|  | 5.5 |

Collisions and Collision Domains in Shared Layer Environments

| 5.5.1 | Shared media environment

Some networks are directly-connected; all hosts share Layer 1. Examples are:

- 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. Occasionally you may hear someone say "all the computers are on the same wire" . It means that they all share the same media - even though the "wire" might be CAT 5 UTP, which has four pairs of wire.

- 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 more users. There are, however, negative aspects to this as well as positive aspects.

- point-to-point network environment - is most widely used in dial-up network connections, and is the one with which you are most likely familiar. It is a shared networking environment in which one device is connected to only one other device via a link, such as you connecting to internet service provider by phone line.

Some networks are indirectly-connected, meaning that some higher layer networking devices and/or some geographical distance is between the two communicating hosts. There are two types:

- circuit-switched - an indirectly-connected network in which actual electrical circuits are maintained for the duration of the communication. The current telephone system is still, in part, circuit-switched, although the telephone systems in many countries is now concentrating less on circuit-switched technologies.

- packet-switched - rather than dedicating a link as an exclusive circuit connection between two communicating hosts, the source sends messages in packets. Each packet contains enough information for it to be routed to the proper destination host. The advantage is that many hosts can share the same link; the disadvantage is that conflicts can occur.