Patent Publication Number: US-8976688-B2

Title: Grant scheduler for optical network devices

Description:
TECHNICAL FIELD 
     The disclosure relates to electronic scheduling of upstream traffic from multiple network devices on a shared telecommunication medium. 
     BACKGROUND 
     An optical network, such as a passive optical network (PON) as an example, often delivers voice, video and/or other data among multiple network nodes. In the case of a PON, the network nodes are often referred to as optical network terminals (ONTs). The PON can deliver data among multiple ONTs using a common optical fiber link. Passive optical splitters and combiners enable multiple ONTs to share the optical fiber link. An optical line terminal (OLT) transmits information downstream to the ONTs, and receives information transmitted upstream from the ONTs. Each ONT terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber or customer premises node. 
     Each ONT is connected to one or more customer premises equipment (CPE) devices, which ultimately receive the voice, video and other data delivered via the PON. Examples of CPE devices include computers, telephones, television set-top boxes or the like. An ONT on a PON may receive traffic from several sources. Some sources may be commonly used among several ONTs on a PON. For example, several ONTs may access a common traffic flow associated with switched digital video (SDV) or other multicast streams. Other sources may produce traffic flows that are unique to an individual ONT. For example, an individual ONT may receive web content from an Internet service provider (ISP) or voice data from the public switched telephone network (PSTN). 
     To schedule delivery of data, such as requests for data stored on a server located in a public network, upstream from ONTs to OLTs over the shared or common optical fiber link, the OLT maintains a scheduler that grants each of the ONTs to which it connects (or, more specifically, so-called traffic containers or “T-conts” storing packets within each of the ONTs) a time slot during which each of the ONTs may transmit their respective upstream data to the OLT. In order to schedule these grants, the OLT may determine how much data each of the ONTs has to transmit upstream. In some instances, the OLT may issue a request to the ONTs for a report of how much data is stored to each of its one or more upstream queues. Those ONTs that support this form of status reporting may respond with an approximate amount of how much data is stored to these one or more upstream queues. For those ONTs that do not support status reporting, the OLT may grant each of these ONTs a set amount of time to transmit upstream and then monitor the amount of traffic sent upstream by these ONTs during their granted set amounts of time. However, in monitoring upstream traffic, the OLT may not proactively schedule time slots to accommodate the upstream data waiting at the monitored ONTs. Instead, the OLT may react to the data sent upstream by these ONTs that do not support status reporting. In this reactive mode, the OLT may fail to schedule an adequate number of time slots to accommodate the amount of data waiting to be sent upstream by the monitored ONTs, which may hamper network operation in certain instances. 
     In any event, based on the reported status and/or the monitored status, the OLT may determine an upstream grant map that grants each of these ONTs one or more time slots during which these ONTs may transmit upstream data to the OLT. The OLT may generate this upstream grant map in a manner that accommodates different levels of service and past delivery of service using, for example, a weighted round robin or weighted fair queuing algorithm. Typically, the OLT determines this upstream grant map after each scheduling round, which is typically a set period of time during which the OLT may receive status reports from or after determining a monitored amount of data to be transmitted by the ONTs. The OLT determines this grant map at the end of each scheduling round as the OLT may not otherwise have enough information to schedule time slots appropriately for each of the ONTs. The period of time during which the OLT has to determine this grant map after receiving the status reports during any given scheduling round and monitoring upstream data before the grant map has to be sent downstream to the ONTs is, for example, approximately 125 micro seconds (μs) in a gigabit PON (GPON). Considering that the OLT also takes into consideration different levels of service and past delivery of service, computing this grant map in this limited amount of time may require significant resources in terms of processing power and storage capacity. 
     SUMMARY 
     In general, this disclosure describes techniques that provide a grant scheduler for an OLT that may pre-compute certain aspects required to determine the upstream grant map so as to reduce the requirements for processing power and storage capacity in comparison to conventional implementations. In accordance with these techniques, the OLT may convert upstream data amounts either reported by status-reporting-enabled ONTs or monitored by the OLT into grant cycles pending (GCPs) without waiting to receive all of the other status reports and/or determine the monitored data amounts for those ONTs that do not support status reporting. By pre-computing the GCPs as these upstream data amounts are received and/or determined, the OLT may reduce the number of computations that need be performed that are tangentially unrelated to generating the grant map, such as converting upstream data amounts to time slots, during the grant map generation period of time between receiving status reports or monitoring the last ONT and sending the upstream grant map downstream to the ONTs (which is commonly a time period of approximately 125 μs, as noted above). 
     By removing at least partially the conversion of data amounts into time slots through the conversion of data amounts into GCPs before the grant map generation time period, the OLT may then perform more computations during the grant map generation time period that may be considered more directly related to actual grant map generation, such as allocating time slots for ONTs based on the GCPs. In this manner, the techniques may enable the OLT to perform more iterations allocating time slots to ONTs during the grant map generation time period, which may result in better scheduling results as measured in terms of fair bandwidth allocation while also reducing burst sizes and latency considering that more bandwidth may be scheduled in comparison to conventional systems that both convert data amounts into time slots and allocate time slots during the grant map generation time period. 
     Moreover, techniques are described to better monitor those ONTs that do not support status reporting so as to improve OLT scheduling of monitored ONTs. These monitoring techniques may involve monitoring traffic sent downstream to these monitored ONTs. In response to detecting a marked increase in bandwidth usage by one of these monitored ONTs, the OLT then, in accordance with the techniques, allocates one or more additional grant cycles pending to that monitored ONT proactively, thereby improving grant response times in certain instances. The allocation of the number of additional grant cycles pending may be based on the extent of the increase in downstream bandwidth usage. While these techniques may be most beneficial for allocating upstream bandwidth to those ONTs that do not support status reporting, the techniques may also be applied with respect to those ONTs that do support status reporting to further improve grant response times in certain instances. 
     In one example, a method comprises determining, with an optical line terminal (OLT) coupled to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT, an amount of upstream data associated with a category of service that is waiting at a first one of the plurality of ONTs to be transmitted upstream to the OLT and computing a number of grant cycles pending (GCPs) with the OLT based on the determined amount of data associated with the category of service that is waiting at the first one of plurality of ONTs to be transmitted upstream to the OLT. The method also comprises computing a number of GCPs for each of the plurality of ONTs based on a determined amount of data associated with the category of service that is waiting to be transmitted upstream to the OLT for each of the plurality of ONTs and, after computing the number of GCPs for each of the plurality of ONTs, granting time slots to the one or more of the plurality of ONTs based on the number of GCPs computed for each of the plurality of ONTs, wherein the time slots comprise time slots for upstream communication form the ONTs to the OLT. 
     In another example, an optical line terminal (OLT) couples to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT. The OLT comprises a control unit that determines an amount of upstream data associated with a category of service that is waiting at a first one of the plurality of ONTs to be transmitted upstream to the OLT, computing a number of GCPs for each of the plurality of ONTs based on a determined amount of data associated with the category of service that is waiting to be transmitted upstream to the OLT for each of the plurality of ONTs and, after computing the number of GCPs for each of the plurality of ONTs, grants time slots to the one or more of the plurality of ONTs based on the number of GCPs computed for each of the plurality of ONTs, wherein the time slots comprise time slots for upstream communication form the ONTs to the OLT. 
     In another example, a network system comprises a plurality of customer network devices and a passive optical network (PON). The PON comprises an optical line terminal (OLT) and a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of the plurality of customer network devices and the OLT. The OLT comprises a control unit that determines an amount of upstream data associated with a category of service that is waiting at a first one of the plurality of ONTs to be transmitted upstream to the OLT, computes a number of grant cycles pending (GCPs) with the OLT based on the determined amount of data associated with the category of service that is waiting at the first one of plurality of ONTs to be transmitted upstream to the OLT, computing a number of GCPs for each of the plurality of ONTs based on the determined amount of data associated with the category of service that is waiting to be transmitted upstream to the OLT for each of the plurality of ONTs and, after computing the number of GCPs for each of the plurality of ONTs, grants time slots to the one or more of the plurality of ONTs based on the number of GCPs computed for each of the plurality of ONTs, wherein the time slots comprise time slots for upstream communication form the ONTs to the OLT. 
     In another example, a non-transitory computer-readable medium comprising instructions that, when executed, cause one or more processors to determine, with an optical line terminal (OLT) coupled to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT, an amount of upstream data associated with the category of service that is waiting at a first one of the plurality of ONTs to be transmitted upstream to the OLT, compute a number of grant cycles pending (GCPs) with the OLT based on the determined amount of data associated with the category of service that is waiting at the first one of plurality of ONTs to be transmitted upstream to the OLT, compute a number of GCPs for each of the plurality of ONTs based on the determined amount of data associated with the category of service that is waiting to be transmitted upstream to the OLT for each of the plurality of ONTs and, after computing the number of GCPs for each of the plurality of ONTs, grant time slots to the one or more of the plurality of ONTs based on the number of GCPs computed for each of the plurality of ONTs, wherein the time slots comprise time slots for upstream communication form the ONTs to the OLT. 
     In another example, a method comprises determining, with an optical line terminal (OLT) coupled to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT, an amount of upstream data that is waiting at one of the plurality of ONTs to be transmitted upstream to the OLT and determining, with the OLT, an amount of downstream data that is transmitted by the OLT to the one of the plurality of ONTs. The method also comprises increasing the determined amount of upstream data based on the determined amount of downstream data that is transmitted by the OLT to the one of the plurality of ONTs, after increasing the determined amount of upstream data, generating an upstream grant map that grants time slots to the one or more of the plurality of ONTs based on the amount of upstream data determined for each of the plurality of ONTs and transmitting, with the OLT, the upstream grant map downstream to the plurality of ONTs. 
     In another example, an optical line terminal (OLT) couples to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT. The OLT comprises a control unit that determines an amount of upstream data that is waiting at one of the plurality of ONTs to be transmitted upstream to the OLT, determines an amount of downstream data that is transmitted by the OLT to the one of the plurality of ONTs, increases the determined amount of upstream data based on the determined amount of downstream data is transmitted by the OLT to the one of the plurality of ONTs and after increasing the determined amount of upstream data, generates an upstream grant map that grants time slots to the one or more of the plurality of ONTs based on the amount of upstream data determined for each of the plurality of ONTs. The OLT also comprises at least one interface that transmits the upstream grant map downstream to the plurality of ONTs. 
     In another example, a network system comprises a plurality of customer network devices and a passive optical network (PON). The PON comprises an optical line terminal (OLT) and a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of the plurality of customer network devices and the OLT. The OLT comprises a control unit that determines an amount of upstream data that is waiting at one of the plurality of ONTs to be transmitted upstream to the OLT, determines an amount of downstream data that is transmitted by the OLT to the one of the plurality of ONTs, increases the determined amount of upstream data based on the determined amount of downstream data is transmitted by the OLT to the one of the plurality of ONTs and after increasing the determined amount of upstream data, generates an upstream grant map that grants time slots to the one or more of the plurality of ONTs based on the amount of upstream data determined for each of the plurality of ONTs. The OLT also comprises at least one interface that transmits the upstream grant map downstream to the plurality of ONTs. 
     In another example, a non-transitory computer-readable medium comprising instructions that, when executed, cause one or more processors to determine, with an optical line terminal (OLT) coupled to a plurality of optical network terminals (ONTs) that facilitate the transfer of data between a corresponding one of a plurality of customer network devices and the OLT, an amount of upstream data that is waiting at one of the plurality of ONTs to be transmitted upstream to the OLT, determine, with the OLT, an amount of downstream data that is transmitted by the OLT to the one of the plurality of ONTs, increase the determined amount of upstream data based on the determined amount of downstream data is transmitted by the OLT to the one of the plurality of ONTs, after increasing the determined amount of upstream data, generate an upstream grant map that grants time slots to the one or more of the plurality of ONTs based on the amount of upstream data determined for each of the plurality of ONTs and transmit, with the OLT, the upstream grant map downstream to the plurality of ONTs. 
     The details of one or more examples of the techniques are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a network system including an example of an optical line terminal (OLT) that implements grant scheduling techniques described in this disclosure. 
         FIG. 2  is a block diagram illustrating the OLT of  FIG. 1  in more detail. 
         FIG. 3  is a flowchart illustrating exemplary operation of an OLT in implementing universal grant scheduling aspects of the techniques described in this disclosure. 
         FIG. 4  is a flowchart illustrating exemplary operation of an OLT in implementing downstream traffic monitoring aspects of the techniques described in this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques that provide a grant scheduler for an OLT that may pre-compute certain aspects required to determine the upstream grant map so as to reduce the requirements for processing power and storage capacity in comparison to conventional implementations. In accordance with these techniques, the OLT may convert upstream data amounts either reported by status-reporting-enabled ONTs or monitored by the OLT into grant cycles pending (GCPs) without waiting to receive all of the other status reports and/or determine the monitored data amounts for those ONTs that do not support status reporting. By pre-computing the GCPs as these upstream data amounts are received and/or determined, the OLT may reduce the number of computations that need be performed during the grant map generation period of time between receiving the last status report or monitoring the last ONT and sending the upstream grant map downstream to the ONTs (which is commonly a time period of approximately 125 μs, as noted above). 
     During this 125 μs time period, which may be referred to generally as a “grant map generation time period,” the OLT may then only need generate the grant map from the pre-computed GCPs rather than both transform the data values to the equivalent of grant cycles and then generate the grant map from these grant cycles, as is common in conventional systems. While described as a two-step sequential process, during the grant map generation time period, the OLT may continue to convert data amounts into GCPs, updating existing GCPs with the computed amount of GCPs. In effect, the techniques may enable computation of GCPs in a manner that is independent from the grant map generation time period. By pre-computing these GCPs, the techniques may reduce the number of operations that need to be performed by the OLT during the grant map generation time period as the OLT may not need to both compute the grant cycles and then generate the grant map but only generate the grant map from the GCPs. By reducing the amount of processing power consumed by the OLTs in this manner, the techniques may enable this grant scheduler to be implemented using less processor intensive hardware, such as a field programmable gate array (FGPA), which may be more cost effective in terms of the actual cost and subsequent maintenance and troubleshooting of the OLT. 
     Moreover, techniques are described to better monitor those ONTs that do not support status reporting so as to improve OLT scheduling of monitored ONTs. These monitoring techniques may involve monitoring traffic sent downstream to these monitored ONTs. Typically, flows of traffic are monitored rather than ONTs, however, for purposes of discussion, monitoring of ONTs may include monitoring of flows directed to the ONTs as each flow identifies its destination ONT. In response to detecting a marked increase in bandwidth usage by one of these monitored ONTs, the OLT then, in accordance with the techniques, allocates one or more additional grant cycles pending to that monitored ONT proactively, thereby improving grant response times in certain instances. The allocation of the number of additional grant cycles pending may be based on the extent of the increase in downstream bandwidth usage. While these techniques may be most beneficial for allocating upstream bandwidth to those ONTs that do not support status reporting, the techniques may also be applied with respect to those ONTs that do support status reporting to further improve grant response times in certain instances. 
       FIG. 1  is a block diagram illustrating a network system  10  including an example of an optical line terminal (OLT)  12  that implements the grant scheduling techniques described in this disclosure. While described in this disclosure with respect to an optical line terminal  12  (“OLT  12 ”), the techniques may be implemented by any type of network device that schedules upstream grants over a shared medium, such as a PON, a cable modem termination system (CMTS), or other networks. 
     As shown in the example of  FIG. 1 , network system  10  includes OLT  12 , an optical splitter  14 , optical network terminals (ONTs)  16 A- 16 N (“ONTs  16 ”) and customer premises equipment (CPE)  18 A- 18 Z (“CPE  18 ”). OLT  12  generally represents an optical network device that resides in a central office of a service provider and serves as a point of origination for fiber-to-the-premises (FTTP) transmissions. OLT  12  generally performs operations to maintain connectivity and schedule data transmissions between OLT  12  and ONTs  16 . OLT  12  may, for example, perform auto-discovery to discover new ONTs  16 , range ONTs  16  to adjust data transmissions to reduce conflicts, schedule upstream data transmission from ONTs  16  to OLT  12 , and otherwise manage fiber line  20  coupling OLT  12  to each of ONTs  16 . 
     Optical splitter  14  represents a device that splits an optical signal into two or more copies of the optical signal. In other instances, optical splitter  14  represents a device that splits an optical signal composed of a number of different wavelengths into at least two different optical signals that each has a different subset of the wavelengths. Optical splitter  14  is presumed to be a passive optical splitter that merely splits an optical signal into two or more copies or replicas of the optical signal. In effect, optical splitter  14  splits optical signals sent over optical line  20  into a number of different sub-signals so that these sub-signals can be sent via optical fiber lines  21 A- 21 N (“optical fiber lines  21 ” or “optical lines  21 ”). 
     OLT  12  generally includes one physical interface to service a number of ONTs, such as ONTs  16 , where OLT  12  multiplexes a number of different optical signals of different wavelengths into a single optical signal that can be sent via the single physical interface to optical fiber line  20 . Optical splitter  14 , in this context, serves to copy the multiplexed optical signal and relay a copy of each of the optical signals to each one of ONTs  16 , where this copying is commonly referred to as “splitting.” This splitting, however, does not involve any active switching, but instead merely refers to creating a copy of any given optical signal and sending a copy of this signal to each of ONTs  16 . Optical splitter  14 , while referred to as a splitter, may also include a combiner that combines the various optical signal sent upstream from ONTs  16  to OLTs  12  into a single optical signal. Optical splitter  14  is commonly not powered and, as a result, may be referred to as “passive” both because it is not powered and because it does not actively route or switch optical signals. 
     Each of ONTs  16  represents an optical network device that terminates a corresponding one of optical fiber lines  21 . ONTs  16  receive their corresponding optical signals and further demultiplex these signals into different component parts, such as a voice component, a video component and a data component. ONTs  16  then forward these different components via various subscriber networks (which, in this case, may include a voice, video and data network or some combination thereof) to CPE  18 . CPE  18  each represents any device capable of receiving and processing the different components of the optical signal, such as a set-top box (STB), a digital recorder, a personal media player (PMP), a cellular phone (including so-called “smart phones”), a voice over Internet protocol (VoIP) telephone, an ordinary plain-old telephone system (POTS) phone, a television, a wireless network enabled television, a desktop computer, a laptop computer, a so-called “netbook,” a personal digital assistant (PDA), a server, a workstation, a wireless access point (WAP), a hub, and a switch. CPE  18  receives these different components of the optical signal and presents these varying components for consumption by customers that own and operate CPE  18 . 
     As noted above, each of CPE  18  typically receives a copy of every optical signal sent downstream from OLT  12  to any one of CPE  18 , as link  20  is shared by CPE  18  and optical splitter  14  does not actively switch or router these optical signals. Given the large amount of bandwidth provided by optical networks systems, such as network system  20  shown in the example of  FIG. 1 , this form of passive networking that involves replication or copying was widely adopted for its ease of maintenance and low cost. This passive form of data distribution is typically considered “easy to maintain” in that optical splitter  14  is not powered and does not require any extensive configuration. Typically, optical splitter  14  is installed in a post or other outside plant enclosure near the customer premises by inserting this splitter  14  in the enclosure and coupling splitter  14  to the proper links. Once properly inserted and coupled, little if any other configuration is required for optical splitter  14  to begin properly “splitting” or copying the optical signals on a passive basis. 
     The passive form of network is lower in cost than actively switching networks because it does not require power, which can be costly when having to power hundreds if not thousands of active switches. Moreover, un-powered devices, such as passive splitter  14 , may be, as noted above, easier to maintain, leading to lower maintenance associated costs over comparable active optical networks. For this reason, passive optical networks were widely deployed by service provider networks to provide so-called “triple-play” package (involving delivery of a combined package of video, data and voice services) to subscribers. 
     In contrast to downstream communications from OLT  12  to ONTs  16  where all such downstream communications are combined over shared link  22  and then replicated to each of ONTs  16 , upstream communications from ONTs  16  to OLT  12  are managed by OLT  12  through implementation of a downstream bandwidth allocation (DBA) algorithm. Initially, service provider networks deployed basic OLT and ONTs that implemented basic DBA algorithms. For example, OLT  12  assigned time slots to ONTs  16  during which the ONTs  16  may communicate upstream based solely on the service agreement between the customer associated with each of ONTs  16  and the service provider. Typically, OLT  12  implemented a static table identifying a subscribed-to amount of bandwidth for each of the services to which each ONT  16  subscribed regardless of whether the customer actually utilized this assigned upstream bandwidth. These time slots may be variable-sized time slots or fixed-sized time slots, although commonly, OLT  12  allocates variable-sized time slots in the manner defined by the GPON or other PON standards. 
     As data services grew in adoption (with the notable almost exponential surge in Internet data traffic on a yearly basis) and more data that was commonly provided via different communication media converging with the offering of triple-play packages, the generous amount of upstream bandwidth offered by passive optical networks was consumed by bursty requests for Internet traffic during certain periods of the day, e.g., such as when consumers arrived home from work or when they got up in the morning. Not wanting to increase the outlay of additional fiber links and deployment of additional OLTs and ONTs to satisfy this relatively bursty and often periodic upstream bandwidth consumption, service providers sought ways to oversubscribe the network while still satisfying minimum bandwidth requirements for services specified in subscription contracts (which are also often referred to as “service agreements”) with consumers. Responding to this request, optical network device manufactures soon provided updated OLTs and ONTs that implemented more advanced DBA algorithms. While this is commonly considered the reasons optical network device manufactures provided these updated OLTs and ONT, there are any number of other reasons and context for developing updated OLTs and ONTs. The above is provided merely for context and should not be considered to limit the techniques described in this disclosure in any way. 
     For example, some PON standards, such as the GPON standard set forth in a standard identified as an International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) G984.2 standard, began introducing a more advanced DBA algorithm that specified a way of polling or querying ONTs for an approximate amount of data waiting to be sent upstream to the OLT. In response to this query, the ONTs issued status reports indicating the approximate amount of data waiting to be sent upstream to the OLTs for a given traffic container (or “T-CONT”). The term “traffic container” generally refers to any container for storing upstream data (such as a buffer, memory, virtual memory, virtual buffer, cache or the like). Typically, T-CONTs are provided for each of the data, voice and television services as well as for other special traffic and may represent a way to differentiate different types of traffic that have different service classes. In any event, this status reporting form of DBA enables OLTs to actively determine an amount of data waiting to be sent upstream for each of its ONTs and then proactively assign or grant time slots to each of the ONTs so as to accommodate oversubscription of shared links, such as link  22 . 
     To illustrate, assuming OLT  12  implements this more advanced form of status reporting (SR) DBA, OTL  12  may poll ONTs  16  and receive status reports from each of ONTs  16  regarding an approximation of a current amount of data waiting to be sent upstream to OLT  12 . In this example, it is also assumed that half of ONTs  16  have data waiting to be sent upstream to OLT  12  while the other half of ONTs  16  do not have data waiting to be sent upstream to OLT  12 . Collectively, ONTs  16  may be entitled to more upstream bandwidth than that physically capable of being provided by shared link  22 , which results in oversubscription of shared link  22 . Yet, considering that this upstream traffic is bursty in nature, shared link  22  may accommodate approximately two thirds of ONTs  16  at any one time when providing ONTs  16  with their maximum upstream bandwidth to which they are entitled under the service agreement. Without SR DBA, OLT  12  may inefficiently allocate bandwidth to the half of ONTs  16  that do not have data waiting to be sent upstream to OLT  12 . By polling these ONTs  16 , however, OLT  12  may more accurately grant upstream time slots to only the half of ONTs  16  that actually have data waiting to be sent upstream to OLT  12 . In this manner, SR DBA accommodates oversubscription of shared link  22 . 
     While SR DBA provides many benefits in terms of granting upstream bandwidth, often legacy ONTs are deployed by service providers alongside these more advanced forms of ONTs. To provide a similar form of DBA with respect to these legacy ONTs, OLTs were deployed or upgraded to support a form of DBA referred to as traffic monitoring DBA. In traffic monitoring DBA, the OLT initially grants these upstream time slots (which refer to time slots during which these legacy ONTs may transmit data upstream to the OLT) in accordance with their service agreements. The OLT then monitors the transmission of data during each of these granted upstream time slots to determine whether each ONT has any data to transmit upstream, and, if they have data to send upstream, how much data was sent. The OLT then maintains a table or other data structure to store this historical upstream data transmission information. Going forward, the OLT refers to this table when granting upstream time slots to the ONTs. ONTs may additionally request upstream time slots in this implementation so as to provoke a monitoring session and thereby gain access to additional time slots. Alternatively, the OLT may periodically grant each of the ONTs an upstream time slot to enable this monitoring and data collection. 
     Such a traffic monitoring DBA may suffer, however, in certain instances or contexts due to the reactive nature of traffic monitoring DBA. That is, this traffic monitoring DBA may be considered “reactive” in that the OLT only learns of upstream data waiting to be sent by the ONT after some data has been sent by that ONT and then reacts to this data to assign the ONT more upstream time slots if the ONT fully utilizes the previously assigned time slots. Such reactive scheduling of upstream bandwidth may result in scheduling increasingly more upstream time slots for an ONT that has a lot of upstream data waiting to be sent to the OLT, which may result in inefficiencies in certain contexts. 
     For example, when a protocol by which this data is to be sent includes some bandwidth limiting or windowing mechanism that determines a number of packets that may be sent prior to receiving an acknowledgement based on a time to respond to downstream data packets sent by the other device terminating the session (such as a transmission control protocol or TCP), the reactive nature of traffic monitoring DBA may result in a grant time that delays transmission of upstream data to the extent that these protocols impose a what limit on the number of packets that may be sent via the windowing mechanism. The OLT then monitors this ONT, noting that the ONT is only transmitting during a portion of granted time slots (due to the TCP packet limit), whereupon the OLT then allocates less time slots despite the fact that this ONT may still have large amounts of data waiting to be sent. As the OLT continues to send this data, TCP may enlarge the number of packets that may be sent (which may be referred to as a “window”), whereupon the OLT may then assign more time slots. This process may continue with TCP expanding while the OLT contracts bandwidth allocations, resulting in bandwidth utilization inefficiencies. 
     In any event, OLTs typically schedule those ONTs that implement SR DBA separately from those ONTs that do not support SR DBA, as these different grant scheduling algorithms are typically implemented differently and rely on different scheduling periods. Moreover, when performing traffic monitoring, the OLT may behave reactively, as noted above. When performing SR DBA, however, the OLT may be considered to act proactively in that the OLT learns of this data before it is sent and may adjust the number of time slots granted accordingly. This fragmented treatment to scheduling grants of upstream time slots often results in overly complicated scheduling algorithms. Often, when attempting to schedule these ONTs collectively, the OLT may provide disjointed treatment of ONTs depending on whether they support SR DBA or not, resulting in varying degrees of service for different customers despite the fact that these different customers may subscribe to the same type and level of service. 
     In addition, when implemented within the same scheduler, the grant scheduler of the OLT usually waits until a scheduling round (during which the OLT may receive data identifying an amount of data to be transmitted upstream from each ONT to which the OLT connects and/or monitor upstream data communications from the ONT) has completed prior to computing the grant map that allocates the upstream time slots to the OLTs during the upcoming upstream transmission period. The grant scheduler waits until the completion of the scheduling round during which it receives or monitors this upstream information as without this information the grant scheduler is unable to adequately assess which of the OLTs require upstream time slots. In the time between the completion of the scheduling round and generating this grant map (which may be approximately 125 microseconds (μs)), the OLT is required to determine and generate this grant map. Yet, computing this grant map involves a number of different mathematical computations, some of which involve multiplications and/or divisions, which may take many processor cycles to complete. 
     Moreover, given the dynamic nature of both SR and traffic monitoring DBA in conjunction with the various weights that may be assigned to accommodate different service classes or levels and the necessity of generally meeting the minimum requirements specified in the service contract, the grant scheduler may have to perform a significant number of computations in what may be considered a very short period of time. Typically, implementation of a grant scheduler of this nature involves significant processing resources and/or specifically designed dedicated hardware, such as application specific integrated circuits (ASICs). While such components are not uncommon, these components increase the cost in terms of both initial purchase price and potential maintenance requirements. 
     Furthermore, various PON standards are frequently being revised or otherwise altered. Given that ASICs are specially designed to implement these standards, changes to these ASICs so as to implement the updated or revised standards are often difficult and may, in some instances, require that completely new ASICs be designed. Given the inflexibility of ASICs to accommodate some PON revisions, ASIC-based OLTs may be expensive to maintain over the life of multiple PON revisions, especially when such revisions require a new ASIC be developed and installed across what may be tens, if not hundreds, of OLTs. ASICs may, therefore, generally be considered inflexible with respect to any changes and result in increased administrative and other maintenance costs. In accordance with the grant scheduling techniques described in this disclosure, OLT  12  includes a universal grant scheduler unit  24  that potentially reduces the number of computations required to be performed during this short period of time between collecting all of the status reports and transmitting the grant map. By reducing the number of computations, universal grant schedule unit  24  may not require the significant hardware resources or specially designed dedicated hardware conventionally required to perform these operations during what is conventionally thought of as the grant map generation period of time between determining the last amount of data to be sent upstream by ONTs  16  and transmitting the grant map, which may reduce costs associated with manufacturing OLT  12 . 
     In some instances, the techniques may reduce the processing requirements during this short period of time post data collection and pre-grant map transmittal such that universal grant scheduler unit  24  may be implemented as programmable hardware, such as a field programmable gate array (FPGA). There are a number of benefits associated with FPGAs including flexibility and rapid prototyping (in comparison to prototyping and development of ASICs), cost in terms of nonrecurring engineering expense (especially in comparison to designing an ASIC), reliability that is on par with dedicated hardware (including ASICs), and long-term maintenance benefits in that FPGAs are field upgradeable to account for changing design requirements. 
     To reduce the number of computation, universal grant scheduler unit  24  is configured to operate on what is referred to in this disclosure as “grant cycles pending” or “GCPs.” GCPs represent an intermediate form of the amount of data waiting to be sent upstream by ONTs  16  in that GCPs represent the number of grant cycles that would be consumed to transmit all of the data waiting to be sent upstream at each of ONTs  16 . By converting the data amounts into GCPs for each of ONTs  16 , universal grant scheduler unit  24  need not perform this conversion during the grant map generation period of time, thereby significantly reducing the number of operations that need be performed during this shortened grant map generation period of time. This form of abstraction from amounts of data to GCPs hides the underlying DBA algorithm employed to learn of these amounts of data from universal grant scheduler unit  24 . Instead, universal grant scheduler unit  24  may be considered “universal” in the sense that it can schedule upstream time slot grants without regard to whether ONTs  16  support SR DBA or not. By implementing the techniques described in this disclosure, universal grant scheduler unit  24  may then base the allocation of upstream bandwidth allocation on grant cycles pending, fully unaware of whether these grant cycles pending correspond to traffic monitored, reported data amounts or any other way by which data amounts are reported. By effectively removing the responsibility of collecting this data and converting this data into grant cycles, universal grant scheduler unit  24  may more efficiently generate the grant map. 
     To illustrate, OLT  12  may first determine an amount of upstream data that is waiting at a first one of ONTs  16  (e.g., ONT  16 A) to be transmitted upstream to OLT  12 . Typically, OLT  12  determines this amount of data with respect to a category of services, which may include a quality or class of service or any other identifier that identifies a category of service to which this data or traffic may be associated. ONTs  16  may generally store data associated with different categories of service to different queues (or traffic containers as they are commonly abstracted in the optical network context), whereupon OLT  12  may determine the amount of data that is associated with this category of service. In some instances, OLT  12  may determine an amount of data regardless of the category of service. 
     OLT  12  may determine this amount via traffic monitoring DBA or via status reports sent by ONT  16 A to OLT  12  in the manner described above. When performing traffic monitoring DBA, OLT  12  may determine a category of service of monitored traffic by determining a protocol or other aspect of the packets that is associated with different types or categories of service. For example, hypertext transfer protocol (HTTP) traffic is generally identified by an HTTP header and use of Internet Protocol (IP) port  80 . OLT  12  may maintain or otherwise store data identifying associations between certain protocol/port pairs and a category of service. 
     Regardless, OLT  12  may then, prior to determining an amount of data that is waiting at a second one of ONTs  16  (e.g., ONT  16 N) to be transmitted upstream to OLT  12 , compute a number of grant cycles pending (GCPs) based on the determined amount of data that is waiting at ONTs  16 A to be transmitted upstream to the OLT  12 . While not shown in the example of  FIG. 1  for ease of illustration purposes, OLT  12  may include a translator unit that converts data amounts into GCPs using a conversion formula. The conversion formula may be statically defined or dynamically specified or configurable by a user. This translator of OLT  12  then stores the computed number of GCPs for the ONT  16 A to a GCP table, which again is not shown in  FIG. 1  for ease of illustration purposes. 
     In this manner, a portion of the process that is conventionally performed during the short period of time after determining the last amount of data waiting to be sent upstream by ONTs  16  and transmitting the grant map may be performed at least in part prior to this shortened grant map generation period of time. As a result, universal grant scheduler unit  24  may not be required to also convert these amounts of data to grant cycles during this shortened period of time, thereby potentially reducing the number of operations that need be performed during this period of time. Instead, universal grant scheduler unit  24 , after the translator stores the computed number of GCPs to the GCP table for each of the plurality of ONTs  16 , determines an upstream grant map that grants time slots (which may also be referred to as “grant cycles”) to one or more of ONTs  16  based on the computed number of GCPs stored to the GCP table for each of ONTs  16 . Universal grant scheduler unit  24  may still implement the weighted round robin algorithm or any other scheduling algorithm only with respect to GCPs instead of amounts of upstream data waiting at ONTs  16  to be sent upstream. Universal grant scheduler  24  then transmits via an interface coupling OLT  12  to link  22  and thereby ONTs  16  the upstream grant map downstream to ONTs  16 . 
     In addition, OLT  12  includes a downstream monitoring unit  26  that may enhance how grant cycles pending are allocated to traffic monitored ones of ONTs  16  in accordance with various aspects of the techniques described in this disclosure. Downstream monitoring unit  26  may represent a unit that monitors downstream traffic (i.e., traffic received by OLT  12  that is destined to one or more of ONTs  16 ) typically for those of ONTs  16  that do not support SR DBA. However, while described as being typically applied to those ONTs  16  that do not support SR DBA, the techniques may implemented such that downstream monitoring unit  26  monitors traffic for one or more and potentially all of ONTs  16  regardless of whether these ONTs  16  support SR DBA or not. 
     Downstream monitoring unit  26  may determine which of ONTs  16  to monitor based on some configuration data input by an administrator. Alternatively, downstream monitoring unit  26  may be configured to automatically monitor downstream traffic for those of ONTs  16  that do not respond to a status request that requests an approximate amount of upstream data is waiting to be sent upstream at each of ONTs  16  to OLT  12 . The term “automatically” is used in this context to mean that the administrator does not indicate for which of ONTs  16  that downstream monitoring unit  26  is to monitor their downstream traffic. Rather, downstream monitoring unit  26  may automatically determine without being specifically configured by the administrator which of ONTs  16  do not support SR DBA based on status reports received from ONTs  16 . Downstream monitoring unit  26  may then generate a list or any other type of data structure defining those of ONTs  16  that did not respond with a status report and monitor those of ONTs  16  specified within the list or other data structure. 
     Downstream monitoring unit  26  may identify downstream traffic intended for one or more of ONTs  16  based on, for example, a layer two (L2) address associated with ONTs  16  or any other identifier associated with ONTs  16 , such as a PON ID, a virtual local area network (VLAN) tag, or a layer three (L3) address. Downstream monitoring unit  26  may maintain a table or other data structure having entries for each of the ONTs  16  for which downstream traffic monitoring has been enabled. To each entry, downstream monitoring unit  26  may store data defining an amount of downstream traffic sent to each of ONTs  16 . Periodically or, in some instances, in response to updating an entry, downstream monitoring unit  26  may forward this updated data to the translator unit, which determines a number of additional GCPs for each of these ONTs  16  based on the amount of downstream traffic sent to each of the corresponding ones of ONTs  16 . Again, this translator unit may employ a conversion formula or any other mechanism for determining how to convert the amount of downstream traffic into additional GCPs to assign to each of these ONTs  16  for delivery of upstream data to OLT  12 . This translator unit may update entries in the GCP table that correspond to each of the downstream monitored ones of ONTs  16  with the additional GCPs so as to allocate additional upstream bandwidth to these ones of ONTs  16 . 
     By attempting to proactively schedule more upstream bandwidth for those of ONTs  16  that do not support SR DBA, the techniques may reduce what may be characterized as the “thrashing” of bandwidth allocation in certain contexts, such as in the TCP windowing context described above. To illustrate, consider the TCP windowing example above, where the OLT fails to grant a time slot in time for the OLT to response to a TCP packet sent by another device that terminates the TCP session. In this example, downstream monitoring unit  12  of OLT  12  may proactively add additional GCPs for those of ONTs  16  receiving downstream data and that do not support SR DBA. Universal grant scheduler unit  24  in this context may then, when generating the grant map, determine that these ones of ONTs  16  have relatively more upstream data waiting to be sent than other ones of ONTs  16 , granting these ones of ONTs  16  additional upstream time slots. These ones of ONTs  16  may then respond more quickly to the downstream data, ensuring that the TCP windowing mechanism does not drastically reduce the window (i.e., the number of packets that these ones of ONTs  16  may send in response to an acknowledgement by the other device terminating the TCP session). In this sense, the enhanced traffic monitoring DBA aspects of the techniques set forth in this disclosure may attempt to add a certain amount of proactivity to what is conventionally considered a reactive form of DBA to potentially reduce or possibly eliminate the thrashing of bandwidth allocation in certain contexts. 
     Although the grant scheduling and downstream monitoring aspects of the techniques are described as being implemented in conjunction with one another, the grant scheduling and downstream monitoring aspects of the techniques may not necessarily be implemented in this manner. In some instances, OLT  12  may implement only the grant scheduling techniques or only the downstream monitoring aspects of the techniques described in this disclosure without implementing the downstream monitoring or grant scheduling aspects of the techniques described in this disclosure. Consequently, while described as being implemented by the same OLT, i.e., OLT  12  in this example, the techniques should not be limited in this respect. 
     While described in this disclosure generally with respect to passive optical networks (PONs), the techniques described in this disclosure may be implemented with respect to any type of passive optical network, such as a broadband PON (BPON), a gigabyte PON (GPON), an Ethernet PON (EPON), and the like. For example, optical transport system  10  may function in accordance with the giga-bit PON (GPON), baseband PON (BPON), or Ethernet PON (EPON) standards, or other standards. The GPON, BPON, and EPON standards are defined by ITU-T G.984.2 and G983.3, ITU-T 983.1, respectively. 
     Moreover, while the techniques are described in reference to optical network terminals, the techniques may be implemented by any network device capable of terminating an optical link or line and providing data transmitted via the optical link or line to a customer or subscriber network, including an optical network unit (ONU). In some instances, the term ONU is used interchangeably with the term ONU, and the techniques should not be limited to either an ONT or ONU but to any network device capable of terminating an optical link or line and providing data transmitted via the optical link or line to a customer or subscriber network. 
       FIG. 2  is a block diagram illustrating OLT  12  of  FIG. 1  in more detail. In the example of  FIG. 2 , OLT  12  includes a control unit  30  and interfaces  32 A- 32 N (“interfaces  32 ”). Control unit  30  may represent one or more processors (not shown in example of  FIG. 2 ) that execute software instructions, such as those used to define a software or computer program, stored to a computer-readable storage medium (again, not shown in the example of  FIG. 2 ), such as a storage device (e.g., a disk drive, or an optical drive), or memory (such as Flash memory, random access memory or RAM) or any other type of volatile or non-volatile memory, that stores instructions to cause a programmable processor to perform the techniques described herein. Alternatively, control unit  30  may represent dedicated hardware, such as one or more integrated circuits, one or more Application Specific Integrated Circuits (ASICs), one or more Application Specific Special Processors (ASSPs), one or more Field Programmable Gate Arrays (FPGAs), or any combination of one or more of the foregoing examples of dedicated hardware, for performing the techniques described herein. 
     Each of interfaces  32  represents an interface for interfacing with a physical communication medium, such as an optical fiber link  20  and optical fiber link  22 . In the example of  FIG. 2 , interface  32 A is assumed to couple to optical fiber link  22  while interface  32 N is assumed to couple to optical fiber link  20 . An interface card (IFC) may comprise one or more of interfaces  32 , which in this context may be referred to as ports. Each IFC may be inserted into a slot of a chassis that couples to control unit  30  via a backplane or other high-speed interconnection. 
     Control unit  30  includes universal grant scheduler unit  24  and downstream monitoring unit  26 , where were described above with respect to the example of  FIG. 1 . Universal grant scheduler unit  24  represents a unit that implements the grant scheduling aspects of the techniques described in this disclosure to generate a grant map scheduling time slots during which upstream data may be sent by one or more of ONTs  16  based on grant cycles pending or GCPs rather than actual or approximated data amounts determined to be waiting at ONTs  16  for upstream transmission. Downstream monitoring unit  26  represents a unit that monitors downstream traffic  40  received by ONT  12  via interface  32 A over link  22  for delivery to one or more of ONTs  16 . 
     Control unit  30  also includes a status reporting downstream bandwidth allocation (SR DBA) unit  34  (“SR DBA unit  34 ”), a traffic monitoring (TR) DBA unit  36  (“TM DBA unit  36 ”) and a translator unit  38 . SR DBA unit  34  represents a unit that implements SR DBA as specified by any applicable PON standard, such as the GPON standard set forth by the ITU-T G.984.2. TM DBA unit  36  represents a unit that monitors upstream traffic  42  received via interface  32  via link  20  from ONTs  16  for one or more of ONTs  16  and generates additional GCPs in accordance with various aspects of the techniques described in this disclosure. Translator unit  38  represents a unit that translates actual or approximate data amounts of traffic waiting upstream (and downstream in instances where both the universal grant and downstream traffic monitoring aspects of the techniques are implemented together) into GCPs. 
     Initially, an administrator or other user may interface with a user interface presented by a user interface module or unit of control unit  30 , which is not shown for ease of illustration purposes. The administrator may configure or otherwise specify monitoring parameters  44  for downstream monitoring unit  26  and monitoring parameters  46  for TM DBA unit  36 . Both monitoring parameters  44  and  46  may specify those of ONTs  16  that downstream monitoring unit  26  should monitor or otherwise configure downstream monitoring unit  26  and TM DBA unit  36  to automatically detect those ONTs  16  that do not support SR DBA based on status reports received from ONTs  16 , such as status reports  50  (“stat rep  50 ”), in the manner described above. Monitoring parameters  44  may also define a threshold or other metric to which downstream monitoring unit  26  compares monitored downstream traffic amounts stored to downstream table  52 . 
     Downstream monitoring unit  26  may also store downstream table  52  such that this table  52  includes an entry for each of those ONTs  16  specified by monitoring parameters  44  and/or that do not support SR DBA (as automatically detected based on status reports  50 . Each entry stores an amount of data sent downstream to corresponding ones of ONTs  16 . Downstream monitoring unit  26  may compare the amount stored to each entry to this threshold and, if the amount exceeds the threshold, downstream monitoring unit  26  may forward this amount to translator unit  38 , which effectively adds additional GCPs to an entry in GCP table  54  associated with that one of ONTs  16 . If the amount does not exceed the threshold or other configurable or pre-defined metric, downstream monitoring unit  26  continues to monitor downstream traffic  40  and update downstream table  52 . 
     Monitoring parameters  44  may also define a wasting or decay function that downstream monitoring unit  26  may apply to clear or otherwise reduce amounts stored to downstream table  52 . Downstream monitoring unit  26  may reduce amounts stored to downstream table  52  as a function of time so that downstream monitoring unit  26  may only report surges in downstream traffic to any given one of ONTs  16  that may require these ones of ONTs  16  to quickly respond so as to avoid thrashing bandwidth allocation when the TCP or other protocols windowing or bandwidth limitation mechanisms interfere with bandwidth allocation by OLT  12 . 
     In any event, once configured, the administrator may enable ONT  12  to begin receiving and forwarding both downstream traffic  40  and upstream traffic  42 . Interface  32 A may receive downstream traffic and replicate this traffic, forwarding this replicated downstream traffic  40  to downstream monitoring unit  26 . Interface  32 A may replicate this traffic  40  so that it can forward the original traffic via interface  32 N to ONTs  16  while downstream monitoring unit  26  performs its monitoring tasks. Likewise, interface  32 N may receive upstream traffic  42  from ONTs  16  and replicate this traffic  42  while sending the original upstream traffic  42  upstream via interface  32 A. For this reason, replicated downstream traffic  40  and replicated upstream traffic  42  is shown as a dashed line to indicate that it represents replicated traffic rather than the original traffic, which may be switches or otherwise forwarded upon receipt to reduce latency. Although described with respect to replicated traffic  40 ,  42 , the techniques may be implemented with respect to OLTs that do not replicate traffic but operate on the original traffic. The techniques should therefore not be limited in this respect to the example of  FIG. 2 . 
     As noted above, OLT  12  does not schedule transmission of downstream traffic  40  downstream to ONTs  16  considering that link  20  is shared by all of ONTs  16  and therefore each of ONTS  16  receive downstream traffic  40  whether destined for that particular one of ONTs  16  or not. ONTs  16  then filter downstream traffic  40  that is not destined for them typically by performing hardware-level layer two address filtering, such as media access control (MAC) address filtering. Consequently, OLT  12  forwards downstream traffic  40  upon receipt of this traffic  40  unless these time slots during which this downstream traffic is to be forwarded has been reserved for upstream transmissions from ONTs  16  to OLT  12 . 
     In order to schedule these upstream time slots, SR DBA unit  34  may initially issue a status request  48  (“stat req  48 ”) over link  20  via interface  32 N to ONTs  16 . Those of ONTs  16  that support SR DBA may respond with one of status reports  50 . SR DBA unit  34  may parse these status reports  50  to extract an amount of data waiting at the one of ONTs  16  that sent the corresponding one of status reports  50 . The amount of data reported may be exact in or approximated. For example, the amount of data may be reported as a number of data blocks, where a block may represent one or more bytes. Thus, if this block is configured to be 4 bytes and an ONT has 42 bytes waiting to be sent upstream, the ONT may report that it has 11 blocks (which the OLT will interpret as 44 bytes) waiting to be sent upstream, which is only an approximate amount of data waiting to be sent upstream as the ONT actually has 42 bytes. Typically, if the amount of data waiting to be sent upstream is small (e.g., smaller than 128 kilobytes (KBs)), those of ONTs  16  that support SR DBA may specify the amount approximately by way of a signaling a number of blocks. However, if the amount of data waiting to be sent upstream is large (e.g., greater than 128 KBs), those of ONTs  16  that support SR DBA may specify a code indicating whether the amount of data waiting to be sent upstream is between 128 KBs and 256 KBs, between 256 KBs and 512 KBs, and so on. 
     SR DBA unit  34  may forward status reported amounts  53  to translator unit  38 , which employs conversion formula  54  to convert status reported amounts  53  into GCP  56 . SR DBA unit  34  may also provide a list  51  of those ONTs  16  that do not support SR DBA to downstream monitoring unit  26  and TM DBA unit  36 , which may enable these units  26 ,  36  to determine which of ONTs  16  to monitor when configured to automatically monitor those ONTs  16  that do not support SR DBA. SR DBA unit  34  may determine this list  51  based on status reports  50 . For those ONTs  16  that SR DBA unit  34  did not receive one of status reports  50 , SR DBA unit  34  may add those ONTs  16  to list  51 . 
     Whether configured to automatically monitor those of ONTs  16  that do not support SR DBA or configured with a list of ONTs  16  to monitor via monitoring parameters  44 , downstream monitoring unit  26  may monitor downstream traffic  40 , updating downstream table  52  to log the amounts of data sent downstream to each of these monitored ones of ONTs  16 . Downstream monitoring unit  26  may periodically or, in response to updating downstream table  52 , apply one or more of the above noted thresholds to determine whether to report this downstream data amount for one or more of the monitored ones of ONTs  16 . 
     As noted above, if a downstream data amount stored to downstream table  52  exceeds one of the thresholds, downstream monitoring unit  26  may forward this downstream data amount  58  to translator unit  38  (in instances where both the downstream monitoring and universal grant scheduling aspects of the techniques are implemented in conjunction with one another). Translator  38  applies conversion formula  54  to convert this downstream data amount  58  to GCPs  56 , which translator  38  then stores to GCP table  54 . If the amounts stored to downstream table  52  do not exceed the threshold, downstream monitoring unit  26  continues to update downstream table  52  based on downstream traffic  40 . Downstream monitoring unit  26  may also apply a wasting or decay function to decrement the amounts stored to downstream table  52  periodically as noted above. 
     Universal grant scheduler unit  24  may, meanwhile, generate an initial grant map  60  allocating each of ONTs  16  (or those of ONTs  16  that are active or otherwise powered on) at least one upstream time slot during which each of ONTs  16  may communicate data upstream. This initial grant map  60  may facilitate upstream bandwidth monitoring performed by TM DBA unit  36 . If all of ONTs  16  support SR DBA, then universal grant scheduler unit  24  may forgo this initial grant map  60  and instead generate a grant map  60  that allocates time slots based on GCPs  56  translated from reported data amounts  53  by translator unit  38 . Otherwise, if not all of ONTs  16  support SR DBA, this initial grant map  60  that allocates at least one time slot to each of ONTs  16  (assuming that all of ONTs  16  are active) provides TM DBA unit  36  an opportunity to monitor upstream bandwidth transmissions for each of ONTs  16  or, at least, those of ONTs  16  specified by list  51 . 
     TM DBA unit  36  receives this initial grant map  60  and uses this initial grant map  60  to determine when to monitor upstream traffic  42 , i.e., which of the upstream time slots correspond to ONTs  16  that do not support SR DBA. Alternatively, TM DBA unit  36  may monitor all upstream traffic  42  without reference to grant map  60 . TM DBA unit  36  then monitors upstream traffic  42 , logging amounts of data sent upstream to upstream table  56 , which may be similar to downstream table  52  only for monitoring upstream traffic  42 . TM DBA unit  36  may then report upstream data amounts  62  stored to upstream table  56  after monitoring those ONTs  16  that do not support SR DBA. While described as involving a table  56 , the techniques may be implemented such that TM DBA unit  36  merely buffers a data amount for each of ONTs  16  and immediately reports this data amount rather than storing a data amount to a table. Whether stored to a table or not, TM DBA unit  36  forwards upstream data amounts  62  to translator unit  38 , which applies conversion formula  54  to these amounts  62  to convert amounts  62  into GCPs  56 . Translator unit  38  then stores GCPs  54  for those of ONTs  16  that do not support SR DBA to a corresponding entry of GCP table  54 . 
     As described above, translator unit  38  converts data amounts  53 ,  58  and  62  to GCPs  56  prior to the grant map generation period of time between receiving the last one of status reports  50  or monitoring and reporting the last one of upstream data amounts  62  and the actual generation of grant map  64 . In converting amounts  53 ,  58  and  62  to GCPs  56  prior to the grant map generation time period, translation unit  38  may effectively reduce the number of operations universal grant scheduler unit  24  need perform during this short grant map generation time period. Translation unit  38  stores these GCPs  56  to GCP table  54 . 
     Universal grant scheduler unit  25  then, during the grant map generation time period, evaluates GCP table  54  in view of parameter table  66  and service level agreement (SLA) table  68  (“SLA table  68 ”). Parameter table  66  may define a maximum bandwidth supported by link  20  as well as other parameters relevant to the delivery of data via the PON, including a total PON bandwidth available to the DBA scheduler, a grant time overhead (which includes a guard band, preamble and delimiter) and a grant size to use for each OLT (actually, for each traffic container or so-called “T-CONT” provided by each OLT). SLA table  68  may define service level agreements or what may be referred to as “service classes” for various services to which customers may describe. Such service classes may define a maximum permitted bandwidth, minimum guaranteed bandwidth, minimum and maximum latency requirements and other service level metrics that may limit or otherwise define a level of service provided to customers that subscribed to that level of services. SLA table  68  may further specify an association between each of ONTs  16  and one or more SLA agreements, where more than one SLA agreement may be associated to one of ONTs  16  when that ONT delivers more than one service, such as voice, data and video services. Universal grant scheduler unit  24  may implement a round-robin scheduling algorithm, a weighted-round-robin scheduling algorithm, a deficit round-robin scheduling algorithm or any other type of scheduling algorithm capable of scheduling utilization of a limited resource, which is link  20  in this instance. 
     To illustrate the generation of grant map  64 , universal grant scheduler unit  24  may first access parameter table  66  to determine a total bandwidth that can be provided via link  20 . Universal grant scheduler unit  24  may then determine a service level agreement associated with a first one of ONTs  16  by accessing SLA table  68  using what is commonly referred to as an allocation identifier (which is commonly referred to as an “allocID”). An allocID may uniquely identify a traffic container (which is commonly referred to as a “T-cont”) of one of ONTs  16 . As noted above, ONTs  16  may include one or more traffic containers, such as one for voice, a second for data and a third for video. Downstream monitoring table  52  may associate monitored data amounts  58  with PON identifiers (IDs) in downstream table  52 , where each ONT is assigned and associated with a different PON ID. Typically, status requests are sent to each allocID, requesting the amount of data waiting at the associated T-cont and status reports are generated that identify the amount od data waiting at that particular T-cont for transmission upstream. 
     Likewise, TM DBA unit  36  may associate monitored data amounts  62  with allocIDs. Downstream monitoring unit  26 , SR DBA unit  34  and TM DBA unit  36  may report amounts  58 ,  53  and  62  respectively with their associated allocIDs. Translator unit  38  typically stores the then converted GCPs  56  to entries of GCP table  54  associated with the allocIDs. Universal grant scheduler unit  24  may maintain SLA table  68  by allocID, associating each allocID with a defined service level agreement. Universal grant scheduler  24  may further maintain data with regard to past delivery of services in SLA table  68 , such as an amount of data allowed to be sent upstream in a previous upstream time slot, for purposes of determining current provisioning of bandwidth via grant map  64 . 
     Analyzing GCP table  54 , universal grant scheduler unit  24  may determine allocIDs that require upstream bandwidth and then analyze SLA table  68  to determine a current amount of bandwidth that should be offered to each of the allocIDs, as well as, to ensure that maximum bandwidth limits are enforced. Universal grant scheduler unit  24  may then schedule the available bandwidth defined by parameter table  66  using one of the above listed scheduling algorithms to allocate one or more GCPs  54  for associated ones of ONTs  16  to time slots defined in grant map  64 . 
     Upon allocating these GCPs  56 , universal grant scheduler unit  24  may update GCP table  54  to decrement the number of GCPs  56  stored to entries associated with allocIDs to reflect the grant of a time slot. Each one of GCPs  56  corresponds to a time slot, meaning that if universal grant scheduler unit  24  scheduled three of GCPs  56  for an allocID identifying a T-cont of ONT  16 A, for example, universal grant scheduler unit  24  decrements three of GCPs  56  stored to the entry associated with that allocID. Considering that universal grant scheduler unit  24  no longer need convert data amounts to grant cycles or time slots during this grant map generation time period but only assign these GCPs to time slots within grant map  64  according to the implemented scheduling algorithm, universal grant scheduler unit  24  may enable a number of benefits. 
     First, by utilizing pre-computed (where pre-computed is used herein to refer to the fact that GCPs may be computed without regard to the grant map generation time period and therefore may be generally pre-computed with respect to any given scheduling round) GCPs, universal grant scheduler unit  24  may work on much smaller number representations. Rather than perform mathematical operations with respect to potentially large (in terms of number of bits) representations of the amount of data waiting to be sent upstream at each ONT, universal grant scheduler unit  24  may compute the grant map from GCPs, which represent the amount of data waiting to be sent upstream in a more compact form. Considering that computation times for mathematical operations are generally directly proportional to the size of the data (in terms of bits) to which the mathematical operation is performed, reducing the size of the data to which the mathematical operation is applied may improve the speed with which universal grant scheduler unit  24  may perform these mathematical operations. As a result, universal grant scheduler unit  24  may perform more iterations of this mathematical operation with respect to GCPs in comparison to the actual amount of data reported or monitored, which may improve scheduling accuracy. 
     Second, use of GCPs may require less storage space, as GCPs represent the amount of data waiting to be sent upstream in a more compact form. Reducing memory requirements to store this amount of data may enable universal grant scheduler unit  24  to implement more GPON MACs in the same amount space as that conventionally used to implement a universal grant scheduler. This reduction in space may also translate into reduced cost and/or more board space for other units. 
     Third, use of GCPs may enable universal grant scheduler unit  24  to scheduler upstream time slots regardless of the way in which these data amounts are collected, which in effect may unify scheduling. The GCP may, in effect, abstract data amounts from the manner in which these data amounts were collected so as to enable a single universal scheduler to schedule upstream time slots. For example, SR DBA reports data amounts using the representation discussed above, while TR DBA monitors actual amounts sent upstream and downstream monitoring monitors an amount sent downstream. All three of these values may be pertinent for performing upstream time slot allocation, but all three may represent their respective data amounts differently. This different representations require additional operations to be performed so as to enable all three to be scheduled. Rather than perform these operations during the grant map generation time period, the techniques facilitate the abstraction of these representations into a single unifying or universal GCP representation with the added benefits discussed above. By using GCPs, the techniques may leverage common scheduling hardware between what would have been three different scheduling implementations to reduce overall size (in terms of board space by providing one universal scheduler) and also improving performance. 
     Universal grant scheduler unit  24  may transmit grant map  64  downstream over link  22  via interface  34 N to ONTs  16 . The process described above may then be repeated only an initial grant map need not be generated and sent. Instead, TM DBA unit  36  may monitor upstream data  42  sent in accordance with grant map  64 , providing monitored upstream data amounts  62  to translator unit  38 . In some instances, universal grant scheduler unit  24  may prepare a similar sort of initial grant map to give ONTs  16  that previously had no data to send a chance to delivery data upstream. This grant map (which may be referred to as a polling grant map) may be substantially similar to the initial grant map described above and the initial grant map may be considered a polling grant map. In some other instances, rather than poll those of ONTs  16  that do not support SR DBA through a polling grant map, these ones of ONTs  16  may be able to request an upstream time slot through a control channel (which in this context represents a dedicated time reserved over link  22  during which ONTs  16  may communicate control, rather than payload, data to OLT  12 ). 
       FIG. 3  is a flowchart illustrating exemplary operation of an optical line terminal, such as OLT  12  of  FIG. 2 , in implementing the universal grant scheduling aspects of the techniques described in this disclosure. Initially, OLT  12  determines an amount of data waiting at ONTs  16  for delivery upstream to OLT  12  in the manner described above (i.e., either using status reporting (SR) and/or traffic monitoring (TM), in this example) ( 70 ). That is, control unit  30  of OLT  12  includes SR DBA unit  34  and TM DBA unit  36 , where SR DBA unit  34  determines an amount of upstream data  53  waiting at ONTs  16  by sending status request  48  to and receiving status reports  50  from ONTs  16  and TM DBA unit  36  determines an amount of upstream data  62  waiting at ONTs  16  by monitoring upstream data sent during time slots allocated to those ONTs  16  via a previously sent grant map  64 . 
     Regardless of how upstream data amounts  53  and  62  are determined, translator unit  38  converts upstream data amounts  53  and  62  to CGPs  56  in accordance with conversion formula  54 , again as described above ( 72 ). Translator unit  38  may perform this translation or conversion from data amounts  53  and  62  to corresponding GCPs  56  while still collecting amounts  53 ,  62  for other ONTs  16  and prior to having to generate the grant map. Translator unit  38  may then store GCPs  56  to corresponding entries of GCP table  54  in the manner described above ( 74 ). If time remains before the grant map needs to be generated, meaning that the current scheduling round is not yet complete and it is not time to generate the grant map (“NO”  76 ), SR DBA unit  34  and TM DBA unit  36  may continue to determine upstream data amounts  53 ,  62  for those active (in the sense of being powered or otherwise activated) ones of ONTs  16 . Translator unit  38  continues to convert these upstream data amounts  53 ,  62  to GCPs  56  and store these GCPs  56  to CGP table  54  ( 72 ,  74 ). However, if the current scheduling round has completed and it is time to generate the grant map (“YES”  76 ), universal grant scheduler unit  24  generates grant map  64  based on GCPs  56  stored to GCP table  54  for each active one of ONTs  16  in the manner described above ( 78 ). In addition, universal grant scheduler unit  24  may determine or otherwise generate grant map  64  based on parameter table  66  and SLA table  68 , as noted above. 
     While universal grant scheduler unit  24  generates grant map  64 , SR DBA unit  34  and TM DBA unit  36  may continue to receive stat reports and monitor upstream data transmission to determine upstream data amounts  53 ,  62 . Thus, while shown in the example of  FIG. 3  as if SR DBA unit  34  and TM DBA unit  36  stop monitoring upstream data amounts  53 ,  62 , converting these data amounts to GCPs and storing these GCPs to the GCP table while universal grant scheduler unit  24  generates grant map  64 , SR DBA unit  34  and TM DBA unit  36  may continue to determine these data amounts and translator unit  36  may continue convert these data amounts to GCPS and potentially store these GCPS to the GCP table while universal grant scheduler unit  24  generates the grant map. In this respect, the techniques should not be strictly limited to the example of  FIG. 3 . 
     In any event, after generating grant map  64 , universal grant scheduler unit  24  transmits grant map  64  to ONTs  16  over link  20  via interface  32 N ( 80 ). Interface  32 N then receives upstream data  42  from ONTs  16  in accordance with grant map  64  and forwards upstream data  42  to its destination (often over link  22  via interface  32 A to a public network, such as the Internet) ( 82 ,  84 ). The techniques may continue in this manner with TM DBA unit  36  monitoring this upstream data  42  while SR DBA unit  34  polls ONTs  16  for their upstream data amounts  53  by sending a status request  48  and receiving status reports  50  in response to status request  48  ( 70 ). Translator unit  36  then converts amounts  53 ,  62  to GCPs  56 , which are stored to GCP table  54  ( 72 ,  74 ). Universal grant scheduler unit  24  may, while SR DBA unit  34  and TM DBA unit  36  may again continue to determine these data amounts and translator unit  36  may continue convert these data amounts to GCPS and potentially store these GCPS to the GCP table, generate another grant map  64  based on GCPs  56  stored to GCP table  54  for each active one of ONTs  16  and transmit grant map  64 . 
       FIG. 4  is a flowchart illustrating exemplary operation of an optical line terminal, such as OLT  12  of  FIG. 2 , in implementing the downstream traffic monitoring aspects of the techniques described in this disclosure. Initially, OLT  12  determines an amount of data waiting at ONTs  16  for delivery upstream to OLT  12  in the manner described above (i.e., either using status reporting (SR) and/or traffic monitoring (TM), in this example) ( 90 ). That is, control unit  30  of OLT  12  includes SR DBA unit  34  and TM DBA unit  36 , where SR DBA unit  34  determines an amount of upstream data  53  waiting at ONTs  16  by sending status request  48  to and receiving status reports  50  from ONTs  16  and TM DBA unit  36  determines an amount of upstream data  62  waiting at ONTs  16  by monitoring upstream data sent during time slots allocated to those ONTs  16  via a previously sent grant map  64 . 
     Regardless of how upstream data amounts  53  and  62  are determined, SR DBA unit  34  and TM DBA unit  36  store upstream data amounts  53  and  62  to GCP table  54  after being converted, in this example to GCPs  56  in accordance with the universal grant scheduling aspects of the techniques described in this disclosure. While described with respect to the grant scheduling aspects of the techniques, the downstream monitoring aspects of the techniques may be implemented separate from the grant scheduling aspects. In these instances, OLT  12  may store upstream data amounts  53 ,  62  to a table within a grant scheduler or similar unit that may be similar to GCP table  54  only, instead of storing GCPs, it stores upstream data amounts. 
     However, assuming the various aspects of the techniques are implemented in conjunction as described in the examples of this disclosure, translator unit  38  converts upstream data amounts  53  and  62  to CGPs  56  in accordance with conversion formula  54 , again as described above. Translator unit  38  may perform this translation or conversion from data amounts  53  and  62  to corresponding GCPs  56  while still collecting amounts  53 ,  62  for other ONTs  16  and prior to determining a last amount  53  or  62  for those active (in the sense of being powered or otherwise activated) ones of ONTs  16 . Translator unit  38  may then store GCPs  56  to corresponding entries of GCP table  54  in the manner described above. In this respect, translator unit  38  stores upstream data amounts  53 ,  62  to GCP table  54  in the form of GCPs  56  ( 92 ). 
     Meanwhile, downstream monitoring unit  26  determines an amount of data  58  sent downstream by OLT  12  to each of active ones of ONTs  16  in the manner described above ( 94 ). Downstream monitoring unit  26  may store these downstream data amounts  58  to corresponding entries of downstream table  52 . Downstream monitoring unit  26  may then compare downstream data amounts  58  to a threshold or other metric defined as one of monitoring parameters  44  ( 96 ). In some instances, rather than compare downstream data amounts  58 , downstream monitoring unit  26  may determine a change in downstream data amounts over any given period of time, where the period of time may be configurable or pre-defined. Downstream monitoring unit  26  may then compare this change in downstream data amounts to the threshold or other metric defined as one of monitoring parameters  44 . 
     In any event, if one or more of downstream data amounts  58  or changes in downstream data amounts (which effectively represents a change in the downstream rate of data delivery) exceeds the threshold (“YES”  98 ), downstream monitoring unit  26  may forward this downstream data amount  58  to translation unit  36 , which translates downstream data amounts  58  to one or more GCPs  56 . Translation unit  36  then updates an entry of GCP table  54  corresponding to the one of ONTs  16  for which downstream data amount  58  was monitored to increase the number of GCPs  56  assigned to that one of ONTs  16 . In this manner, OLT  12  updates an amount of upstream data based on the monitored amount of downstream data ( 100 ). Again, while described in this example with respect to GCPs, the downstream monitoring aspects of the techniques set forth in this disclosure may be implemented with respect to data amounts rather than GCPs to potentially reduce latency when implementing conventional or standard grant scheduling algorithms. 
     If one or more downstream data amounts  58  do (or changes in the downstream data rate of delivery does) not exceed the threshold (“NO”  98 ) or after updating GCPs  56  stored to GCP table  54  based on downstream data amounts  58 , universal grant scheduler unit  24  may generate grant map  64  based on GCPs  56  stored to GCP table  54  for each active one of ONTs  16  ( 102 ). In addition, universal grant scheduler unit  24  may determine or otherwise generate grant map  64  based on parameter table  66  and SLA table  68 , as noted above. After generating grant map  64 , universal grant scheduler unit  24  transmits grant map  64  to ONTs  16  over link  20  via interface  32 N ( 104 ). Interface  32 N then receives upstream data  42  from ONTs  16  in accordance with grant map  64  and forwards upstream data  42  to its destination (often over link  22  via interface  32 A to a public network, such as the Internet) ( 106 ,  108 ). The techniques may continue in this manner until such time that OLT  12  is no longer active, powered or otherwise enabled or operational ( 90 - 108 ). 
     In this way, OLT  12  determines an amount of upstream data that is waiting at one of ONTs  16  to be transmitted upstream to OLT  12  and then determines an amount of downstream data that is transmitted by the OLT to the one of ONTs  16 . OLT  12  then increases the determined amount of upstream data based on the determined amount of downstream data is transmitted by OLT  12  to the one of ONTs  16  and, after increasing the determined amount of upstream data, generates an upstream grant map  64  that grants time slots to the one or more of ONTs  16  based on the amount of upstream data determined for each ONTs  16 . 
     OLT  12  then transmits the upstream grant map downstream to ONTs  16 , receives upstream data in accordance with upstream grant map and forwards this upstream traffic to its intended destination. By monitoring downstream data amounts and then increasing upstream data amounts based on the monitored downstream amounts, OLT  12  may proactively assign upstream bandwidth to those of ONTs  16  that are likely to require this bandwidth to response to TCP or other windowing protocols that limit the amount of data that can be sent. By proactively assigning this upstream bandwidth, OLT  12  may reduce or, in some instances, prevent bandwidth allocation thrashing whereby both OLT  12  and the underlying protocol each misidentify the reason why ONTs  16  fail to fully utilize their allotted bandwidth and then decrease bandwidth at odds with one another. 
     The techniques described herein may be implemented in hardware, firmware, or any combination thereof. The hardware may, in some instances, also execute software. Any features described as modules, units or components may be implemented together in an integrated logic device or separately as discrete but interoperable logic devices. In some cases, various features may be implemented as an integrated circuit device, such as an integrated circuit chip or chipset. If implemented in software, the techniques may be realized at least in part by a computer-readable medium comprising instructions that, when executed, cause a processor to perform one or more of the methods described above. 
     A computer-readable medium may form part of a computer program product, which may include packaging materials. A computer-readable medium may comprise a computer data storage medium such as random access memory (RAM), synchronous dynamic random access memory (SDRAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), FLASH memory, magnetic or optical data storage media, and the like, including non-transitory computer-readable mediums. The techniques additionally, or alternatively, may be realized at least in part by a transitory computer-readable communication medium that carries or communicates code in the form of instructions or data structures and that can be accessed, read, and/or executed by a computer. 
     The code or instructions may be executed by one or more processors, such as one or more DSPs, general purpose microprocessors, ASICs, field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated software modules or hardware modules. The disclosure also contemplates any of a variety of integrated circuit devices that include circuitry to implement one or more of the techniques described in this disclosure. Such circuitry may be provided in a single integrated circuit chip or in multiple, interoperable integrated circuit chips in a so-called chipset. Such integrated circuit devices may be used in a variety of applications. 
     Various examples have been described. These and other examples are within the scope of the following claims.