d-channel backup processing -...

Part Number OL-5466-01 Rev. A0, January 2004

A P P E N D I X C

nd aof theationd ine a

NFAS

r-endnnelThe

rs.

s) areOS

D-Channel Backup Processing

The NFAS option provides D-channel redundancy when both the in service (IS) D-channel fails abackup D-channel is defined for the NFAS group. This appendix provides graphic representationsstate transitions occurring on the primary D-channel and backup D-channel during channel initializand switchover. Channel states are displayed on the NFAS Group Configuration screen describeChapter 5, “Non-Facility Associated Signaling (NFAS).” The figures in this appendix do not providcomplete description of D-channel backup processing. Only the typical states that appear on theGroup Configuration screen are represented.

Figure C-1 represents D-channel initialization as the system attempts to establish a link with the faconnected equipment. During initialization, the system always attempts to bring the primary D-cha(D1) into the in service (IS) state before attempting to make the backup the controlling D-channel.solid lines in Figure C-1 indicate this preferred initialization path.

Figure C-2 demonstrates the state transitions during automatic and manual D-channel switchove

Note Figure C-2 assumes that the primary D-channel (D1) is currently in service (IS) while thebackup D-channel (D2) is in standby (STBY) state. Switchover processing between the ISchannel and the STBY channel is the same regardless of which channel (primary or backup)is currently in IS state.

In both figures, channels that are in IS state (supporting stable calls and allowing new call attemptshown in white. Channels that are unable to support new call attempts (in STBY, WAIT, MB, or Ostate) are shaded.

C-1Cisco VCO/4K ISDN Supplement

Appendix C D-Channel Backup Processing

Figure C-1 Redundant D-Channel Initialization

System brings backupinto STBY when thebackup D-channel (D2)is established

System brings primaryinto STBY when theprimary D-channel (D1)is established

SERV or SERVICEACK message receivedfrom far end on primaryD-channel (D1)

SERV or SERVICEACK message receivedfrom far end on backupD-channel (D2)

Backup D-channel(D2) established.System sends SERVmessage on backup.

PrimaryD-channel

(D1)

offline

BackupD-channel

(D2)

offline

Primary D-channel(D1) established.System sends SERVmessage on primary.

Primary D-channel(D1) established.System sendsSERV message onprimary.Primary

D-channel(D1)

offline

BackupD-channel

(D2)

WAIT

PrimaryD-channel

(D1)

WAIT

BackupD-channel

(D2)

MB

PrimaryD-channel

(D1)

offline

BackupD-channel

(D2)

online

PrimaryD-channel

(D1)

online

BackupD-channel

(D2)

offline

PrimaryD-channel

(D1)

STBY

BackupD-channel

(D2)

online

PrimaryD-channel

(D1)

online

BackupD-channel

(D2)

STBY

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Figure C-2 Channel States During D-Channel Switchover

T321 timer expiresbefore SERV orSERVICE ACK

received from far end,or no backup definedfor NFAS group.

Backup D-channel supports stable and new calls. System brings primaryinto STBY when the primary D-channel (D1) is reestablished.

SERV or SERVICE ACK message received from far end on backupD-channel (D2)

Primary D-channel (D1) fails or the system administrator initiates amanual switchover. T321 (40 seconds) timer started. Stable calls maintained;new call attempts blocked. System sends SERV message on backup.

PrimaryD-channel

(D1)

online

BackupD-channel

(D2)

STBY

Stable calls dropped;new call attempts blocked.

PrimaryD-channel

(D1)

MB

BackupD-channel

(D2)

WAIT

PrimaryD-channel

(D1)

offline

BackupD-channel

(D2)

ofline

PrimaryD-channel

(D1)

offline

BackupD-channel

(D2)

online

PrimaryD-channel

(D1)

STBY

BackupD-channel

(D2)

online

PrimaryD-channel

(D1)

MB

BackupD-channel

(D2)

STBY

Send SERV on D2

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