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Ethernet Switching Basics
Duplex Mode
Store-and-
Forward
Backpressure and
Flow Control
An Ethernet switch interconnects network devices, such as workstations,
printers, routers, and other Ethernet switches, so that they can
communicate with each other by sending and receiving Ethernet frames.
Duplex mode refers to how an end node receives and transmits data. If an
end node can receive or transmit data, but not both simultaneously, it is
operating in half-duplex mode. If an end node can both receive and
transmit data simultaneously, the end node is operating in full-duplex
mode. As such an end node capable of operating in full-duplex can handle
data much faster than an end node that can only operate in half-duplex
mode.
The twisted pair ports on the GS910/XST series switch can operate in half-
or full-duplex mode for 10/100 Mbps. They are IEEE 802.3u-compliant and
use Auto-Negotiation to set the duplex mode setting for you automatically.
Note
In order for a switch port to successfully Auto-Negotiate its duplex
mode with a 10 or 100 Mbps end-node, the end-node should also be
configured for Auto-Negotiation. Otherwise, a duplex mode
mismatch can occur. A switch port using Auto-Negotiation defaults
to half-duplex if it detects that the end-node is not using
Auto-Negotiation. This results in a mismatch if the end-node is
operating at a fixed duplex mode of full-duplex.
The GS910/XST series switch uses store-and-forward as the method for
receiving and transmitting frames. When an Ethernet frame is received on
a switch port, the switch does not retransmit the frame out the destination
port until it has received the entire frame and stored the frame in a port
buffer. It then examines the frame to determine if it is a valid frame. Invalid
frames, such as fragments or runts, are discarded by the switch. This
insures that only valid frames are transmitted out the switch ports and that
damaged frames are not propagated on your network.
To maintain the orderly movement of data between the end-nodes, an
Ethernet switch may periodically need to signal an end-node to stop
sending data. This can occur under several circumstances. For example, if
two end-nodes are operating at different speeds, the switch, while
transferring data between the end-nodes, might need to instruct the faster
end-node to stop transmitting data to allow the slower end-node to catch
up. An example of this would be when a server operating at 100 Mbps is
sending data to a workstation operating at only 10 Mbps.
Chapter 1: Product Description
29
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