Abstract:
The method for converting cell-based ATM traffic to frame-based ATM traffic comprises assembling a group of one or more cells of a input cell stream which corresponds to an AAL5 PDU; extracting a user data payload of the AAL5 PDU, exclusive of any padding; constructing a frame-based ATM adaptation layer protocol data unit (FB-AAL PDU) having no padding using the AAL5 PDU payload; and segmenting the FB-AAL PDU into one or more frames to generate an output frame stream. The method provides an increase in bandwidth efficiency since the padding overhead required by AAL5 PDUs is eliminated when transferring data to, over or across a frame-based ATM network.

Description:
FIELD OF INVENTION 
     The invention relates to a method and apparatus for inter-operating or interworking a cell-based ATM network with a frame-based ATM network. 
     BACKGROUND OF INVENTION 
     Frame-based asynchronous transfer mode (FB-ATM) protocols are currently under development by organizations such as the ATM Forum. FR-ATM differs from the presently well defined cell-relay ATM (CR-ATM) protocols in that the former will employ a variable length frame as the basic unit of data transport whereas the latter employs fixed size cells. An interworking function will be required for seamlessly transporting data between CR-ATM networks and FR-ATM networks. 
     The CR-ATM protocol is layered or stacked as set forth for instance in the B-ISDN protocol stack described in CCIT Recommendations I.362 and I.363. The protocol includes an ATM adaptation layer (AAL) for adapting the services of the connection orientated ATM Layer to those required by higher layers (commonly referred to as AAL users). Some versions of the AAL operate by receiving user data, such as an Internet Protocol (IP) or Frame Relay (FR) packet, constructing an AAL common part convergence sub-layer protocol data unit (CPCS-PDU—hereinafter referred to as “AAL PDU”) based on this data, and segmenting the AAL PDU into plural segmentation and re-assembly protocol data units (SAR PDUs) that form the payload of the transportable fixed-size ATM cell. The AAL also functions in the reverse direction to convert plural SAR PDUs into an AAL PDU. The emerging FB-ATM protocol will likely also be stacked in a manner similar to CR-ATM and include an AAL (hereinafter FB-AAL) equivalent to the CR-ATM AAL. 
     One intuitive approach to interworking when data originates and terminates in a CR-ATM network and is transferred across an intermediary FB-ATM network is to assemble a series of fixed size ATM cells which collectively correspond to a CR-ATM AAL PDU. This assembly of cells could form the input to the FB-AAL. Conversely, an assembly of FB-ATM frames could form the AAL PDU for segmentation and data transfer across a CR-ATM network. Such an approach is, however, not optimally efficient with respect to the conservation of bandwidth. 
     SUMMARY OF INVENTION 
     Broadly speaking, the invention provides a method of interworking between cell-based and frame-based ATM networks. 
     According to one aspect of the invention, a method is provided for converting cell-based ATM traffic to frame-based ATM traffic. The method comprises (a) aggregating a group of one or more cells of an input cell stream which corresponds to an AAL, preferably AAL5, PDU; (b) extracting from the cell group a user data payload of the AAL5 PDU, exclusive of any AAL PDU padding; (c) constructing a frame-based ATM adaptation layer protocol data unit (FB-AAL PDU) having no padding using the AAL PDU payload; and (d) segmenting the FB-AAL PDU into one or more frames to generate an output frame stream. The method provides an increase in bandwidth efficiency since the padding overhead required by AAL5 PDUs is eliminated when transferring data to, over or across FB- ATM. 
     According to another aspect of the invention, a method is provided for converting frame-based ATM traffic to cell-based ATM traffic. The method comprises (a) assembling a group of one or more frames of an input frame stream which corresponds to a FB-AAL PDU; (b)extracting from the frame group a user data payload of the FB-AAL PDU; (c) constructing a CR-ATM AAL PDU using the FB-AAL PDU payload; and (d) segmenting the CR-ATM AAL PDU into one or more cells to generate an output cell stream. 
     Another more general aspect of the invention provides a method for converting a stream of fixed size packets to a stream of variable sized packets in circumstances where the variable sized packets have a maximum size limit. The method comprises: (a) assembling a group of one or more fixed size packets wherein a last packet of the group includes padding for the purposes of aligning a user data payload for transport over the group of packets; (b) extracting the user data payload from the group of fixed size packets, exclusive of the padding; and (c) segmenting the user data payload into one or more variable sized packets to generate the variable sized packet stream. 
     Interworking apparatus for effecting the above methods is also disclosed. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other aspects of the invention will become more apparent from the following description of the preferred embodiments thereof and the accompanying drawings which illustrate, by way of example only, the principles of the invention. In the drawings: 
     FIG. 1A is a diagram of the logical structure of a CR-ATM cell; 
     FIG. 1B is a diagram of the logical structure of a CR-ATM AAL5 PDU; 
     FIG. 2A is a diagram of the logical structure of a FB-ATM frame; 
     FIG. 2B is a diagram of the logical structure of a FB-ATM AAL PDU; 
     FIG. 3 is a schematic diagram of various processing layers employed in an interworking function in the CR-ATM to FB-ATM direction in accordance with the preferred embodiment; and 
     FIG. 4 is a schematic diagram of various processing layers employed in an interworking function in the FB-ATM to CR-ATM direction in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1A shows the structure of a CR-ATM cell  12 . The cell  12  comprises a  5  octet header  14  and a  48  octet data payload  16  for a fixed total size of  53  octets. The header  14  includes a cell loss priority (CLP) field  18  for indicating the priority level of the cell as well known in the art. The header  14  also includes a PT field  20 , the use of which is discussed in greater detail below. 
     FIG. 1B shows the structure of an AAL PDU  22  in accordance with AAL5, a predominant version of the AAL. The AAL5 PDU  22  comprises a variable sized user data payload  24 , a variable sized padding field  26 , and a trailer  28 . AAL5 sizes the padding field  14  such that the size (in octets) of the entire AAL5 PDU  22  is an exact multiple of  48 . This enables the AAL5 PDU  22  to be directly segmented into cell payloads  16  (FIG.  1 A), which correspond to AAL5 SAR-PDUs. 
     A CR-ATM virtual circuit (VC) stream  50  is shown in FIG.  3 . It will be noted that each AAL5 PDU is carried by plural cells  12 . The last cell of a group of a cells which corresponds to an AAL5 PDU has its PT field  20  set to delineate the boundary of the PDU. This cell is labeled as the EoM (end of message) cell in FIG.  3 . 
     FIG. 2A shows the structure of a contemplated FR-ATM frame  32 . The frame  32  comprises a header  34  and a variable sized data payload  36 . At the present time it is contemplated for the frame  32  to have a maximum permissible size Lmax. See Section 3.4 of ATM Forum Technical Committee, “Frame Based User-To-Network Interface (FUNI)” Specification v2.0, AF-SAA0088. 00, Jul. 1997, which is incorporated by reference herein in its entirety. The header  34  includes a frame loss priority (FLP) field  38  and a payload type (PT) field  40  which are respectively intended to function in a manner similar to the CLP field  18  and PT field  20  of CR-ATM cell  10 . 
     FB-ATM contemplates an equivalent to AAL, which is termed herein as FB-AAL. The FB-AAL, as with AAL5, includes an FB-AAL PDU construction function or sub-layer for receiving and encapsulating data from AAL users. FIG. 2B shows the structure of a contemplated FB-AAL PDU  42 , which comprises a variably sized user data payload  44  and a trailer  48 . The PDU  42  does not require a padding field of any kind since the FB-ATM frame  32  comprises a variably sized payload  36 . Nevertheless, due to the limitation on the maximum permissible size of frame  32 , the FB-AAL will require a segmentation function or sub-layer to segment PDU  42  across        N   =     ⌈         size        (     user                 data     )       +   8         L                 max     -   4       ⌉                            
     frames  32 . The details of the segmentation and re-assembly state machines for FB-AAL will be analogous to those for AAL5/CR-ATM. 
     An FB-ATM VC stream  52  is also shown in FIG.  3 . It will be noted that each FB-ATM PDU  42  is carried by one or more frames  32 . The PT field  40  of the last frame of a PDU  42  is set in order to delineate PDU boundaries. This frame is labeled as the EoM (end of message) cell in FIG.  3 . 
     The preferred embodiment employs a “store-and-forward” method for FB-AAL/FB-ATM inter-operability with AAL5/CR-ATM. FIG. 3 illustrates the processing layers in the CR-ATM to FB-ATM direction and FIG. 4 illustrates the processing layers in the FB-ATM to CR-ATM direction. 
     Referring to FIG. 3, the preferred method for interworking in the CR-ATM to FB-ATM direction comprises the following steps: 
     1. An AAL5 assembly function  60 A as well known in the art is implemented to extract each AAL5 PDU  22  from the CR-ATM VC stream  50 . The extraction operation for a given AAL5 PDU terminates upon receipt of the corresponding EoM cell. 
     2. The payload  24  of the AAL5 PDU  22  is extracted. This operation is performed by an extraction function  62 A of the AAL5 common part convergence sub-layer (CPCS). Note that the payload  24  does not include the padding field  26 . 
     3. The AAL5 PDU payload  24  is passed onto the FB-AAL which, as described above, includes an FB-ATM PDU construction function  64 A and an FB-ATM SDU or frame segmentation/function  66 A. The FB-AAL encapsulates the AAL5 PDU payload  24  or user data, absent any padding, within an FB-AAL PDU  42  structure and segments the PDU  42  as necessary to generate an emerging FB-ATM VC frame stream  52 . 
     Thus each AAL5 PDU results in exactly one FB-AAL PDU being constructed as shown in FIG.  3 . 
     If one of the cells being assembled has its CLP field  18  set to 1 (indicating low priority), then all resulting FB-ATM frames  32  have their FLP fields  40  set to 1 (to indicate low priority). 
     Additionally, in the event the CR-ATM VC stream  50  includes resource management (RM) cells, such cells bypass the segmentation and re-assembly functions described above. Instead, each RM cell is directly passed to the FB-AAL sub-layer  64  to be encapsulated in an individual frame  32  (with the header  34  appropriately set) and then inserted into the emerging frame stream  52 . 
     Referring to FIG. 4, the preferred method for interworking in the FB-ATM to CR-ATM direction comprises the following steps: 
     1. An FB-AAL assembly function  66  as described above is implemented to extract each FB-AAL PDU  42  from FB-ATM VC frame stream  52 . The extraction operation for a given FB-AAL PDU  42  terminates upon receipt of the corresponding EoM. 
     2. The payload  44  of the FB-AAL PDU  42  is extracted. This is preferably accomplished by an extraction function  62 B of the FB-AAL. 
     3. The FB-AAL PDU payload  44  is passed to the AAL5 which, as known in the art, includes an AAL5 PDU construction function  62 B in the CPCS and an AAL5 SDU or cell segmentation function  66 A. The AAL5 encapsulates the FB-AAL PDU payload  44  or user data within an AAL5 PDU structure  22  and segments the PDU  22  as necessary to generate an emerging CR-ATM VC cell stream  50 . 
     If one of the arriving frames has its FLP field  40  set to 1, all resulting cells have their CLP fields  18  set to 1. Each frame marked as RM bypasses the segmentation and re-assembly functions described above. Instead, each such frame is converted to a single ATM cell and inserted into the emerging CR-ATM VC cell stream  50 . 
     The key benefit achieved from the above described interworking method is an increase in bandwidth efficiency since the padding overhead required by AAL5 PDUs are eliminated when transferring data to, over or across FB-ATM. On the negative side, the interoperability functions are “store-and-forward” in nature and therefore incur delay and require increased storage. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the preferred embodiment without departing from the spirit and scope of the claims.