Cabletron Systems MMAC-5FNB, Networking Manual

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Technology Basics
3-4 Ethernet
Segmentation
The operation of Ethernet networks is based on the fact that every workstation or
device on the network is connected to every other device on the network. Due to
this treatment, the larger the Ethernet network gets, the more congested it
becomes with added traffic. The added traffic also increases the probability of
collisions occurring. In time, the Ethernet network will grow to encompass so
many stations that it is excessively difficult for a station to receive an opportunity
to transmit frames. The only way to reduce this congestion is by breaking the one
large network up into smaller networks through segmentation.
It is the function of segmentation to separate networks while allowing
information to flow between them when necessary. An Ethernet bridge acts in
much the same way as a bridge in the physical world. It provides a single point of
access between both networks, allowing traffic which must cross to the other
network to do so, while keeping unrelated network activity on one side of the
bridge from impacting the operation of the network on the other side of the
bridge.
Segmentation devices available for Ethernet include bridges, switches, and
routers. While all of these devices perform the same basic function they carry out
the task in different ways and with different results.
Bridges
Bridges operate by reading in frames from one interface to determine if they are
intended for the network they are read from. If the bridge determines that a frame
being examined is, in fact, intended for a station on the network it was received
from, it will discard the frame. After all, the destination station is on the same side
as the transmitting station, and has already received the frame. If, however, the
bridge determines that the destination of the frame is a station which is not on
that network, the bridge will repeat that frame to one of its other interfaces.
In addition, Ethernet bridges can connect an Ethernet network to another
networking technology, provided that the networks are similar. While the
definition of “similar” in respect to bridges is a complex one, we can simplify it
for design purposes: Ethernet, Token Ring, and FDDI are all similar network
technologies from a bridge’s point of view.
This bridging functionality is the basis of creating backbones that use a different
technology than workgroup networks. The most common backbone strategy is to
interconnect several Ethernet workgroups, which operate at a maximum of 10
Mbps, to each other with an FDDI backbone, which operates at 100 Mbps. An
FDDI Management Media Interface Module, or FDMMIM, configured in an
Ethernet chassis would provide the bridging functions necessary to allow the
chassis to connect to such a backbone.