Abstract:
A method and system for protecting data transfers between a switch and physical network ports transfers data from a master network interface to a slave network interface, synchronizing the data transfer through a protection first in first out circuit that references the master clock and the slave clock. The master network interface improves ATM switch efficiency by supporting optical trunk and subtend ports for a digital subscriber line access multiplexer. The slave network interface provides back-up protection on a port basis so that a failed trunk port of the master network interface is backed up by the slave network interface even while the master network interface maintains a subtend port.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to network communications, and more specifically relates to a method and system for protecting a network interface. 
   BACKGROUND OF THE INVENTION 
   Digital subscriber line access multiplexers (DSLAM) typically interface a bank of modems with a broadband network. For instance, an asynchronous transfer mode (ATM) switch card in a DSLAM communicates with optical interfaces, such as trunks that interface with OC-3 fiber to a regional data center or subtends that interface with OC-3 fiber to other DSLAMs. If a network interface fails, then communication with the DSLAM is typically not possible. To improve reliability, redundant network interfaces are typically used. 
   One difficulty with protecting network communications through redundant network interfaces is the synchronization of data transfers when a failed network interface is protected by a backup network interface. Without synchronization, switching to a back-up network interface is more difficult. For instance, failure of a network interface leads to complete switching to another network interface so that each network interface typically only supports a single type of physical port. 
   SUMMARY OF THE INVENTION 
   Therefore a need has arisen for a method and system which synchronizes network interface data transfer for protection upon failure of a network interface with a backup network interface. 
   A further need has arisen for a method and system which protects from a network interface failure on a physical network trunk and subtend basis. 
   In accordance with teachings of the present invention, a method and system is provided that substantially eliminates disadvantages and problems associated with previously developed methods and systems for protecting network interfaces. Synchronization of a backup network interface to protect from failure of a master network interface is provided by slaving the backup network interface to the master and transferring data through a first in first out circuit. 
   More specifically, data transfers between a switch and a physical network are protected from failure with master and slave network interfaces. Data provided to the master network interface is communicated to the slave network interface through a first in first out circuit associated with the slave network interface. Data transfers with the master network interface are made through the protection first in first out circuit of the slave network interface based on a master clock synchronization. The protection first in first out circuit transfers data with the switch and physical network based on a slave clock synchronization. A timer detects failure of the master network interface. 
   In one embodiment, the master and slave network interfaces transfer data between an Asynchronous Transfer Mode (ATM) switch card inserted in a digital subscriber line access multiplexer (DSLAM) and optical networks for trunks and subtends. ATM cells are transferred from the master network interface to the protection first in first out circuit through a bi-directional bus. The timer detects failure of an interface by monitoring the time between cell available (CLAV) signals. Redundancy is provided on a physical network interface basis when plural physical networks communicate with the switch, such as trunk and subtend networks. For instance, if the master network interface fails to an optical trunk, then the slave network interface provides protection for the trunk while the master network interface maintains data transfers with an optical subtend. 
   The present invention provides a number of important technical advantages. One important technical advantage is that the master and slave network interface architecture allows multiple ports to feed an ATM switchcard, thus allowing greater bandwidth into the switchcard. If one port fails to a master network interface, the slave network interface provides data transfer to the network associated with the failed port while the master network interface continues to support operable ports. 
   Another important technical advantage of the present invention is that it allows optical interface redundancy on an ATM switch. The network interfaces share a substantially similar architecture so that either may function as master or slave, reducing complexity and improving flexibility. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  depicts a block diagram of a DSLAM having trunk and subtend redundant network interfaces; 
       FIG. 2  depicts a block diagram of communications between a master and slave network interface; and 
       FIG. 3  depicts a block diagram of first in first out circuits for a master and slave network interface. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the present invention are illustrated in the figures, like numerals being used to refer to like and corresponding parts of the various drawings. 
   ATM switch cards provide bandwidth capacity for modem banks, such as DSL modems deployed in a DSLAM, to communicate at high data rates with regional data centers over optical trunks, such as OC-3 trunks. Similarly, ATM switch cards allow subtend connections so that a series of DSLAMs may communicate with each other over optical interfaces, thus allowing the DSLAMs to use a common trunk. To ensure reliability, redundant network interfaces allow a back-up network interface to maintain data transfers of ATM cells between the ATM switch card and the optical trunks and subtends in the case of a failure of the main network interface. 
   Referring now to  FIG. 1 , a block diagram depicts a digital subscriber line access multiplexer (DSLAM)  10  that provides DSL service to end users  12  through DSL modem cards  14 . The DSL modem cards  14  transfer data from end users  12  to ATM cards  16  through a bus  18 . ATM cards  16  provide broadband communication through network interfaces  20  and optical communication pipelines  22  to either a trunk  24  or subtend  26 . Trunk  24  provides access for DSLAM  10  to a regional data center, the internet or other public network. Subtend  26  allows the daisy chaining of a series of DSLAMS  10 , such as to create a DSL modem bank. Although  FIG. 1  depicts a single trunk and multiple subtends interfaced with the ATM card, in alternative embodiments, different configurations of trunks and subtends may be interfaced with the ATM card depending upon the available bandwidth, each trunk or subtend associated with both a master network interface  20  and a slave network interface  20  to transfer data with the ATM card. 
   Each network interface  20  supports an optical fiber interface, such as with a trunk  24  and two subtends  26 . The network interface  20  provides redundancy for both trunk and subtend optical fiber interfaces, such as OC-3 interfaces. In case of failure of a network interface  20 , a 16 bit, 50 MHz bi-directional bus allows a second network interface on an ATM card  16  to provide redundant communication for either or both of the OC-3 trunk port and an OC-3 subtend ports with full rates for the trunk and subtend interfaces. The number of trunks and subtends supported depend upon the bandwidth of the physical optical interface, e.g., OC-3, OC-12, etc. . . . 
   Referring now to  FIG. 2 , a block diagram depicts the interface between two network interfaces  20  that provide protection for ATM card  16 . Although the network interfaces  20  are architecturally identical, one network interface is configured as a master network interface  30  and the backup network interface is configured as a slave network interface  32 . Master network interface  30  provides a clock reference to slave network interface  32  and a synch pulse to synchronize slave network interface  32  with master network interface  30 . Clock  34  provides a local clock reference for each network interface  20  and provides a clock reference through clock interface  36  from a master network interface  30  to a slave network interface  32 . 
   Each network interface  20  has a 128-state state machine that runs by reference to the clock  34  of the master network interface  30 . A synch interface  40  provides a synch pulse from master network interface  30  to slave network interface  32  to synchronize state machine  38  to the clock  34  of master network interface  30 . For instance a synch pulse occurs at the beginning of a cell time, which is every 2.56 microseconds for a 128-state state machine on a 50 MHz clock. Synchronization of state machine  38  on slave network interface  32  by the clock  34  of master network interface  30  allows cells to be sent from slave network interface  32  to master network interface  30  at the appropriate time. 
   Each network interface  20  also has a 128-state local state machine  42  that runs on the clock  34  for that network interface  20 . For instance, slave network interface  32  has a clock  34  that provides a clock reference to local state machine  42  that is not synched to the clock  34  of master network interface  30 . A first in first out synch circuit  44  between the master network interface  30  and slave network interface  32  provides data synching by reference to the clock of master network interface  30 . A cell available (CLAV) timer  46  tracks the operational status of each network interface  20  so that if an interface is declared inactive, protection is provided. For instance, cell available timer  46  determines that an optical interface is down when a cell available signal is not received for ten microseconds. 
   Master network interface  30  and slave network interface  32  communicate through a bi-directional bus  50 , which includes a clock line  36 , a synch line  40 , a data path  52 , a parity line  54 , a start of cell line  56  and a cell available line  58 . Bi-directional bus  50  is a 16 bit, 50 MHz bus that will support full data transfer rates for OC-3 trunk and subtend ports supported by a network interface  20 . The data and parity signals are driven by the transmit interface of master network interface  30  and the slave received interface of slave network interface  32 . Data transferred through data path  52  includes trunk and subtend cells sent in the following order: transfer master trunk, receive slave trunk, receive slave subtend, and transfer master subtend. The parity, master to slave start of cell, and slave to master cell available signals accompany the data transfers. 
   The slave to master cell available signal is also used for transfer back pressure status for master to slave and data flows in the direction of slave to master. When transmitting transfer cells to the optical interface of the trunk or subtend, a cell is not transmitted until both the master and slave transfer cell available are received. An optical interface is considered down if a cell available signal is not received for ten microseconds, resulting in a time out period. When a time out occurs, the cell available for the timed out interface is ignored when reading from that interface&#39;s cell dual port RAM (CDPRAM) but cells will continue to be passed to the optical interface if the cell available is active. A software monitor monitors the framer state and k-bytes of the receive optical interface to select the appropriate interface through a register of the network interfaces  20 . 
   Referring now to  FIG. 3 , a block diagram depicts circuitry for synchronizing transmission and receive cells between a network interface master  30  and network interface slave  32  with first in first out circuits. Cells for transmission from and to the ATM card  16  are stored in CDPRAM  60 , and cells transferred to and from fiber optic interface  22  are provided to physical interface  62 , such as a utopia bus. Network interface slave  32  provides protection for both transmitted and received cells. The upper portion of  FIG. 3  depicts the transmit path and the lower portion of  FIG. 3  depicts the receive path. Each trunk and/or subtend interface support by the ATM card will have a transmit or receive path as depicted by  FIG. 3 , with the number of trunks and/or subtends limited by the bandwidth of the associated physical network  62 . 
   Cells sent through the transmit path to the physical interface  62  originate in CDPRAM  60  and are sent to transmit port first in first out circuit  64 . Cells are read from transmit port first in first out circuit  64  through a transmit multiplexer  66  and transmit de-multiplexer  68  when the trunk or subtend physical interface  62  can accept the cell. 
   To provide transmit protection, master network interface  30  transfers CDPRAM cells across bi-directional bus  50  to a transmit protect first in first out circuit  70 . To avoid collisions on bi-directional bus  50 , cells from CDPRAM  60  are delayed in delay first in first out circuit  72  before transfer to transmit protect first in first out circuit  70 . The slave network interface  32  writes the cells into the transmit protect first in first out circuit for transfer to the slave network interface  32 &#39;s physical interface  62  through transmit multiplexer  66  and transmit de-multiplexer  68 . Timers associated with the physical devices for trunks and subtends provide cell available signals to read the cells so that if no cell available signal is received from a device for ten microseconds, a timeout occurs and the interface becomes inactive. Transmit protect first in first out circuit  70  writes cells with reference to the clock of the master network interface  30  and reads cells with reference to the clock of the slave network interface  32 . Since the master network interface  30  and slave network interface  32  have identical transmit path architectures, either network interface may be selected as master or slave. 
   The receive path of the master interface provides protection through the slave interface in a manner similar to the transmit path, using first in first out circuits to synchronize received data cells. Cells are received from the physical interface  62  through a physical multiplexer  74  and a receive port first in first out circuit  76 . The cells are read from the receive port first in first out circuit  76  and written to CDPRAM  60  through receive multiplexer  78  for storage in cell buffer random access memory of ATM card  16 . To provide protection, cells read into the master network interface  30  are stored in a receive protect first in first out circuit  80  of slave network interface  32  and read out based on a synch signal received from master network interface  30 . The synch pulse from master network interface  30  synchronizes the local state machine  42  of the slave network interface  32  to the clock reference of the master network interface  30 . Thus, the synch pulse allows the slave network interface  32  to know when the master network interface  30  can accept cells and reads the cells from the receive protect first in first out circuit  80  at the appropriate time. 
   Protection software associated with master network interface  30  selects trunk or subtend cells from the slave network interface  32  or the master network interface  30 &#39;s receive port first in first out circuit  76  based on the OC-3 framer states and k-bytes. Receive protect first in first out circuit  80  synchronizes cells sent from slave network interface  32  by using the slave reference clock to write the cells and the master reference clock to read cells. If a network interface goes down, the operative interface is selected by the protection software. For example, if the master network interface  30  subtend interface times out, the master network interface  30  will switch to accepting cells from the slave subtend physical interface  62  while still accepting cells from the master trunk physical interface  62 . 
   The master network interface  30  and slave network interface  32  have substantially similar architectures so that either may function as master or slave. Each physical network port has a receive and transmit protect first in first out circuit associated with it. By having the ability to interface both subtends and trunks with redundancy through separate protect first in first out circuits, the present invention helps use available bandwidth through the ATM cards in a more efficient manner. 
   Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as