# Rcv_ew Overview

(last revised 16 April, 1998)

Rcv_ew receives triggered or continuous trace data from one or more USNSN stations (collected in Golden, Colorado), reformats it into standard Earthworm format and places it in an Earthworm transport ring. For each channel, the trace data output by rcv_ew will be chronological. There will be no overlap between successive data messages, but there may be gaps. The messages will likely be of variable length, with a maximum length set by the user.

Rcv_ew is a composite Earthworm module. It is made up of two USNSN programs (RCV and STATION, written and maintained by Dave Ketchum) that call a set of user functions (written by Lynn Dietz) which contain all the Earthworm-specific code. RCV is a command-line program which under Earthworm is started by startstop. RCV handles the communication with the USNSN in Golden, receiving USNSN packets via a TCP/IP socket or a VSAT. RCV spawns the STATION process at the end of a pipe and passes data packets to it. STATION can be spawned one for each incoming channel or it can be invoked as one STATION process handling all of the channels coming in on the communications link. The latter mode, referred to as the "multiple" STATION mode, is required for the Earthworm implementation of rcv_ew. STATION processes the data packets and handles all the Earthworm communications by calls to the user functions. As the USNSN produces internal enhancements to RCV or STATION, the new code can be moved directly into rcv_ew. Likewise changes to the Earthworm-side user functions can be made without affecting the USNSN code. As of April, 1998, rcv_ew runs on Solaris only.

Waveform Packet Processing:

RCV receives USNSN packets from prearranged channels via a TCP/IP socket or a VSAT. The data may be either triggered or continuous. The packet stream can include data from multiple stations. RCV performs the recommended input processing for rollbacks as described in the USNSN Satellite Primer (ftp://gldfs.cr.usgs.gov/primer/Primer.eps). The data supplied to STATION is as complete as it can be (except that during rollback some data may be tossed) without duplicating many packets. However, the packets for a given channel may arrive out of sequence or may overlap in time.

In the absence of new waveform data, the USNSN sends a 'keepalive' message to RCV at least once a minute. This keepalive ensures that the communications link is still viable and allows the Earthworm code to issue a heartbeat. RCV also passes the keepalives to STATION. If no data or keepalive is seen for more than 120 seconds, RCV automatically attempts to reconnect to Golden.

After STATION receives the first packet for each channel, it watches for sequencing errors on the channel and reports them. STATION knows that the rollback code in RCV has handed it as good a set of data as can be had. Much of STATION's code deals with "Partial update" records. These are short packets which contain incremental parts of a longer packet. Partial packets are used for mainly with 1 sps long-period data. Since a full 2048 byte compressed packet contains as much as 45 minutes of 1 sps data, it was perceived as desirable to send incremental pieces of this longer packet at shorter intervals (about every 2 minutes). Each partial packet contains the chunk of data that has been created since the last partial. STATION puts these partial packets in buffers. When the buffer contains a whole packet, STATION decompresses the data and calls the user-supplied processing function (user_proc) with the time and in-the-clear data.

The Earthworm processing function (user_proc) is configured to accept only packets from a user-supplied list of channels (specified by a station, channel and network code, or SCN). Packets from any unlisted SCNs are ignored, with an "Ignoring data from SCN" error being issued. Within user_proc, packets for each listed channel are buffered (up to a user-specified limit) to allow for the re-sequencing of packets on rollbacks. Once the buffer for a channel is full, user_proc will "release" (output) the oldest packet on receipt of a new, in-sequence packet. If user_proc is handed an out-of-sequence packet with an earlier-than-expected timestamp (a rollback packet), it looks through that channel's buffer for a packet with an identical sequence number. If such a packet is found, user_proc overwrites it with the newly received packet. Otherwise, user_proc issues a "Rollback cannot be used" error, and discards the new packet. If user_proc is handed an out-of-sequence packet with a later-than-expected timestamp, it issues a "Gap prior to seq=x" error, and buffers the new packet for later release. When user_proc is handed a packet that is marked "end-of-trace-segment" (triggered data), it releases all packets in the buffer for that channel (who knows how long it will be until the next trigger occurs?). To "release" a packet, user_proc first discards any data in the packet which duplicates previously released data, then it produces a chronological stream of standard Earthworm waveform messages (TYPE_TRACEBUF). Most of the waveform messages will contain the number of data samples specified by the user; but some shorter messages will also be produced. These TYPE_TRACEBUF messages are output to the user-configured Earthworm transport ring where any waveform-processing modules can access them.

The Earthworm code also includes a thread which monitors the time since the last data packet was received for each channel. If this time exceeds a user-specified limit ("MaxSilence" minutes) for any channel, rcv_ew will issue an error message. It will continue to issue an error every "MaxSilence" minutes that no data comes in for that channel. Rcv_ew will issue an "un-error" message when it starts receiving data for that channel again. This thread also monitors the Earthworm termination flag in the transport ring so that rcv_ew will exit in a timely manner when the Earthworm system is stopped.

SEED channel names:

RCV and STATION (as well as other USNSN and IRIS codes) use get_name() to get SEED names for channels. This function requires that a file named nsnstation2.dat reside in the directory in which the program was started. A sample of this file is included in rcv_ew's source directory .../vX.X/src/rcv. The best way to get the most up-to-date station list from Golden is to use the autodrm. Send a mail message to autodrm@gldfs.cr.usgs.gov with the body:

> BEGIN
> CLIST
> STOP

The returned mail can then be saved as a text file and named nsnstation2.dat. For rcv_ew to run correctly, a copy of nsnstation2.dat must be placed in the EW_PARAMS directory.

Earthworm Heartbeats:

Rcv_ew beats its Earthworm heart from within STATION. STATION calls the function user_heart_beat() first from within the configuration function (user_proc_cmd), and then on receipt of any data or keepalive from RCV. user_heart_beat() will issue a TYPE_HEARTBEAT message only if the time since the last beat is greater than "HeartBeatInt" seconds (set in config file). The actual heartbeat interval will be irregular since it is driven by data coming from Golden. But since "keepalives" should come every minute, so should heartbeats. If the communication link to Golden goes down, rcv_ew will stop beating its heart. The rcv_ew heartbeat contains a process_id so that the rcv_ew module can be restarted by statmgr/startstop. The process_id in the heartbeat is actually that of RCV, STATION's parent (RCV is the only process that startstop knows about).

Tip:
  Set "HeartBeatInt" to something under 60 seconds to guarantee a heartbeat for each keepalive.

User Functions:

The user functions which contain all the Earthworm-specific code live in the source file user_proc_ew.c. The functions are:

1. user_proc_cmd(argc,argv) called once at startup of STATION for user setup. This is where we read the Earthworm configuration file and start up the time-since-last-packet monitoring thread.
2. user_proc(iy,id,ih,im,is,ms,leap,name,cname,network,eof,rate,seq) called from STATION to deliver one packet of data to the user. This function contains the guts of the Earthworm packet processing.
3. user_heartbeat() called from RCV on receipt of data or keepalive message. This function is a dummy function within rcv_ew.
4. user_heart_beat() called from STATION on receipt of data or keepalive message. This function is the Earthworm heartbeat for rcv_ew.
5. user_shutdown() invoked by RCV and STATION as part of the process termination.

   Module Index | Rcv_ew Commands