8860ch08.fm Draft Document for Review September 23, 2019 1:27 am With z15, STP stratum level 4$^{1}$ is supported. Timekeeping information is transmitted over  coupling links. Figure 8-1 shows a diagram of a CTN.  Note: z15 supports STP timing mode only (can only be part of an STP-only CTN). The z15  cannot be in the same sysplex or CTN with any IBM Z server earlier than a z13. STP retrieves the time from a Network Time Protocol (NTP) server through the NTP client on  the Support Element (SE). STP synchronizes the TOD to the external time source (NTP  server) accessed through the NTP client on the SE. To access the ETS, the SE requires  network connectivity to the NTP server. To provide NTP data to the NTP client on the SE, you have two options: Note:  A phone modem is not supported on the IBM Z anymore. Therefore, you cannot set  up the HMC as ETS by using a dial-out configuration. You must provide network  connectivity to an NTP server. For security reasons, the NTP server should not be directly attached to the SE-HMC network  (firewall separation is recommended). If firewal configuration is not possible, HMC can act as  an NTP server, so HMC can be used as NTP server for STP. For more information, see 8.4,  “Configuring the HMC as an NTP server” on page 166.  GLYPH<SM590000> Use HMC as NTP server. GLYPH<SM590000> Connect NTP server directly to the SE-HMC network. Figure 8-1 STP only CTN with z14 connectivity 8.3.1 Using external time source 1 Alhough STP stratum level 4 is supported, it should not be used for permanent configurations. Stratum 4 should be  used for transitional configurations, during CTN maintenance. Chapter 8. Preparing for Sysplex and configuring Server Time Protocol  165