Accurate time logging

Absolute Time Sources

There are a number of different approaches to absolute time synchronization, a grandmaster clock is connected to one of the following and provides synchronization to the local devices using PPS, PTP or NTP.

DCF77

Long wave frequency clock transmitted from Germany and receivable over most of Europe. Transmits at 77.5 KHz, accuracy is +- 2.5mS. A foot long piece of wire will probably provide enough signal in most locations for this to work. If you are hosted, this may be your only option.

CDMA/GSM

Both of these provide accuracy of around 10uS and need to be in an area receiving a strong mobile network signal. For reliability reasons, a small antenna is usually mounted on an outside wall.

GPS

The Geo Positioning Satellite system relies on a high precision clock source for its accuracy. These are kept to within 100nS accuracy. This is the preferred option, since it is the most accurate. This requires line of sight to the sky for the antennae, which is then attached using co-ax back to the computer room.

Time synchronization network

Unfortunately you cannot just attach these IEEE1588 cards to your existing Ethernet. They require a low jitter Ethernet to maintain accuracy. These can either be stardard switches which have dedicated queues for the IEEE1588 Ethertype or specialist Ethernet switch designed for low jitter and available from some of the above vendors. InfiniBand also has the potential to be used as a synchronization network (IBM Sysplex has done this for years!). It requires QoS configuration to ensure the IEEE1588 packets suffer minimum latency. We've not seen anybody deploy this yet, but would be keen to help someone modify the standard Linux ptpd to accomplish this