Computer Networks: Comparison of Ring and Star Topologies

Ring Topology -

A ring network is network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travel from node to node, with each node along the way handling every packet.

Because a ring topology provides only one pathway between any two nodes, ring networks may be disrupted by the failure of a single link. A node failure of cable break might isolate every node attached to the ring. In response, some ring networks add a "counter-rotating ring" (C-Ring) to form a redundant topology: in the event of a break, data are wrapped back onto the complementary ring before reaching the end of the cable, maintaining a path to every node along the resulting C-Ring. Such "dual ring" network include Spatial Reuse Protocol, Fiber Distributed Data Interface (FDDI), and Resilient Packet Ring. 802.5 network - also known as IBM token ring network - avoid the weakness of a ring topology altogether: they actually use a star topology at the physical layer and a media access unit (MAU) or multistation access uni t(MSAU) to imitate a ring at the data link layer.

A multistation access unit (MSAU) is a hub or concentrator that connects a group of computers ("nodes" in a network terminology) to a token ring local area network. For example, eight computers might be connected to an MSAU in one office and that MSAU would be connected to an MSAU in another office that served eight other computers. In turn that MSAU could be connected to another MSAU in another office which would be connected back to the first MSU. Such a physical configuration is called a star topology. However, the logical configuration is a ring topology because every message passes through every computer one at a time, each passing it on to the next in a continuing circle.

Fig1: Multistation access unit (MSAU/MAU) used in forming ring network topology

If a larger LAN is required, you can hoin MAUs to form a larger ring by connecting the ring-out connector of one MAU to the ring-in connector of another, and continue connecting until the ring-out of the last MAU in series is connected to the ring-in of the first to complete the loop.

Fig 2: MAUs can be connected to each other in order to form a larger LAN.

Characteristics of ring LANs -

• The basic ring topology is shown in the figure 3, which shows that a number of repeaters are used and that the transmission is unidirectional.
• The data is transferred in a sequential manner bit by bit around the ring. Note that each repeater will regenerate and retransmit each bit.

Fig 3: Ring Topology

Advantage -

• Very orderly network where every devices has access to the token and the opportunity to transmit.
• Performs better than a bus topology under heavy network load.
• Does not require a central node to manage the connectivity between the computers.
• Due to the point to point line configuration of devices with a device on either side (each device is connect to its immediate neighbour). It is quite easy to install and reconfigure since adding or removing a device requires moving just two connections.
• Point to Point line configuration makes it easy to identify and isolate faults.

Disadvantages - 

• One malfunctioning workstation can create problems for the entire network. This can be solved by using a dual ring or a switch that closes off the break.
• Moving, adding and changing the devices can affect the network.
• Communication delay is directly proportional to number of nodes in the network.
• Bandwidth is shared on all links between devices.
• More difficult to configure than a star: node adjunction = Ring shutdown and reconfiguration.

Star Topology -

Star network are one of the most common computer network topologies. In its simplest form, a star network consist of one central switch, hub or computer, which act as a conduit to transmit messages. This consist of a central node, to which all other noses are connected: this central node provides a common connections point for all nodes through a hub.

Fig 4: Star topology

• In star topology, every node is connected to a central node called a hub or switch. The switch is the server and the peripherals are the clients. Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of a star.
• The central node can be active and passive.
• If the central node is Passive, the origination 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 network has an active central node that usually has the means to prevent echo-related problems.

Characteristics -

• The star topology reduces the damage caused by the line 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 this 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 on the isolation of that peripheral node from all others, bit the rest of the systems will be unaffected.

Important Points -

• Data on a star network passes through the hub, switch, or concentrator before continuing to its destination.
• The hub, switch or concentrator manages and controls all functions of the network.
• It also acts as repeater for the data flow.
• This configuration is common with twisted pair cable. However, it can also be used with coaxial acble or optical fibre cable.

Advantages - 

• Better performance: Star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices. Although this topology places a huge overhead on the central hub, with adequate capacity, the hub very high utilization by one device without affecting others.
• Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straight forward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.
• Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.
• Easy to detect faults and to remove parts.
• No disruptions to the network when connecting or removing devices.
• Installation and configuration is easy since every one devices only requires a link and one input/output port to connect it to any other device(s).

Disadvantages -

• Failure of the central hub renders the network inoperable.
• Communication between nodes pass through the central hub. Therefore if the central hub is compromised, do is all communication.

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