ISDN PROTOCOL ARCHITECTURE

           ISDN has two different layering mechanisms, one for the B/H channels and the other for the D channel. For the B and the H channel, only layer 1 or the physical layer protocol is defined and all the other six layers of the OSI model are left to the applications running on these channels. This is because the B and H channels are just a physical pipe for transferring of user data at the rate of 64kbps and N*64kbps respectively. Therefore, only the physical layer specifications (containing the electrical characteriestics, the line coding schemes etc.) are defined for these channels, thereby paving a way for a wide variety of applications to run on it.

                                   ISDN has a three layered protocol architecture defined for the D channel. Whereas for the B Channel, only Physical layer is defined (I.430) and the rest of the upper layers are dependant on the application.


 
The three protocol layers for the D Channel are
 

Layer 1 gives the specifications for the physical connection between the TE and the NT, including the connector, line coding scheme, framing and electrical characteristics.The physical connection is synchronous, serial and full-duplex. It may be point-to-point (BRI and PRI) or point-to-multipoint (BRI only). The D and B channels share the physical medium using TDM.  The protocol defines the physical connection between ISDN TE (TE1 or TA) and network termination equipment (NT2 or NT1). ITU-T ISDN recommendations do not describe the physical connection between the NT1 and the LE because the transmission line is considered to be internal to the network.

Layer 2  D channel protocol is called LAPD (or Q.921) and it defines the procedures for ensuring error-free communication over the physical link. It defines the logical connection between the user and the network. The protocol also provides rules for multiplexing multiple TEs on a single physical channel (multi-point) on the BRI environment.

Layer 3 D channel protocol (Q.931) defines the signalling and call control messages used to request from the services. These messages are carried in the Information fields of LAPD frames, which are transmitted bit by bit across the physical link.
                   The layer 2 and layer 3 protocols define the logical link and signalling protocols, respectively, between  ISDN TE (TE1 or TA) or customer premises switching equipment (NT2), and the LE. The NT1 provides only a layer 1 service and , therefore, layers 2 and layers 3 are transparent to it. It is important to note that the layer 2 and layer 3 protocols are specified only  on the D channel. The user may choose any protocol for the bearer services and teleservices on the B or H channels. All channels share the same Physical layer standard since B, H and D channels are time division multiplexed on the same physical line.