Computers communicate by transmitting an electrical signal over wires, with differing electrical characteristics meaning binary 0 and binary 1. For effective communication, both transmit and receive paths are needed; with a single path, the electrical signals would overlap, confusing the meaning of both electrical signals. Also, a twisted pair of wires is used for transmission in each direction because a twisted pair reduces the impact of electromagnetic interference on the wires, which in turn reduces transmission errors.
LAN UTP cabling includes up to four twisted pairs of unshielded wires, typically terminated with RJ-45 connectors. Although four pairs are supported, only two pairs are needed for Ethernet LAN communicationsone pair for each direction of transmission. Depending on which devices are being connected, you might need a cross-over cable or a straight-through cable, ensuring that the pair used for transmission by one device connects to the pair used for receiving data on the other device.
Ethernet hubs allow multiple devices to connect to it, with the result that all devices receive a copy of whatever each device sends to the hub. By doing so, each device needs a single NIC and a single cable connecting it to the LAN hub, while allowing all devices to communicate with all other devices.
Finally, a structured wiring plan helps prevent unnecessary cabling runs inside the difficult-to-reach parts of the physical location (under the floor or in the ceiling). It also allows other personnel to change cabling later, without involving the electrician.