NTP and Stratum

[sarahr202]

Hi everybody

I have few questions about the term " stratum" in NTP.  

Please consider the excerpt from the link given below:

http://www.endruntechnologies.com/stratum1.htm

As you progress through different strata there are network delays involved that reduce the accuracy             of the NTP server in relation to UTC.  Timestamps generated by an EndRun Stratum 1 Time Server will             typically have 10 microseconds accuracy to UTC.  A stratum-2 server will have anywhere from 1/2 to 100 ms             accuracy to UTC and each subsequent stratum layer (stratum-3, etc.) will add an additional 1/2-100 ms of inaccuracy.

Based on the above, A stratum -1 server is more acurate than stratum-2 server, Similarly a stratum-2 server is more acurate than stratum-3 ser

ver.

It makes sense because lower stratum servers are closer to reference clock i.e stratum -0 device and thus more acurate.

But when I read following link, it gives me conflicting information:

http://en.wikipedia.org/wiki/Network_Time_Protocol

NTP uses a hierarchical, semi-layered system of levels of clock sources. Each level of this hierarchy is termed a

stratumand is assigned a layer number starting with 0 (zero) at the top. The  stratum level defines its distance from the reference clock and exists  to prevent cyclical dependencies in the hierarchy. It is important to  note that the stratum is not an indication of quality or reliability, it  is common to find stratum 3 time sources that are higher quality than other stratum 2time sources. This definition of stratumis also different from the notion of clock strata used in telecommunication systems.

So what is it? 

Thanks a lot have a great day.

[John Blakley · Accepted solution]

Sarah,

I'd be careful with wikipedia It is less reliable the farther you go out, and here's a snippet from the RFC:

Loosely following the conventions established by the telephone
   industry, the level of each server in the hierarchy is defined by a
   stratum number.  Primary servers are assigned stratum one; secondary
   servers at each lower level are assigned stratum numbers one greater
   than the preceding level.  As the stratum number increases, its
   accuracy degrades depending on the particular network path and system
   clock stability.  Mean errors, measured by synchronization distances,
   increase approximately in proportion to stratum numbers and measured
   round-trip delay.

You can find the RFC at this link:

http://www.ietf.org/rfc/rfc5905.txt

HTH,

John

[sarahr202]

Thanks John

[Reza Sharifi · Accepted solution]

Hi Sarah,

I can tell you that stratum 1 is more accurate than stratum 2 and 2 is more accurate than 3. I have actually installed GPS for NTP timing.  

Here is a picture of the acual GPS receiver.

http://www.symmetricom.com/products/gps-solutions/gps-time-frequency-receivers/XLi-SAASM-GB-GRAM/

HTH

[sarahr202]

Thanks Reza.

You connected  this gps to a server , shouldn't that sever be considered as stratum-1 ntp server?

thanks

[Reza Sharifi · Accepted solution]

Hi Sarah,

This GPS connects to the actual antenna usually located on the roof of the building via coax cable.  The GPS also connects to the actual NTP server via coax.  From there, you connect the NTP server to one of your switches or routers via Ethernet and that is how we get stratum 1 timing.

HTH

Reza

[sarahr202]

Hi Reza

antenna---GPS----NTP SERVER-1---------rest of network.

GPS is providing the reference clock i.e the current time in UTC zone,  Let assume our NTP-server -1 is located in time zone UTC+6. We want all our networking devices to be synchronized with NTP-server1 so debugging events can be co-related across different routers.

We can acheive the desired results by:

1) Configuring NTP-server-1 and all other networking devices in timezone UTC.

or

2) We can configure NTP server-1 and all other networking devices in time zone: UTC+6

Both options will yeild desired results.

Is my understanding correct ?

==========================================================

What kind of server do we usually connect to reference clock? I mean the manufacture.

thanks and have a great weekend.

[Reza Sharifi · Accepted solution]

Hi Sarah,

Is my understanding correct ?

Yes,

What kind of server do we usually connect to reference clock? I mean the manufacture.

If you are connecting directly to GPS, I usually use Symmetricom 350 series as sync server (NTP server).

Here is the data sheet:

http://www.symmetricom.com/products/ntp-servers/ntp-network-appliances/SyncServer-S350/

I am sure, there are other vendors that provide NTP servers, but I have only used Symmetricom

HTH

[Leo Laohoo · Accepted solution]

Take note that there are two "types" of NTP servers.

The first one is the "shonkiest" kind.  It's actually a Linux-based server that requires to access the internet to "pull" NTP synchronization.

The second one is a genuine NTP server that has a built-in GPS antenna inside.

I strongly recommend anyone who want to setup an NTP server to consider the second one. 

[sanishasunny02@gmail.com]

which is the default stratum in ntp ?

[Joseph W. Doherty · Accepted solution]

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Posting

NTP stratum is a hop count from the authoritative time source.  Since the quality of the links between hops, and the NTP stratum servers, can impact NTP accuracy, lower stratum doesn't always guarantee better time.  I.e. a stratum 4 server might have more accurate time than a stratum 2 server; similar to a network path with 4 hops might offer better throughput and/or latency than a network path with only 2 hops.