Draft Document for Review January 6, 2020 11:24 pm 8480ch02.fm Planning for NTP server with PPS NTP server with PPS redundancy To configure an NTP server with PPS, the NTP output of the NTP server must be connected  to the Support Element LAN, and the PPS output of the same NTP server must be connected  to the PPS input provided on the External Clock Facility (ECF) card of the IBM Z server. The NTP information is provided via the HMC/SE local area network. However, the NTP  server propagates its pulse per second signal via a coaxial cable. The signals are carried over  a copper cable, which is limited in length. The cable length limitation depends on vendor  specifications, and certain vendors might offer a fiber optic cable connection as an option.  The cable is connected to a coaxial connector, which resides on the ECF card. As a minimum, one NTP server with a pulse per second output should be configured at the  server that has the Current Time Server role.  Each IBM Z server has been equipped with two OSC$^{2}$ cards, providing the capability of  attaching one or two NTP servers with pulse per second. Attaching a second NTP server with  pulse per second gives the redundancy that might be required in certain configurations.  When two NTP servers with PPS are configured, the user is responsible for selecting the  preferred NTP server. This NTP server is called the selected NTP server with PPS. The other  is called the non-selected NTP server with PPS. Normally, STP uses the time information  from the selected NTP server to perform the time adjustment. The SNTP client also  compares the quality of the NTP data received from both NTP servers and informs the user if  one of the following conditions is detected: The following planning considerations should be made to provide NTP servers with PPS  redundancy: Figure 2-10 on page 44 illustrates this type of redundancy. GLYPH<SM590000> The selected NTP server with PPS has a stratum level that is lower in the hierarchy than  the non-selected NTP server. (NTP stratum 1 server is a better choice than NTP stratum  2.) GLYPH<SM590000> The time obtained from the selected NTP server with PPS has less accuracy than the  non-selected NTP server. GLYPH<SM590000> The two NTP servers are providing different times. GLYPH<SM590000> When two NTP servers with PPS are configured on the server that has the PTS/CTS role,  STP will automatically access the PPS signal from the non-selected NTP server  configured on the PTS/CTS if the PPS signal from the selected NTP server fails.  Figure 2-9 on page 43 is an example of this redundancy. GLYPH<SM590000> When NTP servers with PPS are configured on the server with the BTS role, the following  is provided: – Access to an NTP server with PPS when the  BTS becomes the CTS as the result of  planned or unplanned recovery – Time adjustments to the CTN when the PT S/CTS does not receive PPS signals from  all of its configured NTP servers with PPS 2 Oscillator (clock) cards. Depending on the server hardware, each sever (z13 and later) have redundant OSC cards  with PPS input. 42  IBM Z Server Time Protocol Guide