Four Classes of Communication Services

Communication services come in four basic groups. With a dedicated circuit, the organization pays a fixed monthly fee, while with the other three, the organization pays on a per-use basis.

Dialed Circuit Services

Dialed circuit services are regular dialed telephone calls from one point to another through the telephone networks. Dialed circuits are usually slow and noisy. The usual voice telephone network is used for data transmission with DDD (direct distance dialing). In this service, users dial the telephone number of the host computers through a modem to be connected.
This service contains an AT&T Megacom WATS (Wide Area Telephone Services), AT&T Megacom services, and DLAL-IT 900 services.

Dedicated Circuit Services

This service is point-to-point circuit service that a leasing corporation can use exclusively to connect two point. A common carrier lends this circuit for the exclusive use. The cable is faster and more noise-free than dialed circuit service.
This service includes voice grade channels, Wideband analog services, digital services, T carrier circuits, SONET, and satellite services.

Switched Circuit Services

A switched circuit service is the circuit in which the corporation installs network connection points at many locations and uses the common carrier's network to connect temporarily (switched connections) between locations when necessary. This service includes SMDS and ISDN. Let's see more details of ISDN.

ISDN

ISDN (Integrated Services Digital Network) is a special type of private leased service called the integrated services digital network (ISDN) is expected to dramatically affect the business environment in coming years. This type of network allows for simultaneous voice, data, text, and video digital communications. ISDN represents an effort to create a universal communications network nationwide. The ISDN consists of two types of channels.

The B-channel transmits user information with a transmission speed of 64 kilobits per second (Kbps). The D-channel provides a control function by initiating, terminating, and directing transmissions. In addition, there are two basic types of ISDN structures: basic access and primary access. Basic access networks have two B-channels and one D-channel with a transmission speed of 16 Kbps. These networks are primarily leased by residential users. Business end users are more likely to use the primary access system that uses 23 B-channels and one 64Kbps D-channel.

The ISDN system offers high-quality digital communications as well as many features that enhance the quality and flexibility of network use. For example, the B-channel enables users to simultaneously access multiple databases and computer systems. At the same time, users can take advantage of a variety of office automation technologies and multimedia systems. The D- channel can perform many useful functions such as transmission forwarding, automatic callback, and caller identification.

Packet Switched Networks

This works very much like a switched circuit, except that the user breaks data transmissions into pre-defined packets that conform to network protocols. This service includes packet switching, Frame relay, ATM, public data networks and software defined networks.