Telecom Stratum Levels

In telecommunications, Stratum Levels define the synchronization hierarchy specified in ANSI T1.101. These levels classify the accuracy, stability, and holdover performance of network clocks used throughout TDM systems, SONET/SDH transport, mobile networks, PBX's, and other timing-dependent infrastructure.

Stratum Levels establish a tiered clock hierarchy in which each device synchronizes to a clock of higher quality. This ensures that timing is consistently distributed across the network. Telecom Stratum Levels are not the same as NTP Stratum values which are based on logical hop distance.

Stratum levels ensure every device is in sync with devices above it, and that the network remains stable even if part of it loses connection to the master clock.

• these slip rates assume the device is in holdover mode (eg, not locked to a higher-stratum clock)
• when properly synchronized, slip rates should approach/become zero, regardless of stratum
• 'slip' means a 1-frame error on a synchronous TDM system due purely to clock mismatch

A Stratum 1 clock may control strata 2, 3E, 3, 4E, or 4 clocks. A Stratum 2 clock may drive strata 2, 3E, 3, 4E, or 4 clocks. A Stratum 3E clock may drive strata 3E, 3, 4E or 4 clocks. A Stratum 3 clock may drive strata 3, 4E or 4 clocks. A Stratum 4E or 4 clock is not recommended as a source of timing for any other clock system.

If a device loses its upstream timing reference, it enters holdover mode. The stability of the internal oscillator determines how long it can maintain correct timing.

Timing Faults:

• Slip- A frame lost or repeated due to timing drift
• Jitter- Short-term, rapid timing variations
• Wander- Long-term, slow timing drift
• Clock failure- Oscillator stops or behaves unpredictably
• Timing loop- Two devices clocking from each other (must be avoided)
• Reference loss- Primary or secondary reference becomes unavailable or unstable

A properly engineered Stratum hierarchy minimizes drift, eliminates timing loops, and keeps all network elements phase-aligned.

Nortel/Avaya PBX Clocks:

• Nortel Meridian Clock Controller (CC) card QPC775C, or CS1000 NTDW12AAE5 (CC) daughterboard provide Stratum 3 clocking.
• Avaya CS1000 NTDW12AAE5 clock controller daughterboard replaces both the NTAK20ADE5 (Stratum 3) and NTAK20BDE5 (Stratum 4).
• NTDW12AAE5 daughterboard can only be mounted on the NTDW79 PRI card. It cannot be mounted on previous T1 or E1 digital trunk cards.

A clock controller will attempt to synchronize with the primary reference source (PREF), which should ideally have a higher Stratum level. If the primary reference is unavailable or unstable, it will switch to the secondary reference (SREF) if defined. If both references are unstable or lost, the controller enters free-run/holdover mode.

Telecom Stratum Levels (ANSI T1.101)

Stratum 1 - Primary Reference Source (PRS)

• The top of the synchronization hierarchy, providing the highest level of clock accuracy
• Autonomous, and extremely stable; serves as the master timing reference for the entire network
• Historically implemented with cesium beam atomic standards, and today often supplemented or replaced by GPS-disciplined oscillators (GPSDOs)
• Usually only a small number of Stratum 1 sources exist across a carrier’s national or regional backbone

Stratum 2 - High-Quality Network Clock

• Used in major central offices and timing distribution nodes
• Common in SONET/SDH line equipment and regional timing hubs
• Requires a very stable Oven-Controlled Crystal Oscillator (OCXO) or Rubidium (Rb) oscillator
• Provides synchronization to Stratum 3 and below

Stratum 3/3E - Standard Network Element Clock

• Found in mid-tier network elements such as SONET/SDH ADMs, digital cross-connects, and legacy T1/E1 equipment
• Typically implemented using a high-quality Temperature-Compensated Crystal Oscillator (TCXO)
• Adequate for access and regional network timing when locked to higher strata
• 3E has improved holdover, and is typically the minimum acceptable clock quality for modern networks

Stratum 4/4E - Customer Premises Equipment Clock

• Lowest tier; used in end-user equipment such as PBXs, CSUs/DSUs, and various CPE
• Typically based on a standard Quartz Oscillator (XO)
• Should not provide timing to the network; must lock to higher-level sources to avoid slip and wander
• 4E has improved holdover, but is still low in the hierarchy

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