Network Interface Cards
The physical connection of workstations to the LAN is achieved through a network interface card (NIC), which fits into one of the expansion slots in the microcomputer. This device provides the electronic circuitry needed for internode communications. The NIC works with the network control program to send and receive messages, programs, and files across the network.
Servers
LAN nodes often share common resources such as programs, data, and printers, which are managed through special – purpose computers called severs, as depicted in Figure 3.17. When the server receives requests for resources, the requests are placed in queue and are processed in sequence.
In a distributed environment, there is often a need t o link networks together. For example, users of one LAN may share data with users on a different LAN. Networks
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Are linked via combinations of hardware and software devices called bridges and gateways. Figure 3.18 illustrates this technique. Bridges provide a means for linking LANs of the anme type, for example, an IBM token ring to another IBM token ring, Gateways connect LANs of different types and are also used to link LANs to WANs. With these definitions in mind, we now turn our attention to the five basic network topologies.
Star Topology
The star toptlogy shown in Figure 3.19 describes a network of computers with a large central computer (the host) at the hub that has direct connections to a periphery of smaller computers. Communications between the nodes in the star are managed and controlled from the host site.
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The star topology is often used for a WAN, in which the central computer is a mainframe. The nodes of the star may be microcomputer workstations, minicomputers, mainframes, or a combination. Databases under this approach may be distributed or centralized. A common model is to partition local data to the nodes and centralize the common data. For example, consider a department store chain that issues its own credit cards. Each node represents a store in a different metropolitan area. In Figure 3.19, These are Dallas, St. Louis, Topeka, and Tulsa. The nodes maintain local databases such as records for customers holding credit cards issued in their areas and records of local inventory levels. The central site-Kansas City-maintains data common to the entire regional area, including data for customer billing, accounts receivable maintenance, and overall inventory control. Each local node is itself a LAN, with point-of-sales (POS) terminals connected to a minicomputer at the store.
If one or more nodes in a star network fail, communication between the remaining nodes is still possible through the central site. However, if the central site fails, individual nodes can function locally, but cannot communicate with the other nodes.
Transaction processing in this type of configuration could proceed as follows. Sales are processed in real time at the POS terminals. Local processing includes obtaining credit approval, updating the customer’s available credit, updating the inventory records, and recording the transaction in the transaction file (journal). At the end of the business day, the nodes transmit sales and inventory information to the central site in batches. The central site updates the control accounts, prepares customer bills, and determines inventory replenishment for the entire region.
The assumption underlying the star topology is that primary communication will be between the central site and the nodes. However, limited communication between the nodes is possible. For example, assume a customer from Dallas was in Tulsa and made a purchase from the Tulsa store on credit. The Tulsa database would not contain the customer’s record, so Tulsa would send the transaction for credit approval to Dallas via Kansas City. Dallas would the return the approved transaction to Tulsa via Kansas City. Inventory and sales journal updates would be performed at Tulsa.
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This transaction processing procedure would differ somewhat depending on the database configuration. For example, if local databases are partial replicas of the central database, credit queries could be made directly from Kansas City. However, this would require keeping the central database current with all the nodes.
Hierarchical Topology
A hierarchical topology is one in which a host computer is connected to several levels of subordinate, smaller com