3Com CoreBuilder 9000, User Manual

Page: 407 / 468

Traffic Management in the CoreBuilder 9000 ATM Enterprise Switch 407
The Back Pressure Cell Storage method works as follows: In Figure 27, a
precongested state is created on the output channel when not all the
cells can be transmitted through the Framer out to the line, and they
begin backing up. This “back pressure” is detected by the output ALP,
which begins storing the backed-up cells in its Cell Buffer (1). When the
output ALP Cell Buffer has exceeded the threshold, the ALP signals the
Cell Buffer (2) in the ATM Switch Fabric Module to begin storing cells.
When it is full, the Cell Buffer (3) on the input channel is signaled to
begin storing cells. This three-tier system acts to prevent output data loss
due to back pressure. Only after all queues are full, data begins to be
discarded according to priority.
A high degree of control of cell storage is achieved by maintaining
independent thresholds on each cell storage queue.
Input Data Loss
Prevention
The CoreBuilder 9000 ATM Enterprise Switch also provides safeguards
against loss of input data. In this situation, the ATM Switch Fabric Module
becomes congested and therefore cannot accept input data coming from
the input ALP of the ATM Interface Module. The following mechanisms
are provided to control loss of input data:
Early Packet Drop
Partial Packet Drop
CLP-based Cell Discard
Early Packet Drop
Early Packet Drop (EPD) avoids traffic congestion in the ATM Switch Fabric
Module by intentionally removing low-priority packets from the incoming
data stream. EPD is a loss-prevention system, because it is activated
before the system has reached the stage congestion. EPD is also a more
efficient system from the point of view of the network than the PPD
system that is described in the next section because whole packets are
dropped, beginning from the first cell.
This method works as follows: In Figure 27, the input ALP senses
precongestion and drops whole frames according to a frame priority that
was initialized by software at connection setup. Different queue level
thresholds are enabled for EPD according to the different priorities
required. The EPD method is only applicable to AAL5 connections.
You can activate the Early Packet Drop feature selectively per connection.