Frame Relay to ATM Network and Service Interworking 12-7
ATM Protocol Stack
Discard selection is based upon the standard CLP bit in the cells. When the routing path enters an
IPX/IGX switch, an AIT/BTM card which supports Interworking traffic is required to convert the
connection data from cells to frames (frames to fastpackets out onto MuxBus to FRP/cell bus to
FRM), and visa versa. Additionally, the AAL-5 framing is removed upon conversion to frames, and
added upon conversion to cells. At the destination (FRP), FastPackets are placed in the port queue
and, when a complete frame has been assembled, the frame is played out the remote port in the
original format (as provided in the frames delivered inside AAL-5 PDUs).
For each connection, only a single dlci can be played out for all traffic exiting the port, and is inserted
into the frame headers. The standard LAPD framing format is played out the port on the FRP/FRM.
At the AIT/FRM card, several additional protocol mappings take place. First, the Interworking Unit
acts as a pseudo endpoint for the purposes of ATM for all constructs which have no direct mapping
into Frame Relay, such as loopbacks and FERF indications. Thus, end-to-end loopback OAM cells
which come to AIT/FRM cards are returned to the ATM network without allowing them to proceed
into the Frame Relay network, which has no equivalent message construct. Further, AIS and
supervisory cells and FastPackets (from the Frame Relay direction) are converted into their
counterparts within the other network.
ATM Protocol Stack
A general view of the ATM protocol layers with respect to the Open Systems Interconnection model
is shown in Figure 12-7. In this example, a large frame might be input into the top of the stacks. Each
layer performs a specific function before passing it to the layer below. A protocol data unit (PDU) is
the name of the data passed down from one layer to another and is the Service Data Unit (SDU) of
the layer below it. For Frame Relay to ATM interworking, a specific convergent sublayer, Frame
Relay Service Specific Convergent Sublayer, FR-SSCS is defined. This is also referred to as FR-CS,
in shortened notation.
Figure 12-7 ATM Layers
Higher layer functions
Convergence sublayer
(CS)
Physical
layer
SAR
ATM layer
TC
PM
AALs
(ATM
adaptation
layers)
Management plane
Control plane User plane
Service specific, e.g., FR-SSCS
Common part convergence
sublayer CPCS
Segmentation and reassembly
Cell header insert/extract
Cell multiplexing/demultiplexing
VPI/VCI addressing and translation
Generic flow control
Transmission convergence
Physical medium
H8021
