|  | 5.1 | ISDN
| 5.1.1 | What is ISDN?

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 viewed as an alternative to leased lines, which can be used for telecommuting and networking small and remote offices into LANs. Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), which is how the screen looks when the user dials into the network, and network capabilities. Standardizing subscriber services makes it more possible to ensure international compatibility. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity, which help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. Basically, the digitizing function is done at the user site rather than the telephone company. ISDN's ability to bring digital connectivity to local sites has many benefits, including the following: ISDN can carry a variety of user traffic signals. ISDN provides access to digital video, circuit-switched data, and telephone network services by using the normal phone network which is circuit-switched. ISDN offers much faster call setup than modem connections because it uses out-of-band (D, or delta, channel) signaling. For example, some ISDN calls can be setup in less than one second. ISDN provides a faster data transfer rate than modems by using the bearer channel (B channel of 64Kbps). With multiple B channels, ISDN offers users more bandwidth on WANs than some leased lines. For example, if you were to use two B channels, the bandwidth capability is 128Kbps because each B channel handles 64Kbps. ISDN can provide a clear data path over which to negotiate PPP links. You should ensure in the design phase that the equipment selected has the feature set that takes advantage of ISDN's flexibility. In addition, you must keep in mind the following ISDN design issues: Security issues -- Because network devices can now be connected over the Public Switched Telephone Network (PSTN), it is crucial to design and confirm a robust security model for protecting the network. Cost and Containment issues -- A primary goal of selecting ISDN for your network is to avoid the cost of full-time data services (such as leased lines or Frame Relay). Therefore, it is very important to evaluate your data traffic profiles and monitor ISDN usage patterns to ensure that your WAN costs are controlled.
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