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Configuring the Backup D Channel

Purpose

This section describes an optional ISDN configuration for the backup D channel.

Before you begin

Download and assign custom PPL protocols and modify timers and configuration bytes as required.

D channel backup supported

ISDN cards support the D Channel backup procedure as defined by Lucent TR 41459. This procedure provides a standby D channel when using NFAS. When an active D channel fails, or if there is a span alarm, the CSP converts to the standby D channel and maintains active calls. This is a provisioned option and is applicable only when using NFAS. D Channel backup is supported on the T1 cards only.

Both D channels must be assigned to the same ISDN card. When a span fails, a D channel fails, or a T1 card is removed, a D channel switchover occurs (not a card switchover).

As both D channels must be assigned to the same ISDN card, there are no extra hardware requirements to enable D Channel Backup. You can logically assign spans on any T1 card in the system.

Configuring D channel backup

To configure the backup D Channel, perform the following steps:

Step | Action
--- | ---
1 | Use the D Channel Assign message to configure the type and location of the D channel. After the primary D channel is configured, configure the backup D channel (using the D Channel Assign message again). Include the span, channel, and facility number on which it resides.
2 | Send the D Channel Facility List Configure message to all other facilities you want to include in the NFAS arrangement. You can also send this message for the same facility that was assigned in the backup D channel assignment. The backup D channel cannot be de-assigned; it becomes de-assigned when the primary D channel is de-assigned.
3 | Use the second facility (1) to configure the backup D channel; (however, 1-19 are permissible).
4 | Bring spans and channels into service. It is not necessary to bring the backup D channel into service. It automatically comes into service when the primary D channel is brought into service.
5 | When the D channels align and process the SERVICE/SERVICE ACK exchange, the host receives a DS0 Status Change message (indicating "In-Service") for the primary D channel.

Call Processing

When one of the D channels becomes active, both incoming and outgoing call processing can begin. Only the active D channel allows information frames to progress to the Layer 3 Channel Released component 8 (where calls are managed). On a D channel switchover, only calls in the Active State (10) are preserved; all other calls are cleared. Messages are dropped in the Wait State when the switchover occurs. The host receives Channel Released messages for calls in transit.

Manual D Channel Switchover

You can initiate the switchover of D channels manually by sending a PPL Event Request message to the L3 D Channel Control component and the D channel to which you want to be switched.

Important! You cannot initiate a switchover by taking the primary D channel out-of-service or the span on which it resides.

Compliance

The CSP software is compliant with the Primary Rate Interface guidelines defined in Lucent TR 41459 specification, as follows:

Layer 2

Layer 2 provides information frames to Layer 3. For both D channels, all unnumbered, supervisor, and information frames are processed as described in the Lucent specification. Since the protocol is run over primary rate, the SAPI and TEI are both 0. Data links do not know if the D Channel Backup is enabled or disabled.

Layer 2 is managed by the following Layer 3 D Channel Control messages (primitives):

• DL Establishes Request

• DL Remove Request

• DL DM Remove Request

• DL Data Request

Layer 3

Layer 3 complies with Lucent specification TR41459. The L3 D Channel Control component (0x0A) handles the D channel backup logic. Each state represents both D channel states. The L3 DSM manages the following:

• Distribution of incoming and outgoing information frames to the other components in Layer 3

• The switchover procedure (sending SERVICE and SERVICE ACK messages on the D channel)

• Distribution of the indication to appropriate component to preserve active calls

• Reporting of D channel failure

• Reporting status to the L3P D Channel Control component (0x06)

• Reporting status to the host using the PPL Event Indication message

When a D channel changes state, the host receives one of the following events in the PPL Event Indication message:

• D is Active (0x00001)

• D is Standby (0x0002)

• D is not aligned (0x0003)

The D channel failure event is sent from L3 D Channel Control component to the L3P D Channel Control component only when both D channels are not aligned. L3P does not know the state of both D channels; it only knows that at least one D channel is accessible, or that both D channels are not.

Any call not in Active State gets cleared on a switchover. The host receives a Channel Released message for the bearer channel to which the call was in transition.

Layer 3 Plus

L3P automatically requests a switchover when either span goes into an alarm condition. The L3P B Channel Control component informs the host about channel service states. When both D channels are not established, it resends the Alarm message to the host to indicate the establishment of a D channel.

See Integrated Services Digital Network.
See Programmable Protocol Language.
Data channel. One of two fundamental components of an ISDN interface, it provides a path (16Kbps for BRI, 64Kbps for PRI) for carrying control signals and call data in a packet-switched mode.
See Non Facility Associated Signaling.
Digital communications link - voice, data, and video at a 1.544 Mbps rate (24 64 Kbps channels)
Positive acknowledgment to a message
Protocol layer of the CSP.
The condition of a component at a given instant. The current "context" of a PPL object, such as Idle, Seizing, Answered. In PPL files, a State is represented by a circle.
A condition that, when met, drives a state machine from one state to another. A condition that drives a Component out of its current state by invoking an associated primitive.
PRI. One of two basic ISDN interface configurations providing 23 (30 outside of North America) Bearer channels (64 Kbps data) and 1 Data signaling channel (16 Kbps control and signaling data). Commonly refered to as ISDN PRI or 23B+D interface (30B+D outside of North America).
In PPL, a condition that, when met, drives a state machine (PPL Component) from one state to another, by invoking an associated Primitive. In PPL files, an Event is marked by an arrow and text.
Bearer channel, 64Kbps (ISDN) The channel that carries the actual payload (as opposed to D-channels, which carry signaling data that helps route the payload).