- 14 -
Switching, Filtering, and Forwarding
Each time a packet arrives at one of the switched ports, a decision is made to
either filter or forward the packet. Packets with source and destination
addresses belonging to the same port segment will be filtered, constraining
those packets to one port, and relieving the rest of the network from the need to
process them. A packet with destination address on another port segment will
be forwarded to the appropriate port, and will not be sent to the other ports
where it is not needed. Packets that are used in maintaining the operation of the
network (such as the occasional multi-cast packet) are forwarded to all ports.
The EDS operates in the store-and-forward switching mode, which eliminates
bad packets and enables peak performance to be achieved when there is heavy
traffic on the network.
Switching and Address Learning
The EDS has an address table that can hold up to 1K node addresses, which
makes it suitable for use with large networks. The address tables are
self-learning, so that as nodes are added or removed, or moved from one
segment to another, the EDS automatically keeps up with new node locations.
An address-aging algorithm causes the least-used addresses to be deleted in
favor of newer, more frequently used addresses. To reset the address buffer,
power down the unit and then power it back up.
Auto-Negotiation and Speed Sensing
All of the EDS’s RJ45 Ethernet ports independently support auto-negotiation
for speeds in the 10BaseT and 100BaseTX modes, with operation according to
the IEEE 802.3u standard. This means that some nodes could be operating at
10 Mbps, while at the same time, other nodes are operating at 100 Mbps.
Auto-negotiation takes place when an RJ45 cable connection is made, and then
each time a LINK is enabled. The EDS advertises its capability for using either
10 Mbps or 100 Mbps transmission speeds, with the device at the other end of
the cable expected to similarly advertise. Depending on what type of device is
connected, this will result in agreement to operate at a speed of either 10 Mbps
or 100 Mbps.
If an EDS RJ45 Ethernet port is connected to a non-negotiating device, it will
default to 10 Mbps speed and half-duplex mode, as required by the IEEE
802.3u standard.
