Abstract:
A method and apparatus for detecting errors in a data packet being transmitted as a set of smaller data cells by performing operations on the Cyclic Redundancy Check (CRC) values of the individual cells. An error detection apparatus initializes a memory area with a first value. The apparatus generates a CRC value for a first data cell. The apparatus combines the first value and the CRC value for the first cell in a XOR operation and the apparatus stores the result in a memory location. For subsequent cells in the data packet, the apparatus generates a cell CRC value, shifts the value in the memory location twelve times, and replaces the shifted value in the memory location with a new value generated from a XOR operation performed on the shifted value and the cell CRC value. The apparatus compares the final value in the memory location with an end comparison value and generates an error signal if the final and comparison values are different.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/275,155 filed Mar. 9, 2001 which is hereby incorporated by reference as if set forth in full herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to communications networks, and more specifically to communications networks transmitting data packets.  
           [0003]    Communications networks often transfer data between applications residing on different nodes of the network. The transmitted data is often of variable length. Some communication network protocols, however, call for the transmission of data packets in cells comprised of fixed length packets. For example, Asynchronous Transfer Mode (ATM) networks in accordance with ATM Adaption Layer  5  (AAL 5 ) generally transmit cells of 53 bytes, with a 5 byte header and a 48 byte payload. In operation, a variable length message provided by an application is broken, or segmented, into a number of ATM cells.  
           [0004]    The variable length message is sometimes termed a Protocol Data Unit (PDU), and the ATM cells forming the message is sometimes referred to as an AAL 5  Common Part Convergence Sublayer (CPCS) PDU. The last cell of AAL 5  CPCS PDU includes a trailer. The trailer forms the last 8 bytes of the payload of the last cell. The last 4 bytes of the trailer is a 32 bit Cyclic Redundancy Check (CRC) value. The CRC value pertains to the entire PDU.  
           [0005]    The CRC is used for error detection in the payload. The CRC is generated by combining, generally using polynomial division, the binary value of the PDU with another binary value, generally termed a polynomial. For ATM processing the polynomial generally used is x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x+1. The remainder after the entire PDU has been processed using polynomial division is the CRC value. On receipt by a receiving node the CRC value is recalculated in order to determine if errors occurred in transmission of the PDU.  
           [0006]    At the receiving node, calculation of the CRC value can be resource intensive. This may be particularly true if a node is receiving multiple messages from multiple nodes over the same time period. In such a circumstance, during the course of reception of cells from a particular PDU, the node may also receive cells from numerous other PDUs. CRC values are calculated for each PDU. Thus, if calculating CRC values on reception, the node potentially will require CRC evaluators for each PDU being received at a given time. As cells for numerous PDUs may be received at any given time, such an implementation may be costly, particularly with respect to area and power in hardware implementations. Alternatively, a node may wait for reception of all of the cells forming a PDU, and then perform CRC calculations for the PDU. This reduces the need for multiple CRC evaluators, but possibly requires storage of cells forming the PDU. Such a solution may unnecessarily increase storage requirements of a node.  
         SUMMARY OF THE INVENTION  
         [0007]    Errors are detected in a data packet being transmitted as a set of smaller data cells by performing operations on the Cyclic Redundancy Check (CRC) values of the individual cells. An error detection apparatus initializes a memory area with a first value. The apparatus generates a CRC value for a first data cell. The apparatus combines the first value and the CRC value for the first cell in a XOR operation and the apparatus stores the result in the memory location.  
           [0008]    For subsequent cells in the data packet, the apparatus generates a cell CRC value, shifts the value in the memory location twelve times, and replaces the shifted value in the memory location with a new value generated from a XOR operation performed on the shifted value and the cell CRC value.  
           [0009]    The apparatus compares the final value in the memory location with an end comparison value and generates an error signal if the final and comparison values are different.  
           [0010]    In one aspect of the invention, an error detection apparatus is provided that comprises a high speed interface, a segmentation module and a reassembly module. The high speed interface calculates a cyclic redundancy check value for incoming data. The segmentation module divides higher layer cells into lower layer cells and calculating intermediate CRC values for each lower layer cell. The reassembly module generates higher layer cells from lower layer cells and calculating intermediate CRC values for each of the lower layer cells. In another aspect of the invention, a register is provided where the high speed interface initializes the register with a predetermined value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    These and other features, aspects and advantages of the present invention will be more fully understood when considered with regard to the following detailed description, appended claims and accompanying drawings wherein:  
         [0012]    [0012]FIG. 1 is a diagram of a system in accordance with aspects of the present invention;  
         [0013]    [0013]FIG. 2 is a flow diagram in accordance with the present invention;  
         [0014]    [0014]FIG. 3 is a diagram depicting a AAL 5  CPCS PDU used within an AAL 5 ;  
         [0015]    [0015]FIG. 4 is a diagram of an ATM cell;  
         [0016]    [0016]FIG. 5 is an exemplary system implementing the reassembly error checking process described in FIG. 2; and  
         [0017]    [0017]FIGS. 6 and 7 depict a process flow diagram of an exemplary process for calculating a CRC value while segmenting a higher layer PDU into an AAL 5  CPCS PDU.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 1 illustrates a block diagram of elements of a node in accordance with aspects of the present invention. The node is adapted to perform operations with respect to the receipt and transmission of Asynchronous Transfer Mode (ATM) cell traffic.  
         [0019]    The node includes a high speed interface (HSI)  11  for reception of ATM cells. The high speed interface provides the ATM cells to a Segmentation and Reassembly (SAR) module  13 . The SAR module includes a reassembly component  15 . The reassembly module or component reassembles the received ATM cells into AAL 5  CPCS PDUs. The AAL 5  CPCS PDUs are provided to further system components, which ultimately provide the received data from the ATM cells to an application on the receiving node.  
         [0020]    The node also performs a CRC check on the received AAL 5  CPCS PDU. The CRC value is calculated by a transmitter, often by a segmentation module  17 , before the AAL 5  CPCS PDU is sent. The CRC value is checked by the receiver when the AAL 5  CPCS PDU is received. In one embodiment, a difference in the CRC value calculated by the segmentation module and the reassembly module indicates that there was an error during transmission of the payload.  
         [0021]    A flow diagram of one embodiment of a process for performing the CRC check is shown in FIG. 2. In one embodiment, the process is implemented in hardware, and those of skill in the art will recognize how to transform the process of FIG. 2 into circuit elements either manually or using an HDL such as VHDL or Verilog. Briefly, the process manipulates ATM cells upon receipt, and maintains an intermediate value for each PDL to allow for CRC checking.  
         [0022]    To assist in the understanding of the present invention, FIG. 3 illustrates an exemplary AAL 5  CPCS PDU. The AALS CPCS PDU is broken into cells. As illustrated, the AAL 5  CPCS PDU includes four cells  300 A-D. Although only four cells are illustrated, actual AAL 5  CPCS PDU may vary in the number of cells.  
         [0023]    Referring now also to FIG. 4, each of the cells, e.g., cells  300 A-D, includes a header portion  200  and a payload portion  210 . The header portion is comprised of five bytes and the payload portion is comprised of 48 bytes of data. The header portion provides routing information used by the ATM layer to route ATM cells through the ATM network. The payload portion comprises user data and AAL 5  layer data.  
         [0024]    The header portion for ATM cells used within a Network-to-Network Interface (NNI) is comprised of a Virtual Path Identifier (VPI) field  212 , a Virtual Circuit Identifier (VCI) field  214 , a Payload Type (PT) identifier field  216 , a Cell Loss Priority (CLP) field  218 , and a Header Error Control (HEC) field  220 . The VPI field is 12 bits in length and is used in conjunction with the VCI field to define a routing path through the physical layer. The VCI field is 16 bits in length. During transmission of a PDU, the VPI/VCI pair is assumed to be unique. The PT identifier field is 3 bits in length and the PT identifier field describes the payload type. The PT identifier field is also used to signify that a cell is the last cell in a PDU. Accordingly, through tracking of VPI/VCI pairs and through examination of the PT identifier field, the receiver may determine when reception of cells for a PDU beings and ends. The CLP field is 1 bit in length and signals whether or not the ATM cell should be dropped during times of network congestion. The HEC field is 8 bits in length and is used by the physical layer for detecting ATM cell transmission errors.  
         [0025]    The last cell, e.g., cell  300 D, includes at most 40 bytes of payload data and 8 bytes of additional information at the end of the payload. If less than 40 bytes of payload data are available, then the last cell is padded with a pad field  310  to allow for a properly sized ATM cell. The last 8 bytes include a CPCS User-to-User (CPCS-UU) field  320 , a Common Part Indicator (CPI) field  330 , a length field ( 340 ), and a Cyclic Redundancy Check (CRC) field  350 .  
         [0026]    The CPCS-UU field comprises data used for user-to-user information. Its content is not specified for the AAL 5  CPCS PDU format. The size of the CPCS-UU field is one byte. The length field  340  comprises a binary representation of the length of the payload field in bytes. The size of the length field is two bytes. The CRC field comprises a binary representation of a CRC value calculated over the entire payload field. The size of the CRC field is 4 bytes.  
         [0027]    Returning to the process of FIG. 2, in Block  201 , cell CRC values are generated on a cell by cell basis upon reception. A cell CRC value is calculated for each cell on receipt. The cell CRC value is calculated using “100000100110000010001110110110111” or in polynomial notation, x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x 1 +x 0 , i.e., the CRC- 32 , and by loading all zeros in a CRC  32  register. In the embodiment of FIG. 5, this calculation is performed in the HSI module  501 . The HSI module includes an XOR tree for performing the calculation. Such XOR trees are known in the art. In an alternative embodiment, a linear shift feedback register (LSFR) is used by the HIS module to perform the cell CRC value calculation.  
         [0028]    Referring back to FIG. 2, in Block  203 , the process determines if the first ATM cell payload of an AAL 5  CPCS PDU is being processed. In one embodiment, for example, the VPI/VCI fields are examined to determine if any PDU&#39;s currently being received have such a VPI/VCI value. Thus, VPI/VCI values may be stored in a table suitable for searching, with values cleared when end of message indications, such as indicated by the PT identifier field, are received.  
         [0029]    If the process determines that the first ATM cell payload is being processed, the process combines the cell CRC value with an initial polynomial value in block  205 . In one embodiment, the initial polynomial value in binary is “01010110001100101110111010110000” which in polynomial notation is x 30 +x 28 +x 26 +x 25 +x 21 +x 20 +x 17 +x 15 +x 14 +x 13 +x 11 +x 10 +x 9 +x 7 +x 5 +x 4 . In the embodiment of FIG. 1, this is accomplished by the reassembly module or component, with the operation being performed using an XOR tree. In an alternative embodiment, an LSFR is used to perform the operation. The resulting value is stored as an intermediate value for further processing.  
         [0030]    If the process is processing an ATM cell payload other than the first ATM cell payload of an AAL 5  CPCS PDU, then the intermediate value previously determined is shifted for 12 cycles in Block  207 , and the shifted intermediate value is combined with the cell CRC value calculated for the ATM cell payload in an XOR operation in Block  209 . In one embodiment this is accomplished in the reassembly component, using the XOR tree used by the reassembly component for the previously described XOR calculation. Moreover, in one embodiment, shifting of the intermediate value is accomplished by appropriately routing the intermediate value bits into appropriate locations in the XOR tree. Thus, in one embodiment, multiplexers are used to route either bits of the initial polynomial value and/or bits of the intermediate value into the XOR tree.  
         [0031]    If the process, in block  211 , determines that the last ATM cell payload of a AAL 5  CPCS PDU has not been reached, then the process continues back to block  201  and processes the incoming ATM payloads until the last ATM cell payload is reached.  
         [0032]    If the process determines that the last ATM cell payload has been processed, then the process compares the intermediate value to a final polynomial value in Block  213 . If the intermediate value and final polynomial values are not equal, then an error flag is set in block  215 . The final polynomial value in binary is “11000111000001001101110101111011” and in polynomial notation is x 31 +x 30 +x 26 +x 25 +x 24 +x 18 +x 15 +x 14 +x 12 +x 11 +x 10 +x 8 +x 6 +x 4 +x 3 +x 1 +x 0 .  
         [0033]    [0033]FIG. 5 illustrates one embodiment of a block diagram of a system for performing operations in accordance with the process of FIG. 2. The system of FIG. 5 includes a cell reception module or HSI  501 . The cell reception module receives incoming cells and calculates a cell CRC value for each cell. The cell and the cell CRC value are provided to a reassembly component  503 . The reassembly component examines the VPI/VCI pair for the cell and compares the pair with a table  505 . If the VPI/VCI pair is not in the table, then the reassembly component adds the VPI/VCI pair to the table and calculates an initial intermediate value. The initial intermediate value is stored in a slot of a table of intermediate values  507  corresponding to the slot or location of the placement of the VPI/VCI pair in the table of currently received PDUs.  
         [0034]    If the VPI/VCI pair is in the table, and the cell is not the last cell of the PDU, as indicated by the PT identifier field, then the reassembly component retrieves the appropriate intermediate value from the table of intermediate values and calculates a new intermediate value as previously described above. The new intermediate value is then stored in the appropriate location in the table of intermediate values.  
         [0035]    If, however, the VPI/VCI pair is in the table and the PT identifier field indicates the end of the PDU, then final processing occurs. The reassembly component clears the VPI/VCI pair from the table and calculates a final CRC value as previously described above. The final CRC value is compared with a constant value, and if the values do not match, an error flag is set.  
         [0036]    [0036]FIGS. 6 and 7 are process flow diagrams of an exemplary process for calculating a CRC value while segmenting a higher layer PDU into an AAL 5  CPCS PDU. The exemplary process for calculating a CRC value can be used by a segmentation component  17  (FIG. 1) within the SAR. The process initializes a CRC value in block  600 . In one embodiment, the CRC value is initialized with all “1”s. The process reads a first ATM cell payload of 48 bytes of the higher layer PDU in block  610 . The process generates a CRC value in block  620  for the 48 byte ATM cell payload. In one embodiment, the polynomial value in binary is “100000100110000010001110110110111” or in polynomial form x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x 1 +x 0  that has a width of 32. This polynomial value is termed the CRC 32  polynomial. If the process determines that 48 bytes or more of the higher layer PDU remain to be processed in block  630 , the process repeats continuing back to block  610 , so that an additional 48 byte ATM cell payload is read and the CRC value is regenerated until less than 48 bytes remain to be processed from the higher layer PDU.  
         [0037]    If the segmentation component is to create a last cell as a pad for the AAL 5  CSPC PDU, then the process creates, in block  700 , 40 bytes of zero data and appends the previously described CPCS-UU, CPI, and payload length values in block  750 . The process then continues to calculate the CRC value on the last cell of the payload but stops calculating the CRC value at the 44 th  byte, in block  760 . The process complements the CRC value in block  770  and appends the CRC value in block  780  as the last field in the trailer of the AAL 5  CPCS PDU.  
         [0038]    Alternatively, the higher layer PDU may contain enough data to create a last cell using the data from the higher layer PDU. If so, the process calculates the CRC value until the end of the 40 th  byte of the last cell in block  742  and appends the previously described CPCS-UU, CPI, and payload length values in block  750 . The process then continues to calculate the CRC value on the last cell of the payload but stops calculating the CRC at the 44 th  byte in block  760 . The CRC value is then complemented by the process in block  770  and is appended, in block  780 , as the last field in the trailer of the AAL 5  CPCS PDU.  
         [0039]    Although this invention has been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that this invention may be practiced otherwise than as specifically described. Thus, the present embodiments of the invention should be considered in all respects as illustrative and not restrictive, the scope of the invention to be determined by any claims supportable by this application and the claims&#39; equivalents rather than the foregoing description.