WiMax Operator's Manual

188 CHAPTER 9 ■ NETWORK SECURITY
Specifically, the network operator should have a policy for controlling human access into
the central office. The central office itself should either be guarded or be equipped with a
secure locking system that will keep out unauthorized individuals. If the central office is not
staffed at all times, then it should be equipped with a surveillance and intrusion-detection system
sufficient to thwart entry until humans can respond. Increasingly, municipal governments
use broadband wireless networks for their own communication needs, and the governments
must be reassured that the network hub is not wide open to attack.
All vital records should be backed up in a secure facility and transmitted to that facility
over a secure virtual private network (VPN) if transmission over public networks is involved.
The VPN should make use of encryption and not just tunneling. In many cases, storage will
take place over a private internal network and stored data will reside in a storage data array
within the central office. Of course, the network can be presumed to be secure if it is dedicated
to storage and is not accessible from the outside. If it is accessible, then the storage network
must be protected with a firewall just as is the case with any other network.
All network elements performing vital functions should be replicated such that a reserve
unit can be immediately pressed into action in the event of a failure. Many carrier-class network
elements have built-in redundancy where every aspect of the system is replicated
internally.
Most network elements made today utilize card and cage construction, and, when that is
the case, individual cards should be hot-swappable so that the entire device need not be shut
down to replace a card. The aim of the network operator must always be to minimize
downtime.
Secure Electrical Systems
Another part of security and good network management is to make certain that high-quality
electrical power will be available at all times even in the event of a power outage. This involves
several distinct measures.
The AC power provided by many electrical utilities is often remarkably inconstant, exhibiting
long-term and short-term voltage sags as well as overvoltage conditions and occasional
spikes where voltage levels may exceed the standard voltage by many multiples. The AC may
also be troubled by the presence of harmonics, distortions in the AC waveform that can disrupt
the functioning of many kinds of electrical or electronic components if sufficiently severe. All
these conditions are undesirable, and some may be catastrophic, and the network operator
must guard against them by appropriately selecting power conditioning and power backup
equipment.
Power conditioning devices take a number of forms.
Passive systems consist of high-frequency filters (of limited usefulness because they cannot
raise or lower voltage or eliminate harmonics), constant voltage transformers, and
switched tap autoformers. Constant voltage transformers and switched tap autoformers are
devices that will maintain constant voltage within certain values, say, 5 percent over and under
the nominal value. While both are essentially passive in their operation, switched tap autoformers
contain logic circuits and relays that select among output taps on the autoformer coil
to compensate for changes in input voltage. Constant voltage transformers operate on a different
principle; the transformer core is partially saturated at the nominal line voltage and will
grow more or less saturated as the input voltage goes up and down, which in turn will cause