Allied Telesis AT-GS2002/SP - Manual (Page 21)

AT-GS2002/SP Gigabit Ethernet to Fiber SFP Media Converter Installation Guide
21
Auto-Negotiation
Mode
When the Auto-Negotiation feature is enabled on the twisted-pair port, the
speed and duplex mode are automatically configured for you. Enabling
this feature assumes that the end-node device connected to a twisted-pair
port is also be configured for Auto-Negotiation mode. This feature insures
that the speed and duplex mode is matched on each end.
When the Auto-Negotiation feature is disabled, the port parameters must
be configured for the same speed and duplex mode as the link-partner.
Note
If an end-node device connected to the AT-GS2002/SP twisted-pair
port is set to a fixed speed and duplex mode and the AT-GS2002/SP
has Auto-Negotiation enabled, a speed and duplex mismatch may
occur which can cause a reduction in the data flow. In this case,
disable the Auto-Negotiation feature on the AT-GS2002/SP.
Note
Refer to “DIP Switches” on page 26 and Table 7, “Rear Panel DIP
Switch Settings” on page 34 for information on how to configure the
Auto-Negotiation feature, Speed and Duplex settings.
Flow Control and
Back Pressure
When two connected Ethernet device ports are set to different speeds, an
Ethernet device with the slower data rate capability may randomly be
flooded with more data than it can process and may need to signal the
opposite end-node device to stop sending data until it is ready to receive
more data again. How a port signals its end-node device to stop
transmitting data differs depending on the duplex mode of the Ethernet
ports.
A port operating in full-duplex mode uses PAUSE frames to momentarily
stop the transmission of data from the opposite end-node device.This is
specified in the IEEE 802.3x standard. Whenever a port wants an end-
node device to stop transmitting data because it is being flooded by data, it
issues a PAUSE frame. This frame instructs the end-node device to
temporarily cease transmission. The port continues to issue PAUSE
frames until it is ready to receive data again from the opposite end-node
device. This is referred to as flow control.
A twisted-pair port operating at half-duplex mode stops its end-node
device 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 the end-
nodes to momentarily stop sending data. When a port has received more
data than it can process, it needs to temporarily stop the opposite end-
node device from transmitting data. The port does this by forcing a
collision, which stops the opposite end-node device from transmitting data.
When the port is ready to receive data again, the media converter stops
forcing collisions. This process is referred to as back pressure.