Patent Publication Number: US-2004045035-A1

Title: Distributed cable modem termination system (CMTS) architecture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is related to the following non-provisional applications:  
     [0002] “A MiniMAC Implementation of a Distributed Cable Modem Termination System (CMTS) Architecture,” U.S. patent application Ser. No. ______ (Attorney Docket No. 1875.2630000), by Scott Cummings et al., filed concurrently herewith and incorporated by reference herein in its entirety.  
     [0003] “A Distributed Cable Modem Termination System (CMTS) Architecture Implementing a Media Access Control Chip,” U.S. patent application Ser. No. ______ (Attorney Docket No. 1875.2560000), by Scott Cummings et al., filed concurrently herewith and incorporated by reference herein in its entirety.  
     [0004] “A Distributed Cable Modem Termination System (CMTS) Architecture Implementing a Media Access Control Chip,” U.S. patent application Ser. No. ______ (Attorney Docket No. 1875.2560001), by Scott Cummings et al., filed concurrently herewith and incorporated by reference herein in its entirety. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0005] 1. Field of the Invention  
       [0006] The present invention is generally related to broadband communications systems. More particularly, the present invention is related to a cable modem termination system (CMTS) in a broadband communications system.  
       [0007] 2. Background Art  
       [0008] In broadband communications architectures, data is transferred between a central location and many remote subscribers. For broadband cable modem systems, the central location may be referred to as a headend and the remote subscriber equipment is referred to as a cable modem (CM). In cable modem systems, the communication path from the headend to the cable modem is called the downstream and the communication path from the cable modem to the headend is called the upstream.  
       [0009] As cable modem systems introduce new services, new ways to increase network capacity at a reasonable cost to the subscriber must be implemented. Thus, cable modem systems are constantly being reconfigured to provide adequate bandwidth to remote subscribers.  
       [0010] A cable modem system is typically housed in a hybrid fiber/coaxial (HFC) plant (also referred to as a HFC system). The hybrid fiber/coaxial plant consists of a fiber portion and a coaxial portion. The headend is housed in the fiber portion of the hybrid fiber/coaxial plant. A Cable Modem Termination System (CMTS), located within the headend, services a plurality of cable modems, located in the coaxial portion of the HFC plant via a plurality of fiber nodes in a point-to-multipoint topology. The network over which the CMTS and the cable modems communicate is referred to as a hybrid fiber/coaxial cable network.  
       [0011] Typically, bandwidth is available to transmit signals downstream from the headend to the cable modems. However, in the upstream, bandwidth is limited and must be arbitrated among the competing cable modems in the system. Cable modems request bandwidth from the CMTS prior to transmitting data to the headend. The CMTS allocates bandwidth to the cable modems based on availability and the competing demands from other cable modems in the system.  
       [0012] In the coaxial portion of the hybrid fiber/coaxial plant, problems may arise with the coaxial cable. Such problems may include loose connectors, poor shielding, and similar points of high impedance. These problems cause noise signals to develop from interference sources such as radio transmissions, electric motors, and other sources of electrical impulses. The point-to-multipoint topology of the cable modem system complicates upstream transmissions by exacerbating the noise. With the multipoint structure of the HFC system, noise is additive in the upstream. Thus, the noise problem is more intense in the upstream as signals approach the headend.  
       [0013] One method of providing additional bandwidth to any one cable modem in the hybrid fiber/coaxial plant requires the fiber node servicing that cable modem to be split. Depending on the frequency stacking in the HFC plant, more upconverters may be required to service the new fiber node resulting from the split. Since all of the signals are combined at the headend, there is a limit to the number of times fiber nodes can be split without causing additional noise sources to enter the system. This makes the CMTS in the headend architecture difficult to expand into available fiber bandwidths.  
       [0014] What is therefore needed is a system and method for maximizing bandwidth allocations to cable modems while minimizing system noise in a HFC plant.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015] The present invention solves the above-mentioned problem by enabling additional bandwidth to be administered to any one cable modem in a cable modem system. The present invention accomplishes this by providing a distributed Cable Modem Termination System (CMTS) architecture in a hybrid fiber/coaxial plant. The distributed CMTS architecture distributes layers of the CMTS throughout the hybrid fiber/coaxial (HFC) plant. Distributing portions of the CMTS throughout the HFC plant lessens the distance in which RF waves must travel through the system and increases the distance for digital transmissions. This minimizes system noise and maximizes HFC bandwidth.  
       [0016] Briefly stated, the present invention is directed to a distributed CMTS in a hybrid fiber/coaxial (HFC) plant. The distributed CMTS comprises at least one network layer, at least one media access control layer, and one or more physical layers. The at least one network layer, at least one media access control layer and one or more physical layers each function as separate modules, enabling them to be in separate locations throughout the HFC plant, yet physically connected.  
       [0017] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
     [0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art(s) to make and use the invention.  
     [0019]FIG. 1 is a high level block diagram of an exemplary cable modem system in accordance with embodiments of the present invention  
     [0020]FIG. 2A is a high level block diagram of an exemplary hybrid fiber/coaxial (HFC) plant in accordance with embodiments of the present invention.  
     [0021]FIG. 2B is another high level block diagram of an exemplary hybrid fiber/coaxial (HFC) plant in accordance with embodiments of the present invention.  
     [0022]FIG. 3 is a high level block diagram of a traditional CMTS.  
     [0023]FIG. 4 is a high level block diagram of a distributed CMTS in accordance with embodiments of the present invention.  
     [0024]FIG. 5 is a block diagram illustrating a configuration for a MAC layer implementing a CMTS MAC chip according to an embodiment of the present invention.  
     [0025]FIG. 6 is a block diagram illustrating an alternative configuration for a MAC layer implementing a CMTS MAC chip according to an embodiment of the present invention.  
     [0026]FIG. 7 is a block diagram illustrating an exemplary embodiment of a distributed CMTS in a hybrid fiber/coaxial (HFC) plant according to an embodiment of the present invention.  
     [0027]FIG. 8 is a flow diagram illustrating a method for determining the placement of a distributed CMTS in a hybrid fiber/coaxial (HFC) plant according to an embodiment of the present invention.  
     [0028] FIGS.  9 - 39  are block diagrams illustrating exemplary embodiments of distributed CMTS configurations in a hybrid fiber/coaxial (HFC) plant. 
    
    
     [0029] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawings in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.  
     DETAIL ED DESCRIPTION OF THE INVENTION  
     [0030] While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art(s) with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
     [0031] Overview of a Cable Modem System  
     [0032]FIG. 1 is a high level block diagram of an example cable modem system  100  in accordance with embodiments of the present invention. The cable modem system  100  enables voice communications, video and data services based on a bi-directional transfer of packet-based traffic, such as Internet protocol (IP) traffic, between a cable system headend  102  and a plurality of cable modems  106 - 108  over a hybrid fiber-coaxial (HFC) cable network  110 . In general, any number of cable modems may be included in the cable modem system of the present invention.  
     [0033] Cable headend  102  is comprised of at least one cable modem termination system (CMTS)  104 . CMTS  104  manages the upstream and downstream transfer of data between cable headend  102  and cable modems  106 - 108 , which are located at the customer premises. CMTS  104  broadcasts information downstream to cable modems  106 - 108  as a continuous transmitted signal in accordance with a time division multiplexing (TDM) technique. Additionally, CMTS  104  controls the upstream transmission of data from cable modems  106 - 108  to CMTS  104  by assigning to each cable modem  106 - 108  short grants of time within which to transfer data. In accordance with this time domain multiple access (TDMA) technique, each cable modem  106 - 108  may only send information upstream as short burst signals during a transmission opportunity allocated to it by CMTS  104 .  
     [0034] As shown in FIG. 1, CMTS  104  further serves as an interface between HFC network  110  and a packet-switched network  112 , transferring IP packets received from cable modems  106 - 108  to packet-switched network  112  and transferring IP packets received from packet-switched network  112  to cable modems  106 - 108  when appropriate. In embodiments, packet-switched network  112  may comprise the Internet, the Intranet, a public switched telephone network, etc.  
     [0035] In addition to CMTS  104 , cable headend  102  may also include one or more Internet routers (not shown) to facilitate the connection between CMTS  104  and packet-switched network  112 , as well as one or more servers (not shown) for performing necessary network management tasks.  
     [0036] HFC network  110  provides a point-to-multipoint topology for the high-speed, reliable, and secure transport of data between cable headend  102  and cable modems  106 - 108  at the customer premises. As will be appreciated by persons skilled in the relevant art(s), HFC network  110  may comprise coaxial cable, fiberoptic cable, or a combination of coaxial cable and fiberoptic cable linked via one or more fiber nodes.  
     [0037] Each of cable modems  106 - 108  operates as an interface between HFC network  110  and at least one attached user device. In particular, cable modems  106 - 108  perform the functions necessary to convert downstream signals received over HFC network  110  into IP data packets for receipt by an attached user device. Additionally, cable modems  106 - 108  perform the functions necessary to convert IP data packets received from the attached user device into upstream burst signals suitable for transfer over HFC network  110 . In example cable modem system  100 , each cable modem  106 - 108  is shown supporting only a single user device  114 - 116 , respectively, for clarity. In general, each cable modem  106 - 108  is capable of supporting a plurality of user devices for communication over cable modem system  100 . User devices may include personal computers, data terminal equipment, telephony devices, broadband media players, network-controlled appliances, or any other device capable of transmitting or receiving data over a packet-switched network.  
     [0038] In example cable modem system  100 , any one or more of cable modems  106 - 108  may represent a conventional DOCSIS-compliant cable modem. In other words, any one or more of cable modems  106 - 108  may transmit data packets to CMTS  104  in formats that adhere to the protocols set forth in the DOCSIS specification. Also, any one or more of cable modems  106 - 108  may be likewise capable of transmitting data packets to CMTS  104  in standard DOCSIS formats. However, in accordance with embodiments of the present invention, any one or more of cable modems  106 - 108  may also be configured to transmit data packets to CMTS  104  using proprietary protocols that extend beyond the DOCSIS specification. Nevertheless, such cable modems are fully interoperable with the DOCSIS-compliant cable modems and with DOCSIS-compliant CMTS equipment.  
     [0039] Furthermore, in example cable modem system  100 , CMTS  104  operates to receive and process data packets transmitted to it in accordance with the protocols set forth in the DOCSIS specification. However, in accordance with embodiments of the present invention, CMTS  104  can also operate to receive and process data packets that are formatted using proprietary protocols that extend beyond those provided by the DOCSIS specification. The manner in which CMTS  104  operates to receive and process data will be described in further detail herein.  
     [0040] Hybrid Fiber/Coaxial Architecture  
     [0041] A hybrid fiber/coaxial (HFC) system (also referred to as a HFC plant) is a bi-directional shared-media transmission system having a configuration that combines both fiber-optic and coaxial cables for handling broadband services. HFC systems use fiber-optic cables between a headend and a plurality of fiber nodes and coaxial cables from the plurality of fiber nodes to a plurality of cable modems or other types of remote subscriber equipment. Such systems are far less expensive than full fiber-to-the-curb (FTTC) or switched digital video (SDV) systems. HFC systems offer increased bandwidth capabilities needed for handling broadband interactive services. Such broadband interactive services may include, but are not limited to, interactive multimedia, telephony, wide-area computer networking, video-on-demand (digital), distance learning, etc. HFC systems also support simultaneous analog and digital transmission with minimal impact on existing plants.  
     [0042] An exemplary HFC system has three main components: (1) network elements, (2) a HFC infrastructure or network, such as HFC network  110  and (3) subscriber access. Network elements are service-specific devices that connect a cable operator to both service origination points and other equipment that places services onto the network. Network elements may include, but are not limited to, local and wide area networks, such as the Intranet and Internet, respectively, IP backbone networks (such as packet switched network  112 ), Public Switched Telephone Networks (PSTN), other remote servers, etc. HFC infrastructure may include, but is not limited to, fiber and coaxial cable, fiber transmitters, fiber nodes, RF amplifiers, taps, and passives. Subscriber access equipment may include, but is not limited to, cable modems, set-top terminals, and units to integrate telephony services.  
     [0043]FIG. 2A illustrates an exemplary high level block diagram of a hybrid fiber/coaxial (HFC) system  200 . HFC system  200  comprises, inter alia, a plurality of primary hubs  202  (A-D), a plurality of secondary hubs  204  (A-C), a plurality of fiber nodes  206  (A-C), a plurality of taps  208  (A-F) and a plurality of cable modems  210  (A-D). Primary hubs  202  are coupled to each other and to secondary hubs  204 . Secondary hubs  204  are coupled to primary hub  202 D, other secondary hubs  204 , and fiber nodes  206 . Fiber node  206 C is coupled to taps  208 . Taps  208  are coupled to cable modems  210 . Although FIG. 2A only illustrates a single branching structure from fiber node  206 C, similar branching structures exist for fiber nodes  206 A and  206 B that service other cable modems in other areas of system  200 . Although not shown, similar coax network branching structures also exist for each connection from fiber node  206 C.  
     [0044] Headend  102  is shown located in one of primary hubs  202 . Headend  102 , primary hubs  202 , secondary hubs  204 , and fiber nodes  206  are interconnected via fiber-optic cables, and therefore represent the fiber portion of HFC system  200 . Everything below fiber nodes  206 , such as taps  208  and modems  210 , are interconnected via coaxial cables, and therefore represent the coaxial portion of HFC system  200 .  
     [0045] Although not shown in FIG. 2A, RF amplifiers may be located between taps  208  and cable modems  210 . In one embodiment, RF amplifiers are bi-directional, requiring only one path between taps  208  and any one cable modem  210  for downstream and upstream transmissions. In an alternative embodiment, RF amplifiers are unidirectional, thereby requiring two paths between taps  208  and any one cable modem  210  to allow for downstream and upstream transmissions, respectively.  
     [0046] Hubs  202  and  204  are communications infrastructure devices to which nodes on a loop are physically connected to improve the manageability of physical cables. Hubs  202  and  204  maintain the logical loop topology of HFC system  200 . In the downstream, hubs  202  and  204  are used to manage the distribution of signals into the plant for delivery to customers at the customer premises. In the upstream, hubs  202  and  204  are used to aggregate signals from the various cable modems  210  for delivery to headend  102 . Hubs  202  and  204  also support the, addition or removal of nodes from the loop while in operation. Primary hubs  202  are differentiated from secondary hubs  204  in that all primary hubs  202  are connected together to form a circle. A link from that circle connects primary hubs  202  to a secondary hub  204 . Secondary hubs  204  may be connected to each other, but not all of secondary hubs  204  need be connected together.  
     [0047] In the topology shown in FIG. 2A, fiber nodes  206  are used to convert optical transmissions into electrical signals for distribution over the coaxial portion of HFC system  200  for downstream transmissions. For upstream transmissions, fiber nodes  206  are used to convert electrical signals into optical signals for transmission over the fiber portion of HFC system  200 .  
     [0048] HFC system  200  originates in headend  102 . Headend  102  obtains information from network sources, such as, for example, packet switched network  112 . Headend  102  distributes the information to hubs  202 ,  204 , and nodes  206  for further distribution to customers that subscribe to such services as CATV, cable phones, Internet via cable, ATM, set top applications, etc. The HFC architecture of system  200  uses fiber to carry voice communications, video and data from headend  102  to fiber nodes  206  for servicing a particular area. At fiber nodes  206 , downstream optical signals are converted to electrical signals and carried via coax to individual subscribers via taps  208 . The carrying capacity of fiber is much higher than that of coax, therefore, a single fiber node  206  may typically support a number of coaxial distribution feeds via taps  208 . Taps  208  allow multiple modems  210  to connect to a single trunk of coax.  
     [0049] When cable operators need additional bandwidth to service cable modems  210  for upstream transmissions, often times they may split fiber nodes  206  to provide increased bandwidth. In other instances they may replicate fiber nodes  206 . The splitting of a fiber node or the replication of a fiber node results in what is termed a post-fiber node. Other terms for post-fiber nodes include, but are not limited to, mini-fiber nodes, micro-fiber nodes, and distributed fiber nodes. FIG. 2B illustrates another exemplary high level block diagram of a HFC system  220 . HFC system  220  in FIG. 2B is similar to HFC system  200  in FIG. 2A, except for the addition of post-fiber nodes  222 . In FIG. 2B, post-fiber nodes  222  are shown coupled to one of fiber nodes  206  and one of taps  208 . As illustrated, the addition of post fiber nodes  222  provides additional bandwidth by lessening the number of cable modems  210  serviced by any one post fiber node  222 . Post fiber node  222 A and  222 B now services the half of cable modems  210  previously serviced by fiber node  206 C.  
     [0050] CMTS  
     [0051] Currently, CMTS units are single units that perform three layers of functions that often overlap. FIG. 3 is a high level block diagram illustrating the three layers of functions in a single CMTS unit  300 . The three layers of functions in CMTS unit  300  include a physical (PHY) layer  302 , a media access control (MAC) layer  304 , and a network layer  306 .  
     [0052] PHY layer  302  enables CMTS unit  300  to physically communicate with subscriber access equipment, such as cable modems  210 . PHY layer  302  transmits and receives signals to and from cable modems  210 , respectively. PHY layer  302  converts electronic signals into digital bits for upstream transmissions to MAC layer  304  and converts digital bits from MAC layer  304  into electronic signals for downstream transmissions.  
     [0053] Media access control layer (MAC layer)  304  is the messaging layer of CMTS  300 . MAC layer  304  decodes the bits from physical layer  302  into packets. If the packets are to be communicated to networks outside HFC system  200  or  220  or are for use in aiding network layer  306  in the performance of its functions, MAC layer  304  will send the packets to network layer  306 . MAC layer  304  also acts as a control mechanism for cable modems  210  communicating with CMTS  300 . Packets that are not communicated to network layer  306  are control packets. Control packets are used to: (1) perform ranging to compensate for different cable losses and cable delays to make sure that bursts coming from different cable modems  210  line up in the right time-slots and are received at the same power level at the CMTS; (2) assign frequencies to cable modems  210 ; and (3) allocate time-slots for upstream transmission.  
     [0054] Network layer  306  interfaces external network devices and internal packet sources. Network layer  306  establishes, maintains, and terminates logical and physical connections between interconnected networks, such as packet switched network  112 . Network layer  306  receives packets from MAC layer  304  for transmission to external network devices. Network layer  306  also receives packets from external network devices for transmission to cable modems  210  via MAC and PHY layers  304  and  302 , respectively. Network layer  306  prioritizes packets, maintains packet rates and controls packet flow. Network layer  306  also performs network functions, such as, but not limited to, routing, bridging, quality of service (QoS), etc.  
     [0055] Conventional CMTS units, such as CMTS unit  300 , may not be split according to functionality. In other words, CMTS units  300  are not modularized according to functionality and, therefore, must contain all three functional layers (i.e., physical, MAC, and network layers) in a single unit. There is some modularity in current CMTS units  300 , but this modularity allows features to be added to CMTS  300 .  
     [0056] Current HFC plants provide a centralized location for conventional CMTS units  300 . This centralized location is typically in headend  102 , as shown in FIGS. 2A and 2B. With the location of CMTS  300  in headend  102 , upstream signals are not converted into digital bits until they reach headend  102 . Thus, RF signals are transmitted from cable modems  210  to fiber nodes  206  and optical signals are transmitted from fiber nodes  206  to headend  102 .  
     [0057] Other centralized locations for CMTS units may include primary hub  202  or secondary hub  204 . If CMTS  300  is located in a primary hub  202 , upstream signals are converted into digital bits in primary hub  202 . Thus, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to primary hub  202 , and digital signals are transmitted thereafter. If CMTS  300  is located in a secondary hub  204 , upstream signals are converted into digital bits in secondary hub  204 . Thus, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 , and digital signals are transmitted thereafter.  
     [0058] Distributed CMTS  
     [0059] The present invention provides functional modularity to CMTS units, and enables the functional units of the CMTS to be dispersed throughout an HFC plant in a modular fashion to provide additional bandwidth to subscriber access equipment, such as cable modems  210 . Distributing the CMTS away from the headend and further into the HFC network provides improved data throughput. For example, a PHY layer converts electronic signals into digital bits during upstream transmissions. Moving the PHY layer away from the headend, deeper into the HFC plant enables more traffic to be sent in digital streams over the fiber portion of the plant. After the PHY layer converts the signals into digital bits, the bits can be sent in a digital format that is far more tolerant to noise. These digital streams may be aggregated to maximize the throughput of any given link in the HFC plant. Having digital traffic on the fiber links provides improved fiber efficiency by enabling more of the fiber to be used to carry traffic. It also allows many different digital transmission techniques to be used. Digital transmission techniques can be used to optimize the cost of the network and therefore make the fiber more cost efficient. Also, moving the PHY layer closer to the subscriber equipment (e.g., modems) reduces analog noise between the CMTS and the subscriber equipment (e.g., cable modems).  
     [0060]FIG. 4 is a block diagram illustrating a distributed CMTS according to an embodiment of the present invention. Distributed CMTS  400  comprises a physical (PHY) layer  402 , a media access control (MAC) layer  404 , and a network layer (NF)  406 . PHY layer  402 , MAC layer  404 , and network layer  406  are each separate modules, capable of performing their respective functions (as described above with reference to FIG. 3). PHY layer  402  is coupled to MAC layer  404 , and MAC layer  404  is coupled to NF layer  406 . The individual functionality of each of layers  402 ,  404 , and  406  combine to perform the total functionality of a traditional CMTS unit, such as CMTS unit  300 . The difference being that each of layers  402 ,  404 , and  406  are not restricted to one location, but may be distributed throughout HFC plants, such as exemplary HFC plants  200  and  220 .  
     [0061] In one embodiment of the present invention, a CMTS MAC chip may be implemented to enable distributed CMTS  400 . The CMTS MAC chip may be a BCM3212 CMTS MAC chip, a BCM3210 CMTS MAC chip, both of which are manufactured by Broadcom Corporation in Irvine, Calif., or any other CMTS MAC chip that includes DOCSIS MAC functionality as well as the capability of operating in a distributed CMTS environment. DOCSIS has the ability to split packets, fragment and concatenate packets, perform header suppression, etc. The CMTS MAC chip performs these DOCSIS functions automatically. For example, if a packet is fragmented, the CMTS MAC chip will wait for all the pieces of the packet to arrive, construct the packet, and send the packet to a control mechanism for further processing. The CMTS MAC chip also has a set of features that enable it to be put in a distributed CMTS. Thus, the CMTS MAC chip eliminates the need for MAC layer  404  to be co-located with PHY layer  402  or network layer  406 . In other words, the CMTS MAC chip enables MAC layer  404  to be miles away from either PHY layer  402  and/or network layer  406 .  
     [0062]FIG. 5 is a block diagram illustrating a distributed CMTS  400  implementation using a CMTS MAC chip. FIG. 5 focuses on PHY layer  402  and a configuration  500  of MAC layer  404  in which a CMTS MAC chip  510  is implemented.  
     [0063] PHY layer  402  includes a downstream module  502 , an upstream module  504 , and a PHY subsystem  506 . Downstream module  502  forms the physical interface between CMTS  400  and the downstream channel(s) of HFC system  200  or  220 . Hence, voice, data (including television or radio signals) and/or control messages that are destined for one or more cable modems  210  are collected at downstream module  502  and transmitted to the respective cable modem  210 . Thus, downstream module  502  compresses and/or formats all information for downstream transmission. Upstream module  504  forms the physical interface between CMTS  400  and the upstream channel(s) of cable modems  210 . All bursts from cable modems  210  are received at upstream module  504 . Upstream module  504  processes the bursts to decompress and/or extract voice, video, data, and/or the like from cable modems  210 . PHY subsystem  506  interacts with both upstream module  504  and downstream module  502  to convert electrical signals into digital bits and vice versa.  
     [0064] MAC layer  404  includes CMTS MAC chip  510 , a CPU  512 , buffer RAMs  514  and  516 , and a network interface subsystem  518 . CMTS MAC chip  510  is coupled to CPU  512  and buffer RAM  514 . CPU  512  is coupled to buffer RAM  516  and network interface subsystem  518 . CMTS MAC chip  510  interfaces with PHY layer  402  and provides the timing to maintain the components of PHY layer  402 . All data coming in to CMTS MAC chip  510  from PHY layer  402  goes through CPU  512 . CMTS MAC chip  510  processes and buffers upstream packets. CPU  512 , in operation with CMTS MAC chip  510 , extracts the buffered upstream packets from memory. CPU  512  then transmits the packets to network layer  406  via network interface subsystem  518 . In embodiments, a few of the network functions performed by network layer  406  may be performed in CPU  512  to make for easier digital transport. Network interface subsystem  518  interfaces to network layer  406  and/or other portions of MAC layer  404  and network layer  406 .  
     [0065] With this implementation of CMTS MAC chip  510  described above, CMTS MAC chip  510  does not require packet level MAC functions to be implemented in the same physical location as CMTS MAC chip  510 . CMTS MAC chip  510  is also not required to be local to network layer  406 . This enables implementation of MAC chip  510  in a distributed CMTS. Note that timing interface constraints between MAC chip  510  and PHY layer  402  components  502 ,  504 , and  506  require CMTS MAC chip  510  to be implemented in closer proximity to PHY layer  402  when implementing a BCM3210 MAC chip vs. a BCM3212 MAC chip.  
     [0066] An alternative configuration  600  for MAC layer  404  is shown in FIG. 6. MAC layer configuration  600  is similar to MAC layer configuration  500  except that CMTS MAC chip  510  is also coupled to network layer interface  518 . In this embodiment, CMTS MAC chip  510  includes a packet portal feature that enables CMTS MAC chip  510  to process all packets destined for network layer  406  and send them directly to network layer interface  518  without burdening CPU  512 . Bypassing CPU  512  results in a faster throughput, but prevents conditioning of the packets that would ordinarily be performed by CPU  512 . This embodiment therefore requires the conditioning normally performed by CPU  512  to be performed by network layer  406 . In this embodiment, CMTS MAC chip  510  may be a BCM 3212 or any other CMTS MAC chip that provides an extra layer of encapsulation to allow a packet to pass to a traditional packet network.  
     [0067] With the CMTS MAC chip  510  implementation shown in FIG. 6, network functions are not required to be local, thereby allowing CMTS MAC chip  510  to be implemented in distributed CMTS  400 . CMTS MAC chip  510  also offers a timing offset feature that enables it to handle timing delays between itself and PHY layer  402 . This enables PHY layer  402  to be remotely located from MAC layer  404 .  
     [0068] As previously stated, the present invention modularizes functional layers  402 ,  404 , and  406  of CMTS  400  (as described in FIG. 4) and distributes functional layers  402 ,  404 , and  406  of CMTS  400  throughout an HFC system, such as HFC system  200  or  220 . Moving distributed CMTS  400  closer to the subscriber access equipment, such as, for example, cable modems  210 , reduces analog noise that exists between the CMTS and the subscriber access equipment. Also, more traffic can be sent in digital streams. The digital streams may be aggregated to maximize the throughput of any given link in HFC system  200  or  220 . By having digital traffic on the fiber links, more of the fiber can be used to carry traffic. Also, many different digital transmission techniques may be used.  
     [0069] Determining the best distributed CMTS for a given cable plant is a function of the existing equipment and/or new equipment that will be added to the existing plant. The most beneficial layer to move is PHY layer  402 . PHY layer  402  is bounded in its throughput by the DOCSIS specification. DOCSIS specifies a given set of bandwidth, modulation techniques, and other physical parameters that limit the amount of bandwidth in an upstream spectrum. For example, the North American version of DOCSIS limits the upstream spectrum to 5-42 MHz. A cable plant operator must divide the 5-42 MHz spectrum into upstream channels. Each upstream channel has a fixed bandwidth. DOCSIS specifies that the symbol rate of an upstream channel may be one of 160K, 320K, 640K, 1280K, 2560K, and 5120K symbols per second. The cable plant operator will assign these symbol rates to the spectrum in an efficient manner. The symbol rate defines the total number of channels in the set of spectrum. The symbol rate does not affect the total throughput. For example, a symbol rate of 160K symbols per second requires 200 KHz. A symbol rate of 320K symbols per second requires 400 KHz. Therefore, in 400 KHz a cable operator could have a single 320K symbol per second channel or two (2) 160K symbols per second channels. The total symbols per second would be 320K in either case.  
     [0070] Throughput is a function of symbols per second as well as bits per symbol. DOCSIS allows for several modulation types: QAM4, QAM8, QAM16, QAM32, QAM64. Each modulation type provides a different number of bits per symbol, as shown below in Table 1.  
                           TABLE 1                                   Modulation Type   Bits per Symbol                                                    QAM4   2           QAM8   3           QAM16   4           QAM32   5           QAM64   6                      
 
     [0071] Any given set of spectrum may not have enough noise immunity to allow the higher orders of modulation (e.g., QAM32 and QAM64). The cable plant operator will divide the spectrum into upstream channels and try to maximize the modulation type per channel. The cable plant operator will then assign cable modems to upstream channels. Using a traditional CMTS, such as CMTS unit  300 , the entire system shown in FIGS.  2 A and/or  2 B would have to be contained in a 5-42 MHz spectrum. As PHY layer  402  moves closer to modems  210  in HFC network  110 , each PHY layer  402  supports fewer modems  210 . Once PHY layer  402  is moved from headend  102  out to fiber node  206  and beyond, the number of PHY layers  402  increases, thereby increasing the system bandwidth. If PHY layer capacity exceeds what a single MAC layer  404  can handle, then MAC layer  404  will also be moved to accommodate the additional MAC layers  404  needed to handle the PHY layer capacity.  
     [0072] For example, an embodiment of distributed CMTS  400 , shown in FIG. 7, may place network layer  406  and MAC layer  404  in headend  102  and a PHY layer  402  in each of fiber nodes  206 . A cable system comprising 40 fiber nodes  206  would require  40  PHY layers  402 . Only one network layer  406  and one MAC layer  404  would be required. The total amount of bandwidth on the cable modem side of fiber nodes  206  would be increased by a factor of 40, yet the total cost would not increase by a factor of 40. PHY layer  402  increased by a factor of 40, but MAC layer  404  and network layer  406  did not increase.  
     [0073] As previously stated, PHY layer  402  sends a stream of bits to MAC layer  404 . This stream of bits must not be delayed in arriving at MAC layer  404 . Any artificial delay between PHY layer  402  and MAC layer  404  may cause the system to be incompatible with DOCSIS. Therefore, MAC layer  404  must be placed in a location that enables signals from PHY layer  402  to be received by MAC layer  404  in a timely fashion. The communication channel between PHY layer  402  and MAC layer  404  needs to be a dedicated worst-case bandwidth channel. This may also be a factor in determining where to place MAC layer(s)  404 . Also, the number of PHY layers  402  to be serviced by MAC layer(s)  404  is another factor that may dictate the number of MAC layer(s)  404  needed and where each MAC layer  404  must be placed.  
     [0074] For instance, in the above example, increasing PHY layer capacity by a factor of 40 may be too much for a single MAC layer  404  to handle. The cable operator will then have to decide how far out into HFC system  200  or  220  to move MAC layer  404 . Depending on the MAC layer components, there may be a fixed MAC to PHY ratio that must be supported. If this is the case, this will dictate how many MAC layers  404  are required. The cable plant operator may then move MAC layer  404  into HFC system  200  or  220  to support the PHY layer bandwidth requirements.  
     [0075] The channel between MAC layer  404  and network layer  406  is not nearly as constrained. There are limits as to how latent this channel can be, but packet buffering is acceptable in this channel. The channel between MAC layer  404  and network layer  406  resembles an Internet channel. Multiple links between network layer  406  and MAC layer  404  can be aggregated to make the most of the channel&#39;s bandwidth. To leverage digital channel technology, other optical components may be required. If the optical components are in place deeper in the HFC system, the operator may push MAC layer  404  and network layer  406  deeper into HFC system  200  or  220 . If the optical components do not exist and there is a budget to improve the HFC system, then the operator may pull these layers back toward the headend. The cost of bandwidth space is vast. This allows for many versions of distributed CMTS  400  in a HFC system.  
     [0076]FIG. 8 is a flow diagram  800  illustrating a method for determining the placement of distributed CMTS  400  in a hybrid fiber/coax plant according to an embodiment of the present invention. The invention, however, is not limited to the description provided by flow diagram  800 . Rather, it will be apparent to persons skilled in the relevant art(s) from the teachings provided herein that other functional flows are within the scope and spirit of the present invention. The process begins in step  802 , where the process immediately proceeds to step  804 .  
     [0077] In step  804 , an assessment of the system is made. The assessment includes a determination of customer bandwidth requirements and a review of the current system configuration. The assessment may also include a review of any new equipment that will be added to the existing plant. The process then proceeds to step  806 .  
     [0078] In step  806 , a determination is made as to where to place PHY layer  402  in HFC system  200  or  220  to satisfy customer bandwidth requirements. The placement of PHY layer  402  will also determine the number of PHY layers  402  needed to provide adequate bandwidth to cable modems  210 . For example, if PHY layer  402  is placed within a first or second hub, then only one PHY layer  402  is needed. Alternatively, if it is determined that PHY layer  402  needs to be placed in fiber node  206 , then multiple PHY layers will be needed, one for each fiber node in the system. The process then proceeds to step  808 .  
     [0079] In step  808 , a determination is made as to where to place MAC layer  404 . As previously stated, transmission delays between PHY layer  402  and MAC layer  404  must be nonexistent. Thus, the location of PHY layer(s)  402  is used to determine the maximum distance allowable to place MAC layer  404  without causing transmission delays. Also, the number of PHY layers placed in step  806  is used to determine the number of MAC layers needed. For example, if 10 PHY layers  402  are placed in step  806  and a single MAC layer  404  can only service  2  PHY layers  402 , then at least 5 MAC layers  404  will be needed, and depending on the location of PHY layers  402 , possibly 10 MAC layers  404  will be needed due to the point-to-multipoint configuration of the network.  
     [0080] In step  810 , network layer  406  is placed. Although the constraints on the location of network layer  406  with respect to the location of MAC layer  404  are minimal, latency limits must be adhered to in order that distributed CMTS  400  operate according to DOCSIS specifications. The process then proceeds to step  812 , where the process ends.  
     [0081] Hybrid fiber/coaxial (HFC) systems may be arranged using a plurality of configurations. Thus, numerous embodiments of distributed CMTS  400  may exist for each cable network. Whether any given embodiment of distributed CMTS  400  will work with any given HFC system will depend on the configuration of the HFC system and the distance between various components of the HFC system. Various embodiments of distributed CMTS  400  will now be described with reference to exemplary HFC systems  200  and  220  (described above with reference to FIGS. 2A and 2B). Although HFC systems  200  and  220  are used to provide a plurality of distributed CMTS configurations, the distributed CMTS configurations presented are not to be limited by HFC systems  200  and  220 . One skilled in the art would know that other distributed CMTS configurations are possible depending on the configuration of the HFC system in which the distributed CMTS is to implemented.  
     [0082] FIGS.  9 - 12  illustrate distributed CMTS configurations in which network and MAC layers  406  and  404  reside in headend  102 , and PHY layer  402  is distributed across the fiber portion of HFC systems  200  and  220 . In FIG. 9, PHY layer  402  resides in primary hub  202 D. In this configuration of distributed CMTS  400 , one network layer  406 , one MAC layer  404 , and one PHY layer  402  are used. This configuration enables digital transmissions in the upstream to begin at primary hub  202 D. Although PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C in system  200 , enabling digital transmission to begin further out from headend  102  will lessen the noise. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to primary hub  202 D, and digital signals are transmitted thereafter.  
     [0083] In FIG. 10, PHY layer  402  resides in secondary hub  204 . This configuration of distributed CMTS  400  also uses one network layer  406 , one MAC layer  404 , and one PHY layer  402 . In this embodiment, upstream digital transmission begins at secondary hub  204 C. With this embodiment, PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C in system  200 , but lessens the analog noise level by enabling digital transmissions in the upstream to occur at an earlier time within the network. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 C, and digital signals are transmitted thereafter.  
     [0084] In FIG. 11, PHY layer  402  resides in fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer cable modems  210 , causing the system bandwidth to increase and the analog noise level to decrease. For example, each PHY layer  402  shown in FIG. 11 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 . In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0085] In FIG. 12, PHY layer  402  resides in post fiber node  222 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 12 moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 12 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0086] FIGS.  13 - 16  illustrate distributed CMTS configurations in which network layer  406  resides in headend  102 , MAC layer  404  resides in primary hub  202 D, and PHY layer  402  is distributed across the fiber portion of hybrid fiber/coaxial system  200  or  220 . In FIG. 13, PHY layer  402  resides in primary hub  202 D. In this configuration of distributed CMTS  400 , one network layer  406 , one MAC layer  404 , and one PHY layer  402  are used. This configuration enables digital transmissions in the upstream to begin at primary hub  202 D. Although PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C in system  200 , enabling digital transmission to begin further out from headend  102  will lessen the noise. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to primary hub  202 D, and digital signals are transmitted thereafter.  
     [0087] In FIG. 14, PHY layer  402  is located in secondary hub  204 C. This configuration of distributed CMTS  400  also uses one network layer  406 , one MAC layer  404 , and one PHY layer  402 . With this embodiment, PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C in system  200 , but lessens the analog noise level by enabling digital transmissions in the upstream to occur at an earlier time within the network. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 C, and digital signals are transmitted thereafter.  
     [0088] In FIG. 15, PHY layer  402  is located in fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer modems, causing the system bandwidth to increase and the analog noise level to decrease. For example, each PHY layer  402  shown in FIG. 15 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 . In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0089] In FIG. 16, PHY layer  402  is placed in post fiber nodes  222 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 16 moves PHY layers  402  closer to modems  210 , thereby enabling each PHY layer to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 16 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0090] FIGS.  17 - 19  illustrate distributed CMTS configurations in which network layer  406  resides in headend  102 , MAC layer  404  resides in secondary hub  202 D, and PHY layer  402  is distributed further into the fiber portion of hybrid fiber/coaxial system  200  and  220 . In FIG. 17, PHY layer  402  is co-located with MAC layer  404  in secondary hub  202 C. This configuration of distributed CMTS  400  uses one network layer  406 , one MAC layer  404 , and one PHY layer  402 . In this embodiment, upstream digital transmission begins at secondary hub  204 C. PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C, but lessens the analog noise level by enabling digital transmissions in the upstream to occur at an earlier time within the network. In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 C, and digital signals are transmitted thereafter.  
     [0091] In FIG. 18, PHY layer  402  is located in fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer cable modems  210 , causing the system bandwidth to increase and the analog noise level to decrease. For example, each PHY layer  402  shown in FIG. 18 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 . In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0092] In FIG. 19, PHY layer  402  is located in post fiber nodes  212 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 19 moves PHY layers  402  closer to modems  210 , thereby enabling each PHY layer to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 19 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0093]FIGS. 20 and 21 illustrate distributed CMTS configurations in which network layer  406  resides in headend  102 , MAC layer  404  resides in fiber nodes  206 , and PHY layer  402  is distributed in the fiber portion of hybrid fiber/coaxial systems  200  and  220 . In FIG. 20, PHY layer  402  is co-located with MAC layer  404  in fiber node  206 C. In this embodiment, one network layer  406  is implemented, and multiple MAC and PHY layers  404  and  402  are implemented, one MAC layer  404  and one PHY layer  402  for each fiber node  206 . This configuration is utilized when one MAC layer  404  cannot adequately handle the number of PHY layers  402  required. Using a plurality of PHY layers  402  at fiber nodes  206  requires each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 20 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. This increases system bandwidth and decreases analog noise levels. Digital transmissions in the upstream now begin at fiber nodes  206 . This configuration enables RF signals to be transmitted from cable modems  210  to fiber nodes  206  and digital signals to be transmitted therefrom.  
     [0094] In FIG. 21, PHY layer  402  is located in post fiber node  222 . In this embodiment, one network layer  406  is implemented and multiple MAC and PHY layers  404  and  402 , respectively, are implemented. A MAC layer  404  is placed in each fiber node  206  and a PHY layer  402  is placed in each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each fiber node  206  to handle the number of PHY layers  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 21 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0095]FIG. 22 illustrates distributed CMTS  400  wherein network layer  406  resides in headend  102  and MAC layers  404  and PHY layers  402  reside in post fiber nodes  222 . In this configuration, one MAC layer  404  and one PHY layer  402  are needed for each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each post fiber node  222  to handle each PHY layer  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 22 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0096] The previous examples of distributed CMTS configurations described above all had at least one distributed CMTS layer residing in headend  102 . The remaining examples of distributed CMTS configurations have pushed all layers  402 ,  404 , and  406  of distributed CMTS  400  away from headend  102 .  
     [0097] FIGS.  23 - 25  illustrate distributed CMTS configurations in which network and MAC layers  406  and  404 , respectively, reside in primary hub  202 D and PHY layer  402  is distributed across the fiber portion of hybrid fiber/coaxial systems  200  and  220 . In FIG. 23, PHY layer  402  resides in secondary hub  204 C. In this configuration of distributed CMTS  400 , one network layer  406 , one MAC layer  404 , and one PHY layer  402  are used. With this embodiment, PHY layer  402  services all cable modems  210  attached to each of fiber nodes  206 A-C in system  200 , but lessens the analog noise level by enabling digital transmissions in the upstream to occur at an earlier time within the network. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 C, and digital signals are transmitted thereafter.  
     [0098] In FIG. 24, PHY layer  402  resides in each of fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer cable modems  210 , causing the system bandwidth to increase and the analog noise level to decrease. For example, each PHY layer  402  shown in FIG. 24 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 . In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0099] In FIG. 25, PHY layer  402  resides in post fiber nodes  222 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 25 moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 25 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0100] FIGS.  26 - 28 , illustrate distributed CMTS configurations in which network layer  406  resides in primary hub  202 D, MAC layer  404  resides in secondary hub  204 C, and PHY layer  406  is distributed across the fiber portion of hybrid fiber/coaxial system  220 . In FIG. 26, PHY layer  402  is placed in secondary hub  204 C. This configuration of distributed CMTS  400  uses one network layer  406 , one MAC layer  404 , and one PHY layer  402 . With this embodiment, PHY layer  402  services all cable modems  210  attached to fiber nodes  206 A-C in system  200 , but lessens the analog noise level by enabling digital transmissions in the upstream to occur at an earlier time within the network. Thus, in the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206 , optical signals are transmitted from fiber nodes  206  to secondary hub  204 C, and digital signals are transmitted thereafter.  
     [0101] In FIG. 27, PHY layer  402  is located in each of fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer cable modems  210 , causing the system bandwidth to increase and the analog noise level to decrease. For example, each PHY layer  402  shown in FIG. 27 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 . In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0102] In FIG. 28, a PHY layer  402  is placed in each post fiber node  222 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 25 moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 28 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0103] FIGS.  29 - 30  illustrate distributed CMTS configurations in which network layer  406  resides in primary hub  202 D, MAC layer  404  resides in fiber nodes  206 , and PHY layer  402  is distributed within the fiber portion of HFC system  220 . In FIG. 29, PHY layer  402  is co-located with MAC layer  404  in fiber nodes  206 . In this embodiment, one network layer  406  is implemented, and multiple MAC and PHY layers  404  and  402 , respectively, are implemented, one MAC layer  404  and one PHY layer  402  for each fiber node  206 . This configuration is utilized when one MAC layer  404  cannot adequately handle the number of PHY layers  402  required. Using a plurality of PHY layers  402  at fiber nodes  206  requires each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 29 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. This increases system bandwidth and decreases analog noise levels. Digital transmissions in the upstream now begin at fiber nodes  206 . This configuration enables RF signals to be transmitted from cable modems  210  to fiber nodes  206  and digital signals to be transmitted therefrom.  
     [0104] In FIG. 30, PHY layer  402  is placed in each post fiber node  222 . In this embodiment, one network layer  406  is implemented and multiple MAC and PHY layers  404  and  402 , respectively, are implemented. A MAC layer  404  is placed in each fiber node  206  and a PHY layer  402  is placed in each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each fiber node  206  to handle the number of PHY layers  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 30 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0105] FIGS.  31 - 32  illustrate distributed CMTS configurations in which network and MAC layers  406  and  404 , respectively, reside in secondary hub  204 C and PHY layer  402  is distributed further into the fiber portion of HFC system  220 . In FIG. 31, a PHY layer is placed in each of fiber nodes  206 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each fiber node  206  in HFC system  200 . This configuration requires one network layer  406 , one MAC layer  404 , and a PHY layer  402  for each fiber node in the system. As the number of PHY layers  402  increases, each PHY layer  402  supports fewer cable modems  210 , causing the system bandwidth to increase. For example, each PHY layer  402  shown in FIG. 31 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. Digital transmissions now begin at fiber nodes  206 , causing the analog noise level to decrease. In the upstream, RF signals are transmitted from cable modems  210  to fiber nodes  206  and digital signals are transmitted thereafter.  
     [0106] In FIG. 32, a PHY layer is placed in each of post fiber nodes  222 . In this configuration of distributed CMTS  400 , a PHY layer  402  is needed for each post fiber node  222  that connects to fiber nodes  206 A-C. This configuration requires one network layer  406 , one MAC layer  404 , and a plurality of PHY layers  402 , one for each post fiber node  222  in the system. The configuration shown in FIG. 32 moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 32 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This also causes the system bandwidth to increase. In the upstream, digital transmission begins at post fiber nodes  222 , thereby decreasing any analog noise resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted thereafter.  
     [0107]FIGS. 33 and 34 illustrate distributed CMTS configurations in which network layer  406  is placed in secondary hub  204 C, MAC layer  404  is placed in fiber nodes  206 , and PHY layer  402  is distributed among fiber nodes  206  or post fiber nodes  222 . In FIG. 33, PHY layer  402  is co-located with MAC layer  404  in fiber nodes  206 . In this embodiment, one network layer  406  is implemented, and multiple MAC and PHY layers  404  and  402 , respectively, are implemented, one MAC layer  404  and one PHY layer  402  for each fiber node  206 . This configuration is utilized when one MAC layer  404  cannot adequately handle the number of PHY layers  402  required. Using a plurality of PHY layers  402  at fiber nodes  206  requires each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 33 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. This increases system bandwidth and decreases analog noise levels. Digital transmissions in the upstream now begin at fiber nodes  206 . This configuration enables RF signals to be transmitted from cable modems  210  to fiber nodes  206  and digital signals to be transmitted therefrom.  
     [0108] In FIG. 34, a PHY layer  402  is placed in each of post fiber nodes  222 . In this embodiment, one network layer  406  is implemented and multiple MAC and PHY layers  404  and  402 , respectively, are implemented. A MAC layer  404  is placed in each fiber node  206  and a PHY layer  402  is placed in each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each fiber node  206  to handle the number of PHY layers  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 34 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0109]FIG. 35 illustrates a distributed CMTS configuration in which network layer  406  is placed in secondary hub  204 C and a MAC and a PHY layer  404  and  402 , respectively, are placed in each of post fiber nodes  222 . Thus, one MAC layer  404  and one PHY layer  402  are needed for each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each post fiber node  222  to handle each PHY layer  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 35 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0110] FIGS.  36 - 39  illustrate configurations of distributed CMTS  400  that require multiple layers of each module of distributed CMTS  400 . Although these configurations provide large amounts of additional bandwidth to service the attached cable modems  210  as well as provide reductions in noise, the cost of equipment needed to service the cable modems  210  is expensive.  
     [0111]FIG. 36 requires network layer  406  and MAC layer  404  to reside in each fiber node  206  and PHY layer  402  to reside in each post fiber node  222 . Thus, one network layer  406  and one MAC layer  404  are required by each of fiber nodes  206 A-C and one PHY layer  402  is required for each of post fiber nodes  222 A-D. In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each fiber node  206 A-C to handle each PHY layer  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 36 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0112]FIG. 37 requires network layer  406  to reside in each fiber node  206  and MAC layer  404  and PHY layer  402  to reside in each post fiber node  222 . Thus one network layer  406  is placed in each of fiber nodes  206 A-C and one MAC layer  404  and one PHY layer  402  are placed in each of post fiber nodes  222 A-D. In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each post fiber node  222  to handle each PHY layer  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 37 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0113]FIG. 38 illustrates a configuration of distributed CMTS  400  wherein distributed CMTS  400  resides solely in each of fiber nodes  206 . Thus, one network layer  406 , one MAC layer  404  and one PHY layer  402  are placed in each of fiber nodes  206 . This configuration may be utilized when one MAC layer  404  cannot adequately handle the number of PHY layers  402  required. Using a plurality of PHY layers  402  at fiber nodes  206  requires each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 38 services cable modems  210  attached to one of fiber nodes  206 A,  206 B, or  206 C. This increases system bandwidth and decreases analog noise levels, but at a high cost since multiple layers of each component of distributed CMTS are required. Digital transmissions in the upstream now begin at fiber nodes  206 . This configuration enables RF signals to be transmitted from cable modems  210  to fiber nodes  206  and digital signals to be transmitted therefrom.  
     [0114]FIG. 39 illustrates a configuration of distributed CMTS  400  wherein distributed CMTS  400  resides solely in post fiber nodes  222 . In this embodiment, each of layers  406 ,  404 , and  402  reside in each post fiber node  222 . Although this embodiment provides a large amount of additional bandwidth to service the attached cable modems, such an embodiment may be expensive since it requires a distributed CMTS  400  for each post fiber node  222 . In this embodiment, a single MAC layer would be unable to handle the requirements of the required number of PHY layers. Therefore, MAC layers  404  are placed at each post fiber node  222  to handle each PHY layer  402  placed at each post fiber node  222 . This configuration moves PHY layers  402  closer to cable modems  210 , thereby enabling each PHY layer  402  to support fewer cable modems  210 . For example, each PHY layer  402  shown in FIG. 39 services cable modems  210  attached to one of post fiber nodes  222 A,  222 B,  222 C or  222 D. This implementation causes the system bandwidth to increase. Digital transmission in the upstream begins at post fiber node  222 , thereby decreasing any analog noise signals resulting from interference sources in the coaxial cables. RF signals are transmitted from cable modems  210  to post fiber nodes  222  and digital signals are transmitted therefrom.  
     [0115] Although a plurality of different distributed CMTS configurations have been shown above, these configurations are not exhaustive. One skilled in the relevant art(s) would know that various other configurations may be utilized without departing from the scope and spirit of the present invention.  
     [0116] Conclusion  
     [0117] The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.