Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
Delay between the external RX_CELL_START and local CELL_START is programmable
through the RX_CELL_START_ALIGN register (refer to “RX_CELL_START_ALIGN (Inter-
nal Structure)” on page 122).
The local CELL_START impacts the start of cell transmission to the fabric. It also determines the
period within a cell time during which the BP_ACK_IN(3:0) at ingress is valid. As such, the pro-
grammable CELL_START delay allows the flexibility to synchronize the QRTs and QSEs in a
system.
2.3 Cell Flow Overview
The QRT functions as a 622 Mbps port for an ATM switch fabric composed of either the SE or
QSE devices. The QRT transfers cells between a UTOPIA Level 2 interface and a switch fabric
interface. The device supports header translation and congestion management. The basic flow of
cells through the QRT is as follows (see Figure 18 on page 19):
1. A cell enters the QRT on the receive side from the UTOPIA interface and the channel number is looked
up.
2. The cell is then either dropped or transferred to the receive cell buffer DRAM and queued in the receive
queue controller depending on six congestion management checks (both maximum and congested
thresholds for the device, Service Class Group (SCG), SC, and connection).
3. When an available cell time occurs, four cells are selected by the receive-side scheduler, which reads
the cells from the receive cell buffer DRAM and transmits them from the QRT into the switch fabric.
4. Once a cell is received from the switch fabric on the transmit side, it is again either dropped or trans-
ferred to the transmit cell buffer DRAM and queued in the transmit queue controller, depending on ten
congestion management checks (both maximum and congested thresholds for the device, VO, SC, Ser-
vice Class Group (SCG),Service Class Queue (SCQ), and connection).
5. When the cell is selected for transmission by the transmit-side scheduler, it is removed from the trans-
mit cell buffer DRAM and processed by the transmit multicast/header mapper for corresponding header
translation and distribution.
18
PMC [ PMC-SIERRA, INC ]
