Cisco Systems 8600 Series manual ATM Protocol Stack, ATM Layers

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

Managementplane

Controlplane Userplane

Higher layer functions

AALs	Convergence sublayer
(ATM	(CS)
adaptation
	SAR
layers)
	ATM layer
Physical	TC
layer	PM

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

Frame Relay to ATM Network and Service Interworking 12-7