| When mainframe
computers dominated the industry, user terminals attached either
directly to ports on the mainframe or to a controller that gave the
appearance of a direct connection. Each wire connection was
dedicated to an individual terminal. Users entered data, and the
terminal immediately transmitted signals to the host (the term host
here refers to the mainframe, a usage that may be confusing because
normally the term is applied to end systems). Performance was driven
by the horsepower in the host. If the host became overworked, users
experienced delays. Note, though, that the connection between the
host and terminal was not the cause of the delay. The users had full
media bandwidth on the link, regardless of the workload of the host
device.
Facility managers installing the
connections between the terminals and the host experienced distance
constraints imposed by the terminal line technology of the host. The
technology limited users to locations that lay within a small radius
of the host. Further, labor to install the cables inflated
installation and maintenance expenses. LANs mitigated these issues
to a large degree. One of the immediate benefits of a LAN was to
reduce the installation and maintenance costs by eliminating the
need to install dedicated wires to each user. Instead, a single
cable pulled from user to user allowed users to share a common
infrastructure instead of having dedicated infrastructures for each
station.
A problem arises when users share a
cable, however. Specifically, how does the network control who uses
the cable and when? Broadband technologies such as cable television
(CATV) support multiple users by multiplexing data on different
channels (frequencies). You can think of each video signal on a CATV
system as a data stream-each data stream is transported over its
own channel.
A CATV system carries multiple
channels on a single cable and can, therefore, carry multiple data
streams concurrently. This is an example of frequency-division
multiplexing (FDM). The initial LANs were conceived as baseband
technologies, which do not have multiple channels. Baseband
technologies do not transmit using FDM. Rather, they use bandwidth
sharing, meaning simply that users take turns transmitting.
Ethernet and other LAN technologies
define sets of rules known as access methods for sharing the cable.
The access methods approach media sharing differently, but have
essentially the same end goal in mind.
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