# Earthworm Modules:
slink2ew overview

(last revised 8 March, 2005)

slink2ew is a SeedLink client module for Earthworm. SeedLink is the data server component of the Seismological Communication Processor, or SeisComP, originally developed at GEOFON.
The SeedLink protocol can be summarized as a simple negotiation scheme followed by the streaming of data packets from the server. SeedLink packets are composed of a small header followed by a 512-byte Mini-SEED record (data only SEED). The negotiation phase allows the client to request only specified data types from the server for each data stream. A data stream is defined by a network and station code pair.
By utilizing sequence numbers for each packet in a data stream the SeedLink protocol allows for connections to be resumed, eliminating most data gaps. The ability to resume data streams is primarily dependant on how much data, time-wise, the remote SeedLink has in it's buffer.
The protocol allows for two different modes of data transmission, uni-station and multi-station modes. Uni-station mode operates by transmitting a single data stream (data from a single station) through one network connection. In this mode the data stream does not need to be specified by the client as it is implied by the internet address and port. Multi-station mode operates by transmitting multiplexed data streams (data from multiple stations) through a single network connection.
The maxium size of the TRACEBUF messages created by slink2ew is limited to the maximum number of samples (using Steim2 compression) that can fit into a 512-byte Mini-SEED record (721 samples = 2884 bytes) plus the size of the TRACEBUF header (64 bytes). So 2948 bytes is a good estimate of the maximum size of TRACEBUF messages created by slink2ew.
Version 1.1 and greater of slink2ew supports the creation of either TRACEBUF or TRACEBUF2 messages depending on which version of Earthworm the module was built for. By default the module will create TRACEBUF2 messages if the system supports them and TRACEBUF messages otherwise. Additionally there is an optional command ForceTraceBuf1 that will force the use of TRACEBUF messages on systems that support TRACEBUF2.
SeisComP and SeedLink are actively supported by the GEOFON Project, ORFEUS Data Center and IRIS Data Management Center in addition to the user community.

Module Index | slink2ew Commands

Contact: chad at iris.washinton.edu