SyncE: frequency synchronization in Ethernet

SyncE: Frequency Synchronization for Ethernet Based on SDH

SyncE provides frequency synchronization in Ethernet networks by replicating the SDH architecture. The technology uses the G.8261 standard for architecture and frequency drift, G.8262 for compatible clock signals, and G.8264 for ESMC—the synchronization messaging channel. In Ethernet, the reference frequency is distributed through a PLL chain, similar to SDH multiplexers.

The physical layer of SyncE relies on a 19.44 MHz PLL, where 19.44 MHz × 8 bits = 155.52 Mbps STM-1. This allows scaling up to STM-4/16/64/256 and WAN-PHY. In Juniper logs, the '19.44 MHz clock failure' error indicates a failure in this PLL.

Synchronization Architecture from SDH

SDH requires synchronization for multiplexing VC-12 into STM-1. The reference frequency from a PRC (stability 10^{-11}) or SSU (10^{-8}) is fed to the central multiplexer. The PLL adjusts to the signal, with SSM in headers transmitting quality status.

Downstream nodes check SSM and settings, adjusting their PLL to the incoming sinusoidal signal. SSUs clean jitter/wander in long chains. The scheme:

• PRC/SSU → central node → PLL → SSM in headers → cascade of nodes.

Inter-operator connections at STM require a hierarchy: the main network sets the frequency, subordinate networks synchronize. On E1/PDH, a plesiochronous mode with buffering is possible.

Types of Synchronization in Networks

Synchronization aligns frequency, phase, or time of day (ToD):

• NTP: only ToD.
• SyncE: only frequency (±4.6 ppm from PRC).
• PTP: frequency (weakly), phase (accurately), ToD.

In asynchronous Ethernet, generators (±100 ppm) produce different pulse durations: 1 Gbps = 1.00001 or 1.00002 Gbps. SyncE/PTP ensures a uniform ratio for consumers (FDD base stations, SDH/PDH).

Phase synchronization (PTP) is needed for TDD LTE/5G, stock exchanges, and TV cameras. Clock = frequency generator; phase = time ≠ ToD.

Drift calculation:

100 ppm = 0.0001 → per day: 8.64 s
4.6 ppm = 0.0000046 → per day: 0.4 s

SyncE Components in Ethernet

SyncE: ESMC messages + synchronization signal. SDH-like architecture:

• PRC: primary reference clock (±4.6 ppm, GPS/GLONASS), in the core.
• SSU: secondary supply unit, regenerating the signal at nodes.
• EEC/SEC: slave clocks of routers/multiplexers.

Limit: ≤20 EECs without an SSU. Standard Ethernet: internal generators ±100 ppm. SyncE: from PRC ±4.6 ppm.

Key Takeaways

• SyncE is a direct adaptation of SDH for Ethernet: 19.44 MHz PLL, SSM/ESMC, topology with PRC/SSU/EEC.
• Frequency synchronization (±4.6 ppm) is essential for FDD/5G base stations, PDH/SDH.
• Phase synchronization (PTP) complements for TDD, finance, video; without SyncE, PTP is inefficient.
• PLL errors are visible in logs (Juniper: 19.44 MHz failure); chain limit is 20 nodes.
• Inter-operator hierarchy resolves frequency conflicts.

— Editorial Team