Patent Publication Number: US-6909716-B1

Title: System and method for switching and routing data associated with a subscriber community

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to and filed concurrently with pending U.S. patent application Ser. No. 09/823,604, entitled “System and Method for Switching and Routing Data Associated with a Subscriber Community”. These applications have been commonly assigned to Netensity, Inc. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to the field of data communications and in particular to a system and method for switching and routing data associated with a subscriber community. 
   BACKGROUND OF THE INVENTION 
   Historically, network bridges and routers have been built as standalone units. However, there are several limitations associated with the use of standalone units in a communications system to support multiple subscribers. Standalone units may have a limited number of available subscriber ports, and the physical reach of the individual subscriber lines coupling the subscribers to the units may also be limited. Further, economic considerations may limit the number of virtual circuits (VCs) that may be supported on each subscriber port. Where a unit is distributed, the number of VCs on the distribution ports between the distributed components of the unit may be as large as or even exceed the number of VCs between the unit and a central office (CO) or other external device. As a result, a large number of units may be required in order to accommodate a large subscriber community or a subscriber community spread over a relatively large area. These and other deficiencies have made previous systems inadequate for providing service to many subscriber communities. 
   SUMMARY OF THE INVENTION 
   According to the present invention, disadvantages and problems associated with previous techniques for communicating data associated with a subscriber community are substantially reduced or eliminated. 
   According to one embodiment of the present invention, a system for switching and routing data associated with one or more subscribers within a subscriber community includes layer-2 switching functionality within a first network access switch. The layer-2 switching functionality receives data units communicated by one or more external devices using one or more external links and switches the received data units for communication to an intermediate device within the first network access switch coupling the layer-2 switching functionality within the first network access switch to layer-3 routing functionality within the first network access switch. The intermediate device receives the switched data units from the layer-2 switching functionality, selectively communicates to the layer-3 routing functionality one or more first data units that have been received by the layer-2 switching functionality on an external link and are to be subsequently switched for communication to layer-2 switching functionality of another network access switch on an associated system link, the first data units containing layer-3 routing information for a data block including the first data units and a plurality of other data units, the first data units being communicated to the layer-3 routing functionality for processing including a determination of a system destination for the data block that includes an address of another network access switch that is associated with an external link coupled to the other network access switch, and selectively stores second data units, the second data units being data units within the data block that have been communicated to the intermediate device after the layer-3 routing functionality has begun to determine a system destination for the data block and before all other data units within the data block ahead of the second data units have been switched for communication to the layer-2 switching functionality of the other network access switch. 
   In another embodiment, a system for switching and routing data associated with one or more subscribers within a subscriber community includes layer-2 switching functionality within a first network access switch. The layer-2 switching functionality receives data units communicated by layer-2 switching functionality of one or more other network access switches using one or more system links and switches the received data units for communication to an intermediate device within the first network access switch coupling the layer-2 switching functionality within the first network access switch to layer-3 routing functionality within the first network access switch. The intermediate device receives the switched data units from the layer-2 switching functionality, selectively communicates to the layer-3 routing functionality one or more first data units that have been received by the layer-2 switching functionality on a system link and are to be subsequently switched for communication to an external device on an associated external link, the first data units containing incoming system destination information specifying an address of the first network access switch that is associated with that external link, the incoming system destination information allowing a data block including the first data units and a plurality of other data units to be switched and routed between two or more network access switches, the first data units being communicated to the layer-3 routing functionality for processing including formatting the first data units as standard layer-2 data units for switching and routing by external devices, and selectively stores second data units, the second data units being data units within the data block that have been communicated to the intermediate device after the layer-3 routing functionality has begun to format the first data units and before all other data units within the data block ahead of the second data units have been switched for communication to the external device. 
   Certain embodiments of the present invention may provide a number of important technical advantages. Unlike previous techniques, the present invention may allow the switching and routing of data in a communications system serving a subscriber community to be distributed among multiple network access switches within the system. Distributed switching and routing eases the burden on individual network access switches, thereby reducing their individual capacity requirements as well as their associated costs. Distributed switching and routing also reduces entry-level costs associated with initially setting up a communications system to serve a subscriber community. A single relatively inexpensive network access switch can accommodate the switching and routing demands of a relatively small subscriber community and, as the number of subscribers in the community grows, the system may be incrementally scaled to include additional network access switches according to particular needs. 
   In addition, performing layer-3 routing of ATM cells at their point of ingress into the system (a first network access switch) and formatting the cells with system destination information for communication within the system may allow the cells to be switched in layer-2 fashion (without layer-3 routing) at all intervening network access switches between the point of ingress into and the point of egress from the system (a second network access switch). In this case, layer-3 routing need only be performed again at the point of egress, where the cells are reformatted as standard ATM cells for communication to external devices. Reducing the amount of layer-3 routing that must be performed in communicating cells through the system may improve speed, conserve network resources, and provide other benefits. 
   The present invention may also allow for increased circuit concentration to reduce or eliminate the need to allocate a VC to each subscriber in the community that has a logical connection to a CO. Such circuit concentration may reduce the expense of the individual network access switches while maintaining their usefulness as the system is scaled to incorporate additional network access switches. For example, it may reduce costs associated with maintenance of the system and of the logical connections to the CO. It may also reduce the hardware requirements and the size of the VC table that the system hardware may use, further reducing costs associated with the system. Additionally, the present invention may allow for simpler switching and routing functionalities, thereby permitting the concentration of complexity associated with a large number of VCs into a single unit providing a connection to the CO. Moreover, the concentration of data traffic flowing bidirectionally between a wide area network (WAN) port and multiple subscriber ports into a single VC over a WAN link, to provide a connection to an Internet service provider (ISP) via the CO for example, may reduce the number of VCs that need to be supported by the CO. 
   When multiple VCs are concentrated into a single VC, it may be difficult to differentially shape traffic flowing to local ports. Since local ports often have little queuing capability, such traffic shaping may be desirable or even necessary. The present invention allows for the differential shaping of traffic flowing to local ports, which may be dynamically adjusted, enabling the local ports to train to varying line speeds. The present invention also allows for the combination of the demultiplexing function (of the concentrated VCs to multiple ports) and traffic-shaping function in a way that reduces the cost of performing both functions. 
   Systems and methods incorporating one or more of these or other technical advantages are well suited for switching and routing data associated with subscriber communities. Other technical advantages are readily apparent to those skilled in the art from the following figures, descriptions, and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To provide a more complete understanding of the present invention and the features and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates an exemplary system for switching and routing data associated with a subscriber community; 
       FIG. 2  illustrates an exemplary network access switch for switching and routing data associated with a subscriber community; and 
       FIGS. 3A through 3C  illustrate exemplary methods for switching and routing data associated with a subscriber community. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an exemplary system  10  for switching and routing data associated with a subscriber community. The subscriber community may include one or more businesses, apartment complexes, retirement communities, correctional facilities, or any other communities in which subscribers share communications infrastructure and resources. Each subscriber in the served community may have one or more associated telephones, personal computers, facsimile machines, or other devices to interface with the public switched telephone network (PSTN) or any other telephone network; a data network such as a local area network (LAN), metropolitan area network (MAN), WAN, or a global network such as the Internet; or any other suitable wireline, wireless, or other network. Such devices may be referred to as subscriber devices  12 . 
   Subscriber devices  12  are coupled to one or more network access switches  14  using subscriber lines  16  and associated subscriber ports  18 . Each network access switch  14  may support traffic associated with one or more subscriber devices  12 , including but not necessarily limited to signals communicated to and from associated subscribers. For example, in exemplary system  10 , network access switch  14   a  handles data associated with subscriber devices  12   aa - 12   an , network access switch  14   b  handles data associated with subscriber devices  12   ba - 12   bn , and so on. In one embodiment, subscriber lines  16  include asymmetric or other digital subscriber lines (xDSL) and communicate xDSL signals that may include, depending on traffic characteristics at the time, voice signals, video signals, multimedia signals, data signals, or other suitable signals in any appropriate combination. Reference to signals or the like is meant to encompass any appropriate calls, traffic, or other signals. 
   In the exemplary configuration of  FIG. 1 , network access switch  14   a  is coupled to WAN  20  using WAN link  22  and associated WAN port  24 . WAN link  22  provides a single logical link between network access switch  14   a  and WAN  20 . In one embodiment, WAN link  22  includes a T1 or other suitable link that supports Asynchronous Transfer Mode (ATM), Internet Protocol (IP), Frame Relay (FR), or any other suitable packet-based protocol. WAN link  22  may include any wireline, wireless, optical or other communications link providing suitable wideband or broadband characteristics over a distance of interest, often the distance between network access switch  14   a  and a CO within the PSTN. In one embodiment, only one network access switch  14  in system  10  supports a WAN link  22 , for cost savings, reduced complexity, or any other suitable reason. However, the present invention contemplates more than one WAN link  22  for system  10  to provide, for example, redundancy and increased fault tolerance. In this case, if one of multiple WAN links  22  fails, the capacity of system  10  may be decreased, but communications to WAN  20  on other WAN links  22  will still be available. One or more WAN links  22  may be associated with system  10  in any appropriate manner. For example, each WAN link  22  may be coupled to a different network access switch  14 . 
   Signals are communicated between network access switches  14  using system links  26  and associated system link ports  28 . Although an exemplary stacked “ring” topology is shown, the present invention contemplates any number of network access switches  14  coupled to one another using a ring, daisy chain, mesh, or any other appropriate system topology. The topology may or may not provide redundancy in the interconnection of network access switches  14 . In one embodiment, network access switches  14  “discover” the configuration of system  10  by exchanging suitable messages across system links  26 . The messages may be used, for example, to build routing tables, establish traffic priorities, and the like. Because network access switches  14  may be configured to appear to external devices as a single distributed router, network bridge, or other unit, individual VCs may be concentrated into larger “carrier” VCs by encoding destination information into the VC number, as described more fully below, in a way that permits signals to be communicated among network access switches  14  using a reduced number of VCs. Such VCs are logically “box-to-box,” providing logical connections between the switching and routing functionalities within network access switches  14  at the end of each of the system links  26 . 
   Each network access switch  14  may handle signals that need to be communicated to other network access switches  14  using corresponding system links  26 . For example, in the exemplary configuration illustrated in  FIG. 1 , network access switch  14   a  may communicate its local signals to adjacent network access switches  14   b ,  14   n , or both, network access switch  14   b  may communicate its local signals to adjacent network access switches  14   a ,  14   n , or both, and so on. Since each network access switch  14  may not support its own WAN link  22  or associated WAN port  24 , signals bound for WAN  20  and signals received on WAN link  22  may need to traverse one or more system links  26  before reaching their destination. Similarly, subscriber devices  12  coupled to network access switch  14  may need to communicate with subscriber devices coupled to other network access switches  14 . Each system link  26  preferably has at least the same bandwidth capacity as WAN link  22 , such that signals received on WAN link  22  are not undesirably slowed or otherwise impeded as they travel to their destinations, possibly through multiple system links  26 , even when traffic associated with one or more network access switches  14  is unusually heavy. Moreover, the switching and routing components of each network access switch  14  are preferably able to process the maximum traffic associated with system  10 , such that the overall traffic flow is not unduly slowed or otherwise impeded within a particular network access switch  14  even when the traffic associated with that network access switch  14  is unusually heavy. Network access switches  14  may each include one or more system link ports  28 , such that any desired number of paths between network access switches  14  may be supported with no shared points of failure other than the network access switches  14  themselves. 
   The coupling of multiple network access switches  14  using system links  26  may provide relatively inexpensive incremental growth, fewer single points of failure, increased fault tolerance, and other benefits. For example, for fault tolerance benefits, network access switches  14  may be coupled in the stacked daisy chain configuration shown, such that each network access switch  14  is coupled to two adjacent network access switches  14  and signals may be communicated between any of the non-failing network access switches  14  (assuming only one network access switch  14  in the configuration has failed). Since each network access switch  14  may support a specified number of subscriber lines  16 , incremental growth may be achieved by adding another network access switch  14  and associated system links  26  to system  10 , rather than replacing one or more network access switches  14  with other network access switches  14  having greater capacity. 
     FIG. 2  illustrates an exemplary network access switch  14  coupled to one or more subscriber devices  12  using associated subscriber lines  16  and subscriber ports  18 , to one or more other network access switches  14  in system  10  using associated system links  26  and system link ports  28 , and to WAN  20  using associated WAN link  22  and WAN port  24 . In one embodiment, switch  14  operates in an ATM environment wherein signals are communicated in cells, which may be concatenated to form frames. Although cells and frames are primarily described, the present invention contemplates signals being communicated using any suitable data units and data blocks, respectively. Accordingly, reference to cells is meant to include any suitable data units and reference to frames is meant to include packets, datagrams, or any other suitable data blocks, where appropriate. Cells may enter and exit network access switch  14  through WAN port  24 , subscriber ports  18 , and any other suitable ports associated with devices external to network access switch  14 . The present invention contemplates any suitable devices coupled to network access switch  14  in any suitable manner. 
   In one embodiment, some or all of the cells communicated between network access switches  14  on system links  26  are formatted such that they contain system destination information specifying destinations for the cells. The system destination information for a cell specifies a system destination associated with a network access switch  14  within system  10 , as opposed to any subsequent non-system destination external to system  10 , such as at a subscriber device  12 , a CO, or another device that is not a network access switch  14 . System destination information may include any suitable information that may be used by components of system  10  to switch and route cells to their intended system destinations. For example, system destination information may specify an address of a particular subscriber port  18 , WAN port  24 , or other suitable port of a particular destination network access switch  14 . The particular port may include a logical port representing multiple physical ports used in coupling a single associated device to the destination network access switch  14 , for example, to provide increased fault tolerance. Reference to port or address may include a port, a network address, a port derived from a network address, or other suitable address identifying a destination, where appropriate. 
   Network access switch  14  includes a layer-2 switch  30 , a layer-2 router  32 , and a layer-3 router  34 . Layer-2 switch  30 , layer-2 router  32 , and layer-3 router  34  may be, in whole or in part, integral to or separate from one another and may be, in whole or in part, logically separate rather than physically separate entities. Layer-2 switch  30  is preferably implemented using one or more application-specific integrated circuit (ASIC) devices, although layer-2 switch  30  may be implemented in software or in any other appropriate manner according to particular needs. Similarly, layer-2 router  32  and layer-3 router  34  may be implemented using one or more ASIC devices, in software, or in any other suitable manner. 
   In general, layer-2 switch  30  provides layer-2 switching functionality. Layer-2 switching functionality is appropriate for cells that are normally transported end-to-end by dedicated ATM switched VCs (SVCs) or permanent VCs (PVCs). Layer-2 switch  30  may switch cells out of network access switch  14  in a number of ways, depending on particular needs. For example, in one embodiment, layer-2 switch  30  switches the first cell of a frame out of network access switch  14  according to system destination information in the first cell and simply switches remaining cells of the frame to the destination of the first cell. In another embodiment, layer-2 switch  30  switches cells out of network access switch  14  according to system destination information in each cell of each frame. 
   Layer-2 router  32  performs tasks such as receiving incoming cells from layer-2 switch  30 , making layer-2 destination decisions, or selectively communicating appropriate cells to layer-3 router  34  for processing, as described more fully below. Reference to the processing of cells by layer-3 router  34  is meant to encompass performance of any appropriate layer-3 task. Layer-2 router  32  may also store appropriate cells waiting to be switched out of network access switch  14  and regulate the rate of transmission of cells exiting system  10 , as described more fully below. 
   Cells communicated to network access switch  14  are received at layer-2 switch  30  and communicated to layer-2 router  32 . Layer-2 switch  30  may receive cells communicated on associated subscriber lines  16 , on associated WAN link  20 , or on associated system links  26 . Layer-2 router  32  selectively communicates certain of the incoming cells to layer-3 router  34  according to any appropriate criteria. Criteria for selectively communicating a cell to layer-3 router  34  may include the presence of layer-3 routing information in the cell, the location of the cell (which may be known to contain layer-3 routing information) in a corresponding frame, the source of the cell, the destination of the cell, the presence of system destination information in the cell, any other suitable criteria, singly or in any suitable combination. The selective communication of certain cells from layer-2 router  32  to layer-3 router  34  provides a number of technical advantages. For example, such selective communication may result in there being fewer cells for layer-3 router  34  to process than if all cells were communicated from layer-2 switch  30  to layer-3 router  34 , improving the efficiency and other characteristics of system  10 . 
   An advantage of the present invention may be that intervening network access switches  14  between the network access switch  14  at the point of entry into system  10  and the network access switch  14  at the point of exit from system  10  need not perform layer-3 processing of cells, improving speed, conserving network resources, and providing other benefits. In one embodiment, at the first network access switch  14  reached by incoming cells entering system  10  (the point of ingress into system  10  for these cells), layer-2 router  32  selectively communicates to layer-3 router  34  those incoming cells that contain layer-3 routing information for the associated frames. A frame may need to be switched out of the same network access switch  14  onto WAN link  22  or a subscriber line  16 . Alternatively, a frame may need to traverse one or more system links  26  before reaching its destination, for example, if the frame is bound for a subscriber device or other external device coupled to another network access switch  14  within system  10 . Layer-2 router  32  may determine whether a frame needs to traverse one or more system links  26  by comparing the information contained in one or more cells within the frame to one or more entries in a lookup table  36  or in any other suitable manner. Regardless of the point of egress form system  10 , layer-3 information is communicated to layer-3 router  34  at the point of ingress into system  10 . 
   Layer-2 router  32  may determine whether cells contain layer-3 routing information in a number of ways. In one embodiment, layer-2 router  32  scans the contents of cells for layer-3 routing information, such as an IP address, for the associated frame. This may be preferable when, for example, the layer-3 routing information associated with the frame is contained in more than one cell within the frame. In another embodiment, it is assumed that only the first cells of frames contain layer-3 routing information, and layer-2 router  32  selectively communicates to layer-3 router  34  those cells that are the first cells of the associated frames. Although described in connection with the first cells of frames, any one or more cell at any location(s) in a frame may contain layer-3 routing information and be communicated to layer-3 router  34  accordingly. 
   Layer-2 router  32  may determine whether a cell is the first cell of a frame in any number of ways. In one embodiment, layer-2 router  32  detects the first cell of a frame by scanning cell payloads for data patterns identifying the first cell of a frame. In another embodiment, layer-2 router  32  detects the last cell of a frame, and the following cell is designated the first cell of a next frame. The last cell of a frame may be detected by scanning cell headers for data patterns identifying the last cell of a frame. In yet another embodiment, a combination of first-cell and last-cell detection may be used. 
   Outgoing cells exiting system  10  may be processed by the layer-3 router  34  within the last network access switch  14  reached by the outgoing cells before exiting system  10 . As discussed above, cells communicated on system links  26  may have been formatted so as to contain system destination information. Since external devices, such as CO devices coupled to WAN link  22  or subscriber devices  12 , may be unable to handle cells formatted in this way, and since the system destination information may only specify destinations associated with network access switches  14  rather than external devices, it may be necessary for layer-3 router  34  format some or all of the cells exiting system  10  such that they may be handled by such external devices. Conversely, cells entering system  10  from WAN link  22  may need to be formatted so as to include system destination information. In one embodiment, layer-2 router  32  selectively communicates to layer-3 router  34  those cells exiting system  10  that contain system destination information for an associated frame. System destination information associated with a frame may be encoded in fewer than all of the cells within the frame. In one embodiment, layer-2 router  32  scans all cells within each frame for system destination information. This may be preferable when, for example, system destination information is encoded in multiple cells within a frame communicated between network access switches  14  on one or more system links  26 . In another embodiment, layer-2 router  32  detects cells that are the first cells of outgoing frames exiting system  10 . This may be preferable when, for example, system destination information may be encoded in only the first cell of a frame communicated between network access switches  14  on one or more system links  26 . 
   Cells exiting system  10  at network access switch  14  may need to be reformatted by layer-3 router  34  such that they can be handled by external devices. In one embodiment, layer-3 router  34  receives cells exiting system  10  at network access switch  14  that contain system destination information. Layer-3 router  34  then reformats the cells as standard ATM cells that can be handled by external devices. Cells may be reformatted in a number of ways. For example, in one embodiment, layer-3 router  34  reformats a cell by removing system destination information attached to the cell. In another embodiment, layer-3 router  34  reformats a cell by removing the system destination information from the VPI VCI, or both VPI and VCI fields of the cell header and replacing it with appropriate VPI, VCI, or both VPI and VCI numbers. 
   Certain cells within a data unit being processed by layer-3 router  34  may be temporarily stored by layer-2 router  32  as they are received, rather than being communicated to layer-3 router  34 . Since layer-3 router  34  may need fewer than all of the cells within an incoming or outgoing frame to process the frame, layer-3 router  34  may complete its processing of a frame before all of the cells within the frame have been received at layer-2 switch  30  or layer-2 router  32 . For example, in one embodiment, layer-2 router  32  communicates to layer-3 router  34  the first cell of a frame entering system  10  at network access switch  14  that needs to traverse one or more system links  26  before reaching its destination. Layer-2 router  32  may store, in memory  38  or otherwise, cells of the frame received while layer-3 router  34  is processing the frame using the first cell of the frame (or any other cell in the frame containing layer-3 routing information). Where cells other than the first cells of a frame contain layer-3 routing information or system destination information, layer-2 router  32  may store the first cells of a frame while waiting to receive those cells containing such information and while those cells are being processed by layer-3 router  34  after having been received. Cells may be stored such that they may be transmitted contiguously, in order, and in accordance with ATM conventions and Request for Comment (RFC) 1483 or any other appropriate standard. Cells may similarly be stored by layer-2 router  32  while layer-3 router  34  is processing an outgoing frame exiting system  10  at network access switch  14 . Where layer-2 router  32  does not store such later-arriving cells within a frame, layer-3 router  34  may process the later-arriving cells in the same manner as earlier-arriving cells or may simply disregard such cells. 
   Layer-2 router  32  may use a traffic shaping device  40  to shape traffic flowing to destination ports on a per-port basis according to particular needs. Different levels of performance may be associated with different ports due to the purchasing of different services, different capabilities of different communication channels, and the like. Traffic shaping device  40  may be used to dynamically adjust traffic shaping, for example, in the event that local ports dynamically train to varying line speeds. Such dynamic adjustment may, for example, reduce the need to re-provision certain CO equipment as traffic conditions change. 
   In one embodiment, before cells exiting system  10  are switched out of network access switch  14 , layer-2 router  32  determines whether the destination device is ready to receive the cells. The destination device may be any device external to system  10  that is the ultimate destination of cells to be switched out of network access switch  14 , such as a CO device coupled to WAN  20 , a subscriber device  12 , or any other suitable device. For example only and not by way of limitation, such an external device may include an xDSL or other suitable modem. A destination device may not be ready to receive the cells if, for example, the device or subscriber port  18  supporting the connection with the destination device is busy handling other cells. Layer-2 router  32  may determine whether the destination device is ready to receive cells by performing a “handshake” or otherwise communicating with the device. Alternatively, layer-2 router  32  may determine whether the destination device is ready to receive cells by analyzing a clock signal to which layer-2 router  32  and the destination device are synchronized according to some existing arrangement. If layer-2 router  32  determines that the destination device is not ready to receive outgoing cells exiting system  10 , layer-2 router  32  may store the cells in memory  38  until the destination device is ready. In this way, as cells are assigned to a destination port, they may be enqueued in a port-specific queue and forwarded to the destination port at a rate that satisfies the traffic-shaping requirements of that port, which may change dynamically. Layer-2 router  32  may also store cells when there are other cells ahead of the cells waiting to be switched out of network access switch  14 , either to an external device or to another network access switch  14 . For example, layer-2 router  32  may store a cell when there are other cells within the same frame ahead of that cell that are waiting to be switched out of network access switch  14 . As discussed above, cells may be stored such that they may be transmitted contiguously, in order, and in accordance with ATM conventions and RFC 1483 or any other appropriate standard. 
   Layer-3 router  34  generally provides layer-3 routing functionality. Layer-3 routing functionality is appropriate for certain types of data blocks, such as ATM frames, FR frames, or IP packets, where the desired routing of the signals may vary from data unit to data unit and is determined by examining a portion of the contents of each data unit. In such cases (again referring to the particular case of ATM frames), the function of layer-3 router  34  may include examining individual data units being transported over inbound ATM VCs that will take them to their intended destination. In addition, layer-3 router  34  may perform tasks such as determining the system destinations of signals entering system  10  at network access switch  14 , communicating system destination information to lower functional layers within network access switch  14 , and reformatting signals exiting system  10  at network access switch  14 . Lower functional layers may include layer-2 switch  30 , layer-2 router  32 , or both. As described above, layer-3 router  34  may need fewer than all of the signals within a data unit entering or exiting system  10  to process the data unit. Accordingly, layer-2 switching and layer-3 routing may be performed within a network access switch without using a full segmentation and reassembly (SAR) unit, eliminating costs associated with the use of such units. 
   Layer-3 router  34  may communicate system destination information to lower functional layers within network access switch  14  in a number of ways. For example, in one embodiment, layer-3 router  34  discards cells communicated to it and communicates only system destination information for the cells to lower functional layers. In another embodiment, layer-3 router  34  encodes system destination information into the cells communicated to it and then communicates the cells to lower functional layers. Layer-3 router  34  may encode system destination information into a cell in a number of ways. For example, layer-3 router  34  may encode system destination information into a cell by adding to the cell one or more bytes containing appropriate system destination information. Alternatively, layer-3 router  34  encodes system destination information in a cell by changing the VPI, VCI, or both VPI and VCI numbers in the cell to reflect appropriate system destination information. 
   As discussed above, cells entering system  10  at network access switch  14  may need to be formatted by layer-3 router  34  such that they contain system destination information specifying a system destination, such as a particular port of a particular destination network access switch  14 . In one embodiment, layer-3 router  34  determines the system destination of a frame entering system  10  at network access switch  14  by analyzing layer-3 routing information in one or more cells of the frame. Once layer-3 router  34  has determined the system destination of the frame, layer-3 router  34  communicates the system destination information to lower functional layers within network access switch  14 , such as layer-2 switch  30 , layer-2 router  32 , or both. As described above, system destination information is information that may be used by devices in system  10  to forward cells to their intended system destinations associated with network access switches  14  within system  10 . System destination information may also be used by the layer-3 router at the network access switch  14  which is the point of exit from system  10  to reformat cells as standard ATM cells that may be handled by external devices. Accordingly, in one embodiment, system destination information includes two fields. One field may be used by network access switches  14  to switch cells to their system destination. The other field may be used by the layer-3 router at the network access switch  14  which is the point of exit from system  10  to reformat cells as standard ATM cells. In one embodiment, such tunneling allows an individual network access switch  14  to support only as many VCs as it may need for locally connected resources. 
     FIGS. 3A through 3C  illustrate exemplary methods for switching and routing data associated with a subscriber community. Although the methods are described primarily with respect to ATM cells and frames, the present invention contemplates signals being formatted in any appropriate manner. Referring to  FIGS. 3A and 3C  for cells entering system  10  at a network access switch  14  which is the entry point for the cells into system  10 , the method begins at step  100 , where layer-2 switch  30  receives a cell communicated on an associate WAN link  22  or subscriber line  16 . Layer-2 switch  30  communicates the cell to layer-2 router  32  at step  102 . At step  104 , layer-2 router  32  determines whether the cell contains layer-3 routing information using any suitable technique. If the cell contains layer-3 routing information, the method proceeds to step  106 , where layer-2 router  32  determines whether the cell needs to traverse one or more of system links  26  before reaching its destination. As discussed above, layer-2 router  32  may make this determination by referring to translation lookup table  36  or in any other suitable manner. If the cell needs to traverse one or more system links  26  before reaching its destination, layer-2 router  32  communicates the cell to layer-3 router  34  at step  108 . In one embodiment, the combined effect of steps  104  and  106  is that layer-3 router  34  receives only those cells entering system  10  at network access switch  14  that contain layer-3 routing information and need to be communicated between network access switches  14  in system  10 . As discussed above, layer-2 router  32  may also selectively communicate to layer-3 router  34  incoming cells that contain layer-3 routing information and are to be switched out of the same network access switch  14  onto WAN link  22  or a subscriber line  16  (without traversing one or more system links  26 ). 
   Layer-3 router  34  receives the cell and, at step  110 , determines the system destination of the frame that includes the cell in the manner described above. Once a system destination has been determined, layer-3 router  34  communicates system destination information to lower functional layers within network access switch  14  by encoding system destination information in the cell at step  112  and communicating the cell to layer-2 router  32  at step  114 . Layer-2 router  32  receives the cell and, at step  116 , communicates the cell to layer-2 switch  30 . At step  118 , layer-2 switch  30  switches the cell toward its destination on the appropriate system link  26  according to the system destination information encoded in the cell, and the method ends. 
   Referring again to step  106 , if the cell does not need to traverse one or more system links  26  before reaching its destination, the method proceeds to step  120 , where layer-2 router  32  determines whether the destination device is ready to receive the cell. As described above, this determination may be made in a number of ways. If the destination device is ready to receive the cell, layer-2 router  32  communicates the cell to layer-2 switch  30  at step  122 , layer-2 switch switches the cell out of system  10  on WAN link  22  or a subscriber line  16  to its destination at step  124 , and the method ends. If the destination device is not ready to receive the cell at step  120 , the method proceeds to step  126 , where layer-2 router  32  temporarily stores the cell until the destination device is ready to receive the cell. When the destination device is ready, the method resumes at step  122 . 
   Referring again to step  104 , if the cell is not the first cell of a frame (used in this particular example as a proxy for whether the cell contains layer-3 routing information), the method proceeds to step  128 , where layer-2 router  32  determines whether the cell needs to traverse one or more system links  26  before reaching its destination. If the cell needs to traverse one or more system links  26 , layer-2 router  32  communicates the cell to layer-2 switch  30  at step  130 . At step  132 , layer-2 switch  30  switches the cell on one of the system links  26  to the destination of the preceding cell, and the method ends. Accordingly, cells after the first cell of a frame needing to traverse one or more system links  26  are switched to the destination of the first cell of the frame. 
   If the cell does not need to traverse one or more system links  26  before reaching its destination at step  128 , the method proceeds to step  134 , where layer-2 router  32  determines whether other cells of the frame are waiting to be switched out of system  10  to the destination. If other cells are waiting to be switched out of system  10  at network access switch  14 , layer-2 router  32  temporarily stores the cell at step  136  until other cells of the frame ahead of the cell have been switched out of system  10  and the destination device is ready to receive the cell. When the destination device is ready to receive the cell, layer-2 router  32  communicates the cell to layer-2 switch  30  at step  138 , and, at step  140 , layer-2 switch  30  switches the cell on WAN link  22  or subscriber line  16  to the destination of the preceding cell. Accordingly, cells after the first cell of a frame exiting system  10  are switched out of system  10  to the destination of the first cell of the frame. 
   If no other cells of the frame are waiting to be switched out of system  10  to the appropriate destination at step  134 , the method proceeds to step  142 , where layer-2 router  32  determines whether the destination device is ready to receive the cell. If the destination device is ready to receive the cell, the method resumes at step  138 . If the destination device is not ready to receive the cell, the method instead proceeds to step  144 , where layer-2 router  32  temporarily stores the cell until the destination device is ready to receive the cell. When the destination device is ready, the method resumes at step  138 . 
   Referring to  FIG. 3C  for cells received at network access switch  14  on one of the system links  26 , the method begins at step  146 , where layer-2 switch  30  receives a cell. At step  148 , layer-2 switch  30  communicates the cell to layer-2 router  32 . At step  150 , layer-2 router  32  determines whether the cell needs to traverse one or more system links  26  before reaching its destination. If the cell needs to traverse one or more system links  26 , layer-2 router  32  communicates the cell to layer-2 switch  30  at step  152 . It is then determined at step  154  whether the cell contains system destination information. If the cell contains system destination information, the method proceeds to step  156 , where layer-2 switch  30  switches the cell out of network access switch  14  on a system link  26  to its destination according to the system destination information in the cell, and the method ends. If the cell does not contain system destination information, the method proceeds to step  158 , where layer-2 switch  30  switches the cell to the destination of the preceding cell on a system link  26 , and the method ends. 
   Referring again to step  150 , if the cell does not need to traverse one or more system links  26  before reaching its destination, the method proceeds to step  160 , where it is determined whether the cell contains system destination information. If the cell does not contain system destination information, the method proceeds to step  134  of FIG.  3 B. If the cell contains system destination information, the method proceeds to step  162 , where layer-2 router  32  communicates the cell to layer-3 router  34 . Layer-3 router then reformats the cell as a standard ATM or other suitable cell at step  164  for switching out of system  10  so that the cell can be handled by an appropriate external device. As described above, cells may be reformatted as standard ATM cells or other suitable layer-2 data units in a number of ways. After the cell has been reformatted, layer-3 router  34  communicates the cell to layer-2 router  32  at step  166 , and the method proceeds to step  120  of FIG.  3 A. 
   Although the present invention has been described with several embodiments, divers changes, substitutions, variations, alterations, and modifications may be suggested to one skilled in the art, and it is intended that the invention encompass all such changes, substitutions, variations, alterations, and modifications as fall within the spirit and scope of the appended claims.