The traditional PSTN was based on an analog connection between the customer premises and the local exchange, also called the local loop. The analog circuits introduce limitations on the bandwidth that can be obtained on the local loop. Circuit restrictions do not permit analog bandwidths greater than approximately 3000 Hz. ISDN technology permits the use of digital data on the local loop, providing better access speeds for the remote users.

Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals. ISDN is generally used for telecommuting and networking small and remote offices into the corporate LAN.

Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), better known as the local loop. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity. These standards help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. In an ISDN network, the digitizing function is done at the user site rather than the telephone company.

ISDN brings digital connectivity to local sites. The following list provides some of the benefits of ISDN:

• Carries a variety of user traffic signals, including data, voice, and video
• Offers much faster call setup than modem connections
• B channels provide a faster data transfer rate than modems
• B channels are suitable for negotiated Point-to-Point Protocol (PPP) links

ISDN is a versatile service able to carry voice, video, and data traffic. It is possible to use multiple channels to carry different types of traffic over a single connection.

ISDN uses out-of-band signaling, the delta (D channel), for call setup and signaling. To make a normal telephone call, the user dials the number one digit at a time. Once all the numbers are received, the call can be placed to the remote user. ISDN delivers the numbers to the switch at D-channel rates, thus reducing the time it takes to set up the call.

ISDN also provides more bandwidth than a traditional 56 kbps dialup connection. ISDN uses bearer channels, also called B channels, as clear data paths. Each B channel provides 64 kbps of bandwidth. With multiple B channels, ISDN offers more bandwidth for WAN connections than some leased services. An ISDN connection with two B channels would provide a total usable bandwidth of 128 kbps.

Each ISDN B channel can make a separate serial connection to any other site in the ISDN network. Since PPP operates over both synchronous and asynchronous serial links, ISDN lines can be used in conjunction with PPP encapsulation.

Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ ito_doc/ isdn.htm