WO2013170479A1

WO2013170479A1 - Methods and systems for scheduling transmissions in networks with optical and coaxial components

Info

Publication number: WO2013170479A1
Application number: PCT/CN2012/075723
Authority: WO
Inventors: Honger NIE
Original assignee: Qualcomm Incorporated
Priority date: 2012-05-18
Filing date: 2012-05-18
Publication date: 2013-11-21

Abstract

A coax media converter is coupled to an optical line terminal by an optical link and to a plurality of coax network units by coax links in a cable plant. A message addressed to a first coax network unit of the plurality of coax network units is received from the optical line terminal. The message specifies a time slot for upstream transmission by the first coax network unit. Based on the time slot, one or more coax physical resource blocks are selected. Each physical resource block corresponds to a time period for a sub-band of a plurality of sub-bands available for upstream transmission in the cable plant. The message is modified to specify the selected plurality of coax physical resource blocks and the modified message is transmitted to the first coax network unit.

Description

Methods and Systems for Scheduling Transmissions in Networks with Optical and Coaxial Components

TECHNICAL FIELD

[0001] The present embodiments relate generally to communication systems, and specifically to communication systems with both optical fiber links and coaxial cable links.

BACKGROUND OF RELATED ART

[0002] A network may use both optical fiber and coaxial cable for respective links. For example, the portions of the network that use optical fiber may be implemented using the Ethernet Passive Optical Networks (EPON) protocol, and the EPON protocol may be extended over coaxial cable plants. EPON over coax is called EPOC. In such a network, the fiber part of the network can potentially support a higher data rate than the coax part of the network. Also, different coax parts of the network (e.g., different cable plants) may have different maximum data rates. Slow coax links thus can limit overall system performance.

[0003] In view of these different data rates, there is a need for methods and systems to coordinate scheduling between the fiber and coax portions of such a network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The present embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings.

[0005] FIG. 1 is a block diagram of a network that includes both optical links and coax links in accordance with some embodiments.

[0006] FIG. 2 illustrates network protocol stacks for the network of FIG. 1 in accordance with some embodiments. [0007] FIG. 3 is a timing diagram showing timing of downstream and upstream packets in a network that has one scheduling domain and two timing domains in accordance with some embodiments.

[0008] FIG. 4A shows an example of filtering and demultiplexing downstream packets in a network that has one scheduling domain and two timing domains and that uses a frequency-division duplexing scheme in accordance with some embodiments.

[0009] FIG. 4B shows an example of upstream packet transmission in a network that has one scheduling domain and two timing domains and that uses a frequency- division duplexing scheme in accordance with some embodiments.

[0010] FIG. 5 illustrates timing of upstream and downstream time windows for time-division duplexing as measured at a coax media converter in accordance with some embodiments.

[0011] FIG. 6 is a schematic block diagram of a coax media converter in accordance with some embodiments.

[0012] FIG. 7A illustrates the allocation of physical resource blocks on a coax medium in accordance with some embodiments.

[0013] FIG. 7B shows the relationship between coax timing and optical timing for upstream transmissions using FDD in accordance with some embodiments.

[0014] FIG. 7C shows the relationship between coax timing and optical timing for transmissions using TDD in accordance with some embodiments.

[0015] FIG. 8 is a flowchart showing a method of operating a coax media converter in accordance with some embodiments.

[0016] Like reference numerals refer to corresponding parts throughout the drawing figures.

DETAILED DESCRIPTION

[0017] 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. Any of the signals provided over various buses described herein may be time-multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit elements or software blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be a single signal line, and each of the single signal lines may alternatively be buses, and a single line or bus might represent any one or more of a myriad of physical or logical mechanisms for communication between components. The present embodiments are not to be construed as limited to specific examples described herein but rather to include within their scopes all embodiments defined by the appended claims.

[0018] FIG. 1 is a block diagram of a network 100 that includes both optical links and coax links in accordance with some embodiments. The network 100 includes an optical line terminal (OLT) 110 coupled to a plurality of optical network units (ONUs) 120-1 and 120-2 via respective optical fiber links. The OLT 110 also is coupled to a plurality of coax media converters (CMCs) 130-1 and 130-2 via respective optical fiber links. The CMCs also may be referred to as optical-coax units (OCUs). The CMCs 130-1 and 130-2 convert optical signals from the OLT 110 into electrical signals and transmit the electrical signals to coax network units (CNUs) via respective coax links. In the example of FIG. 1, a first CMC 130-1 transmits converted signals to CNUs 140-1 and 140-2, and a second CMC 130-2 transmits converted signals to CNUs 140-3, 140-4, and 140-5. The coax links coupling the first CMC 130-1 to CNUs 140-1 and 140-2 compose a first cable plant 150-1. The coax links coupling the second CMC 130-2 to CNUs 140-3 through 140-5 compose a second cable plant 150-2. In some embodiments, the OLT 110, ONUs 120-1 and 120-2, and CMCs 130-1 and 130-2 are implemented in accordance with the Ethernet Passive Optical Network (EPON) protocol. The OLT 110 transmits optical signals using FDD and time-domain multiplexing (TDM), such that different time slots are used to transmit packets addressed to different network units. In some embodiments, packets in coax portions of the network 100 are transmitted using orthogonal frequency-division multiplexing (OFDM) symbols. [0019] In some embodiments, each ONU 120 and CMC 130 in the network 100 receives data at the same data rate. The ONUs 120 and CMCs 130 each receive all of the packets transmitted by the OLT 110. For unicast transmissions, each ONU 120 receives every packet transmitted by the OLT 110, but selects only the packets addressed to it, and discards all packets that are not addressed to it.

[0020] For unicast transmissions, the CMCs 130 also receive every packet transmitted by the OLT 110, but filter out the packets not addressed to CNUs 140 on their respective cable plants 150. For example, the CMC 130-1 receives every packet transmitted by the OLT 110 but forwards only those packets addressed to the CNUs 140-1 and 140-2 on the cable plant 150-1. The CMC 130-1 forwards each packet addressed to one of the CNUs 140-1 and 140-2 on the cable plant 150-1 to every CNU 140-1 and 140-2 in the cable plant 150-1. Each CNU 140-1 and 140-2 selects the packets addressed to it and discards other packets. The CMC 130-2 and CNUs 140-3 through 140-5 function similarly.

[0021] In some embodiments, the optical fiber links in the network 100 can support different (e.g., higher) data rates than the coax links. In one example, the optical links can support data rates of lOGbps, while the coax links can support data rates of IGbps. Despite this difference, the OLT 110 transmits at the higher data rate of the optical links (e.g., lOGbps). The filtering performed by the CMCs 130 prevents the coax links from limiting data rates of the optical links and thus the overall network performance. Because only a portion of the packets transmitted by the OLT 110 are forwarded by the CMCs 130, the coax links can operate at lower data rates than the optical links, which can operate at their maximum potential speed. By allowing the optical links to operate at full speed, the filtering thus avoids wasting bandwidth. Also, each CMC 130 buffers packets to smooth out bursts of packet transmissions from the OLT 110 to CNUs 140 on the CMC 130's cable plant 150.

[0022] FIG. 2 illustrates network protocol stacks 200 for the network 100 (FIG.

1) in accordance with some embodiments. The protocol stacks 200 includes a network protocol stack 202 for the OLT 110 (FIG. 1), a network protocol stack 204 for CMCs 130 (e.g., CMCs 130-1 and 130-2, FIG. 1), and a network protocol stack 206 for CNUs 140 (e.g., CNUs 140-1 through 140-5, FIG. 1). The layers 208-224 are specific to optical communications (e.g., are EPON-compatible) and the layers 226-234 are specific to coax communications (e.g., are EPOC-compatible). The layers in the stack 202 for the OLT 110 thus are entirely optical (e.g., are EPON-compatible), while the layers in the stacks 204 and 206 for the CMCs 130 and CNUs 140 include a mix of optical (e.g., EPON-compatible) and coax (e.g., EPOC-compatible) layers.

[0023] The stack 202 for the OLT 110 and the stack 206 for CNUs 140 both include Operations, Administration, and Maintenance (OAM) layers 208 and Dynamic Bandwidth Allocation (DBA) (e.g., EPON DBA) layers 210. Because OAM layers 208 are present in both the stack 202 for the OLT 110 and the stack 206 for CNUs 140, a single network management domain spans the entire network 100, and thus spans both the optical and coax portions of the network 100. Likewise, because DBA layers 210 are present in both the stack 202 for the OLT 110 and the stack 206 for CNUs 140, a single scheduling domain spans the entire network 100. As discussed further below, the instance of the DBA layer 210 in the OLT 110 assigns time slots for upstream transmission by respective CNUs 140 (and also to respective ONUs 120) and transmits these time slots to the CNUs 140 (and ONUs 120). As discussed further below, a CMC 130 remaps time slots to coax physical resource blocks that the CNUs 140 use for upstream transmission to the CMC 130; the time slots are then used for transmission further upstream to the OLT 110.

[0024] The stack 202 for the OLT 110 and the stack 204 for CMCs 130 both include optical (e.g., EPON) Multi-Point Control Protocol (MPCP) layers 212 that implement an optical timing domain for the OLT 110 and CMCs 130, in accordance with bandwidth allocated by the DBA layer 210 in the stack 202. The stack 204 for CMCs 130 and the stack 206 for CNUs 140 both include coax (e.g., EPOC) MPCP layers that implement a coax timing domain for a CMC 130 and respective CNUs 140 on the cable plant 150 of the CMC 130. (The network 100 thus includes multiple timing domains.) For example, the MPCP layer 226 in the stack 204 assigns physical resource blocks to respective CNUs 140, based on time slots assigned by the MPCP layer 212 in the stack 202. The stacks 202 and 204 also each include the following layers for communications over fiber links: encryption layers 214, Media Access Control (MAC) layers 216, Reconciliation Sub-layers (RS) 218, Physical Coding Sublayers (PCS) 220, Physical Media Attachment (PMA) sub-layers 222, and Physical Media Dependent (PMD) sub-layers 224. The layers 220, 222, and 224 compose the physical layer. The stacks 204 and 206 also each include the following layers for communications over coax links: encryption layers 228, MAC layers 230, RS sublayers 232, and coax physical layers 234.

[0025] FIG. 3 is a timing diagram showing timing of downstream and upstream packets in accordance with some embodiments. In FIG. 3, packet transmission for each of the OLT 110, CMC 130, and CNU 140 is shown above the corresponding axis and packet reception is shown below the corresponding axis. To schedule an upstream transmission by the CNU 140 (e.g., one of CNUs 140-1 through 140-5, FIG. 1), the OLT 110 assigns a time slot for the upstream transmission and transmits, at a time Tl, a downstream packet 302 specifying the time slot. The time Tl is embedded as a time stamp in the packet 302. In some embodiments, the packet 302 is an MPCP message (e.g., a GRANT message).

[0026] The CMC 130 that is coupled to the CNU 140 receives the packet 302 at a time T2, which is set equal to Tl : when the packet 302 is received, a timer in the CMC 130 is set equal to Tl . The timer runs while the CMC 130 processes the packet 302. The CMC 130's processing of the packet 302 results in a modified packet 304, which specifies coax physical resource blocks mapped from the time slot for upstream transmission. The modified packet 304 is transmitted to the CNU 140 at a time T3 = Tl + t2, where t2 is the processing time of the CMC 130. T3 is the value of the counter when the processing is complete. The timestamp in the packet 304 is modified to be T3 instead of Tl .

[0027] The modified packet 304 is received at the CNU 140 at a time T4, which is set equal to T3: when the modified packet 304 is received, a timer in the CNU 140 is set equal to T3 and begins to run. The CNU 140 buffers and processes the modified packet 304 and, in response, transmits an upstream packet 306 using the physical resource blocks specified in the modified packet 304. The packet 306 is transmitted at a time T5, which equals T3 plus the time between receipt of the modified packet 304 and transmission of the packet 306. T5 is the value of the CNU 140's timer when the packet 306 is transmitted. The packet 306 may be a data packet or may be an MPCP message reporting on the status of an upstream transmission queue in the CNU 140 to the OLT 110. In some embodiments, a CNU 140 reports its queue status based on a fixed coax PHY payload rate (e.g., lGbps) and the CMC 130 converts the reported queue status into a value based on the optical PHY payload rate (e.g., lOGbps) and forward the report to the OLT 110. [0028] The packet 306 is received at the CMC 130 at a time T6. T6 is the value of the timer in the CMC 130 when receiving the packet 306. The CMC 130 buffers and modifies the packet 306 and transmits a modified packet 308 upstream to the OLT 110 at a time T7 = T6 + t3, where t3 is the time spent processing the packet 306. T7 thus is the value of the timer when the modified packet 308 is transmitted. The CMC 130's modification of the packet 306 includes replacing the time stamp T5 with T7. The OLT 110 receives the modified packet 308 at a time T8.

[0029] The time stamps shown in FIG. 3 can be used to calculate the optical

(e.g., EPON) and coax round-trip times (RTTs):

EPON RTT = (T8-T1) - (T7-T2) = T8-T7 (1)

Coax RTT = (T6-T3) - (T5-T4) = T6-T5 (2)

Coax RTT includes coax cable propagation time as well as processing time for coax physical layer devices (PHYs) (e.g., instances of coax physical layers 234, FIG. 2). This processing time includes downstream and upstream transmission and receiving processing time. Coax RTT may not be constant if a physical layer synchronization mechanism is used. EPON RTT includes optical fiber propagation time as well as processing time for optical PHYs (e.g., instances of physical layers comprising sublayers 220, 222, and 224, FIG. 2), which includes downstream and upstream transmission and receiving processing time.

[0030] Time tl is the time in advance that the OLT 110 should transmit a

GRANT packet or other packet specifying an upstream time slot before the start of the upstream time slot. Time tl should be at least equal to EPON RTT plus coax RTT plus processing and buffering time in the CMC 130 in both directions:

tl > EPON RTT + Coax RTT + t2 +t3. (3)

In some embodiments, the maximum value of the time spacing tl is less than a DBA cycle.

[0031] The CMC 130 processing and buffering times (t2+t3) are not necessarily constant. The maximum value of CMC 130 downstream buffering and processing time t2 depends on the downstream buffer size (e.g., queue 610, FIG. 6), which determines the maximum downstream burst size. In the downstream direction, MPCP packets are extracted from the packet stream and scheduled with the highest priority (e.g., using strict priority scheduler 614, FIG. 6), thus minimizing the buffering delay of MPCP packets. The maximum value of CMC 130 upstream buffering and processing time t3 depends on the upstream buffer size (e.g., queue 628, FIG. 6), which determines the maximum upstream burst size.

Frequency-Division Duplexing

[0032] In some embodiments, the cable plants 150 (e.g., cable plants 150-1 and

150-2, FIG. 1) use frequency-division duplexing (FDD), in which downstream transmissions use a first frequency band and upstream transmissions use a second frequency band distinct from the first frequency band. Upstream and downstream coax transmissions thus may occur in parallel. The OLT 110 also uses FDD.

[0033] In an FDD scheme, a CMC 130 can filter out packets from the OLT 110 not belonging to the CNUs 140 that are coupled to it, as previously discussed with regard to FIG. 1. Also, a CMC 130 buffers packets that are addressed to the CNUs 140 that are coupled to it and thus are to be forwarded. The downstream buffer (e.g., queues 610 and 612, FIG. 6) in the CMC 130 can smooth packet bursts from the OLT 110 and adapt packets to the coax PHY rate. The buffer size is proportional to the maximum tolerated size of a packet burst and the difference between the optical and coax rates.

[0034] The coax PHY rate may be less than the optical PHY rate. In some embodiments, if several CMCs 130 are connected to the OLT 110 (FIG. 1), packets from the OLT 1 10 are demultiplexed into several packet streams, and each stream is forwarded by a respective CMC 130. The downstream throughput of the network 100 thus is greater than the coax PHY rate. The maximum throughput of the network 100 may be the sum of the coax PHY rates of CMCs 130 connected to the OLT 110.

[0035] FIG. 4A shows an example of filtering and demultiplexing downstream packets in a network that has one scheduling domain and two timing domains and that uses an FDD scheme in accordance with some embodiments. In this example, the throughput of the network is twice the coax PHY rate, as shown by the 2: 1 length ratios of the optical and coax packets. Other examples are possible. In FIG. 4A, packet transmission for each network element is shown above the corresponding axis and packet reception is shown below the corresponding axis. Time t2 is the downstream packet buffering and processing time for the CMCs 130-1 and 130-2 (e.g., as previously shown in FIG. 3). [0036] The OLT 110 transmits a burst of optical (e.g., EPON-compatible) packets 402o, 404o, 406o, 408o, 410o, 412o, 414o, and 416o. (Packets with Ό' appended to their reference numbers in FIG. 4A are optical packets; packets with 'c' appended to their reference numbers are coax packets.) Packets 402o, 406o, 412o, and 414o are each addressed to one of CNUs 140-1 and 140-2. Packets 404o, 408o, 410o, and 416o are each addressed to one of CNUs 140-3 through 140-5. CMC 130-1, which is coupled to CNUs 140-1 and 140-2, converts packets 402o, 406o, 412o, and 414o to corresponding coax packets 402c, 406c, 412c, and 414c and transmits them to CNUs 140-1 and 140-2, where they are received. CMC 130-1 discards packets 404o, 408o, 410o, and 416o. CMC 130-2, which is coupled to CNUs 140-3 through 140-5, converts packets 404o, 408o, 410o, and 416o to corresponding coax packets 404c, 408c, 410c, and 416c and transmits them to CNUs 140-3 through 140-5, where they are received. CMC 130-2 discards packets 402o, 406o, 412o, and 414o.

[0037] FIG. 4B shows an example of upstream packet transmission in a network that has one scheduling domain and two timing domains and that uses an FDD scheme in accordance with some embodiments. In such a network, each CMC 130 (FIG. 1) can buffer packets to be forwarded to the OLT 110 on optical fiber. The upstream buffer (e.g., queue 628, FIG. 6) in each CMC 130 can store packets from CNUs 140 in advance of their transmission time (e.g., as assigned by the OLT 110) and adapt the packets to the optical PHY rate. The buffer size is proportional to the maximum tolerated size of packet bursts and the difference between optical and coax rates. For example, the coax PHY rate may be less than optical PHY rate. If several CMCs 130 are connected to the OLT 110, packets forwarded by different CMCs 130 are multiplexed into one packet stream.

[0038] In FIG. 4B, the CNU 140-1 (FIG. 1), which is coupled to the CMC 130-1, transmits packets 432c, 434c, and 436c to the CMC 130-1, which receives them, buffers them, and converts them to optical packets 432o, 434o, and 436o. The CMC 130-1 transmits the optical packets 432o, 434o, and 436o to the OLT 110, where they are received. The CNU 140-3 (FIG. 1), which is coupled to the CMC 130-2, transmits packets 438c and 440c to the CMC 130-2, which receives them, buffers them, and converts them to optical packets 438o and 440o. (Time t3 is the buffering and processing time in each CMC 130.) The CMC 130-2 transmits the optical packets 438o and 440o to the OLT 110, where they are received. [0039] In the example of FIG. 4B, upstream transmission of the packet 438c by the CNU 140-3 on the cable plant 150-2 (FIG. 1) overlaps with upstream transmission of the packets 434c and 436c by the CNU 140-1 on the cable plant 150-1 (FIG. 1). Upstream transmission by the CMC 130-1, however, does not overlap with upstream transmission by the CMC 130-2. More generally, upstream transmissions may occur simultaneously on different cable plants 150, but only one CMC 130 (or ONU 120, FIG. 1) may transmit upstream to the OLT 110 at a given time. The time slots assigned by the OLT 110 for upstream transmission ensure that only one CMC 130 or ONU 120 transmits upstream to the OLT 110 at a given time.

Time-Division Duplexing

[0040] In some embodiments, the cable plants 150 (e.g., cable plants 150-1 and

150-2, FIG. 1) use time-division duplexing (TDD), in which downstream and upstream transmissions use the same frequency band and the upstream and downstream transmissions are duplexed in time. A first time unit is allocated for upstream transmissions from CNUs 140 to a CMC 130 and a second time unit is allocated for downstream transmissions from the CMC 130 to CNUs 140. These time units are also referred to as time periods or time windows. For example, alternating time windows are respectively allocated for upstream and downstream transmissions.

[0041] FIG. 5 illustrates timing of upstream and downstream time windows for time-division duplexing as measured at a CMC 130 (e.g., CMC 130-1 or 130-2, FIG. 1) in accordance with some embodiments. As shown in FIG. 5, alternating time periods are allocated for upstream and downstream transmissions on the same frequency band. During a first time unit 502, the CMC 130 transmits signals downstream to the CNUs 140 that are coupled to it. The first time unit 502 is followed by a guard interval 504, after which the CMC 130 receives upstream signals from one or more of the CNUs 140 during a second time unit 506. The guard interval 504 accounts for propagation time on the coax links and for switching time in the CMC 130 to switch from a transmit configuration to a receive configuration. The guard interval 504 thus ensures separate upstream and downstream time windows at the CNUs 140. The second time unit 506 is immediately followed by a third time unit 508 for downstream transmission, another guard interval 510, and a fourth time unit 512 for upstream transmission. 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 in FIG. 5. The time allocated for upstream time windows (e.g., time units 506 and 512) may be different than the time allocated for downstream time windows (e.g., time units 502 and 508). FIG. 5 illustrates an example in which more time (and thus more bandwidth) is allocated to downstream time windows 502 and 508 than to upstream time windows 506 and 512.

[0042] In the downstream direction, a CMC 130 can filter out packets from the

OLT 110 that are not addressed to its corresponding CNUs 140 and buffer the packets that are to be forwarded. The downstream buffer (e.g., queues 610 and 612, FIG. 6) in the CMC 130 can smooth packet bursts, store the packets from the OLT 110 during upstream periods, and adapt packets from the OLT 110 to the coax PHY rate. The downstream buffer size is the sum of the maximum tolerated size of packet bursts and the total length transmitted during the coax upstream period at average rate.

[0043] In the upstream direction, the CMC 130 can buffer the packets to be forwarded onto optical fiber. The upstream buffer (e.g., queue 628, FIG. 6) in the CMC 130 can store packets from CNUs 140 in advance of their transmission times and adapt the packets to the optical PHY rate. The upstream buffer size is proportion to the maximum tolerated size of packet bursts and the difference between optical and coax rates.

[0044] Downstream packets thus can be filtered and demultiplexed as shown in

FIG. 4A, except that packets are only transmitted downstream (e.g., on respective coax cable plants 150, FIG. 1) during downstream transmission windows (e.g., windows 502 and 508, FIG. 5). Upstream packets can be transmitted and multiplexed as shown in FIG. 4B, except that packets are only transmitted upstream (e.g., on respective coax cable plants 150, FIG. 1) during upstream transmission windows (e.g., windows 506 and 512, FIG. 5).

[0045] The bi-directional throughput of the network may be much greater than the coax PHY rate. For example, the bi-directional maximum throughput may be the sum of the coax PHY rates of CMCs 130 connected to the OLT 110.

[0046] FIG. 6 is a schematic block diagram of a CMC 130 (e.g., CMC 130-1 or

130-2, FIG. 1) in accordance with some embodiments. In the downstream direction, packets are received at an optical PHY 602 and provided to a decryptor 604 followed by a packet parser and filter 606. The filter portion of the packet parser and filter 606 discards packets that are not addressed to CNUs 140 that are coupled to the CMC 130. The output of the packet parser and filter 606 is split into two streams: one for MPCP packets (e.g., GRANT packets that allocate upstream transmission time slots) and one for data packets. The MPCP packets are processed by a grant processing engine 608, which maps allocated time slots to coax physical resource blocks, and are passed into a control queue 610. The grant processing engine 608 is also referred to as a message processor. The data packets are passed into a data queue 612. A strict priority (SP) scheduler 614 schedules the packets in the control and data queues 610 and 612, with MPCP packets in the control queue 610 being given priority over data packets in the data queue 612. A time-stamping element 616 replaces timestamps carried in MPCP packets with local timestamps (e.g., as described for FIG. 3) and passes packets into an encryptor 618. In some embodiments, the time-stamping element 616 includes a timer that functions as described with regard to FIG. 3. The output of the encryptor 618 is fed into a coax PHY 620, which transmits the packets as modified.

[0047] In the upstream direction, packets are received at the coax PHY 620 and provided to a decryptor 622, followed by a packet parser 624, a report processor 626, and an upstream queue 628. A time-stamping element 630 replaces the timestamps carried in MPCP packets with local timestamps (e.g., as described for FIG. 3) and passes packets to an encryptor 632. In some embodiments, the time-stamping element 630 includes a timer that functions as described with regard to FIG. 3. The output of the encryptor 632 is fed into the optical PHY 602, which transmits the packets upstream to the OLT 110 (FIG. 1).

Mapping Time Slots to Physical Resource Blocks

[0048] Attention is now directed to grant processing performed in the CMC 130

(e.g., by the grant processing engine 608, FIG. 6). The CMC 130 maps an assigned optical time slot (e.g., as specified in a GRANT message) to one or more physical resource blocks. Each physical resource block corresponds to a time period on the coax medium of a cable plant 150 (FIG. 1) for a particular sub-band of a plurality of sub- bands available for upstream transmission. Each physical resource block thus has two dimensions: time period and sub-band.

[0049] To map an optical time slot to physical resource blocks, the CMC 130

(e.g., the grant processing engine 608, FIG. 6) calculates a duration on the coax medium based on the coax PHY payload rate and calculates the receiving window on the optical medium for the time slots according to allowed maximum and minimum delays (e.g., as shown in FIG. 3). The receiving window on the optical medium is converted into a receiving window on the coax medium. Physical resource blocks in the receiving window on the coax medium are selected and thus allocated for upstream transmission in response to the time slot grant, and the packet granting the time slot is modified to specify the allocated physical resource blocks.

[0050] FIG. 7A illustrates the allocation of physical resource blocks on the coax medium in accordance with some embodiments. FIG. 7A shows time lines for the coax and optical media connected to a CMC 130. A plurality of physical resource blocks 700 include blocks 702 that have previously been allocated, blocks 704 that are allocated in a current turn for a coax receiving window 710, and blocks 706 that have yet to be allocated. Each physical resource block 700 corresponds to a particular sub-band during a particular time period 708 on the coax medium. When a physical resource block 700 is allocated to a CNU 140, the CNU 140 can transmit upstream on the corresponding sub-band during the corresponding period. Physical resource blocks 700 are allocated first by sub-band and then by time period 708, as illustrated by the arrows for the blocks 704 in FIG. 7A.

[0051] The coax receiving window 710 is calculated based on an optical receiving window 712, which is calculated based on an optical transmission window 714 and corresponding minimum and maximum delays. If the coax PHY rate is less than the optical PHY rate, there is a minimum delay in advance of the end time of data transmission, which guarantees that the CMC 130 receives data before transmitting the data upstream and thus avoids upstream queue 628 (FIG. 6) underflow. If the coax PHY rate is greater than the optical PHY rate, there is a minimum delay in advance of the time, which is equal to the start time of data transmission plus the duration on the coax medium, which guarantees that the CMC 130 receives data before transmitting the data upstream and thus avoids upstream queue 628 underflow. There is a maximum delay in advance of the start time of data transmission, which prevents the upstream queue 628 from overflowing. In some embodiments, due to coax PHY symbol timing, the optical receiving window 712 is quantized to the time periods for coax physical resource blocks.

[0052] In embodiments using FDD on the coax medium, the physical resource blocks 700 on the coax medium are continuous and unmapped physical resource blocks are wasted. The bandwidth allocated on the coax media of all coax plants 150 in the network 100 (FIG. 1) is less than or equal to the bandwidth on the optical medium, so that all upstream packets from CNUs 140 can be transmitted completely to the OLT 110. Mapping time slots on the optical medium to coax physical resource blocks in coax receiving windows avoids buffer overflow and underflow.

[0053] FIG. 7B shows the relationship between coax timing and optical timing in the upstream direction for some embodiments using FDD. A first group of physical resource blocks 732 is used to transmit data from the CNU 140-3 (FIG. 1) to the CMC 130-2 (FIG. 1). The CMC 130-2 forwards this data to the OLT 110 in a corresponding optical packet 740. A physical resource block 734 is used to transmit data from the CNU 140-4 (FIG. 1) to the CMC 130-2. The CMC 130-2 forwards this data to the OLT 110 in a corresponding optical packet 742. A second group of physical resource blocks 736 is used to transmit data from the CNU 140-5 (FIG. 1) to the CMC 130-2. The CMC 130-2 forwards this data to the OLT 110 in a corresponding optical packet 744.

[0054] In embodiments using TDD on the coax medium, coax physical resource blocks are not continuous; instead, there is guard time between adjacent downstream and upstream periods. Optical time slots granted for upstream traffic are first mapped into coax physical resource blocks 700. Unmapped OFDM symbols between upstream periods are used for downstream traffic. If there was a downstream period between the last mapped OFDM symbol and the OFDM symbol to be mapped in a current turn, guard time is added in advance of the symbol being mapped. Thus, in some embodiments the division between upstream and downstream windows is not fixed but instead varies depending on network traffic.

[0055] FIG. 7C shows the relationship between coax timing and optical timing in the upstream direction for some embodiments using TDD. Coax physical resource blocks 700 are divided between downstream windows 752 and 758 and upstream windows 756 and 762. A guard time 754 comes between the downstream window 752 and the upstream window 756. Likewise, a guard time 760 comes between the downstream window 758 and the upstream window 762. Physical resource blocks 700 in the downstream windows 752 and 758 are used to transmit packets from a CMC 130 (FIG. 1) to corresponding CNUs 140 (FIG. 1). Physical resource blocks 700 in the upstream window 756 are used to transmit data from a CNU 140 to the CMC 130; the CMC 130 forwards the data to the OLT 110 in a corresponding optical packet 766. Similarly, physical resource blocks 700 in the upstream window 762 are used to transmit data from a CNU 140 to the CMC 130; the CMC 130 forwards the data to the OLT 110 in a corresponding optical packet 770. The optical packets 764 and 768 are transmitted by another CMC 130.

OLT Scheduling

[0056] In some embodiments, to prevent a particular CMC 130's downstream buffer (e.g., queues 610 and 612, FIG. 6) from overflowing, the OLT 110 (FIG. 1) shapes and limits the packet stream, including unicast and multicast transmissions, sent to the CNUs 140 connected to the CMC 130. To prevent the CMC 130's upstream buffer (e.g., queue 628, FIG. 6) from underflowing, the OLT 110 limits the maximum duration allocated to a CNU 140 and places time spacing between adjacent time slots allocated to the CNUs 140 connected to the CMC 130.

[0057] In some embodiments, the transmission order of packets granting time slots to CNUs 140 (e.g., GRANT messages) matches the order of the start times of the granted time slots. For example, if the start time of time slot 1 is before the start time of time slot 2, the grant packet carrying the grant of time slot 1 is sent prior to the grant packet carrying the grant of time slot 2.

[0058] In some embodiments, for downstream scheduling, the OLT 110 uses a single-bucket downstream shaper per CNU 140; the maximum burst size is the sum of the burst sizes for all CNUs 140. In some embodiments, the OLT 110 uses a single- bucket downstream shaper per CMC 130; the maximum burst size is the bucket depth (e.g., the maximum value of a token for the bucket). The sum of the CNU 140 average rates should be less than the coax PHY payload rate multiplied by the ratio of downstream duration to DBA cycle (e.g., the ratio of a downstream window duration to the total duration of the downstream window, upstream window, and guard time), which is equal to one if FDD is used. The maximum burst size of a packet stream transmitted by the OLT 110 and forwarded by a CMC 130 is proportional to the downstream buffer size and inversely proportion to the difference between optical and coax rates. For FDD embodiments, the maximum downstream buffering delay equals the time to transmit all packets in the buffer. For TDD embodiments, the maximum downstream buffering delay is equal to the maximum value of the upstream window duration plus the time for transmitting all the packets in the buffer. [0059] In some embodiments, for upstream scheduling, the OLT 110 uses a single-bucket upstream shaper per CNU 140. In some embodiments, the OLT 110 uses a single -bucket upstream shaper per CMC 130.

CMC Operation

[0060] FIG. 8 is a flowchart showing a method 800 of operating a coax media converter (e.g., CMC 130-1 or 130-2, FIG. 1) in accordance with some embodiments. The CMC is coupled to an optical line terminal (e.g., OLT 110, FIG. 1) by an optical link and to a plurality of coax network units (e.g., CNUs 140-1 and 140-2 or CNUs 140- 3 through 140-5, FIG. 1) by coax links in a cable plant 150.

[0061] A message (e.g., an MPCP packet, such as a GRANT message) addressed to a first coax network unit of the plurality of coax network units is received (802) from the optical line terminal. The message specifies a time slot for upstream transmission by the first coax network unit.

[0062] Based on the time slot, one or more coax physical resource blocks are selected (804) (e.g., as shown in FIGS. 7A-7C). Each physical resource block corresponds to a time period for a sub-band of a plurality of sub-bands available for upstream transmission in the cable plant. The message is modified (806) (e.g., by message processor 608, FIG. 6) to specify the selected one or more coax physical resource blocks. The modified message is transmitted (808) to the first coax network unit.

[0063] An upstream transmission is received (810) from the first coax network unit. The upstream transmission uses the selected one or more coax physical resource blocks. The upstream transmission is buffered (e.g., in upstream queue 628, FIG. 6) and forwarded (812) to the optical line terminal at a time corresponding to the time slot specified in the message from the optical line terminal.

[0064] While the method 800 includes a number of operations that appear to occur in a specific order, it should be apparent that the method 800 can include more or fewer operations, which can be executed serially or in parallel. An order of two or more operations may be changed and two or more operations may be combined into a single operation.

[0065] In the foregoing specification, the present embodiments have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

Claims

CLAIMS What is claimed is:

1. A method of operating a coax media converter, wherein the coax media converter is coupled to an optical line terminal by an optical link and to a plurality of coax network units by coax links in a cable plant, the method comprising:

receiving from the optical line terminal a message addressed to a first coax network unit of the plurality of coax network units, the message specifying a time slot for upstream transmission by the first coax network unit;

based on the time slot, selecting one or more coax physical resource blocks, wherein each physical resource block corresponds to a time period for a sub-band of a plurality of sub-bands available for upstream transmission in the cable plant;

modifying the message to specify the selected one or more coax physical resource blocks; and

transmitting the modified message to the first coax network unit.

2. The method of claim 1, further comprising:

after transmitting the modified message, receiving an upstream transmission from the first coax network unit, wherein the upstream transmission uses the selected one or more coax physical resource blocks.

3. The method of claim 2, further comprising:

buffering the upstream transmission; and

forwarding the upstream transmission to the optical line terminal at a time corresponding to the time slot specified in the message from the optical line terminal.

4. The method of claim 3, further comprising:

replacing a timestamp in the message with a first time counter value; and replacing a timestamp in the upstream transmission with a second time counter value.

5. The method of claim 1, wherein the plurality of sub-bands compose a first frequency band available for upstream transmission in the cable plant; and

the method further comprises receiving packets from the plurality of coax network units on the first frequency band and transmitting packets including the modified message to the plurality of coax network units on a second frequency band distinct from the first frequency band, wherein packets are transmitted on the second frequency band while packets are being received on the first frequency band.

6. The method of claim 1, wherein the plurality of sub-bands compose a frequency band available for upstream transmission in the cable plant during upstream transmission windows and available for downstream transmission in the cable plant during downstream transmission windows.

7. The method of claim 6, further comprising:

receiving the message from the optical line terminal during an upstream transmission window; and

buffering the message;

wherein the modified message is transmitted to the first coax network unit during a subsequent downstream transmission window.

8. The method of claim 6, wherein a respective upstream transmission window is separated from a previous downstream transmission window by a guard time.

9. The method of claim 1, further comprising:

receiving from the optical line terminal data packets addressed to respective coax network units of the plurality of coax network units; and

prioritizing the message over the data packets in accordance with a strict priority scheme.

10. The method of claim 1, further comprising:

prior to receiving the message from the optical line terminal, receiving a report message from the first coax network unit reporting a status of an upstream transmission queue in the first coax network unit; and

forwarding the report message to the optical line terminal.

11. The method of claim 10, wherein:

the report message from the first coax network unit reports a status value based on a coax link payload rate;

the method further comprises converting the status value based on the coax link payload rate to a status value based on a payload rate for the optical link; and

the report message as forwarded to the optical link terminal includes the status value based on the payload rate for the optical link.

12. The method of claim 1, wherein the message comprises a GATE message.

13. The method of claim 1, wherein the message comprises a multi-point control protocol packet.

14. A coax media converter to be coupled to an optical line terminal by an optical link and to a plurality of coax network units by coax links in a cable plant, comprising:

an optical physical layer device (PHY) to receive a message addressed to a first coax network unit of the plurality of coax network units, the message specifying a time slot for upstream transmission by the first coax network unit;

a message processor to select one or more coax physical resource blocks based on the time slot and modify the message to specify the selected one or more coax physical resource blocks, wherein each physical resource block corresponds to a time period for a sub-band of a plurality of sub-bands available for upstream transmission in the cable plant; and

a coax PHY to transmit the modified message to the first coax network unit.

15. The coax media converter of claim 14, wherein:

the coax PHY is to receive an upstream transmission from the first coax network unit, wherein the upstream transmission uses the selected one or more coax physical resource blocks; the optical PHY is to forward the upstream transmission to the optical line terminal at a time corresponding to the time slot specified in the message from the optical line terminal; and

the coax media converter further comprises a queue to buffer the upstream transmission

16. The coax media converter of claim 15, further comprising:

a first time-stamping unit to replace a timestamp in the message with a first time counter value; and

a second time-stamping unit to replace a timestamp in the upstream transmission with a second time counter value.

17. The coax media converter of claim 14, wherein:

the plurality of sub-bands composes a frequency band available for upstream transmission in the cable plant during upstream transmission windows and available for downstream transmission in the cable plant during downstream transmission windows; the coax PHY is to transmit the modified message during a downstream transmission window; and

the coax media converter further comprises a queue to buffer the message.

18. The coax media converter of claim 14, wherein:

the plurality of sub-bands compose a first frequency band available for upstream transmission in the cable plant; and

the coax PHY is to receive packets from the plurality of coax network units on the first frequency band and to transmit packets to the plurality of coax network units on a second frequency band distinct from the first frequency band.

19. The coax media converter of claim 14, further comprising a strict-priority scheduler to prioritize the message over data packets received from the optical line terminal and addressed to respective coax network units of the plurality of coax network units.

20. A coax media converter, to be coupled to an optical line terminal by an optical link and to a plurality of coax network units by coax links in a cable plant, comprising:

means for receiving from the optical line terminal a message addressed to a first coax network unit of the plurality of coax network units, the message specifying a time slot for upstream transmission by the first coax network unit;

means for selecting one or more coax physical resource blocks based on the time slot and modifying the message to specify the selected one or more coax physical resource blocks, wherein each physical resource block corresponds to a time period for a sub-band of a plurality of sub-bands available for upstream transmission in the cable plant; and

means for transmitting the modified message to the first coax network unit.