This invention relates to data communication, and more particularly, to data communication over cable television (CATV) networks.
Cable television systems are well known. These systems are usually comprised of a headend with one or more trunk lines extending therefrom with each trunk line having a plurality of feeder lines extending therefrom into subscriber areas where each subscriber is attached via a line tap onto the feeder or service line. If the distances between the headend and subscriber areas are substantial, intervening distribution hubs may be located along the trunk lines to replenish the strength and quality of the signal being provided to subscribers. Distribution hubs simply act as small headends and exist to ensure the quality of delivered signal in large CATV networks. Each distribution hub may, in turn, be coupled to a plurality of service sites by feeder lines. Each service site may have one or more service lines extending therefrom to couple a plurality of subscribers to the service site. In this network, a transmission signal is provided over the trunk lines to the distribution hubs or service hubs. This amplified signal is then provided to the feeder lines extending from the distribution hub or service hub to provide the signal to the service sites. If the distance between a distribution hub and service site is so great as to erode signal strength to an unusable level, another distribution hub may be interposed between the service site and first distribution hub to amplify the signal strength again. Amplification occurs along trunk, feeders, and service lines as necessary to maintain the transmission signal at an adequate level before being provided to subscriber equipment. Taps located at each subscriber site bring the transmission signal into a subscriber""s site.
The transmission signal from the headend may include entertainment signals and data signals. The entertainment signals may be received as broadcast signals received via satellite from a broadcast signals originating location. At the headend, each broadcast signal is placed on its own channel within the spectrum of the trunk, feeder and service lines used in the CATV system. The spectrum of the lines coupling the CATV system together is the range of frequencies supported by the communication conduits used for the lines. In a typical CATV system, this spectrum is divided into a transmission portion and a return portion. The return portion of the spectrum may be used to support data transmissions, telemetry, and/or control information from subscriber sites back to the headend. The data transmissions from subscribers typically include status information about the subscriber""s equipment which may be used by components at the headend to ascertain the status of the cable system or subscriber equipment. The most common types of spectrum splitting methods are called sub-split, mid-split and high split. Sub-split means a lower portion of the spectrum smaller than the transmission spectrum is available for the return spectrum. Mid-split means that the spectrum is allocated one-half to the transmission portion and one-half to the return spectrum. High split means an upper portion of the spectrum smaller than the return spectrum is used for the transmission spectrum.
At the headend, each broadcast signal is allocated to a channel in the transmission spectrum. In a sub-split system, the first channel in the transmission spectrum begins at 55 MHz, for example. The width of the channel varies according to the standard used for the system. In the United States, most CATV systems use National Television System Committee (NTSC) standard which allocates 6 MHz to each channel. In Europe, the Phase Alteration Line (PAL) standard is used which allocates 8 MHz to each channel. The frequency of a broadcast signal may be up-shifted or down-shifted to place the broadcast signal on one of the channels of the transmission portion of the spectrum of the transmission signal provided by the headend. The data signals at the headend may be received from one or more digital data sources (including subscriber equipment) and these signals may also be placed on a channel in the transmission signal for distribution through the network. Typically, display devices such as televisions or the like at the subscriber sites use the broadcast signals to generate audio and video while data devices such as cable modems, or other intelligent devices, convert the data signals for use by computers or the like.
The trunk, feeder and service lines of many CATV systems are all coaxial cables. Because the signals carried by coaxial cables are electrical, these systems are susceptible to electrical and electromagnetic noise from natural phenomena and other electrical or magnetic sources. In an effort to improve the clarity of the signals carried over a CATV system, coaxial cables used for trunk and feeder lines are being replaced by fiber optic cables. Because fiber optic cable carries light signals, the signals are less susceptible to electrical and electromagnetic noise from other sources. Additionally, fiber optic cables carry signals for longer distances without appreciable signal strength loss than coaxial cable. However, the cost of replacing coaxial cable with fiber optic cable has prevented many companies from converting their service lines to fiber optic cable. CATV systems having both fiber optic trunk and feeder lines along with coaxial service lines are typically called hybrid fiber cable (HFC) systems. In HFC systems, the service sites where the light signal from a fiber optic cable is converted to an electrical signal for a coaxial service line is called a fiber node.
Previously known CATV systems have limitations for supporting data communication in the return spectrum of a system. In a typical sub-split CATV system, the return spectrum is in the range of approximately 5 to 42 MHz. This leaves, at best, approximately six (6) channels for data communication back to the headend using the NTSC standard and about four (4) under the PAL standard. However, not all of these channels are equally desirable for data communication. Some of the channels in this range are more susceptible to noise degradation than other channels. As a result there are few good channels for data communications in a sub-split system which is probably the most commonly used system type in the United States. In addition, standards are under development which may define channel widths for forward and return spectrum that are different than NTSC or PAL standards already established.
Even if all the channels in the sub-split range are available for data communication use, other limitations arise as the number of subscribers in the system increase. Allocating the subscribers coupled to a service line to the channels available in a return spectrum may place a reasonable number of subscribers on each channel. At the service site or fiber node, though, all of the service lines are typically merged so all subscribers coupled to the service site or fiber node are allocated to the same available channels in the return spectrum of the cable connecting the service site to the distribution hub. At the distribution hub, the data messages from each service site or fiber node coupled to the distribution hub are merged into the same spectrum of a trunk or feeder line. This merger of data messages from lower network levels to the return spectrum of a single cable continues up to the headend. In an effort to prevent all of the channel capacity being shared by a group of subscribers from being consumed, a time frequency, or other multiplex scheme may be used. While this method allocates a time slot or frequency band on a channel for a subscriber, the time or spectrum available for messages decreases as the number of subscribers decreases. For example, if a fiber node has four lines extending from it with each line having 125 customers, the 500 customers coupled to a service site or fiber node are put on six or fewer channels. At the distribution hub coupled to the fiber node, there may be, for example, three other fiber nodes coupled as well. As a result, 2000 subscribers now contend for data message space on the same six channels. In a large metropolitan area where the number of subscribers may be 200,000 or more, there may be as many as 30,000 subscribers or more per channel. Consequently, message traffic within a channel may become congested and overall performance of the messaging system degraded. Likewise, the response time for messages is significantly increased as each subscriber must contend with a large number of other subscribers for space on a channel within the return spectrum of the system.
What is needed is a way to allocate the available return spectrum in a CATV system to subscribers throughout the network without requiring all of the subscribers to contend for the same channels within the return spectrum of a cable.
The above limitations of previously known CATV systems are overcome by a system and method performed in accordance with the principles of the present invention. The system of the present invention includes a headend for generating a transmission signal having broadcast and data signals, a plurality of service sites, each service site being coupled to the headend by a transmission cable and a return cable, the transmission cable to each service sites providing the transmission signal to the service sites, a plurality of service lines extending from each of the service sites to couple a plurality of subscribers to the service sites and provide the transmission signal to the subscribers, and a spectrum parallel router in each of the service sites, each SPR being coupled to one of the service lines extending from the service site, the SPR receives data messages from the subscribers in the return spectrum of the service lines, the SPR routing data messages from one service line to another service coupled to the SPR which corresponds to a destination address in the received messages and places the received data messages on the return cable for transmission to the headend in response to the destination address in a data message not corresponding to one of the service lines coupled to the SPR so that the data messages from one service site are isolated from data messages from other service sites by the return cable.
The inventive system may also include a plurality of distribution hubs which are coupled between the headend and the service site. More than one service site may be coupled to a distribution hub, however, each service site has its own transmission line and return line to couple the service site to the distribution hub. At the distribution hub, a SPR is provided for each return line and each SPR is coupled to the transmission line for each fiber node. In response to a data message having a destination address that corresponds to one of the service sites coupled to a distribution hub, a SPR sends the data message to the SPR at the distribution hub which is coupled to that service site. For data messages having a destination address which does not correspond to a service site coupled to a distribution hub, the SPR sends the data message to the return cable coupling the SPR to the headend or next higher distribution hub. The return cable for each of the routers within a distribution hub are coupled to a corresponding router in the headend or next higher distribution hub. Data messages which an SPR receives from another SPR at the distribution hub are provided to a transmission cable coupled to the next lower level of the network. In this manner, data messages from a service site are maintained in isolation from data messages from other service sites until a data message is coupled to a transmission cable to a lower network level either at a distribution hub or the headend.
This scheme of isolating data messages from a service site as they are routed upwardly through the network to the headend or to the distribution hub where a message may be coupled to a transmission cable to a lower level, is applicable to systems where the transmission and return lines are strands of a coaxial cable or fiber optic cable. Preferably, the SPRs at the service sites also include a frequency stacker so that data messages from each service line may be provided on a separate channel of the return cable. For example, if three service lines are coupled to a service site, the frequency stacker may place all of the data messages from a first service line onto a first channel of the return cable, the data messages of the second service line onto a second data channel, and the data messages of the third service line onto a third data channel. A corresponding frequency destacker at the next higher level in the network places the data messages in the separate data channels in a common return spectrum for conversion and processing by the SPR at that level. By separating the data messages for each service line on a single return cable, isolation of the data messages for a service line is possible.
Most preferably, the SPRs of the present invention include a switch for routing data messages based on a destination address in the data messages. Each switch is an intelligent device having programmed logic which may be stored in non-volatile memory or hardwired. To route a message, the switch compares the destination address in a data message to addresses stored in an address table of the switch. If the destination address corresponds to an address in the table, the switch routes the data message to the switch at the same level corresponding to the destination address. If the address is not in the table, the SPR receives a data message from a switch at the same network level, it sends the data message to a transmission line coupled to the next lower network level. preferably, the SPR compares a source address in data messages sent by switches at the same level to a channel address table. The data message is then sent to the input of a frequency stacker corresponding to the switch which corresponds to data channel for the source address. In this manner, separation and isolation of data messages in the transmission cables of the network may also be obtained.
At the headend (or even at the distribution hub), destination addresses not corresponding to an address in the address table of a switch preferably correspond to destination addresses for other networks. Preferably, the headend or distribution hub of the present invention is provided with a gateway device which couples to other networks and routes such data messages to the other networks, including the Internet. The headend, preferably, also includes an ad server which may be used to overlay portions of broadcast and data signals in the transmission signal before it is provided to the network.
The present invention may be used in CATV systems in which the transmission cables, return cables and services lines are either all fiber optic cables or coaxial cables. In HFC systems, the invention is preferably implemented with a SPR having a group transceiver for each coaxial service line at a service site and a fiber optic transmitter and fiber optic coaxial receiver for coupling the SPR of the service site to the return cable and transmission cable to the next higher level, although other implementations are with the scope of the invention. if the service lines are also fiber optic cables, a SPR may also be used at a subscriber site to route broadcast signals to display device and data signals to data devices. Each switch of a SPR at a subscriber site may return data messages on a return cable which is a strand of the fiber optic cable not used by the other subscriber sites coupled to the service line. In this way, the data messages of subscribers may be isolated from one another. Additionally, a SPR at a subscriber site may include a frequency stacker that places data messages from different data devices at the subscriber site onto data channels of a return cable.
These and other objects and advantages of the present invention may be ascertained by reviewing the detailed specification below in conjunction with the drawings.