3.2 Network Reliability

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A network or resource is reliable when it continues to operate despite the failure of a critical element. The critical elements are different for each network topology: star, ring, and bus. Thus, each topology can be evaluated in terms of its reliability, as well as its suitability for specific applications.

3.2.1 Star Topology

When it comes to link availability, the star topology is highly reliable. In the star topology, all network devices (i.e., nodes) or LAN segments connect to a central hub. Although the loss of a link prevents communication between the hub and the affected node, all other nodes will continue to operate as before unless the hub itself suffers a catastrophic failure.

To ensure a high degree of reliability, the hub has redundant subsystems at critical-points: the control logic, backplane, and power supply. The hub’s management system can enhance the fault tolerance of these redundant subsystems by continuously monitoring their operation and reporting any anomalies. With the power supply, for example, monitoring may include hotspot detection and fan operation to detect trouble before it disrupts hub operation. Upon the failure of the main power supply, the redundant unit switches over automatically or manually under the network manager’s control without disrupting the network.

The flexibility of the hub architecture lends itself to varying degrees of fault tolerance, depending on the criticality of the applications. For example, workstations running non-critical applications may share a link to the same LAN module at the hub. Although this configuration might seem economical, it is disadvantageous in that a failure in the LAN module will put all of the workstations on that link out of commission. A slightly higher degree of fault tolerance may be achieved by distributing the workstations among two LAN modules and links. That way, the failure of one module would affect only half the number of workstations. A one-to-one correspondence of workstations to modules offers an even greater level of fault tolerance, because the failure of one module impacts only the workstation connected to it. However, this configuration is also a more expensive solution than the others.

A critical application may demand the highest level of fault tolerance. This can be achieved by connecting the workstation to two LAN modules at the hub with separate links. The ultimate in fault tolerance would be achieved by connecting one of those links to a different hub. In this arrangement, a transceiver is used to split the links from the application’s host computer, enabling each link to connect with a different module in the hub or to a different hub. All of these levels of fault tolerance are summarized in Figure 3.1.

Figure 3.1: A fault tolerant hub-based network.

3.2.2 Ring Topology

In its pure form, the ring topology offers poor reliability to both node and link failures. The ring uses link segments to connect adjacent nodes together. Each node is actively involved in the transmissions of other nodes through token passing. The token is received by each node, at which time it can transmit data before passing the token to the adjacent node. The loss of a link not only results in the loss of a node but brings down the entire network as well. Enhancing the reliability of the ring topology requires adding redundant links between nodes as well as bypass circuitry. Adding such components, however, makes implementing the ring topology more expensive.

3.2.3 Bus Topology

The bus topology also provides poor reliability. If the link fails, that entire segment of the network is rendered useless. A redundant link for each segment will increase the reliability of the bus topology, but at extra cost. Unlike the ring topology, where each node is dependent on the others adjacent to it, the nodes in a bus topology are independent and contend for access to the LAN. If a node fails, the rest of the network continues to operate.

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