Synchronization in (telecom) networks is the process of aligning the time scales of transmission and switching equipment, so equipment operations occur at the correct time and in the correct order. Synchronization requires the receiver clock to acquire and track the periodic timing information in a transmitted signal.
The transmitted signal consists of data that is clocked out at a rate determined by the transmitter clock. Signal transitions between zero and peak values contain the clocking information and detecting these transitions allows the clock to be recovered at the receiver. The recovered clock is used to write the received data into a buffer, also called elastic store or circular shift register, to reduce jitter. The data is then read out of the buffer onto a digital bus for further multiplexing or switching.
Synchronization at AimValley
Synchronization is a fundamental technology building block within AimValley for many years already. Practically every service provider network requires it, not because networks require the synchronization but mainly to provide, at the edge of the network, a frequency or phase/Time of Day synchronization singal to end-customers.
The performance of the traditional wired T1/E1, SONET and SDH networks are based upon constrained clock control theory and PLL design and deliver a frequency synchronization to the edge of the network. Also Circuit Emulated Services (CES) require frequency synchronization at the edge of the network to recover the emulated service via Differential Clock Recovery (DCR) methods.
In today’s networks end-equipment, like 4G and 5G base stations, require Time of Day (ToD) synchronization which allows to synchronize all base stations to a common wall-clock. Also standards like Audio Video Bridging (AVB) and Time Sensitive Networking (TSN) require ToD synchronization at the edge of the network to overcome Packet Delay Variation (PDV) and recover signals based on a provisioned ‘presentation time’.
The synchronization signal is disciplined by electronic circuits to provide the frequency accuracy, stability, phase movement control, wander filtering and edge jitter (phase noise) levels specified in volumes of various Standards documents. Each ‘end customer service’ has a specific set of performance requirements detailed in these documents. This performance is defined by the various international standards bodies and is well understood.
AimValley provides a wide range of expertise regarding synchronization. From helping you to interpret and understand the various Standards to architectural support.
Synchronization Standards
Within the synchronization architecture a vast number of standards have been defined:
- ITU standards for SONET/SDH/OTN
- IEEE1588 standard for PTP ToD synchronization
- ITU-T G.826x for frequency synchronization
- ITU-T G.827x for Time of Day synchronization
Precision Time Protocol (PTP)
Precision Time Protocol (PTP), specified in IEEE 1588, is a protocol used for synchronizing clocks across networked devices, with nanosecond-level accuracy. It’s commonly used in industries requiring precise timing, such as telecommunications, finance, and industrial automation. Here’s how PTP works in Ethernet switches and routers, as well as its key specifications:
How PTP works
PTP operates by exchanging timing messages between devices to establish a synchronized reference time. It involves a few key components:
Master and Slave Clocks
PTP operates with a hierarchical clock structure.
The Grandmaster clock is the main time source, usually highly accurate (i.e. synchronized with GPS)
Slave cocks receive timing information from the Grandmaster or another upstream clock.
Timing Messages
Devices exchange a sequence of time-stamped messages to synchronize the clocks.
Sync Message: the master clock sends a Sync message to the slave clock, which contains a timestamp of when it was sent.
Follow-up Message: the master sends a follow-up message with an accurate time stamp.
Delay Request and Delay Response: the slave sends a Delay Request message to the master, which responds with a Delay Response. These two messages help the salve calculate network delays and offset.
Clock Adjustment
The slave clock calculates the difference (offset) between its own clock and the master clock, then adjusts its time accordingly.
Boundary and Transparent Clocks
Switches and routers can either actively participate in the timing mechanism or just pass along PTP messages.
Boundary Clock: participates in the PTP protocol by synchronizing with a master clock and serving as a master to downstream devices. This helps to avoid errors due to network delays.
Transparent Clock: switches or routers do not directly participate in clock synchronization but instead measure and correct for the delay they introduce in PTP messages. They modify the timing information, known as correction fields, to improve accuracy.
Key PTP Specifications
Here are some important aspects and specifications from IEEE 1588 (PTP):
Accuracy
PTP can achieve sub-microsecond to nanosecond accuracy, depending on network configuration, hardware support, and clock stability.
Operating Modes
One-Step Mode: all time information is included in the Sync message, reducing the need for a separate Follow-up message.
Two-Step Mode: the Sync and Follow-up messages are separate, which improves accuracy for certain networks.
Synchronization Types
End-to-End (E2E): measures delay between master and slave clocks through delay request/response message.
Peer-to-Peer (P2P): measures delay only between directly connected devices, ideal for networks with transparent clocks.
Profiles
PTP defines various profiles tailored for specific applications to optimize performance.
Telecom Profile: used for telecom networks to support synchronized data across mobile networks.
Power Profile: common in the energy sector for synchronizing power grids.
Enterprise Profile: for use in data centers and local enterprise networks.
Hardware Support
Ethernet switches and routers supporting PTP often have dedicated PTP hardware, allowing for timestamping directly on network interfaces. This reduces timing jitter and improves accuracy.
Network Design
For high accuracy, PTP often requires carefully designed networks with limited hops, minimal network jitter, and either Boundary or Transparent clocks.
In summary, PTP provides high-precision timing synchronization by leveraging master-slave hierarchies, timestamped message exchanges, and dedicated hardware in networking devices.
AimValley Synchronization Expertise
SONET/SDH/CES/CEP
- Timing architecture of various systems
- Core ASICs used in various systems
- SEC/Stratum-3 built into AimValley ASICs (Maximux, Matterhorn)
- IP blocks with Circuit Emulation Services (FPGA)
- Smart SFPs with CES (E1, DS1, 155Mb/s, 622Mb/s, 2488Mb/s
Ethernet Switches
- Architecture and implementation of products
- EEC-1/EEC-2 clocks
- IEEE 1588 TC, BC, BMC
- Calculation of PLL bandwidth for synchronization function
Audio Video Bridging & Time Sensitive Networking
- Standard 802.1AS
- Standard 802.1Qat
- Standard 802.1Qav
