This variable indicates the type of the system timing generated by the module. If set to loopA/loopB, the module will provide packet based timing references to the system. The module can only be configured to provide either loopA or loopB timing reference and not both. Some access modules can provide card level packet based timing reference to the system. These reference sources are referred as Loop A and Loop B packet references. Only two such sources, Loop A and Loop B, should exist within the system at any given time. The source(s) for the card level timing is provisioned on each access module that desires to provide timing. Separate OIDs are used for this provisioning. The phase difference between the system backplane timing and the card timing reference is then computed by the detector hardware. The access module reads the phase difference value from the detector and formats a timing packet for transmission to the system.