GNSS Timing Services for Data Centers Using ViaLite RF over fiber
This white paper on GPS / GNSS timing and distribution in Data Centers was written by ViaLite’s EMEA Sales Account Manager, Amair Khan. The paper is based on Amair’s recent experiences working with leading European Data Centers.
GPS – Global Positioning System
GNSS – Global Navigation Satellite System
MiFID – Markets in Financial Instruments Directive
NTP – Network Time Protocol
UTC – Coordinated Universal Time
SNMP – Simple Network Management Protocol
U – (or RU) Unit of measurement for 19 inch rack equipment, 1.75 inches
Timing and synchronization in today’s world of cloud based and hosted data services are becoming an increasing requirement. Different organizations need to provide accurate timing to conform to standards that are being set by national or international regulatory authorities. One of the latest regulations, where timing and timestamping are critical requirements, is within the financial sector. The Markets in Financial Instruments Directive (MiFID) is an EU legislation that regulates firms who provide data and hosting services to clients linked to the financial products and venues where those services are provisioned. MiFID came into effect in November 2007 and has now been revised and superseded by MiFID II, which came into effect in January 2018.
MiFID II extended the MiFID requirements in a number of areas including:
- new market structure requirements
- new and extended requirements in relation to transparency
- new rules on research and inducements
- new product governance requirements for manufacturers and distributers of MiFID ‘products’
- introduction of a harmonized commodity position limits regime.
In recent years there has been an increase in regulations for financial traders to improve the level of accuracy of business clocks and timestamps. This is essential for post-trade transparency and detecting instances of market abuse. One such European-wide regulation is MiFID II, including clock synchronization standard RTS 25. Automated and algorithmic trading can create thousands of trades per second which need to be timestamped against a common time standard. For MiFID II, this is Coordinated Universal Time (UTC). For high frequency trading, maximum divergence from UTC must be less than 100 microseconds, with a 1 microsecond granularity.
For data centers a typical PC clock will not meet this standard and instead a UTC synchronized timestamp must be supplied. The most common way of doing this is by deriving UTC from a GNSS/GPS signal. Furthermore, data centers are often broken down into discreet customer-specific rooms or zones that provide an additional level of security for each customer and contain unique assets for the user. A large data center can have many customer rooms or zones all needing their own UTC supply and specific timing requirements.
Taking a GNSS/GPS signal (typically from a roof top) and distributing it out to many rooms or zones can create a number of design challenges:
- The roof could end up looking like you are a farmer growing a field load of antennas. If you have 24 rooms to supply, you don’t want 24 antennas on the roof. One antenna (or possibly a main antenna and a back-up antenna) is ideally the maximum.
- GNSS/GPS signals are very small (even with active amplification in the antenna) and routing small – high frequency signals (~1.5 GHz / L1 / L2 band) for 100’s of feet/meters through coaxial is a problem. Coaxial is lossy at this frequency, has a high footprint for many cables and has a limited shielding effectiveness.
Traditional connection method of GPS antenna to NTP time server through coaxial cable
General installations of GPS within these facilities would follow this simplistic setup through to one or more NTP time servers:
Due to longer lengths (typically greater than 50m/150ft) needing to be supported where losses in coaxial cable are too high, ViaLite has been replacing the coaxial element of any setup between the antenna and the NTP server with its fiber transmitters and receivers specifically designed for operations within the timing and synchronization sector.
Using fiber optic cable to connect GPS antenna to the NTP time server
The following setup can replace and maintain longer length cabling over fiber, compared to the coaxial setup:
Design considerations of RF over fiber – Losses
Losses in any GPS system have to be taken into account. Many customers ask where these losses are experienced in a GPS setup. The simple answer is that losses will be experienced throughout the system and a proper link budget calculation should be done from the top end where the antenna is located, all the way through to where the NTP server is installed. Any device connected into the system needs to be taken into account.
The main components which need their loss to be calculated into any GPS setup are:
- GPS Antenna
- RF cable loss
- Lightning arrestors
- GPS splitter
- GPS combiners
- Fiber optic cabling
- Fiber optic converters E/O (Transmitter) and O/E (Receiver)
Always check the manufacturer’s specification of any of the above devices for losses in your calculations.
Fiber losses along any length of fiber optic cable are dependent of the wavelength of the laser used on the E/O transmitter. Typical laser wavelengths used for this application are 1310 nm and 1550 nm. Other wavelengths are available but are typically not used unless a CWDM or DWDM system has to be installed.
For general calculations for losses along a fiber, use the below figures:
1310 nm general loss is 0.4 dB/km
1550 nm general loss is 0.2 dB/km
With any fiber system installed, it is important to note that any loss in optical terms is normally equivalent to 2x loss in RF terms i.e. 5 dB optical loss is equivalent to 10 dB in RF loss.
Design considerations of RF over fiber – System Timing Delays
The ViaLiteHD link introduces a small amount of delay to the system, similar to the contribution of an amplifier. The typical delay contribution is shown below. Delay should also be accounted for through RF and fiber cables, as these are likely to be much higher than the delay in the ViaLite fiber optic modules.
- Fiber optic link transmit and receive pair 13.5 ns
- Fiber optic cable 5 ns/m, check manufacturers specification
- RF cable 3.5– 5.5 ns/m dependent on cable type
- LMR-195 4.0 ns/m
- LMR-400 3.9 ns/m
- LMR-500 3.9 ns/m
- LMR-600 3.9 ns/m
- RG-58 5.1 ns/m, dependent on dielectric
- RG-59 4.1 ns/m, dependent on dielectric
- RG-213 5.1 ns/m
- RG-316 4.2 ns/m
Delays quoted are typical for these cable types and construction of cables of the same type will vary between manufacturers (i.e. foam/ PTFE/solid polyethylene dielectric). If delay is critical please check your manufacturer’s cable specification or electrically test the cables.
GPS timing offset example:
Meinberg 35 MHz timing offset example:
Design considerations of RF over fiber – Frequency Bands & Timing
Design considerations of RF over fiber – simple example
A simple RF over fiber extension for a NTP Time Server (Weak GPS signal strength):
The Microsemi SyncServer S650 is shown in the above diagram. The complete RF over fiber extension kit is available from Microsemi (part number 093-15203-001) or ViaLite (part number VCS6FEK).
Introduction to ViaLite GPS/GNSS link timing and synchronization range
ViaLite introduced RF over fiber modules more than 20 years ago with applications in the timing and synchronization industry. The modules are designed to cover many different standards for GPS and timing signals from around the globe.
ViaLite supports outdoor applications with the use of IP rated enclosures specifically designed and built for various environments. The ODE-MINI and ODE-A4 are IP65 rated enclosures used in varying climate conditions. The ODE-MINI supports up to 2 GPS antennas and the ODE-A4 supports up to 4 GPS antennas inside of the unit, with dual redundant power supply operation existing for those disaster recovery applications.
GPS signal splitting – multiple clients
GPS signals can be split both optically and electrically. ViaLite’s extensive range of splitting solutions allows customers to maintain their timing and synchronization requirements to multiple locations, with little to no loss in signal strength.
GPS signal splitting – ViaLiteHD “Wide area” Multizone Distribution Lossless Splitter
ViaLiteHD Multizone Distribution Lossless Splitters allow for multiple receive points to be supplied by one transmitter with zero optical loss. The Lossless Splitter can be used with any ViaLite RF over fiber frequency band, but is typically used in any fan-out application in satcom or timing applications, such as GPS or 10 MHz reference. The Lossless Splitter utilizes the low loss 1550 nm wavelength or any of the ITU DWDM channels for longer distances.
Combined with our Multizone Distribution Combiner; fan-in, fan-out and combined transmit and receive systems become possible with zero loss for data center timing, satcom, cellular or public safety applications.
- Optical connectors: SC/APC, FC/APC
- 8, 16, 32 or 64 outputs
- Local SNMP control.
GPS signal splitting ViaLite’s “Local” Lossless Splitter Chassis
The ViaLiteHD HRK-1C-xxx-Gx-xx 1U chassis with built in GPS splitters is designed to minimize rack space. The chassis is available with 1-4 RF over fiber receivers with 8-32 outputs (depending on configuration), all in 1RU of rack space. All 8 to 32 channels can be lossless when coupled with gain on the ViaLiteHD GPS RF over fiber receiver. All RF over fiber cards are blind mate which, coupled with ViaLite’s long service life RF over fiber cards, ensures five nines (99.999%) reliability.
The 1RU chassis with GPS splitting has dual redundant PSU’s as well as built in SNMP control. It has been designed for easy installation in a GPS fan-out system where the user needs multiple GPS signals in one area. The chassis can be used with a direct GPS link to the roof or combined with the ViaLiteHD Multizone distribution lossless splitter. The solution can be used to populate multiple floors or rooms with up to 32 local GPS connections with no loss.
- 1 – 4 RF over fiber inputs
- 8, 16, 32 or 64 outputs
- Local SNMP control.
Data center configuration for multiple clients in multiple locations within building
0 dB gain optical fiber link:
+20 dB gain optical fiber link:
Vendor Profile – Microsemi (a Microchip company)
Microsemi Corporation, a wholly owned subsidiary of Microchip Technology Inc. (Nasdaq: MCHP), offers a comprehensive portfolio of semiconductor and system solutions for communications, defense and security, aerospace and industrial markets. Products include high-performance and radiation-hardened analog mixed-signal integrated circuits; FPGAs, SoCs and ASICs; power management products; timing and synchronization devices and precise time solutions. These set the world’s standard for time; voice processing devices; RF solutions; discrete components; enterprise storage and communication solutions; security technologies and scalable anti-tamper products; ethernet solutions; Power-over-Ethernet ICs and midspans, as well as custom design capabilities and services. Microsemi’s headquarters are in Aliso Viejo, California.
Microsemi’s SyncServer S6xx Series NTP Server/Network Time Server family provides accurate and reliable time and synchronization services, with unprecedented flexibility. The SyncServer S600 GPS/GNSS, Stratum 1 network time server also includes the security-hardened NTP ReflectorTM Technology, which is a real-time, hardware-based NTP packet identification and time stamping engine.
Vendor Profile – Meinberg – 35 MHz GPS over fiber System
Meinberg was founded in 1979 by Werner and Gunter Meinberg, pioneering radio-controlled systems that spurred development of longwave DCF77 receivers vital for industrial applications. Today the company is a global technology leader with more than 90 employees developing and manufacturing time and frequency synchronization systems for modern industrial applications. Their objective is complete customer satisfaction, and they are committed to delivering the highest level of quality, technical innovation, and flexibility in all products and services.
Meinberg products are comprised of high quality components that can be combined and configured to address varied and complex customer requirements. This adaptable approach to product management is well-established in the LANTIME NTP Time Servers, and adds impact to the recently launched Intelligent Modular Synchronization (IMS) product line. IMS solutions are focused on the growing field of synchronization technologies, offering a range of high performance timing solutions for networks of any size and industry. Meinberg offers a broad spectrum of proven GPS receivers; WWVB, DCF77 (PZF) and MSF radio clocks; bus-level timing cards and associated accessories.
The Meinberg GPS over fiber setup is different compared to others as it uses a GPS IF signal. The GPS L1 signal from either an active L1 antenna or L1 distribution system is fed into the Meinberg GPSCON which converts the L1 signal to the Meinberg GPS IF (@35 Mhz). The Meinberg GPSAV4 has the ‘master’ channel connected to a Lantime/M1000/ IMS system, providing the necessary signals to the GPSAV4, GPSCON and ViaLite transmitter on Channel 2 of the GPSAV4. The ViaLite transmitters convert these into optical signals which are distributed to the multiple customer enclosures via the ViaLite optical splitter. Single mode fiber optic cable is used to connect each enclosure to the splitter. At the customer enclosure, a ViaLite receiver and Meinberg interface converts the optical signal back into RF, which is connected to a Meinberg Lantime. The Lantime is synchronized and locked to the GPS signal, providing time synchronization to customer equipment.
The Meinberg IF signal is lower frequency (35 MHz vs L band) and much higher signal power compared to traditional GPS signals. This means a much higher signal level to route longer distances.
Vendor Profile – Spectracom (Orolia)
Spectracom, an Orolia brand, was founded in 1972 and has employed a business model grounded in collaboration with their customers, markets and industries. Spectracom extend their core competencies with partnerships so that they can rapidly deliver an end-to-end portfolio of solutions to commercial markets throughout the world. Their heritage is as a world leader of reliable time and frequency solutions for communications equipment and networks.
Orolia was founded in 2006 and became the world leader in Resilient PNT in just a few years. With 10 acquisitions in the US and Europe successfully integrated in less than 7 years, Orolia rapidly became one of the few global providers of precise, rugged and reliable positioning, navigation, timing (PNT) solutions that are critical for governments, military, aerospace and defense markets.
Orolia time servers offer the additional advantage of accuracy and traceability. Orolia offers time and frequency solutions supporting synchronization with an array of GNSS signals including GPS, Galileo, GLONASS, BeiDou, and QZSS.
SecureSync® combines Spectracom’s precision GPS master clock technology and secure network-centric approach with a compact modular hardware design in a 1RU chassis to result in a powerful, scalable and flexible NTP appliance.
Supporting a wide variety of network synchronization and management protocols, the base model comes with a built-in 10/100 base-T Ethernet port for network management and high-performance NTP which can serve more than 7,500 NTP requests per second. It includes all the latest functions of NTPv4. It can be deployed in combination with other stratum-1 NTP server(s), used in a stratum-2 configuration, and expanded through option modules.
Vendor Profile – U-blox
U Blox Evaluation kit
To check availability of satellites in your GNSS setup, the U-blox evaluation kit is an ideal kit to understand how many satellites are being seen and being locked onto from your rooftop through to your time servers. The evaluation kit provides visualization and evaluation features.
The recommended evaluation kit is the EVK-M8N with the use of the U-Center software.