Network Topologies
In networking, the term topology refers to the layout of connected devices on a network. One can thing of a topology as a network’s shape”.
Network topologies are categorized into the following basic types:
Bus, Ring, Star, Tree and Mesh.
More complex networks can be built as hybrids of two or more of the above basic topologies.
Bus: Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium, that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.
Ethernet bus topologies are relatively easy to install and don’t require much cabling compared to the alternatives. 10base-2 (“ThinNet”) and 10Base-5 (“ThickNet”) both were popular Ethernet cabling options years ago. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a bus, performance problems will likely result, in addition, if the backbone cable fails, the entire network effectively becomes unusable.
CSMA/CD (Darrier Sense Multiple Access/ Collision Detection): it is a local area network access method in which contention between two or more stations is resolved by collision detection. When two stations transmit at the same time, they both stop and signal a collision has occurred. Each then tries again after waiting a predetermined time period. To avoid another collision, the stations involved each choose a random time interval to schedule the retransmission of the collided frame. To make sure that the collision is recognized, Ethernet requires that a station must continue transmitting until the 50 microsecond period has ended. If the station has less than 64 bytes of data to send, then it must pad the data by adding zeros at the end. Used with Ethernet.
CSMA/CS (Carrier Sense Multiple Access/ Carrier Sense)” A node listens to the bus for a predetermined amount of time before transmitting and waits until the talking node has completed transmission.
CSMA/CR (Carrier Sense Multiple Access/ Collision Resolution): it allows multiple devices to talk at once, a protocol determines which device receives priority.
Ring: in a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (effectively either “clockwise” or “counterclockwise”). A failure in any cable or device breaks the loop and can take down the entire network. Token passing scheme is used in this topology, which has been discussed earlier.
Star: Many home networks use the star topology. A star network features a central connection point called a “hub” that may be an actual hub or a switch. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet. Compared to the bus topology, a star network cable will only take down one computer’s network access and not the entire LAN. If the hub fails, however, the entire network also fails.
Tree: Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the “root” of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub ports) alone.
Mesh: Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that in a ring, although two cable paths exist, messages can only travel in one direction). Some WANs, like the Internet, employ mesh routing. A mesh offers several advantages over other network topologies.
- Mesh network has high fault tolerance due to multiple links.
- Due to multiple links mesh network can work fine even under severe loads.
- Troubleshooting of mesh network is easy as compared to other networks. If data is not communicated between any two computer then it means that there is some fault in direct links between them.
Disadvantages include the difficulty of installation and reconfiguration, as well as the cost of maintaining redundant links.
