• What Is ISDN & How Its Work?

    What Is ISDN


    Integrated Services Digital Network (ISDN) is a set of communication standards for digital telephone connection and the digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.


    In recent years, ISDN has been replaced by broadband internet access connections like DSL, WAN, and Cable Modems. That said, it is often still used as a backup in the instance that the mainline fails.
     It provides a packet-switched connection for data in increments of 64 kilobits/s. It provided a maximum of 128 kbit/s bandwidth in both upstream and downstream directions.

    In short, ISDN is a telephone the network-based infrastructure that allows the transmission of voice and data simultaneously at a high speed with greater efficiency. This is a circuit switched telephone network system, which also provides access to Packetswitched networks.

    How ISDN Work

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    How ISDN Work?

    A single transmission channel for communication is available in the analog network or regular telephone lines that are provided to us by telephone companies. Therefore only one service can be carried at a time, i.e. voice, data, video at a single time. Whereas in ISDN lines there the same pair of telephone lines are logically divided into multiple channels.

    There are two types of the channel that are Mostly found within ISDN:-
    • B or Bearer channels:   The bearer channels are used to carry the payload data which may be a voice and / or data.
    • D or Delta channels:   The D channels are intended for signaling and control, although it may also be used for data under some circumstances.

    The ability to transfer information between the user and ISDN structured in the form of information transfer channels:-

    • • Channel A: It is an analog channel of 4 Khz.
    • • Channel B: It is a 64 Kbps digital channel that intended for the transport of user information.
    • • Channel C: It is an 8 or 16 Kbps digital channel.
    • • Channel D: It is a digital channel of 16 or 64 Kbps intended primarily for the transmission of user-network signaling information for communication control, although it can also use under certain conditions for the transfer of user information in services selection (telealarm,telecontrol, and Telemedia) and low capacity data transmission.
    • • Channel E: It is a 64 Kbps digital channel (used for internal ISDN signals).
    • • Channel H: It is a digital channel of 384, 1,536 or 1,920 kbps that provides the user with an information transfer capability.
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    Types Of ISDN –

    • Basic Rate Interface (BRI)
    • Primary Rate Interface (PRI)
    • Narrowband ISDN
    • Broadband ISDN

    Basic Rate Interface (BRI) -

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    BRI ISDN

    • There are two data-bearing channels (‘B’ channels) and one signaling channel (‘D’ channel) in BRI to initiate connections.
    • The B channels operate at a maximum of 64 Kbps while the D channel operates at a maximum of 16 Kbps it is used for transmitting control information.
    • For example, one channel is used as a TCP/IP connection to a location while the other channel is used to send a fax to a remote location. In iSeries ISDN supports basic rate interface (BR).

    Primary Rate Interface (PRI) -

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    ISDN PRI

    • PRI is not supported on the iSeries.
    • It supports 30 B channels and two additional channels in a single E1 connection, providing a data transfer rate of 2,048 kbps.

    Broadband ISDN - 

    • B-ISDN relies mainly on the evolution of fiber optics. According to CCITT B-ISDN is best described as ‘a service requiring transmission channels capable of supporting rates greater than primary rate.
    • It uses broadband transmission and can support transmission rates of 1.5 Mbps. It is not widely available.
    • The Transmission is related to ATM i.e.,Asynchronous Transfer Mode.

    Narrowband ISDN -

    • Narrowband ISDN has been designed to operate over the current communications infrastructure.
    • This is actually an attempt to digitize the analog voice information. This uses 64kbps circuit switching.

    ISDN Usage

    • With most companies now opting for DSL, or fiber connections, ISDN is used in some countries where the system has not migrated over fully yet. It can also be used as a backup in case the DSL or other digital systems fail.

    Principle of ISDN - 

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    Basic Principle of ISDN

    The ISDN works based on the standards defined by ITU-T (formerly CCITT). The Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. 

    It became a United Nations specialized agency in 1947, and the International Telegraph and Telephone Consultative Committee (CCITT), (from the French name "Comite Consultatif International Telephonique et Telegraphique") was created in 1956. It was renamed ITU-T in 1993.

    • To support switched and non-switched applications - ISDN supports both circuit switching and packet switching. Also, ISDN supports non-switched services in the form of dedicated lines.
    • To support voice and non-voice applications - The the main feature of the ISDN concept is the support of a wide range of voice (for e.g. Telephone Calls, digital data exchange).
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    • Reliance on 64-kbps connections - ISDN provides circuit switched and packet-switched connections at 64 kbps.
    • Layered protocol architecture - A layered protocol structure should be used for the specification of access to an ISDN. Such a structure can be mapped into the OSI model.
    • Intelligence in the network- An ISDN is expected to provide sophisticated services beyond the simple setup of circuit-switched calls.
    • Variety of configurations - Several configurations are possible for implementing ISDN.

    ISDN Services  

    The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into some categories-

    Bearer Services: -
    • Bearer services provide the means to transfer information (voice, data, and video) between users without the network manipulating.
    • Bearer services belong to the first three layers of the OSI model.  
    • They can be provided using circuit-switched, packet-switched, frame-switched, or cell-switched networks.
    Teleservices:-
    •  In teleservices, the network may change or process the contents of the data.
    • These services correspond to layers 4-7 of the OSI model. 
    • Teleservices include telephony, teletex, telefax, videotex, telex, and teleconferencing.
    Supplementary Service –
    • Additional functionality to the bearer services and teleservices are provided by supplementary services. Examples of these services are reverse charging, call waiting, and message handling, all familiar from today's telephone company services.

    Advantages of ISDN –

    • As the services are digital, there is less chance for errors.
    • The the connection is faster.
    • The bandwidth is higher.
    • Call Forwarding
    • Call Waiting
    • Voicemail
    • Speed Line
    • Voice, data and video − all of these can be sent over a single ISDN line.

    Disadvantages of ISDN-

    • It required digital services.
    • ISDN lines are more expensive 

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