Communication

presented at the

WORKSHOP

High Quality Seismic Stations and Networks for Small Budgets

 

 Volcan, Panama 8-13. March 2004

by

Jens Havskov, Department of Earth Science

University of Bergen

Norway

 

 

Computer Data storageAnalog to

Digitalconverter

Powersupply

GPS

Communication

Sensor

input

Main units of a seismic recorder. There are no flow arrows between the units since all can have 2 way communication. The GPS can be connected to the digitizer or the recorder. The power supply may be common for all elements or each may have its own regulator, but usually the power source is unique (e.g. a battery).

Common ways of communication

Telephone lines. The telephone lines are ordinary dialup, leased analog or leased digital lines.

ADSL (Asymmetric Digital Subscriber Line), which uses the standard telephone cable pair and offers a permanent (no dial-up) connection at a speed of up to 2 Mbit/s.

Leased digital lines. These lines are standard 2 way digital serial links. Usually the user has the choice between several transmission speeds, which is cost dependent. On long distances, this types of line are usually very expensive. There is usually no error correction built in.

Cellular phones. For many areas of the world, this might be the simplest way of setting up communication for a seismic network.

Satellite links. Satellite links function like radio links with the advantage that no line of sight is required. The most common system to use is the Very Small Aperture Terminal (VSAT) system, which has been in operation since 1985

Radio links, can be VHF, UHF or spread spectrum

 

RS-232-C is an interface standard for serial data transmission The maximum allowed cable length depends on the data rate, but cannot be longer than 20 m, except for very low rates..

RS-422 interface uses a differential signal and usually a twisted-pair cable. The interface permits cable lengths of more than 1 km. The maximum theoretical data rate is 10 Mbit/s, although this is not possible with long cables. A rate of 19200 bits/s is achievable in practice with a cable of 2 km.

RS-485 is a superset of RS-422.

The common way to specify the data rate or transmission rate is in bit/s, that is the number of bits of information transmitted per second. The baud rate is different and refers to the modulation rate. Standard baud rates are, among others, 4800, 9600, 14400, 19200, 28800, 38400, 57600 and 115200 bauds.

 Ethernet is a standard for LAN (Local Area Network) at a rate of 10 Mbit/s (later extended to 100 Mbits/s as Fast Ethernet.

 The physical interface may be of several types:

- 10BASE2 Thin coaxial cable (RG-58), with maximum segment length of 185 m (a repeater is needed for larger length).

- 10BASE5 Thick coaxial cable (RG-213), with maximum segment length 500 m.

- 10BASE-T Unshielded twisted-pair (UTP) cable, with maximum segment length 100 m, node-to-node connection

- 10BASE-F Fiber optics cable, with connector ST, maximum segment length of 2 km.

 TCP/IP (Transmission Control Protocol/Internet Protocol) is a standard protocol, which runs on a higher level than the data exchange level. It includes the utilities for virtual terminals (Telnet) and for file transfer (FTP).

  

Compression of digital data

Because of the high data rates, data is often compressed before transmission.

The compression can generally be expected to halve the quantity of seismic data.

After transmission, data must be uncompressed unless it is stored directly without processing.

There are several compression routines in use.

If communication is by a telephone line with modem, the compression can take place in the modem.

With many compression algorithms, the degree of data compression depends on the amplitude of the seismic signal.

Therefore the efficiency of the compression falls sharply during strong (and therefore also long lasting) earthquakes.

Error correction methods used with seismic signals

All digital communications experience errors.

In transmission of seismic signals this is particularly fatal since just one bit of error might result in a spike in the data with a value a million times the seismic signal.  

One of the principles of error correction is that the data is sent in blocks, e.g. 1 s long, and along with the block of data there is some kind of checksum.

If the checksum does not tally with the received data, a request is sent to retransmit that particular block of data.

A checksum can simply be the sum of all the sample values in one block or more sophisticated algorithms can be used.

Error correction requires duplex transmission lines and local data memory at the remote station

Error correction is built in to TCP/IP

SensorCentralrecorder

SensorPermanent

connection

Sensor

Sensor

Sensor

Sensor

A physical seismic network. The sensors are connected to a central recorder through a permanent physical connection like a wire or radio link. In this example, transmission is analog and digitization takes place centrally, but the analog to digital converter could also have been placed with the sensor and transmission would then be digital.

Drumrecorder

Digital recorderDigital toanalog

convertor

Field station

Time markgenerator

GPS

Field station Field station Field station

Digital transmission

GPS GPSGPS

Typical digital network. The digital data are transmitted to the central site over fixed digital communication channels. At reception, the signals enter the recorder directly. Timing is normally taking place at the field station although some systems also time the signal on arrival. In this example, one station is recorded on paper and the data therefore has to be converted from digital to analog. The time mark generator for the drum can use the recorder GPS, if it has one, or it has its own timing reference.

Transceiver Transceiver

Point to point

Transceiver Transceiver

Point to multi point

Transceiver

Transceiver 1

2

3

Point to point or point to multi point system. To the left is shown the traditionally point to point system while to the right is shown a point to multipoint system. Here only one frequency is used to communicate to 3 stations, one at a time. A solid line shows that there is a continuous communication.

 

A Spread Spectrum central station communicates to two data acquisition systems (left). Right is shown the Data-Linc Spread Spectrum radio modem model SRM6000 (RS232) or model SMR6000E (Ethernet).The range is 40 km with whip antenna and 55 km with Yagi antenna (40 and 50 km respectively for model E). Figure from www.data-linc.com

Theoretical calculation of the height h required at a distance d over a flat topography to get line of sight. From the geometric scheme on the left, the formula for h is obtained (center) as a function of R (Earth radius) and the distance. The table on the right list the heights needed for some distances.

d (km) h(m)

10 8

20 31

30 71

50 196

70 38

100 785

Repeater 1

Stationtransmitter

Receiver

Stationtransmitter

transmitter

Repeater 2

Receiver

Central station

Receiver

Receiver Receiver Receiver

Multiplexer

Transmitter

Stationtransmitter

Stationtransmitter

Network of 4 stations with 2 repeater stations using simplex transmission of analog or digital signals. The signals from 3 stations are combined at repeater 1 and retransmitted. At repeater 2, the signal is simply received and retransmitted. Note that the receiver and transmitter at the repeater stations must have different frequencies. Communication is simplex since the flow of data only goes from the field stations to the central station so for the case of digital transmission, there is no error correction, only error check.

Transceiver

Repeater 1

Stationtransceiver

Transceiver

Transceiver

Stationtransceiver

Transceiver

Transceiver

Stationtransceiver

Transceiver

Transceiver

Stationtransceiver

Transceiver

Repeater 2

Transceiver Transceiver Transceiver Transceiver

Central recorder

Network of 4 stations with 2 repeater stations using duplex or half duplex transmission. The transceiver can be understood as a transceiver (half duplex) or a receiver and transmitter using different frequencies. The signals from all stations are simply received and retransmitted. Note that the receiver and transmitter at the repeater stations must have different frequencies. If full duplex is required, there will be 12 antennas at repeater 1 and 4 at repeater 2 and half that much for half duplex.

 

  

Seismic stationwith local memory

Modem

Dialupphone

RS232ComputerModem

Dialupphone

RS232

Manual dial up to a seismic station for data inspection and/or download. The computer dialing can be any type of computer with a terminal emulator program like Hyper terminal in Windows.

Computer withdata collection

software

Seismicrecorder

Communication network

Seismicrecorder

Seismicrecorder

Seismicrecorder

A virtual seismic network. The thick line is the communication network, which can have many physical solutions. The data collection computer can collect data from some or all of the recorders connected to the network provided that it knows the protocol used by the recorder.

TCP/IP based datacollection system forreal time data and

triggeed data

Sensors

Internet and/or local area network

BridgeRS232 to TCP/IP

Digitizer

RS232

Field station

Router

ISDN

Dial up PPP

modemField station

Router

Modem

Router

ISDN

Router

ISDN

Field station Field station

Router

Modem

Field station

Router

Modem

Field station

NetworkWindows

Field station

Broad bandLinux

SensorsSensors

Dial up connection

Different ways of getting a TCP/IP connection to a central data collection system. The thick solid lines indicate permanent Ethernet connections.

Future trend

-All stations TCP/IP connected

-Physical medium

- ADSL

- Mobile phone

- Spread spectrum radio

- Digital telephone

Mobile phone with internet, TCP/IP (56 kb), operating system and 0.5 gb memory