IBM z13s Technical Manual page 210

Coupling links and Server Time Protocol
All external coupling links can be used to pass time synchronization signals by using STP.
STP is a message-based protocol in which timing messages are passed over data links
between servers. The same coupling links can be used to exchange time and coupling facility
messages in a Parallel Sysplex.
Using the coupling links to exchange STP messages has the following advantages:
By using the same links to exchange STP messages and coupling facility messages in a
Parallel Sysplex, STP can scale with distance. Servers exchanging messages over short
distances, such as IFB or ICA SR links, can meet more stringent synchronization
requirements than servers that exchange messages over IFB LR links (distances up to
100 km (62 miles)). This advantage is an enhancement over the IBM Sysplex Timer
implementation, which does not scale with distance.
Coupling links also provide the connectivity that is necessary in a Parallel Sysplex.
Therefore, you can minimize the number of cross-site links that is required in a multi-site
Parallel Sysplex.
Between any two servers that are intended to exchange STP messages, configure each
server so that at least two coupling links exist for communication between the servers. This
configuration prevents the loss of one link from causing the loss of STP communication
between the servers. If a server does not have a CF LPAR, timing-only links can be used to
provide STP connectivity.
The z13s server does not support attachment to the IBM Sysplex Timer. A z13s server
cannot be added into a Mixed CTN and can participate only in an STP-only CTN.
STP recovery enhancement
All coupling host channel adapters (ICA SR, HCA3-O (12xIFB), and HCA3-O LR (1xIFB)) are
designed to send a reliable and unambiguous "going away signal." This signal indicates that
the server on which the ICA SR or HCA3 is running is about to enter a failed (check stopped)
state. The "going away signal" that is sent by the Current Time Server (CTS) in an STP-only
CTN is received by the Backup Time Server (BTS). The BTS can then safely take over as the
CTS. The BTS does not have to rely on the previous Offline Signal (OLS) in a two-server
CTN, or the Arbiter in a CTN with three or more servers.
This enhancement is exclusive to z Systems and zEnterprise CPCs. It is available only if you
have an ICA SR, HCA3-O (12x IFB), or HCA3-O LR (1x IFB) on the CTS communicating with
an ICA SR, HCA3-O (12x IFB), or HCA3-O LR (1x IFB) on the BTS. However, the previous
STP recovery design is still available for the cases when a "going away signal" is not received
or for failures other than a server failure.
A new Enhanced Console Assisted Recovery (ECAR) is introduced in z13s servers that can
help to speed up the progress of BTS take over when the CTS encounters a check stopped
condition. For more information about the ECAR design, see "Enhanced Console Assisted
Recovery" on page 419.
Important: For more information about configuring an STP CTN with three or more
servers, see the white paper that is found at the following website:
http://www.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/WP101833
If the guidelines are not followed, it might result in all the servers in the CTN becoming
unsynchronized. This condition results in a sysplex-wide outage.
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IBM z13s Technical Guide