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What are ISDN layers and components?

About ISDN Layers

– For establishing a physical point – to – point connection that is permanent between the source and the destination, circuit switching is used by the ISDN.
– The standards defined by the ITU are employed in the ISDN protocol for encompassing the 3 bottom layers of the OSI model i.e., the physical, data – link and the network layers.
– The user network interface has been defined at the physical level by the ITU for both primary rate access and basic rate access.
– However, this interface has also been defined by the ANSI.
– The ordinary telephone lines is used by the physical layer for its cabling structure.
– High level data link control or PPP frames like protocols that are point – to – point protocols actually are utilized by the ISDN B channel.
– However, sometimes other methods of encapsulation might be used like the frame relay.
– This methodology can operate up on BRI and PRI D channel.
– One can observe the IP packets at the layer 3.
– ISDN is operated in the full duplex mode i.e., it can thus receive as well as transmit the traffic at the same time. – Different types of signaling protocol are utilized by the ISDN – D channel at the third and second layer of the OSI model.
– At the layer 2, the signaling used is the LAP – D (link access procedure – D channel) and at the layer 3 the digital subscriber signaling system no. 1 or DSS1 is used.

About The Components of ISDN

– As per the ISDN standards, many types of devices are connected to the network which are commonly called as TE or the Terminal Equipment and NTE or Network Termination Equipment.
– There exist some reference points that are used for defining the connections existing among the several equipment employed within the ISDN network.
– Various types of TE and NTE are discussed below:

1. TE1 or terminal equipment type 1: This device can be plugged in directly in to the network and works well on the ISDN standards.
2. TE2 or terminal equipment type 2: Predating the ISDN standards is the major purpose of these devices and make use of a TA or a terminal adapter for plugging in to the network. These may be routers having only one serial interface instead of an ISDN WIC. This adapter plugs in to the interface allowing the router to connect to the network. PCs can also act as TE2.
3. NT1 or network termination 1: This device belongs to the customer and the physical layer specifications are implemented by it in to the network. The U reference point is nothing but this device only used for connecting to telco. Layer 1 is the level at which this device operates in the OSI model.
4. NT2 or network termination 2: This is the typical telco device and is involved with the termination of the NT1 device before the ISDN network is hit by the traffic. These devices can operate at the third and second OSI layers and are quite at performing switching.
5. TA or terminal adapter: The signaling of a TE2 device is converted in to other kind of signaling that the ISDN switch would use.

Different reference points are:
1. R: This point specifies the point lying between the TA and TE2 device.
2. S: This point specifies the point lying between the NT2 device and router of the customer.
3. T: It specifies the point lying between the NT2 and NT1 Device. Both T and S points can be used for performing the same functions and are therefore referred to as the S/ T reference point.
4. U: This point specifies the point lying between the Telco’s TE and the NT1 device in the ISDN carrier network. The customer owns the U interface. The telco is responsible for the maintenance of the NT1 devices.

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