Mainframe Timekeeping with the IBM 9037 Sysplex

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Mainframe Timekeepingwith the IBM 9037 Sysplex

• David L. Mills
• University of Delaware
• http//www.eecis.udel.edu/mills
• mailtomills_at_udel.edu

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IBM 9037 Parallel Sysplex

• The IBM 9037 Parallel Sysplex provides time
  synchronization for one or more CPUs using
  redundant connections.

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Multiple redundant connections

• Multiple 9037s can be connected for enhanced
  redundancy. Only one will be used at a time with
  the other serving as hot backup .

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Diverse premisses for disaster recovery

• Multiple 9037s can be interconnected by fiber and
  separated up to 40 km. The operator console(s)
  can be on the same or different LANs.

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Relationship between TAI, UTC and LT timescales

• International Atomic Time (TAI) is based on an
  ensemble of cesium oscillators in various
  national laboratories.
• It runs at constant rate and is uniform
  throughout the solar system.
• This is the basic timescale used by the 9037,
  even though it has to be derived from available
  UTC time sources.
• Coordinated Universal Time (UTC) is based on
  Ephemeris Time, which depends on astonomical
  observations.
• It runs at the TAI rate, but must be adjusted
  from time to time due to Earth rotation wobble.
• Adjustments are in units of one second at the end
  of June or December as notified by the
  International Earth Rotation Service (IERTS). The
  last leap second ocurred in December 1999.
• Local Time (LT) is based on geographic longitude
  and politics.
• Even worse, it jumps back and forth in one-hour
  increments in March and October.
• The ISO timestamp format provides for unambiguous
  representation.

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Relationship between UTC and ETR time
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Timescale offsets and adjustments

• UTC time is obtained from TAI time by subtracting
  the UTC offset.
• The UTC offset was initialized at 10 seconds on 1
  January 1972.
• A leap event is armed by the operator to occur
  at a selected day and time. At that time the UTC
  offset is increased by one second and the leap
  event is disarmed. There have been 22 leap events
  since 1972, but none since 1999.
• LT time is obtained from UTC time by adding the
  timezone offset.
• The LT offset is specified by the operator.
• A standard/daylight event is armed by the
  operator to occur at a selected day and time. At
  that time the LT offset is increased or decreased
  by one hour and the event is disarmed.
• In general, jumps forward in the spring are
  almost tolerable, but jumps backward in the fall
  are not. Recommended practice is to shut down for
  two hours and go for a bite to eat.
• NTP runs exclusively in UTC, but has provisions
  for leap events.

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How 9037 time is reckoned
TAI (9037)
TAI
UTC Increment (e.g., 1 s) at designated time
UTC offset (now 32 s)
UTC
LT Increment (e.g., 1 hr) at designated time
LT offset (e.g., -5 h)
LT

• The UTC and LT offsets can be set at any time by
  the 9037 or in each partition of each CPU.
• One-time increments or decrements can be
  programmed to occur at a designated day and time
  from the 9037 or by the operator in each
  partition.
• Usual practice is to run from the 9037
  exclusively.

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External time sources

• It is possible to set the time by hand, but not
  recommended if consistency with NTP time is
  required.
• The time can be set via RS232 serial and any of
  three radio clock timecode formats.
• Only the Spectracom format is usable radios that
  use the other two are no longer manufactured.
• It is not practical to change the format in the
  9037.
• The time can be set by telephone modem and the
  Automated Computer Time Service (ACTS) operated
  by NIST.
• ACTS operates from Boulder, CO, and Hawaii, soon
  from Ft. Collins, CO.
• Current experience suggests call failure less
  than one in ten. This is considered acceptable
  with good retry strategy.
• To maintain accuracy to within 100 ms with 9037
  specified frequency tolerance of 1 PPM, the ACTS
  call frequency should be in the order of once per
  day.

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Three ways to synchronize a 9037 to national time

• Connect to a resident GPS receiver.
• A good choice is one of the Spectracom GPS
  receivers integrated with a NTP server for other
  network clients.
• Accuracy is generally better than 1 ms.
• This may be the ultimate plug-n-play solution
  and centralizes all NTP functionality in one box.
• Currently available GPS receivers support SNMP
  monitoring and MD5 authentication.
• Dial out to ACTS once per day.
• In the present telecommunications climate, this
  is probably a free call.
• Accuracy is reported better than 10 ms.
• Use the SHARE GPS receiver emulator with NTP
• This uses a dedicated NT machine synchronized to
  other NTP servers and providing a timecode string
  to the 9037 on demand.
• Accuracy depends on the NTP subnet quality,
  probably in the low milliseconds.

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GPS radio clocks

• This is a Spectracom TTS240, one of several
  models available from companies such as
  Symmetricom and Spectracom.
• They cost from 3500 to 5500 and provide
  accuracies to 100 ms or better. This one has two
  serial ports and a NTP server built in.
• The GPS antenna must have line-of-sight
  visibility over substantial portions of the sky,
  which usually means it has to be on the rooftop.
• The downlead length is critical, inline
  amplifiers are required every 100 feet or so.

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Alternative radio clocks

• Top Spectracom WWVB Netclock/2, proud descendent
  of the 8170 in use here for the last 23 years. It
  uses the same timecode format as the GPS but
  costs far less.
• WWVB radios are vulnerable to power line
  radiation from UPS boxes.
• Bottom Traconex WWV Clock, used today only in
  Los Angeles traffic signals. GPS has displaced
  this and many other radio/satellite services,
  which are becoming extinct.

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The UDel Master Time Facility

• Its possible to get carried away with redundancy
  objectives. The Udel Master Timing Facility (MTF)
  has two GPS primary receivers, two WWVB backup
  receivers, a primary cesium clock and a backup
  quartz oscillator.
• The MTF serves several primary time servers for
  department, campus and the Internet at large.
• Each time server is connected to at least two of
  the four radio serial ports and PPS signals.

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Other issues

• On using the mainframe as a NTP server for other
  clients.
• A program to do this is available from SHARE. The
  program does not operate as a client.
• This is not recommended if the 9037 synchronizes
  to ACTS as the time can drift significantly
  between calls.
• If the 9037 is connected to a GPS receiver, this
  might be possible, but only if operating in UTC
  and not as a primary (stratum 1) server.
• On using a backup if the GPS receiver or antenna
  fail.
• Some GPS receivers already call ACTS if GPS
  fails.
• If the SHARE GPS receiver emulator runs
  full-blown NTP as a client, it already includes
  this capability.