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SYNCHRONIZATION OF DIGITAL FACILITIES
DIGITAL TRANSMISSION FACILITY
PRIMARY FREQUENCY REFERENCE
SECONDARY (BACKUP) FREQUENCY REFERENCY
Figure 5-14. Improper Use of Backup Facilities
Availability is defined as the ratio of the mean time between failures (the average time between
successive system failures abbreviated MTBFs) to the sum of the MTBFs and the mean time to repair
(the total maintenance time divided by the total number of failures during the same amount of time
abbreviated MTTR). Low availability (poor transmission performance) is used as a criterion to reject
a facility as a reference candidate or to reassign one already chosen.
Characteristics that primarily determine the availability of a transmission facility are rank-ordered as
follows:
1.
2.
3.
4.
5.
6.
7.
8.
History record
Activity
Facility length
System rate (that is, whether the facility supports T1 or fiber)
Protection switching
Physical type (that is, whether the facility is physically installed as an underground, buried, or
aerial paired cable, a coaxial cable, or a radio link). (Satellite facilities should not be used to
supply synchronization.)
Number and type of regenerative repeaters
Number of digital terminal products and other intermediate office equipment, if any