Stratum 1 time Sync for TDD network

High availability and reliability of time and Frequency are very important in the telecom sector where the growing trend of Big Data is a challenge. Big Data carriers need to deliver a high-speed transport, real-time processing, and secure data storage. SDH and LTE mobile networks have placed even greater demands on network synchronization, where transmission quality is directly related to the accuracy and performance of network timing. Time synchronization via GPS, GLONASS, Galileo, BeiDou, PTP, NTP, SyncE, 1PPS or E1 guaranteed high reliability. Maintain accurate time and frequency is mandatory for a high performance of traffic is a must. Keeping any telecom facility running efficiently is dependent on accurate time and frequency. There are several mechanisms to carry timing information throughout the networks: SyncE (synchronous ethernet), BITS (Building Integrated Timing Supply), IRIG (Inter Range Instrumentation Group) time code type B (IRIG-B), 1PPS (1 Pulse per Second). All these technologies are dedicated timing signals requiring a physical connection specifically for timing. One exception is that SyncE can coexist in a physical connection of a packetize network; in other words, same port of an ethernet switch can implement SyncE and transport packets for shared physical link. A point to note here is that both NTP and PTP supports TOD, phase and frequency synchronization making them ideal for today’s packetized networks. NTP is less suitable for applications and network where sub nano seconds to microsecond accuracy is needed e.g., 5G TDD deployments such as CBRS, mmWave etc. Apart from dedicated timing signals, there are other packet base solutions for timing distribution as well, e.g., NTP (Network Transport Protocol) and PTP (Precision Time Protocol). Both protocols require no specific connection for timing and best suited for packetized networks. Since inception, PTP has gained increase attention due to the possibility of achieving sub nanosecond accuracy when used in conjunction with ePRS for primary clock source and SyncE to distribute Timing information. 

Mobile Network Operators are facing with unparalleled challenges to comply with stringent requirements for the synchronization or sync plane across the network. We are witnessing transport networks in two distinct parts: RAN (Radio Access Network) and Packet core. However, this transformation in technologies and the implementations thereof requires that timing information distributed in a precisely frequency and phase-aligned networks for advanced applications to function. The problem is that 5G requires precision time distribution due to TDD spectrums and time critical applications. In traditional 4G, time synchronization requirements are not as stringent as 5G and thus reliance on GPS and GNSS is good enough.

PRS - FUNCTIONAL OVERVIEW

SyncE (Synchronous Ethernet) is a good example of frequency plane timing distribution. Similarly, PTP (a protocol set defined by IEEE1588) timing distribution can be better understood by applying time/phase plane characteristics and requirements set forth in ITU-T recommendations G.8271 & G.8272. Please note, frequency and time/phase sync planes can be managed and routed independently. A sync loss in TDD network deployment such as CBRS could be disastrous. Similarly, all other time critical applications such as V2X and IIOT etc. would suffer catastrophic failure. This implies that TDD network need careful sync plane design, one that cope with GNSS/GPS sync loss and capable of providing high precision time distribution across the network from a PRS (Primary Reference source) or enhanced Primary Reference Time Clock (ePRTC).

PRODUCT BY STRATUM ONE

It is a stand-alone Primary Reference Source (PRS) with integrated GPS receiver. Configured with telecom clock outputs to meet the requirements of ANSI T1.101 and ITU-T G.811 for primary reference source operation. It is an Ideal solution for synchronizing systems that require a reliable T1/E1 signal. It has Two outputs of any combination of E1, T1 or Composite clock. It provides two to eight telecom outputs in a 1U chassis. It was specially Designed to provide high-stability Building Integrated Timing Supply (BITS) reference clock signals directly to digital equipment.

System can be securely managed using the serial or dual gigabit Ethernet ports. Meridian II PRS comes in 1U and 2U. It provides Frequency accuracy better than 6x10-14. Its Timing accuracy: < 10 nanoseconds RMS to UTC(USNO). Pulse Shape: Conforms to ITU-T G.703. It combines the benefits with accuracy, stability and holdover which reside at the upper most echelon in the industry, when configured with 5071A cesium clock module. Data are registered and monitored via various Meridian II PRS user interface SSH, HTTP, SNMP and front panel display. Serial port RS-232 of cesium clock module allow to connect with PRS for bi-directional data communication. BNC input connector and coaxial cable allow the 10 MHz output signal.

Key Features

Software option available at meridian II PRS

PTP IEEE 1588 - it enables Precision Time Protocol (PTP) grandmaster clock operation with eight nanosecond hardware time stamping. The PTP option can be installed on one or both ports and works in conjunction with the standard Network Time Protocol (NTP).

Real time Ionospheric correction output module- it enhances the stability and accuracy by directly measuring and compensation for ionospheric delay. This provide the L1/L2 performance in L1 receiver, alternately saves the operational cost of dual band receiver.

SyncE- When locked to GPS, the Meridian II can operate as a Synchronous Ethernet Primary Reference Clock (PRC). It is an ITU-T standard for computer networking that facilitates the transference of clock signals over the Ethernet physical layer (Layer 1). In addition to the physical clock signals, the Synchronization Status Messaging (SSM) is transmitted on the data link layer

(Layer 2). The Ethernet Synchronization Messaging Channel (ESMC) contains the clock Quality Level (QL) with the embedded SSM code. The Meridian II is configured as a Synchronous Ethernet Primary Reference Clock (PRS) when tightly locked to GPS (TFOM <= 4).

Synchronous Ethernet is defined in the ITU-T:

G.8261 Architecture and wander performance G.8262 Timing characteristics

G.8264 Ethernet Synchronization Message Channel (ESMC)

The Ethernet Synchronization Messaging Channel (ESMC) contains the clock Quality Level (QL) with the embedded SSM code. The ESMC data will be sent at a rate of once per second.

Hardware option available at meridian II PRS

5071 A cesium clock module (CCM)- it combines the benefits of meridian II, 5071 A, ionospheric correction to provide time and frequency standard with accuracy stability and holdover that reside at the uppermost echelon in the industry. Its data are registered and monitored to determine the overall function state of the 5071 A as well as the internal diagnostic parameter. All the information is recorded via the various meridian user interface SSH, HTTP, SNMP and front panel display. The module DB9M connector and cable allow the RS-232 serial port of the 5071 A to be connected to the meridian II for the bi-directional data communication. The module BNC input connector and coaxial cable allow the 10 MHz output signal from port 1. This 10 MHz signal become the reference to all meridian II frequency and timing signal including GPS timing receiver.

Direct digital synthesizer (DDS) sinewave output module- it adds four output to the meridian. It produces a sine wave that is generated from the system DDS. Frequencies from 1 Hz to 10 MHz in 1 Hz steps are available, including 1.544 MHz or 2.048 MHz. The selected frequency is phase locked to the system oscillator and is not aligned with system time.

Digital output module- it adds four buffered TTL output to the meridian II, this is a “plug- and-play” option.

There are several options available

1.   PPO (programmable pulse output)- Provides four on-time pulse rates from 1 PPS to 10 MPPS. Each output can be individually selected by using the front-panel keypad and display (Meridian II only), network ports, or RS-232 serial port. Available rates are: 1, 10, 100, 1k, 10k, 100k, 1M, 5M, 10M PPS, 1PPM, 1PP2S. Inverted 1 PPS available in 1U chassis only.

2.      1PPS- Provides four on-time 1 Pulse-Per-Second (PPS) outputs.

3.      10MPPS: Provides four on-time 10M Pulse-Per-Second (PPS) outputs.

4.      Time Code: Provides four digital outputs of the time code format configured for the standard AM Code output (IRIG-B, IEEE-1344/C37.118-2005, NASA36, or 2137).

Dual redundant power supply option- The Meridian II can be configured with an assortment of DC power supply options in place of the standard AC power supply, or as a redundant power source. The power supplies are factory-installed and not field-swappable in the 1U chassis. The 2U chassis has modular power supplies that are field-swappable. The primary and secondary power supplies are connected in a dual-redundant configuration with hitless automatic primary-to-secondary and secondary-to-primary switchover. A fault detector monitors the status of each redundant power supply. When a fault is detected, it will trigger a system alarm and light the front-panel Alarm LED.

CPU Module Options- The CPU Module is always present in the Meridian II providing several outputs as standard features. These are AM Code (Time Code), 1PPS, RS-232 Serial Port, and Dual Network Ports. The Network Ports include all the standard network protocols, including a high- bandwidth Network Time Protocol (NTP) server.

The CPU Module also supports additional, optional output(s) via a spare BNC connector on the 1U chassis (upper left image) and three spare BNCs on the 2U chassis (upper right image). These include Programmable Pulse Rates, DC-Level Shift Time Code, Direct Digital Synthesizer (DDS), and Alarm outputs. A second RS-232 serial port option provides a once-per-second, on-time, serial time string. The Network Ports supports a PTP/IEEE-1588 option with 8-nanosecond- resolution hardware timestamping per the specifications below.

NOTE - Other options and configurations are available - call us with your requirements.

TOPOLOGY AND USE

The job of a ePRS is to continually sync with Universal Coordinated Time (UTC) and distribute it across the networks. It may use a combination of GPS/GNSS, Atomic Clock or Cesium Clock and timing distribution mechanisms to achieve this. All time distribution methods should adhere to respective standards e.g. ANSI, Telcordia and ITU-T requirements for PRC (Primary Reference Clock) or PRS (Primary Reference source) and time synchronization mechanisms. In a typical deployment, Stratum 1 level clock is considered as PRC/ PRS for the network. A Stratum 1 is part of clock hierarchy level defined by ANSI for which Stratum 0 is atomic clock that provides input to Stratum 1. Where atomic clock inputs are not available, the PRC/PRS may take input from GPS/GNSS or Cesium Clock and a combination thereof as required. At Stratum 2 level, time servers generally get time reference from Stratum 1. The sync plane design should consider respective ANSI and ITU-T standards together for optimal outcomes. It is also useful to define a sync plane that is backward compatible. The figure above shows a relative map between ANSI clock hierarchy and ITU-T recommendations for frequency plane and time/phase plane.

Stratum 1 clocks for digital switches require low levels of jitter and wander because timing signal at the source need to be good enough, to avoid sporadic buffer overflows of downstream network elements.

By starting with a stable source, a usable clock can be recovered even after multiple hops. Holdover accuracy is important because a persistent frequency mismatch at the two ends of a T1/E1 link ultimately causes persistent buffer overruns and data loss.

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