within a building are the easiest to control because, depending on the distance between each com- municating device, they can be totally organization owned and in- spected. The most common transmission media include twisted wire, coaxial cable, and fiber optic lines. The intrabuilding link security problem increases when the organization leases lines or uses voice dial-up circuits. Here the security problems become similar to those of the local area category because the organization has relinquished control. The local area network may be entirely within a large building, an industrial complex, or a small area of city. Its communications links can be user-owned cables (including fiber optics), microwave circuits, leased lines, and voice dial-up. The threat of data alteration via line taps is greater than in intrabuilding. Line taps are easy to install and and difficult to prevent. Domestic networks via commercial carrier, including value-added networks, greatly increase link security problems. Unless private lines are used, data are transported totally by vendor media. This may be TELECOMMUNICATIONS a plus where packet-switching networks route various pieces of in- formation via different circuits, but it is a drawback where satellite transmission is concerned. Satellites provide the least degree of data security because transmissions may be intercepted by anyone in the area with an appropriate antenna. International networks have all the security vulnerability of do- mestic networks with two additional problems: they are subject to transborder dataflow limitations and, sometimes, data encryption is not allowed. In addition, many foreign countries have poorer-quality communications networks than those available in the U.S. Data encryption (Fig. 5-7) is the process of changing original data (cleartext) into ciphertext so that they cannot be understood until the ciphertext is decrypted into cleartext at this distant end of a link. The NBS formal description of the data encryption standard (DES) is the DES algorithm-