3Com CoreBuilder 9000, Руководство пользователя

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408 C
HAPTER
16: T
RAFFIC
M
ANAGEMENT
Partial Packet Drop
Partial Packet Drop (PPD) in the CoreBuilder 9000 ATM Enterprise Switch
reduces traffic congestion by dropping incoming data frames. PPD is
employed when the input queues are overflowing, cells are being lost,
and the frame is corrupted. This method is known as Partial Packet Drop
because the “tail” or remainder of the corrupted frame is dropped. The
PPD method is a cell drop method. The PPD method is only applicable to
AAL5 connections.
This method works as follows: The input ALP senses cell-loss in an
incoming data frame of the ATM Interface Module and then removes the
rest of the frame. This way the input queues are cleared of corrupted
frames, making room for the others. Different queue level thresholds are
enabled for PPD according to the different priorities required.
CLP Cell Drop
The Cell Loss Priority (CLP) marking in a cell header signifies the priority of
the cell: a value of 0 denotes high priority, a value of 1 denotes low
priority. When congestion is sensed, the cells with value 1 are discarded in
favor of those with a value of 0. This method is normally applied to the
input data stream.
In Figure 27, the input ALP (3) senses congestion and begins to drop cells
with a CLP value of 1.
Activating Methods
by Threshold
In the CoreBuilder 9000 ATM Enterprise Switch, the Traffic Management
methods described previously are individually activated in combination by
a simple threshold system in the cell storage queues. Each method is
associated with a threshold. The number of cells that are stored in the
queue is compared to the threshold values. When a threshold is
exceeded, the corresponding method is activated. The thresholds can be
static, set by the CPU, or adjusted and interchanged by an ALP-specific
algorithm to create different traffic management regimes. The threshold
system operates in both input and output queues.