Introduction: Latency represents
a measure of the time between when a sample of ground motion
is made by a field acquisition system and when the data becomes
available somewhere downstream. While the concept seems clear, there
are many different ways in which latency could be measured. This
document identifies how the IRIS DMC measures latency.
The following diagram shows a series of recorded data blocked
into discrete data records. The terminology used in the
following definitions is specified by this diagram.
where
TD represents the time of the last sample in a data record
TA represents the time the data record became available. This
could be the time a data record was written to disk or the time a data record
became available in memory.
TM represents the wall clock time a feed latency measurement was made.
There are six specific latencies that apply at the DMC
Input (Data being received at the DMC)
- Data Latency
- Feed Latency
- Total Latency
For input, TA refers to the time the BUD system receives a data record.
Output (Data being served from the DMC)
- Data Latency
- Feed Latency
- Total Latency
For output, TA refers to the time that a specific data record becomes available
from a DMC data server.
All latency measurements are made on specific time series channels. The Network,
Station, Location and channel descriptors will always be specified.
Input Latency Measurements
Measures of Latency for data entering the DMC. These measures are from
the perspective of data entering the IRIS BUD system.
DMC Input Data Latency (TA - TD) is defined to be the difference
in time from when the BUD system receives a data record and the time of the
last sample in the data record for which latency is being measured. The measure
of Input Data Latency does not change for a given data record.
DMC Input Feed Latency (TM – TA) is defined to be the
difference in time from when the feed latency measurement is made to the last
time some data from a given channel becomes available in the BUD system. The
Input Feed latency can change with time and depends upon when the feed latency
measurement is made (TM).
DMC Input Total Latency (TM – TD) is defined to be
the difference in time from when the feed latency measurement is made and the
time of the last sample in the data record. It should be obvious that the Input
Total Latency is the sum of the Input Data Latency and the Input Feed Latency.
Output Latency Measurements
Measures of Latency for data leaving the DMC. Output Latency measurements are
specific to a particular IRIS DMC data server, such as the SeedLink server.
Operational downtime may result in latency being introduced within the DMC. Server
downtime or telecommunication problems could introduce latencies of this type.
DMC Output Data Latency (TA - TD) is defined to be the difference
in time from when a data record is available from a data server at the IRIS
DMC and the time of the last sample in the data record in question. The measure
of Output Data Latency does not change for a given data record.
DMC Output Feed Latency (TM – TA) is defined to be
the difference in time from when the feed latency measurement is made to the
last time some data from a given channel was available from the data server.
The measure of Output Feed Latency will change with time and depends upon when
the feed latency measurement is made (TM).
DMC Output Total Latency (TM – TD) is defined
to be the difference in time from when the feed latency measurement is made
and the time of the last sample in the data record. It should be obvious that
the Output Total Latency is the sum of the Output Data Latency and the Output
Feed Latency.
Frequency of Measurements
As of July 2007 the IRIS DMC was receiving data from approximately 15,000 seismic
channels. It is computationally impossible to make latency measurements
for every data record for all of these channels. For this reason, it is
the DMC’s approach to take snapshots of latencies for all channels on the
order of every few hours. The rate at which latency is measured may vary
within different systems. |
