The Isdn Layers

It is difficult to apply the simple seven layer architecture specified by the OSI to the ISDN. One reason is that the ISDN specifies two different channels with different functionalities. B channels are for user to user communication.

D channels are predominantly for user to network signaling. The subscriber uses the D channel to connect to the network, then the B channel to send information to another user. These two functions require different protocols from each other at many of the OSI layers.

The ISDN also differs from the OSI standard in its management needs. A primary consideration of the ISDN is global integration. Maintaining the flexibility required to keep the network truly integrated using public services a great deal of management.

Instead of single seven-layer architecture like the OSI, the ISDN is defined in three separate planes, the user plane, the control plane, and the management plane. The user plane defines the functionality of the B channel and H channel. The control plane defines the functionality of the D channel when used for signaling. The management plane encompasses both the user and control planes and is used for managing the whole network.

All three planes are divided into seven layers that correspond to the OSI model. When a D channel is used for data, it behaves like a B channel. Eventually D channels will be used for services like telemetry, but the protocols that will make those services possible are still under study. D channel is used primarily for user-to-network signaling. Its functions are therefor confined to the first three layers. Layers 4 to 7 are concerned with end to end user signaling.

At the physical layer, the B and D channels are alike. They use either the BRI or PRI interface. At the data link layer, the B channel uses LAPB or some version of it. At the network layer, the B channel has many options. B channel can connect to circuit switched networks, packet switched networks, frame relay networks, and ATM networks, among others. The user-plane options for layers 4 to 7 are left to the user and are not defined specifically in the ISDN.

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The ISDN physical layer specifications are defined by two ITU-T standards. These standards define all aspects of the BRI and PRI. Standards which are of primary importance are the mechanical and electrical specifications of interfaces R, S, T, and U, and encoding, Multiplexing channels to make them movable by the BRI and PRI digital pipes, and power supply.