connect to the Internet as a point-of-presence (POP) on a T1 line owned by a major telephone network. Many businesses also use T1 (and T3) lines to connect to an Internet access provider.

ISDN provides digital transmission over ordinary telephone copper wire as well as over other media.

Conceptually, ISDN integrates both analog or voice traffic with digital data over the same network. Home and business users receive data at rates up to twice the rate of a modem connection.
ISDN requires adapters at both ends of the transmission (customer premises and service provider end). Most telephone companies in urban areas in North American and Europe can provide ISDN. In many areas where service providers offer digital subscriber line (DSL) and cable modem service, ISDN is no longer a popular option.
There are two levels of ISDN service. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services at a rate of 64 kbps. Each D-channel carries control and signaling information:
- Basic Rate Interface (BRI) is for home and small business use. BRI contains two B-channels and one D-channel.
- Primary Rate Interface (PRI) is for larger users. PRI includes 23 B-channels in North America and Japan (30 in Europe) and one D-channel.
Service providers usually transmit PRI service through a T1 line in North America and an E1 line in Europe.
From a consumer point of view, and as a data connection service, DSL replaces ISDN.
ISDN is also used as a smart network technology intended to add new services to the PSTN by giving users direct access to end-to-end circuit-switched digital services.
ISDN PRI is commonly used in recording studios when a voice-over actor is in one studio but the director and producer are in a studio at another location. ISDN is used because of its "guaranteed" real-time service that does not rely on the Internet and audio fidelity that is superior to POTS service. A few companies make video conference call equipment that combines 3 BRI lines (six 64K channels), to create a good quality picture.

Content 1.1 The Evolution of Telephony in the Enterprise 1.1.4 Digital Telephone Services When the computerization of electronic telephone exchanges began in the 1970s and 1980s, many new network services became available. Here are some of the digital telephone services that were developed:

Voice mail
Caller ID
Call waiting
Reminder calls
(Three-way) conference calling
Enhanced 911 (U.S.)
Central office exchange service (Centrex)

Content 1.1 The Evolution of Telephony in the Enterprise 1.1.5 PBXs and Centrex Services Many enterprises use a private branch exchange (PBX). Medium-sized and larger companies use a PBX because it is much less expensive than connecting an external telephone line to every telephone in the organization. In addition, it is easier to make internal calls within a PBX using only three or four digits. Many telephones can be attached to a PBX, but users can only share a certain number of outside lines for external calling. Usually to secure an outside line, the caller dials the digit ”9” before the full outside telephone number. For companies that are not willing or able to invest in PBX technology, many telephone companies offer central office exchange service (Centrex). Centrex is a virtual PBX with all the switching occurring at the telephone office instead of at the company's premises. Typically, the telephone company owns and manages all the communications equipment that is needed to implement the PBX and then sells various services to the company. Here is a list of some typical Centrex services:

Call Transfer
Call Divert—on no reply or on busy calls
Call Waiting
Three-Party Conference
Call Pick Up (Group)
Ring Back
Reminder or Alarm Call
Last Number Redial
Centrex Hotline (non-dialed connection)
Centrex Warm Line (delayed Hotline)
Centrex Hunt Groups, with optional bypass numbers

Centrex customers can choose from a variety of special services and features:

Packaged Centrex: Packaged Centrex customers are offered a fixed set of features in different packages. These packages are offered at a relatively low cost, as little or no customization is permitted. The lack of customization minimizes the operational costs of programming and maintaining the services.
Centrex Data: Centrex Data services can provide relatively low speed (56 and 64 kbps) data services using the circuit-switched telephone network. Although they are now overshadowed by Internet and other data networks, Centrex Data services can offer very flexible and wide-reaching network configurations since connections can be made almost anywhere within the reach of the telephone network.
Customized Centrex: Customized Centrex offers a highly customizable set of options that require specialized programming and troubleshooting skills to maintain. A typical Customized Centrex setup allows four digit dialing between people in the same network, called "locals" (sometimes even if the locals are located in different parts of a city), customized routing through the telephone network (such as Least Cost Routing or Time of Day Routing), and customized codes for invoking features. Customized Centrex is the most flexible, but usually the most expensive service offering.

Content 1.1 The Evolution of Telephony in the Enterprise 1.1.6 Long-Distance Services Enterprises increasingly rely on their ability to communicate directly with customers, suppliers, partners, branches, and teleworkers. Long-distance services represent a significant expense. When long-distance costs were relatively high, companies kept control over long-distance access. Figure shows a historical example of how operator intervention was required for a caller inside the PBX to access long-distance service. Long-distance trunk lines connecting exchanges usually use TDM technologies and T1 or E1 transport. The most common long-distance service that is offered in North America is Wide Area Telephone Service (WATS). Other parts of the world have similar services, see Figure . WATS-type plans provide access to long-distance telephone lines for commercial use at reduced rates. WATS is outward (OUT-WATS), inward (IN-WATS), or both:

With outward WATS, the calling party can make an unlimited number of long-distance calls (toll calls) for a fixed price within pre-determined time and distance constraints. Most telephone companies now market OUT-WATS as something called a flat-rate plan. Initially, operator assistance was necessary to control access to OUT-WATS from within a PBX. Following company policy, callers asked the company operator at their PBX to connect them to the “tie-line,” or the OUT-WATS line that tied into the long distance system. However, as long distance calling rates fell, operator intervention has almost disappeared. Callers making a call from inside a PBX usually only need to add a prefix to gain access to outside long-distance numbers.
With IN-WATS, subscribers have a toll-free telephone number. IN-WATS service reduces the time that operators spend processing toll-collect calls for businesses. The IN-WATS area codes in North America are 800, 888, 877, and 866. More 800 numbers are reserved for the future (855, 844, 833 and 822). Telephone users within a designated area can call the IN-WATS telephone number of an organization without having to pay a toll charge. Many businesses have toll-free 800 numbers that can ring at multiple call-centers.

Content 1.1 The Evolution of Telephony in the Enterprise 1.1.7 The Concept of Convergence Before advanced networking technologies allowed convergence, enterprises