A convenient way to discuss local area networks is by their physical layout, or topology. To a certain extent, the topology of a certain network will reflect the cable type used and the actual architecture of the network (such as Ethernet or IBM Token Ring). And althoughcontinue the different types of topologies have been assigned particular characteristics (a bus topology, for instance, is considered to be a passive, contention-based network), the actual behavior of a particular network is better defined by the architecture used for the network. A short descriptionof each basic network topology and a diagram of that topology type follow.
Bus Network
A bus network is characterized by a main trunk or backbone line with the networked computers attached at intervals along the trunk line (see Figure 1.6). Bus networks are considered a passive topology. Computers on the bus sit and listen. When they are ready to transmit, they make sure that no one else on the bus is transmitting, and then they send their packets of information.Passive, contention-based bus networks (contention-based because each computer must contend for transmission time) would typically employ the Ethernet network architecture.
Bus networks typically use coaxial
hooked to each computer
using a T-connector. Each end of the network is terminated using a terminator specific to the cable type (if you use 50 Ohm cable, you use 50 Ohm terminators). Because the bus network is really just a collection of cable, connectors, and terminators, there is no amplification of the signal as it travels on the wire.break.
Bus networks are easy to assemble and extend. They require a fairly limited amount of cabling when compared to other topologies. Bus networks are prone to cable breaks, loose connectors,and cable shorts that can be very difficult to troubleshoot. One physical problem on the network,such as a detached connector, can actually bring down the entire bus network.
Star Networks
In a star topology, the computers on the network connect to a centralized connectivity device called a hub. Each computer is connected with its own cable (typically twisted-pair cable) to a port on the hub . Even though the star topology uses a hub (special hubs—multiport repeaters—can actually enhance the packet signals before passing them onto the network), this type of network still employs the passive, contention-based method of moving information on the wire that is embraced by the bus topology. Computers listen to the wire and then contend for transmission time.
Because the star topology uses a separate cable connection for each computer on the network,stars are easily expandable, with the main limiting factor being the number of ports available on the hub (although hubs can be daisy-chained together to increase the number of ports available).
Expanding a star topology network is also very unobtrusive; adding a computer to the network is just a matter of running a wire between the computer and the hub. Users on the network will be pretty much unaware that the expansion is taking place.Disadvantages of the star topology revolve around cabling needs and the hub itself. Because each computer on the network requires a separate cable, cable costs will be higher than a bus topology network (although twisted pair, the cable type used for stars, is the least expensive cable).
Purchasing a hub or hubs for your network does add additional costs when you are building a network based on the star topology, but considering the benefits of this type of topology in terms of managing the physical aspects of your network, it is probably well worth it. (Hub prices have
fallen to a point where even computer users with a small home network will probably want to use a hub to connect computers.)
The most negative aspect of the star topology is related to the central hub. If the hub fails, so does the network. You will find that many network administrators who don’t like crisis management keep an extra hub squirreled away just in case.
Ring Topology
A ring topology connects the networked computers one after the other on the wire in a physical circle . The ring topology (an example of an architecture that uses a ring topology is Fiber Distributed Data Interface—FDDI) moves information on the wire in one direction and is considered an active topology. Computers on the network actually retransmit the packets they receive and then send them on to the next computer in the ring.
Access to the network media is granted to a particular computer on the network by a token. The token circulates around the ring and when a computer wants to send data, it waits for the token to come around and then takes possession of it. The computer then sends its data onto the wire.After the computer that sent the data receives verification from the destination computer that the message was received, the sending computer creates a new token and passes it onto the next computer in the ring, beginning the token passing ritual again.
The fact that a computer must have the token to send data means that all the computers on the network have equal access to the network media. Token passing rings provide a more timely transmission of data (because of the level playing field provided by the token passing strategy) when compared to contention-based networks like thecontinue
bus or star. Token Rings actually degrade more gracefully (in terms of performance) during times of high traffic when compared to passive topologies, which can go down quickly in very high traffic situations due to increased packet collisions.True ring topologies can be difficult to troubleshoot, and the failure of one computer on the ring can disrupt the data flow because data circulates around the ring in one direction. Adding or removing computers from this type of topology also can disrupt the operation of the network.
Mesh Topology
The mesh topology uses redundant connections between computers on the network as a fault tolerance strategy. Each device on the network is connected to every other device. In short, this type of topology requires a lot of cable . This type of topology also can weather a broken segment or two and still continue to operate as a network because of all the redundant lines.
Mesh networks, obviously, would be more difficult and expensive to install than other network topologies because of the large number of connections required. In most cases, networks that use this redundant connection strategy will actually be comprised of a hybrid mesh. In a hybrid mesh only highly important servers and mission-critical computers are configured with redundant connections. This protects the most important parts of the company wide network but doesn’t require multiple lines to every computer.
Source: http://www.adminkernel.com/networking-concepts/understanding-network-topologies