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Reliability and Availability Design Issues for Lans

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Reliability and Availability Design Issues for Lans 51-20-40 Reliability and Availability Design Issues for Previous screen LANs Nathan Muller Payoff Data communications managers with responsibility for LANs should know the distinction between network availability and reliability. This article briefly explains the LAN operating principles that most have an effect on availability and reliability and describes topologies and hardware that can be used to ensure that these two parameters are at their maximum in any particular network. Introduction In many businesses, LANs have displaced older, paper-based methods of doing work, and these organizations now depend on the reliability and availability of their computer networks for the continuity and efficiency of daily operations. Reliability refers to a LAN's capacity to operate despite failures in communications links and equipment. Availability is a measure of performance; it refers to the LAN's capacity to meet the communications needs of its users. High availability indicates that services are provided immediately; low availability indicates either that users have to wait for network access or that network response is poor. This article discusses some of the technologies that contribute to LAN reliability and availability; by applying these, LAN managers can ensure that their networks serve to enhance productivity in their organizations, not detract from it. Discussions in this article refer to LANs according to their topology. Simple versions of the basic LAN topologies are shown in Exhibit 1. Simple Versions of the Basic Network Topologies Reliability Star topologies score high in LAN reliability. The loss of a link prevents communication between the hub and the node attached to that link, but all other nodes operate as before. The hub is the weak link; the reliability of the network is dependent on the reliability of the central hub, and if the hub malfunctions, all nodes will go down. If a peripheral node fails, only that node is affected. A ring network topology, in its pure form, offers poor reliability in case of both node and link failures. Rather than each node being connected to a central hub, in ring networks link segments connect adjacent nodes to each other. Each node passes the data it receives to the adjacent node. The loss of one link not only cuts off one node but can bring down the entire network. Improving the reliability of the ring requires the addition of redundant links between nodes and bypass circuitry in the nodes. Reliability is improved but at great expense. Although link failures also cause problems on bus networks, node failure will not bring down an entire network. A redundant link for each segment increases the reliability of the network, but as with ring topologies, it raises the cost. Mesh topologies are used on the internetworks, the networks that connect geographically separated LANs. Mesh networks are the most reliable because there is always more than one route between nodes. But route diversity entails adding more Previous screen physical links and the equipment (e.g., routers) to support them, again raising the cost of the network. Availability Availability is a measure of performance dealing with the LAN's capacity to support all users who wish to access it. A network that is highly available provides services immediately to users, whereas a network that suffers from low availability typically forces users to wait for access or degrades overall network performance when too many users are allowed onto the network at the same time. Availability on a bus network depends on load, the access control protocol used, and the length of the bus. Under light load, availability is virtually assured for any user who wishes to access the network. As the load increases, however, so does the probability of information collisions. When a collision occurs, the transmitting nodes back off and repeat the attempt to transmit after a short interval. The chance of collisions also increases with bus length. A mesh topology's multiple paths make it the most reliable of networks. In addition, because it provides the highest degree of interconnectivity between users, this type of network is always available to users who require access. The capacity of a network based on a star topology is limited to the capacity of the central hub. Under heavy load conditions, users can be denied access. Hubs equipped with multiple processors can improve access in high-demand networks. Although the ring topology does not provide the same degree of availability as a mesh topology, it is an improvement over the startopology. Its ring availability is lower than a mesh network's because each node on the ring must wait to receive the token (i.e., permission to send) before transmitting data. As the number of nodes on the ring increases, the time interval allotted to each station for transmission must, by necessity, decrease. Access Methods All LANs employ shared media, but the way sharing is done is determined by the access method, which plays a key role in determining the reliability and availability of the network. Bus Ethernet, the major bus topology, is contention-based, which means that nodes compete with each other for access to the network. Each terminal listens to the network to determine whether it is idle. Upon sensing that no traffic is currently on the line, the terminal is free to transmit. The trouble with this access method is that several terminals may try to transmit simultaneously, causing data collision. The more terminals are connected to the network, the higher the probability that such collisions will occur. To avoid loss of data, the transceivers listen as they send, comparing what is sent with what is heard. If these are not the same, a collision has occurred. The transceivers notifies the attached node with a collision signal and sends error messages onto the Ethernet so that all nodes know of the collision. Packets travel at nearly the speed of light over coaxial cable, so collisions may appear to be unlikely. In fact, during the very brief interval that it takes for a packet to traverse the network, terminals at the far end cannot know that the network is in use. This collision window imposes a practical limit on the length of a bus network that does not use Previous screen repeaters. In its 802.3 standard for Ethernets, the Institute of Electrical and Electronic Engineers (IEEE) recognizes 2,500 meters as the maximum bus length, regardless of data rate or cable type. Ethernet is relatively simple to implement, so it typically costs less than other types of network. Because each node functions independently, the failure of one does not disrupt the operation of the others, and nodes may be added or removed without disrupting the network. Ethernet is media independent, functioning well overtwisted-pair wire, coaxial cable, and optical fiber. The choice of media depends on desired data rate, range, and immunity to interference from external sources of electrical noise. Ring Token-Ring networks use a more efficient topology. The token-passing access method ensures all terminals an equal share of network time. Logically the network forms a ring; however, a particular wiring scheme may be physically configured as a star. The token consists of several bytes of data (also referred to as a packet)that circulate around the ring, giving each terminal in sequence a chance to put information on the network. To transmit, the station holds the token, replacing it with a packet, and then reinserts the token on the ring. Only the addressee retains the message, and only the station that put the message on the ring can remove it. Because each terminal regenerates data packets and the token, Token-Ring networks do not have the size and distance limitations of Ethernets. A further advantage of Token-Ring networks is that traffic can be assigned different priorities. Only if a station has traffic equal to or higher in priority than the priority indicator embedded in the token is it allowed to transmit a packet. The Token Ring is not without liabilities. Special procedures must be followed to add terminals to the network without breaking the ring and to ensure that the new station is recognized by the others and is granted a proportionate share of network time. Because failed repeater circuits in a node can break the ring, bringing down the whole network, each station must be equipped with bypass circuitry. A logical ring can be wired as a star, and bypass circuitry, if used with this configuration, will make it easier to correct failures of network equipment than if the bypass circuits are in the nodes. Each node is wired to a centrally located panel, which contains the bypass circuits. If a nodal or link failure occurs, the bypass circuit is activated, preventing the ring from being broken. Centralizing the bypass circuits in this way also facilitates moves and changes as well as fault isolation, because it is much easier to identify connections at a central point than to perform traces between offices and floors. Token-Ring networks are vulnerable to anomalies that can tie up the network until the network manager figures out the problem. If a terminal fails before it has a chance to pass the token, the whole network goes down until a new token can be inserted. The token may even be corrupted by noise, to the point of becoming unrecognizable to the stations. The network can also be disrupted by the occasional appearance of two tokens, or by the presence of continuously circulating data packets. The latter can occur when data is sent and the originating terminal fails before it can remove the packet from the ring. To ensure network availability, one terminal is typically designated as the control station to continually monitor network operations and do such necessary housecleaning chores as reinserting lost tokens, taking extra tokens off the network, or disposing of “lost” packets.
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