Abstract:
A PPP termination device allowing PPP termination at a location where a decreased number of subscriber&#39;s signals are multiplexed is disclosed. The PPP termination is performed in an access gateway including a switch in the access network system. The access gateway includes a first interface to a DSLAM, a second interface to a backbone network, and a header processor for converting between a network-layer packet and a PPP data packet including the network-layer packet. The switch switches a connection among the first interface, the second interface, the PPP link controller, and the header processor, depending on which one of a PPP data packet and a link control packet is inputted. The link control packet is used for link establishment and disconnection.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an access network system for access to a backbone network such as Internet, and in particular to a Point-to-Point Protocol (PPP) termination device thereof. 
   2. Description of the Related Art 
   PPP is a data link protocol that is very popular for data communication between two computer systems. Typically, the PPP protocol is used for computer dial-up access to an Internet Service Provider (ISP) over telephone lines. PPP also provides Authentication, Accounting, Service Management System (SMS), bandwidth allocation for each subscriber, etc. 
   In the case of Internet access, it is necessary to terminate PPP to transfer IP (Internet Protocol) packets to the Internet. Conventionally, a dedicated device is installed at the gate to the ISP to perform PPP termination. 
     FIG. 1  shows an example of a conventional access network system. In the access network system, a personal computer (PC)  1  is connected to an ADSL Transceiver Unit-Remote (ATU-R)  2 , which is connected to a Digital Subscriber Line Access Multiplexer (DSLAM)  3  through ADSL/VDSL line  1101 , wherein ADSL stands for Asymmetric Digital Subscriber Line and VDLS for Very high-speed DSL. The DSLAM  3  is connected to an ATM switch (SW)  4  through ATM OC-3c transmission service  2101 . The ATM SW  4  is connected to PPP termination device  5  through ATM OC-12c transmission service  3101 . In this example, the PPP termination device  5  is the dedicated device for terminating PPP  501  to transfer IP packets to a backbone network  6  such as Internet. Therefore, the PPP termination device  5  is substantially a gate to the ISP. 
   A packet communication system based on the PPP protocol has been disclosed in Japanese Patent Application Unexamined Publication No, 10-322399. In this packet communication system, IP packets are converted to PPP packets according to the PPP protocol and then the PPP packets are in turn converted to ATM cells and vice versa. Such an arrangement allows a plurality of ATM logical lines to be multiplexed into a single physical line without loss of PPP functions and ATM additional functions. 
   However, according to the prior art, as shown in  FIG. 1 , it is necessary to provide the PPP termination device  5  at the gate of the backbone network  6 , where a large number of subscriber&#39;s signals are multiplexed. It is also necessary to increase the number of PPP termination devices depending on an increasing number of subscribers. 
   Alternatively, it can be considered that the ATM layer and the higher AAL5 layer processing and PPP termination are implemented in the DSLAM  3 . However, the virtual Channel (VC) itself must be processed at a layer higher than the AAL5 layer, resulting in more complicated structure of the DSLAM  3 , increased system cost, and complicated system management. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a PPP termination device in an access network system, allowing PPP termination at a location where a decreased number of subscriber&#39;s signals are multiplexed. 
   Another object of the present invention is to provide a PPP termination device in an access network system, which can flexibly deal with an increase in the number of subscribers. 
   According to the present invention, the Point-to-Point Protocol (PPP) termination is performed in a switch in the access network system by which a subscriber gets access to a backbone network. 
   A PPP termination device includes: a first interface to a Digital Subscriber Line Access Multiplexer (DSLAM); a second interface to a backbone network; a Point-to-Point Protocol (PPP) link controller for establishing and disconnecting a PPP link; a header processor for converting between a network-layer packet and a PPP data packet including the network-layer packet; and a switch for switching a connection among the first interface, the second interface, the PPP link controller, and the header processor, depending on which one of a PPP data packet and a link control packet is inputted, wherein the link control packet is used for link establishment and disconnection. 
   According to an aspect of the present invention, a PPP termination device includes: a first interface for inputting and outputting PPP packets in Asynchronous Transfer Mode (ATM) cells from and to a corresponding Digital Subscriber Line Access Multiplexer (DSLAM); a second interface for inputting and outputting network-layer packets in ATM cells from and to the backbone network; a PPP link controller for establishing and disconnecting a PPP link; a packet processor for converting between a network-layer packet and a PPP data packet including the network-layer packet and controlling cell forwarding; and an ATM switch for switching ATM cells among the first interface, the second interface, the PPP link controller, and the packet processor, depending on which one of a PPP data packet in ATM cells and a link control packet in ATM cells is inputted, wherein the link control packet is used for link establishment and disconnection. 
   The packet processor may include: a header processor; and a first AAL5-SAR (ATM Adaptation Layer type 5—Segmentation And Reassembly) section, wherein the header processor determines which one of a PPP data packet in ATM cells and a link control packet in ATM cells is inputted from the first interface through the first AAL5-SAR section and, when the PPP data packet has been inputted through the first AAL5-SAR section, generates a network-layer packet by removing a PPP header of the PPP data packet, and the first AAL5-SAR section segments the network-layer packet into ATM cells to be forwarded to the second interface by the ATM switch. 
   When the link control packet has been inputted from the first interface through the first AAL5-SAR section, the first AAL5-SAR section segments the link control packet into ATM cells to be forwarded to the PPP link controller by the ATM switch, wherein the PPP link controller performs a link establishment procedure based on the link control packet in ATM cells. 
   When receiving a network-layer packet in ATM cells from the backbone network, the packet processor performs PPP encapsulation to generate a PPP data packet from the network-layer packet and the first AAL5-SAR section segments the PPP data packet into ATM cells to be forwarded to the first interface by the ATM switch. 
   The PPP link controller may include: a second AAL5-SAR section; and a processor, wherein, when receiving the link control packet from the first interface through the second AAL5-SAR section, the processor generates a response control packet in response to the link control packet and the second AAL5-SAR section segments the response control packet into ATM cells to be forwarded to the first interface by the ATM switch. 
   According to another aspect of the present invention, a Point-to-Point Protocol (PPP) termination device in an access network system by which a subscriber gets access to a backbone network, include: a first interface for inputting and outputting PPP packets over SONET (Synchronous Optical NETwork) from and to a corresponding Digital Subscriber Line Access Multiplexer (DSLAM), wherein the first interface includes a first packet processor for determining which one of a PPP data packet and a link control packet is inputted, converting between a packet and ATM cells, and controlling cell forwarding; a second interface for inputting and outputting network-layer packets over SONET from and to the backbone network, wherein the second interface includes a second packet processor for converting between a network-layer packet and a PPP data packet including the network-layer packet, converting between a packet and ATM cells, and controlling cell forwarding; a PPP link controller for establishing and disconnecting a PPP link; and an ATM switch for switching ATM cells among the first interface, the second interface, and the PPP link controller, depending on which one of a PPP data packet and a link control packet is inputted. 
   The first packet processor may include: a first header processor; and a first AAL5-SAR (ATM Adaptation Layer type 5—Segmentation And Reassembly) section, wherein the first header processor determines which one of a PPP data packet and a link control packet is inputted from the first interface and, when the PPP data packet has been inputted, generates a network-layer packet by removing a PPP header of the PPP data packet, and the first AAL5-SAR section segments the network-layer packet into ATM cells to be forwarded to the second interface by the ATM switch. 
   When the link control packet has been inputted from the first interface, the first AAL5-SAR section segments the link control packet into ATM cells to be forwarded to the PPP link controller by the ATM switch, wherein the PPP link controller performs a link establishment procedure based on the link control packet. 
   The second packet processor may include: a second header processor; and a second AAL5-SAR section, wherein, when receiving a network-layer packet over SONET from the backbone network, the second header processor performs PPP encapsulation to Generate a PPP data packet from the network-layer packet and the second AAL5-SAR section segments the PPP data packet into ATM cells to be forwarded to the first interface by the ATM switch. 
   The PPP link controller may include: a third AAL5-SAR section; and a processor, wherein, when receiving the link control packet from the first interface through the third AAL5-SAR section, the processor generates a response control packet in response to the link control packet and the third AAL5-SAR section segments the response control packet into ATM cells to be forwarded to the first interface by the ATM switch. 
   An access network system according to an embodiment of the present invention, includes: a plurality of subscriber computers; a plurality of Remote ADSL Transceiver Units (ATU-Rs) each connected to the subscriber computers; a plurality of Digital Subscriber Line Access Multiplexers (DSLAMs), each of which accommodates a plurality of ATU-Rs; an access gateway to a backbone network, the access gateway accommodating a plurality of DSLAMs, wherein the access gateway comprises: 
   a first interface to a corresponding Digital Subscriber Line Access Multiplexer (DSLAM); 
   a second interface to the backbone network; 
   a Point-to-Point Protocol (PPP) link controller for establishing and disconnecting a PPP link with an ATU-R; 
   a header processor for converting between a network-layer packet and a PPP data packet including the network-layer packet and; and 
   a switch for switching a connection among the first interface, the second interface, the PPP link controller, and the header processor, depending on which one of a PPP data packet and a link control packet is inputted, wherein the link control packet is used for link establishment and disconnection. 
   As described above, according to the present invention, a PPP termination device allows PPP termination at a location where a decreased number of subscriber&#39;s signals are multiplexed. This may prevent the network from confusion due to unauthorized accesses. In addition, the PPP termination device can flexibly deal with an increase in the number of subscribers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing an example of a conventional access network system; 
       FIG. 2  is a block diagram showing the configuration of an access network system according to a first embodiment of the present invention; 
       FIG. 3  is a block diagram showing an access gateway of the first embodiment so as to explain a termination operation of PPP control packet; 
       FIG. 4  is a block diagram showing the access gateway of the first embodiment so as to explain a termination operation of PPP data packet; 
       FIG. 5  is a block diagram showing the configuration of an access network system according to a second embodiment of the present invention; 
       FIG. 6  is a block diagram showing an access gateway of the second embodiment so as to explain a PPP termination operation; 
       FIG. 7A  is a diagram showing a HDLC PPP frame format; 
       FIG. 7B  is a diagram showing ATM cell segmentation of a PPP frame based on AAL5; 
       FIG. 8  is a diagram showing the values of protocol field of a PPP packet and a corresponding content of the packet data; 
       FIG. 9  is a diagram showing PPP link processing; and 
       FIG. 10  is a diagram showing PPP processing sequence. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   Referring to  FIG. 2 , it is assumed for simplicity that an access network system according to a first embodiment of the present invention includes a subscriber&#39;s personal computer (PC)  11  connected to an ADSL Transceiver Unit-Remote (ATU-R)  12 , which is connected to a Digital Subscriber Line Access Multiplexer (DSLAM)  13  through ADSL/VDSL line  1101  on a metallic cable. The DSLAM  13  is connected to an access gateway  14  through ATM OC-3c (155 Mbps) transmission service  2101 . The access gateway  14  is connected to the backbone network  6  through ATM OC-12c (622 Mbps) transmission service  3101 . 
   The ATU-R 12 has the ATM Adaptation Layer type 5 (AAL5) function and an IP-over-ATM protocol. The DSLAM  13  accommodates a plurality of ATU-Rs  12  to multiplex respective, ATM cell flows into an ATM OC-3c signal and demultiplex an ATM OC-3c signal into respective ATM cell flows. The access gateway  14  accommodates a plurality of DSLAMs  13  and, as will be described later, plays a role as a PPP termination point, or a gate to the ISP, to perform controls including Authentication, Accounting and the like. 
   PPP Termination 
   The PPP termination will be described briefly with reference to  FIGS. 7–10 . 
   It is necessary to establish a link between the ATU-R  12  and the access gateway  14  according to the PPP protocol before IP packets are transferred in ATM cell form between them. 
   A HDLC (high-level Data Link Control) PPP frame format is shown in  FIG. 7A . The HDLC PPP frame is composed of a 1-byte flag field, a 1-byte address field, a 1-byte control field, a payload, a 16/32-bit FCS field, and a 1-byte flag field. A PPP packet is composed of a 32-bit protocol field and a packet data field and is inserted into the payload of the HDLC PPP frame. A network-layer packet such as IP packet or ICMP (Internet Control Message Protocol) packet is inserted into the packet data field. Typically, the network-layer packet is an IP packet. 
   As shown in  FIG. 7B , the HDLC PPP frame obtained by PPP encapsulation is converted into a plurality of ATM cells each composed of a header and a payload according to the AAL5 function. The ATU-R  12  performs such ATM cell conversion based on the IP-over-ATM protocol. 
   As shown in  FIG. 8 , the content of the packet data in the PPP packet of  FIG. 7A  is determined depending on the value of the protocol field. When ‘0021’ is stored in the protocol field, the PPP packet is a PPP data packet storing IP data in the packet data field. When ‘8021’ or ‘c021’ is stored in the protocol field, the PPP packet is a link control packet or a LCP packet. 
   As shown in  FIG. 9 , in Link Control Protocol (LCP) processing, after a link has been established (Link Establishment Phase), Authentication is performed (Authentication Phase). When the authentication has been successfully completed, a Network Layer Protocol Phase is started. When closing, a Link Termination Phase is started in the LCP processing. 
   In summary, as shown in  FIG. 10 , on a peer-to-peer line, a LCP packet having ‘c021’ stored in the protocol field is used to establish the link (LCP Configuration). Thereafter, a NCP packet having ‘8021’ stored in the protocol field is used to perform configuration for upper layer data transfer (NCP Configuration). Then, user packets (IP packets) are transferred (Data Transfer). 
   Since the PPP frame as shown in  FIG. 7B  is transmitted in the form of ATM cells using AAL5, the PPP termination cannot be performed without AAL5-SAR (Segmentation And Reassembly) processing. The SAR function is used to reassemble an original PPP frame, which allows the link establishment processing. 
   According to the present embodiment, the PPP termination is performed in the access gateway  14 . The details will be described hereinafter. 
   Access Gateway 
   1.1) Structure 
   As shown in  FIG. 3 , the access gateway  14  is composed of a DSLAM-side ATM interface board  101 , a switch board  201 , a packet processing board  301 , a backbone-side ATM interface board  401 , and a CPU board  501 . 
   The ATM interface board  101  receives and transmits optical ATM cells from and to the DSLAM  13  at OC-3c transmission speed. A received ATM cell flow  1102  is converted from optical to electrical, and is transferred to the switch board  201 . A transmitted ATM cell flow  1103  is converted from electrical to optical, and is transmitted to the DSLAM  13 . 
   The ATM interface board  401  receives and transmits optical ATM cells from and to the backbone network  6  at OC-12c transmission speed. As shown in  FIG. 4 , a received ATM cell flow  4402  is converted from optical to electrical, and is transferred to the switch board  201 . An ATM cell flow  4403  received from the switch board  201  is converted from electrical to optical, and is transmitted to the backbone network  6 . 
   The switch board  201  has an ATM switch module  202  implemented thereon. The ATM switch module  202  forwards an ATM cell received from one input port to an appropriate output port depending on the routing information VPI/VCI of the ATM cell. 
   In this embodiment, the ATM switch module  202  has a throughput of the order of 5 Gbps. 
   The packet processing board  301  has a field check and header process section  303  and an AAL5-SAR section  305  implemented thereon. The AAL5-SAR section  305  reassembles a flow of ATM cells received from the ATM switch module  202  to output an original packet flow  302  to the field check and header process section  303 . The AAL5-SAR section  305  also segments a packet flow  304  received from the field check and header process section  303  to produce a flow of ATM cells under control of the field check and header process section  303 . The AAL5-SAR section  305  has a capability to process an OC-12c flow of ATM cells received from the backbone network. The number of ATM connections the AAL5-SAR section  305  is capable of concurrently processing is several tens thousands. 
   The CPU board  501  has an AAL5-SAR section  502  and a central processing unit (CPU)  503  implemented thereon, which terminates the PPP protocol with reference to  FIG. 3 . 
   1.2) Link Control 
   As shown in  FIG. 3 , an ATM cell flow  1102  received from the DSLAM  13  at the ATM interface board  101  is forwarded to the packet processing board  301  through the ATM switch module  202 . The AAL5-SAR section  305  reassembles the received ATM cells into an original PPP packet. 
   When receiving a PPP packet from the AAL5-SAR section  305 , the field check and header process section  303  looks at the protocol field of the PPP packet (see  FIG. 7A ). If the value of the protocol field is ‘8021’ or ‘C021’, then the field check and header process section  303  outputs a control signal  306  indicating that the received PPP packet is a link control packet (see  FIG. 8 ) and forwards the link control packet  304  to the AAL5-SAR section  305 . Tho AAL5-SAR section  305  segments it into ATM cells each having VPI/VCI added to the header thereof so as to be forwarded to the CPU board  501  by the ATM switch module  202 . Accordingly, the ATM switch module  202  switches a flow of ATM cells  3301  to the CPU board  501 . 
   The AAL5-SAR section  502  of the CPU board  501  reassembles the received ATM cells from the ATM switch module  202  to produce an original PPP packet (link control packet) and outputs it to the CPU  503 . The CPU  503  executes a PPP processing program to perform the PPP processing as shown in  FIGS. 9 and 10 . In the PPP processing, as described before, the CPU  503  generates a response packet from the link control packet and the AAL5-SAR section  502  segments it into a flow of ATM cells  5504 , each cell having VPI/VCI directed to the subscriber PC originating the flow of ATM cells  1102  through the ATM interface board  101  by the ATM switch module  202 . Accordingly, the ATM switch module  202  switches the flow of ATM cells  5504  to the ATM interface board  101 , from which the flow of ATM cells  5504  is transmitted as an optical OC-3c ATM signal to the DSLAM  13 . The DSLAM  13  demultiplexes it into a plurality of flows of ATM cells for respective ADSL/VDSL lines depending on the VPI/VCI information of each ATM cell, each or which is forwarded to a corresponding ATU-R  12 . In this manner, the link between the ATU-R  12  and the access gateway  14  is established. 
   1.3) Data Transfer 
   As shown in  FIG. 4 , an ATM cell flow  1102  received from the DSLAM  13  at the ATM interface board  101  is forwarded to the packet processing board  301  through the ATM switch module  202 . The AAL5-SAR section  305  reassembles the received ATM cells into an original PPP packet. 
   When receiving a PPP packet from tho AAL5-SAR section  305 , the field check and header process section  303  looks at the protocol field of the PPP packet (see  FIG. 7A ). If the value of the protocol field is ‘0021’, then the field check and header process section  303  outputs a control signal  306  indicating that the received PPP packet is a PPP data packet including an IP packet (see  FIG. 8 ) and terminates the PPP protocol to forward the IP packet  304  included in the PPP data packet  302  to the AAL5-SAR section  305 . 
   The AAL5-SAR section  305  segments the IP packet  304  to produce a flow of ATM cells  3301 , each cell having VPI/VCI added to the header thereof so as to be forwarded to the backbone-side ATM interface board  401  by the ATM switch module  202 . Accordingly, the ATM switch module  202  switches the flow of ATM cells  3301  and then outputs it as an ATM cell flow  4403  to the ATM interface board  401 . The ATM interface board  401  converts the ATM cell flow  4403  from electrical to optical to transmit it as an OC-12c optical ATM cell flow to the backbone network  6 . 
   In the case where an ATM cell flow  4402  received from the backbone network  6  at the ATM interface board  401  is forwarded to the packet processing board  301  through the ATM switch module  202 . The AAL5-SAR section  305  reassembles the received ATM cells into an original IP packet. 
   When receiving the IP packet from the AAL5-SAR section  305 , the field check and header process section  303  performs PPP encapsulation to produce a PPP data packet  304  having ‘0021’ stored in the protocol field of the PPP packet (see  FIG. 7A  and  FIG. 8 ). The field check and header process section  303  outputs a control signal  306  indicating that the PPP packet is a PPP data packet including an IP packet (see  FIG. 8 ) and forwards the PPP data packet  304  to the AAL5-SAR section  305 . 
   The AAL5-SAR section  305  segments the PPP data packet  304  to produce a flow of ATM cells  3302 , each cell having VPI/VCI added to the header thereof so as to be forwarded to the DSLAM-side ATM interface board  101  by the ATM switch module  202 . Accordingly, the ATM switch module  202  switches the flow of ATM cells  3302  and then outputs it as an ATM cell flow  1103  to the ATM interface board  101 . The ATM interface board  401  converts the ATM cell flow  1103  from electrical to optical to transmit it as an OC-3c optical ATM cell flow to the DSLAM  13 . 
   The DSLAM  13  demultiplexes it into a plurality of flows of ATM cells for respective ADSL/VDSL lines depending on the VPI/VCI information of each ATM cell, each of which is forwarded to a corresponding ATU-R  12 . The ATU-R  12  reassembles the received ATM cells into a PPP packet and terminates the PPP protocol to output the IP packet included in the PPP packet to the subscriber PC  11 . 
   Second Embodiment 
   Referring to  FIG. 5 , it is assumed for simplicity that an access network system according to a second embodiment of the present invention includes a subscriber&#39;s personal computer (PC)  41  connected to an ADSL Transceiver Unit-Remote (ATU-R)  42 , which is connected to a Digital Subscriber Line Access Multiplexer (DSLAM)  43  through ADSL/VDSL line  1101  on a metallic cable. In this embodiment, the ATU-R  42  is connected to the DSLAM  43  by packet-base interface. The DSLAM  43  is connected to an access gateway  44  through Packet-Over-SONET (POS) OC-3c transmission service  2101 . The DSLAM  43  multiplexes a plurality of packet flows received from ATU-Rs  12  to produce a 155 Mbps POS frame signal (OC-3c), which is transmitted to the access gateway  44 . The DSLAM  43  also demultiplexes a POS OC-3c frame signal received from the access gateway  44  into respective packet flows. The access gateway  44  is connected to a POS-ready backbone network  6  through POS OC-12c transmission service  3101 . 
   Access Gateway 
   2.1) Structure 
   As shown in  FIG. 6 , the access gateway  44  is composed of a DSLAM-side POS (OC-3c) interface board  5101 , a switch board  5201 , a backbone-side POS (OC-12c) interface board  5401 , and a CPU board  5501 . 
   In the second embodiment, the DSLAM-side POS (OC-3c) interface board  5101  has a field check and header process section  5104  and an AAL5-SAR section  5105  implemented thereon. The backbone-side POS (OC-12c) interface board  5401  has a field check and header process section  5404  and an AAL5-SAR section  5405  implemented thereon. The switch hoard  5201  has an ATM switch module  5202  implemented thereon. The CPU board  5501  has an AAL5-SAR section  5502  and a CPU  5503  implemented thereon, which terminates the PPP protocol as described before. 
   2.2) Link Control 
   Referring to  FIG. 6 , a PPP packet flow  8102  is received from the DSLAM  43  at the POS interface board  5101 . When receiving the PPP packet flow  8102 , the field check and header process section  5104  looks at the protocol field of a PPP packet (see  FIG. 7A ). If the value of the protocol field is ‘8021’ or ‘c021’, then the field check and header process section  5104  outputs a control signal  5108  indicating that the received PPP packet is a link control packet (see  FIG. 8 ) and forwards the link control packet to the AAL5-SAR section  5105 . The AAL5-SAR section  5105  segments it into ATM cells each having VPI/VCI added to the header thereof so as to be forwarded to the CPU board  5501  by the ATM switch module  5202 . Accordingly, the ATM switch module  5202  switches a flow of ATM cells  1107  to the CPU board  5501 . 
   The AAL5-SAR section  5502  of the CPU board  5501  reassembles the received ATM cell flow  1107  from the ATM switch module  5202  to produce an original PPP packet (link control packet) and outputs it to the CPU  5503 . The CPU  503  executes a PPP processing program to perform the PPP processing as shown in  FIGS. 9 and 10 . In the PPP processing, as described before, the CPU  5503  generates a response packet from the link control packet and the AAL5-SAR section  5502  segments it into a flow of ATM cells  5504 , each cell having VPI/VCI directed to the subscriber PC originating the flow of ATM cells  1107  through the POS interface board  5101  by the ATM switch module  5202 . Accordingly, the ATM switch module  5202  switches the flow of ATM cells  5504  to the POS interface board  5101 . 
   When receiving the ATM cell flow  5504  from the ATM switch module  5202 , the AAL5-SAR section  5105  reassembles it into a response PPP packet  8103 , which is transmitted as an optical OC-3c POS frame signal to the DSLAM  43 . The DSLAM  43  demultiplexes it into a plurality of packet flows for respective ADSL/VDSL lines depending on the IP address of each PPP packet, each of which is forwarded to a corresponding ATU-R  42 . In this manner, the link between the ATU-R  42  and the access gateway  44  is established. 
   2.3) Data Transfer 
   Referring to  FIG. 6 , a PPP packet flow  810  is received from the DSLAM  43  at the POS interface board  5101 . When receiving the PPP packet flow  8102 , the field check and header process section  5104  looks at the protocol field of a PPP packet (see  FIG. 7A ). If the value of the protocol field is ‘0021’, then the field check and header process section  5104  outputs a control signal  5108  indicating that the received PPP packet is a PPP data packet including an IP packet (see  FIG. 8 ) and terminates the PPP protocol to forward the IP packet included in the PPP data packet to the ALL5-SAR section  5105 . 
   The AAL5-SAR section  5105  segments the IP packet to produce a flow of ATM cells  1106 , each cell having VPI/VCT added to the header thereof so as to be forwarded to the backbone-side POS interface board  5401  by the ATM switch module  5202 . Accordingly, the ATM switch module  5202  switches the flow of ATM cells  1106  and then outputs it to the POS interface board  5401 . When receiving the ATM sell flow  1106  from the ATM switch module  5202 , the AAL5-SAR section  5405  of the POS interface board  5401  reassembles it into an original IP packet  8104 , which is transmitted as an optical OC-12c POS frame signal to the backbone network  6 . 
   In the case where a POS OC-12c signal is received from the backbone network  6  at the POS interface board  5401 , an IP packet  8103  is output to the field check and header process section  5404 . The field check and header process section  5404  adds the protocol field to the received IP packet (see  FIG. 7A ) to produce a PPP data Packet (PPP encapsulation). The PPP data packet is output to the AAL5-SAR section  5405 . The AAL5-SAR section  5405  segments the PPP data packet including the received IP packet into ATM cells each having VPI/VCI added to the header thereof so as to be forwarded to the DSLAM-side POS interface board  5101  by the ATM switch module  5202 . 
   When receiving an ATM cell flow  4406  from the ATM switch module  5202 , the AAL5-SAR section  5105  of the POS interface board  5101  reassembles it into an original PPP data packet  8103 , which is transmitted as an optical OC-3c POS frame signal to the DSLAM  43 . 
   The DSLAM  43  demultiplexes the optical OC-3c POS frame signal into a plurality of flows of PPP packets for respective ADSL/VDSL lines depending on the IP address of each PPP packet, each of which is forwarded to a corresponding ATU-R  12 . The ATU-R  12  terminates the PPP protocol to output the IP packet included in the PPP packet to the subscriber PC  11 .