In this review of LAN switching, you will
see that one way to help reduce network congestion is to whenever
possible, use switches instead of hubs. In addition, to increase the
efficiency of the switch you may wish to utilize some or all of these
switching features:
- Fast Ethernet (100 Mbps) switch
ports and host NIC cards
- Full-duplex communications
- Cut-through switching
There are two types of Ethernet
communications:
Half-duplex communications allows for
two or more devices to communicate with each other, but only one
device at a time.
If multiple devices attempt to communicate at the same time, a
collision will occur, those devices back-off, and a random algorithm
within each NIC determines which device will send first.
Hosts that are connected to a hub must
operate in half-duplex. This is because the host must be able to
detect when a collision occurs to stop transmitting.
Full-duplex communications allows two
devices to communicate with each other simultaneously. One of the
limitations of full-duplex Ethernet is that there must only be one
device connected to the switch-port. That device may be a computer, a
printer, a router, or another switch. If you have single devices
attached to switch ports (no hubs) it is a good idea to have them
operate in full-duplex.
Because full-duplex doubles the amount
of throughput on that link. For example, on a standard Ethernet 10
Mbps link, the throughput would be 20 Mbps, 10 Mbps transmitted plus
10 Mbps received. There are virtually no collisions on a full-duplex
connection, because there are only two devices in the collision
domain. 
Configure both ends
It is important that both the switch
port and the device connected to the switch are using the same mode of
communications, either half-duplex or full-duplex. Both the NIC card
in the host, (or in the case of a router, the Ethernet interface) and
the switch port must both be using the same mode.
Answer:
How does the computer’s NIC card get configured for full-duplex or
half-duplex operation?
Question:
In older NIC cards it was done manually, either by software which came
with the NIC card or with hardware on the NIC card itself. Today, most
NIC cards are auto-sensing, and will adapt to the mode the switch is
operating in.
Switches can be auto-sensing or
software configurable, depending upon the vendor and model of the
switch. If both devices, the host and the switch, are auto sensing and
there is only a single device on that switch port, then the link will
most likely be configured to be full-duplex.
Configuring a Router
If a router is connected to a switch,
you may need to configure the router’s Ethernet interface to be
either half or full-duplex. This is a good item to check if you have
to troubleshoot a problem with an Ethernet interface on a router.
Mismatch between two switches
Another problem you may run across is
when two switches are interconnected, and the link seems to be slower
than it should be. The problem might be that there is a mismatch
between themodes on the ports that link the two switches together.
Where one switch is operating in half-duplex and the other switch is
in full-duplex,. You may notice the collision light flashing
frequently on the device running in half-duplex. The full-duplex
device is sending at-will, not attempting to sense whether the other
device is sending frames or not. This will cause a large number of
collisions to occur as the switch operating in half-duplex senses no
traffic on the link, forwards the frame at the same time the switch
operating in full-duplex sends a frame.
Note:
This same problem may also occur if a host and a switch are operating
in different duplex modes, one in half and the other in full.
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