Physically protecting a computer presents many of the same problems that arise when protecting typewriters, jewelry, and file cabinets. As with a typewriter, an office computer is something that many people inside the office need to access on an ongoing basis. As with jewelry, computers are valuable and generally easy for a thief to sell. But the real danger in having a computer stolen isn't the loss of the system's hardware but the loss of the data that was stored on the computer's disks. As with legal files and financial records, if you don't have a backup?or if the backup is stolen or destroyed along with the computer?the data you lost may well be irreplaceable. Even if you do have a backup, you will still need to spend valuable time setting up a replacement system. Finally, there is always the chance that the stolen information itself, or even the mere fact that information was stolen, will be used against you.
Your computers are among the most expensive possessions in your home or office; they are also the pieces of equipment that you can least afford to lose. We know of some computer professionals who say, "I don't care if the thief steals my computer; I only wish that he would first take out the hard drive!" Unfortunately, you can rarely reason in this manner with would-be thieves.
To make matters worse, computers and computer media are by far the most temperamental objects in today's homes and offices. Few people worry that their television sets will be damaged if they're turned on during a lightning storm, but a computer's power supply can be blown out simply by leaving the machine plugged into the wall if lightning strikes nearby. Even if the power surge doesn't destroy the information on your hard disk, it still may make the information inaccessible until the computer system is repaired.
Power surges don't come only during storms: one of the authors once had a workstation ruined because a vacuum cleaner was plugged into the same outlet as the running workstation. When the vacuum was switched on, the power surge fatally damaged the workstation's power supply. Because the computer was an aging Digital Pro 350 workstation with a proprietary disk interface and filesystem, it proved to be cheaper to throw out the machine and lose the data than to attempt to salvage the hardware and information stored on the machine's disk. This proved to be an expensive form of spring cleaning!
There are several measures that you can take to protect your computer system against physical threats. Many of them will simultaneously protect the system from dangers posed by nature, outsiders, and inside saboteurs.
Computers often require exactly the right balance of physical and environmental conditions to operate properly. Altering this balance can cause your computer to fail in unexpected and often undesirable ways. Even worse, your computer might continue to operate erratically, producing incorrect results and corrupting valuable data.
In this respect, computers are a lot like people: they don't work well if they're too hot, too cold, or submerged in water without special protection.
Computers are notoriously bad at surviving fires. If the flames don't cause your system's case and circuit boards to ignite, the heat might melt your hard drive and all the solder holding the electronic components in place. Your computer might even survive the fire, only to be destroyed by the water used to fight the flames.
You can increase the chances that your computer will survive a fire by making sure that there is good fire-extinguishing equipment nearby.
Gas-charged fire extinguishers are popular for large corporate computer rooms. These work by physically blocking oxygen from coming into contact with the burning materials. Unfortunately, gases may also asphyxiate humans in the area. For this reason, all automatic gas discharge systems have loud alarms that sound before the gas is discharged. Commonly used gases include nitrogen, argon, and, less frequently, carbon dioxide.
 Older systems used a gas called Halon. Halon is currently banned from general use because of its effects on ozone in the environment. One of the replacements for Halon is marketed under the name HF200.
Here are some guidelines for fire control:
Make sure that you have a hand-held fire extinguisher near the doorway of your computer room. Train your personnel (and yourself) in the proper use of the fire extinguisher. This training should ideally include the actual use of a fire extinguisher?surprisingly, few people have ever discharged a fire extinguisher! One good way to do this is to have your employees practice outdoors with extinguishers that need to be recharged (usually once every year or two). Repeat the training at least once a year.
Check the recharge state of each fire extinguisher every month. Extinguishers with gauges will show if they need recharging. All extinguishers should be recharged and examined by a professional on a periodic basis (sometimes those gauges stick in the "full" position!).
If you have a gas-discharge system, make sure everyone who enters the computer room knows what to do when the alarm sounds. Post warning signs in appropriate places.
If you have an automatic fire-alarm system, make sure you can override it in the event of a false alarm.
Ensure that there is telephone access for your operators and users who may discover a fire. If you have a PBX, make sure that there is at least one backup telephone that goes directly to the phone company.
Many modern computers will not be damaged by automatic sprinkler systems, provided that the computer's power is turned off before the water starts to flow (although disks, tapes, and printouts left out in the open may suffer). Consequently, you should have your computer's power automatically cut if the water sprinkler triggers. If you have an uninteruptable power supply, be sure that it automatically disconnects as well.
Getting sensitive electronics wet is never a good idea. But if your computer has been soaked after the power was cut, you can possibly recover the system by completely drying the system and then carefully reapplying the power. If your water has a very high mineral content, you may find it necessary to have the computer's circuit boards professionally cleaned before attempting to power up. In some cases, you may find it easier to simply remove your computer's disk drives and put them into a new computer. You should immediately copy the data onto new disks, rather than attempting to run with the salvaged equipment.
Because many computers can now survive exposure to water, many fire-protection experts now suggest that a water sprinkler system may be as good as (or better than) a gas discharge system. In particular, a water system will continue to run long after a gas system is exhausted, so it's more likely to work against major fires. Such a system is also less expensive to maintain, and less hazardous to humans.
If you choose to have a water-based sprinkler system installed, be sure it is a "dry-pipe" system. These systems keep water out of the pipes until an alarm is actually triggered, rather than keeping the sprinkler heads pressurized all the time. Because they are not continuously pressurized, dry-pipe systems tend to be resistant to leaks.
 We know of one instance when a maintenance man accidentally knocked the sprinkler head off with a stepladder. So much water came out that the panels for the raised floor were floating before the water was shut off. The mess took more than a week to clean up.
Be sure that your wiring is protected, in addition to your computers. Be certain that smoke detectors and sprinkler heads are appropriately positioned to cover wires in wiring trays (often above your suspended ceilings) and in wiring closets.
Smoke is very damaging to computer equipment. Smoke is a potent abrasive and collects on the heads of unsealed magnetic disks, optical disks, and tape drives. A single smoke particle can cause a severe disk crash on some kinds of older disk drives that lack a sealed drive compartment.
Sometimes smoke is generated by computers themselves. Electrical fires?particularly those caused by the transformers in video monitors?can produce a pungent, acrid smoke that may damage other equipment and may also be poisonous or a carcinogen. Several years ago, an entire laboratory at Stanford had to be evacuated because of the toxic smoke caused by a fire in a single video monitor.
Another significant danger is the smoke that comes from cigarettes and pipes. Such smoke is a hazard to people and computers alike. Besides the known cancer risk, tobacco smoke can cause premature failure of keyboards and require that they be cleaned more often. Nonsmokers in a smoky environment will not perform as well as they might otherwise, both in the short and long term. In many locales, smoking in public or semi-public places is now illegal.
Here are some guidelines for smoke control:
Do not permit smoking in your computer room or around the people who use the computers.
Install smoke detectors in every room with computer or terminal equipment.
If you have a raised floor, mount smoke detectors underneath the floor as well.
If you have suspended ceilings, mount smoke detectors above the ceiling tiles.
Get a Carbon Monoxide Detector!
Carbon monoxide (CO) won't harm your computer, but it might silently kill any humans in the vicinity. One of the authors of this book became quite sick in February 1994 when his home chimney was inadvertently plugged and the furnace exhaust started venting into his house. Low-cost carbon monoxide detectors are readily available. You should install them wherever coal, oil, or gas-fired appliances are used.
If you think this warning doesn't apply to your computer environment, think again. Closed office buildings can build up high concentrations of CO from faulty heater venting, problems with generator exhaust (as from a UPS), or even trucks idling outside with their exhaust near the building's air intake.
Dust destroys data. As with smoke, dust can collect on the heads of magnetic disks, tape drives, and optical drives. Dust is abrasive and will slowly destroy both recording heads and media.
Many kinds of dust are somewhat conductive. The designs of many computers leads them to suck large amounts of air through the computer's insides for cooling. Invariably, a layer of dust will accumulate on a computer's circuit boards, covering every surface, exposed and otherwise. Eventually, the dust may cause circuits to short, fail, or at least behave erratically.
Here are some guidelines for dust control:
Keep your computer room as dust-free as possible.
If your computer has air filters, clean or replace them on a regular basis.
Get a special vacuum for your computers and use it on a regular basis. Be sure to vacuum behind your computers. You may also wish to vacuum your keyboards. Ideally, your vacuum cleaner should have a microfilter (HEPA or ULPA) so that dust removed from the computers is not simply blown back into your computer room.
In environments with dust that you can't control, consider getting keyboard dust covers to use when the keyboards are idle for long periods of time. However, don't simply throw homemade covers over your computers?doing so can cause computers to overheat, and some covers can build up significant static charges.
While some parts of the world are subject to frequent and severe earthquakes, nearly every part of the planet experiences the occasional temblor. In the United States, for example, the San Francisco Bay Area experiences several noticeable earthquakes every year; a major earthquake that may be equal in force to the great San Francisco earthquake of 1906 is expected within the next 20 years. Scientists also say there is an 80% chance that the Eastern half of the United States will experience a similar earthquake within the next 30 years. The only unknown factor is where it will occur. One of the most powerful U.S. earthquakes in the last 200 years didn't occur in California, but along the New Madrid fault?the quake actually changed the course of the Mississippi River! Recent earthquakes have also been felt in New York City and Chicago. As a result, several Eastern cities have enacted stringent anti-earthquake building codes modeled on California's. These days, many new buildings in Boston are built with diagonal cross-braces, using the type of construction that one might expect to see in San Francisco.
While some buildings collapse in an earthquake, most remain standing. Careful attention to the placement of shelves and bookcases in your office can increase the chances that you and your computers will survive all but the worst disasters.
Here are some guidelines for earthquake remediation:
Avoid placing computers on any high surfaces; for example, on top of file cabinets.
Do not place heavy objects on bookcases or shelves near computers in such a way that they might fall on the computer during an earthquake.
To protect your computers from falling debris, place them underneath strong tables when an earthquake is possible.
Do not place computers on desks next to windows?especially on higher floors. In an earthquake, the computer could be thrown through the window, destroying the computer and creating a hazard for people on the ground below.
Consider physically attaching the computer to the surface on which it is resting. You can use bolts, tie-downs, straps, or other implements. (This practice also helps deter theft.)
Although computers are not prone to explosions, buildings can be?especially if the building is equipped with natural gas or is used to store flammable solvents.
If you need to operate a computer in an area where there is a risk of explosion, you might consider purchasing a system with a ruggedized case. Disk drives can be shock-mounted within a computer; if explosions are a constant hazard, consider using a ruggedized laptop with an easily removed, shock-resistant hard drive.
Here are some guidelines for explosion control:
Consider the real possibility of an explosion on your premises. Make sure that solvents, if present, are stored in appropriate containers in clean, uncluttered areas.
Keep your backups in blast-proof vaults or off-site.
Keep computers away from windows.
Computers, like people, operate best within certain temperature ranges. Most computer systems should be kept between 50 and 90 degrees Fahrenheit (10 to 32 degrees Celsius). If the ambient temperature around your computer gets too high, the computer cannot adequately cool itself, and internal components can be damaged. If the temperature gets too cold, the system can undergo thermal shock when it is turned on, causing circuit boards or integrated circuits to crack.
Here are some basic guidelines for temperature control:
Check your computer's documentation to see what temperature ranges it can tolerate.
Install a temperature alarm in your computer room that is triggered by a temperature that is too low or too high. Set the alarm to go off when the temperature gets within 15 to 20° Fahrenheit of the limits your system can take. Some alarms can even be connected to a phone line and programmed to dial predefined phone numbers and tell you, with a synthesized voice, "Your computer room is too hot."
Pay attention to the way your systems discharge heat and the air flow pattern within the machines and within the room. Evaluate the need for additional cooling equipment, and if you choose to use it, install it properly. One of this book's reviewers told us about seeing room fans installed to direct air at the rear of some machines. Unfortunately, these were machines that were designed to vent hot air out the back, so the new fans were pushing the warm air back toward the machines, instead of venting it out. When the fans were repositioned, the room temperature dropped by 10 degrees.
Be careful about placing computers too close to walls, which can interfere with air circulation. Most manufacturers recommend that their systems have 6 to 12 inches of open space on every side. If you cannot afford the necessary space, lower the computer's upper-level temperature by 10° Fahrenheit or more.
If you are transporting a computer (such as a laptop) outside in very cold or hot weather, give it a chance to reach room temperature before starting it.
Sometimes insects and other kinds of bugs find their way into computers. Indeed, the very term bug, used to describe something wrong with a computer program, dates back to the 1950s, when Grace Murray Hopper found a moth trapped between a pair of relay contacts on Harvard University's Mark 1 computer.
Insects have a strange predilection for getting trapped between the high-voltage contacts of switching power supplies. Others have insatiable cravings for the insulation that covers wires carrying line current, and the high-pitched whine that switching power supplies emit. Spider webs inside computers collect dust like a magnet. For all these reasons, you should take active measures to limit the amount of insect life in your machine room.
Motors, fans, heavy equipment, and even other computers generate electrical noise that can cause intermittent problems with the computer you are using. This noise can be transmitted through space or nearby power lines.
Electrical surges are a special kind of electrical noise that consists of one (or a few) high-voltage spikes. As we've mentioned, an ordinary vacuum cleaner plugged into the same electrical outlet as a workstation can generate a spike capable of destroying the workstation's power supply.
Here are some guidelines for electrical noise control:
Make sure that there is no heavy equipment on the electrical circuit that powers your computer system.
If possible, have a special electrical circuit with an isolated ground installed for each computer system.
Install a line filter on your computer's power supply. Some UPS systems are built to act as power filters. UPSs are affordable for even home systems, and some include integrated signalling that can (with appropriate software) shut down your computer gracefully after a prolonged power outage.
If you have problems with static, you may wish to install a static (grounding) mat around the computer's area, or apply antistatic sprays to your carpet.
Walkie-talkies, cellular telephones, and other kinds of radio transmitters can cause computers to malfunction when they are transmitting. Powerful transmitters can even cause permanent damage to systems. Transmitters have also been known to trigger explosive charges in some sealed fire-extinguisher systems (e.g., Halon). All radio transmitters should be kept at least five feet from the computer, cables, and peripherals. If many people in your organization use portable transmitters, consider posting signs instructing them not to transmit in the computer's vicinity.
Lightning generates large power surges that can damage even computers with otherwise protected electrical supplies. If lightning strikes your building's metal frame (or hits your building's lightning rod), the resulting current can generate an intense magnetic field on its way to the ground.
Here are some guidelines for lightning control:
If possible, turn off and unplug computer systems during lightning storms.
Make sure that your backup tapes, if they are kept on magnetic media, are stored as far as possible from the building's structural steel members.
Surge suppressor outlet strips will not protect your system from a direct strike, but may help if the storm is distant. Some surge suppressors include additional protection for sensitive telephone equipment; however, this extra protection may be of questionable value in most areas because by law, telephone circuits must be equipped with lightning arresters.
In some remote areas, modems can still be damaged by lightning, even though they are on lines equipped with lightning arresters. In these areas, modems may benefit from additional lightning protection.
Do not run copper network cables (e.g., Ethernet or Category 5 cables) outdoors unless the cables are in a metal conduit. Specifically, do not run a network cable out an office window, across the wall or roof of a building, and into another office. If you run a cable outdoors and lightning hits within a few thousand feet of your location, there is an excellent chance that the lightning will induce a surge in the network cable, and this surge will then be transmitted directly into your computer system?or worse, channel a direct lightning strike to the system and users.
Vibration can put an early end to your computer system by literally shaking it apart. Even gentle vibration, over time, can work printed circuit boards out of their connectors and integrated circuits out of their sockets. Vibration can cause hard disk drives to come out of alignment and thus increase the chance for catastrophic failure and resulting data loss. Here are some guidelines for vibration control:
Isolate your computer from vibration as much as possible.
If you are in a high-vibration environment, place your computer on a rubber or foam mat to dampen vibration, but make sure the mat does not block ventilation openings.
Laptop computers are frequently equipped with hard disks that are better at resisting vibration than are desktop machines.
Don't put your printer on top of a computer. Printers are mechanical devices; they generate vibration. Desktop space may be a problem, but the unexpected failure of your computer's disk drive or system board is a bigger problem.
Humidity is your computer's friend?but as with all friends, you can get too much of a good thing. Humidity prevents the buildup of static charge. If your computer room is too dry, static discharge between operators and your computer (or between the computer's moving parts) may destroy information or damage your computer itself. If the computer room is too humid, you may experience condensation on chilled surfaces. Collected condensate can short out and damage the electrical circuits.
Here are some guidelines for humidity control:
For optimal performance, keep the relative humidity of your computer room above 20%, but keep it well below the dew point (which depends on the ambient room temperature).
In environments that require high reliability, you may wish to have a humidity alarm that will ring when the humidity is out of your acceptable range.
Some equipment has special humidity restrictions. Check your manuals.
Water can destroy your computer. The primary danger is an electrical short, which can happen if water bridges between a circuit-board trace carrying voltage and a trace carrying ground. A short will cause too much current to be pulled through a trace, heat up the trace, and possibly melt it. Shorts can also destroy electronic components by pulling too much current through them.
Water usually comes from rain or flooding. Sometimes it comes from an errant sprinkler system. Water may also come from strange places, such as a toilet overflowing on a higher floor, vandalism, or the fire department.
Here are some guidelines for water control:
Mount a water sensor on the floor near the computer system.
If you have a raised floor in your computer room, mount water detectors underneath the floor and above it.
Do not keep your computer in the basement of your building if your area is prone to flooding, or if your building has a sprinkler system.
Because water rises, you may wish to have two alarms located at different heights. The first water sensor should ring an alarm; the second should automatically cut off power to your computer equipment. Automatic power cutoffs can save a lot of money if the flood happens during off hours, or if the flood occurs when the person who is supposed to attend to the alarm is otherwise occupied. More importantly, cutoffs can save lives. Electricity, water, and people shouldn't mix.
To detect spurious problems, continuously monitor and record your computer room's temperature and relative humidity. As a general rule of thumb, every 1,000 square feet of office space should have its own recording equipment. Log and check recordings on a regular basis.
In addition to environmental problems, your computer system is vulnerable to a multitude of accidents. While it is impossible to prevent all accidents, careful planning can minimize the impact of accidents that will inevitably occur.
People need food and drink to stay alive. Computers, on the other hand, need to stay away from food and drink. One of the fastest ways of putting a desktop keyboard out of commission is to pour a soft drink or cup of coffee between the keys. If this keyboard is your system console (as is the case with most PCs), you may be unable to reboot the computer until the console is replaced (we know this from experience).
Food?especially oily food?collects on people's fingers and from there gets on anything that a person touches. Often this includes dirt-sensitive surfaces such as magnetic tapes and optical disks. Sometimes food can be cleaned away; other times it cannot. Oils from foods also tend to get onto screens, increasing glare and decreasing readability. Some screens are equipped with special quarter-wavelength antiglare coatings: when touched with oily hands, the fingerprints will glow with an annoying iridescence. Generally, the simplest rule is the safest: keep all food and drink away from your computer systems.
 Perhaps more than any other rule in this chapter, this rule is ignored.
Simple common sense will tell you to keep your computer in a locked room. But how safe is that room? Sometimes a room that appears to be safe is actually wide open.
In many modern office buildings, internal walls do not extend above dropped ceilings or beneath raised floors. This type of construction makes it easy for people in adjoining rooms, and sometimes adjoining offices, to gain access.
Here are some guidelines for dealing with raised floors and dropped ceilings:
Make sure that your building's internal walls extend above your dropped ceilings so intruders cannot enter locked offices simply by climbing over the walls.
Likewise, if you have raised floors, make sure that the building's walls extend down to the real floor.
If the air ducts that serve your computer room are large enough, intruders can use them to gain entrance to an otherwise secured area.
Here are some guidelines for dealing with air ducts:
Areas that need large amounts of ventilation should be served by several small ducts, none of which is large enough for a person to traverse.
As an alternative, screens can be welded over air vents, or even within air ducts, to prevent unauthorized entry. (This approach is not as good as using small ventilation ducts because screens can be cut; think about all the various adventure movies you've seen.)
The truly paranoid administrator may wish to place motion detectors inside air ducts.
Although glass walls and large windows frequently add architectural panache, they can be severe security risks. Glass walls are easy to break; a brick and a bottle of gasoline thrown through a window can cause an incredible amount of damage. An attacker can also gain critical knowledge, such as passwords or information about system operations, simply by watching people on the other side of a glass wall or window. It may even be possible to capture information from a screen by analyzing its reflective glow.
Here are some guidelines for dealing with glass walls:
Avoid glass walls and large windows for security-sensitive areas. Large windows can also increase your cooling bill by letting in more sunlight.
If you must have some amount of natural light, consider walls made of translucent glass blocks.
Glass walls are good for rooms that must be guarded but in which the guard is not allowed to enter. For these situations, glass walls are preferable to closed-circuit TV because glass walls are harder to spoof.
Computer systems are good targets for vandalism. Reasons for vandalism include:
Intentional disruption of services (e.g., a student who has homework due)
Revenge (e.g., a fired employee)
Political or ideological statement
Entertainment for the feebleminded
Computer vandalism is often fast, easy, and tremendously damaging. Sometimes vandalism is actually sabotage presented as random mischief.
In principle, any part of a computer system?or the building that houses it?may be a target of vandalism. In practice, some targets are more vulnerable than others.
Several years ago, 60 workstations at the Massachusetts Institute of Technology were destroyed in a single evening by a student who poured Coca-Cola into each computer's ventilation holes. Authorities surmised that the vandal was a student who had not completed a problem set due the next day.
Computers that have ventilation holes need them. Don't seal up the holes to prevent this sort of vandalism. However, a rigidly enforced policy against food and drink in the computer room?or a 24-hour guard, in person or via closed-circuit TV?can help prevent this kind of incident from happening at your site.
Local and wide area networks are exceedingly vulnerable to vandalism. In many cases, a vandal can disable an entire subnet of workstations by cutting a single wire with a pair of wire cutters. Compared with Ethernet, fiber optic cables are at the same time more vulnerable (they can be more easily damaged), more difficult to repair (they are difficult to splice), and more attractive targets (they often carry more information).
One simple method for protecting a network cable is to run it through physically secure locations. For example, Ethernet cable is often placed in cable trays or suspended from ceilings with plastic loops. But Ethernet can also be run through steel conduits. Besides protecting against vandalism, this practice protects against some forms of network eavesdropping, and may help protect your cables in the event of a small fire.
Some high-security installations use double-walled, shielded conduits with a pressurized gas between the layers. Pressure sensors on the conduit break off all traffic or sound a warning bell if the pressure ever drops, as might occur if someone breached the walls of the pipe.
Many universities have networks that rely on Ethernet or fiber optic cables strung through the basements. A single frustrated student with a pair of scissors or a pocketknife can stop thousands of students and professors from working.
Some organizations believe that an alternative to physically protecting their network cables is to have redundant connections between various locations on their campus. While it is true that redundant connections will protect an organization from a single failure, if redundancy is the only protection against cable cuts, all an aggressive attacker needs to do is cut the cable in several locations.
We also have heard stories about a fiber optic cable suffering small fractures because someone stepped on it. A fracture of this type is difficult to locate because there is no break in the coating. Once again, it pays to be careful where you place your cables.
In the late 1990s, an off-campus research institute at the University of California at Berkeley faced an unusual vandalism problem. The building was connected to the university's campus network via a microwave transmitter on the building's roof. Over the course of several weeks, this transmitter was repeatedly smashed, painstakingly replaced and aligned, then smashed again. After one of the incidents, police found a note by the transmitter proclaiming that a blow had been struck against invasion of privacy. Apparently, the transmitters were similar in appearance to video cameras, and the idea of video monitoring didn't sit well with someone in the local area. A new transmitter was installed, along with a small sign explaining its function ("THIS IS NOT A CAMERA..."). This one stayed intact.
In addition to cutting a cable, a vandal who has access to a network's endpoint?a network connector?can electronically disable or damage the network. All networks based on wire are vulnerable to attacks with high voltage. At one university in the late 1980s, a student destroyed a cluster of workstations by plugging a thin-wire Ethernet cable into a 110VAC wall outlet. (The student wanted to simulate a lightning strike because he realized that he wasn't going to complete his assignment by the time it was due the next morning.)
In many buildings, electrical, gas, or water cutoffs may be accessible?sometimes even from the outside of the building. Because computers require electrical power, and because temperature control systems may rely on gas heating or water cooling, these utility connections represent points of attack for a vandal.
The successful attack on New York's World Trade Center demonstrated that even computers that are not used by the military and are not operated in a war zone may be the object of terrorist attacks. Because computers are attractive targets, you may wish to consider additional structural protection for your computer room. If your computers are in any way involved in support of something that might inspire violent protest?e.g., university research with animal subjects, oil exploration, fashion design using furs, lumber production?you should definitely consider extra protection for them.
Although protection is important, it is simply impossible to defend against many attacks. In many cases, you should devise a system of hot backups and mirrored disks and servers. With a reasonably fast network link, you can arrange for files stored on one computer to be simultaneously copied to another system on the other side of town?or the other side of the world. Sites that cannot afford simultaneous backup can make hourly or nightly incremental dumps across the network link. Although a tank or suicide bomber may destroy your computer center, your data can be safely protected someplace else.