System Modes , Attributes And Tuning; Forwarding Modes - 3Com 7600 User Manual

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YSTEM ODES TTRIBUTES
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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.
The 7600 Card converts Ethernet frames to ATM sized cells (48 bytes)
Cell Switching
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.