3Com 7600 manual Tuning, Store-and-Forward Mode

15	SYSTEM MODES, ATTRIBUTES,
	AND TUNING

This chapter explains the administrative and management functions in the 7600 Card including system modes, attributes and tuning.

Forwarding Modes Data frames received from the Fast Ethernet ports pass through the ZipChip ASIC and are stored in the shared CRAM (Cell RAM) memory. The CRAM discards all received frames that are not required to be transmitted on any output (filtered frames). The shared memory structure provides for optimal utilization of the available memory. Head of Line blocking, which is caused when frames are queued on input queues, is eliminated by the fact that there is no input queuing on data frames.

Cell Switching The 7600 Card converts Ethernet frames to ATM sized cells (48 bytes) upon receipt and then uses cell switching to forward the frames to their destination addresses. This common format of frames in CRAM makes switching straightforward. CRAM, under control of ZipChip, is optimized for storing data cells. Each frame is organized as a linked list of cells. The cells in the CRAM are either converted back to Ethernet frames when transmitted on an Ethernet port or to LAN Emulation frames when transmitted on the ATM port. In this scheme, cell data is never copied because it is converted to the appropriate format by ZipChip on its way in and out of CRAM. For more information about Cell Switching in ATM refer to Chapter 5.

As a frame passes through ZipChip on its way to CRAM, ZipChip makes a forwarding decision for that frame. The 7600 Card supports two forwarding modes which are set individually for Ethernet and ATM:

Store-and-Forward Mode

In store-and-forward mode, the frame cannot be forwarded to its destination address before the last bit has been received. This allows for the rejection of bad frames.