived from the 62.5 MHz clocks, includes fewer errors than the 62.5 MHz clocks, which results in a more accurate clock. Since the reference 8 kHz telecom clock can be traced back to the 8 kHz telecom-based clock of the OLT  112 , the reference 8 kHz telecom clock is synchronized with the 8 kHz telecom-based clock of the OLT. 
     The reference 8 kHz telecom clock is used by the synchronizer  510  of the ONU  114  to generate one or more telecom clocks for telecom-related devices included in the ONU. In one embodiment, the synchronizer may generate a telecom clock that is a multiple of the T 1  clock rate, i.e., an n×1.544 MHz clock, where n=1, 2, 3 . . . . As an example, the synchronizer may generate a 1.544 MHz telecom clock (T 1  clock) from the reference 8 kHz telecom clock for a T 1  interface module  512 , as illustrated in FIG.  4 . As another example, the synchronizer may generate a 43.232 MHz (T 3 /DS 3  clock), which is 28 times the T 1  clock rate. In another embodiment, the synchronizer may generate a telecom clock that is a multiple of the E 1  clock rate, i.e., an n×2.048 MHz clock, where n×1, 2, 3 . . . . As an example, the synchronizer may generate a 2.048 MHz clock (E 1  clock) from the reference 8 kHz telecom clock for one or more E 1  interface modules (not shown). As another example, the synchronizer may generate a 4.096 MHz telecom clock from the reference 8 kHz telecom clock for devices (not shown) related to PCM and echo cancellation. As another example, the synchronizer may generate a 32.768 MHz telecom clock (E 3  clock), which is 16 times the E 1  clock rate. In still another embodiment, the synchronizer may generate a telecom clock that is a multiple of the OC 1  clock rate, i.e., an n×51.84 MHz clock, where n=1, 2, 3 . . . . As an example, the synchronizer may generate a 155.520 MHz clock (OC 3  clock) from the reference 8 kHz telecom clock for a DS 3  terminal (not shown) included in the ONU. Using the reference 8 kHz telecom clock, the synchronizer can generate any signal that is based on an 8 kHz clock rate. The number and frequencies of the telecom clocks generated by the synchronizer are dependent on the various telecom-related devices that may be included in the ONU. Since these telecom-related clocks at the ONU are generated from the reference 8 kHz telecom clock, which is synchronized with the 8 kHz telecom-based clock at the OLT  112 , the telecom-related clocks at the ONU are also synchronized with the 8 kHz telecom-based clock at the OLT  112 . 
     An advantageous feature of the Ethernet-based PON system  100  is that only a single master clock, i.e., the 8 kHz telecom-based clock, is needed to provide synchronized telecom-related clocks to all the ONUs  114  of the system. The master clock is derived from a single external source, i.e., the central office  102 , and then distributed to the OLTs of the system. Each OLT then “distributes” the clock to the ONUs that are optically connected to that OLT. Thus, the telecom-related clocks at the ONUs are all derived from the single master clock. 
     A method of synchronizing telecom clocks throughout the Ethernet-based PON system  100  in accordance with the present invention is described with reference to FIG.  6 . At step  602 , a telecom-based clock is derived from an external source, for example, the central office  102 , at the access module  106  of the PON system. The telecom-based clock is derived by dividing the received clock from the external source to a lower frequency clock. As an example, the clock from the external source may be a 43.232 MHz clock (T 3 /DS 3  clock). In an exemplary embodiment, the telecom-based clock is an 8 kHz clock. At step  604 , the telecom-based clock is distributed to the OLTs  112  of the Ethernet-based PON system by the switch  100  of the access module. Next, at step  606 , a data transmission clock is generated from the telecom-based clock at an OLT such that the data transmission clock is synchronized with the telecom-based clock. In the exemplary embodiment, the data transmission clock is a 125 MHz clock. The data transmission clock is used by the MAC module  402  and the physical layer module  404  of the OLT to transmit downstream data to the ONUs that are optically connected to the OLT. 
     Next, at step  608 , the downstream data is optically transmitted in variable-length packets in accordance with a prescribed protocol using the data transmission clock. The downstream data is transmitted at a predetermined transmission rate, which is defined by the data transmission clock. In the exemplary embodiment, the prescribed protocol is Gigabit Ethernet and the data transmission rate is 1.25 Gbps. The use of the data transmission clock, which is synchronized with the telecom-based clock, to transmit the downstream data has the effect of embedding timing information from the telecom-based clock into the data transmission clock. At step  610 , the downstream data is received at an ONU of the Ethernet-based PON system  100 . Next, at step  612 , two phase-shifted transmission-based clocks are derived from the received downstream data, which are received at the transmission rate defined by the data transmission clock of the transmitting OLT. The phase-shifted clocks are generated by the physical layer module  504  of the receiving ONU. Since these phase-shifted clocks are derived from the downstream data that was transmitted using the data transmission clock, the phase-shifted clocks are synchronized with the telecom-based clock at the transmitting OLT. In the exemplary embodiment, the two phase-shifted clocks are two 62.5 MHz clocks, which are phase shifted by 180 degrees with respect to each other. 
     Next, at step  614 , an ONU reference clock is generated from the two phase-shifted clocks by the frequency divider  508  of the ONU. Since the phase-shifted clocks can be traced back to the telecom-based clock at the OLT, the ONU reference clock is synchronize with the telecom-based clock. At step  616 , one or more telecom-related clocks are generated by the synchronizer  510  of the ONU using the ONU reference clock. The telecom-related clocks may include a 1.544 MHz clock for a T 1  interface, a 2.048 MHz clock for an E 1  interface, or a 43.232 MHz clock for a DS 3  interface. In addition, the telecom-related clocks may include a 4.096 MHz clock for devices related to PCM and echo cancellation, as well as other clocks that may be needed for telecommunications. Since these telecom-related clocks are derived from the ONU reference clock, which is synchronized with the telecom-based clock at the OLT, the telecom-related clocks at the ONU are also synchronized with the telecom-based clock at the OLT.