ISDN Architecture


This section describes the modules that form the ISDN PRI software architecture. These modules are:

Layer 3 Plus

Layer 3

Layer 2




ISDN PRI Software Architecture illustrates the functional modules involved in the PRI implementation on an ISDN card.

Figure 8-5 ISDN PRI Software Architecture


Layer 3 Plus

The Layer 3 Plus (L3P) module is an interface between Layer 3 and Layer 4 Central Call Processing (CCP) on the Matrix Controller, and Layer 5 (host). L3P formats information from Layer 3 into the programmed format for the host. It also manages application specific variants for call control. It is made up of the following PPL components:

L3P Call Control [Component (0x05), per D channel]

This component is the interface between Layer 3 and Layer 4 central call processing, which resides on the Matrix Controller. This component manages which messages to send internally for call processing as well as notifying the host of various events related to the protocol.

L3P D Channel Control [Component (0x06), per D channel]

This component manages the enabling/disabling of the D channel per terminal (for PRI ISDN there is only one terminal). It manages the sending of the Alarm message when the D channel is establishing a connection, and informs the L3P B channel control component of the availability of D channel.

L3P B Channel Control [Component (0x07), per B channel)

This component manages the service state of the channel based upon the following:

Host service states

• Layer 1 states (span framed or not)

• D channel availability

• B channel availability from the network

Layer 3

The Layer 3 (L3) module provides ITU-T Q.931 implementation. The L3 state diagrams mirror the SDL diagrams of the interface specifications. It consists of the following PPL components:

L3 Call Control [Component (0x08) per D channel]

This component implements Q.931 variants, which has state tables that are SDL equivalents with some validation logic embedded. This component is managed per call reference, and manages B channel bearer capability as well as B channel allocations.

L3 Global Call Control [Component (0x09) per D channel]

This component manages the protocol for the restarting logic. It implicates active calls per a channel RESTART message, ACKs them, and then arbitrates sequencing with the B channel control components.

L3 D Channel Control [Component (0x0A) per D channel]

This component manges the D channel availability as defined by the Data Link Connection state diagram in ITU-T Q.931. It communicates with the L3P DSM upon D channel failure.

L3 B Channel Control [Component (0x0B) per B channel]

There is one state machine per B channel to manage the SERVICE messages for the protocol using the protocol discriminator (0x03). This state machine manages availability for the B channels as well as in-use status.

Layer 2

The Layer 2 (L2) module is the ITU-T Q.921 (LAPD) implementation. It performs flow control, sequence numbering (module 128), retry logic, and it also guarantees message delivery.


The Configuration (CFG) module manages all configuration of parameters and options for the ISDN protocol. This task communicates with the Matrix Controller during configuration. It also verifies battery backed data and card level configuration.


The Management (MGMT) module performs terminal endpoint identifier (TEI) management logic for Basic Rate Interface (BRI) access and it synchronizes Layers 2 and 3 correctly. It also acts as an alarm manager for the L2 protocol.