The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

7.2. Full Duplex Enablers

Two factors are responsible for enabling LANs to operate in full duplex mode:

• The use of dedicated media, as provided by the popular structured wiring systems deployed today.

• The use of microsegmented, dedicated LANs, as provided by switches.

7.2.1. Dedicated Media

Ethernets migrated from using coaxial cable (10BASE5 or 10BASE2) to using structured wiring with twisted pair in the early 1990s, with the development of 10BASE-T [IEEE98e, Clause 14]. The higher-speed versions of Ethernet (Fast and Gigabit Ethernet) developed during the 1990s support only dedicated media connections both for twisted pair and optical fiber. There are no commercial shared-medium implementations of Ethernet at data rates above 10 Mb/s.

Interestingly, the underlying channel for Token Ring LANs has always provided dedicated connections between each station and a central hub [IBM82]. Initially these connections were made with shielded twisted pair (STP), and then later with unshielded twisted pair (UTP), at data rates of 4, 16, and later 100 Mb/s. It is simply not possible to build a Token Ring using a shared-medium channel; by definition such a system constitutes a Token Passing Bus (not a ring) [IEEE90c].

As depicted in Figure 7-2, the use of a structured star-wired system with hubs at the center of the star changes the fundamental assumption that the underlying medium cannot support full duplex operation. Unlike coaxial cables, most twisted pair Ethernet systems (10BASET, 100BASE-TX, ...