Btm Interworking And The Atm Protocol Stack - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

BTM Interworking and the ATM Protocol Stack

BTM Interworking and the ATM Protocol Stack
ATM to Frame Relay interworking (ATF) performs these tasks:
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Figure 22-8 depicts the function of the protocol stack layers in the interworking between ATM and
Frame Relay PDUs. Interworking by the BTM card in the IGX switch includes these functions:
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Cisco BPX 8600 Series Installation and Configuration
22-8
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
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 IGX switch, Incoming Frames are converted to FastPackets by the FRM card. The
FastPackets are then routed to the FRM card via the IGX switch bus and converted back into Frame
Relay Q.922 frames by the BTM card. The BTM 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.
Chapter 22 Configuring Frame Relay to ATM Network and Service Interworking
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001