12-8 Cisco BPX 8600 Series Reference
AIT/BTM Interworking and the ATM Protocol Stack
AIT/BTM Interworking and the ATM Protocol Stack
ATM to Frame Relay interworking (ATF) performs various tasks including the following:
• Conversion of PDUs between the frame relay and ATM virtual circuits of the frame relay and
ATM user devices.
• Conversion between frame relay traffic service and ATM quality of service parameters.
• Mapping of management status, including connection, port, line, and trunk status and events.
Figure 12-8 depicts the function of the protocol stack layers in the interworking between ATM and
Frame Relay PDUs. Interworking by the AIT/BTM card in the IPX/IGX switch includes the
following functions:
• Translating the ATM pvc identifier (vpi.vci) to the frame relay pvc identifier (dlci) and vice versa.
• Mapping the Protocol Data Unit (PDU), which is essentially the data, between the Frame Relay
Service Specific Convergence Sublayer (FR-SSCS) and the Frame Relay Q.922 core protocol,
and vice versa.
• On the IPX switch, Incoming Frames are converted to FastPackets by the FRP card. The
FastPackets are then routed to the AIT card via the IPX switch MUX bus and converted back into
Frame Relay Q.922 frames by the AIT card. The AIT card interworking function executes four
layers to convert the Frame PDU to ATM cells:
— FRCS layer (Frame Relay Service Specific Convergence Sublayer, FRSSCS, or FRCS for in
shortened notation) which uses a PDU format identical to the Q.922 core (without CRC-16
or flags).
— CPCS layer (Common Part Convergence Sublayer) which appends a CS-PDU trailer to the
FR-PDU to create a CS-PDU.
— Segmentation and Reassembly layer (SAR) which segments the CS-PDU (Protocol Data
Unit) into SAR-PDUs (48 byte data entities).
— ATM layer which attaches an ATM header to each SAR-PDU to create an ATM-SDU
(Service Data Unit). The same process is performed in the reverse order by the AIT card
when transforming cells to frames.
