# D channel backup

When NFAS is employed, the reliability of the signaling performance for the ISDN interfaces controlled by the D channel can be improved by employing a standby D channel: the D channel backup. The D channel backup feature allows a customer continued access to the ISDN network if one of the D channels fails by transferring most of the signaling information to the backup D channel.

The designated primary D channel (labeled D1) is always present on one trunk. A backup or standby D channel (labeled D2) is present on a different trunk:

At any point in time, only one of the D channels, D1 or D2, conveys B channel signaling information. The other D channel remains in a standby role and is active at the LAPD layer (layer 2) only. While the backup D channel is on standby, any layer 3 messages received on it are ignored.

Neither D1 nor D2 can serve as a B channel while designated as a backup D channel. Also, each D1/D2 pair provides signaling only for the set of B channels assigned to it, and cannot backup any other D channel(s) on a different interface.

When both D channels are out of service, D1 has priority as the channel to carry call control signaling. If D1 cannot be established, then D2 is chosen.

## Setting up D channel backup

The D channel to use as the backup is specified in the configuration file. For more information, see the NMS ISDN Installation Manual.

Note: Both primary and backup D channels must be defined on the same board and belong to the same NFAS group.

In order to preserve active call signaling information in the event of the D channel failure, the application can enable the t309 parameter defined in ISDN_PROTOCOL_PARMS_Q931CC or ISDN_PROTOCOL_PARMS_CHANNELIZED. This parameter enables timer T309 (described in the NSF IE structure).