Abstract:
The present invention discloses a host receiver synchronizer for passive optical networks, and in particular a burst clock data recovery circuit in a host receiver in a bursty asynchronous communication system having a non-data preamble of less than 250 ns, for recovering a clock signal from a subscriber data burst. The circuit comprises: an adjustable oscillator for generating an output clock signal in response to a signal at an input thereof; a first comparator for comparing a frequency and phase of the output clock signal to that of a reference signal and feeding back a first feedback signal to the oscillator input; and a second comparator for comparing the frequency and phase of the output clock signal to that of the data burst and feeding back a second feedback signal to the oscillator input once the output clock signal is locked in frequency with the reference signal. The output clock signal is locked in frequency and phase to the data burst before receipt of the last bit of the preamble. The present invention is advantageous in that the receiver circuit improves synchronized jitter performance over the prior art solutions so that additional timing margin is provided, thereby allowing longer fiber lengths to be supported.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to passive optical networks and in particular to a novel host receiver synchronizer for passive optical networks. 
         [0003]    2. Description of the Prior Art 
         [0004]    Passive optical networks (PONs) are optical networks in which there is a shared host transceiver and a plurality of subscriber nodes. Each of the subscriber nodes are connected to the host transceiver by a single shared optical fiber through optical splitters. 
         [0005]    Transmission in the downstream direction, that is, from the host transmitter to each subscriber node is accomplished by broadcasting a signal on a designated wavelength from the host transmitter through a series of optical splitters to each of the subscriber nodes. The data stream in the downstream direction contains messages which may be intended for any of the subscribers and each subscriber reads the downstream data. The data stream is encoded to identify to which subscriber a portion thereof is intended and that subscriber recognizes when it is the intended recipient of the data and extracts and processes it. 
         [0006]    Transmission from the subscriber nodes back to the host (in the “uplink” direction) is done through the same optical splitter network and fiber, but on a second optical wavelength. The uplink bandwidth is shared between subscribers using time division multiplexing (TDM). Accordingly, the uplink data stream is necessarily bursty, as subscribers only transmit when they have data to send and at specific times to avoid collisions with other subscribers. 
         [0007]    Because a single host optical transceiver is shared by many subscribers, PONs, including networks complying with the EPON (Ethernet PON) or GPON (Gigabit PON) standards are a relatively low cost solution while providing substantially higher data rates than the present state of the art for either wired or wireless access solutions. 
         [0008]    In the more aggressive PON standard, GPON, maximum permissible data rates are 2.48832 Gbit/s in both the downstream and uplink directions, though in practice only 1.24416 Gbit/s is presently feasible for the uplink direction. Discussion is presently underway for future systems with 9.95328 Gbit/s and beyond. 
         [0009]    Moreover, PON systems provide substantial economy over direct dedicated point to point solutions such as Ethernet or SONET. Accordingly, PONS are rapidly becoming the emerging access solution for the delivery of broadband services to the business and residential market. 
         [0010]    Under the PON standard specifications, in the uplink direction, subscribers transmit their data in bursts at a frequency synchronized to the downstream data frequency. The subscribers recover the transmit clock signal from the downstream data and use this clock to retime their upstream data back to the host making the entire system frequency synchronous. 
         [0011]    Even though all subscriber transmitters are synchronous with the host, they are all at different distances from the host and may also have different latencies before transmission occurs so the phase of the upstream data arriving at the host will vary widely from burst to burst. 
         [0012]    Since the phase of every burst may be different, the phase of the host receiver sampling clock must be realigned with every burst of upstream data. Until the phase of the sampling clock has been successfully realigned to the data, any data received will be lost. To avoid loss of usable bandwidth it is preferable to minimize the portion of the burst that is lost during phase realignment. 
         [0013]    Because of the bursty nature of upstream transmissions, conventional feedback mechanisms for clock recovery and resynchronization are not generally applicable as they typically take considerable time to approach the received frequency and then match its phase. Rather, any clock resynchronization mechanism must adapt to each burst in turn in a time much shorter than the burst duration. 
         [0014]    In the various PON standards, each burst is preceded by a preambular period of short duration relative to the burst length in which a pattern providing expendible data transitions is transmitted for the purpose of allowing phase alignment of the receiver sampling clock to the burst. The use of a tone in particular ensures that there are a large number of rising and falling transitions to aid in the resynchronization process. 
         [0015]    In GPON systems in particular, phase alignment should occur within the preambular period since a delimiter follows the preamble supplying framing information for the remainder of the burst. If the delimiter is incorrectly received, then the burst will be lost. 
         [0016]    Conventional synchronizers for optical systems may require on the order of hundreds of microseconds to resynchronize to a data stream after a prolonged period without data applied to the input. However, the brevity of the preambular period mandates that resynchronization be completed in substantially less than 100 nanoseconds. 
         [0017]    In current PON systems, digital oversampling techniques are applied to choose the optimal sampling point from among many samples per data bit period, using a digital state machine. 
         [0018]    The granularity of sampling limits the accuracy of phase alignment. Improved granularity of sampling costs power and chip area and can be limited by matching concerns. 
         [0019]    Accordingly, such prior art methodologies are only appropriate for relatively low uplink data rates, presently 1.24432 Gbit/s and below, but would not be feasible using currently available technology at rates of 2.48832 Gbit/s or above. Even at the preferred 1.24416 Gbit/s uplink data rate for GPON, the receiver performance would be limited by the oversample resolution. 
         [0020]    This is because oversampling techniques suffer numerous limitations such as bandwidth, noise, and matching, all of which degrade the ability of the resynchronization circuit to synchronize at an optimal sampling position. This makes PON systems embodying such techniques very sensitive to signal degradation and may even result in loss of data when using longer fiber lengths. 
         [0021]    Linear clock and data recovery (CDR) circuits, that is, a CDR circuit that continuously adjusts the phase of the recovered clock with infinite precision adapting to the centre of the data eye, have been conventionally used to adjust the frequency and phase of a local clock to optimally sample the data, using a feedback loop that compares the period of the received data to the period of a local clock and continuously adjusts the phase and period of the local clock to maintain a proper sample point even as the phase of the data changes. For the purposes of the discussion below, such maintaining of a proper sample point is referred to as frequency and/or phase locking to the local clock. 
         [0022]    However, linear CDR circuits first match the frequency of the incoming data in an initial frequency lock state and it is only when frequency lock is achieved that the linear CDR circuits deal with the phase of the incoming data in a subsequent phase lock state. Unfortunately, typically, the length of the frequency lock state is, in such conventional systems, much longer than the time available for locking to a PON burst. Thus, such conventional systems are unsuited to burst mode applications. 
         [0023]    Accordingly, it is desirable to provide a novel resynchronizer for resynchronizing the host receiver sampling clock in a PON system. 
         [0024]    It is further desirable to provide a resynchronizing circuit that will automatically centre in the middle of the data eye without oversampling. 
         [0025]    It is still further desirable to provide a resynchronizing circuit that is capable of operating at a present preferred 1.24416 Gbit/s uplink data rate and future 2.48832 Gbit/s rate as well higher rates that may be specified in the future. 
       SUMMARY OF THE INVENTION 
       [0026]    The present invention accomplishes these aims by providing a modified linear CDR circuit to adjust the frequency and phase of a local clock oscillator to sample the data at an optimal point. 
         [0027]    A feedback loop compares the data period to the period of the local clock oscillator and continually adjusts both the phase and the clock period to maintain a proper sampling point even as the phase of the data varies. 
         [0028]    The inventive receiver circuit synchronizes to incoming burst data sufficiently quickly to comply with the highest data rate specified in the GPON standard while meeting the needs of anticipated future higher rates. The GPON standard presently provides the most stringent synchronization time requirements, namely 44 preamble bit periods at 1.24416 Gbit/s. 
         [0029]    Advantageously, the inventive receiver circuit improves synchronized jitter performance over existing solutions so that additional timing margin is provided. Potentially, this could allow longer fiber lengths to be supported or the design of cheaper network elements in other parts of the system. 
         [0030]    According to a first broad aspect of an embodiment of the present invention, there is disclosed a burst clock data recovery circuit in a host receiver in a bursty asynchronous communication system having a non-data preamble of less than 250 ns, for recovering a clock signal from a subscriber data burst, the circuit comprising:
       (a) an adjustable oscillator for generating an output clock signal in response to a signal at an input thereof;   (b) a first comparator for comparing a frequency and phase of the output clock signal to that of a reference signal and feeding back a first feedback signal to the oscillator input; and   (c) a second comparator for comparing the frequency and phase of the output clock signal to that of the data burst and feeding back a second feedback signal to the oscillator input once the output clock signal is locked in frequency with the reference signal;
 
whereby the output clock signal is locked in frequency and phase to the data burst before receipt of the last bit of the preamble.
       
 
         [0034]    According to a second broad aspect of an embodiment of the present invention, there is disclosed a method of recovering a clock signal from a subscriber data burst to a host receiver in a bursty asynchronous communication system having a non-data preamble of less than 250 ns before receipt of the last bit of the preamble, the method comprising the steps of:
       (a) generating an output clock signal in response to an oscillator input;   (b) comparing the output clock signal against a reference signal substantially locked in frequency to that of the data burst;   (c) feeding back a first feedback signal to the oscillator input corresponding to a difference between a frequency and phase of the output clock signal and that of the reference signal until the output clock signal is substantially locked in frequency and phase to the reference signal;   (d) waiting for detection of the data burst;   (e) comparing the output clock signal to the data burst; and   (f) feeding back a second feedback signal to the oscillator input corresponding to a difference between the frequency and phase of the output clock signal and that of the data burst until the output clock signal is substantially locked in frequency and phase to the data burst.       
 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
           [0042]      FIG. 1  is a system block diagram of a conventional PON system; and 
           [0043]      FIG. 2  is a system block diagram of a linear burst mode receiver according to the present invention for use in the PON Host Transceiver shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Referring now to  FIG. 1 , there is shown a system block diagram of a PON system  10  in which the present invention might find application. 
         [0045]    The system  10  comprises a PON host  20 , a plurality of substantially identical (for purposes of the present discussion) PON subscribers  30 , of which an exemplary subscriber is shown in detail, a PON host optical module  40 , at least one optical splitter  50 , a plurality of fibers  60 ,  61  and a plurality of PON subscriber optical modules  70 . 
         [0046]    The host  20  is connected to the host optical module  40  by a pair of serial connections  25  and  26 . Serial connection  26  conveys serial data from the host  20  to the host optical module  40  (downstream data) while serial connection  25  conveys serial data from the host optical module  40  to the host  20  (uplink data). As mandated by the PON standard specifications, the downstream data is transmitted in continuous fashion while the uplink data is received in bursts. 
         [0047]    As shown in  FIG. 1 , the host  20  may accept data for transmission as downstream data from an external source (not shown) through a parallel data interface  21  and forward on the received uplink data to the external source through the parallel data interface  21 . 
         [0048]    The host  20  accepts as an input a global frequency reference  24 , which it uses to time its downstream data. 
         [0049]    As is discussed below, the host  20  also comprises a burst CDR circuit  23  in accordance with the present invention, with which the host  20  maintains a receiver sampling clock that is synchronized to each uplink data burst received along serial connection  22 . 
         [0050]    Each subscriber  30  is connected to a corresponding subscriber optical module  70  by a pair of serial connections  31  and  32 . Serial connection  31  conveys serial data from the subscriber  30  to the subscriber optical module  70  (uplink data) while serial connection  32  conveys serial data from the subscriber optical module  70  to the subscriber  30  (downstream data). As mandated by the PON standard specifications, the downstream data is received in continuous fashion while the uplink data is transmitted in bursts. 
         [0051]    The subscriber  30  recovers a clock signal from the downstream data and uses it to retime its uplink data intended for the host  20 , thus ensuring relative identity of frequency for all uplink data intended for the host  20 . 
         [0052]    The host optical module  40  is connected to the host  20  by serial connections  25  and  26  and to a single optical fiber  60  feeding into the optical splitter  50 . The host optical module  40  converts the downstream serial data received along serial connection  26  from the electrical domain into the optical domain and transmits it along host optical fiber  60  at a first downstream wavelength. Similarly, it converts the uplink optical data it receives along host optical fiber  60  at a second upstream wavelength back down to the electrical domain and transmits it in serial form along serial connection  25  to the host  20 . 
         [0053]    The optical splitter  50  is connected to the host optical module  40  by the host optical fiber  60  and to a plurality of subscriber optical fibers  61  each feeding into a corresponding subscriber optical module  70 . The optical splitter  50  divides downstream signals arriving along the host optical fiber  60  at the downstream wavelength to each of the subscriber optical fibers  61  and collects uplink signals arriving along the various subscriber fibers  61  at the uplink wavelength and merges them into the host fiber  60 . 
         [0054]    Those having ordinary skill in the art will readily recognize that depending upon the number of subscribers in the PON, a plurality of optical splitters, either in parallel or cascaded may be implemented to accommodate the subscribers, with additional fibers (not shown) interconnecting them. 
         [0055]    Turning now to  FIG. 2 , the structure of the inventive burst CDR  23  is shown in detail. The CDR  23  comprises a modified linear CDR circuit constructed from known silicon technology components, although in an innovative combination. 
         [0056]    It comprises a voltage controlled oscillator (VCO)  231 , which is preferably but not limited to an inductive/capacitive (LC) VCO, acting as a variable phase/frequency clock source. The VCO  231  is adjustable in frequency as a function of either input voltage or input current. Those having ordinary skill in this art will readily recognize that the LC VCO is an oscillation source with variable phase/frequency. Alternatively fixed phase VCO sources followed by phase rotators to provide phase interpolation could be used. Those having ordinary skill in this art will readily recognize that a ring oscillator with calibrated parameters related to PLL bandwidth could also be used. 
         [0057]    At all times, the oscillator used in the inventive burst CDR circuit  23  to generate the receiver frequency is highly bandwidth constrained. Preferably, this is achieved by use of an LC VCO, which is chosen for its tight control of bandwidth-related parameters in conventional silicon processes. Those having ordinary skill in this art will readily recognize that any controllable oscillator with tight control of bandwidth related parameters could be suitably used. 
         [0058]    Those having ordinary skill in this art will also recognize that conventional PON receivers using an oversampled synchronization scheme do not face such bandwidth constraints, so that ring oscillators are more typically used. Ring oscillators are well understood, low cost and of simple implementation. While they also produce much wider bandwidth variation, this is not significant in conventional oversampled synchronization approaches. 
         [0059]    The inventive burst CDR circuit  23  modifies a conventional linear CDR circuit to overcome the timing constraints endemic to such circuits. Specifically, before a burst of uplink data arrives, the inventive linear CDR circuit  23  is synchronized to the same local frequency reference source  24  that times the downstream data stream, so that all of the subscribers  30  are already synchronized to this frequency and the time to achieve frequency synchronization is minimized. 
         [0060]    In order to frequency lock the VCO to the master frequency reference, a frequency comparison circuit  232  compares the output  237  of the VCO  231  with the PON master frequency reference  24 . The master frequency reference  24  is the same local frequency reference source that times the downstream data stream, so that all of the subscribers  30  are already synchronized to this frequency. Accordingly, the time to achieve frequency synchronization can be minimized. Typically, since the master frequency reference  24  operates at a lower frequency than the LC VCO  231 , a divided version of the VCO  231  is actually compared against the master frequency reference  24 . 
         [0061]    Those having ordinary skill in this art will recognize that either the local sample clock, the reference, or both signals may be divided prior to the frequency/phase comparison. Accordingly, for the purposes of this invention, the term “frequency and/or phase lock” incorporate comparing a divided first signal with a higher rate second signal. In such circumstances, frequency and/or phase lock is said to have occurred when a pair of similar transitions occur at the same time (or within a predetermined tolerance) in both signals, even though in the higher rate signal the transitions are not necessarily consecutive. 
         [0062]    The frequency comparison circuit  232  provides a correaction signal  236  to be used in a feedback loop to adjust the sample clock frequency during an initial lock to reference state. In the lock to reference state, the correaction signal  236  is fed through a PLL selector  233  and into a charge pump  237  that drives an integrating filter circuit  238  that feeds back into an input of the LC VCO  231 . The integrating filter circuit  238  provides noise immunity and stability for the feedback loop. In the lock to reference state the local sample clock is locked in both frequency and phase to the master reference. 
         [0063]    Preferably, the frequency comparison circuit  232  comprises a phase-frequency detector that locks the output signal  237  of the LC VCO  231  to the PON master frequency reference  24 . Those having ordinary skill in the relevant art will recognize that such a phase-frequency detector will not lock the LC VCO  231  to data, but only to a tone, such as the master frequency reference  24 . 
         [0064]    The frequency comparison circuit  232  causes the LC VCO  231  to remain locked to the master reference frequency  24  between uplink bursts. When an uplink data burst arrives, a PLL selection switch  233  causes the burst CDR circuit  23  to switch to a lock to data state in which the correaction signal for a feedback phase locked loop (PLL) comprising the VCO  231 , integrating filter circuit  238  and charge pump  237  is derived from the data, not the master reference frequency  24 . 
         [0065]    Preferably the arrival of the uplink data burst is detected and signalled by a data detection circuit  241  that monitors the uplink data stream from serial connection  25 . Alternatively, the host  20  may provide an external signal (not shown) when it obtains information about the anticipated arrival time of the burst. Still further, a signal may be obtained from the optics (not shown) to initiate phase lock. 
         [0066]    The data detection circuit  241 , or an external signal indicating the presence of a data burst, transmits a signal  242  to a system controller  235  when data is received along the serial connection  25 , so that the LC VCO  231  should be locked to data as opposed to the master reference source  24 . 
         [0067]    The system controller  235  is preferably a state machine consisting of a plurality of timers (not shown). In order to optimize phase lock to the data burst, the receiver remains in the lock to data state only when uncorrupted data is present. The timers ensure that this is the case. 
         [0068]    A first timer provides a delay before initiating data lock after data has been signalled to be present by either the data detection circuit  241  or an external signal. This first delay provides a settling period for the data (not shown) after laser turn on at the subscriber  30 . During this delay period, the VCO  231  remains locked to the master reference  24 . If phase lock to data is attempted before the data has settled, the burst CDR  23  could be driven out of frequency lock before uncorrupted data arrives. In such an event, the long time period to reacquire frequency lock would result in the loss of the data in the data burst. 
         [0069]    Prior to arrival of the burst, the inventive CDR  23  is placed in a high bandwidth state. This state speeds phase lock during the preambular period. The initiation of the higher bandwidth state is achieved by supplying control signals to both the charge pump  237  and the integrating filter circuit  238 , thus increasing the gain of each while still meeting PLL requirements for stability and linearity. The bandwidth setting for the lock to data state is beyond the range usually associated with broadband standards. 
         [0070]    The time with expanded bandwidth is limited by a second timer in the system controller  235  to limit jitter effects caused by the expanded bandwidth. A high bandwidth CDR will be subject to high pattern-dependent recovered clock jitter when recovering a clock from random data. If a tone is used for the preamble, this higher pattern-dependent jitter is not present. For this reason the inventive burst receiver  23  is only placed in its high bandwidth state when bursts are not present or during a tonal preamble. Following the timeout of the second timer (set to expire prior to the arrival of payload data) the CDR will be forced to a low bandwidth state. Again, this bandwidth optimization is accomplished using control signals to the charge pump  237  and the integrating filter circuit  238 . 
         [0071]    Eventually, once the uplink data burst has been determined to have ended, the inventive burst CDR  23  will be returned to a high bandwidth state. The end of the burst is determined by the system state machine detecting that a sufficient number of 0s have occurred to guarantee that the subscriber laser is off and that the pattern is not simply a valid occurrence of a long string of 0s within the burst. This is implemented with a third timer in the state machine. An external control signal from the host can also be used to signal the end of the burst. When data has been determined to be no longer present, the VCO  231  is locked to the master reference  24  in preparation for the next burst. 
         [0072]    If frequency lock is lost, as a result of drift in the absence of data it will take a considerable period of time to reacquire lock. It is for this reason, after the burst is complete, that the system controller  235  causes the burst CDR  23  to lock to the master reference frequency  24  so that the absence of data will not cause frequency lock to be lost. 
         [0073]    Those having ordinary skill in this art will also readily recognize that a collision of two bursts may also cause the linear CDR to lose frequency lock. For example, if during normal operation a subscriber sends data during an invalid timing window (for that subscriber) or during ranging mode when multiple subscribers simultaneously send bursts to allow system calibration, a conventional CDR will lose frequency lock. To regain frequency lock, the system will either have to wait for CDR recovery or the host must recognize the delivery of corrupt data and immediately force the CDR to lock to the master reference. As collision events are rare, a CDR recovery period is allowable within the standard. 
         [0074]    When the CDR is in the lock to data state, the phase comparison circuit  239  compares the output signal  237  of the LC VCO  231  to the received data stream delivered from serial connection  25  and generates an output voltage or current  240  that is a linear function of the phase difference between a transition in the uplink data stream and a transition in the output signal  237  of the LC VCO  231 . The output signal  240  is fed into the PLL selection switch  233  and in the lock to data state, through the charge pump  237  and the integrating filter circuit  238  and back into the LC VCO  231 . Thus, the feedback circuit fixes the LC VCO  231  in a stable state at an optimal phase that corresponds with the center of the data eye. Those having ordinary skill in this art will recognize that the center of the eye depends on the shape of the eye. The phase of the CDR may be adjusted slightly to optimize the bit error rate of the system and the center of the eye should be understood as that phase that is most optimal for transmission of data. A CDR may also be centered slightly off center and still provide sufficient performance for some systems. 
         [0075]    Preferably, the phase comparison circuit  239  comprises a modified Hogge-type phase detector circuit. The Hogge-type circuit works in conjunction with the charge pump  237  to drive a linear current output as an input into the integrating filter circuit  238  and ultimately providing a correction signal to the LC VCO  231 . Alternatively, those having ordinary skill in this art will readily recognize that many other types of linear and non-linear phase detectors could be used, including but not limited to a bang-bang phase detector, SAW filter, mixer, and sample&amp;hold phase detector. 
         [0076]    Finally, a demultiplexing circuit  243  converts the recovered high speed serial data stream from the phase comparison circuit  239  at a data rate suitable for conventional digital processing by the host  20 . 
         [0077]    In operation, at all times the system controller  235  monitors the frequency of the VCO  231  and compares it to that of the master reference  24  or divided versions thereof. This is done by counting transitions of the divided VCO clock and the master reference within a fixed time duration. The difference between counts translates to the frequency deviation between the two clocks. If the count period is long enough to provide sufficient resolution and the counts differ beyond a threshold that defines frequency lock (˜1 loop bandwidth) then the burst CDR  23  will be forced into frequency lock with the master reference  24  to resynchronize the VCO with the PON master reference frequency. This monitor function overrides all other system controller  235  functions because no other states are valid if there is excessive frequency deviation between the frequency of the master reference  24  and the frequency of the VCO  231 . 
         [0078]    The frequency monitoring time to provide a resolution of 1 loop bandwidth is substantially longer than the time to achieve frequency lock. Transition from the lock to reference state to the lock to data state can only occur after the frequency comparison circuit  232  has completed a measurement. In order to speed frequency lock after a collision (or other event forcing loss of frequency lock), the frequency comparison circuit  232  resolution is relaxed below the traditional value for transport systems of 1 loop bandwidth. The monitor measures gross loss of frequency lock but has its resolution set low enough so that it does not gate the readiness of the CDR for the next burst. The time for frequency lock is known and the monitor resolution is set so that frequency lock will be guaranteed in the time that is takes to monitor the frequency. 
         [0079]    Following the transition to the lock to reference state, the system controller  235  forces the inventive burst CDR receiver  23  to remain in the lock to reference state until the VCO  231  frequency matches that of the master frequency reference  24 . While in the lock to reference state, the LC VCO  231  will be in its high bandwidth setting so as to promote fast frequency locking. 
         [0080]    Once frequency lock has been acquired, and when data is thereafter detected by the data detection circuit  241 , the system controller  235  will delay for a pre-determined delay to ensure that the data has settled. Thereafter, the system controller  235  will signal the PLL selection switch  233  using timer signal  236  to lock to data in an attempt to acquire phase lock. 
         [0081]    Thereafter, after a further pre-determined delay, the bandwidth of the CDR will be lowered for optimal circuit jitter performance. This second delay is chosen to ensure that sufficient phase lock has occurred. 
         [0082]    Presence of data is determined by the presence of optical output at the receiver of the host  20  when the subscriber optical module laser (not shown) is activated and transmitting a minimum number of 1s in a given time period. The minimum number is determined from analysis of typical data, which is permitted to have a limited sequence of consecutive 0s under the transmission protocol. The PON standards statistically permit up to 72 consecutive 0s in real data, however those persons having ordinary skill in the art will readily recognize that 72 is just an example. Accordingly, if the system controller  235  times out because of an excessive number of 0s in the pattern, for example, after a period corresponding to 72 consecutive 0s, it will conclude that data has been lost and revert to the lock to reference state. 
         [0083]    It is possible to have a time between bursts of less than 72 bits. If this is true then the burst CDR  23  relies on external input from the host to determine the expected beginning and end of a burst. In order for the PON to function without collisions, the latency between the host and each subscriber must be calibrated in a ranging/discovery period within the PON protocol. During this ranging period the host compiles the timing data that is necessary to supply an external host signal to the burst receiver indicating a gap between burst. This signal allows the use of the timers in the system controller  235  to optimize bandwidth, expanding it for phase acquisition, and after phase lock, contracting it for jitter immunity. If an external signal is not available then the burst CDR  23  will be subject to effects including lack of filtering at laser turn on and the inability to have a different bandwidth during the preamble and the payload data. The burst receiver  23  can still function with these restrictions but performance will be sub-optimal. 
         [0084]    Because the inventive burst CDR  23  does not employ oversampling, it is better suited to enable host receiver synchronization at higher PON data rates, for example, at 1.24416 Gbit/s and higher. Indeed, as data rates increase, synchronization times improve because there are more data transitions available to more rapidly force phase lock. Also there is a fundamental stability ratio relation for a linear CDR between the CDR clock rate and the CDR bandwidth. Higher clock rates allow higher bandwidth and therefore faster locking time. By contrast, conventional oversampling systems are constrained to relatively lower data rates because sampling must take place at a significant multiple of the data rate. In today&#39;s 0.13 μm silicon technology, 10 Gbit/s conventional (non burst) linear receivers are widely available. The same technology has only delivered 1.25 Gbit/s oversampled receivers with sufficient resolution to be used in the longer reach optical links required in PON standards such as GPON. Baud rate sampling will always allow a higher maximum receiver rate within a given technology as compared to oversampling. 
         [0085]    Simulations have predicted that the inventive burst CDR  23  will achieve frequency lock to the master reference within 12 μs and will typically achieve phase lock to data within 10 ns. In particular without bandwidth widening during the preambular period, the inventive burst CDR  23  will achieve lock in less than 250 ns at data rates below 1.2 Gb/s. Above this data rate, other components in the system overwhelm the ratio between the preambular time and the bit period, so that the standards will fix the phase lock period to less than 250 ns. This provides significant margin in the GPON timing budget that may now be used by other components in the system and compares favourably to the lock time period of approximately 15,000 bit periods using a conventional (CDR or oversampling) system. 
         [0086]    When the inventive CDR  23  has a variable bandwidth that may be increased as previously discussed during the pre-ambular period, the inventive burst CDR  23  will require a maximum of 30 bits to achieve lock. 
         [0087]    The inventive burst mode receiver  23  is constructed in conventional and economic CMOS processes and it allows other elements in the systems to be designed with relaxed specifications thus providing multiple cost advantages. 
         [0088]    The present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and methods actions can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. 
         [0089]    Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in ASICs (application-specific integrated circuits). 
         [0090]    The system may comprise a processor, a random access memory, a hard drive controller, and an input/output controller coupled by a processor bus. 
         [0091]    It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention. 
         [0092]    Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein. 
         [0093]    Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.