MC92501 Datasheet, PDF (8/52 Pages) Motorola, Inc – ATM Cell Processor

SECTION 3. PACKET-BASED UPC DISCARD ALGORITHMS
3.1. Introduction
The MC92501 UPC function performs cell-based discard or
packet-based discard according to ATM Forum TM 4.0. It
supports packet discard on VC connections AAL5 packets (not
including OAM cells). The MC92501 also performs Partial
Packet Discard or Early Packet Discard.
The MC92501 offers four modes of UPC operation on a per-
connection basis: Cell-Based UPC, Partial Packet Discard
(PPD), Early Packet Discard (EPD), and Limited Early Packet
Discard (Limited EPD). These modes are selected on a per-
connection basis using the IUOMÑIngress UPC Operation
Mode bit in the Common Extension Parameters Table. Packet-
based UPC is enabled globally by the IPCVÑIngress Features
Enable bit in the ACR register.
3.1.1. AAL5 Packet Definition
A packet is deÞned as a stream of user cells belonging to
the same virtual connection that has a series of one or more
cells with the PTI[0] bit set to 0 and the last PTI[0] bit set to 1.
(See Figure 3.)
3.2. Cell-Based UPC
This is the default mode. The MC92501 discards cells on a
per-cell basis as deÞned in MC92500/D.
3.3. Partial Packet Discard (PPD)
According to the PPD algorithm, if a cell is discarded then
all subsequent cells belonging to that packet are discarded up
to but not including the last cell. Following is a detailed
explanation of the UPC function.
Â¥ The UPC is a two-state machine: discarding and not-
discarding. See Figure 4.
Â¥ While the UPC is in the not-discarding state, it performs
normal cell-based operation with tagging and policing
counter updates.
Â¥ The UPC transitions from the not-discarding to the
discarding state on the Þrst discarded cell.
Â¥ While the UPC is in the discarding state, it does not update
the UPC bucket but it does increment the policing discard
counter.
Â¥ When in the discarding state and the last cell of a packet
is received, there are two options:
Ñ If all the cells belonging to that packet were discarded,
then this last cell is discarded.
Ñ If not all the cells belonging to that packet were
discarded, then this means that the packet was
truncated and this last cell is admitted in order to
delineate the corrupted packet from the next packet.
There is however one exception: if this last cell is
violating cell-based UPC then it is discarded.
Figure 5 illustrates an example for the PPD algorithm. A
UPC policy violation occurs during the transmission of the Þrst
packet. The UPC detects the violation and discards the
remainder of the packet except for the last cell. The last cell
of the Þrst packet is transmitted to avoid the concatenation of
the corrupted packet with the subsequent Packet #2. If the
UPC detects that the Þrst cell of Packet #3 violates its policy
then Packet #3 is truncated. Packet #3Õs last cell is not
transmitted because it cannot be admitted by the cell-based
UPC. Packet #4 is not transmitted either because its Þrst cell
violates the UPC policy.
CELL STREAM
PTI[0] = 1
PTI[0] = 0
PTI[0] = 0
PTI[0] = 1
PTI[0] = 0
LAST CELL OF
PACKET 0
PACKET 1
FIRST CELL OF
PACKET 2
Figure 3. Delineation of a Packet Within a Cell Stream
LAST CELL OF PACKET ARRIVES
If all cells within a packet are discard
OR
If the last cell is violating UPC policy
then discard the last cell
If not all calls within packet discarded, then admit last cell
NOT-DISCARDING
DISCARDING
Figure 4. UPC Discarding State Machine
CELL VIOLATES
UPC POLICY
MC92501
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