PXB4340E Datasheet, PDF (25/185 Pages) Infineon Technologies AG – ICs for Communications

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The downstream transmit buffer has 2 thresholds for each queue:
• the UTOPIA backpressure threshold:
beyond this threshold the backpressure signal is given to the downstream receive interface for
this PHY (see VHFWLRQ  page 91)
• the OAM cell insertion threshold:
beyond this threshold the insertion of OAM cells is disabled (see VHFWLRQ  page 91). As
with the upstream receive buffer in this state OAM cells to be inserted are lost.
The two thresholds are identical for all queues. The UTOPIA backpressure threshold should be
programmed to a value lower than the OAM cell insertion threshold, this difference guarantees
a cell storage space for OAM cells (see ILJXUH , part b).
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if a queue exceeds
this threshold :
no more OAM cells
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are lost.
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The ATM cell load should be selected by the user in a way that the probability to loosing OAM
cells is almost zero (e.g. 10^-11). This is done by reserving bandwidth for the inserted OAM cells.
OAM cell bandwidth is mainly depending on the selected PM block size (128, 256, 512, 1024).
In worst case, when F4 and F5 level PM cell streams are generated, the overhead is still less
than 2%.
Lost OAM cells do not lead to system malfunction. If e.g. an FM cell is lost the PM processor
continues to count user cells and BIP-16 checksums. The correct values will be sent out with the
next block. Thus block size would be e.g. 256 instead of 128 in case of a lost cell. The insertion
of AIS/RDI/CC cells will only be halted temporarily during the (very unlikely) case of a buffer
overflow. An LB cell to be looped, however, will be lost. Here only the repeat function would help.
Data Sheet
2-25
04.2000