This would be my understanding about the Broadband telecommunications particularly about the ISDN or the Integrated Services Digital Network. Base on my research that it is a system of digital phone connections which allows voice and data to be transmitted simultaneously across the world by using end-to-end digital connectivity. All central and end office switching is performed by digital switches, and signaling system or the call establishment occurs through digital protocols. ISDN in concept is the integration of both analog or voice data together with digital data over the same network. So definitely, ISDN is a digital transmission from the customer-premises equipment such as telephones, data terminals and fax machines over the other media at the same network. The voice data and the digital data has been integrated for fast transmission and provides a raw data rate of 144 kbps for a single telephone company. With that data rate, it would sufficiently enough teleconferencing and would be enough for a a data to be transmitted. ISDN has two levels of services, the Basic Rate Interface (BRI), which is intended for individual users and small enterprises while the Primary Rate Interface (PRI), provides high capacity service for larger users. In BRI, the initial concept had this as being everything from analog telephone calls which is been digitized for teleconferencing data and this would be switched channels. The BRI has two switched channels of 64 kbps B-channel and one 16 kbps D-channel. PRI is intended for users with greater capacity requirements. PRI  has 23 B-channels and one 64 kbps D-channel. B-channel can carry any type of digital information such as voice, data, or video with no restrictions on format or protocol imposed by the ISDN carrier. D-channel carries the information needed to connect or disconnect calls and to negotiate special calling parameters such as automatic number ID, call waiting, data protocol. ISDN also provide internet service that works over telephone lines. Generally, internet service supports data rates of 128 kbps. ISDN emerged as an alternative to traditional dial-up networking. Advantages of ISDN is that it allows multiple digital channels to be operated simultaneously through the same regular phone wiring used for analog lines. For multiple device, ISDN allows multiple devices to share a single line. It is possible to combine many different digital data sources and the information routed to the proper destination. ISDN integrates all services by providing a small set of standard interfaces and access protocols that apply to all services because ISDN is an international standard, the same interfaces and access protocols should be available anywhere in the world, across international boundaries, and among equipment from any set of vendors.


sources:
http://www.techfest.com/networking/wan/isdn.htm

http://www.jet.net/isdn/isdnintro.html

http://compnetworking.about.com/od/internetaccessbestuses/g/bldef_isdn.htm

http://www.ralphb.net/ISDN/defs.html

http://searchenterprisewan.techtarget.com/definition/ISDN