Abstract:
A power manager for a passive optical network, including a network statistics collector, for collecting data regarding traffic in a passive optical network (PON) including a plurality of optical network units (ONUs) on the downstream side of the PON, wherein each ONU can be in at least a sleep state and an active state, and wherein the PON transmits data in packets of data frames, a buffer for storing downstream data frames for each ONU while the ONU is in the sleep state, an activity detector for processing the data collected by the network statistics collector to generate indicators of activity levels for each ONU, and a protocol manager including a plurality of state machines for the respective plurality of ONUs, wherein each state machine governs state transition of its respective ONU to the sleep state when the activity detector indicates a low activity level for the ONU, and to the active state when the activity detector indicates a high level of activity for the ONU.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority benefit of U.S. Provisional Application No. 61/381,452, entitled NOVEL APPROACH TO POWER SAVE IN PASSIVE OPTICAL NETWORKS, filed on Sep. 10, 2010 by inventors Danny Greenberg and Oren Spector. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to passive optical networks. 
       BACKGROUND OF THE INVENTION 
       [0003]    A passive optical network (PON) is a point-to-multipoint fiber-optic network, in which unpowered optical splitters are used to enable a single optical fiber to service multiple premises, typically 16-128 homes. Reference is made to  FIG. 1 , which is a simplified diagram of a prior art PON  100 . As seen in  FIG. 1 , PON  100  includes a central office node, referred to as an optical line terminal (OLT)  140 , a number of user nodes, referred to as optical network units (ONUs)  120 , which are near user premises  130 , and fibers  150  and splitters  160  between them. 
         [0004]    Data is transmitted within PON  100  in packets referred to as data frames. Downstream signals (i.e., signals transmitted from left to right in  FIG. 1 ) originate from network services, such as an Internet service  110 A, a voice over IP service  1108 , a cable TV service  110 C and other such services  110 D. The downstream signals are broadcast to all premises  130  that share a single fiber. Bandwidth for upstream signals (i.e., signals transmitted from right to left in  FIG. 1 ) is allocated by time-division multiplexing among ONUs  120 . Current PONS  100  operate at 1 Gbit/s and 2.5 Gbit/s rates, and 10 Gbit/s PONs will be available in the near future. 
         [0005]    A drawback of conventional PONs  100  is the large amounts of power that they consume. Moreover, as traffic rates in a PON increase, the power consumption of the PON also increases. As such, as PONs improve to operate at higher rates, the need to reduce their power consumption is ever more pressing. 
       SUMMARY OF THE DESCRIPTION 
       [0006]    Aspects of the present invention relate to methods and systems for managing power for PONs, so as to save energy, reduce heat dissipation, meet power limit regulations, and save operational expenses. The present invention includes a novel hardware architecture, a novel software architecture and application programming interface, and a novel protocol for power management. 
         [0007]    Embodiments of the present invention include a distributed power management protocol that runs on the OLT side and on the ONU side of a PON. These embodiments track PON activity, and transition ONUs in and out of power saving states based on activity statistics and configurable parameters. 
         [0008]    There is thus provided in accordance with an embodiment of the present invention a power manager for a passive optical network, including a network statistics collector, for collecting data regarding traffic in a passive optical network (PON) including a plurality of optical network units (ONUs) on the downstream side of the PON, wherein each ONU can be in at least a sleep state and an active state, and wherein the PON transmits data in packets of data frames, a buffer for storing downstream data frames for each ONU while the ONU is in the sleep state, an activity detector for processing the data collected by the network statistics collector to generate indicators of activity levels for each ONU, and a protocol manager including a plurality of state machines for the respective plurality of ONUs, wherein each state machine governs state transition of its respective ONU to the sleep state when the activity detector indicates a low activity level for the ONU, and to the active state when the activity detector indicates a high level of activity for the ONU. 
         [0009]    There is additionally provided in accordance with an embodiment of the present invention a method for managing power of a passive optical network, including collecting data regarding traffic in a passive optical network (PON) including a plurality of optical network units (ONUs) on the downstream side of the PON, wherein each ONU can be in at least a sleep state and an active state and wherein the PON transmits data in packets of data frames, storing downstream data frames for each ONU in a buffer, while the ONU is in the sleep state, processing the data collected by the collecting to generate indicators of activity levels for each ONU, and determining state transitions of each ONU, wherein an ONU transitions to the sleep state when the indicators of activity level for the ONU indicate a low activity level for the ONU, and an ONU transitions to the active state when the indicators of activity level for the ONU indicate a high level of activity for the ONU. 
         [0010]    There is further provided in accordance with an embodiment of the present invention a system for power management of a passive optical network, including a plurality of optical network units (ONUs) on the downstream side of a passive optical network (PON), wherein each ONU transmits data in packets of data frames, wherein each ONU can be in at least a sleep state and an active state, and wherein each ONU includes an ONU network statistics collector, for collecting data regarding traffic at the ONU, an ONU activity detector for processing the data collected by the network statistics collector to generate indicators of activity level of the ONU, and an ONU protocol manager including a state machine for the ONU, wherein the state machine governs state transition of the ONU to the sleep state when the ONU activity detector indicates a low activity level for the ONU, and an optical line terminal (OLT) on the upstream side of the PON, including an OLT network statistics collector, for collecting data regarding traffic at each of the ONUs, an OLT buffer for storing downstream data frames for each ONU while the ONU is in the sleep state, an OLT activity detector for processing the data collected by the OLT network statistics collector to generate indicators of activity levels for each ONU, and an OLT protocol manager including a plurality of state machines for the respective plurality of ONUs, wherein each state machine governs state transition of its respective ONU to the active state when the OLT activity detector indicates a high level of activity for the ONU. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: 
           [0012]      FIG. 1  is a simplified block diagram of a prior art PON; 
           [0013]      FIG. 2  is a simplified block diagram of an enhanced PON, with power management in accordance with an embodiment of the present invention; 
           [0014]      FIG. 3  is a simplified block diagram of an OLT-side power management module, in accordance with an embodiment of the present invention; 
           [0015]      FIG. 4  is a simplified block diagram of an OLT-side power management framework, in accordance with an embodiment of the present invention; 
           [0016]      FIG. 5  is a simplified diagram of a protocol message structure, in accordance with an embodiment of the present invention; 
           [0017]      FIG. 6  is a simplified block diagram of an ONU-side power management module, in accordance with an embodiment of the present invention; 
           [0018]      FIG. 7  is a simplified block diagram of an ONU-side power management framework, in accordance with an embodiment of the present invention; 
           [0019]      FIG. 8  is a simplified flow chart of a protocol for power management, in accordance with an embodiment of the present invention; and 
           [0020]      FIG. 9  is a simplified power management state transition diagram for an ONU, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Aspects of the present invention relate to power management of PONs. 
         [0022]    Reference is made to  FIG. 2 , which is a simplified block diagram of an enhanced PON  200 , with power management in accordance with an embodiment of the present invention. PON  200  includes an OLT  240  with a novel power management (PM) module  260 , and ONUs  220  with novel PM modules  280 . 
         [0023]    Each ONU PM module  280  maintains its own state machine, and OLT PM module  260  maintains separate state machines for each ONU  220 . Each ONU  220  may operate in one of three possible states; namely, an “Active” state, which is its normal operational state, a “Sleep” state, which is a low-power mode, and an interim “Waking Up” state, during which ONU PM module  280  sends a report regarding its upstream buffer occupancy to OLT PM module  280 , and OLT PM module  260  and ONU PM module  280  jointly decide whether ONU  220  should transition to Sleep state or to Active state. The state machines in OLT PM module  260  and ONU PM module  280  are activated periodically. The inputs to each ONU PM state machine include the current state of the ONU, various PON activity statistics described hereinbelow, and system-wide configurable parameters described hereinbelow. The output of each ONU PM state machine includes the next state that the ONU should transition to, and relevant control signals as described hereinbelow. 
         [0024]    While one or more ONUs  220  are in Sleep state, downstream, data for the ONUs is stored in one or more buffers/queues in PM module  260 , and upstream data for the ONUs is stored in buffers/queues in PM module  280 . OLT PM module  260  and ONU PM modules  280  implement logic of a power management protocol described hereinbelow with reference to  FIG. 8 . 
         [0025]    Reference is made to  FIG. 3 , which is a simplified block diagram of OLT PM module  260 , in accordance with an embodiment of the present invention. As seen in  FIG. 3 , OLT PM module  260  includes one or more controllable buffers/queues  261 , for storing downstream data for each ONU that is in Sleep state. Storage of data in buffers/queues  261  and extraction of data from buffers/queues  261  is controlled by control signals of the power management protocol described hereinbelow with reference to  FIG. 8 . 
         [0026]    OLT PM module  260  further includes a management data frames receiver channel  264  and a management data frames transmitter channel  265 . In accordance with an embodiment of the present invention, power management protocol messages are encapsulated within management data frames. Channels  264  and  265  are hardware buffers for receiving and transmitting such management data frames. OLT PM module  260  writes a management data frame, for downstream transmission to a destination ONU PM module  280 , into transmitter channel  265 . After writing the complete management data frame, OLT PM module  260  indicates to transmitter channel  265  that the data frame is ready for transmission. Channel  265  then transmits the data frame downstream to the destination ONU PM module  280 , at a higher than standard priority. 
         [0027]    OLT PM module  260  writes management frame data, arriving from an ONU PM module  280 , into receiver channel  264 . OLT PM module  260  polls a hardware flag to determine when a complete management data frame is available in receiver channel  264 . Alternatively, OLT PM module  260  is interrupted by receiver channel  264  when a complete management data frame is available in receiver channel  264 . OLT PM module  260  then reads the content of the received management data channel, and parses the content to extract a power management protocol message. 
         [0028]    OLT PM module  260  further includes a statistics collector  266 . Statistics collector  266  uses counters and accumulators to track PON data, including the following time series statistics. 
         [0000]    Upstream received bytes—bytes received by each ONU  220  from OLT  240 
 
Incoming frames—frames received by each ONU  220  from OLT  240 
 
Downstream received bytes—bytes received by each ONU  220  from premises  130 
 
Outgoing frames—frames prepared by each ONU  220  for upstream transmission
 
PON clock—time, measured in units of time quanta
 
OLT Downstream buffer occupancy—number of bytes in downstream data buffer/queue  261 
 
ONU upstream buffer occupancy—number of bytes in each ONU upstream buffer, obtained by parsing each ONU&#39;s report messages
 
         [0029]    OLT PM module  260  further includes a downstream activity detector  262  and an upstream activity detector  263 , for calculating respective downstream and upstream metrics from the statistical data collected by statistics collector  266 . Downstream activity detector  262  generates the following metrics for each ONU  220 . 
         [0000]    Downstream rate—a decaying time series average of the ONU downstream received bytes, as described hereinbelow
 
Downstream frames gap—a decaying time series average of gaps between ONU outgoing frames, as described hereinbelow
 
         [0030]    Similarly, upstream activity detector  263  generates the following metrics for each ONU  220 . 
         [0031]    Upstream rate—a decaying time series average of the ONU upstream received bytes, as described hereinbelow 
         [0000]    Upstream frames gap—a decaying time series average of gaps between ONU incoming frames, as described hereinbelow 
         [0032]    The downstream and upstream rates are calculated by respective activity detectors  262  and  263  according to the following function. 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 calc_rate( current_rx_bytes, prev_rx_bytes, prev_average ) 
               
               
                   
                 { 
               
               
                   
                   new_bytes = current_rx_bytes − prev_rx_bytes; 
               
               
                   
                   prev_rx_bytes = rx_bytes; 
               
               
                   
                   prev_average = smoothing_factor * new_bytes + 
               
               
                   
                     (1 − smoothing_factor) * prev_average; 
               
               
                   
                   rate = 8 * prev_average / time_step // in bits per second 
               
               
                   
                   return rate; 
               
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0033]    The function calc_rate is called every time_step seconds. The smoothing factor parameter is a configurable parameter between 0 and 1. The time series of byte arrivals is smoothed by smoothing_factor to derive traffic arrival rates. The time_step parameter is a configurable parameter in units of seconds 
         [0034]    The downstream and upstream frame gaps are calculated by respective activity detectors  262  and  263  according to the following function. 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 calc_gap( current_rx_bytes, prev_rx_bytes, prev_gap, 
               
               
                   
                 prev_average ) 
               
               
                   
                 { 
               
               
                   
                   if (current_rx_bytes &gt; prev_rx_bytes ) { 
               
               
                   
                     if (smoothing factor == 1.0 ) { 
               
               
                   
                       prev_average = 0; 
               
               
                   
                     } else { 
               
               
                   
                       prev_average = smoothing_factor * prev_gap + 
               
               
                   
                         (1 − smoothing_factor) * prev_average; 
               
               
                   
                     } 
               
               
                   
                     current_average = prev_average; 
               
               
                   
                     prev_gap = 0; 
               
               
                   
                   } 
               
               
                   
                   else { 
               
               
                   
                     prev_gap = prev_gap + time_step; 
               
               
                   
                     current_average = prev_average; 
               
               
                   
                   if ( prev_gap &gt; current_average ) 
               
               
                   
                     current_average = smoothing_factor * prev_gap + 
               
               
                   
                        (1 − smoothing_factor) * current_average; 
               
               
                   
                   } 
               
               
                   
                   gap = current_average; // in seconds 
               
               
                   
                   return gap: 
               
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0035]    The function calc_rate is also called every time_step seconds. If a new data frame has arrived since the previous call to calc_rate, then prev_gap is reset to zero. Otherwise, prev_gap is incremented by time_step. 
         [0036]    Activity detectors  262  and  263  determine state transitions for ONUs  220  based on thresholds of activity. Specifically, an ONU  220  is deemed to have sufficiently low activity to transition to Sleep state at a given time, if the above metrics for ONU  220  at the given time satisfy all of the following conditions. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 Sleep Conditions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 upstream_rate &lt; upstream_rate_threshold 
               
               
                   
                 downstream_rate &lt; downstream_rate_threshold 
               
               
                   
                 upstream_frame_gap &gt; upstream_frame_gap_threshold 
               
               
                   
                 downstream_frame_gap &gt; downstream_frame_gap_threshold 
               
               
                   
                   
               
             
          
         
       
     
         [0037]    It is noted that low rates and large gaps indicate a low level of activity, whereas high rates and small gaps indicate a high level of activity. 
         [0038]    An ONU  220  is deemed to have accumulated a sufficient backlog of activity to transition to Active state at a given time, if the statistics for ONU  220  at the given time satisfy both of the following conditions. 
         [0000]    
       
         
               
             
               
             
           
               
                   
               
               
                 Wakeup Conditions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 upstream_buffer_occupancy &gt; upstream —  occupancy_threshold 
               
               
                 downstream_buffer_occupancy &gt; downstream —  occupancy_threshold 
               
               
                   
               
             
          
         
       
     
         [0039]    The various threshold parameters, upstream_rate_threshold, downstream_rate_threshold, upstream,_frame_gap_threshold, downstream_frame_gap_threshold, upstream_occupancy_threshold and downstream_occupancy_threshold are configurable parameters which control transitions of ONUs  220  into Sleep and Active states. 
         [0040]    Reference is made to  FIG. 4 , which is a simplified block diagram of an OLT-side power management framework  270 , in accordance with an embodiment of the present invention. As seen in  FIG. 4 , power management framework  270  includes an OLT API  271 , which provides methods and data structures for use by OLT PM module  260 . 
         [0041]    TABLE I describes methods and TABLES II and III describe respective parameter and statistical data structures provided by OLT API  271 . 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 API Methods used by OLT PM Module 260 
               
             
          
           
               
                 Method 
                 Description 
               
               
                   
               
               
                 Protocol initialization 
                 Used to initialize internal states and variables of 
               
               
                   
                 the PM protocol 
               
               
                 Message handling 
                 Used to process protocol messages transmitted 
               
               
                   
                 from ONU PM modules 280 to OLT PM module 
               
               
                   
                 260 
               
               
                 State machine process 
                 Used to decide whether to transition an ONU 
               
               
                   
                 220 from one state to another state. OLT PM 
               
               
                   
                 module 260 is responsible to subsequently call 
               
               
                   
                 the state machine of ONU PM module 280 when 
               
               
                   
                 this method is called. 
               
               
                 Get parameters 
                 Read the protocol parameters 
               
               
                 Get statistics 
                 Read the statistics and decisions of activity 
               
               
                   
                 detectors 262 and 263 
               
               
                 Send message 
                 Send a protocol message from OLT PM module 
               
               
                   
                 260 to an ONU PM module 280 
               
               
                 Set downstream buffer 
                 Enable or disable downstream buffer 261 for a 
               
               
                   
                 specific ONU. Buffer 261 is enabled for ONUs 
               
               
                   
                 that are in Sleep state and disabled for ONUs 
               
               
                   
                 that are in Active state. 
               
               
                 Set ONU state 
                 Used to set the next state of an ONU 220. 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 API Parameter Data Structures used by OLT PM Module 260 
               
             
          
           
               
                 Data Structure 
                 Description 
               
               
                   
               
               
                 ONU List 
                 List of ONUs for which the PM protocol is 
               
               
                   
                 implemented 
               
               
                 Number of ONUs 
                 Size of the ONU List 
               
               
                 PM cycle time 
                 Time between consecutive sleep cycles when a 
               
               
                   
                 periodic PM protocol is implemented 
               
               
                 Initiator 
                 Flag indicating whether OLT 240 or an ONU 220 
               
               
                   
                 initiates a PM cycle 
               
               
                 Downstream 
                 Threshold for bytes in downstream buffer 261. 
               
               
                 occupancy threshold 
                 Used to decide if an ONU 220 should transition 
               
               
                   
                 to Active state. 
               
               
                 Downstream buffer 
                 Flag indicating if downstream buffer occupancy 
               
               
                 occupancy enable 
                 should be used by the PM protocol logic 
               
               
                 Upstream 
                 Threshold for bytes in an ONU 220 upstream 
               
               
                 occupancy threshold 
                 buffer 281. Used to decide if ONU 220 should 
               
               
                   
                 transition to Active state. The occupancy of 
               
               
                   
                 ONU 220 upstream buffer is known to PM 
               
               
                   
                 module 260 via the ONU reporting mechanism 
               
               
                   
                 and report frames. 
               
               
                 Upstream buffer 
                 Flag indicating if upstream buffer occupancy 
               
               
                 occupancy enable 
                 should be used by the PM protocol logic 
               
               
                 Rate threshold 
                 Threshold rate (in bits/s) used to decide if an 
               
               
                   
                 ONU 220 should transition to Active state. 
               
               
                   
                 Upstream and downstream rate thresholds may 
               
               
                   
                 have the same or different values. 
               
               
                 Rate threshold enable 
                 Flag indicating if rate threshold should be used 
               
               
                   
                 by the PM protocol logic 
               
               
                 Gaps between frames 
                 Threshold gap between consecutive frames 
               
               
                 threshold 
                 going downstream or coming upstream from an 
               
               
                   
                 ONU 220, measured in units of PON time 
               
               
                   
                 quanta. Used to decide if an ONU 220 should 
               
               
                   
                 transition to Active state. 
               
               
                 Gaps between frames 
                 Flag indicating if gaps between frames should be 
               
               
                 enable 
                 used by the PM protocol logic 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 API Statistical Data Structures used by OLT PM Module 260 
               
             
          
           
               
                 Data Structure 
                 Description 
               
               
                   
               
               
                 Downstream buffer 
                 Number of bytes in downstream buffer 261 for 
               
               
                 occupancy 
                 a given ONU 220 
               
               
                 Reported bytes 
                 Number of bytes in an ONU 220 upstream 
               
               
                   
                 buffer 281, as reported by ONU 220 
               
               
                 Upstream rate 
                 Arrival rate of upstream data for a given ONU 
               
               
                   
                 220 
               
               
                 Downstream rate 
                 Arrival rate of downstream data for a given ONU 
               
               
                   
                 220 
               
               
                 Gaps between 
                 Gaps between arrivals of upstream data frames 
               
               
                 upstream frames 
                 for a given ONU 220, measured in units of PON 
               
               
                   
                 time quanta 
               
               
                 Gaps between 
                 Gaps between arrivals of downstream data 
               
               
                 downstream frames 
                 frames for a given ONU 220, measured in units 
               
               
                   
                 of PON time quanta 
               
               
                 ONU active 
                 Flag indicating decision of activity detectors 262 
               
               
                   
                 and 263 for a given ONU 220 
               
               
                 ONU state 
                 Current state of a given ONU 220 
               
               
                   
               
             
          
         
       
     
         [0042]    As seen in  FIG. 4 , power management framework  270  further includes a protocol frames composer  276 . Protocol frames composer  276  generates a PON management frame, having a PM protocol message as its payload. Reference is made to  FIG. 5 , which is a simplified diagram of a protocol message structure, in accordance with an embodiment of the present invention. Shown in  FIG. 5  is a PON management frame  300  that carries a PM protocol message. PON management frame  300  has a management frame header  310 , and a management Frame Check Sequence (FCS)  330 . As seen in  FIG. 5 , the protocol management payload includes an OPCODE  320 A and a data block  320 B. The OPCODE indicates one of the following frames: SLEEP_INITIATE, SLEEP, WAKE_REQ, SLEEP_READY and WAKE_ACK. Use of these frames is described hereinbelow with reference to  FIG. 8 . The data block of the payload stores the next wake-up time for an ONU that is in Sleep state. 
         [0043]    Use of protocol frames composer  276  ensures that the PM protocol is independent of management frame type. As such, the frame type may be an ethernet Operations Administration and Maintenance (OAM) frame, a Mufti-Channel Pulse Code Modulation (MPCM) frame, an ONU Management and Control Interface (OMCI) frame, a Physical Layer Operations Administration and Maintenance (PLOAM) frame or such other type of frame. 
         [0044]    As seen in  FIG. 4 , power management framework  270  further includes a protocol frames parser  275 . Protocol frames parser  275  extracts a PM protocol message from management frame  300 , and passes it to the PM protocol. 
         [0045]    As seen in  FIG. 4 , power management framework  270  further includes an event timer  274 . Event timer  274  serves as the clock for the protocol state machines. OLT PM module  260  calls the process function of the protocol once per time period measured by this clock. The time period is set to be approximately an order of magnitude shorter than the quickest event period. Generally, the time period is on the order of 100 microseconds. 
         [0046]    As seen in  FIG. 4 , power management framework  270  further includes a statistics and activity detection manager  273 . Statistics and activity detection manager  273  is used to configure PON statistics and detection hardware in accordance with user and system defined parameters. Such parameters include: 
         [0000]    enable upstream rate measurement;
 
enable downstream rate measurement;
 
enable upstream frames gaps measurements;
 
enable downstream frames gap measurements; and
 
set rate thresholds.
 
         [0047]    As seen in  FIG. 4 , power management framework  270  further includes a downstream buffer manager  272 . Downstream buffer manager  272  is used to enable and disable downstream buffer/queue  261  for a given ONU  220 . 
         [0048]    Reference is made to  FIG. 6 , which is a simplified block diagram of ONU PM module  280  for an ONU  220 , in accordance with an embodiment of the present invention. As seen in  FIG. 6 , ONU PM module  280  includes an upstream data buffer  281 , a downstream activity detector  282 , an upstream activity detector  283 , a management frames receiver channel  284 , and a management frames transmitter channel  285 . Upstream data buffer  281  stores incoming data from ingress ports while ONU  220  is in Sleep state. The other ONU-side components are similar to respective OLT-side components  262 ,  263 ,  264  and  265  of OLT PM module  260 , which are described hereinabove. 
         [0049]    Reference is made to  FIG. 7 , which is a simplified block diagram of an ONU-side power management framework  290  for an ONU  220 , in accordance with an embodiment of the present invention. As seen in  FIG. 7 , power management framework  290  includes an ONU API  291 , which provides methods and data structures for use by ONU PM module  280 . 
         [0050]    TABLE IV describes methods and TABLES V and VI describe respective parameter and statistical data structures provided by ONU API  291 . 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 API Methods used by ONU PM Module 280 
               
             
          
           
               
                 Method 
                 Description 
               
               
                   
               
               
                 Protocol initialization 
                 Used to initialize internal states and variables of 
               
               
                   
                 the PM protocol 
               
               
                 Message handling 
                 Used to process protocol messages transmitted 
               
               
                   
                 from OLT PM modules 260 to ONU PM module 
               
               
                   
                 280 
               
               
                 State machine process 
                 Used to decide whether to transition ONU 220 
               
               
                   
                 from one state to another state 
               
               
                 Get parameters 
                 Read the protocol parameters 
               
               
                 Get statistics 
                 Read the statistics and decisions of activity 
               
               
                   
                 detectors 282 and 283 
               
               
                 Send message 
                 Send a protocol message from ONU PM module 
               
               
                   
                 280 to an OLT PM module 260 
               
               
                 Set ONU state 
                 Used to set the next state of ONU 220. 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE V 
               
             
             
               
                   
               
               
                 API Parameter Data Structures used by ONU PM Module 280 
               
             
          
           
               
                 Data Structure 
                 Description 
               
               
                   
               
               
                 Initiator 
                 Flag indicating whether OLT 240 or ONU 220 
               
               
                   
                 initiates a PM cycle 
               
               
                 Upstream 
                 Threshold for bytes in an ONU 220 upstream 
               
               
                 occupancy threshold 
                 buffer 281. Used to decide if ONU 220 should 
               
               
                   
                 transition to Active state. 
               
               
                 Upstream buffer 
                 Flag indicating if upstream buffer occupancy 
               
               
                 occupancy enable 
                 should be used by the PM protocol logic 
               
               
                 Rate threshold 
                 Threshold rate (in bits/s) used to decide if an 
               
               
                   
                 ONU 220 should transition to Active state. 
               
               
                   
                 Upstream and downstream rate thresholds may 
               
               
                   
                 have the same or different values. 
               
               
                 Rate threshold enable 
                 Flag indicating if rate threshold should be used 
               
               
                   
                 by the PM protocol logic 
               
               
                 Gaps between frames 
                 Threshold gap between consecutive frames 
               
               
                 threshold 
                 going downstream or coming upstream from an 
               
               
                   
                 ONU 220, measured in units of PON time 
               
               
                   
                 quanta. Used to decide if an ONU 220 should 
               
               
                   
                 transition to Active state. 
               
               
                 Gaps between frames 
                 Flag indicating if gaps between frames should be 
               
               
                 enable 
                 used by the PM protocol logic 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE VI 
               
             
             
               
                   
               
               
                 API Statistical Data Structures used by ONU PM Module 280 
               
             
          
           
               
                 Data Structure 
                 Description 
               
               
                   
               
               
                 Upstream rate 
                 Arrival rate of upstream data for ONU 220 
               
               
                 Bytes in upstream 
                 Occupancy (in bytes) of ONU upstream buffer 
               
               
                   
                 buffer 
               
               
                 Downstream rate 
                 Arrival rate of downstream data for ONU 220 
               
               
                 Gaps between 
                 Gaps between arrivals of upstream data frames 
               
               
                 upstream frames 
                 for ONU 220, measured in units of PON time 
               
               
                   
                 quanta 
               
               
                 Gaps between 
                 Gaps between arrivals of downstream data 
               
               
                 downstream frames 
                 frames for ONU 220, measured in units of PON 
               
               
                   
                 time quanta 
               
               
                 ONU state 
                 Current state of ONU 220 
               
               
                   
               
             
          
         
       
     
         [0051]    As seen in  FIG. 7 , power management framework  290  further includes a statistics and activity detection manager  293 , an event timer  294 , a protocol frames parser  295 , and a protocol frames composer  296 . These ONU-side components are similar to respective OLT-side components  273 ,  274 ,  275  and  276  of power management framework  270 , which are described hereinabove. 
         [0052]    Reference is made to  FIG. 8 , which is a simplified flow chart of a PM protocol, in accordance with an embodiment of the present invention. The PM protocol runs on the OLT-side and the ONU-side of PON  200 . OLT PM module  260  and ONU PM modules  280  use the PM protocol via the respective OLT API  271  and ONU API  291 . The PM protocol on the OLT-side includes protocol state machines for the respective ONUs  220 , and protocol message handlers and senders for the respective ONUs. Each PM protocol on the ONU-side includes a protocol state machine and a protocol message handler and sender. 
         [0053]    Each ONU may be in one of three states; namely, an Active state, which is its normal operational mode, a Sleeping state, which is a low power mode, and a “Waking Up” state, which is an intermediate state between sleep cycles. During the Waking Up state, ONU PM module  280  sends a report regarding its upstream buffer occupancy to OLT PM module  260 . The PM protocol includes the following messages, used for communication between OLT  240  and an ONU  220 . 
         [0000]    SLEEP_INITIATE—sent by OLT  240  to a specific ONU  220  to indicate that OLT  240  is initiating a sleep cycle for ONU  220 
 
SLEEP—sent by OLT  240  to a specific ONU  220  to instruct ONU  220  to transition to Sleep state
 
WAKE_REQ—sent by OLT  240  to a specific ONU  220  to transition to Active state
 
SLEEP_READY—sent by an ONU  220  to OLT  240  to indicate that ONU  220  is ready to transition to Sleep state
 
WAKE_ACK—sent by an ONU  220  to OLT  240  in indicate that ONU  220  has transitioned to Active state
 
         [0054]      FIG. 8  is divided into two columns. The left column indicates operations performed by OLT  240 , and the right column indicates operations performed by an ONU  220 .  FIG. 8  is further divided, by dashed lines, into a top portion and a bottom portion. 
         [0055]    The top portion of  FIG. 8  shows the PM protocol for an OLT-initiated sleep transition for an ONU  220  (operations  1010 - 1030 ), and an ONU-initiated sleep transition for ONU  220  (operations  1040  and  1050 ). At operation  1010 , OLT  240  checks the sleep conditions described hereinabove with reference to activity detectors  262  and  263 . If the sleep conditions are satisfied, OLT  240  sends a SLEEP_INITIATE message to ONU  220 . At operation  1020 , ONU  220  receives the SLEEP_INITIATE message and checks the sleep conditions. If the sleep conditions are satisfied, ONU  220  sends a SLEEP_READY message to OLT  240 . If the sleep conditions are not satisfied, ONU  220  ignores the SLEEP_INTIIATE message. At operation  1030 , OLT  240  receives the SLEEP_READY message. OLT  240  checks the sleep conditions. If the sleep conditions are satisfied, OLT  240  sends a SLEEP message to ONU  220 , with a next wakeup time. When ONU  220  receives the SLEEP message, it transitions to Sleep state until the next wakeup time. If the sleep conditions are not satisfied, OLT  240  ignores the SLEEP_READY message. 
         [0056]    At operation  1040 , an ONU  220  checks the sleep conditions. If the sleep conditions are satisfied, ONU  220  sends a SLEEP_READY message to OLT  240 . At operation  1050 , OLT  240  receives the SLEEP_READY message. OLT  240  checks the sleep conditions. If the sleep conditions are satisfied, OLT  240  sends a SLEEP message to ONU  220 , with a next wakeup time. When ONU  220  receives the SLEEP message, it transitions to Sleep state until the next wakeup time. If the sleep conditions are not satisfied, OLT  240  ignores the SLEEP_READY message. 
         [0057]    When ONU  220  is in Sleep state, it remains in this state until the next wakeup time, at which time ONU  220  transitions to Waking Up state. 
         [0058]    The bottom portion of  FIG. 8  shows the PM protocol when ONU  220  is in Waking Up state. At operation  1060 , ONU  220  checks the sleep conditions. If the sleep conditions are satisfied, ONU sends a SLEEP-READY message to OLT  240 . At operation  1070 , OLT  240  receives the SLEEP_READY message. OLT  240  checks the sleep conditions. If the sleep conditions are satisfied, OLT  240  sends a SLEEP message to ONU  220 , with a next wakeup time. When ONU  220  receives the SLEEP message, it transitions to Sleep state until the next wakeup time. If the sleep conditions are not satisfied, OLT  240  ignores the SLEEP_READY message. 
         [0059]    At operation  1080 , ONU  220  checks the wakeup conditions described hereinabove with reference to activity detectors  262  and  263 . If the wakeup conditions are satisfied, ONU  220  sends a WAKE_ACK message to OLT  240  and transitions to Active state. When OLT  240  receives the WAKE_ACK message, it knows that ONU  220  is in Active state. 
         [0060]    At operation  1090 , OLT  240  checks the wakeup conditions. If the wakeup conditions are satisfied, OLT  240  sends a WAKE_REQ message to ONU  220 . At operation  1100 , ONU  220  receives the WAKE_REQ message. ONU  220  checks the wakeup conditions. If the wakeup conditions are satisfied, ONU  220  transitions to Active state, and sends a WAKE_ACK message to OLT  240 . When OLT  240  receives the WAKE_ACK message, it knows that ONU  220  is in Active state. If the wakeup conditions are not satisfied, ONU  220  ignores the WAKE_REQ message. 
         [0061]    If a timeout period has elapsed and OLT  240  has still not received an expected SLEEP_READY or WAKE_ACK message from ONU  220 , then at operation  1110  OLT  240  checks the sleep conditions. If the sleep conditions are satisfied, OLT  240  sends a SLEEP message to ONU  220 . Otherwise, OLT  240  checks the wakeup conditions. If the wakeup conditions are satisfied, OLT  240  sends a WAKE_REQ message to ONU  220 . At operation  1120 , ONU  220  receives the WAKE_REQ message. ONU  220  checks the wakeup conditions. If the wakeup conditions are satisfied, ONU  220  transitions to Active state, and sends a WAKE_ACK message to OLT  240 . When OLT  240  receives the WAKE_ACK message, it knows that ONU  220  is in Active state. If the wakeup conditions are not satisfied, ONU  220  ignores the WAKE_REQ message. 
         [0062]    Reference is made to  FIG. 9 , which is a simplified power management state transition diagram for an ONU  220 , in accordance with an embodiment of the present invention. Shown in  FIG. 9  are the three ONU states; namely, Active state  10 , Sleep state  20  and Waking Up state  30 . While in Active state  10 , ONU downstream and upstream activity detectors  282  and  283  track the respective upstream and downstream data traffic rates for ONU  220 , and the respective upstream and downstream frame gaps for ONU  220 , and use the rates and gaps to check whether or not the sleep conditions for ONU  220  are satisfied. OLT activity detectors  262  and  263  also track these respective rate and gaps, and use the rates and gaps to check whether or not the sleep conditions for ONU  220  are satisfied. When either activity detectors  262  and  263 , or activity detectors  282  and  283 , determine that the sleep conditions are satisfied, then ONU  220  transitions into Sleep state  20  by a respective OLT-initiated or ONU-initiated transition. The transition to Sleep state  20  includes designation of a next wakeup time. 
         [0063]    While in Sleep state  20 , ONU  220  is in a low power mode. Buffer  261  stores downstream data for ONU  220 , and buffer  281  stores upstream data for ONU  220 . When the wakeup time for Sleep state  20  arrives, or when ONU PM module  280  receives a wakeup request from OLT PM module  280 , ONU  220  transitions to Waking Up state  30 . 
         [0064]    While in Waking Up state  30 , ONU PM module  280  sends a report to OLT PM module  260  regarding the occupancy of ONU upstream data buffer  281 . Activity detectors  262  and  263 , and activity detectors  282  and  283  use this information to determine if the wakeup conditions are satisfied. If so, then ONU  220  transitions into Active state  10 . Otherwise, ONU  220  transitions back into Sleep state  20 . 
         [0065]    As seen in  FIG. 9 , when ONU  220  is in Waking Up state, ONU PM module  280  sends a report regarding its upstream buffer occupancy to OLT PM module  260 . Thus OLT PM module  260  is able to determine whether or not the upstream buffer occupancy exceeds its threshold, which is one of the wakeup conditions. 
         [0066]    In an alternative embodiment of the present invention ONU  220  undergoes a “fast wakeup”, which is a direct transition from Sleep state  20  to Active state  10 , if local wakeup conditions are satisfied. Upon waking up, ONU PM module  280  responds to report requests from OLT PM module  260 . As such, OLT PM module  280  may send report requests to ONU PM module  280  while ONU  220  is in Sleep state  20 , for ONU PM module  280  to report occupancy of upstream buffer  281 . Such a fast wakeup mechanism enables ONU PM module  280  to respond to local events in the middle of a sleep cycle. Such a fast wakeup is indicated by a dashed arrow between states  20  and  10  in  FIG. 9 . 
         [0067]    The present invention has broad application to power management for point-to-multipoint networks. Such applications include inter alia: 
         [0000]    security camera networks, wherein cameras are similar to ONUs and the control center is similar to an OLT;
 
sensory arrays, wherein smart sensors are similar to ONUs and the control center is similar to an OLT; and
 
smart house management, wherein home appliances are similar to ONUs and the main control computer is similar to an OLT.
 
         [0068]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific exemplary embodiments without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.