Abstract:
IGMP is a protocol for managing and controlling multicast of data streams to a plurality of end users connected to a telecommunications network through an access system. A novel implementation of IGMP for multicasting data streams in an xDSL access network system is disclosed. DSLAM (Digital Subscriber Line Asynchronous Multiplexer) or ASAM (ATM Subscriber Access Multiplexer) is used in the access network system and interfaces between the telecommunications network and the plurality of end users. IGMP signals are terminated at LT (Line Termination). A better scalability of the access system can be achieved, resulting in increased numbers of end users.

Description:
FIELD OF INVENTION 
   The invention resides in the field of multicasting data streams to multiple destinations through telecommunications networks. In particular, it is directed to providing Internet Group Management Protocol (IGMP for short) on a Digital Subscriber Loop (DSL for short) access system. 
   BACKGROUND OF INVENTION 
   Data is commonly transported through networks in packets, frames or cells; these terms e.g., packets, frames, cells, data streams, packet streams, frame streams and cell streams, are used interchangeably throughout this specification. In multicasting, copies of data from a single source are sent to multiple destinations over a network supporting the Internet protocol (IP for short). IP multicast uses a multicast group destination address which is shared by a group of hosts (end users). A multicast source sends packets to that address and when a multicast router or switch (commonly called as “routing device”) receives such a packet (addressed to the multicast address), it replicates and delivers the packet to multiple receiving hosts within an internet routed infrastructure. The multicast router compares the received multicast packets to a multicast forwarding table and sends a copy of the received packets out all the interfaces named in that multicast forwarding table. DSL access systems can be used to deliver multicast data streams from the routing device to a host or a group of hosts. A variety of DSL (xDSL) systems are available, e.g., HDSL (High bit rate DSL), SDSL (Symmetric DSL), RADSL (Rate Adaptive DSL), ADSL (Asymmetric DSL) and VDSL (Very high bit rate DSL). 
   Internet Group Management Protocol (IGMP) is the preferred signaling protocol used in IP networks supporting multicast communications. IGMP messages are separate from packets used for data transfer. Hosts and routing devices use IGMP messages to report their IP multicast group memberships to any neighboring multicast routing devices. For example, when the host joins or leaves the multicast group, the IGMP message indicating this fact is transmitted to the neighboring routing devices 
   Following U.S. patent documents describe related subject matters. 
   U.S. Pat. No. 6,654,371 Nov. 25, 2003 Dustan et al describes “Method and Apparatus for Forwarding Multicast Data by Relaying IGMP Group Membership” 
   U.S. patent application Ser. No. 2002/0097728 published Jul. 25, 2002 Hinderks et al describes “Method and Apparatus for Injection of IP Multicast Content into an ATM DSL Network”. 
   U.S. patent application Ser. No. 2002/0191631 published Dec. 19, 2002 Couty describes “Method of and a System for Lightening the Signaling Load of a Multicast Protocol applied to a Network of Terminals using a Transmission Medium that does not support Mutual Listening between Terminals”. 
   U.S. patent application Ser. No. 2003/0145102 published Jul. 31, 2003 Keller-Tuberg describes “Facilitating Improved Reliability of Internet Group Management Protocol through the Use of Acknowledge Messages” 
   Referring to  FIG. 1 , a source  30  is injecting a multicast data stream  32  to a network  34  which can be, for example, an ATM network or an IP network. Downstream, a DSL access system, such as a DSLAM  38  (Digital Subscriber Line Asynchronous Multiplexer) delivers the data stream  32  to multiple end users through DSLs  36 . Each end user generally has a host computer or a set-top box  40  with a viewing device that is connected to a DSL by way of a CPE  42  (Customer Premise Equipment). In other cases, as shown in  FIG. 1 , one CPE  44  may interface between a DSL  46  and multiple of end users on a local area network (LAN)  48  of any type e.g., an ATM network, an Ethernet, etc. The CPE includes a DSL modem which bridges the customers system and the DSL. 
   Referring further to  FIG. 1 , the DSLAM  38  interfaces between network  34  and CPEs  42 ,  44 . The DSLAM  38  includes an NT (Network Termination)  45  and one or more LTs (Line Termination)  47 . The NT manages termination of a point-to-multipoint connection between DSLAM and CPEs. Each LT is essentially a DSL card which manages termination of DSL connections of data streams towards its CPE. A suitable transport medium, such as a bus, switching fabric or point-to-multipoint connections, for example, is provided between NT and LTs. 
   For the connection over the DSL link  36 , ATM has been widely regarded as the communication protocol for use between a DSLAM to a DSL modem. The Alcatel 7300 ASAM (Advanced Services Access Manager) is an example of a DSLAM that employs ATM. 
   It is noted that while the following description may emphasize the IP multicast over an ATM network, the invention to be described in detail will be equally applicable to other multicasting environment over any DSL access system which may be implemented in other forms of networks, e.g., Ethernet, fiber networks, etc, where virtual circuits or multicast groups can be formed between the host and routing device. 
   The prior art provides IGMP on a router (IP Server) in an ASAM. The router comprises an IP service module (ISM) and a controller (Network Termination—NT), each being on separate circuit cards. This approach spans two cards. Consequently it is not as efficient as a one-card solution would be. A present Applicant&#39;s patent application, Ser. No. 10/878,132 filed on Jun. 8, 2004, describes a solution which uses IGMP termination on the NT for IGMP processing of multicast data streams. This can be called an IGMP on NT architecture. According to the described invention, multicast of the data stream is performed at the ATM level by use of a point-to-multipoint connection, and controlled through IGMP control messages terminating on the NT without usage of the IP Server. This solution is simpler and more efficient than the known systems. It has, however, some performance limitations in that the number of STB (set-top boxes) per NT is limited due to the fact that all the IGMP functions are performed at NT for all the CPEs of the group. Each LT may have many ports, each connected to a CPE (STB) and there are many LTs connected to one NT. As a termination point of IGMP, NT handles all the IGMP control signals for all the connected CPEs. For example, if many end users simultaneously surf video channels, this may overload the NT. Therefore this approach is only appropriate for a very limited video deployment. It is desirable to be able to provide IGMP channel connections in a manner that is scalable. 
   The invention therefore relates to the problem of providing IGMP services on a DSL access system, such as an DSLAM or ASAM. The invention is applicable equally to the cases where the DSL access system in ATM environment or non ATM environment. 
   SUMMARY OF INVENTION 
   The invention resides in the field of integrated IGMP service, particularly to deliver multicast services of data streams to a plurality of end users through a network which uses DSLs. The network can be an ATM network or other type such as Ethernet by the use of DSLs. 
   An aspect of the invention provides that IGMP signaling can be terminated on LTs preferably for the processing of video channels. This can be called an IGMP on LT architecture (or LT architecture for short). For example, busy functions such as “join/leave” operation are shared by a plurality of LTs without NT&#39;s involvement. This will lessen the load on the NT and may result in ease of scaling up the number of end users. It should, however, be noted that both technologies can co-exist in that certain LTs will terminate their IGMP signaling channel on the NT and other LTs will terminate their own IGMP signaling channels. 
   According to another aspect, the invention also uses a dedicated IGMP signaling VC that can be formed between each LT and CPE. The invention, in this embodiment however, contemplates IGMP termination that is performed at LTs (at each of the DSL cards). 
   According to still further aspects, the LT architecture can be implemented in an ATM environment or in a non-ATM environment which can be of Ethernet type. 
   Yet a further aspect, the invention can be achieved by employing a combination of the NT and LT architecture. In such a configuration, certain LTs will terminate their IGMP signaling channel on the NT and other LTs can terminate their own IGMP signaling channels. This would be a hybrid approach. In this hybrid approach, the proportion of LT termination to NT terminations is variable, as determined by scalability requirements of each specific deployment. 
   In accordance with another aspect, the invention includes a system for providing multicasted communication services on IGMP channel connections. The invention is distinguished over the prior art in that it is capable of terminating IGMP channel connections on line termination (LT) subsystems of the system (i.e. it can use a distributed termination approach for all, or some, of its IGMP channel connections). 
   According to the method of the invention, the xDSL access system initiates an ATM point-to-multipoint connection with a source connected to the ATM network responsive to an IGMP control message received from one of the end user systems requesting a particular data stream. The source formats data from the particular data stream into IP packets, each IP packet having an IP multicast address. These IP packets are first encapsulated into Ethernet frames and then the source ATM encapsulates the Ethernet frames into ATM cells and launches the cells into the ATM network on the ATM point-to-multipoint connection. The xDSL access system receives the ATM cells over the ATM point-to-multipoint connection and transmits the cells in an xDSL modulated signal to the end user system. An xDSL modem connected between the xDSL access system and the end user system receives the xDSL modulated signal, reassembles the Ethernet frames from the ATM cells and sends the frames to the end user system; and then the end user system receives the IP packets encapsulated in the Ethernet frames and re-formats them into a particular data stream. In this context, the invention is characterised in that the multicasting is performed at the ATM level using a point-to-multipoint connection and controlled through the IGMP control messages which are terminated on the LT (of the xDSL system). 
   According to another aspect, the invention is directed to a method for providing multicasted data streams to end user systems connected to a telecommunications network by means of a digital subscriber line (xDSL) access system having a NT module and one or more LT modules. The method includes steps of multicasting said data streams using point-to-multipoint connection and controlling said multicasting through internet group management protocol (IGMP) control messages terminated at one or more of the LT modules of the xDSL access system and in the absence of an internet protocol (IP) server. 
   In accordance with a further aspect, the invention is directed to a method for providing multicasted video data streams to end user systems connected to an asynchronous transfer mode (ATM) network by means of a digital subscriber line (xDSL) access system having an NT module and one or more LT modules. The method includes steps of multicasting said data streams at said ATM network level using a point-to-multipoint connection and controlling said multicasting through internet group management protocol (IGMP) control messages terminated at one or more LT modules of the xDSL access system and in the absence of an internet protocol (IP) server. 
   The xDSL system may further include an IGMP management entity for controlling the line termination modules to exchange IGMP messages with end users and controlling IGMP multicast functions, i.e., join and leave processing, on the multicast connections in response to the IGMP messages. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic illustration of delivery of multicast streams to a plurality of end users where IGMP is used in conjunction with the use of DSLAM and DSLs. 
       FIG. 2  is a schematic illustration delivery of multicast streams to a plurality of end users where IGMP is used in conjunction with the use of ASAM and DSLs in an ATM network. 
       FIG. 3  shows how an ASAM interfaces with external elements within networks. 
       FIG. 4  is a schematic illustration of a construction of an ASAM in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   According to one embodiment of the invention, an ASAM (ATM Subscriber Access Multiplexer) is used to deliver IP multicast data streams through an ATM network to a group of end users connected to ASAM. ASAM combines functionality of DSLAM (delivery of multicast data streams) and IGMP processing (IGMP termination). ASAM dispenses the use of an “IP-Server”, e.g., router.  FIG. 2  illustrates schematically an ATM environment in which IGMP is implemented using DSL and ASAM for delivering and managing multicasting data streams to group membership. The figure shows a connection to one CPE. 
   Referring to  FIG. 2 , a source (data stream head end)  50  provides statically configured channels  52  (shown in dotted line) to NT  54  in ASAM  56 . The source  50 , therefore, sends out data streams  58  to an ATM network  60  which delivers them to ASAM  56  through an interface  62 . The above assumes that the data streams in interface  62  going into ASAM are properly formatted in the correct IP Multicast Address and ATM encapsulation. The data streams therefore are formatted by the source prior to be injected into the ATM network. Alternatively, they can be formatted at somewhere else along the transport path. ASAM  56  therefore receives properly encapsulated IP multicast streams on interface  62 . The data streams are cross connected through a transport medium such as a bus to a plurality of LTs  55 , (only one is shown). A plurality of end users (set-top-boxes)  64  are connected to LT  55  in ASAM  56  through a CPE  66  by way of DSL  68  and LAN  70 . The data stream connection  72  (shown in dotted line), therefore, is made between ASAM  56  and each end user  64 , wherein the connection over DSL  68  is in ATM and that over LAN  70  is IP over Ethernet. In one example, the source and ASAM are managed on the ATM layer for all the ATM functions including such connection as “a point-to-point” or “a point-to-multipoint”. The invention uses IGMP between the end users and the ASAM for group management of multicasting. IGMP control signals  74  are exchanged between the ASAM and end users&#39; CPE  66  so that the multicast data streams are properly transported to an appropriate end user. 
     FIG. 3  illustrates the ASAM external interfaces. Referring to the Figure, ASAM  80  may receive ATM connections from an ATM network  82 . ATM network typically communicates with ASAM through an SDH interface for management signal exchange and data transfer. The management interface statically configures the IGMP signaling channels and the multicast IGMP tables. The interface between ASAM  80  and the CPE  84  carries the IGMP signaling information and the video channels that have been subscribed to. The user data interfaces are often SONET/SDH because the broadcast TV channels require bandwidth per channel in the order of 3 Mbps. End users  86  receive data stream, e.g., video streams in a video channel, through their locally located CPEs  84  and end users&#39; STB (set-top-box)  86 . A STB is attached to a display device e.g., a TV, to send the content of the stream for display and at the same time to interact with a remote control unit to respond to customer&#39;s commands. It is also possible to manage ASAM remotely by exchanging management signals through Ethernet interface and IP network  88 . 
   Referring now to  FIG. 4 , an ASAM  100  typically comprises an NT module  102  and a plurality of LT modules  104 , in addition to other associated functional modules, e.g., an EMS (Element Management System)  106  supporting management related functions for the ASAM. A transport medium, such as a system bus of a sufficient bandwidth, connects the NT and LTs modules. Each LT module includes a DSL card  108  (or Line Interface Module or LIM for short), which generally have up to a maximum number of ports, e.g.,  96 , each port being hard-wired to a respective customer&#39;s CPE  110  for delivering selected multicast data streams as well as exchanging IGMP message signals. A STB  112  connects CPE  110  and a TV  114  for display. In place of the STB and TV, a PC can be used for display and interaction with a user. A handheld controller  116  such as a remoter controller interacts with STB  112  for user inputs. Data multicast is performed on the ATM level by the use of a point-to-multipoint connection. IGMP is used between ASAM and CPE to control the group multicasting through a dedicated IGMP signaling VC (virtual circuit) provided on the DSL between ASAM and each CPE. Unlike multicasting in the router environment, the NT  102  does not perform IP multicast routing such as DVMRP/PIM (Distance Vector Multicast Routing Protocol/Protocol Independent Multicast), etc. 
   IGMP is implemented in the ATM DSL environment on the ASAM. NT  102  at ASAM  100  is configured with the addresses of the source of the multicast streams. On the upstream, the NT terminates certain IGMP control channels from each service subscriber and uses the IGMP control messages, such as, join and leave to initiate and terminate cross-connections with corresponding ATM point-to-multipoint connections. These IGMP control channels are shown by a dotted line  118  as a dedicated VC between CPE and NT. At the subscriber premises, the CPE  110  includes an xDSL modem which terminates the ATM point-to-multipoint flows and bridges the ATM packets onto the end user&#39;s Ethernet LAN. Once on the LAN, the end user&#39;s PC or STB receives the multicast flows and an application on either of these devices presents the video as appropriate. 
   According to an embodiment of the invention, IGMP channels are terminated on LTs for certain IGMP functions, including join and leave functions, which require quick reaction in response to user inputs. Referring back to  FIG. 4 , in addition to IGMP protocol at NT  102 , IGMP protocol functionality can be supported at one or more LTs  104 ′ (e.g. individual DSL cards  108 ); an IGMP state machine is provided and run at each such LT. Each LT  104 ′ has an IGMP signaling channel (e.g. dedicated VC)  118 ′ to its corresponding CPE  110 ′. Moreover, each LT  104 ′ maintains an IP multicast source table similar to the NT  102 . The IP multicast source table provides a mapping or translation between IP multicast addresses and respective internal (ASAM) interfaces for the network connections carrying video channels corresponding to the multicast addresses. EMS  106  may configure the IP multicast source table on the NT  102  and LTs  104 ′. Alternatively, the NT  102  may provide the IP multicast source table to the LTs  104 ′. 
   In operation, ASAM  100  receives properly encapsulated IP multicast data streams, carried on point to multipoint connections assuming ATM is the transport network, through one or more network interface at the NT  102 . The multicast data steams may be video channels, for example. Each of these IP multicast streams can be provided by the NT  102  through a corresponding internal interface, cross connected via an internal transport medium, to any of the LTs  104 ′. The transport medium can be, for example, a bus over which the cross connections between the NT  102  and LTs  104 ′ can be established. When an IGMP join message is received by an LT  104 ′ from its associated CPE  110 ′ via their signaling channel  118 ′, a lookup operation is performed for the multicast address from the join message in the IP multicast source table. The IP multicast source table provides the identifier of the internal interface corresponding to the multicast address. The LT  104 ′ then initiates a cross connection to the internal interface whereby the corresponding IP multicast stream is received from the NT  102  over the bus. When the LT  104 ′ receives a leave message identifying the same multicast address, the LT tears down the cross connection. 
   In general, an advantage of the present invention is to alleviate the workload on the NT  102 . Furthermore, it is more scalable than the centralized approach in which only the NT  102  supports IGMP functionality. The IGMP on LT model is therefore a cost-effective solution for providing IGMP channel connections, which are useful for applications such as providing broadcast television over DSL services. One variant of the invention is to provide IGMP processing of join and leave messages on the LT  104 ′ instead of the NT  102 . Alternatively, in the hybrid approach, IGMP channels can be terminated on the NT  102  for some LTs  104 , and other LTs  104 ′ terminate their own IGMP channels. In this last model, a selected subset of LTs  104 ′ is provided with IGMP state machines.