content_image/1012138.jpg
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. 

<img src='content_image/1012138.jpg'>

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. 

## 8.3.1 Using external time source 

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: 

GLYPH<SM590000> Connect NTP server directly to the SE-HMC network. 

GLYPH<SM590000> Use HMC as NTP server. 

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. 

