To keep a network operating, to diagnose failures or degradation, and to circumvent problems, networks may have a wide-ranging amount of ancillary equipment.
Providing Electrical Power
Individual network components may have surge protectors - an appliance designed to protect electrical devices from voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltage
above a safe threshold
Beyond the surge protector, network elements may have uninterruptible power supplies (UPS), which can be anywhere from a line-charged battery to take the element through a brief power dropout, to an extensive network of generators and large battery banks that can protect the network for hours or days of commercial power outages. A network as simple as two computers linked with a crossover cable has several points at which the network could fail: either network interface, and the cable. Large networks, without careful design, can have many points at which a single failure could disable the network.
When networks are critical the general rule is that they should have no single point of failure. The broad factors that can bring down networks, according to the Software Engineering Institute [6] at Carnegie-Mellon University:
1. Attacks: these include software attacks by various miscreants (e.g., malicious hackers, computer criminals) as well as physical destruction of facilities.
2. Failures: these are in no way deliberate, but range from human error in entering commands, bugs in network element executable code, failures of electronic components, and other things that involve deliberate human action or system design.
3. Accidents: Ranging from spilling coffee into a network element to a natural disaster or war that destroys a data center, these are largely unpredictable events. Survivability from severe accidents will require physically diverse, redundant facilities. Among the extreme protections against both accidents and attacks are airborne command posts and communications relays[7], which either are continuously in the air, or take off on warning. In like manner, systems of communications satellites may have standby spares in space, which can be activated and brought into the constellation.
Dealing with Power Failures
One obvious form of failure is the loss of electrical power. Depending on the criticality and budget of the network, protection from power failures can range from simple filters against excessive voltage spikes, to consumer-grade Uninterruptible Power Supplies(UPS) that can protect against loss of commercial power for a few minutes, to independent generators with large battery banks. Critical installations may switch from commercial to internal power in the event of a brownout,where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full
power outage.
Some network elements operate in a manner to protect themselves and shut down gracefully in the event of a loss of power. These might include noncritical application and network management servers, but not true network elements such as routers. UPS may provide a signal called the "Power-Good" signal. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present, the computer shuts down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself
To help standardize approaches to power failures, the Advanced Configuration and Power Interface (ACPI) specification is an open industry standard first released in December 1996 developed by HP, Intel, Microsoft, Phoenix and Toshiba that defines common interfaces for hardware recognition, motherboard and device configuration and power management
.
Monitoring and Diagnostic Equipment
Networks, depending on their criticality and the skill set available among the operators, may have a variety of temporarily or permanently connected performance measurement and diagnostic equipment. Routers and bridges intended more for the enterprise or ISP market than home use, for example, usually record the amount of traffic and errors experienced on their interfaces. Diagnostic equipment, to isolate failures, may be nothing more complicated than a spare piece of equipment. If the problem disappears when the spare is manually replaced, the problem has been diagnosed. More sophisticated and expensive installations will have spare elements that can automatically replace a failed unit. Failures can be made transparent to user computers with techniques such as the Virtual Router Redundancy Protocol (VRRP), as specified in RFC 3768.
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