Allied Telesis AT-GS905 - Manual (Page 17)

AT-GS905 Gigabit Ethernet Switch Installation Guide
17
If the switch receives a packet with a destination address that is on the
same port on which the packet was received, it discards the packet
without forwarding it on to any port. Since both the source end-node
and the destination end-node for the packet are located on the same
port on the switch, there is no reason for the switch to forward the
packet.
Duplex Mode 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,
the end-node is operating in what is referred to as half-duplex mode. If
an end-node can both receive and transmit data simultaneously, the
end-node is said to be operating in full-duplex mode. Naturally, 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 AT-GS905 switch can operate in half- or
full-duplex mode for 10/100 Mbps and full-duplex for 1000 Mbps. They
are IEEE 802.3u-compliant and use Auto-Negotiation to set the duplex
mode setting for you automatically.
Store and
Forward
This Gigabit Ethernet Switch uses store and forward as the method for
receiving and transmitting frames. When a 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.
Backpressure
and Flow
Control
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.
How a switch signals an end-node to stop transmitting data differs
depending on the duplex mode of the end-node and switch port. A
twisted pair port operating in half-duplex mode stops an end-node from
transmitting data by forcing a collision. A collision on an Ethernet
network occurs when two end-nodes attempt to transmit data using the
same data link at the same time. A collision causes an end-node to stop
sending data, wait for a brief period of time, and then retransmit the
same data. Once the switch is ready to receive data again, the switch
stops forcing collisions. This is referred to as backpressure.