參數(shù)資料
型號(hào)： MVTX2802AG
廠商： ZARLINK SEMICONDUCTOR INC
元件分類： 網(wǎng)絡(luò)接口
英文描述： Managed 4-Port 1000 Mbps Ethernet Switch
中文描述： DATACOM, LAN SWITCHING CIRCUIT, PBGA596
封裝： 40 X 40 MM, 2.33 MM HEIGHT, MS-034, HSBGA-596
文件頁數(shù)： 31/154頁
文件大?。?/td> 1936K
代理商： MVTX2802AG
MVTX2802
Data Sheet
31
Zarlink Semiconductor Inc.
7.9 Flow Control Basics
Because frame loss is unacceptable for some applications, the MVTX2802AG provides a flow control option.
When flow control is enabled, scarcity of buffer space in the switch 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.
In the MVTX2802AG, each source port can independently have flow control enabled or disabled. For flow
control enabled ports, by default all frames are treated as lowest priority during transmission scheduling. This is
done so that those frames are not exposed to the WRED Dropping scheme. Frames from flow control enabled
ports feed to only one queue at the destination, the queue of lowest priority. What this means is that if flow
control is enabled for a given source port, then we can guarantee that no packets originating from that port will
be lost, but at the possible expense of minimum bandwidth or maximum delay assurances. In addition, these
“downgraded” frames may only use the shared pool or the per-source reserved pool in the FDB; frames from
flow control enabled sources may not use reserved FDB slots for the highest six classes (P2-P7).
The MVTX2802AG does provide a system-wide option of permitting normal QoS scheduling (and buffer use) for
frames originating from flow control enabled ports. When this programmable option is active, it is possible that
some packets may be dropped, even though flow control is on. The reason is that intelligent packet dropping is
a major component of the MVTX2802AG’s approach to ensuring bounded delay and minimum bandwidth for
high priority flows.
7.9.1 Unicast Flow Control
For unicast frames, flow control is triggered by source port resource availability. Recall that the MVTX2802AG’s
buffer management scheme allocates a reserved number of FDB slots for each source port. If a programmed
number of a source port’s reserved FDB slots have been used, then flow control Xoff is triggered. Xon is
triggered when a port is currently being flow controlled, and all of that port’s reserved FDB slots have been
released.
Note that the MVTX2802AG’s per-source-port FDB reservations assure that a source port that sends a single
frame to a congested destination will not be flow controlled.
7.9.2 Multicast Flow Control
In unmanaged mode, a global buffer counter triggers flow control for multicast frames. When the system
exceeds a programmable threshold of multicast packets, Xoff is triggered. Xon is triggered when the system
returns below this threshold. MCC register programs the threshold.
In managed mode, per-VLAN flow control is used for multicast frames. In this case, flow control is triggered by
congestion at the destination. The MVTX2802AG checks each destination to which a multicast packet is
headed. For each destination port, the occupancy of the lowest-priority transmission queue (measured in
number of frames) is compared against a programmable congestion threshold. If congestion is detected at even
one of the packet’s destinations, then Xoff is triggered.
In addition, each source port has an 4-bit port map recording which port or ports of the multicast frame’s fanout
were congested at the time Xoff was triggered. All ports are continuously monitored for congestion, and a port is
identified as uncongested when its queue occupancy falls below a fixed threshold. When all those ports that
were originally marked as congested in the port map have become uncongested, then Xon is triggered, and the
4-bit vector is reset to zero.
The MVTX2802AG also provides the option of disabling multicast flow control.
Note:
If port flow control is on, QoS performance will be affected.
相關(guān)PDF資料
PDF描述
MVTX2803 Unmanaged 8-Port 1000 Mbps Ethernet Switch
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MVV200 Analog IC
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
MVTX2803 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:Unmanaged 8-Port 1000 Mbps Ethernet Switch
MVTX2803A 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Unmanaged 8 port Gigabit Ethernet switch
MVTX2803AG 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:Unmanaged 8-Port 1000 Mbps Ethernet Switch
MVTX2803AG2 制造商:Microsemi Corporation 功能描述:
MVTX2804 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:8-Port 1000 Mbps Ethernet Distributed Switch
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