ISDN

ISDN BRI is specifically used to solve the low bandwidth problems that small offices or dial-in users have with traditional telephone dial-in services.
Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN was developed to use the existing telephone wiring system, and it works very much like a telephone.
When you make a data call with ISDN, the WAN link is brought up for the duration of the call and is taken down when the call is completed.

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.

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. It provides access to digital video, packet-switched data, and telephone network services.
ISDN offers much faster call setup than modem connections because it uses out-of-band (D, or delta, channel) signaling.
Some ISDN calls can be setup in less than one second.