Abstract:
An access network system, an address resolution protocol proxy method and an IP bridging forwarding method for the access network system are disclosed. The access network system comprises: one or more access network edge nodes that connect to one or more access nodes; one or more access nodes that connect to user terminals to the access network, and a reference point that is introduced between two adjacent access nodes for the access nodes interconnection. With this system, the multi-cast or broadcast flow in the network can be reduced and the communication delay and path can also be decreased. Therefore the communication bottleneck brought by BRAS/BNG can be avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2007/070393, filed Jul. 31, 2007, which claims priority to Chinese Patent Application No. 200610104208.3, filed Aug. 1, 2006 and Chinese Patent Application No. 200610153985.7, filed Sep. 15, 2006, all of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the technical field of communications, and particularly to an access network system, an access device in the system, and an address resolution protocol proxy and an IP bridging forwarding method thereof. 
     BACKGROUND OF THE INVENTION 
     The Digital Subscriber Line (DSL) network architecture is evolving from the Point to Point Protocol over Asynchronous Transfer Mode (PPP over ATM) to an enabling architecture of IP Quality of Service (QoS) based on Ethernet aggregation and connection. Under this background, a general reference architecture of the DSL is illustrated in  FIG. 1 . 
     In the DSL reference architecture as illustrated in  FIG. 1 , T is a reference point between a User Equipment (UE) and a Residential Gateway (RG) in a Customer Premises Network (CPN); U is a reference point between an RG and an Access Node (AN, i.e., DSL Access Multiplexer, DSLAM); in an Access Network (AN), there is an Aggregation Network between the AN (i.e., the DSLAM) and a Broadband Remote Access Server (BRAS) or a Broadband Network Gateway (BNG); V is an Ethernet Aggregation reference point between the AN (i.e., DSLAM) and the BRAS/BNG in the access network; A 10  is a reference point between the access network and a Service Provider (SP). The reference point A 10  may connect an Application Service Provider (ASP) to a Network Service Provider (NSP) including an access network, or connect the NSP to an access network in a visiting area in the scenario of roaming. The CPN is connected with the access network in a DSL access technique. For a Passive Optical Network (PON), the AN is an Optical Line Termination (OLT), and the CPN is connected with the access network in a PON access technique, etc. 
     At present, as illustrated in the DSL reference architecture of  FIG. 1 , there is no reference point between ANs (i.e., DSLAMs) of the DSL network, and only star or tree networking can be adopted between the ANs (i.e., DSLAMs) and the BRAS/BNG of the DSL network. In the procedure of attempting to achieve the present invention, the inventor finds that the DSL reference architecture of  FIG. 1  has at least the following disadvantages: 
     1. For providing multicast services such as IP TV, the BRAS/BNG and the aggregation network should support multicast copy, thereby the existing DSL network should be modified. A schematic diagram of a multicast and broadcast model of a DSL network with star or tree networking is illustrated in  FIG. 2 . Because star or tree networking paths (BNG-&gt;AN 1 , BNG-&gt;AN 2 , BNG-&gt;AN 3 , . . . , BNG-&gt;ANn) are adopted to transmit multicast or broadcast data streams, which results in a large amount of multicast or broadcast traffic in the DSL network. 
     2. If a network single point of failure occurs in the connection between an AN and the BRAS/BNG as illustrated in  FIG. 3 , the AN disconnects with the BRAS/BNG, and all users connected to the AN are provided with service. 
     3. The traffic between users covered by different ANs should be transferred by the BRAS/BNG. A path for communication between users covered by different ANs is illustrated in  FIG. 4 . The path is long with a long delay. The BRAS/BNG becomes a bottleneck for the communication and can not meet the requirements of future VoIP and Peer-to-Peer communications. 
     SUMMARY OF THE INVENTION 
     The present invention is to provide an access network system, an access device in the system, an address resolution protocol proxy and IP bridging forwarding method thereof. The present invention may be applied to a digital subscriber line network and a passive optical network, so as to reduce multicast or broadcast traffic in such a network, to reduce the delay, and to shorten the communication path, thereby preventing the BRAS/BNG from becoming a bottleneck for the communication. 
     An object of the present invention is achieved by the following technical solutions. 
     An access network system includes one or more access network edge nodes, each of which connected with one or more access nodes; and the one or more access nodes, adapted to enable a user terminal to access an access network, where a reference point is introduced between the access nodes to support connections between the access nodes. 
     An access device in an access network system, includes (1) a user-side port module, adapted to communicate with a user terminal; (2) a network-side port module, adapted to communicate with an access network edge node; (3) an access device-side port module, adapted to communicate with another access device in the access network system; (4) an address resolution protocol proxy module, which is connected to the user-side port module, the network-side port module and the access device-side port module, adapted to proxy-forward an address resolution protocol message; and (5) an IP bridging forwarding module, which is connected to the user-side port module, the network-side port module and the access device-side port module, adapted to forward a packet. 
     An address resolution protocol proxy method in an access network system, applied to address resolution protocol proxy between a first access node and a second access node of multiple access nodes, includes (1) forwarding, by the first access node, an address resolution protocol request from an address resolution protocol requestor to the second access node, and sending, by the first access node, the MAC address of the first access node in a first address resolution protocol in response to the address resolution protocol request; and (2) sending, by the second access node, the MAC address of the second access node in a second address resolution protocol in response to another address resolution protocol request from the first access node. 
     An IP bridging forwarding method in an access network system, applied to IP bridging forwarding between a first access node and a second access node of multiple access nodes, includes (1) creating an IP bridging forwarding table in the first access node and the second access node; (2) searching, by at least one of the first access node and the second access node, in the IP bridging forwarding table according to an IP address in a received packet to obtain at least one of IP session information and IP service connection information; (3) updating, by the at least one of the first access node and the second access node, MAC frame header information in the received packet according to the at least one of IP session information and IP service connection information; and (4) forwarding, by the first access node, the updated packet to the second access node, or forwarding, by the second access node, the updated packet to the first access node. 
     The present invention reduces multicast or broadcast traffic in the network, reduces the delay and shortens the communication path, thereby preventing the BRAS/BNG from becoming a bottleneck for communications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings provide further understanding of the present invention, and are incorporated in the present application to constitute a part of the present application. The drawings together with the description show embodiments of the present invention to make clear of the principle of the present invention. 
         FIG. 1  is a schematic diagram of a multimedia broadcast and multicast service function layer; 
         FIG. 2  is a schematic diagram of a multicast and broadcast model with star or tree networking; 
         FIG. 3  is a schematic diagram of a network single point of failure; 
         FIG. 4  is a schematic diagram of a path of the communication between users; 
         FIG. 5   a  is a structure diagram of an access network system according to the present invention; 
         FIG. 5   b  is a structure diagram of an access device in an access network system according to the present invention; 
         FIG. 6  is a flowchart of an address resolution protocol proxy method utilized in an access network system according to the present invention; 
         FIG. 7  is a flowchart of an IP bridging forwarding method utilized in an access network system according to the present invention; 
         FIG. 8   a  is another flowchart of an IP bridging forwarding method utilized in an access network system according to the present invention; 
         FIG. 8   b  is another flowchart of an IP bridging forwarding method utilized in an access network system according to the present invention; 
         FIG. 9   a  is a further flowchart of an IP bridging forwarding method utilized in an access network system according to the present invention; 
         FIG. 9   b  is a further flowchart of an IP bridging forwarding method utilized in an access network system according to the present invention; 
         FIG. 10  is a schematic diagram of a single-edge access network reference architecture according to the present invention; 
         FIG. 11  is a schematic diagram of a multi-edge access network reference architecture according to the present invention; 
         FIG. 12   a ,  FIG. 12   b ,  FIG. 12   c  and  FIG. 12   d  are schematic diagrams of chain or ring networking multicast and broadcast models according to the present invention; 
         FIG. 13   a  and  FIG. 13   b  are schematic diagrams of chain networking multicast and broadcast failure models according to the present invention; 
         FIG. 14   a ,  FIG. 14   b ,  FIG. 14   c  and  FIG. 14   d  are schematic diagrams of dual-ring networking multicast and broadcast failure models according to the present invention; 
         FIG. 15  is a schematic diagram of a networking unicast failure model 1 according to the present invention; 
         FIG. 16  is a schematic diagram of a networking unicast failure model 2 according to the present invention; 
         FIG. 17   a  and  FIG. 17   b  are schematic diagrams of paths for communications between users; 
         FIG. 18  is a schematic diagram of IP bridging in an access network reference architecture according to the present invention; 
         FIG. 19  is a schematic diagram of an ARP proxy in an access network reference architecture according to the present invention; and 
         FIG. 20  is a schematic diagram of IP bridging in reference architecture according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described in further detail with reference to the attached drawings and embodiments as follows. 
       FIG. 5   a  is a structure diagram of an access network system according to the present invention. The access network system includes: one or more access network edge nodes, and one or more access nodes. Each access network edge node is connected with one or more access nodes. The access node is adapted to enable user terminals to access the access network. Reference points are introduced between the access nodes for supporting connections between the access nodes. 
     The above access network edge node includes at least one of a broadband remote access server and a broadband network gateway. 
     The networking between the access network edge nodes and the access nodes may be implemented in at least one of the following modes: star, tree, chain, and ring. The access nodes may be connected with each other in a ring mode. 
     In the case of unicast service, the networking between the access network edge nodes and the access nodes is implemented in a star or tree mode. In the case of multicast or broadcast service, the networking between the access network edge nodes and the access nodes is implemented in a chain, ring or dual-ring mode. 
     The access node may be at least one of the following: an access node supporting layer-3 routing, an access node supporting layer-2, an access node supporting IP bridging, and an access node supporting address resolution protocol proxy. 
     In normal cases, user data is transmitted between the access nodes directly, to support the communication between users in different access nodes. If a single point of failure occurs between an access node and an access network edge node, the access node is connected with the access network edge node via another access node. If a failure occurs between an access node and another access node, the access node is connected with another access node to transmit data via a reverse ring path between the access node and a third access node. 
       FIG. 5   b  is a block diagram of an access device of an access network system according to the present invention. As illustrated in  FIG. 5   b , the access device of the access network system according to the present invention includes: a user-side port module  502 , adapted to communicate with a user terminal; a network-side port module  504 , adapted to communicate with an access network edge node; an access device-side port module  506 , adapted to communicate with one or more other access devices in the access network system; an address resolution protocol proxy module  508 , which is connected to the user-side port module, the network-side port module and the access device-side port module, adapted to proxy-forward an address resolution protocol message; and an IP bridging forwarding module  510 , which is connected to the user-side port module, the network-side port module and the access device-side port module, adapted to forward a packet. The packet is data including an IP header and a MAC frame. 
     The above access network edge node includes at least one of the following: a broadband remote access server and a broadband network gateway. If the access device receives an address resolution protocol request, the address resolution protocol proxy module in the access device forwards the address resolution protocol request to a next access device, and forwards the MAC address of the access device in an address resolution protocol in response to the address resolution protocol request. 
     The above IP bridging forwarding module stores an IP bridging forwarding table. IP session information in the above IP bridging forwarding table includes: a user-side physical port, a user IP address, and a user MAC address. IP service connection information in the IP bridging forwarding table includes: the MAC address of an access network edge node. The IP service connection information further includes at least one of the following: a network-side physical port and a virtual local area network identifier. 
     In the IP bridging forwarding table of the access device, the item of a user MAC address of the access device is the MAC address of a previous access device or user terminal connected with the user-side physical port. 
     In the IP bridging forwarding table of the access device, the item of the MAC address of the access network edge node is the MAC address of a next access device or access network edge node connected with the network-side physical port. 
     If the access device receives a packet from a user terminal/an access network edge node/another access device, the access device searches in the IP bridging forwarding table according to the IP address in the packet to obtain IP session information and/or IP service connection information, updates MAC frame header information in the received packet according to the IP session information and/or the IP service connection information, and forwards the updated packet to a next access device. 
       FIG. 6  is a flowchart of an address resolution protocol proxy method utilized in the above access network system according to the present invention.  FIG. 6  illustrates an address resolution protocol proxy method between an access node  1  and an access node  2  of multiple access nodes. If packet information is transmitted from a user terminal to a network edge access node in an uplink direction, the method includes the following steps. 
     In step S 602 , the access node  1  forwards a first address resolution protocol request from an address resolution protocol requestor to the access node  2 , and sends the MAC address of the access node  1  in an address resolution protocol in response to the address resolution protocol request. 
     In step S 604 , the access node  2  sends its MAC address in an address resolution protocol in response to the address resolution protocol request from the access node  1 . 
       FIG. 7  is a flowchart of an IP bridging forwarding method utilized in the above access network system according to the present invention. The IP bridging forwarding method between an access node  1  and an access node  2  of multiple access nodes according to the present invention includes the following steps. 
     In step S 702 , an IP bridging forwarding table is created in the access node  1  and the access node  2 . 
     In step S 704 , the access node  1  and/or the access node  2  searches in the IP bridging forwarding table according to an IP address in a received packet to obtain IP session information and/or IP service connection information. 
     In step S 706 , MAC frame header information in the received packet is updated according to the IP session information and/or the IP service connection information. 
     In step S 708 , the access node  1  forwards the updated packet to the access node  2 , or the access node  2  forwards the updated packet to the access node  1 . 
     Particularly, the access node obtains the MAC address of a user device and address information obtained by an ARP proxy via a Dynamic Host Configuration Protocol (DHCP) message initiated from the user device, so as to create the IP bridging forwarding table. 
     The IP session information in the IP bridging forwarding table includes: a user-side physical port, a user IP address, and a user MAC address. 
     In the IP bridging forwarding table of the access node  2 , the item of the user MAC address corresponding to the user-side physical port connected with the access node  1  is the MAC address of the access node  1 . 
     The IP service connection information in the IP bridging forwarding table includes: the MAC address of an access network edge node. The IP service connection information further includes at least one of the following: a network-side physical port and a virtual local area network identifier. 
     In the IP bridging forwarding table of the access node  1 , the item of the MAC address of the access network edge node corresponding to the network-side physical port connected with the access node  2  is the MAC address of the access node  2  directed to the access network edge node. 
     As illustrated in  FIG. 8   a , if a packet is sent from a user terminal to an access network edge node (in an uplink direction) and an access node  1  forwards the packet to an access node  2 , an IP bridging forwarding method may include the following steps. 
     In step S 802   a , the access node  1  searches in an IP bridging forwarding table according to a source IP address in the packet from the user terminal, to obtain a network-side physical port, a virtual local area network identifier and the MAC address of the access network edge node. 
     In step S 804   a , the access node  1  updates a destination MAC address in the packet with the MAC address of the access node  2 , updates a source MAC address in the packet with the MAC address of the access node  1 , and adds or updates the virtual local area network identifier information in the packet. 
     In step S 806   a , the access node  1  forwards the updated packet to the access node  2  via the obtained network-side physical port. 
     As illustrated in  FIG. 8   b , if a packet is sent from a user terminal to an access network edge node (in an uplink direction) and an access node  2  forwards the packet to the access network edge node, an IP bridging forwarding method may include the following steps. 
     In step S 802   b , the access node  2  searches in an IP bridging forwarding table according to a source IP address in the packet from the user terminal, to obtain a network-side physical port, a virtual local area network identifier and a MAC address of the access network edge node. 
     In step S 804   b , the access node  2  updates a destination MAC address in the packet with the MAC address of the access network edge node, updates a source MAC address in the packet with the MAC address of the access node  2 , and adds or updates the virtual local area network identifier information in the packet. 
     In step S 806   b , the access node  2  forwards the updated packet to the access network edge node via the obtained network-side physical port. 
     As illustrated in  FIG. 9   a , if a packet is sent from an access network edge node to a user terminal (in an downlink direction) and an access node  2  forwards the packet to the access node  1 , an IP bridging forwarding method may include the following steps. 
     In step S 902   a , the access node  2  searches in an IP bridging forwarding table according to a destination IP address in the packet from a network-side physical port, to obtain a user-side physical port and a user MAC address. 
     In step S 904   a , the access node  2  updates a destination MAC address in the packet with the MAC address of the access node  1 , and updates a source MAC address in the packet with the MAC address of the access node  2 . 
     In step S 906   a , the access node  2  forwards the updated packet from the obtained user-side physical port to the access node  1 . 
     As illustrated in  FIG. 9   b , if a packet is sent from an access network edge node to a user terminal (in an downlink direction) and an access node  1  forwards the packet to the user terminal, an IP bridging forwarding method may include the following steps. 
     In step S 902   b , the access node  1  searches in an IP bridging forwarding table according to a destination IP address in the packet from a network-side physical port, to obtain a user-side physical port and a user MAC address. 
     In step S 904   b , the access node  1  updates a destination MAC address in the packet with the user MAC address, and updates a source MAC address in the packet with the MAC address of the access node  1 . 
     In step S 906   b , the access node  1  forwards the updated packet to the user terminal via the obtained user-side physical port. 
     As can be seen from the above, a reference point R is introduced between ANs according to the present invention, so that the ANs can connect with each other to constitute a new single-edge or multi-edge access network reference architecture. 
     For multicast or broadcast service, the present invention may adopt a chain, ring or dual-ring networking path (BNG-&gt;AN 1 -&gt;AN 2 -&gt;AN 3  . . . -&gt;ANn) to transmit broadcast or multicast data streams. For unicast service, the present invention may adopt a star or tree networking path to transmit unicast data streams. The BRAS/BNG and the aggregation network do not need to support multicast copy, and multicast or broadcast service can be provided only if the ANs are connected with each other, thereby reducing multicast or broadcast traffic greatly in the network. 
     If a single point of failure occurs in a connection between an AN and the BRAS/BNG, the connection can be switched to another AN link because the ANs can be connected with each other, thereby implementing a multi-homing function. 
     The present invention can meet the requirements of future VoIP and Peer-to-Peer communications. The traffic between users covered by different ANs may be transferred at the data plane not via the BRAS/BNG but via interfaces between the ANs. Therefore, the delay can be reduced and the communication path can be shortened evidently, to prevent the BRAS/BNG from becoming a bottleneck for communications. 
     An AN supporting layer-3 routing or an AN supporting IP bridging may be adopted in the present invention. 
     Technical solutions of the present invention are described with reference to the attached drawings as follows. 
     According to the present invention, a reference point R is introduced between ANs, so that the ANs can connect with each other. A single-edge access network reference architecture is illustrated in  FIG. 10 . In addition, a multi-edge access network reference architecture is illustrated in  FIG. 11 , in which a reference point R is introduced similarly. 
     In the access network architecture according to the present invention, star or tree and chain or ring hybrid networking may be adopted between an AN (i.e., DSLAM or OLT) of the DSL network and the BRAS/BNG, as illustrated in  FIG. 12 . As illustrated in  FIG. 12(   a ) and  FIG. 12(   b ), a chain, ring or dual-ring networking path (BNG-&gt;AN 1 -&gt;AN 2 -&gt;AN 3  . . . -&gt;ANn) may be adopted to transmit multicast or broadcast data streams, and a star or tree networking path may be adopted to transmit unicast data streams. As illustrated in  FIG. 12(   c ) and  FIG. 12(   d ), a ring networking path may be adopted to transmit multicast or broadcast data streams. The BRAS/BNG and the aggregation network do not need to support multicast copy, and multicast or broadcast service can be provided only if the ANs are connected with each other, thereby reducing multicast or broadcast traffic greatly in the network. 
     For multicast or broadcast service, assuming that if a single point of failure occurs on the BNG-&gt;AN 1  path as illustrated in  FIG. 13(   a ) and  FIG. 13(   b ), a chain networking path (BNG-&gt;AN 2 -&gt;AN 3  . . . -&gt;Ann; AN 2 -&gt;AN 1 ) according to the present invention may be adopted to transmit multicast or broadcast data streams, here AN 2  is a multicast copy point; or as illustrated in  FIG. 14(   a ) and  FIG. 14(   b ), a second ring networking path of a dual-ring (BNG-&gt;ANn-&gt; . . . AN 3 -&gt;AN 2 -&gt;An 1 ) may be adopted to transmit multicast or broadcast data streams; if a single point of failure occurs on the AN 1 -&gt;AN 2  path as illustrated in  FIG. 14(   c ) and  FIG. 14(   d ), an inverse networking path of a ring (BNG-&gt;AN 1 -&gt;ANn . . . AN 3 -&gt;AN 2 ) may be adopted to transmit multicast or broadcast data streams. This shows the flexibility of chain or ring networking which supports multicast or broadcast service. 
     For unicast service, if a single point of failure occurs on the BNG-&gt;AN 1  path as illustrated in  FIG. 15 , a networking path (BNG-&gt;AN 2 -&gt;AN 1 ) according to the present invention may be adopted to transmit unicast data streams to implement a multi-homing function; or as illustrated in  FIG. 16 , a networking path (NSP 2 -&gt;BNG-&gt;AN 2 -&gt;AN 1 ) according to the present invention may be adopted to transmit unicast data streams to implement a multi-homing function. 
     For meeting the requirements of future VoIP and Peer-to-Peer communications, for the traffic between users covered by different ANs, control signaling may still passes through the BRAS/BNG, but packet data on the data plane may be transferred not via the BRAS/BNG but via interfaces between the ANs directly as illustrated in  FIG. 17(   a ) and  FIG. 17(   b ). Therefore, the delay can be reduced and the communication path can be shortened evidently, to prevent the BRAS/BNG from becoming a bottleneck of communications. 
     In the unicast models in  FIG. 15  and  FIG. 16 , ANs supporting layer-3 routing or ANs supporting IP bridging may be adopted. As illustrated in  FIG. 18 , assuming that the RG is a layer-2 bridging device, the IP address of UE 1  is IPa, the IP address of BNG/ER is IPr (a default gateway of a user), the IP address of a party peer to the UE (the Server in  FIG. 18 ) is IPx, the MAC address of UE 1  is MAC 1 , the MAC address of AN 1  is MAC 2 , the MAC address of AN 2  is MAC 3 , the MAC address of BNG/BRAS/ER is MAC 4 ; the interworking port n of AN 1  is connected with the interworking port m of AN 2 , the AN 1  user port connected with the UE 1  is port x, the AN 1  is connected uplink via the port t, the AN 2  is connected uplink via the port r. If a single point of failure occurs on the BNG-&gt;AN 1  path, the connection will be switched to the BNG-&gt;AN 2  path. 
     The AN provides an Address Resolution Protocol Proxy (ARP Proxy) function. As illustrated in  FIG. 19 , the AN 1  forwards an ARP request (ARP-req) from the AN 1  user port x to the AN 2 , and the AN 2  responds an ARP reply (ARP-reply) including the MAC address MAC 3  of AN 2  to the AN 1 ; the AN 2  forwards an ARP request (ARP-req) from the port r to the AN 1 , the AN 2  responds an ARP reply (ARP-reply) including the MAC address MAC 3  of AN 2  via the port r, and the AN 1  responds an ARP reply (ARP-reply) including the MAC address MAC 2  of AN 1  to the AN 2 . 
     The AN may obtain the MAC address of a user device and address information obtained by an ARP proxy via a Dynamic Host Configuration Protocol (DHCP) message initiated by the user device, so as to create an IP bridging forwarding table. Therefore, an IP bridging forwarding table based on IP session in the AN, as illustrated in tables 1 to 2 Particularly, the IP session is represented by a use-side physical port, a user IP address and a user MAC address; an IP service connection is represented by a network-side physical port, a virtual local area network identifiers S-VLAN and the BNG MAC address. 
     The S-VLAN may be configured statically or dynamically. In table 1, the MAC address corresponding to the IP address of a user connected with the interworking port m of AN 2  is not the user&#39;s MAC address, but the MAC address of the AN covering the user. In table 2, the MAC address corresponding to the IP address of a user connected with the interworking port n of AN 1  is not the MAC address of the BNG which the user belongs to, but the MAC address of the AN connected with the interworking port n of AN 1 . 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 IP bridging forwarding table of AN2 
               
             
          
           
               
                 IP session 
                 IP service connection 
               
             
          
           
               
                 User-side 
                   
                   
                   
                   
                 BNG 
               
               
                 physical 
                 User IP 
                 User MAC 
                 Network-side 
                   
                 MAC 
               
               
                 port 
                 address 
                 address 
                 physical port 
                 S-VLAN 
                 address 
               
               
                   
               
               
                 m 
                 IPa 
                 MAC2 
                 r 
                 1011 
                 MAC4 
               
               
                   
                   
                 (MAC 
               
               
                   
                   
                 address of a 
               
               
                   
                   
                 next AN) 
               
               
                 y 
                 . . . 
                 . . . 
                 r 
                 1011 
                 MAC4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 IP bridging forwarding table of AN1 
               
             
          
           
               
                 IP session 
                   
               
             
          
           
               
                 User-side 
                   
                 User 
                 IP service connection 
               
             
          
           
               
                 physical 
                 User IP 
                 MAC 
                 Network-side 
                   
                 BNG MAC 
               
               
                 port 
                 address 
                 address 
                 physical port 
                 S-VLAN 
                 address 
               
               
                   
               
               
                 . . . 
                 . . . 
                 . . . 
                 t 
                 1011 
                 MAC4 
               
               
                 x 
                 IPa 
                 MAC1 
                 n 
                 0111 
                 MAC3 
               
               
                   
                   
                   
                   
                   
                 (MAC 
               
               
                   
                   
                   
                   
                   
                 address of a 
               
               
                   
                   
                   
                   
                   
                 next AN) 
               
               
                   
               
             
          
         
       
     
     The IP bridging procedure in the access network reference architecture according to the present invention is illustrated in  FIG. 20 , in which the parentheses denote a packet of a certain layer. Particularly, the packet including IP addresses is an IP packet, and the packet including MAC addresses is a MAC frame. 
     In the Uplink Direction: 
     For a packet (including a MAC frame and an IP packet header) from a user-side port of the AN 1 , the AN 1  searches in an IP bridging forwarding table of the AN 1  (i.e., table 2) according to a source IP address of the IP packet (IPa in this example) to obtain a network-side physical port (port n in this example), S-VLAN (0111 in this example) and a BNG MAC address (MAC 3  in this example), modifies a destination address of the MAC frame of the packet to MAC 3 , modifies the source address to the MAC address of AN 1 , MAC 2 , and adds or modifies the S-VLAN in the MAC frame to ‘0111’; then forwards the packet to the AN 2  via the port n, thereby accomplishing IP bridging at this node. Alternatively, the port n is bound with a specific S-VLAN value in a default manner, e.g., ‘0111’. The IP service connection is represented only by the S-VLAN and the BNG MAC address. If the S-VLAN ‘0111’ is obtained by search, it is default that the packet is forwarded by the port n; if another S-VLAN is obtained by search, it is default that the packet is forwarded by the port t. 
     For a packet (including a MAC frame and an IP packet header) from a user-side port of the AN 2 , the AN 2  searches in an IP bridging forwarding table of the AN 2  (i.e., table 1) according to a source IP address of the IP packet (IPa in this example) to obtain a network-side physical port (port r in this example), S-VLAN (1011 in this example) and a BNG MAC address (MAC 4  in this example), modifies a destination address of the MAC frame of the packet to MAC 4 , modifies the source address to the MAC address of AN 2 , MAC 3 , and adds or modifies the S-VLAN in the MAC frame to ‘1011’; then forwards the packet to the BNG via the port r, thereby accomplishing IP bridging at this node. 
     In the Downlink Direction: 
     For a packet (including a MAC frame and an IP packet header) from a network-side port of the AN 2 , the AN 2  searches in an IP bridging forwarding table of the AN 2  (i.e., table 1) according to a destination IP address of the IP packet (IPa in this example) to obtain a user-side physical port (port m in this example) and a user MAC address (MAC 2  in this example), modifies a destination address of the MAC frame of the packet to MAC 2 , modifies a source address of the MAC frame to the MAC address of AN 2 , MAC 3 , and forwards the packet to the AN 1  via the port m, thereby accomplishing IP bridging at this node. 
     For a packet (including a MAC frame and an IP packet header) from a network-side port of the AN 1 , the AN 1  searches in an IP bridging forwarding table of the AN 1  (i.e., table 2) according to a destination IP address of the IP packet (IPa in this example) to obtain a user-side physical port (port x in this example) and a user MAC address (MAC 1  in this example), modifies a destination address of the MAC frame of the packet to MAC 1 , modifies a source address of the MAC frame to the MAC address of AN 1 , MAC 2 , and forwards the packet to the user via the port x, thereby accomplishing IP bridging at this node. 
     The above description is for the embodiments of the present invention, which shall not limit the scope of the present invention. It is apparent to those skilled in the art that various variations and substitutes may be easily made to the present invention within the scope of the present invention as defined in the appended claims.