BusWorks
®
900EN-S005 Ethernet Switch User’s Manual Ethernet I/O
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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:
http://www.acromag.com
48
Recall that the flow of traffic on a network can be either half-duplex (one
direction at a time), or full-duplex (simultaneous in both directions).
In 1995, the 802.3 Ethernet standard was modified to include provisions for
802.3u 100Mbps Fast Ethernet. Fast Ethernet operates using full-duplex
communication in lieu of half-duplex with CSMA/CD. Thus, fast Ethernet
implies a point-to-point Ethernet connection between two devices, with no
possibility of collisions, allowing the stations at each end to transmit
simultaneously (full-duplex).
Similarly, if only one device is connected to a switch port (such as an
Ethernet host adaptor or another switch port), this is referred to as
microsegmentation
and full-duplex operation is possible without the
occurrence of collisions. If however, a collision domain is shared on a switch
port (multiple host adaptors or repeating hubs connected on a segment),
then only half-duplex communication is allowed and use of the
interconnection medium must be arbitrated among the nodes (using
CSMA/CD). Flow controls are methods used to help arbitrate
communication along this medium. For full-duplex segments, the
Pause
Scheme
is used. For half-duplex segments, the
back pressure
approach is
used.
Full-Duplex Flow Control
Full-duplex flow control is enabled by default, but may be disabled via DIP
switch S1-2. Per IEEE 802.3x, flow control refers to the use and handling of
pause frames to address short term traffic overload on a full-duplex link, for
both the transmit and receive channels.
On a receive channel, if a pause control frame is received, the switch will not
transmit the next normal frame until the timer (specified in the pause control
frame) expires. If another pause frame is received before the original time
expires, the timer will be updated with the new time value in the second
received pause frame. During the flow control period (timer running), only
flow control packets may be transmitted.
On a transmit channel, the switch determines when to invoke flow control
based on the availability of switch resources, considering available buffer
space, the transmit queues, and the receive queues.
The 900EN-S005 will apply flow control on any port which just received a
packet if the destination port resources are being used fully. The switch will
also flow control all ports if its receive queue becomes full. A flow control
frame (XOFF) will be issued which contains the maximum pause time
defined by IEEE 802.3x. Once its required resource is freed up, it will send
out the opposite flow control frame (XON) with a zero pause time to turn off
the flow control (and thereby turn on transmission to the port). Additionally,
hysteresis is employed to prevent the flow control mechanism from being
activated and deactivated too many times.
Traffic Flow Controls
