PXF4333 Datasheet, PDF (69/361 Pages) Infineon Technologies AG – ABM 3G ATM Buf fer Manager

ABM-3G
PXF 4333 V1.1
Functional Description
The Buffer Manager attempts not to corrupt a packet, once it has accepted the first cell.
This means that for EPDen=1, the maximum thresholds TrafClassMax, SBMax and
QueueMax are disabled for the rest of the packet. Only the thresholds BufMax,
BufMaxNg and QueueLimit can corrupt an accepted packet.
Partial Packet Discard (PPD)
Under the rare circumstances described at the end of the previous section, it may
happen that a cell is discarded from within a packet although the EPD algorithm has
accepted it. In this case it is meaningful to discard also all following cells of the packet.
However, the last cell of a partially discarded packet should be buffered if possible, since
the reassemble mechanism at the receiver is triggered by the last cells of user data
packets. This mechanism is referred to as Partial Packet Discard (PPD).
In packet discard mode, if at cell arrival the related cell counter exceeds this threshold,
and the exceeding cell is not an end of packet or an OAM cell, then the cell is discarded
and the flag DiscardRestOfPacket is enabled in the LCI table. All subsequently arriving
cells of the packet, excluding the last cell of the packet, are discarded without taking into
consideration the cell counter.
PPD may only be applied to non real-time connections. The mechanism is enabled by
the software configurable flag PPDen, specified per traffic class in the TCT.
Note: EPD/PPD functionality is offered by the ABM-3G on a per VC basis. Hence, these
functions can be supported also for connections sharing a queue.
Note: Cell discarding due to EPD and PPD does not apply to non-user cells, e.g. an
OAM cell within a packet is not discarded.
GFR Packet Discard
The EPD mechanism in combination with the flag GFRen is used to support the GFR
service. GFR packet discard works only in conjunction with EPDen = 1 and discards only
a well defined subset of the packets normally eligible for EPD.
In particular, when EPDen = 1 and GFRen = 1, a packet is discarded only if:
[(BufEPDNg or SBMax or TrafClassMax) and QueueMax] or
any of the EPD CLP1 thresholds is exceeded.
GFRen and PPDen are independent. GFRen has no influence on PPD and PPDen has
no influence on GFR.
GFRen has no influence on the discard of CLP=1 frames. Therefore there is no
difference between EPD and GFR packet discard regarding CLP=1 frames.
3.4.1.6.4 Minimum Buffer Reservation
A minimum buffer reservation is provided on a per queue basis by setting parameter
MinBG. As long as the queue length has not reached this value, an incoming cell can be
Data Sheet
69
2001-12-17