Network Topologies : Most LANs are based on simple structured topologies, such as bus, ring or star. Some of the basic characteristics of these topologies are discussed in the following sections.
Network Topologies
A Network Topology is the arrangement with which computer systems or network devices are connected to each other. Topologies may define both physical and logical aspect of the network. Both logical and physical topologies could be same or different in a same network.
Network topologies can be classified by how their elements are interconnected. Depending on the type of nodes and links, topologies can vary from centralized, decentralized to distributed.
Bus Topologies
Bus topologies are multipoint electrical circuits that can be implemented using coaxial cable JTP, or STP. Data transmission is bi-directional, with the attached devices transmitting in both direct ions. While operating at a raw data rate of 10 mbps, actual throughput is much less.
This is employed frequently in the LANs with distributed control. In this all nodes. as shown in Figure share the common bus. Messages placed on the bus are transmitted to all nodes. Nodes must be able to recognise their own address in order to receive messages. However. unlike nodes in a ring. they do not have to repeat and forward massages intended for other nodes. As a result, the delay and overheads associated with retransmitting messages at each intervening node are eliminated.
Because of the passive role that node play in transmission on the bus, network operations will continue in the event of node failures. This makes distributed bus network’s inherently resistive to single point failures. Bus networks employ a decentralised method of the media access control known as CSMA (Carrier Sense Multiple Access) that allows the attached devices to make independent decisions relative to media access and initiation of transmission.
Ring Topologies
The ring architecture is a distributed architecture with minimal connectivity and a topology
Of two links connected to every node as shown in Figure and forms unbroken circular configurations. Figure shows a network laid out in a physical ring or closed loop, configuration. Transmitted messages travel from node to node around the ring. Each node must be able to recognize its own address in order to accept messages. Information travels around the ring in only one direction, with each attached station or node serving as a repeater. Rings generally are coaxial cable or fiber in nature, Operating at raw transmission rates of 4, 16, 20 or 100 mbps or more. Rings are deterministic in nature, employing token passing as the method of media access control to ensure the ability of all nodes to access the network within a predetermined time interval.
Ring networks with centralised control are known as loops. When ring networks with distributed control are employed, some form of control strategies must be used to avoid conflicting demands for the shared channel. The popular control strategies are token passing circulating and registering insertion techniques.
Star Topologies
The distinguishing feature of star topology is that all nodes are joined at a single point, as shown in Figure. This single point is called the central node, hub, or switch, to which all other devices are attached directly, generally via UTP or STP. This topology is frequently used for networks in which control of the network is located in the central node. This method is optimal when the bulk of communication is between the central and the outlying nodes. If traffic is high between the outlying nodes, an undue switching burden is placed on the central node.
Transmission rates vary with AT&T’s Star LAN operating at I to 10 mbps, and both 100Base-T and 100VG-AnyLAN at 100 mbps. The biggest advantage of star is that a
disruptive or failed station can be isolated: thereby eliminating any negative effect it may have on LAN performance. Additionally, each node has access to the full bandwidth of the LAN, at least in a LAN switch environment. The disadvantage is that a hub failure is catastrophic as all connectivity is provided through the central hub; its failure affects the entire LAN. ATM (Asynchronous Transmission Mode) can be considered as one example.
AIM cell has a fixed length of 53 bytes. Being of fixed length allows the information to be transported in a predictable manner. This predictability accommodates different traffic types on the same network. The cell is broken into two main sections: the header and the payload. The payload (48 bytes) is the portion which carries the actual information i.e., voice, data, or video. The header (5 bytes) is the addressing mechanism.
Mesh Topology
Mesh topology configures in such a way that every device on the network is physically connected to every other device on the network. It provides better reliability and performance with complexity and difficulty in creating a mesh network when it increases geometrically with the increase of nodes on the network. A smaller mesh network consisting of three or four nodes is relatively easy to handle, while it is impractical to up a mesh network of several hundred nodes. They find their place in WAN rather than in LAN. In case of WAN, reliability is paramount and the number of sites being connected together is fairly small.
Hybrid Topology
Hybrid topology includes a combination of different topologies as described here and configures a network including a star-bus hybrid, star-ring network, and mesh networks with connections between various computers on the network.