An optical-coax unit (OCU) includes an optical PHY to receive and transmit optical signals and a coax PHY to receive and transmit coax signals. The OCU also includes a media-independent interface to provide a first continuous bitstream from the optical PHY to the coax PHY and a second continuous bitstream from the coax PHY to the optical PHY. The first continuous bitstream corresponds to received optical signals and transmitted coax signals, and the second continuous bitstream corresponds to received coax signals and transmitted optical signals.

TECHNICAL FIELD

The present embodiments relate generally to communication systems, and specifically to communication systems that include both optical and coaxial portions.

BACKGROUND OF RELATED ART

The Ethernet Passive Optical Networks (EPON) protocol may be extended over coaxial (coax) links in a cable plant. The EPON protocol as implemented over coax links is called EPoC. Implementing an EPoC network or similar network over a coax cable plant presents significant challenges. For example, EPON-compatible systems traditionally achieve full-duplex communications using frequency-division duplexing (FDD). However, cable operators may desire to use time-division duplexing (TDD) instead of FDD for communications between a coax line terminal and coax network units.

Like reference numerals refer to corresponding parts throughout the drawings and specification.

DETAILED DESCRIPTION

Embodiments are disclosed in which an optical-coax unit (OCU) is implemented as a repeater. An optical physical-layer device (PHY) in the OCU may be coupled to a coax PHY in the OCU without an intervening media-access controller (MAC).

In some embodiments, an OCU includes an optical PHY to receive and transmit optical signals and a coax PHY to receive and transmit coax signals. The OCU also includes a media-independent interface to provide a first continuous bitstream from the optical PHY to the coax PHY and a second continuous bitstream from the coax PHY to the optical PHY. The first continuous bitstream corresponds to received optical signals and transmitted coax signals, and the second continuous bitstream corresponds to received coax signals and transmitted optical signals.

In some embodiments, a method of data communications is performed in an OCU. In the method, optical signals are received and transmitted in an optical PHY. Coax signals are received and transmitted in a coax PHY. A first continuous bitstream, which corresponds to received optical signals and transmitted coax signals, is provided from the optical PHY to the coax PHY over a media-independent interface. A second continuous bitstream, which corresponds to received coax signals and transmitted optical signals, is provided from the coax PHY to the optical PHY over the media-independent interface.

In the following description, numerous specific details are set forth such as examples of specific components, circuits, and processes to provide a thorough understanding of the present disclosure. Also, in the following description and for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present embodiments. However, it will be apparent to one skilled in the art that these specific details may not be required to practice the present embodiments. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present disclosure. The term “coupled” as used herein means connected directly to or connected through one or more intervening components or circuits. The present embodiments are not to be construed as limited to specific examples described herein but rather to include within their scope all embodiments defined by the appended claims.

FIG. 1Ais a block diagram of a coax network100(e.g., an EPoC network) in accordance with some embodiments. The network100includes a coax line terminal (CLT)162(also referred to as a coax link terminal) coupled to a plurality of coax network units (CNUs)140-1,140-2, and140-3via coax links. A respective coax link may be a passive coax cable, or may also include one or more amplifiers and/or equalizers. The coax links compose a cable plant150. In some embodiments, the CLT162is located at the headend of the cable plant150and the CNUs140-1,140-2, and140-3are located at the premises of respective users.

The CLT162transmits downstream signals to the CNUs140-1,140-2, and140-3and receives upstream signals from the CNUs140-1,140-2, and140-3. In some embodiments, each of the CNUs140-1,140-2, and140-3receives every packet transmitted by the CLT110and discards packets that are not addressed to it. The CNUs140-1,140-2, and140-3transmit upstream signals at scheduled times specified by the CLT162. For example, the CLT162transmits control messages (e.g., GATE messages) to the CNUs140-1,140-2, and140-3specifying respective future times at which respective CNUs140-1,140-2, and140-3may transmit upstream signals.

In some embodiments, the CLT162is part of an optical-coax unit (OCU)130-1or130-2that is also coupled to an optical line terminal (OLT)110, as shown inFIG. 1B.FIG. 1Bis a block diagram of a network105that includes both optical links and coax links in accordance with some embodiments. The network105includes an OLT110(also referred to as an optical link terminal) coupled to a plurality of optical network units (ONUs)120-1and120-2via respective optical fiber links. The OLT110also is coupled to a plurality of OCUs130-1and130-2via respective optical fiber links. OCUs are sometimes also referred to as fiber-coax units (FCUs), media converters, or coax media converters (CMCs).

Each OCU130-1and130-2includes an ONU160coupled with a CLT162. The ONU160receives downstream packet transmissions from the OLT110and provides them to the CLT162, which forwards the packets to the CNUs140(e.g., CNUs140-4and140-5, or CNUs140-6,140-7, and140-8) on its cable plant150(e.g., cable plant150-1or150-2). In some embodiments, the CLT162filters out packets that are not addressed to CNUs140on its cable plant150and forwards the remaining packets to the CNUs140on its cable plant150. The CLT162also receives upstream packet transmissions from CNUs140on its cable plant150and provides these to the ONU160, which transmits them to the OLT110. The ONUs160thus receive optical signals from and transmit optical signals to the OLT110, and the CLTs162receive electrical signals from and transmit electrical signals to CNUs140.

In the example ofFIG. 1B, the first OCU130-1communicates with CNUs140-4and140-5, and the second OCU130-2communicates with CNUs140-6,140-7, and140-8. The coax links coupling the first OCU130-1with CNUs140-4and140-5compose a first cable plant150-1. The coax links coupling the second OCU130-2with CNUs140-6through140-8compose a second cable plant150-2. A respective coax link may be a passive coax cable, or alternately may include one or more amplifiers and/or equalizers. In some embodiments, the OLT110, ONUs120-1and120-2, and optical portions of the OCUs130-1and130-2(e.g., including the ONUs160) are implemented in accordance with the Ethernet Passive Optical Network (EPON) protocol.

In some embodiments, the OLT110is located at a network operator's headend, the ONUs120-1and120-2and CNUs140-4through140-8are located at the premises of respective users, and the OCUs130-1and130-2are located at the headend of their respective cable plants150-1and150-2or within their respective cable plants150-1and150-2.

In some embodiments, communications on a respective cable plant150are performed using time-division duplexing (TDD): the same frequency band is used for both upstream transmissions from the CNUs140to the CLT162and downstream transmissions from the CLT162to the CNUs140, and the upstream and downstream transmissions are duplexed in time. For example, alternating time windows are allocated for upstream and downstream transmissions. A time window in which a packet is transmitted from a CNU140to a CLT162is called an upstream time window or upstream window, while a time window in which a packet is transmitted from a CLT162to a CNU140is called a downstream time window or downstream window.

FIG. 2illustrates timing of upstream and downstream time windows as measured at a CLT162(FIGS. 1A and 1B) in accordance with some embodiments. As shown inFIG. 2, alternating windows are allocated for upstream and downstream transmissions. During a downstream time window202, the CLT162transmits signals downstream to CNUs140. The downstream time window202is followed by a guard interval204, after which the CLT162receives upstream signals from one or more of the CNUs140during an upstream time window206. The guard interval204accounts for propagation time on the coaxial links and for switching time in the CLT162to switch from a transmit configuration to a receive configuration. The guard interval204thus ensures separate upstream and downstream time windows at the CNUs140. The upstream time window206is immediately followed by another downstream time window208, another guard interval210, and another upstream time window212. Alternating downstream and upstream time windows continue in this manner, with successive downstream and upstream time windows being separated by guard intervals and the downstream time windows immediately following the upstream time windows, as shown inFIG. 2. The upstream and downstream transmissions during the time windows202,206,208, and212use the same frequency band. The time allocated for upstream time windows (e.g., windows206and212) may be different than the time allocated for downstream time windows (e.g., windows202and208).FIG. 2illustrates an example in which more time (and thus more bandwidth) is allocated to downstream time windows202and208than to upstream time windows206and212.

FIG. 3is a block diagram of a system300in which a CLT302is coupled to a CNU312by a coax link310in accordance with some embodiments. The CLT302is an example of a CLT162(FIGS. 1A-1B) and the CNU312is an example of one of the CNUs140-1through140-8(FIGS. 1A-1B). The CLT302and CNU312communicate via the coax link310using TDD. The coax link310couples a coax physical layer device (PHY)308in the CLT302to a coax PHY318in the CNU312. The coax PHY308transmits signals to the CNU312during downstream time windows (e.g., windows202and208,FIG. 2) and receives signals from the CNU312(or from other CNUs on a corresponding cable plant150that includes the coax link310) during upstream time windows (e.g., windows206and212,FIG. 2). Likewise, the coax PHY318transmits signals to the CLT302during upstream time windows (e.g., windows206and212,FIG. 2) and receives signals from the CLT302during downstream time windows (e.g., windows202and208,FIG. 2).

The coax PHY308in the CLT302is coupled to a full-duplex media access controller (MAC)304by a media-independent interface306. The media-independent interface306continuously conveys signals from the MAC304to the PHY308and also continuously conveys signals from the PHY308to the MAC304. The data rate of the media-independent interface in each direction is higher than the data rate for the coax link310, allowing the PHY308to perform TDD communications despite being coupled to the full-duplex MAC304(e.g., as described below with respect toFIGS. 5A-5B,6A-6B,7A-7B,8A-8B, and/or9A-9B). TDD functionality for the CLT302is thus achieved entirely in the coax PHY308in accordance with some embodiments.

The coax PHY318in the CNU312is coupled to a full-duplex MAC314by a media-independent interface316. The media-independent interface316continuously conveys signals from the MAC314to the PHY318and also continuously conveys signals from the PHY318to the MAC314. The TDD functionality of the CNU312is achieved entirely in the coax PHY318in the same manner as for the coax PHY308of the CLT302.

FIG. 4provides a high-level illustration of downstream data transmission in the system300(FIG. 3) in accordance with some embodiments. The data transmission uses a TDD scheme implemented at the PHY level. A continuous bitstream400is provided from the full-duplex MAC304to the coax PHY308. The bitstream400includes data402-1provided during a TDD period from times 0 to TD, data402-2provided during a TDD period from times TDto 2TD, and data402-3provided during a TDD period from times 2TDto 3TD. A TDD period is the total period of time associated with a guard interval404, an upstream window406, and a downstream window408-1,408-2, or408-3in sequence. The duration of each TDD period equals TD, as shown inFIG. 4. The guard intervals404are examples of guard intervals204or210(FIG. 2). The upstream windows406are examples of upstream time windows206or212(FIG. 2). The downstream windows408-1,408-2, and408-3are examples of downstream time windows202and208(FIG. 2).

The PHY308(FIG. 3) converts the data402-1into a first downstream transmission signal that is transmitted during a first downstream (DS) window408-1. Likewise, the data402-2is converted into a second downstream transmission signal that is transmitted during a second downstream window408-2, and the data402-3is converted into a third downstream transmission signal that is transmitted during a third downstream window408-3. In this example, T1represents the processing time for the PHY308to perform this conversion. Each downstream window408-1,408-2, and408-3is included in a respective TDD period that also includes an upstream (US) window406and a guard interval404. The PHY318(FIG. 3) in the CNU312receives the downstream transmission signals and reconstructs a continuous bitstream410that includes the data402-1,402-2, and402-3. Starting at a time T2, the PHY318passes the continuous bitstream to the MAC314(FIG. 3). In this example, T2represents the channel delay on the coax link310plus processing time in both the PHY308and PHY318.

WhileFIG. 4illustrates downstream transmission, a similar scheme may be used for upstream transmission. For example, the MAC314in the CNU312(FIG. 3) may provide a continuous bitstream to the PHY318, which converts the data in the bitstream into discrete transmission signals that are transmitted upstream during successive upstream transmission windows406(assuming the successive upstream windows406are allocated to the CNU312and not to other CNUs on the cable plant). The PHY308in the CLT302(FIG. 3) receives the transmission signals, reconstructs the continuous bitstream, and provides the reconstructed bitstream to the MAC304.

To convert the continuous bitstream400into the discrete signals transmitted during the transmission windows408-1,408-2, and408-3, the PHY308performs symbol mapping and maps the symbols to corresponding time slots and physical resources in the transmission windows408-1,408-2, and408-3. A single carrier or multi-carrier transmission scheme may be used.

A more detailed example of TDD operation for downstream transmissions is now provided with reference toFIGS. 5A and 5B. InFIG. 5A, a TDD PHY (e.g., coax PHY308,FIG. 3) includes a physical coding sublayer (PCS)508, a physical medium attachment sublayer (PMA)514, and a physical medium dependent sublayer (PMD)516. The PCS508is coupled to a full-duplex MAC502(e.g., MAC304,FIG. 3) through a media independent interface (xMII)506and a reconciliation sublayer (RS)504. In some embodiments, the media-independent interface506is a 10 Gigabit Media-Independent Interface (XGMII) operating at 10 Gbps. (The term media-independent interface may refer to a family of interfaces but also to a particular type of media-independent interface in the family. As used herein, the term refers to the family of interfaces and is abbreviated xMII to distinguish it from specific media-independent interfaces such as XGMII.) The media-independent interface506is shown symbolically inFIG. 5Aas arrows but in practice includes first interface circuitry coupled to the RS504, second interface circuitry coupled to the PCS508in the PHY, and one or more signal lines connecting the first and second interface circuitry.

In some embodiments, the PHY ofFIG. 5A, including the PCS508, PMA514, PMD516, and the PHY's portion of the xMII506, is implemented in hardware in a single integrated circuit. The full-duplex MAC502may be implemented in a separate integrated circuit or the same integrated circuit.

FIG. 5Bis aligned withFIG. 5Ato show downstream signals provided between the various sublayers ofFIG. 5Ain accordance with some embodiments. The signals ofFIG. 5Bthus correspond to the solid downward arrows ofFIG. 5A. The MAC502transmits a continuous bitstream520across the media-independent interface506to the PCS508. The media-independent interface506runs at a fixed rate RxMIIthat is higher than the rates of other interfaces in the system ofFIG. 5A. The bitstream520includes data packets522(in corresponding frames) and idle packets524; the idle packets524are included in the bitstream520to maintain the fixed rate RxMII.

The PCS508includes one or more upper PCS layers510that remove the idle packets524and perform a forward error correction (FEC) encoding process that inserts parity bits in the data packets (D+P), resulting in a bitstream530that includes data packets532and idle characters534that act as packet separators. The upper PCS layers510provide the bitstream530to a TDD adapter512in the PCS508at a downstream baud rate of RPCS,DS. The TDD adapter512adapts the bitstream530to a higher baud rate RPMAand inserts pad bits546, resulting in a bitstream540that is provided to the PMA514at RPMA. The bitstream540includes data packets542and idle characters544that correspond respectively to the data packets532and idle characters534of the bitstream530. The pad bits546correspond to time slots552during which the PMA514and PMD516cannot transmit downstream. The time slots552correspond, for example, to guard intervals404and upstream windows406(FIG. 4).

The PMA514(or alternatively, the PMD516) converts the packets542into downstream signals550that the PMD516transmits during downstream windows408(e.g., windows408-1,408-2, and408-3,FIG. 4). Each downstream window408has a duration TDSand each time slot552has a duration TUS+TGI, where TUSis the duration of an upstream window406(FIG. 4) and TGIis the duration of a guard interval404(FIG. 4).

The baud rates RPCS,Dsand RPMAare related as follows:

RPCS,DS=RPMA×TDSTDS+TUS+TGI(1)
Equation (1) shows that RPCS,DSis a fraction of RPMAas determined by the ratio of TDSto an entire TDD cycle. (InFIG. 5B, the indices n and n+1 are used to index successive TDD cycles.)

WhileFIG. 5Bdescribes downstream transmissions, upstream transmissions may be performed in a similar manner (e.g., in the coax PHY318of the CNU312,FIG. 3).

An example of TDD operation for upstream transmissions is now provided with reference toFIGS. 6A and 6B. The MAC502and PHY ofFIG. 6Aare the same MAC502and PHY inFIG. 5A.FIG. 6Bis aligned withFIG. 6Ato show upstream signals provided between the various sublayers ofFIG. 6Bin accordance with some embodiments. The signals ofFIG. 6Bthus correspond to the solid upward arrows ofFIG. 6A. The PMD516receives analog upstream signals during upstream windows406(FIG. 4) and converts them to digital upstream (US) signals630, which are provided to the PMA514. No upstream signals630are present during time slots632, each of which includes a downstream window408and a guard interval404(FIG. 4).

The PMA514inserts pad bits622during the time slots632, resulting in a bitstream620that also includes data packets (with parity bits)624and idle characters626that separate the data packets624. The data packets624contain data extracted from the upstream signals630. The PMA514provides the bitstream620to the TDD adapter512at the baud rate RPMA, which is the same RPMAas for downstream communications. The TDD adapter512discards the pad bits622and adapts the bitstream620to a baud rate RPCS,US, resulting in the bitstream610. The bitstream610includes data packets612and idle characters614that correspond respectively to the data packets624and idle characters626as adapted to RPCS,US. RPCS,USis defined as:

RPCS,US=RPMA×TUSTDS+TUS+TGI(2)
Equation (2) shows that RPCS,USis a fraction of RPMAas determined by the ratio of TUSto an entire TDD cycle. In general, RPCS,USis not equal to RPCS,DS, although they will be equal if TDSequals TUS.

The TDD adapter512provides the bitstream610to the upper PCS layers510, which discard the parity bits, fill the resulting empty spaces, and adapt the bitstream610to RxMIIby inserting idle packets604, resulting in the bitstream600. The data packets602of the bitstream600correspond to the data packets612with the parity bits removed, as adapted to RxMII. In some embodiments, RxMIIis the same in the upstream and downstream directions. The upper PCS layers510provide the bitstream600at RxMIIto the full-duplex MAC502via the media-independent interface506and RS504. The combination ofFIGS. 5B and 6Billustrate the full-duplex nature of the MAC502: it simultaneously transmits the continuous downstream bitstream520(FIG. 5B) and receives the continuous upstream bitstream600(FIG. 6B).

WhileFIG. 6Bshows upstream reception, downstream reception may be performed in a similar manner (e.g., in the coax PHY318of the CNU312,FIG. 3).

FIGS. 5A-5Band6A-6B thus illustrate how to implement TDD in the PCS sublayer508by adding a TDD adapter512to the PCS sublayer508. As described, the TDD adapter512performs rate adaptation to ensure that the amount of data in the bitstreams520and530(or600and610) during a TDD cycle equals the amount of data in the bitstream540(or620) during a downstream (or upstream) window. In some embodiments, the other sublayers of the PHY ofFIGS. 5A and 6A(e.g., the upper PCS layers510, PMA514, and PMD516) function as defined in the IEEE 802.3 family of standards.

In some embodiments, an adapter for implemented TDD is included in the PMD instead of the PCS.

InFIG. 7A, a TDD PHY (e.g., coax PHY308or318,FIG. 3) includes a PCS708, PMA710, and PMD712. The PCS708is coupled to the full-duplex MAC502(e.g., MAC304or314,FIG. 3) through the xMII506and RS504. In some embodiments, the PHY ofFIG. 7A, including the PCS708, PMA710, PMD712, and the PHY's portion of the xMII506, is implemented in hardware in a single integrated circuit. The full-duplex MAC502may be implemented in a separate integrated circuit or the same integrated circuit as the PHY.

FIG. 7Bis aligned withFIG. 7Ato show signals provided between the various sublayers ofFIG. 7Awhen transmitting in accordance with some embodiments. The signals ofFIG. 7Bthus correspond to the solid downward arrows ofFIG. 7A. The MAC502transmits a continuous bitstream520across the media-independent interface506, as described with respect toFIGS. 5A and 5B. The media-independent interface506runs at a fixed rate RxMII. The PCS708receives the continuous bitstream520, which includes data packets522and idle packets524.

The PCS708removes the idle packets524and performs an FEC encoding process that inserts parity bits in the data packets522, resulting in a mixture of data and parity bits (D+P). For example, the PCS708generates encoded data frames (D+P)732separated by idle characters734that fill the inter-frame gaps and act as packet separators. In some embodiments, the PCS708deletes some idle characters from the idle packets524, leaving idle characters to fill the gaps between the data frames732, and performs stream-based FEC encoding on the data and remaining idle characters of the bitstream520, producing parity bits that take the place of the deleted idle characters. Alternatively, the PCS708performs block-based FEC encoding. The PCS708generates a bitstream730in which the encoded data frames732and idle characters734are grouped into bursts. The PCS708inserts pad bits736into the bitstream730; the pad bits736separate respective bursts. (Alternatively, instead of inserting pad bits736, the PCS708leaves gaps in the bitstream730, such that the bitstream730is not continuous.) In some embodiments, the pad bits736(or alternatively, the gaps) have a fixed length (i.e., duration) TPADand the bursts have a fixed length (i.e., duration) TBURST. In other embodiments, the values of TPADand TBURSTvary about fixed averages and the PCS708, PMA710, and/or PMD712perform buffering to accommodate this variation.

The PCS708provides the bitstream730to the PMA710at a rate RPCSthat equals the rate RxMII. The PMA710processes the bitstream730(e.g., in accordance with IEEE 802.3 standards) and forwards the bitstream730to the PMD712at a rate RPMAthat equals the rates RxMIIand RPCS. The xMII506, PCS708, and PMA710thus all operate at the same rate (e.g., 10 Gbps).

(The term “bitstream” as used herein includes all signals described as such that are transmitted between respective PHY sublayers as shown in the figures. It therefore is apparent that the term “bitstream” may include streams of samples and/or streams of symbols as well as streams of individual bits.)

The PMD712includes a coax rate adapter714and one or more lower PMD layers716. The coax rate adapter714receives the bitstream730from the PMA710at the rate RPMA, removes the pad bits736, adapts the encoded data frames732and idle characters734to a lower rate RPMD,TX, and periodically inserts gaps746of duration TGAP. The result is a bitstream740with data frames742and idle character separators744. The data frames742and idle character separators744between two gaps746have a total length (i.e., duration) of TDATA. TDATAmatches the length TTXof a transmission window752in a TDD Cycle of duration TD. The PHY ofFIG. 7Acan transmit during each transmission window752, which may be a downstream window202or208(FIG. 2) for a CLT162(FIGS. 1A-1B) or an upstream window206or212(FIG. 2) for a CNU140(FIGS. 1A-1B). The PHY ofFIG. 7Acannot transmit, however, during times754that correspond to reception windows (e.g., upstream windows206and212,FIG. 2, for a CLT162or downstream windows202or208,FIG. 2, for a CNU140) and guard intervals (e.g., guard intervals204or210,FIG. 2).

The rates RPMD,TXand RPMAare related as follows:

In some embodiments, TBURSTmay be substantially shorter than TDATA. For example, a burst may be a single FEC code word (e.g., in embodiments using stream-based FEC) or a single frame (e.g., a single Ethernet frame). Furthermore, the period TBURST+TPADmay be less than the period TDATA+TGAP. Also, the values of TBURST, TPAD, and TBURST+TPADmay vary (e.g., about fixed averages).FIGS. 8A and 8Billustrate an example in which TBURSTis less than TDATA, TBURST+TPADis less than TDATA+TGAP, and the values of TBURST, TPAD, and TBURST+TPADvary. The bitstream830ofFIG. 8Bis an example of the bitstream730ofFIG. 7B. In this example, the rates RPMD,TXand RPMAare related as follows:

The lower PMD layers716convert the data frames742into transmit signals750that are transmitted onto a coax link (e.g., link310,FIG. 3) during transmission windows752. The gaps746in the bitstream740correspond to times754between transmission windows752(e.g., to a combination of guard intervals and reception windows). The start of a transmission window752may be aligned with the end of a sequence of pad bits736or with the start of a burst, but is not necessarily so aligned.

An example of TDD operation for data reception is now provided with reference toFIGS. 9A and 9B.FIG. 9Ashows the same MAC and PHY asFIGS. 7A and 8A.FIG. 9Bis aligned withFIG. 9Ato show signals provided between the various sublayers ofFIG. 9Awhen receiving in accordance with some embodiments. The signals ofFIG. 9Bthus correspond to the solid upward arrows ofFIG. 9A. The lower PMD layers716receive signals902during receive windows906of duration TRX(e.g., downstream windows202and208,FIG. 2, for a CNU140or upstream windows206and212,FIG. 2, for a CLT162) and convert them to a bitstream910that includes data frames912and idle character separators914in time periods of duration TDATAthat are separated by gaps of duration TGAP. The data frames912are encoded and include parity bits. TDATAcorresponds to receive windows906and equals TRX; TGAPcorresponds to periods904of TDD cycles in which the PHY does not receive (e.g., periods904that are a combination of a transmission window752,FIGS. 7B and 8B, and a guard interval). The bitstream910is provided to the coax rate adapter714at a rate RPMD,RX, which may be calculated using an equation analogous to Equation (3) or (4).

The rate RPMD,RXmay differ from RPMD,TXdue to asymmetry between upstream and downstream bandwidth. In some embodiments, fewer sub-carriers are available in the upstream direction than in the downstream direction, resulting in less upstream bandwidth than downstream bandwidth. As a result, RPCS,RXis less than RPCS,TXin the CLT162and is greater than RPCS,TXin a CNU140. (The difference between RPCS,RXand RPCS,TXcauses the relative values of TBURSTand TPADfor transmission to differ from the relative values of TBURSTand TPADfor reception.) However, RPMAis constant with the same value in both directions in accordance with some embodiments.

The coax rate adapter714inserts pad bits922(or alternatively leaves gaps) in the bitstream910, resulting in a bitstream920that is provided to the PMA710at a rate RPMA. In addition to the pad bits922, the bitstream920includes encoded data frames924and idle character separators926that correspond to the data frames912and separators914. The PMA710processes the bitstream920(e.g., in accordance with IEEE 802.3 standards) and forward the bitstream920to the PCS708at the rate RPCS, which equals RPMA.

The PCS708decodes the data frames924and removes the parity bits, resulting in data packets602. The PCS708also removes the pad bits922and inserts idle packets604, resulting in a bitstream600(FIG. 6B). The bitstream600is transmitted across the xMII506to the RS504and MAC502at the rate RxMII, which equals RPCSand RPMA. Furthermore, these rates may be the same as the corresponding rates for data transmission as described with respect toFIGS. 7A-7Band8A-8B.

In some embodiments, the PHY ofFIGS. 5A and 6Aand the PHY ofFIGS. 7A,8A, and9A (e.g., the PHYs308and318,FIG. 3) are orthogonal frequency-division multiplexing (OFDM) PHYs that transmit and receive OFDM symbols using TDD.FIG. 10illustrates the operation of such an OFDM PHY1006in accordance with some embodiments. The PHY1006is coupled to a full-duplex MAC (e.g., MAC502,FIGS. 5A,6A,7A,8A, and9A; MAC304or314,FIG. 3) by a media-independent interface1004(e.g., xMII506,FIGS. 5A,6A,7A,8A, and/or9A; interface306,FIG. 3). In the downstream direction, the MAC provides a continuous bitstream1000to the PHY1006. Downstream processing circuitry1008(including, for example, downstream portions of the PCS508, PMA514, and PMD516,FIGS. 5A and 6A, or of the PCS708, PMA710, and PMD712,FIGS. 7A,8A and9A) collects data from the bitstream1000in a buffer1009. Once enough data has been collected for processing (e.g., for encoding/OFDM symbol construction), the data are converted to time-domain samples1012to be transmitted in OFDM symbols. The samples1012are buffered in a buffer1018until a switch1020is set to couple the buffer1018to a physical medium interface1024, thus beginning a downstream transmission window. In the example ofFIG. 10, two downstream OFDM symbols1022are transmitted during the downstream (DS) window of each TDD cycle. (InFIG. 10, data in the bitstreams1000and1002have the same fill patterns as their corresponding OFDM symbols.)

During upstream windows, the switch1020is set to couple the interface1024to a buffer1014in upstream processing circuitry1010. The upstream processing circuitry1010includes, for example, upstream portions of the PCS508, PMA514, and PMD516(FIGS. 5A and 6A) or of the PCS708, PMA710, and PMD712(FIGS. 7A,8A and9A). The buffer1014buffers time-domain samples1016in received OFDM symbols. In the example ofFIG. 10, two upstream OFDM symbols1022are received during the upstream (US) window of each TDD cycle. Once the buffer1014collects enough samples1016for processing (e.g., FFT processing, demodulation, or decoding), the upstream processing circuitry1010converts the samples1016into bitstream data, thereby recovering a continuous bitstream1002that is provided to the full-duplex MAC via the media-independent interface1004.

WhileFIG. 10shows downstream transmission and upstream reception, downstream reception and upstream transmission may be performed in a similar manner (e.g., in a CNU312,FIG. 3).

FIG. 11is a block diagram of a system1100in which a CLT1102with a full-duplex MAC1104and coax TDD PHY1108is coupled to a CNU1116with a full-duplex MAC1118and coax TDD PHY1122in accordance with some embodiments. The system1100is an example of the system300(FIG. 3). A coax link1114couples the PHYs1108and1122. A media-independent interface1106couples the MAC1104with the PHY1108in the CLT1102, and a media-independent interface1120couples the MAC1118with the PHY1122in the CNU1116. In the downstream direction, the PHY1108performs mapping to convert data in a continuous bitstream1110to OFDM symbols1112that are transmitted to the PHY1122during downstream windows, and the PHY1122performs mapping to recover the data from the received OFDM symbols1112and recreate the continuous bitstream1110. In the upstream direction, the PHY1122performs mapping to convert data in a continuous bitstream1110to OFDM symbols1112that are transmitted to the PHY1108during upstream windows, and the PHY1108performs mapping to recover the data from the received OFDM symbols1112and recreate the continuous bitstream1110. (WhileFIG. 11shows a single bitstream1110for simplicity, in practice there are separate upstream and downstream bitstreams that are continuously sent in both respective directions between the MAC1104and PHY1108in the CLT1102, and also between the MAC1118and PHY1122in the CNU1116.)

FIG. 12further illustrates downstream transmissions in the system1100(FIG. 11) in accordance with some embodiments. The PHY1108of the CLT1102receives a continuous bitstream of data from the full-duplex MAC1104(FIG. 11) during a series of DBA cycles1202. (DBA stands for dynamic bandwidth allocation; a DBA cycle1202is another term for a TDD cycle. Each DBA cycle1202includes a downstream window1204and an upstream window1206, as well as a guard interval, which is not shown inFIG. 12for simplicity.) Each DBA cycle1202is divided into four periods1208,1210,1212, and1214(or, more generally, a plurality of periods) of duration Ts. In the examples ofFIGS. 10-12, two OFDM symbols are transmitted downstream during each DBA cycle1202. Therefore, the bitstream data for each period1208,1210,1212, and1214is data for half an OFDM symbol.

The data for the first and second periods1208and1210of the first DBA cycle1202are provided to a queue1216(e.g., buffer1009,FIG. 10), where they are buffered. Once all the data for the first and second periods1208and1210have been collected, inverse fast Fourier transform (IFFT) processing1218is performed to convert them to samples from which a first OFDM symbol is constructed. (Other processing, such as channel coding performed in the PCS508,FIGS. 5A and 6A, or the PCS708,FIGS. 7A,8A, and9A, is omitted fromFIG. 12for simplicity.) The first OFDM symbol is then transmitted from the PHY1108of the CLT1102to the PHY1122of the CNU1116during a portion of a downstream window1204that occurs during the first period1208of the second DBA cycle1202. During receive (RX) processing1220, the PHY1122recovers the bitstream data from the first OFDM symbol and delivers1222the recovered bitstream data to the MAC1118. The duration of this delivery1222equals the duration of two periods (i.e., 2*Ts), as shown.

The data for the third and fourth periods1212and1214of the first DBA cycle1202are provided to the queue1216, where they are buffered. Once all the data for the third and fourth periods1212and1214have been collected, inverse fast Fourier transform (IFFT) processing1218is performed to convert them to samples from which a second OFDM symbol is constructed. (Again, other processing, such as channel coding performed in the PCS508,FIGS. 5A and 6A, or the PCS708,FIGS. 7A,8A, and9A, is omitted fromFIG. 12for simplicity.) The second OFDM symbol is then transmitted from the PHY1108of the CLT1102to the PHY1122of the CNU1116(FIG. 11) during a portion of the downstream window1204that occurs during the second period1210of the second DBA cycle1202. During receive (RX) processing1220, the PHY1122(FIG. 11) recovers the bitstream data from the second OFDM symbol. The PHY1122then buffers1224the recovered bitstream data before delivering1222the recovered bitstream data to the MAC1118(FIG. 11). This delivery1222immediately follows delivery1222of the data received in the first OFDM symbol.

Downstream transmission continues in this manner, with the result that a continuous recovered bitstream is delivered from the PHY1122to the MAC1118of the CNU1116, even though OFDM symbols are only transmitted downstream during a portion of each DBA cycle1202.

WhileFIG. 12illustrates downstream transmissions, upstream transmissions may be performed in an analogous manner.

Attention is now directed to an OCU implemented as a TDD repeater. Examples of OCUs130-1and130-2(FIG. 1B) have been provided above in which the CLT162in the OCU130-1or130-2includes a full-duplex MAC. For example, the CLT302(FIG. 3) includes a full-duplex MAC304and the CLT1102(FIG. 11) includes a full-duplex MAC1104. In some embodiments, however, an OCU may be implemented as a repeater that lacks a MAC coupled to the OCU's coax PHY. The repeater repeats received signals by converting them from an optical format to a coax format and vice-versa. An OCU implemented as a receiver does not include the ONU160and CLT162of the OCUs130-1and130-2ofFIG. 1B. Again, OCUs are sometimes also referred to as fiber-coax units (FCUs), media converters, or coax media converters (CMCs).

FIG. 13Ais a block diagram of an OCU1300implemented as a repeater in accordance with some embodiments. The OCU1300includes an optical PHY1304that connects to a fiber link1302(and thereby to an OLT110,FIG. 1B) and a coax PHY1308that connects to a coax link1312(and thereby to a plurality of CNUs140on a cable plant150,FIG. 1B). The optical PHY1304is a frequency-division duplexing (FDD) PHY that transmits optical signals on a first frequency or band of frequencies and receives optical signals on a second frequency or band of frequencies distinct from the first frequency or band of frequencies. In some embodiments, the optical PHY1304is an EPON PHY. The optical PHY1304transmits upstream on the fiber1302in a bursty fashion; it does not transmit during idle frame periods.

The coax PHY1308is a TDD PHY (e.g., coax PHY308,FIG. 3, or1108,FIG. 11). In some embodiments, the coax PHY1308includes the PCS508, including the upper PCS layers510and the TDD adapter512; the PMA514; and the PMD516(FIGS. 5A and 6A). In some embodiments, the coax PHY1308includes the PCS708, the PMA710, and the PMD712, including the coax rate adapter714and lower PMD layers716(FIGS. 7A,8A, and9A). In some embodiments, the coax PHY1308is an OFDM PHY (e.g., PHY1006,FIG. 10) that functions as described with respect toFIGS. 10-12, except that instead of providing a continuous bitstream to and receiving a continuous bitstream from a MAC, the coax PHY1308provides a continuous bitstream to and receives a continuous bitstream from the optical PHY1304.

A bit buffer1306couples the optical PHY1304with the coax PHY1308. In some embodiments, the optical PHY1304provides a first continuous bitstream to the coax PHY1308in a format corresponding to a media-independent interface (e.g., in XGMII format), which the coax PHY1308processes in a fixed predefined manner. Similarly, the coax PHY1308provides a second continuous bitstream to the optical PHY1304in the same format. The bit buffer1306buffers the first and second continuous bitstreams. The bit buffer1306thus is part of a media independent interface1310that couples the optical PHY1304with the coax PHY1308. (The media independent interface1310also includes interface circuitry in the PHYs1304and1308, which is not shown inFIG. 13Afor simplicity.) In some embodiments, the bit buffer1306drops packets that are not addressed to any of the CNUs140on the cable plant150(FIGS. 1A-1B) corresponding to the coax link1312. For example, such packets are replaced with idle frames. The bit buffer1306may optionally include a reconciliation sublayer to perform this filtering in accordance with some embodiments.

FIG. 13Billustrates a bitstream1320created by the optical PHY1304based on downstream optical signals received via the fiber link1302. The bitstream1320includes first data1322-1, second data1322-2, and third data1322-3. The bitstream1320is queued in the bit buffer1306and provided to the coax PHY1308. The coax PHY1308creates OFDM symbols based on the bitstream1320that are transmitted downstream during downstream windows, as shown inFIG. 13Cin accordance with some embodiments. A first pair of OFDM symbols corresponding to the first bitstream data1322-1is transmitted during a first downstream window1330-1, a second pair of OFDM symbols corresponding to the second bitstream data1322-2is transmitted during a second downstream window1330-2, and a third pair of OFDM symbols corresponding to the third bitstream data1322-3is transmitted during a third downstream window1330-3. In this manner, coax TDD communications are made compatible with optical FDD communications in an OCU1300designed as a repeater.

FIG. 14is a block diagram of a network1400that is identical to the network105ofFIG. 1B, except that the OCUs130-1and130-2ofFIG. 1Bhave been replaced with OCUs130-3and130-4implemented as repeaters1300(FIG. 13A). Because the OCUs130-3and130-4only perform PHY-layer processing and do not perform MAC or higher-layer processing, the OCUs130-3and130-4are invisible to the CNUs140-4through140-8and the OLT110from a protocol perspective.

FIG. 15is a flowchart showing a method1500of data communications in accordance with some embodiments. The method1500is performed (1502) in an OCU1300(FIG. 13A) (e.g., OCU130-3or130-4,FIG. 14) that includes an optical PHY1304, a coax PHY1308, and a media-independent interface1310coupling the optical PHY1304with the coax PHY1308(FIG. 13A). In some embodiments, the coax PHY1308includes a PCS, PMA, and PMD (e.g., PCS508, PMA514, and PMD516,FIGS. 5A and 6A, or PCS (e.g., PCS508, PMA514, and PMD516,FIGS. 5A and 6A, or PCS708, PMA710, and PMD712,FIGS. 7A,8A, and9A).

In the method1500, optical signals are received (1504) in the optical PHY1304. A first continuous bitstream (e.g., analogous to bitstream520,FIG. 5B,7B, or8B) is provided (1506) from the optical PHY1304to the coax PHY1308over the media-independent interface1310. The first continuous bitstream corresponds to received optical signals. Coax signals are transmitted (1508) in the coax PHY1308. The transmitted coax signals correspond to the first continuous bitstream and the received optical signals. In some embodiments, the coax PHY1308transmits the coax signals in the manner shown for the downstream signals550(FIG. 5B) or the transmitted signals750(FIG. 7Bor8B).

Also in the method1500, coax signals are received (1510) in the coax PHY1308. In some embodiments, the coax PHY1308receives the coax signals in the manner shown for the upstream signals630(FIG. 6B) or the received signals902(FIG. 9B). A second continuous bitstream (e.g., analogous to bitstream600,FIG. 6Bor9B) is provided (1512) from the coax PHY1308to the optical PHY1304over the media-independent interface1310. The second continuous bitstream corresponds to the received coax signals. Optical signals are transmitted (1514) in the optical PHY1304. The transmitted optical signals correspond to the second continuous bitstream and the received coax signals.

In some embodiments, the optical PHY1304receives (1504) and transmits (1514) optical signals by performing FDD, while the coax PHY1308receives (1510) and transmits (1508) coax signals by performing TDD. For example, the coax PHY1308transmits coax signals during a first plurality of time windows (e.g., including downstream time windows202and208,FIG. 2) and receives coax signals during a second plurality of time windows (e.g., including upstream time windows206and212,FIG. 2) that is distinct from the first plurality of time windows.

In some embodiments, providing (1506) the first continuous bitstream from the optical PHY1304to the coax PHY1308includes buffering the first continuous bitstream in the bit buffer1306. Similarly, providing (1512) the second continuous bitstream from the coax PHY1308to the optical PHY1304may include buffering the second continuous bitstream in the bit buffer1306.

In some embodiments of the method1500, the coax PHY1308generates a third bitstream based on the first continuous bitstream and corresponding to the transmitted coax signals, adapts a rate of the third bitstream, and inserts pad bits or gaps into the third bitstream. The pad bits or gaps are inserted into the third bitstream in locations corresponding to times during which the coax PHY1308does not transmit coax signals. These times include the second plurality of time windows and guard intervals that separate respective time windows of the first and second pluralities of time windows. The PCS may perform these operations. For example, the upper PCS layers510(FIG. 5A) generate the bitstream530(FIG. 5B) as the third bitstream and the TDD adapter512adapts the rate of the bitstream530and inserts pad bits546, resulting in the bitstream540(FIG. 5B). Alternatively, the PCS generates the third bitstream; the PMD adapts the rate of the third bitstream and inserts the gaps. For example, the PCS708(FIGS. 7A and 8A) generates the bitstream730(FIG. 7B) or830(FIG. 8B) as the third bitstream. The coax rate adapter714of the PMD712(FIGS. 7A and 8A) adapts the rate of the bitstream730or830and inserts gaps746, resulting in the bitstream740(FIGS. 7B and 8B).

In some embodiments of the method1500, the coax PHY1308converts the received coax signals to a fourth bitstream with pad bits in locations corresponding to times during which the coax PHY1308does not receive coax signals. These times include the first plurality of time windows and guard intervals that separate respective time windows of the first and second pluralities of time windows. The coax PHY1308generates a fifth bitstream based on the fourth bitstream. Generating the fifth bitstream includes adapting a rate of the fourth bitstream and deleting the pad bits from the fourth bitstream. The coax PHY1308further generates the second continuous bitstream based on the fifth bitstream. For example, the fourth bitstream (e.g., bitstream620,FIG. 6B) is generated in the PMA (e.g., PMA514,FIG. 6A); the fifth bitstream (e.g., bitstream610,FIG. 6B) and second continuous bitstream (e.g., bitstream600,FIG. 6B) are generated in the PCS (e.g., PCS508,FIG. 6B).

In some embodiments of the method1500, the coax PHY1308converts the received coax signals to a fourth bitstream (e.g., bitstream910,FIG. 9B) with gaps in locations corresponding to times during which the coax PHY1308does not receive coax signals. These times include the first plurality of time windows and guard intervals that separate respective time windows of the first and second pluralities of time windows. The coax PHY1308further adapts a rate of the fourth bitstream and removes the gaps from the fourth bitstream (e.g., resulting in the bitstream920,FIG. 9B). These operations are performed, for example, in the PMD (e.g., PMD712,FIG. 9A).

While the method1500includes a number of operations that appear to occur in a specific order, it should be apparent that the method1500can include more or fewer operations, which can be executed serially or in parallel. An order of two or more operations may be changed, performance of two or more operations may overlap, and two or more operations may be combined into a single operation. For example, the operations1504,1506,1508,1510,1512, and1514may be performed simultaneously in an ongoing manner.