star network

Star network
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Star network layout

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.

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Advantages
Performance: Data packets do not travel through any unnecessary nodes. Communication between any two devices on the network involves at most three devices and two links. The isolation of traffic between nodes means that heavy utilization from one device is invisible to other devices on the network, provided that the central hub retains adequate capacity.
Isolation: Each device is isolated on its own link. This makes it easy to isolate individual devices from the network by disconnecting them from the hub. Any non-centralized failure will have very little effect on the network.
Centralization: The network can easily be scaled or expanded by adding to the capacity of the hub node, or by adding additional devices to the star. The fact that all traffic passes through the central hub means that the hub can easily be used to inspect or control traffic through the network.
Simplicity: The topology is easy to understand, establish, and navigate. Complex routing or message passing protocols are generally unnecessary. Individual nodes can easily be added or removed, and fault detection is simplified, as each link/device can be probed individually.

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Disadvantages

The primary disadvantage of a star topology is the high dependence of the system on the functioning of the central hub. While the failure of an individual link only results in the isolation of a single node, the failure of the central hub renders the network inoperable, immediately isolating all nodes. The performance and scalability of the network also depend on the capabilities of the hub. Network size is limited by the number of connections that can be made to the hub, and performance for the entire network is capped by its throughput. While in theory traffic between the hub and a node is isolated from other nodes on the network, other nodes may see a performance drop if traffic to another node occupies a significant portion of the central node's processing capability or throughput.