Patent Publication Number: US-7583898-B1

Title: Signal-detect-based ranging technique for burst-mode optical systems

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to data transmission in a passive optical network (PON) and, more specifically, to an improvement in the ranging process that is performed prior to initiating (time-division multiplexed) upstream transmissions. 
     2. Description of the Related Art 
     Most digital telecommunications networks (i.e., networks that facilitate the communication of data, voice, video, etc., between parties or between a content distribution service and subscribers) typically comprise active components, such as repeaters, relays and other such devices that consume power, in the path between an exchange and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the exchange and subscribers. A single fiber can run from the exchange to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers. 
     The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). These standards and recommendations are well-known to persons skilled in the art to which the invention relates and are therefore not described in further detail herein (i.e., in this patent specification). 
     In accordance with these standards and recommendations, a PON comprises an optical line terminator (OLT) at the exchange or central office and a number of optical network units (ONUs), also known as optical network terminals (ONTs), each located at or near the subscriber&#39;s premises (e.g., home, office building, etc.), with optical fiber and splitters between the OLT and ONUs. In the downstream direction, i.e., data transmitted from the exchange to a subscriber, the data packets (also referred to as cells) are broadcast from the OLT to all of the ONUs in the network, and an ONU can select the data to receive by matching the address embedded in the data units to a selected address. In the upstream direction, i.e., data transmitted from a subscriber to the exchange, the data units are time-division multiplexed with those transmitted from other subscribers. These systems are sometimes referred to as burst-mode PON technologies because they transmit bursts of data packets at relatively high bit rates. 
     The amplitude of the upstream signal received at the OLT generally varies from ONU to ONU. The amplitude differences occur for a number of reasons, including the number of splits in the paths and the different distances from the OLT at which the ONUs may be located. Bit errors can occur if the OLT receiver threshold is too high or too low to detect the bits of a packet. To account for differences in amplitudes of signals received from different ONUs, the G984 specifications provide for the OLT to “train” its receiver to each upstream packet, i.e., adjust itself to the amplitude range of that packet, in order to receive the packet data without errors. The specifications provide for inclusion of a preamble preceding the data bits of each packet to use in training the OLT receiver. After an initialization procedure, described below, involving applying a reset signal, the OLT adjusts its receiver&#39;s detection threshold upward or downward until the threshold is centered half way between the logic “1” legal and the logic “0” level. When the detection threshold has been successfully centered between the logical levels, the received data will be recovered without duty cycle distortion, which is necessary to the process of recovering the clock signal from the combined clock-data signal. 
     In accordance with the ITU standards and recommendations, in the upstream direction each ONU is to transmit a data packet only during its assigned timeslot. Nevertheless, because some ONUs may be located at different distances from the OLT than other ONUs, differences in propagation delays may cause data packet transmissions to overlap slightly in time and become garbled with each other at splitters/combiners. A synchronization measure known as “ranging” is employed to prevent such overlap or collisions. Ranging is a method that comprises the OLT calculating a propagation delay between it and each ONU in the PON. The OLT does this by transmitting a ranging window message indicating that ranging is to begin. Any ONU that has not already been ranged responds by transmitting a reply. (Additional steps, not described here for purposes of clarity, are taken to ensure that replies from different ONUs do not collide.) The reply, much like any data packet, includes a delimiter or predetermined bit pattern between the preamble and the data bits that follow the preamble. When the received bits match the expected delimiter, the OLT calculates the time differential between the sending of the ranging grant and the receipt of the ranging response. From the time differential, the OLT can calculate an equalization (EQ) delay for the ONU that will allow the ONU to adjust its upstream transmissions so that they arrive at the OLT precisely within the time slot assigned to that ONU. After the ONU receives its calculated EQ delay from the OLT, ranging is deemed completed for that ONU. 
     Ranging is generally performed as soon as an ONU is powered on or otherwise brought into the PON, or upon initialization of PON operation, but it can also be performed periodically thereafter to ensure that all ONUs in the PON have been ranged. 
     As shown in  FIG. 1 , a prior or conventional OLT has a media access controller (MAC)  10  that controls the majority of OLT functions and thus is analogous to a central processor. The burst-mode transceiver circuitry of the OLT (only the receiver portion  12  of which is shown in  FIG. 1  for purposes of clarity) includes an optical module  14 , a clock processing device (CPD)  16 , and a data de-serializer  18 . Optical module  14  receives the optical signal from the PON (i.e., transmitted by an ONU) and detects the waveform transitions that represent the packet preamble and data bits. To detect the data bits without errors, it performs the above-described training process on each packet preamble. That is, it adjusts itself to respond to bit transitions in the amplitude range of the preamble. In PON architectures in which each ONU generates a clock signal that it maintains at the same frequency as the reference clock signal that the OLT generates, CPD  16  is typically a clock phase aligner (CPA) device that merely determines the phase difference between the ONU clock and OLT clock. In other PON architectures, CPD  16  may be a clock-data recovery (CDR) device that recovers a clock signal from the received signal and uses the recovered clock signal to sample the data bits from the received signal. In either case, CPD  16  receives the detected bit stream and outputs a clock signal and an accompanying (serial) data bit stream. In architectures in which CPD  16  is a CPA, it is generally unable to perform its function unless the information it receives is completely undistorted. Regardless of CPD type, all architectures have limits on the amount of duty cycle distortion and jitter they can tolerate. To minimize the amount of distorted preamble information that reaches CPD  16 , it is important that the optical module  14  train itself as quickly as possible. De-serializer  18  converts the bit stream to multi-bit words, synchronized with the clock, for use by MAC  12 . Note that other portions of the OLT that do not directly relate to the present invention are not shown in  FIG. 1  for purposes of clarity. 
     In a conventional OLT, MAC  12  can assert a reset signal  20  (“RST”) that causes optical module  14  to re-adjust its detection threshold in preparation for receiving a data packet. In other conventional OLT architectures (not shown), the MAC can assert another reset signal that causes the CPD to re-acquire the clock signal. In normal operation, i.e., when the ONUs are transmitting ordinary data packets to OLT  10 , MAC  12  can determine with some precision the beginning of a timeslot in which an ONU signal is expected to be received. A problem arises, however, when the packet is not an ordinary data packet, but rather a ranging reply, because MAC  12  cannot determine when a ranging reply will be received. A reply could be received at any time within the ranging window, the length of which is set to a relatively large predetermined worst-case length (e.g., 200 microseconds) to account for ONUs that may be far from the OLT. Due to this uncertainty, it has been suggested that MAC  12  simply assert reset signal  20  immediately after OLT  10  transmits the ranging window message. In some architectures, however, the reset signal  20  causes the optical module  14  to enter a high-gain mode. In the high-gain mode, optical module  14  is unduly sensitive to noise spikes and is thus very susceptible to producing spurious bit outputs. Although not all burst-mode optical receiver architectures necessarily behave in this manner with regard to entering a high-gain mode, essentially all such receivers suffer from the problem that the longer an input (light) signal is absent, the higher the probability that the receiver will produce a spurious output, which will almost always cause the MAC to produce errors. Determining the optimal condition or conditions under which to assert such a reset signal during ranging has been problematic in the art. The present invention addresses this problem and others in the manner described below. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and system in which a burst-mode receiver of an optical line terminator (OLT) is initialized during an initialization phase by asserting one or more reset signals in response to detecting the presence of a packet signal. In one form of the invention, the OLT transmits an initialization message to an optical network unit (ONU), such as a ranging grant or other message indicating the initiation of ranging. When the OLT detects the presence of a packet signal received from the ONU in response to the initialization message, it asserts one or more reset signals that are provided to the burst-mode receiver. 
     In commercially available burst-mode receivers, there are typically one or more electronic elements that need to be reset (and thus have a pin or similar contact to which a reset signal is to be applied). One such reset signal can be, for example, a reset signal provided to the element known as the “optical module.” Alternatively or in addition, such a reset signal can be a reset signal provided to a receiver clock processing device (CPD). The CPD can be a clock phase aligner (CPA), clock-data recovery (CDR) device, or similar device. The reset signal or signals can be asserted immediately upon detection of a packet signal or after a predetermined delay. In embodiments of the invention in which more than one reset signal is asserted, they can be asserted simultaneously, or alternatively, one reset signal can be delayed from another reset signal. For example, there can be a predetermined delay between asserting the reset signal applied to an optical module and asserting the reset signal applied to a CPD. 
     Asserting burst-mode receiver reset signals when the presence of a packet signal (i.e., light) is first detected minimizes the length of time that a signal is absent from the receiver input before the arrival of actual packet data and thereby minimizes the probability that the receiver will produce a spurious output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generalized block diagram of a prior art burst-mode receiver for a passive optical network (PON). 
         FIG. 2  is a generalized block diagram of a PON that includes an optical line terminator (OLT) in accordance with an exemplary embodiment of the invention. 
         FIG. 3  is a generalized block diagram of an exemplary burst-mode receiver of the OLT of  FIG. 2 . 
         FIG. 4  is a flow diagram illustrating an exemplary method for asserting one or more reset signals provided to portions of the receiver illustrated in  FIG. 2 . 
         FIG. 5  is a flow diagram illustrating an alternative exemplary method for asserting one or more reset signals provided to portions of the receiver illustrated in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 2 , in an exemplary embodiment of the present invention a passive optical network (PON) includes an optical line terminator (OLT)  200  and a number of optical network units (ONUs)  202 ,  204 ,  206 , etc. The OLT  200  is interconnected in the conventional manner with each ONU  202 ,  204 ,  206 , etc., by optical fibers  208 ,  210 ,  212 ,  214 , etc., and one or more optical splitters  216 , etc. Although only one such splitter  216  and three such ONUs  202 ,  204  and  206  are shown for purposes of illustration, the PON can have any other suitable topology and number of ONUs, splitters, fibers, etc. The OLT  200  can be located at, for example, an exchange or central office from which services are provided, such as distribution of television programming and provision of Internet access. The ONUs  202 ,  204 ,  206 , etc., can be located at, for example, residences or other premises occupied by subscribers to such services. Although data communication in the PON is bidirectional, the present invention relates primarily to the ranging process that is performed as an initialization step in preparation for data communication in the upstream direction, i.e., from any of ONUs  202 ,  204 ,  206 , etc., to OLT  200 . The communication of data on the PON occurs in the manner well-understood in the art, using any of a number of suitable conventional technologies, such as asynchronous transfer mode (ATM) protocol, and is therefore not described herein in further detail. The present invention relates not to the communication protocols or content of what is communicated, but rather to the ranging process, described below, that typically occurs when the PON is initially brought into operation, or at such time as it is re-configured, modified, augmented, etc., or at any other suitable time. 
     The OLT  200  includes a media access controller (MAC)  218  that controls the majority of OLT functions and thus is analogous to a central processor. MAC  218  is programmed or configured in accordance with the present invention to include “signal-detect (SD)-to-reset” control logic  220 , such as suitable software or firmware, which controls the method of operation described below with regard to  FIGS. 4-5 . SD-to-reset control logic  220  can be included as part of ranging logic  222 . SD-to-reset control logic  220  and ranging logic  222  are shown in generalized form for purposes of illustration as residing within MAC  218 , but as persons skilled in the art will appreciate, this depiction is intended to indicate only that MAC  218  is programmed or configured to perform the corresponding functions of those elements and is not intended to imply any limitations as to where any corresponding software, firmware or other logic must actually or physically reside. As persons skilled in the art understand, such logic can be in any suitable form and can be disposed or distributed in any suitable manner, such as among a number of elements. 
     As well understood in the art, ranging is a method or process that comprises the OLT  200  calculating a propagation delay between it and each ONU  202 ,  204 ,  206 , etc., in the PON. Ranging logic  222  and the well-known ranging process in general are not described in further detail herein but can include any suitable methods known in the art for ranging and related functions of the types typically performed in a PON or similar optical network. MAC  218  is also programmed or configured with other logic for controlling other OLT functions, but only SD-to-reset control logic  220  and ranging logic  222  are shown for purposes of clarity. In addition to MAC  218 , OLT  200  includes an optical transmitter  224 , optical (burst-mode) receiver  226 , and other OLT logic  228 . Other OLT logic  228  represents logic elements, such as processors, memories, data encoders and decoders, etc., that are conventional and typically included in prior OLTs of the type known in the art. The structure and function of such elements are well known in the art and therefore not described herein in further detail. In other embodiments of the invention, SD-to-reset control logic  220  can be separate from ranging logic  222  and instead included as part of other OLT logic  228 . Transmitter  224  is similarly of the type well-known in the art and therefore not described herein. Transmitter  224  and receiver  226  together define an optical transceiver, although they can be physically integrated or not integrated with one another to any suitable extent. 
     Receiver  226  is shown in further detail in  FIG. 3  with MAC  218  and together they define a receiver control system. Receiver  226  includes an optical module  302 , a clock processing device (CPD)  304 , and a de-serializer  306 , each of which is well-known in the art and commercially available. Other portions of receiver  226  that are not directly relevant to the present invention are not shown for purposes of clarity and can include any elements known in the art of the types typically included in burst-mode optical receivers. Receiver  226  has a well-known architecture in which the optical module  302  receives upstream optical signals transmitted on the PON and, in response, provides an output signal to CPD  304 . CPD  304  in turn provides clock and data output signals to de-serializer  306 , which provides the data in the form of parallel words to MAC  218 . Note that optical module  302  has an output that produces a Signal Detect (SD) signal  308 . Many commercially available optical modules  302  have such an output for indicating the presence or absence of an optical signal, but the output is typically either unused in prior receiver architectures or used only to report a loss of signal (LOS), i.e., that a malfunction has occurred in the fiber or upstream equipment. Optical module  302  also has an input that receives a Reset signal  310 . CPD  304  similarly has an input that receives another Reset signal  312 . CPD  304  can be a clock phase aligner (CPA), clock-data recovery (CDR) device, or other suitable clock processing device of a type that has a reset signal input. 
     As illustrated by the flow diagram of  FIG. 4 , MAC  218  ( FIG. 3 ) asserts Reset signals  310  and  312  in response to SD signal  308 . The flow diagram represents an exemplary method that can be embodied in ranging logic  222  or other OLT logic  228  ( FIG. 2 ). As indicated above with regard to  FIG. 2 , SD-to-reset control logic  220  causes Reset signals  310  and  312  to be asserted in response to SD signal  308 . Persons skilled in the art to which the invention relates will readily be capable of programming or configuring MAC  218  to effect such methods in view of the teachings herein. 
     At step  402 , OLT  200  ( FIG. 2 ) transmits an initialization message indicating initiation of an initialization phase, such as ranging. For example, it can transmit a ranging grant message or other message indicating opening of a ranging window. 
     At step  404 , OLT  200  listens or monitors for a response from one of ONUs  202 ,  204 ,  206 , etc. ( FIG. 2 ). In a conventional PON, an ONU responds to a message indicating initiation of ranging by transmitting a reply message. Optical module  302  ( FIG. 3 ) detects the arrival of the reply message (i.e., the beginning of the data packet preamble) in a suitable manner, such as by comparing the power or other characteristic of the incoming optical signal with a predetermined threshold. As soon as the reply message begins to arrive at OLT  200  (as indicated by a characteristic that exceeds the threshold), optical module  302  asserts SD signal  308 . In accordance with SD-to-reset control logic  220 , MAC  218  responds to the assertion of SD signal  308  by asserting one or both of Reset signals  310  and  312 , as indicated by step  406  in  FIG. 4 . For example, step  406  can comprise asserting Reset signal  310  at step  408 , delaying some suitable predetermined time interval at step  410 , and then asserting Reset signal  312  at step  412 . The delay can be on the order of nanoseconds or tens of nanoseconds or any other suitable non-zero amount. Although in the illustrated embodiment of the invention both of Reset signals  310  and  312  are asserted (with a delay in between), in other embodiments only one of them is asserted. In still other embodiments, the various reset signals can be asserted simultaneously, with zero intentional delay in between, or they can be asserted in another order. 
     Although not shown for purposes of clarity, OLT  200  also trains itself on the preamble at this time in the conventional manner so that it can properly receive the data bits that follow the preamble. As indicated by step  414 , when MAC  218  detects the preamble delimiter, indicating the end of the packet preamble and beginning of the packet data, it calculates the time differential between the sending of the ranging grant and the receipt of the ranging reply and otherwise completes the ranging process in the conventional manner. If MAC  218  fails to detect the preamble delimiter (within some predetermined timeout interval), it re-initiates another attempt at ranging at step  402 . 
     As illustrated in  FIG. 5 , an alternative method may enhance performance in some instances. For example, especially in an embodiment of the invention in which optical module  302  receives no Reset signal  310 , optical module  302  might assert SD signal  308  while it is still outputting distorted data. In such an instance, CPD  304  may be unable to acquire the correct phase lock, causing it to output garbled data and preventing MAC  218  from detecting the delimiter. It may enhance performance in such instances or others if the assertion of Reset signal  312  is delayed. (Note that it is delayed at step  410  in  FIG. 4 .) The method illustrated in  FIG. 5  provides an adjustable or incremental delay. Steps  502  and  504  are identical to steps  402  and  404 , described above. Step  506  is similar to step  410 , described above, in that there is some suitable predetermined amount of delay between assertion of SD signal  308  and assertion of a reset signal (at step  508 ). In an embodiment of the invention in which optical module  302  receives no Reset signal  310 , Reset signal  312  is thus asserted at step  508 . Step  510  is identical to step  414 , described above. If MAC  218  ( FIG. 3 ) does not detect the preamble delimiter at step  510 , however, then at step  512  it increments or otherwise increases the delay (to which step  506  refers) before returning to step  502  and re-initiating ranging. The increased delay provides more time for the opto-electronics of optical module  302  to settle and thus for distortion in the data it outputs to subside. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to this invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of any claims and their equivalents. With regard to the claims, no claim is intended to invoke the sixth paragraph of 35 U.S.C. Section 112 unless it includes the term “means for” followed by a participle.