Most corporations do not have their own circuit or long distance communication route for use in MANs and WANs. They usually rent or lease them from common carriers. Common carriers provide a variety of telecommunications services for the corporations to use them for their LANs and MAN/WANs.

Many computer-based communications services are now being provided by common carriers. A common carrier is a private company (usually a telephone company) which offers computer networking services for a fee. These fees are regulated by agencies such as the Federal Communications Commission (FCC) and the Public Utility Commission (PUC). Two types of services are commonly provided by common carriers: public-measured service and private leased service.

Two Types of Services

Public-Measured Service

In a public-measured service, various network systems share common communications channels with other networks. For example, an ordinary telephone line that users pass on your street every day may be carrying data communications generated by a variety of computers in your neighborhood.

Private Leased Service

In a private leased service arrangement, the common carrier provides its customer with an exclusive service. An illustration of this is when the common carrier allows a customer to have access to a dedicated line - one reserved for the exclusive use of the customer. Common carriers figure out their fees for communications services in much the same manner as phone companies decide your monthly phone bill. A dial-up arrangement represents the simplest billing system used by common carriers. Users can establish connection with other terminals by dialing the receiver's number. In this arrangement, customers pay a flat fee for the right to engage in an unlimited number of transactions.

Long distance transmissions are charged on a per-transmission basis. Businesses that conduct many network activities with long distance transmissions may consider using a Wide Area Telecommunications Service (WATS). In this service, as long distance usage goes up, rates go down. Both WATS lines and dial-up arrangements are examples of voice grade circuits designed to transmit ordinary voice messages over telephone lines. These systems may be too noisy for computer networking systems that transmit vary large amounts of data in short periods. Therefore, many common carriers attempt to provide higher quality circuits by offering special private lines which filter out excess sound. These lines are known as above-voice grade circuits.