The process of checking and verifying that the system clocks of computers are in sync with the time source is known as time synchronization. Nowadays, with a large number of contemporary computers spanning across locations and are performing time-critical operations, it is essential to have the clocks are synchronized and accurate with in the order of few tens of Nano-seconds. Some of the use cases for such a it might be necessary to time stamp event occurrences, co-ordination of media broadcasting, phase corrections in small cell base stations, power generation, air traffic control, timing stock trading. One of the easiest and proven mechanisms is the use the constellations of GPS/GNSS satellites and create a globally acceptable clock with high accuracy. 

The key components of Time synchronization are Grandmaster, Master and slave. 

• The Grandmaster clock is the major time source in a multi-clock network, sending time downstream to other master clocks. It has exceptionally accurate timing synchronization.
• There could be options Master clocks acting as distributor aligning Grandmaster and slave clocks.  
• A slave clock is a device or clock that synchronizes with the master clock but does not provide timing.

In this blog, with the advent of powerful embedded systems, we will discuss in detail about the GPS grandmaster and realizing it with a low-cost ARM based Embedded Linux system and the associated technologies. 

GPS Grandmaster

As mentioned, Grandmaster clock is the primary source of clock in the network. Some of the major features expected of a Grandmaster includes:

• Accuracy – This is the most important feature of Grandmaster, and it is determined by system design, timestamping accuracy, and many algorithms such as BMCA (Best Master Clock Algorithm) and processes that run in the system (e.g., filtering, servo, etc). 
• Scalability – It refers to the overall number of physical interfaces a Grandmaster can have as well as the number of clock instances it can handle. 
• Resiliency– It is the capacity to handle numerous timing inputs that act as alternate time sources.  
• Portability – Sometimes it is essential to have the Grandmaster mobile. 

Today, every part of the world is practically covered with Global navigation satellite systems (GNSS)such as USA’s NAVSTAR Global Positioning System (GPS), Europe’s Galileo

Today, every part of the world is practically covered with Global navigation satellite systems (GNSS)such as USA’s NAVSTAR Global Positioning System (GPS), Europe’s Galileo, Russia’s Global’naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), Indian Regional Navigation Satellite System (IRNSS), China’s BeiDou Navigation Satellite System. These satellites not only provide navigation data but also are time-transfer systems. Even a low-cost GPS receiver can provide accurate time information with stability very close to one part in ten to the fourteenth over one day (1ns/day).

Grandmasters can be created with such GNSS/GPS based receivers. A typical, GPS Grandmaster architecture looks like as shown in the below diagram 

GPS-PPS Synchronization 

Typically, GPS receivers provides the time of the day (ToD) information over a serial interface such as RS232/USB Serial as NMEA text. As this is not sufficient to synchronize, the GPS receivers provide a synchronization mechanism called pulse per second (PPS). This pulse, which has a rising edge synchronized with the GPS second, is of high accuracy, and can be used to discipline local clocks in order to keep them in sync with Universal Time (UT).

With a capable timing system inside the embedded Linux, it is possible to maintain the system clock with in few Nano-seconds of UT. With the system clock synchronized, now it has to be transferred to the slaves via a standard mechanism such as PTP or NTP.

NTP Server 

One of the early and widely used Time Synchronization protocols is NTP – Network Time Protocol. The hierarchical architecture of NTP is divided into strata. Atomic clocks, like those in GNSS satellites, and GPS are examples of stratum 0 devices at the very top. Stratum 1 servers, also known as primary time servers, have a one-on-one direct link with a Stratum 0 clock, can achieve microsecond-level synchronization with Stratum 0 clocks, and can connect to other Stratum 1 servers for quick sanity checks and data backup. For tighter synchronization and increased accuracy, Stratum 2 servers can link to numerous primary time servers. NTP can support up to 15 strata, although each one reduces client synchronization by a little amount compared to Stratum 0. 

Because NTP networks are software-based, all timestamp queries must wait for the local operating system, they have more latency and poorer accuracy. NTP provides a precise enough time resolution for most enterprises to settle conflicts quickly, but those requiring a much higher level of synchronization need to go for more precise PTP. 

PTP Server 

PTP, or Precision Time Protocol, is a network-based time synchronization standard that aims at nanosecond or even picosecond-level synchronization rather than millisecond-level synchronization of NTP. 

Vis a vis NTP’s software-based approach, PTP timestamping is particularly precise because it uses hardware timestamping. 

A total of four messages are exchanged between the master and slave in every PTP sequence:  

• The master’s first sync message to the slave 
• A slave to master sync message is sent as a follow-up
• A message from the slave to the master requesting a postponement
• The master sends a final delay response message to the slave

There are four different timestamps produced by this sequence: 

• T1 is the time when the master sends the first sync message
• T2 is the time when the slave receives the first sync message
• T3 is when the slave requests a delay 
• T4 is when the delay request is received by the master 

During the delay response phase, the master delivers all four timestamps to the slave, and the slave can calculate the network latency between the master and slave in both directions. 

IEEE 1588 enabled Ethernet PHY 

As mentioned earlier, PTP needs a dedicated hardware time stamping mechanism. This is possible with single-chip Ethernet Physical Layer Transceiver (PHY) that are provided with IEEE 1588 based timestamping. While these are very similar to conventional Ethernet PHY’s, they have high precision timer that can timestamp transmission/receive packets in pico-second resolution. Some of the 1588 enabled PHY’s includes are Renesas UPD60611, Microchip KSZ8441, TI DP83869HM, Broadcom BCM81384 etc.

Embedded Linux based Grandmaster  

Earlier days, it called for very powerful dedicated system to achieve time synchronization. Nowadays, even low-cost systems have enough power to act as Grandmasters. It is possible to achieve the high precision with some support from hardware such as IEEE1588 based timestamping. There is a plethora of open-source projects addressing the needs and it is quiet easy to create Embedded Linux based Grandmaster systems.

Some of the utilities that can be used are :

Ptp4l 

Ptp4l is an IEEE-compliant implementation of the PTP. It implements both network master and slave clocks. For Grandmaster implementation, the master functionality can be used which will consider system clock as reference clock. Typical output of Ptp4l running as master on eth0 port is as follows: 

ptp4l[1760.714]: port 1 (eth0): LISTENING to MASTER on ANNOUNCE_RECEIPT_TIMEOUT_EXPIRES 

ptp4l[1760.715]: selected local clock 0e8a76.fffe.6b8917 as best master 

ptp4l[1760.715]: port 1 (eth0): assuming the grand master role 

Phc2sys 

Phc2sys is an application that synchronizes the system clock with a PTP hardware clock (PHC). 

PHC follows PTP time in hardware time stamping mode, while the system clock follows UTC time. phc2sys maintains the time difference between these two clocks in nanoseconds 

Ts2PHC 

ts2phc can be used to synchronizes PTP Hardware Clocks (PHC) to external time stamp signals. A single source may be used to distribute time to one or more PHC devices.  

 In addition to above tools, testp2p utility can be used to perform various operations such as driving external signal at PPS, setting/getting PTP time and date etc.,

With the GPS/PPS inputs, it will be possible to synchronize the Realtime clock to globally accepted accurate time. The PTP server can serve this time to other slaves.  

Running phc2sys will produce the following output :

CLOCK_REALTIME phc offset 1635162324159518479 s0 freq +0 delay 160546 

CLOCK_REALTIME phc offset 1635162324159518096 s1 freq –375 delay 160606 

CLOCK_REALTIME phc offset 0 s2 freq -375 delay 160606 

CLOCK_REALTIME phc offset 8 s2 freq -367 delay 160545 

 CLOCK_REALTIME phc offset 12 s2 freq -368 delay 160540 

 CLOCK_REALTIME phc offset -25 s2 freq -365 delay 160541 

…

As it can be seen the time offset between the PHC and the System clock is shown in the offset information. The System clock is synchronized if the offset is continuously less than 30 ns.  

The offset is kept within a nanosecond range of +/-50 nanoseconds. The clock servo states are indicated by the s0, s1, and s2 strings:  

• s0- unlocked 
• s1- clock step 
• s2- locked 

The clock will not be stepped until the Servo state is locked (s2) (slowly adjusted). The freq value is the frequency adjustment of the clock in parts per billion (ppb). 

NTP servers too, synchronize the NTP client with server time (Stratum 1). 

Other management protocols such as SNMP can be used to monitor various clocks in the system and their characteristics like accuracy, precision, resolution, current synchronization states etc. 

About Embien

Embien Technologies is a high-tech services provider in the embedded systems segment catering to such niche requirements. We have helped customers achieve sub- 30 nanoseconds compliance for PTP and sub-100 microseconds accuracy for NTP with Linux based embedded systems. Our other credentials include GPS anti-jamming system development, MIR-DIAL- Mid-infrared differential absorption LIDAR systems etc.