Technology Reference Guide

	Technology Reference Guide 7. Wide-Area Networks B. Wide-Area Carrier Services/Tariffs

What is it?
With very few exceptions, most companies who need wide-area network connections purchase the services of a public carrier. The original public carrier was AT&T/Bell Telephone, and the original carrier service was simply a voice-grade phone line. Modems were used to transmit data across these lines, and the circuit was established by picking up the phone and dialing the number of the remote system. This arrangement had some advantages, in that one could connect locations easily, and only pay for the time the connection was used. The major down-side was that the telephone system was designed to carry voice traffic, not data, and could only handle data traffic that was transmitted at a relatively slow rate.

Realizing the limitations of voice-grade lines, and the demand for high data throughput, the Bell system, and later their competitors, began to offer higher-speed lines that were designed specifically to handle data reliably. The capacity of a data line is often defined by its bandwidth. The bandwidth of a voice-grade line is 5 Kbits/sec, whereas data-grade lines are available at 56 Kbits/sec, 1.54 Mbits/sec and above.

What does it do?
A wide-area carrier service provides a circuit for data traffic between two or more geographic locations. The circuits are available as either switched circuits (dial-up) or dedicated lines (constant connection). A common switched circuit is a standard voice-grade phone line. There are also switched 56K data lines available. Dedicated lines are private point-to-point connections leased from the common carrier, with lease rates outlined in a tariff schedule. Tariffs are generally based on the distances and geographies covered by the leased line, and traditionally were a major consideration in setting up a wide-area network. Frequently, compromises were made between line speed and cost. With the breakdown of the AT&T monopoly, and the emergence of competing phone companies, as well as carriers specializing in data traffic, tariffs have come down significantly, although they are still an issue.

The most common types of lines available are voice-grade, which is seldom used for serious network traffic, both switched and dedicated 56k lines, and T-1 lines, which are leased lines capable of 1.544 megabits/sec. There are also some leased T-3 lines available, which run at 44 megabits/sec. It is also possible to lease fractional T-1 services, in which the 1.544 Mb/s line is divided into 24 channels of 64Kb/s each. A customer can purchase as many channels as necessary to meet his data requirements, and several customers can share the cost of a single T-1. T-1 and T-3 lines are also referred to as DS-1 and DS-3, although this actually refers to the physical layer protocol used to transmit the data across these lines.

How is it used?
Carrier services have traditionally provided a circuit to the customer at a defined maximum data rate. The implementation of transmission protocols across this circuit were the responsibility of the customer. This is analogous to providing the wiring in a LAN, and nothing else. These services are switched, which means they establish a point-to-point circuit between two end-points by dial-up, or dedicated. There is no routing involved in this type of link, because either the connection is established by dialing a phone number, or the link exists permanently between the two end points.

Where do I get more information?
Contact your local CBV Office.