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参数资料
| 型号： | ZL50408GDG2 |
| 厂商： | CONEXANT SYSTEMS |
| 元件分类： | 网络接口 |
| 英文描述： | DATACOM, LAN SWITCHING CIRCUIT, PBGA208 |
| 封装： | 17 X 17 MM, 1.40 MM HEIGHT, LEAD FREE, MO-192, LBGA-208 |
| 文件页数： | 89/144页 |
| 文件大小： | 1779K |
| 代理商： | ZL50408GDG2 |
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ZL50408
Data Sheet
49
Zarlink Semiconductor Inc.
Figure 12 - Buffer Partition Scheme
See Buffer Allocation application note, ZLAN-47, for more information.
7.6.1
Dropping When Buffers Are Scarce
As already discussed, the WRED mechanism may drop frames on output queue status. In addition to these
reasons for dropping, we also drop frames when global buffer space becomes scarce. The function of buffer
management is to make sure that such dropping causes as little blocking as possible.
If a received frame is dispatched to the P0 (“best effort”) queue, the buffer management will check on the overall
buffer situation plus the output queue status to decide the frame drop condition. If the source port has not enough
buffer for it, the frame will be dropped. If the output queue reaches PCC (packet congest control) and the shared
buffer has run out, the frame may be dropped by RED (at B%). If the output queue reaches PCC and the source
port reservation is lower than the buffer low threshold, the frame will be dropped.
For multicast frames, global thresholds are used to manage scarce resources. When a multicast frame is received,
the buffer management will check on the overall buffer situation. If total multicast resources reaches MCCTH
(multicast congest control threshold), the frame may be dropped by RED (at B%). If total multicast resources
reaches MCC, the frame will be dropped.
All the 100% dropping functions are disabled if the source port is flow control capable, however, RED (at B%) will
still be applied.
7.7
Flow Control Basics
Because frame loss is unacceptable for some applications, the ZL50408 provides a flow control option. When flow
control is enabled, scarcity of source port buffer space may trigger a flow control signal; this signal tells a source
port sending a packet to this switch, to temporarily hold off.
While flow control offers the clear benefit of no packet loss, it also introduces a problem for quality of service. When
a source port receives an Ethernet flow control signal, all microflows originating at that port, well-behaved or not,
are halted. A single packet destined for a congested output can block other packets destined for un-congested
outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be assured with high
confidence when flow control is enabled.
On the other hand, the ZL50408 will still prioritize the received frame disregarding the outgoing port flow control
capability. If a frame is classified as high priority, it is still subjected to the WRED, which means the no-loss on the
high priority queue is not guaranteed. To resolve this situation, the user may set the output port WRED threshold so
high that may never be reached, or program the priority mapping table in the queue manager to map all the traffic to
“best effort” (P0) queue on the flow control capable port. The first method has side impact on the global resource
management since the port may hold too much per class resource that is scarce in the system. The second
method, by nature, lost the benefit of prioritization.
Per Class
Reservation
Per Source Port
Reservation
Temporary reservation
Rpri1
Rpri2
Rpri3
Shared Pool S
Rp0
Rp1
Rp2
Rp3
Rp4
Rp5
Rp6
Rp7
Rp8
(CPU)
Rp9
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相关代理商/技术参数 |
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|---|---|
| ZL50409 | 制造商:ZARLINK 制造商全称:Zarlink Semiconductor Inc 功能描述:Managed 9-Port 10/100M Ethernet Switch |
| ZL50409GDC | 制造商:ZARLINK 制造商全称:Zarlink Semiconductor Inc 功能描述:Managed 9-Port 10/100M Ethernet Switch |
| ZL50409GDC1 | 制造商:Microsemi Corporation 功能描述: |
| ZL50410 | 制造商:ZARLINK 制造商全称:Zarlink Semiconductor Inc 功能描述:Managed 8-Port 10/100M + 1-Port 10/100/1000M Ethernet Switch |
| ZL50410GDC208 | 制造商:ZARLINK 制造商全称:Zarlink Semiconductor Inc 功能描述:Managed 8-Port 10/100M + 1-Port 10/100/1000M Ethernet Switch |
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