Logical topology of a network is closely related to the mechanism used to manage network access. Access methods provide the procedures to manage network access so that all stations have access. When several entities share the same media, some mechanism must be in place to control access. Access methods are applied to networks to regulate this media access.
Some network topologies share a common medium with multiple nodes. At any one time, there may be a number of devices attempting to send and receive data using the network media. There are rules that govern how these devices share the media.
There are two basic media access control methods for shared media:
- Contention-based access: All nodes compete for the use of the medium but have a plan if there are collisions. Figure 1 shows contention-based access.
- Controlled access: Each node has its own time to use the medium. Figure 2 shows controlled access.
The data link layer protocol specifies the media access control method that will provide the appropriate balance between frame control, frame protection, and network overhead.