Abstract:
Raman interference (also known as Raman scattering) during an idle frame transmission state in a passive optical network (PON) having radio frequency video overlay is reduced by generating and transmitting data packets having both random data and random length. Randomly varying both the packet data content and the packet length can achieve significant improvement in Raman interference reduction. The random packet data and length in effect spreads the interference across a spectrum wide enough that there is no interference effect perceptible to a television viewer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/747,936, filed May 22, 2006, entitled “METHOD AND APPARATUS TO REDUCE THE IMPACT OF RAMAN INTERFERENCE IN PON NETWORKS WITH RF VIDEO OVERLAY,” is hereby claimed and the specification thereof incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to passive optical networks (PONs) and, more specifically, to avoiding interference in PONs in which radio frequency video signals are overlayed on packet data signals. 
     2. Description of the Related Art 
     Telecommunication service providers are using their networks to deliver an ever-broadening array of services to their subscribers&#39; or customers&#39; homes, businesses or other premises. Whereas once a digital subscriber line (DSL) or cable television line was used solely to provide subscribers with World Wide Web and e-mail access, today&#39;s service providers wish to bundle Internet service with voice (telephony) and video (television) services. Such bundled Internet, voice and video service is sometimes colloquially referred to as “triple play” service. The desire to bundle video service along with voice and Internet access has driven telecommunication service providers to turn increasingly to optical fiber-based technologies, such as the passive optical network (PON). Fully optical telecommunications service networks, which some have referred to as “fiber-to-the-premises” (FTTP), are increasingly being developed and deployed. 
     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 a central office (or exchange, as its sometimes referred to) 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 central office and subscribers. A single fiber can run from the central office 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). The IEEE has also adopted Ethernet-based (i.e., IEEE 802.3-based) PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). 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 in this patent specification. 
     In accordance with these standards, a PON comprises an optical line terminal (OLT) (also known as optical line terminator), which is typically located at the central office, and a number of optical network terminators (ONTs) (also known as optical network terminals and optical network units), each located at the subscriber&#39;s premises (e.g., home, office building, etc.), with optical fiber and one or more splitters between the OLT and ONTs. In the downstream direction, i.e., data transmitted from the OLT (e.g., located at the central office) to an ONT (e.g., located at a subscriber&#39;s premises), the data units are broadcast from the OLT to all of the ONTs on the PON, and an ONT can select the data to receive by matching the address embedded in the data units to a previously provisioned or learned address. In other words, an ONT only “listens” to data units having a matching address. Thus, the OLT can transmit data to a particular or selected ONT by addressing it to that ONT. In GPON, the address typically comprises a GPON Encapsulation Method (GEM) Port-ID. In the upstream direction, i.e., data transmitted from an ONT to the OLT, the data units are time-domain multiplexed. 
     Two methods for delivering video services over a PON are being explored. In one method, the video is digitally encoded using the same transport mechanism and protocol (e.g., Internet Protocol) and transmitted in the same manner as any other digital data transmitted on the PON. In the other method, radio frequency (RF) signals that represent video information are overlayed, using optical wavelength division multiplexing (WDM) techniques, on the digital packet data signals that represent voice, Internet communications and all such other information. In other words, at the service provider&#39;s central office or PON head end, the video signal is coupled onto the fiber at a wavelength that is different from the wavelength at which data traffic from the OLT is transmitted on the fiber. Downstream, at each subscriber&#39;s ONT, the RF video signal and digital packet data are separated by a triplexer or similar device. In a case in which the RF video signal is transmitted in analog form, the separated video signal can be sent directly to a television set. In a case in which the RF video signal is transmitted in digital form, a set-top box first decodes the signals. 
     A periodic signal in the optical data stream can interfere with the video signal, causing interference lines on the television screen and thereby degrading the subscriber&#39;s viewing experience. This type of interference is due to an effect known as Raman interference or Raman scattering. Transmission of a stream of uniform packets can result in visible degradation of an RF video signal due to Raman interference. As illustrated in  FIG. 2 , the OLT conventionally transmits non-informational or placeholder “idle” frames bearing some predetermined data pattern to maintain ONT synchronization when no information is to be transmitted, i.e., when the data source is idle. The predetermined pattern is commonly referred to as an idle pattern. (Although the IEEE Ethernet standard (802.3ah) defines idle patterns for Ethernet packets, a simple pattern of alternating ones and zeroes is shown in  FIG. 2  for purposes of illustration.) All idle frames have the same length (“L” in  FIG. 2 ). Transmission of a stream of idle frames is problematic in a PON with video overlay because the repetitive idle pattern contains frequency components that overlap the frequencies of video signals. 
     Solutions for minimizing Raman interference during idle frame transmissions include substituting random data for the idle pattern. Nevertheless, the noise reduction that these solutions provide is not always sufficient to make the effects of Raman interference unnoticeable to a television viewer. 
     It would be desirable to provide a method and apparatus that reduces Raman interference in a PON with RF video overlay to a level that is unnoticeable to a television viewer. The present invention addresses these problems and deficiencies and others in the manner described below. 
     SUMMARY OF THE INVENTION 
     The present invention relates to reducing interference in a passive optical network (PON) having radio frequency video overlay during an idle frame transmission state by generating and transmitting frames having both random data and random length during the idle state. Randomly varying both the packet data content and the packet length can achieve significant improvement in Raman interference reduction. The random frame data and length in effect spreads the interference across a frequency spectrum wide enough that there is no interference effect perceptible to a television viewer. 
     In an exemplary embodiment of the invention, a pseudo-random pattern generator (PSRPG) can be used. Some of the output bits of the PSRPG can be used to determine the packet length, and some of the output bits can be used to determine the data content. Also, the idle frames can bear a destination address that is not recognized by any optical network terminal (ONT) in the PON, causing all ONTs to discard or ignore such frames. 
     Alternatively, the invention can entail generating and transmitting frames having random length during the idle state, instead of having both random data and random length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a passive optical network in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  illustrates transmission of idle frames in a conventional or prior art PON. 
         FIG. 3  illustrates transmission of idle frames in a PON in accordance with the exemplary embodiment of the present invention. 
         FIG. 4  is a flow diagram of a method for generating idle frames in accordance with the exemplary embodiment. 
         FIG. 5  is a block diagram of frame generation logic in the optical line terminal of  FIG. 1  in accordance with the exemplary embodiment. 
         FIG. 6  is a block diagram of a linear-feedback shift register for generating random values in accordance with the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , in an exemplary embodiment of the invention, a passive optical network (PON) includes an optical line terminal (OLT)  100  and a number of optical network terminators (ONTs)  102 ,  104 ,  106 , etc. The OLT  100  is interconnected in the conventional manner with each ONT  102 ,  104 ,  106 , etc., by optical fibers  108 ,  110 ,  112 ,  114 , etc., and one or more optical splitters  116 , etc. Although only one such splitter  116  and three such ONTs  102 ,  104  and  106  are shown for purposes of illustration, the PON can have any other suitable topology and number of ONTs, splitters, fibers, etc. The OLT  100  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 ONTs  102 ,  104 ,  106 , 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 GPON encapsulation method (GEM) that is performed on data packets before they are transmitted in the downstream direction, i.e., from OLT  100  to any of ONTs  102 ,  104 ,  106 , etc. The communication of data on the PON occurs in the manner well understood in the art and is therefore not described herein in further detail. 
     The OLT  100  includes a media access controller (MAC)  118  that, as known in the art, controls the majority of OLT functions and thus is analogous to a central processor. MAC  118  is programmed or configured in accordance with the present invention to include idle frame generation logic  120 , which controls the method of operation described below with regard to  FIG. 4 . Idle frame generation logic  120  can be included as part of GPON encapsulation method (GEM) logic  122 . Idle frame generation logic  120  and GEM logic  122  are shown in generalized form for purposes of illustration as residing within MAC  118 , but as persons skilled in the art will appreciate, this depiction is intended to indicate only that MAC  118  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. For example, in an instance in which MAC  118  is embodied in an application-specific integrated circuit (ASIC), it is contemplated that the above-described logic can reside in a suitable portion or portions of the ASIC. 
     As well understood in the art, GEM is a method or process that comprises encapsulating a data packet (e.g., Ethernet packet) to be transmitted downstream to the PON in a GEM frame. MAC  118  is also programmed or configured with other logic for controlling other OLT functions that do not directly relate to the present invention, but only idle frame generation logic  120  and GEM logic  122  are shown for purposes of clarity. In addition to MAC  118 , OLT  100  includes an optical transceiver  124  and other OLT logic  126 . Other OLT logic  126  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, idle frame generation logic  120  can be separate from GEM logic  122  and instead included as part of other OLT logic  126 . 
     As noted above with regard to  FIG. 2 , in a conventional OLT (not shown) the GEM logic encapsulates normal packet data (e.g., Ethernet packets) in GEM frames as the packet data becomes available from the external data source (not shown) from which the MAC obtains data to be transmitted downstream on the PON. The GEM frame length is variable. A GEM frame consists of a header and the payload, i.e., packet data. The header includes, among other information, the length of the payload or packet data. The MAC causes the GEM frames to be transmitted on the PON as part of a burst transmission in which the frames are preceded by a control block that aids synchronization and provides other information. When no data is available from the source for transmission on the PON, conventional idle frame generation logic (not shown) can substitute packets having a predetermined idle pattern for the (unavailable) packet data to maintain synchronization. It has also been suggested in the prior art to generate a random data pattern (not shown in  FIG. 2 ) instead of a predetermined idle pattern. 
     As illustrated in  FIG. 3 , in accordance with an exemplary embodiment of the present invention, when no data is available from the source for transmission on the PON, idle frame generation logic  120  can substitute packets having both random lengths and random data patterns for the (unavailable) packet data. Except as indicated herein, GEM logic  122  otherwise functions in the conventional manner. 
     As illustrated in  FIG. 4 , the exemplary method by which idle frame generation logic and GEM logic  122  frame packets for transmission on the PON comprises steps  402 - 410 . At step  402 , it is determined whether the state of operation indicates that idle frames are to be transmitted (e.g., when no packet data is available). If idle frames are not to be transmitted (e.g., packet data is available), then at step  404  GEM logic  122  uses the available packet data to form a GEM frame payload (see  FIG. 3 ) in the conventional or normal manner. At step  406 , GEM logic  122  adds a header to the payload to complete the encapsulation. The header includes, among other information, an address (e.g., GEM Port-ID) that indicates the normal destination of the packet data on the PON. The header also includes the payload length. However, if at step  402  it is determined that idle frames are to be transmitted (e.g., packet data is not available), then at step  408  GEM logic  122  uses the available packet data to form a GEM frame payload (see  FIG. 3 ) having random data and a random length. At step  410 , GEM logic  122  adds a header that includes, among other information, the randomly generated length value and an unused or dummy address that will not be recognized on the PON as a valid destination. Thus, ONTs  102 ,  104 ,  106 , etc., will ignore the data. Following either step  406  or step  410 , the GEM frame is ready for transmission on the PON in the conventional manner. As the manner in which MAC  100  ( FIG. 1 ) causes such frames to be transmitted is well understood by persons skilled in the art, it is not described herein. 
     As illustrated in  FIG. 5 , in an exemplary embodiment, the relevant portions of GEM logic  122  and idle frame generator  120  can comprise a pseudo-random pattern generator (PSRPG)  502 , a GEM header generator  504 , a GEM control state machine  506 , and a number of selectors  508 ,  510 ,  512  and  514 . Together, these elements effect the method described above with regard to  FIG. 4 . GEM control state machine  506  responds to signals indicating that packet data is available and indicating the end of a GEM frame by asserting signals that control selectors  508 ,  510 ,  512  and  514 . Specifically, when the GEM frame header is being generated and when the signal indicates that packet data is available, GEM control state machine  506  causes selector  508  to select the Port-ID of the packet destination, causes selector  510  to select the length of the packet, and causes selector  514  to select the header that GEM header generator generates in response to the selected inputs. Accordingly, when the GEM frame payload is being generated and the signal indicates that packet data is available, GEM control state machine  506  causes selector  512  to select the available packet data and causes selector  514  to select the output of selector  512  as the GEM frame payload. In contrast, when the GEM frame header is being generated and the signal indicates that packet data is not available, GEM control state machine  506  causes selector  508  to select the unused or dummy Port-ID, causes selector  510  to select a random number generated by PSRPG  502  as the packet length, and causes selector  514  to select the header that GEM header generator  504  generates in response to these selected inputs. Accordingly, when the GEM frame payload is being generated but the signal indicates that packet data is not available, GEM control state machine  506  causes selector  512  to select a random number generated by PSRPG  502  as the packet data and causes selector  514  to select the output of selector  512  as the GEM frame payload. Note that the randomly generated length is reflected both in the length of the payload and in the corresponding length field that is included in the header. 
     PSRPG  502  can comprise, for example, a linear-feedback shift register of the well-known type shown in  FIG. 6 . A 23-bit linear-feedback shift register having 23 corresponding registers or flip-flops  602 ,  604 ,  606 ,  608 , etc., with feedback bits from several registers combined through exclusive-OR logic  610  has been used with success in simulations, but PSRPG  502  can have any other suitable arrangement and number of registers. As well understood in the art, an N-bit linear-feedback shift register, initialized to some seed state, will cycle through a sequence of 2 N −1 states before repeating the sequence, thereby producing a seemingly random pattern. Some of the output bits  612  can be used as the input to selector  510  to provide the random packet length, and other output bits  612  can be used as the input to selector  512  to provide the random packet data. Although in the exemplary embodiment of the invention PSRPG  502 , comprising a linear feedback shift register, is used to generate the random data, in other embodiments any other suitable logic or other means can be used. 
     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.