Physical bus topology

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The Physical Bus Topology

The physical bus topology is one of the oldest topologies in LANs, and was largely associated with early Ethernet environments implementing coaxial cable. A bus is an environment where the cable plant is an expanse of cable, terminated at both ends, to which LAN adapters are passively attached. The cable plant typically extended through the ceilings and/or floors of a building. Every LAN adapter was attached to the cable via a connector, also known as a medium-dependent interface (MDI). The specific nature of the MDI differed depending on the specific medium being used. A LAN adapter transmitted a signal on to the bus where the signal propagated in both directions. The terminator at each end of the bus was a resistor responsible for preventing a signal from reflecting back onto the bus, where it would corrupt subsequent signals.

The use of the word “passive” describes an environment where no attached device (i.e., a LAN adapter) has responsibility (at the Physical Layer) for regenerating or repeating another adapter's transmission. They simply passively monitor the bus and are capable of transmitting signals on to, and reading signals from, the bus. As a consequence, the failure of any LAN adapter on a bus has no impact on the physical viability of the bus.

Because of the passive station attachment, physical bus topologies were originally described as being highly reliable environments. The argument suggested that the bus is a distributed environment, and hence does not have a single point of failure. The validity of this view is questionable. The loss of a single terminator or the presence of a single cable short could destroy the viability of the entire LAN. Not only was the cable plant a single point of failure, it was a single point of failure that could be several hundred meters long and run through walls, ceilings, and floors, making it extremely difficult to troubleshoot when failures inevitably occurred. Historically, the most popular medium for a physical bus topology was coaxial cable. This was largely due to the early popularity of coaxial cable for deploying Ethernets. LocalTalk was another type of LAN that was deployed as a physical bus, but this LAN typically implemented unshielded twisted pair (UTP). When twisted pair was deployed in a physical bus topology, a strategy called daisy chaining was typically used. In daisy chaining, one or two twisted pairs are extended between adjacent systems. The systems or connectors at either end of the chain must provide the termination. For those who owned or have seen Macintosh computers connected using LocalTalk, daisy chaining is a familiar concept.

Although an amount of coax-based Ethernet and UTP-based LocalTalk is still deployed (few technologies ever truly die completely), physical bus topologies have largely disappeared as a strategy for LAN cabling to the desktop.