Abstract:
A system and method for optical transmission that provides an efficient manner of increasing transmission capacity, especially in the downstream direction, with minimal changes to existing components. An optical network node includes an optical transmitter, a plurality of optical media access controllers, and a bit interleaver for interleaving the output of each of the plurality of optical media access controllers into a single bit stream and providing the bit stream to the optical transmitter. In operation, the optical network node, which may be an OLT, thereby is capable of transmitting at a rate equal to the sum of the output rates of the individual media access controllers. A corresponding deinterleaver in a receiving node, such as an ONT, separates the interleaved bit stream into its constituent bit streams, one or more of which may be selected for use in the receiving device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
       [0001]    The present disclosure is related to and claims priority from U.S. Provisional Patent Application Ser. No. 61/190,746, filed on 2 Sep. 2008, the entire contents of which are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to the field of optical communication networks, and, more particularly, to a method and system for efficiently transmitting signals via an optical network such as a passive optical network (PON). 
       BACKGROUND 
       [0003]    The following abbreviations are herewith defined, at least some of which are referred to within the following description of the state-of-the-art and the present invention.
   FTTH fiber to the home   FTTU fiber to the user   GEM GPON encapsulation method   GPON gigabit-capable PON   IETF Internet Engineering Task Force   ITU International Telecommunication Union   LOS/LOF Loss of Signal/Loss of Frame   MAC medium access control   MDU multi-dwelling unit   ODN optical distribution network   OLT optical line termination   ONT optical network termination   ONU optical network unit   PLL phase-locked loop   PON passive optical network   SN serial number   SNI service node interface   TDMA time division multiple access   UNI user-network interface   WDM wave division multiplexing   
 
         [0024]    Optical telecommunication networks have been widely built out in recent years and are gaining in popularity. Optical fibers are capable are capable of carrying a high volume of traffic at a reasonable cost. Although previously the “last mile” between an optical communication network and the end user was still spanned with copper wire, now fiber optic cables are often run directly up to houses, apartment complexes, and business locations. One arrangement for spanning this portion of the communication link is referred to as a PON. 
         [0025]    Generally speaking, a PON includes an OLT, typically located in a central office, which communicates via a fiber-optic cable system with one or more ONUs, with each ONU being located on or near a customer premises. An ONT is an ONU that typically serves a single user and may, for example, be located at the user&#39;s residence. An MDU is an ONU that serves a multi-dwelling unit such as an apartment complex or small business. Each ONU is capable of segregating the downstream signals from the OLT and directing them to the proper user, and of transmitting upstream signals back to the OLT. In addition to forming a part of one or more PONS, the OLT is also connected to the larger telecommunication system through which the various services such as telephony, 
         [0026]    Internet access, and broadcast media are accessible so that they can be made available to the users associated with the OLT. 
         [0027]    Standards have been promulgated for PON operations. For example, many current implementations are configured in accordance with a family of specifications including ITU-T G.984 and related standards. Such systems currently provide for transmission speeds of up to (approximately) 2.5 Gbps in the downstream direction and 1.24 Gbps upstream. The directional difference in transmission speeds is due in part to practical consideration of the cost of facilitating higher upstream transmission speeds, coupled with the fact that, as a general rule, more content needs to be transmitted downstream than back to the OLT. 
         [0028]    With increased utilization of optical network services, however, there is a need to increase existing transmission speeds, at least in the downstream direction and eventually in both directions. One solution, of course, is to simply replace or upgrade all PON and related equipment with the components necessary to accommodate the higher transmission speeds. This, however, may be too expensive or difficult, especially in the near term. In some cases there may be technical obstacles as well. There is a need, therefore, for a way to facilitate an increase in transmission speed without having to replace or upgrade all of the components in each OLT and ONU. 
       SUMMARY 
       [0029]    The present invention is directed to a manner of efficiently transmitting optical signals in an optical network such as a PON to extend the transmission capability of existing resources while necessitating only a relatively-small, if any, modification of existing hardware. 
         [0030]    In one aspect, the present invention is an optical network node that include an optical transmitter, a plurality of optical media access controllers, and a bit interleaver for interleaving the output of each of the plurality of optical media access controllers into a single bit stream and providing the bit stream to the optical transmitter. The node may also include a phase-lock loop coupled to the bit interleaver and to each of the plurality of optical media access controllers. In a preferred embodiment, the network node is an OLT in a PON, and transmits downstream at a transmission rate approximately equal to the sum of the operating rates of the plurality of media access controllers. In one embodiment he network node interleaves the plurality of bit streams into a single bit stream by selecting a bit from each of the media access controllers in turn, but in some implementations another scheme may be used. In most embodiments the network node includes an optical receiver and upstream MAC for receiving and processing upstream transmissions. In some implementations it will have a plurality of optical receivers and upstream MACs for receiving WDM-separated upstream transmissions from a plurality of MACs. 
         [0031]    In another aspect, the present invention is a method of transmitting an optical signal within a passive optical network including receiving, by a bit interleaver of an optical network node, the output of a plurality of media access controllers, interleaving the output of the plurality of media access controllers to create a single bit stream, and providing the single bit stream to an optical transmitter. Again, in a preferred embodiment, the optical network node is an OLT operating in a PON, such as a GPON. The method may further include transmitting the bit stream via a passive optical network and receiving the bit stream in an optical receiver where it is deinterleaved. 
         [0032]    In yet another aspect, the present invention is a system for optical communication that includes an OLT having a bit interleaver, and at least one ONU having a deinterleaver in communication. The ONU preferably includes at least one channel selector. 
         [0033]    Additional aspects of the invention will be set forth, in part, in the detailed description, figures and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    A more complete understanding of the present invention may be obtained by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
           [0035]      FIG. 1  is a simplified block diagram illustrating selected components of an exemplary optical network in which the present invention may be implemented; 
           [0036]      FIG. 2  is a simplified block diagram illustrating selected components of an OLT according to an embodiment of the present invention; 
           [0037]      FIG. 3  is a simplified block diagram illustrating selected components of an optical network according to an embodiment of the present invention; 
           [0038]      FIG. 4  is a simplified block diagram illustrating selected components of an optical network according to embodiment of the present invention; 
           [0039]      FIG. 5  is a simplified block diagram illustrating the interleaving of a plurality of bit streams according to an embodiment of the present invention; 
           [0040]      FIG. 6  is a simplified block diagram illustrating the deinterleaving of a bit stream according to an embodiment of the present invention; 
           [0041]      FIG. 7  is a simplified block diagram illustrating selected components of an OLT according to an alternate embodiment of the present invention; 
           [0042]      FIG. 8  is flow diagram illustrating a method of transmitting via an optical network according to an embodiment of the present invention; 
           [0043]      FIG. 9  is a simplified block diagram illustrating selected components of an optical network according to an alternate embodiment of the present invention; and 
           [0044]      FIG. 10  is a state diagram illustrating a process of channel assignment according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    The present invention is directed to a manner of facilitating higher transmissions speed in a PON operating according to a current standard, for example ITU-T G.984 and related specifications.  FIG. 1  is a simplified block diagram illustrating selected components of an exemplary optical network, in this case a PON,  100  in which the present invention may be implemented. 
         [0046]    In the embodiment of  FIG. 1 , PON  100  includes an OLT  150  configured according to the present invention and operable to communicate with a telecommunication system (not shown) providing services to a number of users (also not shown) that are associated with OLT  150  and accessible via PON  100 . PON  100  also includes an ONT  110 , which serves a single user, and MDU  130 , which is capable of serving four users. Note that the number of users is exemplary, and could vary from implementation to implementation. The number of ONUs such as ONT  110  and MDU  130  may also vary, though a typical implementation may have as many as thirty-two or sixty-four. The OLT  150  is connected to ONT  110  and MDU  130  via an optical fiber system  101 . Optical fiber system  101  includes a fiber-optic cable and typically one or more un-powered optical splitters that enable a single OLT to communicate with many ONUs, as implied in  FIG. 1  although only ONT  110  and MDU  130  are illustrated. 
         [0047]    In some cases the present invention may be implemented in other types of networks that are similar in operation, and the invention is not intended to be limited to only those networks or components that are labeled or referred to by now-current terminology. 
         [0048]      FIG. 2  is a simplified block diagram illustrating selected components of an OLT  250  configured according to an embodiment of the present invention. Generally speaking, downstream transmission components are shown in  FIG. 2 . In accordance with this embodiment of the present invention, communication capacity over a fiber optic cable system is increased by bit interleaving the input of multiple MACs prior to transmission. In the embodiment of  FIG. 2 , there are shown four MACs, referred to as  256  through  259 , each providing input from an external communication system (not shown), that is to be transmitted by OLT  250  to one or more users. In  FIG. 2 , MACs also receive input from bandwidth mapper  255  so that downstream transmissions also include instructions for timing upstream transmissions. A PLL  265  works in conjunction with a multiplexer  260  and the MACs to insure proper synchronization and timing. 
         [0049]    In this embodiment, each of the MACs  256  through  259  provide their output to multiplexer  260 , which bit interleaves the output from each of the MACs into a single bit stream. In a preferred embodiment, the bits from each MAC are selected in turn so that the input from each MAC is equally represented in the output bit stream, although in some implementations a different selection scheme may be used. The bit stream output is then provided to an optical transmitter  270  for transmission on an optical fiber. 
         [0050]    In a preferred embodiment, the optical transmission capacity of the optical transmitter  270  is approximately equal to or greater than the sum of the capacities of the individual MACs. For example, if each MAC is capable of outputting approximately 2.5 Gbps in the downstream directions, the optical transmitter would be capable of transmitting at 10 Gbps to take full advantage of the increased capacity. Note that herein all transmission speeds mentioned are taken to be approximate values, as is currently customary in describing telecommunication system capacity. 
         [0051]    Naturally, this embodiment also presumes that the multiplexer  260  is capable of outputting a 10 Gbps bit stream, as is the case in a preferred embodiment. Of course, even a smaller improvement over the 2.5 Gbps that transmitter  270  would normally transmit using a single MAC is still advantageous. It should be noted here that while the use of 2.5 Gbps MACs is consistent with many current implementations, the same configuration may be used with four 10 Gbps MACs to produce a 40 Gbps bit stream, given an appropriate multiplexer and optical transmitter. Finally, it is noted that more or less than four MACs may be used to provide input to the multiplexer. In one embodiment, the available MAC capacity may be dynamically adjustable. The use of OLT  250  in an exemplary network will now be illustrated. 
         [0052]      FIG. 3  is a simplified block diagram illustrating selected components of an optical network  200  according to an embodiment of the present invention. In this embodiment, optical network  200  is a PON and includes OLT  250 , described above. Also shown are two ONUs, ONT  210  and MDU  230 . As is implied by the depiction of optical fiber system  201 , which interconnects these components, a number of other ONUS may be, and usually are present as well. ONT  210  and MDU  230  are exemplary of the other ONUS, but it is not necessary that all ONUS are identically configured. 
         [0053]    In this embodiment, ONT  210  includes an optical receiver  215 , which receives downstream transmissions from OLT  250  via optical fiber system  201 . Naturally, optical receiver  215  is capable of receiving transmissions from optical transmitter  270  of ONT  250  (shown in  FIG. 2 ). For example, if optical transmitter  270  transmits downstream at 10 Gbps, then optical receiver  215  receives at the same rate. Once received, the bit stream from OLT  250  is provided to demultiplexer  218  where it is separated into its original constituent bit streams. In this embodiment, these correspond to the four MACs of OLT  250  (also shown in  FIG. 2 ), and a divide-by-four function  226  is present to allow the demultiplexer  218  to recreate the constituent streams. As illustrated in  FIG. 3 , the four bit streams are then provided to channel selector  220 , which selects one of the bit streams at the direction of downstream MAC  225 . The selected bit steam is then provided to MAC  225  and electronically processed as usual. 
         [0054]    Similarly, MDU  230  includes optical receiver  235 , which also receives the optical transmission from optical transmitter  270  (shown in  FIG. 2 ). Once received, the bit stream from OLT  250  is provided to demultiplexer  238  where it is separated into its original constituent bit streams. In this embodiment, these correspond to the four MACs of OLT  250  (also shown in  FIG. 2 ), and a divide-by-four function  237  is present to allow the demultiplexer  218  to recreate the constituent streams. MDU  230  serves multiple users (not shown), and includes multiple downstream MACs  245  through  248 . Each of MACs  245  through  248  direct a respective channel selector  240  though  243  to select a bit stream from the four provided by demultiplexer  238 . Note that the four MACs  245  through  248  of MDU  230  may but do not necessarily correspond respectively to the four MACs  256  though  259  of OLT  250  (shown in  FIG. 2 ). 
         [0055]    In this manner, the present invention provides an efficient network-capacity upgrade in the downstream direction. The basic electronics and MACs functions may remain the same while an increase in transmission rate is achieved via the bit interleaving technology, supported (if necessary) by ensuring that the optics modules of each respective component are able to accommodate the increased rate. Adding only the bit interleaving components is in most implementations a much lower cost operation than upgrading all components and making corresponding protocol adjustments. Existing optical fiber systems, including the optical splitters, will in most cases support operation of the present invention without expensive upgrade. 
         [0056]    In the upstream direction, one alternative is simply to employ existing techniques in conjunction with the downstream enhancements according to the present invention. Although upstream capacity improvements may be needed more urgently in the future, present demand remains manageable with existing systems. One embodiment of the network of the present invention is illustrated in  FIG. 4 .  FIG. 4  is a simplified block diagram illustrating selected components of an optical network  200  according to an embodiment of the present invention. In this embodiment, OLT  250  includes a single bandwidth mapper  255  for managing transmissions in the upstream direction. Optical receiver  275  receives the upstream signals and provides the received transmission to upstream MAC  254 . A PON delay function  253  in this embodiment accounts the differences in physical distance from the OLT  250  and may adjust the bandwidth map generated by bandwidth mapper  255  accordingly. 
         [0057]    This bandwidth map, for example, provides a transmission window for upstream transmissions from MAC  213  of ONT  210  via the optical transmitter  214 . The same is true for MDU  230 , which includes an OR function  239  between its multiple upstream MACs  231  through  234  and the optical transmitter  236  of MDU  230 . This embodiment thereby provides for traditional TDMA upstream transmission, with each ONU transmitting in burst mode over optical fiber system  201  during its assigned time slot. 
         [0058]    The downstream transmission interleaving process is illustrated in more detail in  FIGS. 5 and 6 .  FIG. 5  is a simplified block diagram illustrating the interleaving of a plurality of bit streams according to an embodiment of the present invention. In the embodiment of  FIG. 5 , MAC  256  produces a bit stream and provides it to multiplexer  260 . For convenience, the bit stream produced by MAC  256  is represented as bits A 1  through A 4 . Similarly, downstream bit streams from MACs  257  through  259  are respectively shown as bits B 1  through B 4 , C 1  through C 4 , and D 1  through D 4 . In the exemplary implementation referred to above, each of these separate bit streams are received at a rate of approximately 2.5 Gbps. Multiplexer  260  than creates a single 10 Gbps bit stream by selecting one bit from each of the separate streams, in order, and interleaving them as illustrated in  FIG. 5 . The interleaved bit stream is then provided to optical transmitter  270  (see  FIG. 2 ) for transmission from OLT  250 . As alluded to above, in some implementations (not shown) a different bit selection or interleaving scheme may be employed. 
         [0059]      FIG. 6  is a simplified block diagram illustrating the deinterleaving of a bit stream according to an embodiment of the present invention. When this interleaved bit stream arrives at a destination, for example optical receiver  215  (shown in  FIG. 3 ), it is again divided into its four separate bit streams by demultiplexer  218 . This separation in effect creates four “channels”. As shown in  FIG. 6 , when downstream MAC  225  selects channel  1 , selector  220  provides it with (exemplary) bits A 1  through A 4 . MAC  225  may have determined that this bit stream is intended for it, or may have made an arbitrary selection which may be modified later if the wrong bit stream is selected. 
         [0060]      FIG. 8  is a flow diagram illustrating a method  400  of transmitting via an optical network, such as a PON, according to an embodiment of the present invention. The method  400  presumes that the components for performing the method, for example those described above, are available and operational. The method then begins with the receipt of content (step  405 ) in the plurality of MACs in an optical network node, such as an OLT. “Content” is used here to refer broadly to information and data to be provided to users via the optical network. In some implementation it may refer to control signaling as well. The manner in which this content is provided to each of the MACs (illustrated, for example, in  FIG. 2 ) is otherwise outside of the scope of this disclosure. Each of the MACs then creates a bit stream representing the content it has received and provides it to a multiplexer (step  410 ). 
         [0061]    In this embodiment, upon receiving the bit streams from each MAC interleaves them into a single bit stream (step  415 ). This single bit stream represents the output of each of the individual MACs of the node, it is transmitted as a single bit stream (step  420 ) by an optical transmitter to an ONU, where it is received (step  425 ). When the single interleaved bit stream is received, it, is provided to a demultiplexer and separated into its constituent bit streams associated with the downstream MACs of the OLT. Taking as an example the channel selector  220  of  FIG. 6 , in the embodiment of  FIG. 8  a channel selector then selects (step  435 ) one of these constituent bit streams and provides it to a MAC for further processing. 
         [0062]    In this embodiment, the MAC, for example MAC  225  shown in  FIG. 6 , has selected a channel arbitrarily. The MAC is not aware which bit stream will appear on this channel, but it is able to recognize the bit stream that is intended for it. When the channel selector provided the bit stream associated with the selected channel to the MAC, the MAC determines whether the channel has been selected properly (step  440 ). If not, the channel selection (step  435 ) and verification (step  440 ) is performed again. In most implementations, the channel selector simply steps through the available channels until the correct bit stream is chosen. Thereafter, the intended content is provided to the MAC (step  445 ) until it has all been transmitted or some event (not shown) necessitates that the channel selection process needs to be performed again. 
         [0063]    Note that the operations illustrated in  FIG. 8  are exemplary of one embodiment; in alternate embodiments other operations may be added or in some cases removed without departing from the spirit of the invention. In addition, the sequence shown is not necessarily mandatory, and the operations may in other embodiments be performed in any logically-consistent order. 
         [0064]    In some implementations, it will be necessary or desirable to provide a manner of identifying a channel for selection by the receiving MAC. This may be done in a number of ways. For example, in a GPON network operating under the G.984 protocol, the bits of the Psync frame may be modified to distinguish between channels. Other fields may be used as well. 
         [0065]    The optical network components are not limited to the configurations illustrated above. For example,  FIG. 7  illustrates an alternate OLT configuration.  FIG. 7  is a simplified block diagram illustrating selected components of an OLT  350  according to an alternate embodiment of the present invention. In the embodiment of  FIG. 7 , OLT  350  includes four MACs  361  through  364 , each of which output to a multiplexer  360  in the downstream direction, and each of which receive input from an associated PLL  365 . Each of the MACs  361  through  364 , however, is associated with a respective one of the bandwidth mappers  355  through  358 . This can create a one to one correspondence with downstream components, each of which may then transmit simultaneously in the upstream direction using WDM. Note that in the downstream direction, however, the multiplexer  360  again interleaves the output of each MAC  361  thorough  364  and provides the resulting single interleaved bit stream to the optical transmitter  370 . 
         [0066]      FIG. 9  is a simplified block diagram illustrating selected upstream components of an optical network  500  according to an alternate embodiment of the present invention. This embodiment is compatible with the OLT  350  of  FIG. 7 , and OLT  550  may be but is not necessarily an upstream counterpart. In this embodiment, ONT  510  and MDU  530  are again in communication with OLT  550  via a optical fiber system  501 . ONT  510  includes an upstream MAC coupled to an optical transmitter, which transmits upstream according to the instructions of a bandwidth map received from the OLT  550 . In this sense, the ONT  510  is similar to the ONT  210  of  FIG. 4 . 
         [0067]    Returning the embodiment of  FIG. 9 , MDU  530  includes four optical transmitters  531  through  534 , each corresponding to one of the MACs  535  through  538 . In this embodiment, each of the transmitters  531  through  534 , in cooperation with their associated MAC, transmits upstream using a different wavelength. For this reason they can (but are not required to) each transmit via optical fiber system  501  at the same time. Corresponding receivers  564  through  567 , each attuned to a different one of these wavelengths, accept the transmission only from the appropriate MAC. Each receiver  564  through  567  then provides the received signal to an associated upstream MAC for further processing. Note that each of the MACs  560  through  563  is associated with a respective bandwidth mapper  551  through  554  and PON delay function  555  through  558 . In this manner, four virtual PONs are created for transmission in the upstream direction. Note that while each of the four virtual PON channels associated with transmission in the upstream direction may transmit (at, for example,  2 . 5  Gbps) at the same time, these transmissions are separated by WDM and not (in this embodiment) combined using bit interleaving as in the downstream direction. 
         [0068]      FIG. 10  is a state diagram illustrating a process of channel assignment according to an embodiment of the present invention. It is noted that the state diagram of  FIG. 10  is in some respect similar to  FIG. 10-1  of the G.984.3 specification, from which it is derived. Returning to  FIG. 10 , at initial state ( 01 ) occurs when an ONU configured according to the present invention is switched on. At this state, the ONU asserts LOS/LOF, which clears when it receives downstream frames. The ONU then enters standby state ( 02 ) and waits for network parameters. Once the Upstream_Overhead Parameters message is received, the ONU reconfigures accordingly and moves to SN state ( 03 ), where it waits for a SN Request. According to the embodiment of  FIG. 10 , when the ONU receives an SN request and responds to the OLT, it receives an ONU-ID and VPON-ID. The VPON_ID may in some embodiments be used to distinguish upstream transmissions from the ONU, and to allow the ONU to identify the channel on which it is to receive downstream transmissions. If the received VPON_ID matches the ONU-ID, the ONU transitions to ranging state ( 04 ). If the VPON_ID is a new assignment, the ONU returns to initial state ( 01 ). The ONU preferably retains its VPON_ID in a non-volatile memory so that it&#39;s remembered on restart and subsequent ranging will be able to select correct the correct channel. In one embodiment, an unspecified Assign_ONU-ID field is used to make the VPON-ID assignment, and the Modify Serial_Number_ONU message may be used to pass the current VPON-ID upstream to the OLT. 
         [0069]    According to this embodiment, when the ONU reaches the ranging state ( 04 ), it waits for a ranging request and a determination is made of the required equalization delay. Once this is determined and the ONU receives an equalization delay message, it enters operation state ( 6 ) and may begin transmitting data in the upstream direction. 
         [0070]    Although multiple embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims. For example, in some embodiments, it may be desirable to employ the described downstream or upstream techniques in the other direction, as well or in lieu of the direction for which they are described herein.