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Data Communications

Integrated Services Digital Network (ISDN)

     

  1. ISDN was developed by ITU-T in 1976. It is a set of protocols that combines digital telephony and data transport services. The goal of ISDN is to form a wide area network that provides universal end-to-end connectivity over digital media.

     

  2. Services -- provide fully integrated digital services to users.

       

    • Bearer services provide the means to transfer information (voice, data, and video) between users without the network manipulating the content of that information. Belong to the first three layers of the OSI model.

       

    • Teleservices may change or process the contents of the data. These services correspond to layers 4-7 of the OSI model. Includes telephony, teltex, telefax, videotex, telex, and teleconferencing.

       

    • Supplementary services provide additional functonality to the bearer services and teleservices.

       

  3. Integrates customer services with IDN. Integrated Digital Networks(IDNs) are a combination of networks available for different purposes. Access to these networks is by digital pipes, which are time-multiplexed channels sharing very-high-speed paths.

     

  4. Defines three channel types, each with a different transmission rate.

     

    • Bearer channels (B channels) are defined at a rate of 64 Kbps. Carries transmission end-to-end.

       

    • Data channels (D channels) can be either 16 or 64 Kbps. Primary function is to carry control signaling for the B channels. Uses out-of-band signaling.

         

      • In-band signaling is where control information is carried by the same channel that carries the message.

         

      • Out-of-band signaling is where control information is carried by a channel that is different from the one that carries the message.

         

    • Hybrid channels (H channels) are available with data rates of 384 Kbps (H0), 1536 Kbps (H11), or 1920 Kbps (H12). These channels are for high data-rate applications such as video, teleconferencing, etc.

       

  5. User interfaces

     

    • The basic rate interface (BRI) specifies a digital pipe consisting of two B channels and one 16 Kbps D channel. The same twisted-pair local loop that delivers analog transmission can be used to handle digital transmission. Supported by either a bus or star topology.

       

    • The primary rate interface (BRI) specifies a digital pipe with 23 B channels and one 64 Kbps D channel. 23 B channels of 64 Kbps each, plus one D channel of 64 Kbps equals 1.536 Mbps. In addition, the PRI service uses 8 Kbps of overhead. PRI therefore requires a digital pipe of 1.544 Mbps.

     

  6. Functional grouping

     

    • A network termination 1 (NT1) device controls the physical and electrical termination of the ISDN at the user's premises and connects the user's internal system to the digital subscriber loop. It performs the basic multiplexing functions of byte interleaving, but it is not a multiplexer. An NT1 synchronizes the data stream with the frame- building process in such a way that multiplexing occurs automatically.

       

    • A network termination 2 (NT2) device performs functions at the physical, data link, an network layers of the OSI model. NT2s provide multiplexing (layer 1), flow control (layer 2), and packetizing (layer 3). An NT2 provides intermediate signal processing between the data-generating devices and an NT1. NT2s are used primarily to interface between a multiuser system and an NT1 in a PRI.

       

    • Terminal equipment 1 (TE1) is any device that supports the ISDN standards. Examples are digital telephones, integrated voice/data terminals, and digital facsimiles.

       

    • Terminal equipment 2 (TE2) equipment is any non-ISDN device, such as a terminal, workstation, host computer, or regular telephone.

       

    • Terminal adapters (TA) converts information received in non-ISDN format from a TE2 into a format capable of being carried by the ISDN.

     

  7. ISDN Layers -- three planes that are divided into seven layers

     

    • User plane

       

    • Control plane

       

    • Management plane

       

     

  8. The ISDN physical layer specifications are defined by two ITU-T standards, I.430 for BRI access and I.431 for PRI access

     

  9. B and D channels use different data link protocols. B channels use the LAPB protocol. The D channel uses link access procedure (LAPD), which is HDLC with a few modifications.

     

  10. On the network layer, once a connection has been established by the D channel, the B channel sends data using circuit switching, X.25, or other similar protocols.

     

  11. Broadband ISDN -- provides subscribers to the network with data rates in the range of 600 Mbps, almost 400 times faster than the PRI rate.

     

    • Interactive services require two-way exchanges between either two subscribers or between a subscriber and a service provider.

       

    • Disbributive services are unidirectional services sent from a provider to subscribers without the subscriber having to transmit a request each time a service is desired. These services can be without or with user control.

     

  12. N-ISDN (narrowband ISDN) has replaced telephones in some European countries. In the US, this has been delayed and new technologies such as cable modem and ADSL has evolved.