Abstract:
Processing a received optical signal in an optical communication network includes equalizing a received optical signal to provide an equalized signal, demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and decoding the inner-decoded signal according to an outer code. Other aspects include other features such as equalizing an optical channel including storing channel characteristics for the optical channel associated with a client, loading the stored channel characteristics during a waiting period between bursts on the channel, and equalizing a received burst from the client using the loaded channel characteristics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/477,845 filed Jun. 10, 2003, incorporated herein by reference, and U.S. Provisional Application No. 60/480,488 filed Jun. 21, 2003, incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to optical fiber communications. 
   BACKGROUND OF THE INVENTION 
   Line coding is a process by which a communication protocol arranges symbols that represent binary data in a particular pattern for transmission. Conventional line coding used in fiber optic communications includes non-return-to-zero (NRZ), return-to-zero (RZ), and biphase, or Manchester. The binary bit stream derived from these line codes can be directly modulated onto wavelengths of light generated by the resonating frequency of a laser. Traditionally direct binary modulation based transmission offers an advantage with regard to the acceptable signal-to-noise ratio (SNR) at the optical receiver, which is one of the reasons direct binary modulation methods are used in the Datacom Ethernet/IP, Storage Fiber-Channel/FC and Telecom SONET/SDH markets for transmission across nonmultiplexed unidirectional fiber links. 
   The performance of a fiber optic network can be measured by the maximum data throughput rate (or information carrying capacity) and the maximum distance between source and destination achievable (or reach). For Passive Optical Networks (PONs) in particular, additional measures of performance are the maximum number of Optical Networking Units (ONUs) and/or Optical Networking Terminals (ONTs) possible on a network and the minimum and maximum distance between the Optical Line Terminator (OLT) and an ONU/ONT. These performance metrics are constrained by, among other things, amplitude degradation and temporal distortions as a result of light traveling through an optical fiber. 
   Amplitude degradation is substantially a function of length or distance between two end points of an optical fiber. Temporal distortion mechanisms include intramodal (chromatic) dispersion and intermodal (modal) dispersion. Intramodal dispersion is the dominant temporal dispersion on Single-mode fiber (SMF), while intermodal dispersion is dominant on Multi-mode fiber (MMF). Both types of temporal distortions are measured as functions of frequency or rate of transmission (also referred as line rate of a communication protocol) over distance in MHz·km. Temporal distortions are greater, hence a constraint on network performance, with increasing frequency transmission. 
   SUMMARY OF THE INVENTION 
   In general, in one aspect, the invention includes a method for processing a received optical signal in an optical communication network, the method including: determining a first set of coefficients to equalize a portion of an optical signal received over a first optical link including using a blind equalization method that does not use a known training sequence to equalize the portion of the optical signal, equalizing the portion of the optical signal using the determined coefficients, and demodulating the equalized portion of the optical signal according to an m-ary modulation format. 
   Aspects of the invention may include one or more of the following features. The method includes determining a second set of coefficients to equalize a portion of an optical signal received over a second optical link. The method includes selecting one of the first or second set of coefficients based on a source of the portion of optical signal being equalized. The portion of the optical signal includes a burst within a time slot of the first optical link. The method includes storing the determined coefficients. The method includes retrieving the stored coefficients for equalizing a second portion of the optical signal corresponding to a portion received from a same source as generated the first portion of the optical signal. The coefficients are retrieved between signal bursts on the first optical link. The stored coefficients are retrieved for respective portions of the optical signals that correspond to respective signal sources. The first optical link includes a link in a point-to-multipoint passive optical network. The m-ary modulation format is selected from the group consisting of quadrature amplitude modulation and pulse amplitude modulation. The method includes demodulating a received first data stream and demodulating a second data stream received in the optical signal, and multiplexing the first and second data streams. 
   In general, in another aspect, the invention includes optical communication system including: a first transceiver coupled by an optical network to a second transceiver and third transceiver, the first transceiver including an equalization block and a modulation block, the equalization block operable to determine a first set of coefficients to equalize a portion of an optical signal received over the optical network from the second transceiver and a second set of coefficients to equalize a portion of the optical signal received over the optical network from the third transceiver, the equalization block including a blind equalization routine that does not use a known training sequence to equalize the portions of the optical signal, the equalization block operable to equalize the portions of the optical signal using the determined coefficients, and the modulation block operable to demodulate equalized portions of the optical signal according to an m-ary modulation format. 
   Aspects of the invention may include one or more of the following features. The optical network includes a first optical link for coupling the first and second transceiver, and a second optical link for coupling the first and third transceivers and where the equalization block is operable to select one of the first or second set of coefficients based on a source of the portion of optical signal being equalized. The equalization block is operable to store the first and second sets of coefficients for later retrieval and use to equalize portions of the optical signal. The portion of the optical signal includes a burst within a time slot on the optical network. The equalization block is operable to retrieve the sets of coefficients between signal bursts on the optical network. The optical network includes a link in a point-to-multipoint passive optical network. The m-ary modulation format is selected from the group consisting of quadrature amplitude modulation and pulse amplitude modulation. The system includes a multiplexer, the modulation block operable to demodulating a received first data stream and a second data stream received in the optical signal, and the multiplexer operable to multiplex the first and second data streams. The system includes a transmission convergence layer block for processing data streams received by the first transceiver, the transmission convergence layer block operable to control the demultiplexing of data streams including control of the multiplexer. The optical network is an optical distribution network. The first transceiver is an optical line terminator. The second and third transceivers are optical network terminals or optical network units. 
   In general, in another aspect, the invention includes a method for processing data for transmission in an optical communication network, the method including: demultiplexing a data stream into a first demultiplexed data stream and a second demultiplexed data stream, modulating each of the first and second data streams according to an m-ary modulation format, transmitting the first modulated data stream over a first optical link; and transmitting the second modulated data stream over a second optical link. 
   In general, in another aspect, the invention includes an optical communication system including: a demultiplexer operable to demultiplex a data stream into a first demultiplexed data stream and a second demultiplexed data stream, a modulation block operable to modulate each of the first and second data streams according to an m-ary modulation format, transmitting means operable to transmit the first modulated data stream over a first optical link and the second modulated data stream over a second optical link. 
   In general, in another aspect, the invention includes a method for processing a received optical signal in an optical communication network, the method including: equalizing a received optical signal to provide an equalized signal, demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and decoding the inner-decoded signal according to an outer code. 
   Aspects of the invention may include one or more of the following features. The m-ary modulation format is selected from the group consisting of quadrature amplitude modulation and pulse amplitude modulation. Equalizing the received optical signal includes equalizing the received optical signal using a blind equalization routine that does not use a known training sequence. Equalizing the received optical signal includes equalizing the received optical signal using a known training sequence. The known training sequence is multiplexed in a frame within the received optical signal. The inner code includes a trellis code. The outer code includes an error correction code. The outer code includes a Reed Solomon code. 
   In general, in another aspect, the invention includes a transceiver including: an equalizer for equalizing a received optical signal to provide an equalized signal, a demodulator in communication with the equalizer for demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, an inner-decoder in communication with the demodulator for decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and an outer-decoder in communication with the inner-decoder for decoding the inner-decoded signal according to an outer code. 
   Aspects of the invention may include one or more of the following features. The transceiver includes an optical module including a first bi-directional optical fiber interface including a first detector and a first driver, and a second bi-directional optical fiber interface including a second detector and a second driver, and management means for managing data flow across the first bi-directional optical fiber interface and across the second bi-directional optical fiber interface. The transceiver includes an optical module including a first bi-directional optical fiber interface including a first detector and a first driver, and a second bi-directional optical fiber interface including a second detector and a second driver, and a multiplexer for multiplexing a first demultiplexed data stream received over the first bi-directional optical fiber interface and a second demultiplexed data stream received over the second bi-directional optical fiber interface into a multiplexed data stream for transmission. The transceiver includes an optical module including a first bi-directional optical fiber interface including a first detector and a first driver, and a second bi-directional optical fiber interface including a second detector and a second driver, and a queue manager for managing traffic for a first bi-directional link associated with the first bi-directional optical fiber interface independently from traffic for a second bi-directional link associated with the second bi-directional optical fiber interface. 
   In general, in another aspect, the invention includes a transceiver including: an optical module including a first bi-directional optical fiber interface including a first detector and a first driver, and a second bi-directional optical fiber interface including a second detector and a second driver, and management means for managing data flow across the first bi-directional optical fiber interface and across the second bi-directional optical fiber interface. 
   Aspects of the invention may include one or more of the following features. The management means includes a multiplexer for multiplexing a first demultiplexed data stream received over the first bi-directional optical fiber interface and a second demultiplexed data stream received over the second bi-directional optical fiber interface into a multiplexed data stream for transmission. The management means is configured to demultiplex a data stream over a plurality of fiber links that excludes one or more failed fiber links. The management means includes a queue manager for managing traffic across the first bi-directional fiber interface independently from traffic for the second bi-directional fiber interface. The management means is configured to change the alignment of received data bits to adjust for an order of optical fiber connections to the first bi-directional optical fiber interface and the second bi-directional optical fiber interface. 
   In general, in another aspect, the invention includes a method for equalizing an optical channel including: storing channel characteristics for the optical channel associated with a client, loading the stored channel characteristics during a waiting period between bursts on the channel, and equalizing a received burst from the client using the loaded channel characteristics. 
   Aspects of the invention may include one or more of the following features. The method includes determining that the waiting period occurs before a burst from the client based on a schedule. The method includes updating the stored channel characteristics. The method includes providing a grant window, transmitting an identification number to the client in response to receiving a serial number from the client after the grant window. The method includes determining a distance from an upstream device to the client. The method includes compensating for communication delays between the upstream device and the client based on the determined distance. 
   In general, in another aspect, the invention includes a method for communicating data on a fiber optic network, the method including: modulating and demodulating data traffic on an optical link in the network in an m-ary modulation format; encoding and decoding data traffic on an optical link in the network according to an inner coding routine and an outer coding routine, demultiplexing data traffic from an optical link in the network and transmitting the data traffic across a plurality of optical fiber links in the network, multiplexing the data traffic from the plurality of optical fiber links, and equalizing a receive channel in the network to remove temporal distortions. 
   Aspects of the invention may include one or more of the following features. The method includes equalizing the receive channel according to a blind equalization routine. The method includes equalizing the receive channel according to a decision directed equalization routine. The method includes saving and loading coefficients for equalizing the receive channel for each of a plurality of transmitting sources. The method includes saving and loading an inner coding state for the receive channel for each of a plurality of transmitting sources. The method includes saving and loading an outer coding state for the receive channel for each of a plurality of transmitting sources. The method includes excluding one or more failed optical fiber links from the plurality of optical fiber links. The method includes conveying a training sequence for a decision directed equalization routine as part of an in-use communication protocol. A training sequence for a decision directed equalization routine is conveyed as part of the activation process for an optical network terminal or optical network unit. An incorrect connection of an optical fiber link is corrected without having to physically change the connection. 
   In general, in another aspect, the invention includes a method for communicating on a passive optical network between a central transmission point and a plurality of receiving client end points, the method including: preparing downstream data for transmission and transmitting an optical downstream continuous mode signal demultiplexed across a plurality of bi-directional fibers using a plurality of wavelengths of light, receiving an optical downstream continuous mode signal demultiplexed from the plurality of bi-directional fibers using the plurality of wavelengths of light and recovering a downstream data transmission, preparing upstream data for transmission and transmitting an optical upstream burst mode signal demultiplexed across the plurality of bi-directional fibers using the plurality of wavelengthss of light, and receiving an optical burst mode signal demultiplexed from the plurality of bi-directional fibers using the plurality of wavelengths of light and recovering an upstream data transmission. 
   Aspects of the invention may include one or more of the following features. The central transmission point includes an optical line terminal, and the end points are operative as transceivers in a passive optical network. The upstream and downstream data for transmission are conveyed by respective different industry-standard services. 
   Implementations of the invention may include one or more of the following advantages. 
   A system is proposed that provides for high-speed communications over fiber optic networks. The system may include the use of the one or more of the following techniques either individually or in combination: m-ary modulation; channel equalization; demultiplexing across multiple fibers, coding and error correction. M-ary modulation allows for increased data throughput for a given line rate due to an increase in the number of bits per symbol transmitted. Channel equalization reduces the effects of temporal distortions allowing for increased reach. Demultiplexing across multiple fibers allows lower lines rates for a given data throughput rate due to the increased aggregate data throughput from the multiplexing. Coding and error correction allows for a greater selection of qualifying optical components that can be used in the network and complements m-ary modulation and channel equalization for overall system performance improvement as measured by transmit energy per bit. These methods when combined (in part or in total) increase the data throughput and reach for fiber optic networks. For PONs in particular, these methods may increase the number of ONU/ONTs and the distance between OLT and ONU/ONT without increasing the line rate as compared to a conventional communication system of equivalent data throughput. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a fiber optic data network. 
       FIG. 2  illustrates a block diagram of a passive optical network. 
       FIG. 3  illustrates a block diagram of a high-speed communication system for fiber optic networks. 
       FIG. 4  illustrates a block diagram of an alternative high-speed communication system for fiber optic networks. 
       FIG. 5  illustrates a block diagram of an alternative high-speed communication system for fiber optic networks. 
       FIG. 6  illustrates a block diagram of an alternative high-speed communication system for fiber optic networks. 
       FIG. 7  illustrates a block diagram of an alternative high-speed communication system for fiber optic networks. 
       FIG. 8  is a flow diagram for an upstream burst mode communication equalization process. 
       FIG. 9  is a flow diagram for a downstream continuous mode communication equalization process. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , wherein like reference numerals designate identical or corresponding parts throughout the several views, a high-level fiber optic data network  50  includes a first transceiver  100  in communication with a second transceiver  101  via a fiber  108 . The first transceiver  100  and the second transceiver  101  include transmitter circuitry (Tx)  134 ,  135  to convert electrical data input signals into modulated light signals for transmission over the fiber  108 . In addition, the first transceiver  100  and the second transceiver  101  also include receiver circuitry (Rx)  133 ,  136  to convert optical signals received via the fiber  108  into electrical signals and to detect and recover encoded data and/or clock signals. First transceiver  100  and second transceiver  101  may contain a micro controller (not shown) and/or other communication logic and memory  131 ,  132  for network protocol operation. Although the illustrated and described implementations of the transceivers  100 ,  101  include communication logic and memory in a same package or device as the transmitter circuitry  134 ,  135  and receiver circuitry  133 ,  136 , other transceiver configurations may also be used. 
   First transceiver  100  transmits/receives data to/from the second transceiver  101  in the form of modulated optical light signals of known wavelength via the optical fiber  108 . The transmission mode of the data sent over the optical fiber  108  may be continuous, burst or both burst and continuous modes. Both transceivers  100 ,  101  may transmit a same wavelength (e.g., the light signals are polarized and the polarization of light transmitted from one of the transceivers is perpendicular to the polarization of the light transmitted by the other transceiver). Alternatively, a single wavelength can be used by both transceivers  100 ,  101  (e.g., the transmissions can be made in accordance with a time-division multiplexing scheme or similar protocol). 
   In another implementation, wavelength-division multiplexing (WDM) may also be used. WDM is herein defined as any technique by which two optical signals having different wavelengths may be simultaneously transmitted bi-directionally with one wavelength used in each direction over a single fiber. In yet another implementation, dense wavelength-division multiplexing (DWDM) may be used. DWDM is herein defined as any technique by which more than two optical signals having different wavelengths may be simultaneously transmitted bi-directionally with more than one wavelength used in each direction over a single fiber with each wavelength unique to a direction. For example, if wavelength division multiplexing is used, the first transceiver  100  may transmit data to the second transceiver  101  utilizing a first wavelength of modulated light conveyed via the fiber  108  and, similarly, the second transceiver  101  may transmit data via the same fiber  108  to the first transceiver  100  utilizing a second wavelength of modulated light conveyed via the same fiber  108 . Because only a single fiber is used, this type of transmission system is commonly referred to as a bi-directional transmission system. Although the fiber optic network illustrated in  FIG. 1  includes a first transceiver  100  in communication with a second transceiver  101  via a single fiber  108 , other implementations of fiber optic networks, such as those having a first transceiver in communication with a plurality of transceivers via a plurality of fibers (not shown), may also be used. 
   Electrical data input signals (Data IN  1 )  115 , as well as any optional clock signal (Data Clock IN  1 )  116 , are routed to the transceiver  100  from an external data source (not shown) for processing by the communication logic and memory  131 . Communication logic and memory  131  process the data and clock signals in accordance with an in-use network protocol. Communication logic and memory  131 ,  132  provides management functions for received and transmitted data including queue management (e.g., independent link control) for each respective link, demultiplexing/multiplexing and other functions as described further below. The processed signals are transmitted by the transmitter circuitry  134 . The resulting modulated light signals produced from the first transceiver&#39;s  100  transmitter  134  are then conveyed to the second transceiver  101  via the fiber  108 . The second transceiver  101 , in turn, receives the modulated light signals via the receiver circuitry  136 , converts the light signals to electrical signals, processes the electrical signals using the communication logic and memory  132  (in accordance with an in-use network protocol) and, optionally, outputs the electrical data output signals (Data Out  1 )  119 , as well as any optional clock signals (Data Clock Out  1 )  120 . 
   Similarly, the second transceiver  101  receives electrical data input signals (Data IN  1 )  123 , as well as any optional clock signals (Data Clock IN)  124 , from an external data source (not shown) for processing by the communication logic and memory  132  and transmission by the transmitter circuitry  135 . The resulting modulated light signals produced from the second transceiver&#39;s  101  transmitter  135  are then conveyed to the first transceiver  100  using the optical fiber  108 . The first transceiver  100 , in turn, receives the modulated light signals via the receiver circuitry  133 , converts the light signals to electrical signals, processes the electrical signals using the communication logic and memory  131  (in accordance with an in-use network protocol), and, optionally, outputs the electrical data output signals (Data Out  1 )  127 , as well as any optional clock signals (Data Clock Out  1 )  128 . 
   Fiber optic data network  50  may also include a plurality of electrical input and clock input signals, denoted herein as Data IN N  117 / 125  and Data Clock IN N  118 / 126 , respectively, and a plurality of electrical output and clock output signals, denoted herein as Data Out N  129 / 121  and Data Clock Out N  130 / 122 , respectively. The information provided by the plurality of electrical input signals may or may not be used by a given transceiver to transmit information via the fiber  108  and, likewise, the information received via the fiber  108  by a given transceiver may or may not be outputted by the plurality of electrical output signals. The plurality of electrical signals denoted above can be combined to form data plane or control plane bus(es) for input and output signals respectively. In some implementations, the plurality of electrical data input signals and electrical data output signals are used by logic devices or other devices located outside (not shown) a given transceiver to communicate with the transceiver&#39;s communication logic and memory  131 ,  132 , transmit circuitry  134 ,  135 , and/or receive circuitry  133 ,  136 . 
     FIG. 2  illustrates an implementation of a passive optical network (PON)  52 , where the functions described above associated with the first transceiver  100  and the second transceiver  101  of  FIG. 1 , are implemented in an optical line terminator (OLT)  150  and one ore more optical networking units (ONU)  155 , and/or optical networking terminals (ONT)  160 , respectively. PON(s)  52  may be configured in either a point-to-point network architecture, wherein one OLT  150  is connected to one ONT  160  or ONU  155 , or a point-to-multipoint network architecture, wherein one OLT  150  is connected to a plurality of ONT(s)  160  and/or ONU(s)  155 . In the implementation shown in  FIG. 2 , an OLT  150  is in communication with multiple ONTs/ONUs  160 ,  155  via a plurality of optical fibers  152 . The fiber  152  coupling the OLT  150  to the PON  52  is also coupled to other fibers  152  connecting the ONTs/ONUs  160 ,  155  by one or more passive optical splitters  157 . All of the optical elements between an OLT and ONTs/ONUs are often referred to as the Optical Distribution Network (ODN). Other alternate network configurations, including alternate implementations of point-to-multipoint networks are also possible. 
     FIG. 3  shows a system block diagram for an implementation of transceiver  100 . The following is a description of the functions and responsibilities that are part of an implementation of the Communication Logic &amp; Memory  131  of transceiver  100 . The Communication Logic &amp; Memory  131  includes an asynchronous or synchronous system transmit (TX) interface  301  and receive (RX) interface  302  that is supported by the TX Path  303  and RX Path  304  blocks. System interfaces can be selected from conventional interfaces including serial, serial XFI, parallel, GMII, XGMII or XAUI or some other interface may be used. TX Path  303  and RX Path  304  blocks manage the TX and RX interfaces  301 ,  302  and feed data into and get data from the transmission convergence layer (TC-Layer) block  305 . TX Path  303  and RX Path  304  blocks may perform line code adaptation functions (e.g., line coding used outside the transceiver can be terminated by a TX Path block  303  or sourced by a RX Path block  304  to allow a bit stream, cell and/or packet formatted data to be adapted for processing by a TC-Layer block  305 ). The TC-Layer  305  block creates the transport system that the data traffic, management and control agents will exploit. TC-Layer  305  block includes a TC-layer protocol stack such as specified in the ITU G.984 specification (incorporated herein by reference), IEEE 802.3ah MAC protocol stack specification (incorporated herein by reference) or a derivative thereof. A variety of other protocol stacks may also be used. The TC-Layer  305  block may perform the additional functions of equalizer, coding, queue and demultiplexing management. The TC-Layer  305  block sends transmit data to a DeMux  306  block, which splits the transmitting data into a plurality of data paths (two paths shown in  FIG. 3 ) for demultiplexing data across multiple fibers. Some implementations need not include DeMux  306  block (and hence do not support demultiplexing data across multiple fibers). DeMux  306  block may demultiplex data across a subset of fibers to exclude fibers experiencing link failure to ensure data throughput. The exclusion of fiber links experiencing failure is controlled by the TC-Layer  305  block as part of the demultiplexing management function. 
   After DeMux  306  block, in one implementation, the transmit paths have analogous processing blocks. In an alternative implementation, independent signal processing is can be supported in each path.  FIG. 3  shows two transmit paths, though more can be included. In a transmit path, the transmit data is provided to the outer coder  307   a ,  307   b  block. In one implementation, outer coder  307   a  performs a reed-solomon coding. Other coding methods can be used. The outer coder  307   a ,  307   b  block provides data to the inner coder  308   a ,  308   b  block. In order to improve the energy per bit required to deliver the transmitting data, an inner coder  308   a ,  308   b  is used. Outer coder  307   a ,  307   b  may be used to support forward error correction (FEC) recovery of bit(s) errors. In one implementation, inner coder  308   a ,  308   b  implements a trellis coding method. Other coding methods may be used. Data from the inner coder  308   a ,  308   b  is provided to Modulation (MOD)  309   a ,  309   b  block. Alternatively, in some implementations, the outer coder  307   a ,  307   b  and inner coder  308   a ,  308   b  blocks are not used, and the output of the DeMux  306  block is provided directly to the MOD  309   a ,  309   b  block. 
   To increase the number of bits per symbol transmitted, m-ary modulation is performed in the MOD  309   a ,  309   b  block. In one implementation, an m-ary modulation method such as Quadrature Amplitude Modulation (QAM), 32-QAM, 256-QAM, Pulse Amplitude Modulation (PAM) or 5-PAM is used. Other m-ary modulation methods can be used. After processing by the MOD  309   a ,  309   b  block, the transmit data is converted to an analog signal by a Digital to Analog Converter (DAC)  310   a ,  310   b . In one implementation, DAC  310   a ,  310   b  is configured to shape, condition or emphasize the signal for improved transmission performance. The DAC  310   a ,  310   b  passes the transmit data via electrical signals  311   a ,  311   b  to the laser driver (Driver)  312   a ,  312   b  as part of an implementation of TX  134  in an Optical Module  326 . The driver  312   a ,  312   b  drives the Laser Diode (LD)  313   a ,  313   b , which transmits light in response to transmit data signals received from the driver  312   a ,  312   b . The light emitted from LD  313   a ,  313   b  is directed into the fibers  314   a ,  314   b  with the aid of a fiber optic interface (not shown). The fiber optic interface may include the necessary components (e.g., filters) to implement WDM or DWDM functions. 
   On the receive side of the transceiver  100  as part of an implementation of RX  133  in an Optical Module  326 , light propagated across an ODN (not shown in  FIG. 3 ) travels over fibers  314   a ,  314   b  through a fiber optic interface (not shown) and is received by the photo diode (PD)  315   a ,  315   b . In response, the PD  315   a ,  315   b  provides a photocurrent to the TransImpedance Amplifier (TIA)  316   a ,  316   b  that converts the photocurrent into an electrical voltage signal. The electrical voltage signal from the TIA  316   a ,  316   b  is then transmitted to a Linear Amplifier (LA)  317   a ,  317   b  as a differential signal or a single-ended signal  318   a ,  318   b . The LA  317   a ,  317   b  performs signal conditioning on the received electrical voltage signal to provide increased resolution and system performance. The LA  317   a ,  317   b  provides an electrical signal  319   a ,  319   b  to a Clock Data Recovery (CDR)  320   a ,  320   b  block that recovers clock and data signals, which are provided to an Analog to Digital Converter (ADC)  321   a ,  321   b . The ADC  321   a ,  321   b  converts analog signals to digital signals, which are provided to an Equalization (EQ)  322   a ,  322   b  block. The EQ  322   a ,  322   b  block performs equalization on the received data, which is then provided to a De-Mod &amp; Inner Decoder  323   a ,  323   b . An Equalization  322   a ,  323   b  block may implement a blind equalization method or decision-directed equalization method. Blind equalization is discussed further below. Other equalization methods may be used. The De-Mod &amp; Inner Decoder  323   a ,  323   b  block performs complementary de-modulation to the m-ary modulation performed in the MOD  309   a ,  309   b  block as well as a complementary decoding method to the coding method performed in the Inner Coder  308   a ,  308   b  block. In one implementation, De-Mod &amp; Inner Decoder  323   a ,  323   b  includes a Viterbi decoder. Other decoding means may be used. Received data is then provided to the outer decoder  324   a ,  324   b  block, which performs a complementary decode to the error detection and/or recovery method chosen in the outer coder  307   a ,  307   b  block. After demodulation and decoding, the received data is then provided to the Mux  325  block that performs a complementary function to the DeMux  306  block. The combined received data is then provided to the TC-Layer  305 . In implementations without Outer Coder  307   a ,  307   b  and Inner Coder  308   a ,  308   b  blocks, the output of the EQ  322   a ,  322   b  block is provided directly to the Mux  325  block. 
   The RX  133 ,  136  and TX  134 ,  135  circuitry of transceivers  100 ,  101 , or portions thereof, for example, PD  315   a ,  315   b  and LA  317   a ,  317   b , can be combined within industry standard optical modules. Common optical module standards are 300pin, XENPAK, X2, and XPAK transponders and XFP or SFP transceivers. These optical modules include unidirectional fiber links with one fiber link for transmit path and a second fiber link for the receive path. However, implementations of optical modules  326 ,  401 ,  501  incorporate a plurality of bi-directional fiber links for transmitting demultiplexed data on separate fiber links. Any of a variety of optical couplers may be used to separate and/or combine light propagating into or out of the fiber links. These optical modules  326 ,  401 ,  501  used herein can conform to a form factor of standard optical modules such as the 300pin, XENPAK, X2, XPAK, XFP or SFP. Other form factors may also be used. 
   Alternatively, in other implementations of transceiver  100 , functions described above may be integrated into various different components. For example, in the implementation of transceiver  100  shown in  FIG. 4 , various functions may be incorporated into optical module  401  such as: digital to analog conversion  310   a ,  310   b ; analog to digital conversion  321   a ,  321   b ; clock data recovery  320   a ,  320   b ; m-ary modulation  309   a ,  309   b ; m-ary de-modulation  323   a ,  323   b ; channel equalization  322   a ,  322   b ; inner coder  308   a ,  308   b ; inner de-coder  323   a ,  323   b ; outer coder  307   a ,  307   b ; outer de-coder  324   a ,  324   b , and the De-Mux  306  and Mux  325  functions that enable demultiplexing across multiple fibers. The optical module  401  may have an interface that can connect to existing TC-Layer or MAC implementations currently produced. In another alternative implementation the digital to analog conversion  310   a ,  310   b ; analog to digital conversion  321   a ,  321   b , and the clock data recovery  320   a ,  320   b  functions are incorporated into an optical module (not shown). In yet another alternative implementation of the transceiver  100  as shown in  FIG. 5 , an optical module  501  includes the De-Mux  306  and Mux  325  functions enabling demultiplexing across multiple fibers. The optical module  501  may have an interface that can connect to existing TC-Layer or MAC implementations currently produced. 
   An alternative implementation of transceiver  100  utilizing a single fiber link  314   a  (without demultiplexing across multiple fibers) is illustrated in  FIG. 6 . Alternatively, an implementation of the transceiver  100  may utilize multiple fiber links  314   a ,  314   b  while not performing demultiplexing across multiple fibers, as illustrated in  FIG. 7 . In this implementation, the TC-Layer  701  block manages the fiber links as independent fiber links that all connect to the same end point(s) on the network. In one implementation, TC-Layer  701  block is a derivative of the Transmission Convergence Layer specified in ITU G.984 or MAC specified in IEEE 802.3ah, with the added functionality of queue management of the traffic across the plurality of independent fiber links. The TC-Layer  701  block may exclude use of one or more fiber links if the fiber link experiences a failure. This exclusion of failed fiber links enables the TC-Layer  701  block (i.e., queue management function) to continue providing service across a PON using the remaining active links. Each fiber can be deployed across physically different paths to provide optical fiber distribution path diversity and improved protection against failures. Failures may originate in the optical module or across elements of the ODN such as fiber or connector breaks. 
   Channel Equalization 
   An implementation for a channel equalization routine executed in the EQ  322   a ,  322   b  block includes determining coefficients or weights that are applied to the received data to remove undesired information or noise from the received data. Channel equalization can include a training or convergence period in which characteristics of the channel are learned or accounted for and coefficients or weights are adapted before processing the received data. Decision-directed equalization is an equalization method in which a known training sequence is sent during the training period and the receiver/transceiver uses the knowledge of the training sequence to learn about the channel characteristics. The training sequence can be multiplexed within a PON&#39;s TC-Layer framing protocol. Blind equalization is a process during which an unknown input data sequence is recovered from the output signal of an unknown channel (i.e., current equalization data for a given channel is unknown or otherwise unavailable). 
   One mode of communications used by a PON, e.g., for upstream data traffic (ONU/ONT to OLT direction), is “burst mode” communications. For example, upstream communications on a PON may include a link shared among multiple clients or ONUs/ONTs via time division multiplexing under control by an OLT. The upstream direction is divided into time slots; each time slot includes a defined number of bits. A given ONU/ONT is granted some number of time slots during which to transmit an upstream frame of data to an OLT. The upstream direction uses an orchestrated collection of bursts from the different ONU/ONTs, coordinated by the OLT that tries to maximize upstream traffic bandwidth efficiency by minimizing empty slots. 
   A flow chart for an exemplary upstream burst mode communication equalization process is shown in  FIG. 8 . To read or interpret the upstream data traffic from a client ONU/ONT, an OLT trains and/or equalizes the channel for that client ONU/ONT. Since the ONU/ONTs may be at different distances from the OLT and all do not share the same fiber, different channel characteristics result. Communication efficiencies may be obtained by determining  800  a set of equalization coefficients for a channel during a burst from a client, saving  801  the determined equalization coefficients, entering a wait period  802  (also known as a PON&#39;s silence period when no client ONU/ONTs are transmitting upstream), and loading  803  the stored equalization coefficients before a next burst from the client (during the wait period), to avoid re-training or re-equalizing on every burst communication. The OLT has prior knowledge of which ONU/ONT will be transmitting data during which time slots and can use this knowledge during the time between burst communications (during the wait period) to load  803  an appropriate set of coefficients pertaining to the particular ONU/ONT transmitting prior to receiving  804  its next upstream burst. This process continues for subsequent bursts. In one implementation, periodic (though not coincident with each communication burst) updates to the channel characteristics may be made (and stored). The OLT can save  801  coefficients that have converged or have been trained after receiving burst communications from the first ONU/ONT and load  803  a new set of coefficients during the wait period between bursts (i.e., before an incoming upstream burst from a second ONU/ONT). In one implementation, in addition to or alternative to storage of coefficient data, the OLT may also save and load inner and/or outer coding states between bursts improving the efficiency of communication, similar to the equalization process of  FIG. 8 . 
   Another mode of communications used by a PON, e.g., for downstream data traffic (OLT to ONU/ONT direction), is “continuous mode” communications. In one implementation, a receiver, such as an ONU/ONT, equalizes a received data channel using either one of a blind equalization or a decision directed equalization method. 
   A flow chart for an exemplary PON activation process is shown in  FIG. 9 . In a PON in which a decision directed method is used for training an ONU/ONT receiver, a continuous mode transmitter, such as an OLT transmitter, sends a training sequence  900  multiplexed within a PON&#39;s TC-Layer downstream frame protocol. In a PON in which a blind equalization method is used, the OLT needed not send this training sequence  900 . An ONU/ONT equalizes its received downstream channel  901  before it is able to receive and interpret PON network parameters  902 . If the OLT has not been previously informed of the existence of the ONU/ONT then the ONU/ONT awaits an upstream grant window  903  available for new ONU/ONTs to respond to the OLT with its serial number  904 . After the ONU/ONT has received an upstream grant window and processed PON system parameters, the ONU/ONT sends a training sequence  905  and then its serial number  904  to the OLT. In a PON in which blind equalization is used the ONU/ONT need not send a training sequence  905 . After the OLT has received the ONU/ONT serial number the OLT will assign and send the ONU/ONT an identification number. If the ONU/ONT does not receive an identification number  906   a , the ONU/ONT returns to waiting for an upstream grant window for new ONU/ONTs  903 . Once the ONU/ONT receives an identification number  906   b , the OLT performs ranging  907  to determine the distance between the OLT and ONU/ONT and then compensates for the communication timing delays. The ONU/ONT can perform updates continuously or periodically depending on the equalization method employed. After the downstream continuous mode channel and the upstream burst mode channel have been equalized, both ends of the PON transmission link are equalized and the ONU/ONT enters its normal operating state  908 . 
   Link Connection Errors 
   A system has been proposed that includes demultiplexing across multiple fibers as is shown above with reference to  FIGS. 3–6 . In systems using demultiplexing across multiple fibers, fibers can be connected incorrectly at installation. For example, a first transceiver  100 , such as is shown in  FIG. 3 , with fibers  314   a  and  314   b  can be connected to a second transceiver  101  with fiber  314   b  connected in place of fiber  314   a , and fiber  314   a  connected in place of fiber  314   b . The incorrect connection in this example may cause the first and second transceivers to not establish communications due to misalignment of bits during multiplexing of received data. 
   Information in a frame is used to synchronize a receiver (e.g., transceiver  101 ) with the beginning of a frame (e.g., a “frame delimiter”). The process of discovering the beginning of a frame is called “frame synchronization.” In specific protocols such as G.984, the downstream frame delimiter is called Psync, the upstream frame delimiter is called Delimiter and the process of frame synchronization in the downstream is called the HUNT. In one implementation, TC-Layer  305  block performs frame synchronization. In one implementation, specific bit patterns or values for frame delimiters are used that are unique for each fiber to differentiate one fiber from another or the order of fiber connections to correctly multiplex received data. The use of unique frame delimiters allows the TC-Layer  305  block to change the alignment of received data bits during multiplexing to adjust for the order of the fiber connections, without having to physically change the connections. Management of the bit alignment in this implementation forms part of the TC-Layer&#39;s  305  block demultiplexing management responsibilities and functions. 
   Alternatively, the TC-Layer  305  block may assume an order for the fiber connections to determine the alignment of bits for multiplexing the received data and attempt frame synchronization. After a period of time with no frame synchronization success, the TC-Layer  305  block may assume a different order for the fiber connections and change the alignment of bits during multiplexing and attempt frame synchronization again. The process may repeat, including changing the alignment of bits to reflect other configurations during the multiplexing, and frame synchronization attempts continue until frame synchronization succeeds. In yet another alternative implementation, the TC-Layer  305  block may assume and attempt frame synchronization on all possible combinations of bit alignments in parallel, one of which will succeed in achieving frame synchronization. 
   Although the invention has been described in terms of particular implementations, one of ordinary skill in the art, in light of this teaching, can generate additional implementations and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.