What is ISDN?
ISDN, which stands for integrated services digital network, is a system of digitizing phone networks which has been in the works for over a decade. This system allows audio, video, and text data to be transmitted simultaneously across the world using end-to-end digital connectivity.
The original telephone system used analog signals to transmit a signal across telephone wires. The voice was carried by modulating an electric current with a waveform from a microphone. The receiving end would then vibrate a speaker coil for the sound to travel back to the ear through the air. Most telephones today still use this method. Computers, however, are digital machines. All information stored on them is represented by a bit, representing a zero or a one. Multiple bits are used to represent characters, which then can represent words, numbers, programs, etc. The analog signals are just varying voltages sent across the wires over time. Digital signals are represented and transmitted by pulses with a limited number of discrete voltage levels. [Hopkins]
The modem was certainly a big breakthrough in computer technology. It allowed computers to communicate with each other by converting their digital communications into an analog format to travel through the public phone network. However, there is a limit to the amount of information that a common analog telephone line can hold. Currently, it is about 28.8 kbit/s. [Hopkins] ISDN allows multiple digital channels to be operated simultaneously through the same regular phone jack in a home or office. The change comes about when the telephone company's switches are upgraded to handle digital calls. Therefore, the same wiring can be used, but a different signal is transmitted across the line. [Hopkins] Previously, it was necessary to have a phone line for each device you wished to use simultaneously. For example, one line each for the phone, fax, computer, and live video conference. Transferring a file to someone while talking on the phone, and seeing their live picture on a video screen would require several expensive phone lines. [Griffiths] Using multiplexing (a method of combining separate data signals together on one channel such that they may be decoded again at the destination), it is possible to combine many different digital data sources and have the information routed to the proper destination. Since the line is digital, it is easier to keep the noise and interference out while combining these signals. [Griffiths] ISDN technically refers to a specific set of services provided through a limited and standardized set of interfaces. This architecture provides a number of integrated services currently provided by separate networks.
ISDN adds capabilities not found in standard phone service. The main feature is that instead of the phone company sending a ring voltage signal to ring the bell in your phone, it sends a digital package that tells who is calling (if available), what type of call it is (data/voice), and what number was dialed (if multiple numbers are used for a single line). ISDN phone equipment is then capable of making intelligent decisions on how to answer the call. In the case of a data call, baud rate and protocol information is also sent, making the connection instantaneous. [Griffiths] ISDN Concepts:
With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth of 64 kbit/s each. A delta channel (D channel) handles signalling at 16 kbit/s or 64 kbit/s. H channels are provided for user information at higher bit rates. [Stallings] There are two types of ISDN service: Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI).
BRI: consists of two 64 kbit/s B channels and one 16 kbit/s D channel for a total of 144 kbit/s. The basic service is intended to meet the needs of most individual users. PRI: intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kbit/s D...
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