2.1 Legacy Media Types
2.1.1 Legacy Ethernet
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.