Allied Telesyn International Corp TurboStack AT-S6 User's Guide download pdf (Page 60)

Fault Tolerance

Department Concentrator Reconfiguration

Each module in a department concentrator of multiple hubs knows its own

slot position within the chassis, and which other slot positions are

occupied. From this information, each module determines whether it

should act in the capacity of a master, backup, or slave. This process is

called role determination.

Role determination occurs as soon as you power up or reset the

department concentrator. Each module then begins to operate in its

appropriate capacity. In addition, all of the modules constantly check the

conﬁguration of the department concentrator. If anything changes, role

determination is repeated to make sure each module is performing its

appropriate function. If you remove or add any modules, the entire

department concentrator readjusts itself to the new conﬁguration as

necessary. Conﬁguration changes are detected and the new role

determination performed in less than two seconds.

If you remove the master, role determination causes the backup to take

over as the new master. The module below the old backup (now the new

master) takes on the role of a new backup. This process repeats if the new

master is itself removed. Similarly, removing a backup causes a new

module (the next module below the master) to take over as backup.

If you add a new master, that is, if you insert a new module above the

current master, role determination causes the current master to

relinquish control to the new module and to take on the role of backup; the

previous backup, if any, then becomes a slave. Similarly, inserting a new

module in an empty slot between the master and the backup causes the

new module to take over as the department concentrator’s backup.

Note that the AT-TS12FS/XX, AT-TS24TS, and the AT-TS24TRS models

are manufactured as slave modules and can never assume the master

function.

Module Failures

The masters and backups frequently exchange “handshakes” in order to

verify proper operation. If the backup does not receive a handshake from

the master after about two seconds, the backup concludes that the master

has failed and then takes over the master’s functions.

This takeover process is not the same as the role determination process

described previously. In the case of the master’s failure (not removal), the

backup can take over the master’s functions, but the chassis does not

allow another module to take over as a new backup. Therefore, only a

single level of redundancy exists in cases where the master fails but is not

removed from the chassis.

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Allied Telesyn International Corp TurboStack AT-S6 User's Guide Page 60