NEC Electra Elite IPK II System User Manual page 546

Document Revision 4C
Back Pressure
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For congestion on a half-duplex port, back pressure flow control is supported. Back pressure
generates collision-detection signals in the CSMA/CD MAC-layer (media access control) protocol
of Ethernet when a port is overloaded. These collision-detection signals from the congested port
trick the sending port into believing a collision is imminent, and cause it to back off. This, in turn,
forces a throttling back of the LAN segments experiencing congestion. When the sending ports try
to transmit after the standard Ethernet delay time, the congested port may again send
collision-detection signals or accept the incoming frames if ready.
Flow Control
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For congestion on a full-duplex port, the ETU supports IEEE 802.3x flow control that allows a
device to momentarily stop the flow of traffic from a transmitting device, by sending a pause frame.
VLAN Configuration
VLAN Tagging

A Virtual LAN (VLAN) subdivides a local area network by logical, rather than physical or
geographical, means. For example, voice traffic can be put on one VLAN, and data traffic can be
put on another VLAN. Ports on the same switching hub assigned to different VLANs are logically
separated, and a router is required to pass traffic from one VLAN to another. A router would be put
on a port configured to allow traffic from multiple VLANs.
Quality of Service
Using the Quality of Service (QoS) related memory blocks, the VLAN carrying voice traffic could be
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assigned to the high priority queue and data traffic to other VLANs destined for the low priority
queue.
Traffic with VLAN tags already assigned by the sending device can be directed to either the High
Priority queue or the Low Priority queue according to thresholds to be determined for each HUB
port.
Traffic without VLAN tags may be tagged with a default priority assigned to that port.
Port Mirroring
For troubleshooting network issues with a protocol analyzer, port mirroring can be enabled to have
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traffic on a source port reflected to the target port.
Conditions
When Auto Negotiation is denied and port speed is set to 100Mbps, the yellow LED

(located on the RJ45 connector) is ON. When port speed is set to 10Mbps, this LED is
OFF.
Only one HUB(8)-U( ) ETU can be assigned and installed per cabinet.
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Electra Elite IPK II
Internal Hub