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B-6 SYNCHRONIZATION OF DIGITAL FACILITIES
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The public digital network nodes and services that the AT&T private digital switches and digital terminal
products can connect to are as follows:
• DDS
• Digital serving office (DSO), also called a digital toll office, such as a 4ESS
• Digital central office (DCO) such as a 5ESS
• Digital-access and cross-connect system (DACS)
The AT&T private digital network nodes are the following:
• System 75
• System 85
• DEFINITY Communications System Generic 1
• DEFINITY Communications System Generic 2
The digital terminal products include the following:
• D4- and D5-channel banks
• Channel-division multiplexer (CDM)
• Bit-compression multiplexer (BCM-32000)
• Digital data multiplexer (DDM-1000)
All public and private digital network nodes have internal clocks. Figure B-3, Stratum Levels for the
Synchronization Hierarchy, shows the synchronization hierarchy and the node’s internal clock stratum
level.
Each node is compelled to take its timing from the following:
1. A higher stratum level
2. A node equal to its own stratum level
3. A free-running timing clock (generated internally) that meets or exceeds the requirements for its level
The minimum clock accuracy for each stratum level is as follows:
• Stratum 1 (± 0.00001 ppm, most accurate)
• Stratum 2 (± 0.016 ppm, more accurate than stratum 3)
• Stratum 3 (± 4.6 ppm, more accurate than stratum 4)
• Stratum 4 (± 32 ppm, least accurate)
NOTE: When a hierarchical public digital network is implemented (and when full network
synchronization is obtained), each node clock, regardless of its stratum level, will have an average
frequency identical to its master and to the AT&T standard reference frequency.