- What is time synchronization and how does it work?
- Which solutions exist for telecom networks and attached networks?
- Which accuracy and stability is required?
- Why should manufacturers and operators of equipment for telecom networks care about these terms?
This document has been created to answer these questions and offers you
a comprehensive and easy-to-understand introduction. Read on ...
Many Thanks to Mr. Gerd Backhaus for his friendly support at this publication!
Diagram of Clock Stability and Accuracy
Communication requires excellent quality of service in the network!
Synchronization equipment, planning and installation account for 1% of the overall project equipment costs. Synchronization must be considered and planned from the very beginning of a project. If the manufacturer cannot deliver synchronization equipment, he must inform the customer so that the customer can take steps to:
- Identify appropriate synchronization equipment
- Plan system synchronization appropriately
- Seek a Time & Frequency Specialist if necessary
After an inventory of the hardware available, a preliminary network overview is made including the available timing reference sources. This can be used to plan the synchronization of switching devices. For transmission network synchronization (SDH), a plan must be developed with a special sync planning tool.
In ITU-Recommendation G.811 (Timing Characteristics of Primary Reference Clocks) a network clock stability of 1 x 10 E-11
for the Primary Reference Clock (RPC) is recommended.
This accuracy is derived from 1 acceptable slip in a 2 MBit/s transmission in an observation time of 70 days.
1 (frame) = 125 us
125 us / 70 d x 24 h/d x 3600 s/h = 2 x 10 E-11 or +-1 x 10 E-11
This clock stability can be generated by atomic clocks only!
There are different possible approaches to achieve the required accuracy.
Different solutions are possible and can be realized using MEINBERG timing equipment.
with MEINBERG Equipment
Meinberg NTP Time Servers deliver a highly precise and stable clock for the system, and provide precise NTP time for the network.
For example, reliable time stamps are necessary for diagnosis of system error messages.
Synchronization in Telecom - Networks with MEINBERG NTP Server
LANTIME GPS - NTP Server
Examples of Use
Time Synchronization Terms and Glossary
SONET / SDH Technical Specs
SONET and SDH are related standards for synchronous data transmission via fiber optic networks. SONET is an acronym for Synchronous Optical NETwork, SDH is short for Synchronous Digital Hierarchy. SONET is the US version of the American National Standards Institutue (ANSI) standard. SDH is the international version of the International Telecommunications Union (ITU) standard.
SONET / SDH Digital Hierarchy
This table shows the SONET/SDH data rate hierarchy:
|Optical Level||Electrical Level||Line Rate (Mbps)||Payload Rate (Mbps)||Overhead Rate (Mbps)||SDH Equivalent|
The "line rate" describes the raw bit rate transferred via the optical fiber. A certain amount of the bit rate are the overhead, that includes information that provides OAM&P: Operations, Administration, Maintenance, and Provisioning information like framing, trace, multiplexing, status, and performance monitoring. The payload rate which is the available bandwidth for user data as packets or ATM cells is calculated by the line rate minus the overhead rate.
Other rates like OC-9, OC-18, OC-24, OC-36, OC-96 were never widely implemented. Maybe higher rates like OC-3072 will defined in future times.
The SONET/SDH level designations including a "c" suffix (e.g. "OC-48c") indicate a "concatenated" or "clear" channel. This shows, that the entire payload rate is available as a single channel of communications. In these channels the entire payload rate can be used by a single flow of packets. Opposed to the concatenated or clear channel is the "channelized". In a channelized link the payload rate is divided into several fixed rate channels. E.g. the payload of an OC-48 link can be subdivided into four OC-12 channels. In this scenario the data rate of a single cell or packet flow has its limitations in the bandwidth of the individual channel.
ANSI SONET Standards
The ANSI coordinates and approves the SONET standards. These standards are developed by the T1 Committee that is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) and accredited by ANSI to develop network and interoperability standards for the US. T1X1, dealing with "digital hierarchy and synchronization" and T1M1, dealing with "internetworking operations, administration, maintenance, and provisioning" are the primary T1 Subcommittees developing SONET.Below are some important standards from ANSI. For a complete set of SONET standards please visit the ANSI homepage.
- ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats
- ANSI T1.105.01: SONET - Automatic Protection Switching
- ANSI T1.105.02: SONET - Payload Mappings
- ANSI T1.105.03: SONET - Jitter at Network Interfaces
- ANSI T1.105.03a: SONET - Jitter at Network Interfaces - DS1 Supplement
- ANSI T1.105.03b: SONET - Jitter at Network Interfaces - DS3 Wander Supplement
- ANSI T1.105.04: SONET - Data Communication Channel Protocol and Architectures
- ANSI T1.105.05: SONET - Tandem Connection Maintenance
- ANSI T1.105.06: SONET - Physical Layer Specifications
- ANSI T1.105.07: SONET - Sub-STS-1 Interface Rates and Formats Specification
- ANSI T1.105.09: SONET - Network Element Timing and Synchronization
- ANSI T1.119: SONET - Operations, Administration, Maintenance, and Provisioning (OAM&P) - Communications
- ANSI T1.119.01: SONET: OAM&P Communications Protection Switching Fragment
ITU-T SDH Standards
The International Telecommunications Union (ITU) (formerly known as the CCITT), coordinates the development of the actual SDH standards. It is financed by the United Nations and coordinates the development of telecom standards for the whole world. Below are the most important SDH standards available from ITU. Please visit the ITU homepage for a list of the complete SONET standards.
- ITU-T G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)
- ITU-T G.781: Structure of Recommendations on Equipment for the Synchronous Digital Hierarchy (SDH)
- ITU-T G.782: Types and Characteristics of Synchronous Digital Hierarchy (SDH) Equipment
- ITU-T G.783: Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks
- ITU-T G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)