## Planning for NTP server with PPS 

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. 

## NTP server with PPS redundancy 

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: 

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. 

The following planning considerations should be made to provide NTP servers with PPS redundancy: 

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 

Figure 2-10 on page 44 illustrates this type of redundancy. 

