# Earthworm Modules:
Ew2liss Overview

(last revised 4 March, 2000)

Ew2liss is the Earthworm module for serving Earthworm trace data in LISS (Live Internet Seismic Server) format. The LISS is a very simple service that provides trace data in miniSEED format in near-real-time over the Internet. The ew2liss module will convert selected SCNs (station-component-network) of trace data from an Earthworm transport ring, convert that data to miniSEED format, and serve the data to a single LISS client (such as liss2ew) over a TCP/IP network connection.

One LISS normally has data for a single station. One or more components are sent together in the data stream. In some cases, one component is provided in more than one sample-rate. The `location code' is used in this case to label the different kinds of data. Note that nothing prevents ew2liss from handling SCNs from many stations except this LISS convention. It may be that ew2liss and liss2ew could be used to exchange data between Earthworm installations. Since LISS uses Steim-2 compression, this format should use less network bandwidth than Earthworm's export_scn/import pair. Performing the compression and decompression will use more CPU time, however.

#### A comment about timing

Earthworm trace data (TRACE_BUF format) includes "starttime" and "endtime" (8-byte floating-point numbers) to mark the start and end times of each message. It also includes sample count (an integer) and samplerate (an 8-byte floating-point number). Frequently this samplerate value has been set to the nominal sample rate; the start and end times are assumed to be actual measured values.

The miniSEED format includes only a start time for each record. The sample rate is encoded in the data fixed record as two two-byte integers. These two integers are treated as multipliers if positive, divisors if negative; this a wide range of sample rate values can be expressed. In converting from Earthworm TRACE_BUF messages to miniSEED, errors would be introduced if the nominal sample rate was used to set the miniSEED sample rate factors. Instead, a best rate is determined from the starttime, endtime, and sample count values. Then this new sample rate is rounded to the nearest 0.0001 samples per second. This rounded is intended to reduce the discretization introduced by the conversion to the short integer rate factors. Finally, this new sample rate is used to set the miniSEED sample rate factors.

Module Index | Ew2liss Commands