Without electricity, you have no network at all, and computers require a well-conditioned electrical source to function properly. The power supply in a computer can't handle an incoming spike of electricity caused by a lightning strike, for example. Similarly, a brownout, in which the voltage level drops for a short period, can cause a computer to crash. Large-scale computer systems used in corporate environments, such as minicomputers or mainframes, also need a good source of power. To ensure this, most large computer rooms use a heavy-duty UPS to interface between the outside source of electricity and the computers and other devices in a computer room. In most large computer rooms, for example, you'll find that computers ”whether they're PC servers or larger systems ”are rack-mounted in cabinets , along with tape drives, disk drives , and other peripheral equipment. The cabinet usually contains one or more power distribution units that are used to supply power to components mounted in the cabinet (see Figure 5.1). Figure 5.1. Several points in the power supply can be constructed to prevent a single point of failure for powering the network.
As you can see from this figure, several computer systems and the tape drives they use are housed in a single cabinet. Two power distribution units located at the bottom of the cabinet supply power to all devices in the cabinet. These two power distribution units are configured in a dual-redundant manner so that if one fails the other continues to supply power to the cabinet. Each of these power distribution units is connected to a separate UPS in the computer room. This is important for several reasons. First, not all power failures are due to outside problems, such as a downed power line. Sometimes, UPSs themselves fail. An electrician might disconnect the wrong cable during routine maintenance or installation tasks . A mouse could chew into the wires, causing a short. Sometimes, things just happen. You need to prepare for the unexpected. To carry the concept further, each UPS in the computer room that the system uses is connected to a separate outside source of power. Thus, if a tree falls and knocks down a power line, an alternative power line is still feeding electricity into the computer room to redundant UPS systems. Because of this second source of power, computers and other devices on the network stay up and running. Power Is MoneyThere is an old saying that "money is power." The opposite also is true. The setup described in the preceding section might seem extreme to a network administrator running a small network of PCs in which some downtime can be tolerated. However, in a high-availability computer environment ”such as in a large corporation ”the cost of downtime can be prohibitively expensive for several reasons:
If you operate in a large-scale environment like this, you are probably already aware of how important it is to keep computer systems up and running. If you don't provide a steady, secure source of power up front, all your other preventive maintenance measures might prove of little value the next time the power goes out. A UPS is not an eternal source of power. It is a conduit through which your external power source is routed before it gets to your computer systems. UPSs operate by storing electricity in one or more batteries so that when the outside source of electricity is unexpectedly lost, the batteries can be switched into use in a few milliseconds . However, batteries can be used only for a limited amount of time. If you are using only a single UPS connected to a single power source, the UPS buys you the time needed to notify users to log off the systems affected and gracefully shut down the computers so that no data is compromised. Although you'll still have idle employees, you won't have to recover data after power has been restored. For a large network consider planning for power outages one step further: Diesel generators. Although these would be too expensive to keep a large number of desktop users working, they can be used to keep servers up so that you can relocate users to a disaster recovery site that has been set up in advance to give users a temporary workplace. For example, in most situations not all network applications are critical and require 24/7 uptime. Thus, you should provide backup power systems for servers that are critical. If your operation is a retail one, for example, you would want to keep servers running that interface with your customers ”such as your Web site. Other applications, such as word processing for the legal department, and other job functions that can wait can be recovered later. Advanced Configuration and Power Interface (ACPI) and Standalone UPS SystemsFor PCs and small servers, you can buy inexpensive UPS systems for a few hundred dollars that can be used in an environment in which downtime can be tolerated but data corruption cannot. A typical UPS, such as one from American Power Conversion Corp. (APC), can be installed in just a few minutes. Depending on the model, it can provide both a battery backup and some power conditioning. Be aware that power strips , even those that claim to be able to prevent power spikes from getting through, don't always work as claimed by the manufacturer. The inexpensive models you buy at local discount stores are notorious for not providing the protection they claim to provide. If you depend on the simple mechanics of a fuse or breaker in a cheap power strip to protect your computer, also plan on buying a new computer the next time lightning strikes. For true protection, you really need to spend the extra hundred dollars to get a small UPS for your server or other network devices if you don't use a large-scale UPS.
To allow you to gracefully shut down the operating system when the power goes out and the batteries take over, an industry initiative (involving major players such as Intel, Microsoft, and others) developed the Advanced Configuration and Power Interface (ACPI). ACPI covers a lot of territory, including power management for laptops and other computers. However, ACPI also allows a standard way for a standalone UPS to communicate with a computer and instruct it to shut down when the UPS battery supply takes over from the outside source of electricity.
This communication is accomplished by connecting the power cable from the computer to the power UPS, and also attaching a small cable (usually a serial cable) to the UPS and the computer system, and then enabling the UPS service in the operating system. Windows 2000 and Windows 2003 servers have a UPS service you can run in conjunction with an attached UPS that supports ACPI. You'll even find this service in Windows 2000 Professional as well as Windows XP, and some Linux/Unix systems. The UPS communicates with the service and instructs the system to perform an orderly shutdown when the loss of power is detected and the batteries take over. Things to look for in a small UPS include the following:
Although a large number of vendors manufacture and sell small UPS systems of this type, the Web site for American Power Conversion Corp. (www.apcc.com) has information (including documentation) for products that scale from the desktop to full-fledged computer-room UPS systems. As with power strips, when making a purchasing decision about a UPS system, you generally get what you pay for. Balance the cost of the unit with what it would cost you to replace the devices you are going to use it to protect, as well as the cost of downtime, data corruption, and so on. Network DevicesUPS systems aren't just for computers. After all, this book is about networking. Don't forget the routers, switches, and other devices in your network. Although it might be acceptable to let a printer be offline for a while during a power problem, it won't matter whether your computers are up and running if users can't access them through the network. In a large computer room, routers and other such devices should be connected to plugs that terminate in the UPS. In a small office or home-office environment, don't forget to connect your broadband switch/router to the UPS just as you do your computer. |