Some networks are directly connected and all hosts share Layer 1. Examples are listed in the following:

• Shared media environment – Occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire, optical fiber, or share the same airspace, they all share the same media environment.
• Extended shared media environment – Is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple access or longer cable distances.
• Point-to-point network environment – Is widely used in dialup network connections and is the most familiar to the home user. It is a shared networking environment in which one device is connected to only one other device, such as connecting a computer to an Internet service provider by modem and a phone line.

It is important to be able to identify a shared media environment, because collisions only occur in a shared environment. A highway system is an example of a shared environment in which collisions can occur because multiple vehicles are using the same roads. As more vehicles enter the system, collisions become more likely. A shared data network is much like a highway. Rules exist to determine who has access to the network medium, but sometimes the rules simply cannot handle the traffic load and collisions occur.

Web Links Carrier Sense Multiple Access with Collision Detection (CSMA/CD) http://www.erg.abdn.ac.uk/users/gorry/course/ lan-pages/ csma-cd.html