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.Thiscan use to communicate with eachvalue represents the address of the nodeother.The most commonly usedsending the packet.Depending on the typeprotocols are LU 6.2 and HLLAPI.of address, this field may be 4 or 6 bytes oreven longer.Common Applications: This is concernedwith developing common frameworksCOMPAREfor the same kinds of applications run-DA (Destination Address)ning in different environments.Thiscomponent is largely product-oriented,SAA (Systems Applicationand is more for marketing and appear-Architecture)ance than a substantive part of SAA.Infact, some armchair architects do notSAA is an effort on the part of IBM to stan-regard it as part of SAA.dardize the conventions, interfaces, andprotocols used by applications in all IBMThe figure SAA components showsoperating environments.The intent was tohow these components fit together withprovide a unified, logical architecture foran operating system in a particularapplications running on machines rangingenvironment.from a PS/2 up to a System/370.SAA has four main components:SAA COMPONENTSCUA (Common User Access): This com-ponent defines standard interfaces forapplications that are window- orcharacter-based.The user will interactwith this interface.CUA includes spec-ifications for screen and keyboard lay-out, and for selection methods usingeither a keyboard or mouse.CPI (Common Program Interface): Thiselement defines APIs (ApplicationProgram Interfaces) that are consis-tent across all systems.These areused by developers in their applica-tions.The CPI standards relatingto languages and databases followANSI specifications. Safety Device 895Electrical ThreatsaSAC (Simplified Access Control)As in other areas of life, electrical dangersIn the CCITT X.500 Directory Servicesbcan come from having too much or toomodel, the more restricted of two sets oflittle.Collectively, such disturbances arecaccess control guidelines.The other set isknown as overvoltages and undervoltages,BAC (Basic Access Control).drespectively.SEE ALSOOvervoltages include spikes and surges.eX.500Undervoltages include blackouts andbrownouts (or sags).According to studies by fIBM and by AT&T Bell Labs, undervoltagesSAC (Single-Attachmentgaccount for over 90 percent of electrical dis-Concentrator)turbances, with brownouts accounting forhIn FDDI, a concentrator that serves as a ter-about 87 percent and blackouts for about 5mination point for single-attachment sta-ipercent of the total.Overvoltages accounttions (SASs) and that attaches to the FDDIfor the remaining 8 percent, with spikesjring through a dual-attachment connectoraccounting for 7 percent and surges for only(DAC).about 1 percent of all electrical disturbances.kSee the Power Disturbances article for moreSEE ALSOlinformation about these types of electricalFDDI (Fiber Distributed Data Interface)disturbances.mIn most cases, such disturbances occurSafety Devicensporadically.In some areas, voltage varia-Safety devices are designed to keep a file tions may be unnervingly frequent.Thisoserver or other piece of hardware running can be the case if the power supply passessmoothly, regardless of power fluctuations through old lines or if there are malfunction- por loss.The protection devices buffer, or ing components along the line.In someqshield, the hardware from the harsh world cases, your power company may be able toof the electrical power line.clean up the power supply.rSafety devices operate between the power In addition to these variations in theline and the hardware s circuitry.For a net- power supply, several types of noise, or ran- swork, the biggest natural threat to the hard- dom elements in the power supply, also existtware comes from the electrical lines.Drastic in an electrical system:deviations or fluctuations in the electricaluCommon mode noise is noise arisingpower supply can cause various types ofbecause of voltage differences betweenvdamage, from minimal data loss to friedthe neutral and ground wires in a sys-hardware.wtem.This type of noise is relatively rarefor computer systems, and is almostxcompletely eliminated by noise filters.yz 896 Safety DeviceNormal mode noise is noise arising is no voltage at all during blackouts.because of voltage differences between Most UPSs also provide at least somethe hot and neutral wires in a system.protection against surges, spikes, andThis type of noise has various sources sags.During an outage, the connected(other electrical activity on the line, device runs off the UPS s battery.motors being turned on or off, and soSPS (standby power supply) deviceson).Some of this noise is handled byare similar to UPSs, except that thenoise filters, some is eliminated by var-power does not go through the SPSious tricks and offsets (such as twistingbattery during normal operations
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