Early Ethernet and legacy Token Ring LAN technologies included two other types of topologies:
• Bus — All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Legacy Ethernet networks were often bus topologies using coax cables because it was inexpensive and easy to set up.
• Ring — End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated, unlike in the bus topology. Legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks used ring topologies.
Figure 31-8 illustrates how end devices are interconnected on LANs. It is common for a straight line in networking graphics to represent an Ethernet LAN including a simple star and an extended star.
Figure 31-8 LAN Physical Topologies
Check Your Understanding – Topologies (31.1.6)
Refer to the online course to complete this activity.
Media Access Control Methods (31.2)
How nodes on a network communicate is determined by the topology of the network. This topic will provide an overview of how data access to the media is regulated.
Half and Full Duplex Communication (31.2.1)
Understanding duplex communication is important when discussing LAN topologies because it refers to the direction of data transmission between two devices. There are two common modes of duplex.
Both devices can transmit and receive on the media but cannot do so simultaneously. WLANs and legacy bus topologies with Ethernet hubs use the half-duplex mode. Half-duplex allows only one device to send or receive at a time on the shared medium. In Figure 31-9, the server and hub are operating in half-duplex.
Figure 31-9 Half-Duplex Communication
Both devices can simultaneously transmit and receive on the shared media. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but they can operate in half-duplex if connecting to a device such as an Ethernet hub. Figure 31-10 shows an example of full-duplex communication.
Figure 31-10 Full-Duplex Communication
In summary, half-duplex communications restrict the exchange of data to one direction at a time. Full-duplex allows the sending and receiving of data to happen simultaneously.
It is important that two interconnected interfaces, such as a host NIC and an interface on an Ethernet switch, operate using the same duplex mode. Otherwise, there will be a duplex mismatch creating inefficiency and latency on the link.
Access Control Methods (31.2.2)
Ethernet LANs and WLANs are examples of multiaccess networks. A multiaccess network is a network that can have two or more end devices attempting to access the network simultaneously.
Some multiaccess networks require rules to govern how devices share the physical media. There are two basic access control methods for shared media:
• Contention-based access
• Controlled access