Network Topologies are in essence the way the network is laid out as it relates to the devices that are connected to it. If you could view all the devices connected to the network, the arrangement would suggest which type the network topology it falls within. The topologies can be described as either a physical or logical design in their defined layout. Network topologies are grouped in the following design categories, Mesh, Bus, Ring and Star Topologies.
A mesh topology comes in two varieties, Full mesh and Partial mesh. Full mesh consists of connectivity between every node on the network, meaning each has a direct path to the other. A partial mesh topology consists of the same type of connectivity as a full mesh except you may have to cross nodes to reach another, meaning that direct connectivity may not exist. Mesh topologies rely on routing to be able to choose the correct path between the hosts. One of the largest mesh topologies is the Internet itself. While you may not have a direct connection to a web server, the routers in the path know exactly the best path to get you there. Mesh topologies are considered highly available or redundant. With multiple paths to a destination, one can fail and your connectivity will remain. This leads to mesh networks being very complex. Being complex makes them harder to troubleshoot when issues arise. Along with additional routing overhead and connections, mesh topologies tend to be the most expensive to deploy of the topologies listed. Bus topologies can be described as the trunk of a tree where the branches are the nodes. Again each node has connectivity to the other but there is no interdependency between them. This type of topology was popular with 10base2 and 5 media types. While each node has connectivity to one another, there is no routing within the bus; it is one large broadcast domain. While the bus topology is simple and cheap to deploy it can only support a limited amount of nodes per bus or segment. This is because of the bus length and traffic that is broadcasted throughout it. Broadcast storms are common with this type of topology.
The Ring topology is defined exactly by its name, a ring. Within this topology every node has 2 neighbors, the one directly ahead and behind it on the ring. The data flow in this scenario is unidirectional around the ring. If a single node fails the ring fails, and the network as a whole is broken. Token ring is the most common form of this topology. Although you can still find this type of topology in some ISP’s through the use of FDDI and Sonet rings, Token ring is quickly being replaced by 802.3 Ethernet technologies.
The Star topology is designed in the shape of star. Every node in the network is connected through a centralized point. This makes the connectivity between the nodes indirect. While this type of network dedicates connectivity to each node, it also is vulnerable to complete network failure should the central point fail.
Many of today’s LAN and WAN networks share many of the features from the different topologies. Every Switch port is an arm of the star type topology. Every subnet could be defined as a bus topology, one large broadcast domain. Sonnet rings are commonly found among MANS and the Internet is one gigantic mesh topology. The important thing about topologies is to understand the theory. In order to properly design networks you must have a firm understanding of broadcasts, multicasts and routing.
The Institute of Electrical and Electronics Engineers (IEEE) specify 802.3 for the physical media characteristics of Ethernet. It defines the physical and data link layer for the media access control (MAC). It lays out the media types and their correspondent characteristics. The main advantage of Ethernet technology is its use as a communication media type between various types of communication protocols. Bandwidth speeds typically range from 10Mbs to 1Gbs. Ethernet is the most common communication method on today’s LANS.
Token Ring is the second most common protocol used in LANS today. While it will soon become obsolete, it still exists in larger environments. This protocol uses a token to establish who the speaker on the network is. This reduces data collisions between multiple computers. IEEE 802.5 defines its characteristics, and while it is a reliable transport it is only capable of 16Mbs. In information technology today its speed alone will render it obsolete.
Fiber Distributed Data Interface (FDDI) is the standard for transmitting data over Fiber optic line. This allowed for LANS to extend further distances. FDDI is commonly found in the backbones of WANS and some LANS, because it can support many types of traffic and its distance limitations are greater than that of copper. The major drawback of FDDI is its expense.
Wireless technologies are defined by IEEE 802.11a, b, g, and now n standards. This is the standards created to implement wireless communications for LANS. While wireless used to be limited by its bandwidth speeds, with the introduction of the N standard they are capable of 600Mbs. This technology is expensive as a whole but offers much cost savings when deployed where copper has geographical limitations.
Transmission control protocol (TCP) functions at layer 4 or transport layer of the OSI model. It receives encapsulated data from layers 5 through 7 of the OSI model and compiles it into a segment. It then transmits this segment to the network layer 3 or Internet protocol (IP) of the OSI model. The IP network layer 3 takes the segments from layer 4 and encapsulates them in an IP datagram or packet for transmission. The next layer of the OSI model the transport layer takes the IP packet and places it into an Ethernet frame. The Ethernet frame is sent on to the physical layer 1 for transmission across the particular media type. The exact opposite occurs when the physical layer one receives a transmission and hands it up to the corresponding layers each stripping off their part as it is passed up along the OSI model.
All of these topologies and protocols have laid the ground work for what we experience everyday on the Internet. Without them the technologies of today would not exist.