iber-optic cables are not susceptible to electromagnetic radiation, problems such as ground loops, crosstalk, and lightning interference are eliminated. No electrical signals are transmitted between equipment intercon- nected by the glass fibers, thereby eliminating the possibility of elec- trical surges or short circuits. Moreover, it is almost impossible to tap into detection, a security advantage over coaxial cable. 7.2.2 Measured Use Services The DDD service we know best is the telephone company's direct distance dialing. This service charge is based on distance and connect time. If the data to be transmitted each week only amounts to an hour or two, DDD will be hard to beat. Wide area telecommunications service (WATS) is available on either a measured time rate (10 hours per month included in the base rate; or full business day rate with 240 hours/month in the base rate. Overtime usage is accumulated in 6 minute increments. WATS would be a good choice if the requirement dictates contacting multiple lo- cations on a fairly regular but not a full time basis. SATELLITE AND CARRIER SERVICES 115 TWX and Telex lines are used for data transmissions only and are limited to speeds of 150 and 50 bps respectively. DDS and WATS are offered by the telephone company; TWX and Telex are both offered by Western Union. Specialized carriers, such as MCI Southern Pacific, and others of- fer measured use of their networks on a dial-in basis. For a low monthly fee ($10 to $35) you receive an access code and phone num- bers in the various served cities. Usage is charged at a relatively low rate of 20 to 30 cents per minute regardless of distance. These ser- vices should be given serious consideration when the cities served fit your requirement. Tymnet provides a "virtual connection" network accessed by DDD, WATS, or Tymnet provided access lines. Speeds are up to 4800 bps and rates are based on connect time, number of 1000 char- acter block units, and any network interface equipment required. Packet switching services like Telenet 