|  | 8.1 | Basic Networking Knowledge
| 8.1.1 | Star, bus, mesh, and ring topologies
Star Topology In a star topology, all devices are connected to a common central location, typically a hub or a switch. When a node sends data to the central location, the central device retransmits the information and sends it to the destination. Because all cabling is connected to a central device, if one link fails, only that portion of the network will fail. The rest of the network will not be affected. However, if the central device fails, the entire network will also fail. A star topology can have a maximum of 1,024 nodes on a LAN and is commonly used for 10BASE-T (IEEE 802.3) and 100BASE-TX (IEEE 802.12) Ethernet. Advantages of star topologies include reliability along with the ease of maintenance and installation. Monitoring and troubleshooting can be maintained at the central device, providing easier maintenance. Star topologies allow for greater reliability because each node is connected to the central device by a segment. If one segment breaks, only that node loses access to the network, hence the rest of the network is not affected. Because each node is connected to the central device, star topologies also allow for an easy network layout, providing the network administrator easier installation over the other topologies. A disadvantage of this topology is cost. With each device being connected to the central location, more cabling is required than other topologies. In addition there is the cost of the central device. Bus Topology A bus topology connects multiple devices onto one main cable and is sometimes referred to as a backbone, trunk, or segment. Terminators must be connected at each end of the topology to absorb any reflected signals. If coaxial cable is used, without terminators, reflected signals will echo across the network, causing the entire network to be unusable. An advantage of a bus topology is cost and ease of installation. Because this topology uses a simple cable layout, it costs less and is easier to implement than the other topologies. A disadvantage is that if a cable segment or the backbone breaks or fails, the network will fail. Another disadvantage is only one node can transmit data onto the network at a time. If two or more nodes attempt to send data at the same time, a collision will occur; which will require a recovery procedure thereby slowing down the network. Once the collision has occurred, all the data must be re-sent. A process called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) prevents the occurrence of another collision. CSMA/CD is a process where each node waits its "turn" to retransmit data. Mesh Topology Normally used for WANs, a mesh topology connects every device on the network together and provides a path to and from each device. An advantage is that because all the devices are connected to each other, the network has a higher fault tolerance and reliability. If a cable segment breaks along the network, the devices will find the quickest way to reroute the packet to its destination. Therefore, the data will most often always reach its destination. Disadvantages of this topology are cost and difficulty in management. Because there are numerous connections, to and from each device, there is a large number of cabling requirements, causing a mesh topology to be somewhat expensive. If a segment breaks on the network, with the complex design of the mesh topology, finding the exact problem location can be very difficult. Therefore, maintaining the network can be very complex. Ring Topology Ring topologies consist of each device on the network being connected with two other devices. There is no beginning or end of the cable. This particular topology forms a complete ring. The devices on this network use a transceiver to communicate with their neighbors. Transceivers also act like repeaters to regenerate each signal as it is passed through the device. Advantages include better performance because each device receives a "turn" to transmit signals and has equal access to the network. An additional advantage is that the signal is regenerated by each device it passes through, thereby preventing the signal from degrading. A disadvantage of using this topology is that if one device on the cable fails, the entire network will also fail. Locating the failure can sometimes be difficult. Another disadvantage is that if any changes are made to the network, including adding to or moving devices, the disruption will cause the network will fail.
| Web Links
| CompTIA Home/Net + Certification
| Cisco Connection Online
| Cisco Documentation
| Search Cisco