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參數(shù)資料
| 型號(hào)： | PM73488 |
| 廠(chǎng)商： | PMC-SIERRA INC |
| 元件分類(lèi)： | 數(shù)字傳輸電路 |
| 英文描述： | 5 Gbit/s ATM Switch Fabric Element |
| 中文描述： | ATM SWITCHING CIRCUIT, PBGA596 |
| 封裝： | EPBGA-596 |
| 文件頁(yè)數(shù)： | 29/135頁(yè) |
| 文件大?。?/td> | 1735K |
| 代理商： | PM73488 |
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PMC-Sierra, Inc.
PM73488 QSE
L
PMC-980616
Issue 3
5 Gbit/s ATMSwitch Fabric Element
Released
Datasheet
31
For example, consider a congested 3-stage switch fabric where unicast cells and multicast cells of equal priorities col-
lide at each stage in the fabric, without any biasing. A unicast cell must make it from ingress to egress in one cell time
and the chances of doing so would be a little more than (1/2)
3
= 12.5%. However, each multicast cell would have a
50% chance of advancing to the next stage in the switch fabric.
2.6
The BP_ACK_OUT(31:0) lines are used to send information from a QSE to upstream QSEs or QRTs. These lines are
used to send two types of information:
BP_ACK Drivers
Backpressure information (for unicast cells).
Transmit acknowledge information (for multicast cells).
Backpressure information is sent for multicast cells. This information indicates to an upstream QRT or QSE if the
QSE can accept another multicast cell in the next cell time. Backpressure information also indicates what multicast
cell priorities the QSE can accept.
Cell transmit acknowledge information is sent for unicast cells. This information signals whether or not the unicast
cell transmitted in the current cell time made it to its destination QRT. If the cell makes it to the destination QRT, an
Acknowledgment (ACK) is sent. If the cell has been dropped in the switch fabric, information is sent back indicating
if the cell was dropped internally Mid Switch Negative Acknowledgment (MNACK) or at the output of the switch
fabric Output Negative Acknowledgment (ONACK). The MNACK and ONACK is used by the QRT to determine
when to retry sending the given cell.
2.7
All input and output ports can be configured in groups of four to directly connect to either QRT devices or other QSE
devices. This allows considerable flexibility in the switch fabric types and sizes that can be constructed using the
entire PMC chip set.
Interdevice Interconnectability
2.8
For many switch fabric architectures using the QSE, a single metric called the Speedup Factor (SF) allows compari-
son of different network topologies, which is independent of traffic load and type. The SF also allows for predictions
about the network performance.
Network Topologies and the Speedup Factor (SF)
Before describing the SF metric, we will briefly discuss the network philosophy and the different network topologies.
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