7.2.3 Media Access Control for Non-Shared Media Page 1:

Media access control protocols for non-shared media require little or no control before placing frames onto the media. These protocols have simpler rules and procedures for media access control. Such is the case for point-to-point topologies.

In point-to-point topologies, the media interconnects just two nodes. In this arrangement, the nodes do not have to share the media with other hosts or determine if a frame is destined for that node. Therefore, Data Link layer protocols have little to do for controlling non-shared media access.

Full Duplex and Half Duplex

In point-to-point connections, the Data Link layer has to consider whether the communication is half-duplex or full-duplex.

Click the tabs in the figure to see the differences in the two methods.

Half-duplex communication means that the devices can both transmit and receive on the media but cannot do so simultaneously. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time.

In full-duplex communication, both devices can transmit and receive on the media at the same time. The Data Link layer assumes that the media is available for transmission for both nodes at any time. Therefore, there is no media arbitration necessary in the Data Link layer.

The details of a specific media access control technique can only be examined by studying a specific protocol. Within this course, we will study traditional Ethernet, which uses CSMA/CD. Other techniques will be covered in later courses.

7.2.4 Logical Topology vs Physical Topology Page 1:

The topology of a network is the arrangement or relationship of the network devices and the interconnections between them. Network topologies can be viewed at the physical level and the logical level.

The physical topology is an arrangement of the nodes and the physical connections between them. The representation of how the media is used to interconnect the devices is the physical topology. These will be covered in later chapters of this course.

A logical topology is the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network independent of their physical layout. These logical signal paths are defined by Data Link layer protocols. The Data Link layer "sees" the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and media access control used.

The physical or cabled topology of a network will most likely not be the same as the logical topology.

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. Access methods will be discussed in more detail later.

Logical and physical topologies typically used in networks are:

• Point-to-Point

• Multi-Access

• Ring

The logical implementations of these topologies and their associated media access control methods are considered in the following sections.