The foundation for Ethernet technology was first established in 1970 with a program called Alohanet. Alohanet was a digital radio network designed to transmit information over a shared radio frequency between the Hawaiian Islands.
Alohanet required all stations to follow a protocol in which an unacknowledged transmission required re-transmitting after a short period of waiting. The techniques for using a shared medium in this way were later applied to wired technology in the form of Ethernet.
Ethernet was designed to accommodate multiple computers that were Interconnected on a shared bus topology.
The first version of Ethernet incorporated a media access method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD managed the problems that result when multiple devices attempt to communicate over a shared physical medium.
Early Ethernet Media
The first versions of Ethernet used coaxial cable to connect computers in a bus topology. Each computer was directly connected to the backbone. These early versions of Ethernet were known as Thicknet, (10BASE5) and Thinnet (10BASE2).
10BASE5, or Thicknet, used a thick coaxial that allowed for cabling distances of up to 500 meters before the signal required a repeater. 10BASE2, or Thinnet, used a thin coaxial cable that was smaller in diameter and more flexible than Thicknet and allowed for cabling distances of 185 meters.
The ability to migrate the original implementation of Ethernet to current and future Ethernet implementations is based on the practically unchanged structure of the Layer 2 frame. Physical media, media access, and media control have all evolved and continue to do so. But the Ethernet frame header and trailer have essentially remained constant.
The early implementations of Ethernet were deployed in a low-bandwidth LAN environment where access to the shared media was managed by CSMA, and later CSMA/CD. In additional to being a logical bus topology at the Data Link layer, Ethernet also used a physical bus topology. This topology became more problematic as LANs grew larger and LAN services made increasing demands on the infrastructure.
The original thick coaxial and thin coaxial physical media were replaced by early categories of UTP cables. Compared to the coaxial cables, the UTP cables were easier to work with, lightweight, and less expensive.
The physical topology was also changed to a star topology using hubs. Hubs concentrate connections. In other words, they take a group of nodes and allow the network to see them as a single unit. When a frame arrives at one port, it is copied to the other ports so that all the segments on the LAN receive the frame. Using the hub in this bus topology increased network reliability by allowing any single cable to fail without disrupting the entire network. However, repeating the frame to all other ports did not solve the issue of collisions. Later in this chapter, you will see how issues with collisions in Ethernet networks are managed with the introduction of switches into the network.
Ethernet Collision Management
Legacy Ethernet
In 10BASE-T networks, typically the central point of the network segment was a hub. This created a shared media. Because the media is shared, only one station could successfully transmit at a time. This type of connection is described as a half-duplex communication.
As more devices were added to an Ethernet network, the amount of frame collisions increased significantly. During periods of low communications activity, the few collisions that occur are managed by CSMA/CD, with little or no impact on performance. As the number of devices and subsequent data traffic increase, however, the rise in collisions can have a significant impact on the user's experience.
A good analogy is when we leave for work or school early in the morning, the roads are relatively clear and not congested. Later when more cars are on the roads, there can be collisions and traffic slows down.
Current Ethernet
A significant development that enhanced LAN performance was the introduction of switches to replace hubs in Ethernet-based networks. This development closely corresponded with the development of 100BASE-TX Ethernet. Switches can control the flow of data by isolating each port and sending a frame only to its proper destination (if the destination is known), rather than send every frame to every device.
The switch reduces the number of devices receiving each frame, which in turn reduces or minimizes the possibility of collisions. This, and the later introduction of full-duplex communications (having a connection that can carry both transmitted and received signals at the same time), has enabled the development of 1Gbps Ethernet and beyond.
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