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Black Box Modular Switches
Installation and User Guide (8/99)
34
• NOTE – some NIC cards only auto-negotiate when the computer system that they
are in is powered up. These are exceptions to the “negotiate at LINK – enabled” rule
above, but may be occasionally encountered.
When operating in 100Mb half-duplex mode, cable distances and hop-counts may be
limited within that collision domain. The Path Delay Value (PDV) bit
-times must account for all
devices and cable lengths within that domain. For Black Box LE14XXA
-Series Fast Ethernet
switched ports operating at 100Mb half
-duplex, the bit time delay is 50BT.
4.5
Auto-negotiation for 10 Mb ports, half- or full-duplex mode
Full-duplex Ethernet provides separate Transmit and Receive data paths, enabling
simultaneous bi
-directional collision-free data movements on a port. The network topology must
be a “star” type, not a “bus” type. With full
-duplex mode, the cable distance is only limited by
the physical layer line driver and cable attenuation. There are no collision
-domain restrictions or
limitations.
The LE14XXA-Series Switches perform half- or full-duplex mode auto-negotiation
independently on all switched ports. If the device or node on the other end of a port’s attached
cable supports F/H mode auto
-negotiation or is set to operate as full-duplex, the 4-Port Switch
will negotiate to run full
-duplex. If the attached device or node doesn’t support F/H mode auto-
negotiation (for example, if it is a 10 Mb repeater or a standard 10 Mb hub), the 4
-Port-Switch’s
RJ
-45 ports will default to operate at half-duplex.
4.6
Flow-control, IEEE 802.3x standard
Black-Box LE14XXA- Series Switches incorporate a flow-control mechanism for Full-
Duplex mode. The purpose of flow
-control is to reduce the risk of data loss if a long burst of
activity causes the switch to save fr
ames until its buffer memory is full. This is most likely to
occur when data is moving from a 100Mb port to a 10 Mb port, and the speed difference makes
the 10Mb port unable to keep up. It can also occur when multiple 100Mb ports are attempting
to transmit
to one 100Mb port, and in other protracted heavy traffic situations.
LE14XXA-Series Fiber Switches implement the 802.3x flow control (non-blocking) on
Full
-Duplex ports, which provides for a “PAUSE” packet to be transmitted to the sender when
the packet
buffer is nearly filled and there is danger of lost packets. The transmitting device is
commanded to stop transmitting into the 4
-switch port for sufficient time to let the Switch