Abstract:
An Internet Group Multicast Protocol (IGMP)-based communications system shares the digital television channels output by two different head ends to increase the number of television channels that can be provided to the end users by forwarding multicast data packets that represent the television channels from one access system to another access system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a system and method of utilizing video head ends and, more particularly, to a system and method of sharing video head ends.  
         [0003]     2. Description of the Related Art  
         [0004]     In a communications network that supports the Internet Group Multicast Protocol (IGMP), Version 1 (RFC 1112) and/or Version 2 (RFC 2236), multicast sources, such as video head ends, output streams of multicast data packets to the network. Each multicast data packet, in turn, has a multicast group address.  
         [0005]     The multicast data packets can represent, for example, television channels that are identified by the multicast group addresses. For example, a multicast data packet of television channel  1  can have a first multicast group address, while a multicast data packet of television channel  2  can have a second multicast group address.  
         [0006]     The multicast data packets (television channels) are forwarded through the public internet, or a similar communications network. The multicast data packets can then be received by an access system that provides an interface to a large number of end users, such as set top boxes.  
         [0007]      FIG. 1  shows an example of a prior-art access system  100 . As shown in  FIG. 1 , access system  100  includes a number of multicast line cards  110 , and a number of host line cards  112 . In addition, access system  100  has a fabric switch module card  114 , a primary control module card  116 , and a control bus  118 .  
         [0008]     Each multicast line card  110  is connected externally to receive multicast data packets that represent a number of television channels (which have a corresponding number of multicast group addresses), and internally to the host line cards  112  via fabric switch module card  114 , where the host line cards  112  are forwarding destinations for the multicast data packets.  
         [0009]     In addition, each of the multicast line cards  110  has a multicast forwarding table that determines how the received multicast data packets are forwarded. A multicast forwarding table lists all of the television channels (multicast group addresses) that are received by a multicast line card  110 .  
         [0010]     Further, for each received television channel (multicast group address), the multicast forwarding table lists of all of the host line cards  112  that are to receive the multicast data packets of the television channel, along with path information for fabric switch module card  114 . For example, the multicast forwarding table can include a list of all of the host line cards  112  which are to receive television channel  3 .  
         [0011]     The host line cards  112 , which can be implemented with xDSL line cards, are each connected to a number of end users  120 , such as set top boxes. Each host line card  112  also has a multicast forwarding table that lists the television channels (multicast group addresses) that are to be forwarded to the end users  120  (e.g., set top boxes).  
         [0012]     Fabric switch module card  114 , which is connected to the multicast line cards  110  and the host line cards  112 , passes data packets between the line cards at up to OC12 speeds. Primary control module card  116  receives non-packet control messages from, and outputs non-packet control messages to, the multicast line cards  110 , the host line cards  112 , and fabric switch module card  114  over control bus  118 .  
         [0013]     In operation, each multicast line card  110  receives multicast data packets of television channels from a forwarding unit, such as a router or a video head end. When a multicast data packet is received by a multicast line card  110 , the multicast line card  110  determines the multicast group address of the packet, and checks the multicast forwarding table to determine which host line cards  112  are to receive the television channel represented by the multicast group address. In addition, the multicast line card  110  forwards the multicast data packet via fabric switch module card  114  to the host line cards  112  as indicated by the entries in the multicast forwarding table.  
         [0014]     For example, a multicast line card  110 A can receive television channels  1 - 50  (with corresponding multicast group addresses of 1-50) from a video head end A. When the end users  120  connected to a host line card  112 A, as a group, receive all of the channels  1 - 50 , the multicast forwarding table of the multicast line card  110 A associates host line card  112 A with all 50 of the received television channels. Thus, in this example, multicast line card  110 A forwards all of the television channels  1 - 50  to host line card  112 A to be forwarded to the end users  120 .  
         [0015]     When a company, such as the company which outputs television channels  1 - 50  to multicast line card  110 A, wishes to increase the number of channels provided to the end users  120 , the company must obtain a source for the additional television channels, and route the additional television channels to another multicast line card  110  in access system  100 .  
         [0016]     Constructing the routing can be a complex process, particularly when access system  100  is connected to a designated router which, in turn, is connected to a router that functions as a rendezvous point in a protocol independent multicast (PIM) architecture. Thus, there is a need for a simple approach to providing additional television channels.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides an access system that includes a host line card that is connectable to an end user, and a multicast line card that is electrically connected to the host line card to pass packets of data to the host line card. The multicast line card receives a multicast data packet that has a multicast group address. The multicast line card has a multicast forwarding table that includes the multicast group address and an indication of whether the multicast data packet with the multicast group address is to be forwarded to the host line card.  
         [0018]     The present invention also provides an access system that includes a plurality of host line cards. Each host line card, in turn, is connected to a plurality of end users. The access system also includes a multicast line card that is electrically connected to the host line cards to pass packets of data to the host line cards. The multicast line card receives a plurality of multicast data packets that have a plurality of multicast group addresses. The multicast line card has a multicast forwarding table that defines the host line cards that are to receive the multicast data packets with the multicast group addresses. The access system additionally includes an intersystem line card. The multicast line card is electrically connected to the intersystem line card to pass data packets to the intersystem line card. The intersystem line card is not connected to any end users.  
         [0019]     The present invention further includes a communications network that includes a first access system and a second access system. The first access system includes a plurality of first host line cards and a first multicast line card. The first multicast line card is electrically connected to the first host line cards to pass packets of data to the first host line cards. The first multicast line card receives a plurality of first multicast data packets that have a plurality of first multicast group addresses. The first multicast line card has a first multicast forwarding table that defines the first host line cards that are to receive the first multicast data packets with the first multicast group addresses. The first access system also includes a first intersystem line card. The first multicast line card is electrically connected to the first intersystem line card to pass packets of data to the first intersystem line card.  
         [0020]     The second access system includes a plurality of second host line cards and a second multicast line card. The second multicast line card is electrically connected to the second host line cards to pass packets of data to the second host line cards. The second multicast line card receives a plurality of second multicast data packets that have a plurality of second multicast group addresses. The second multicast line card has a second multicast forwarding table that defines the second host line cards that are to receive the second multicast data packets with the second multicast group addresses. The second access system further includes a second intersystem line card. The second multicast line card is electrically connected to the second intersystem line card to pass packets of data to the second intersystem line card.  
         [0021]     The present invention also includes a method of providing digital television service to an end user. The method includes the step of receiving a multicast data packet with a multicast line card. The multicast data packet has a multicast group address. The method also includes the step of forwarding the multicast data packet from the multicast line card to a host line card in accordance with a forwarding table that associates the multicast group address with the host line card.  
         [0022]     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a block diagram illustrating a prior-art access system  100 .  
         [0024]      FIG. 2  is a block diagram illustrating an example of a communications system  200  in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 2  shows a block diagram that illustrates an example of a communications system  200  in accordance with the present invention. As described in greater detail below, communications system  200  shares video head ends to increase the number of television channels that can be provided to the end users by forwarding multicast data packets from one access system to another access system.  
         [0026]     As shown in  FIG. 2 , communication system  200  includes a first access system  210  which has a multicast line card  212 , one or more host line cards  214 , and an intersystem line card  216 . In addition, access system  210  also has a fabric switch module card  218 , a primary control module card  220 , and a control bus  222 .  
         [0027]     Multicast line card  212  is connected externally to receive multicast data packets that represent a number of first television channels (which have a corresponding number of multicast group addresses), and internally to the host line cards  214  and intersystem line card  216  via fabric switch module card  218  as forwarding destinations for the multicast data packets.  
         [0028]     In addition, multicast line card  212  has a multicast forwarding table that determines how the multicast data packets are forwarded. The multicast forwarding table lists all of the first television channels (multicast group addresses) that are received by multicast line card  212 . Further, for each received television channel (multicast group address), the multicast forwarding table includes a list of all of the line cards that are to receive the multicast data packets of the television channel, along with path information for fabric switch module card  218 .  
         [0029]     The host line cards  214 , which can be implemented with xDSL line cards, are connected to a number of end users  224 , such as set top boxes. The host line cards  214  also have a multicast forwarding table that lists the television channels (multicast group addresses) that are to be forwarded to the end users  224  (e.g., set top boxes).  
         [0030]     Intersystem line card  216  is connected externally to receive multicast data packets that represent a number of second television channels (which have a corresponding number of multicast group addresses), and to output multicast data packets that represent the first television channels.  
         [0031]     Intersystem line card  216  is also connected internally to the host line cards  214  via fabric switch module card  218  as a forwarding destination for the multicast data packets of the second television channels, and internally to multicast line card  212  via fabric switch module card  218  to receive the multicast data packets of the first television channels. Further, in the present example, intersystem line card  216  is not connected to any end users  224 , and can never be connected to any end users  224 .  
         [0032]     In addition, intersystem line card  216  has both an outgoing multicast forwarding table and an incoming multicast forwarding table. The outgoing multicast forwarding table lists all of the first television channels (multicast group addresses) that are received from multicast line card  212  and, for each received television channel, provides a forwarding destination for the multicast data packet.  
         [0033]     The incoming multicast forwarding table lists all of the second television channels (multicast group addresses) that are received by intersystem line card  216  and, for each television channel (multicast group address), the multicast forwarding table includes a list of all of the host line cards  214  that are to receive the television channel, along with path information for fabric switch module card  218 .  
         [0034]     Fabric switch module card  218 , which is connected to multicast line card  212 , host line card  214 , and intersystem line card  216 , passes packets of data between the line cards at up to OC12 speeds. Primary control module card  220  receives non-packet control messages from, and outputs non-packet control messages to, multicast line card  212 , the host line cards  214 , intersystem line card  216 , and fabric switch module card  218  over control bus  222 .  
         [0035]     As further shown in  FIG. 2 , communication system  200  includes a second access system  230  which, in the present example, is structurally identical to access system  210 . As a result, access system  230  has a multicast line card  232 , one or more host line cards  234 , and an intersystem line card  236 . In addition, access system  230  also has a fabric switch module card  238 , a primary control module card  240 , and a control bus  242 .  
         [0036]     Multicast line card  232  is connected externally to receive multicast data packets that represent the second television channels, and internally to the host line cards  234  and intersystem line card  236  via fabric switch module card  238  as forwarding destinations for the data packets of the second television channels.  
         [0037]     In addition, multicast line card  232  has a multicast forwarding table that determines how the data packets are forwarded. The multicast forwarding table lists all of the second television channels (multicast group addresses) that are received by multicast line card  232 . Further, for each television channel (multicast group address), the multicast forwarding table includes a list of all of the line cards that are to receive the multicast data packets of the television channel, along with path information for fabric switch module card  238 .  
         [0038]     The host line cards  234 , which can also be implemented with xDSL line cards, are connected to a number of end users  244 , such as set top boxes. The host line cards  234  also have a multicast forwarding table that lists the television channels (multicast group addresses) that are to be received by the end users  244  (e.g., set top boxes).  
         [0039]     Intersystem line card  236  is connected externally to receive multicast data packets that represent the first television channels from intersystem line card  216 , and to output multicast data packets that represent the second television channels to intersystem line card  216  of access system  210 .  
         [0040]     Intersystem line card  236  is also connected internally to the host line cards  234  via fabric switch module card  238  as forwarding destinations for the data packets of the first television channels, and internally to multicast line card  232  via fabric switch module card  238  to receive the multicast data packets of the second television channels. Further, in the present example, intersystem line card  236  is not connected to any end users  244 , and can never be connected to any end users  244 .  
         [0041]     In addition, intersystem line card  236  has both an outgoing multicast forwarding table and an incoming multicast forwarding table. The outgoing multicast forwarding table lists all of the second television channels (multicast group addresses) that are received from multicast line card  232  and, for each received television channel, defines intersystem line card  216  as the forwarding destination for the multicast data packet.  
         [0042]     The incoming multicast forwarding table lists all of the first television channels (multicast group addresses) that are received by intersystem line card  236  from intersystem line card  216  and, for each television channel (multicast group address), the multicast forwarding table includes a list of all of the host line cards  234  that are to receive the television channel, along with path information for fabric switch module card  238 .  
         [0043]     Fabric switch module card  238 , which is connected to multicast line card  232 , host line card  234 , and intersystem line card  236 , passes data packets between the line cards at up to OC12 speeds. Primary control module card  240  receives non-packet control messages from, and outputs non-packet control messages to, multicast line card  232 , host line card  234 , intersystem line card  236 , and fabric switch module card  238  over control bus  242 .  
         [0044]     As additionally shown in  FIG. 2 , communication system  200  includes a fiber optic cable  250  that is connected to both intersystem line card  216  of access system  210  and intersystem line card  236  of access system  230 .  
         [0045]     In operation, multicast line card  212  receives the first television channels from a forwarding unit  252 , such as a router or a video head end, and forwards the first television channels to the host line cards  214  and intersystem line card  216  via fabric switch module card  218  as indicated by the entries in the multicast forwarding table stored in multicast line card  212 .  
         [0046]     When a multicast data packet is received by multicast line card  212 , multicast line card  212  determines the multicast group address of the packet, checks the multicast forwarding table to determine which line cards are to receive the television channel represented by the multicast group address, and forwards the data packet to the line cards which are to receive the television channel using the path information.  
         [0047]     For example, multicast line card  212  can receive first television channels  1 - 50  (with corresponding multicast group addresses of 1-50) from a video head end A  252 . When the end users  224  connected to a host line card  214 A, as a group, receive all of the channels  1 - 50 , the multicast forwarding table of multicast line card  212  associates the host line card  214 A with all of the received television channels. Thus, in this example, multicast line card  212  forwards all of the first television channels  1 - 50  to host line card  214 A, to be forwarded on to the end users  224 .  
         [0048]     In accordance with the present invention, the multicast forwarding table of multicast line card  212  also associates intersystem line card  216  with each received television channel. As a result, multicast line card  212  forwards all of the television channels  1 - 50  to intersystem line card  216 .  
         [0049]     When a multicast data packet for one of the first television channels ( 1 - 50 ) is received by intersystem line card  216 , line card  216  determines the multicast group address of the packet, checks the outgoing multicast forwarding table to determine which line cards are to receive the television channel represented by the multicast group address, and forwards the multicast data packet to intersystem line card  236  as indicated by the outgoing multicast forwarding table.  
         [0050]     In further accordance with the present invention, intersystem line card  216  also receives the second television channels from intersystem line card  236 , and forwards the second television channels to the host line cards  214  via fabric switch module card  218  as indicated by the incoming multicast forwarding table stored in intersystem line card  216 .  
         [0051]     When a multicast data packet for one of the second television channels ( 51 - 100 ) is received by intersystem line card  216 , line card  216  determines the multicast group address of the packet, checks the incoming multicast forwarding table to determine which host line cards  214  are to receive the television channel represented by the multicast group address, and forwards the data packet to the host line cards  214  which are to receive the television channel using the path information.  
         [0052]     Thus, in accordance with the present invention, host line card  214  can receive multicast television channels  1 - 100  by receiving channels  1 - 50  from multicast line card  212  and channels  51 - 100  from intersystem line card  216 .  
         [0053]     In addition to the above, multicast line card  232  receives the second television channels from a forwarding unit  254 , such as a router or a video head end, and forwards the second television channels to the host line cards  234  and intersystem line card  236  via fabric switch module card  238  as indicated by the entries in the multicast forwarding table stored in multicast line card  232 .  
         [0054]     When a multicast data packet is received by multicast line card  232 , multicast line card  232  determines the multicast group address of the packet, checks the multicast forwarding table to determine which line cards are to receive the television channel represented by the multicast group address, and forwards the multicast data packet to the line cards which are to receive the television channel using the path information.  
         [0055]     For example, multicast line card  232  can receive second television channels  51 - 100  (with corresponding multicast group addresses of 51-100) from a video head end B  254 . When the end users  244  connected to a host line card  234 A, as a group, receive all of the channels  51 - 100 , the multicast forwarding table of multicast line card  232  associates host line card  234 A with all of the received television channels. Thus, in this example, multicast line card  232  forwards second television channels  51 - 100  to host line card  234 A, to be forwarded on to the end users  244 .  
         [0056]     In accordance with the present invention, the multicast forwarding table of multicast line card  232  also associates intersystem line card  236  with each received television channel. As a result, multicast line card  232  forwards all of the second television channels  51 - 100  to intersystem line card  236 .  
         [0057]     When a multicast data packet for one of the second television channels ( 51 - 100 ) is received by intersystem line card  236 , line card  236  determines the multicast group address of the packet, checks the outgoing multicast forwarding table to determine which line cards are to receive the television channel represented by the multicast group address, and forwards the data packet to intersystem line card  216  as indicated by the outgoing multicast forwarding table.  
         [0058]     In further accordance with the present invention, intersystem line card  236  also receives the first television channels from intersystem line card  216 , and forwards the first television channels to the host line cards  234  via fabric switch module card  238  as indicated by the incoming multicast forwarding table stored in intersystem line card  236 .  
         [0059]     When a multicast data packet for one of the first television channels ( 1 - 50 ) is received by intersystem line card  236 , line card  236  determines the multicast group address of the packet, checks the multicast forwarding table to determine which host line cards  234  are to receive the television channel represented by the multicast group address, and forwards the multicast data packet to the host line cards  234  which are to receive the television channel using the path information.  
         [0060]     Thus, in accordance with the present invention, the host line cards  234  can receive multicast television channels  1 - 100  by receiving channels  1 - 50  from intersystem line card  236  and channels  51 - 100  from multicast line card  232 .  
         [0061]     One of the advantages of the present invention is that communications system  200  allows company A, which sources television channels  1 - 50  via head end A, and company B, which sources television channels  51 - 100  via head end B, to easily share television channels, thereby allowing company A and company B to each provide 100 channels of video content.  
         [0062]     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.