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
17
Unlike other Ethernet devices, such as an Ethernet host adapter or Network
Interface Card (NIC), the port of a switch does not require its own MAC
address. During retransmission of a received packet, the switch port will
instead look like the originating device by having assumed its source
address. This is why the Ethernet collision domain is said to terminate at the
switch port. That is, a two-port switch will effectively break a network into
two distinct data links or segments. A five port switch like the Acromag
900EN-S005 can break a network into 5 distinct data links or segments (also
called
collision domains
). Since all Ethernet nodes are able to recognize the
occurrence of a collision, and since the detection of a collision is principal to
the way Ethernet arbitrates media access, large domains containing many
nodes can become quite cumbersome. Thus, using an Ethernet switch to
subdivide a large network into separate collision domains will certainly help
to increase throughput.
Each port of a switch forwards data to another port based on the MAC
address contained in the received data packet/frame. In order to know
which port to forward a data packet to, the switch will learn and store the
MAC addresses of every device it is connected to, along with the associated
port number (up to 1024 MAC addresses are stored in high speed SRAM).
However, until the switch actually learns the port a particular address resides
at (the first packet), it forwards this traffic to all ports. The switch will use this
internal look-up table to quickly determine the location (port) of a node,
establish a temporary connection between itself and the node, then
terminate the connection once a packet is transferred. In this way, it
increases network bandwidth and provides the network determinism
required for critical control applications.
This switch uses a
store and forward
algorithm to process Ethernet frames.
That is, it first stores the Ethernet frame and examines it for errors before
forwarding it to its destination. Although this method may seem to increase
the forwarding time (latency) and possibly cause fragmentation, it effectively
reduces the occurrence of error frames and improves overall throughput.
This is particularly useful when there is heavy network traffic and or greater
potential for noise and interference.
Refer to the simplified schematic shown below to gain a better
understanding of the circuit. Note that the network transmit and receive
channels of each port include transient suppression. Further, the metal
shield of the network ports are terminated with an isolation capacitor and
TVS, which effectively isolates the shield connection, minimizes emissions,
and enhances transient protection.
HOW IT WORKS
The current tendency in critical
industrial control applications
is to connect one Ethernet
device per switch port. This
will produce the most
deterministic mode of
operation as the switch can
operate full-duplex, with no
chance of collisions. This
ensures determinism, helping
critical control applications to
remain predictable and on-
time.
