Segmentation
5-15
Ethernet
As all of the modules listed in Table 5-4, above, are Ethernet Channel A modules,
they are fully interchangeable in configuration terms. A configuration like the
example given previously could have utilized three TPMIM-34 modules if RJ21
connectors were to be used, or included one FOMIM-26 to connect six stations
with fiber optic cabling.
Even though many simple Ethernets will occupy only one MMAC chassis, it is
important to recognize that any links made from one simple Ethernet chassis to
another that is not made through a bridge or router will expand the one network.
Without the bridge or router present to perform the filtering or forwarding
services for traffic between the two chassis, the modules and stations will act as
though they were configured in a single MMAC. Although the two chassis and
their associated modules are operating on the same network, each modular
chassis to be configured in simple Ethernet fashion requires an Ethernet repeater
module. Without the module, the stations attached to the connectivity modules
will not be able to transmit and receive signals to and from the network.
To illustrate, if a location required two MMAC chassis configured as in the
previous example, connected to one another through a fiber optic connection, we
would begin by placing an IRM3 in each of the MMAC chassis. These IRM3
modules would provide the necessary Ethernet repeating functions to allow the
connectivity modules, TPMIM-24s once again, to operate and communicate with
one another. The fiber optic link between the two hubs (assuming multimode
fiber optics and ST connectors) would be made using the fiber optic connectors on
the front panel of each IRM3. While the stations connected to the TPMIM modules
of each hub would be located in physically distinct areas, the network would treat
all the stations as being on the same logical network, sharing the same 10 Mbps of
bandwidth available in an Ethernet network.
Segmentation
Segmentation is the incorporation of bridging or routing between individual
networks or segments. The segmentation of a network is something that must be
approached cautiously, and extended discussions of this planning are presented
in Chapter 4, Network Design. One of the important design guidelines for
segmentation is to try and follow the 80/20 rule. You want, ideally, to keep 80% of
your traffic local and pass about 20% of your traffic across segmentation devices.
It makes perfect sense, therefore, to put departments that share fileservers on the
same segment. That way users needing access to the file server in question do not
have to cross the bridge, cutting into the speed of the bridge and reducing the
speed of network operations.
This segmentation is based on the extent to which distinct groups of users or
departments are related and require transmission of information betweenone
another. For example, the Payroll, Personnel, and Accounting departments of
most businesses transfer information on a very regular basis. It makes sense from
a network design perspective, therefore, to attempt to keep these departments
from having to pass more than 20% of their traffic across a bridge or other
segmentation device.
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