The only communication link between two stations. Either of the two stations can initiate communication or can be configured to be one who controls the other.
Topology is the geometric arrangement of nodes and links that form a network. The following topologies are most used by a SCADA system:
A) PEER TO PEER
The only communication link between two stations. Either of the two stations can initiate communication or can be configured to be one who controls the other.
Stations can be connected using cables or dedicated public standing. The dedicated line can be analog or digital. You can also use temporary connections such as dial-up lines, microwave, radio or satellite transmissions.
Select this topology if you need a direct connection to the remote station. For these cases, redundant systems should be seen as part of system design.
Two-Wire means that the means of transmission uses two wires for transmission and reception. For example, public telephone line uses two wires to connect between two users.
B) point to multipoint (multidrop)
The communication loop with three or more stations and who initiates the communication is the master station that controls communication with the remote stations.
Communication can be established using an analog or digital public line. You can also use the atmospheric environment such as microwaves, radio or satellite.
This topology is the most used by SCADA. Four lines means that the transmission medium used to transmit two lines and two lines for the reception. Public service providers normally enable the use of the four lines of communication.
C) multipoint to multipoint
The communication link is between three or more stations where not necessarily the master station initiates communication with other stations.
This is a special topology of some suppliers of radio modems. This easy-to-peer communication between stations.
TRANSMISSION MODE SELECTION
The mode of transmission is the way the information is sent and received between devices on the network. Scada systems on the network topology usually determines the mode of transmission of data.
When you have a point to multipoint topology, the transmission mode is half duplex. This means:
When you have a point to point topology, the transmission mode is full duplex. This means:
When you have a multipoint to multipoint topology, the transmission mode is full-duplex between station and modem half duplex between modems. This means:
SELECTION OF A MEDIA
Select a media taking into consideration the following:
Needs of transmission of the application.
Locations of the control center and remote sites.
Distances between sites.
Services available.
Budget allocated to the project.
Media types available:
a) PUBLIC TELEPHONE LINE (PSTN Public Switched Telephone Network)
The network of dial-up is provided by a telephone company. The phone line we use every day and allows the transmission of voice and data.
In Peru, the provider is Telefonica del Peru, but now we have to other providers such as BellSouth, TIM, etc.
Advantages and capabilities:
Useful if required little data collection.
You have access to the use of a public line.
The telephone company monthly charges for the service. The cost depends on the number of connections and connection time.
The communications network achieved a speed of up to 56 000 bps.
The network is a two-wire connection that supports half-duplex and full duplex if the connection is point to point.
Disadvantages:
Transmission is expensive if it requires greater volumes of information from remote sites.
The modem connection can cause transmission errors is 1 error per one million bits.
The media can not be used in locations where this service has not.
It takes time for the connection (checked) and the establishment of the connection.
An additional logic is required for an automatic connection.
Uses standard Bell or the International Consultative Committee for Telephones and modems (CCITT).
b) dedicated private line (PRIVATE Leased Line PLL)
It is a dedicated telephone line that is in permanent connection between two or more locations using an analog transmission. The line is available 24 hours a day, usually for sending voice and data.
c) DIGITAL DATA SERVICES (DIGITAL DATA SERVICES DDS)
It is a private line with a bandwidth special uses digital technology to transfer data at high speeds and low error rates using a dedicated private line. The line is available 24 hours a day.
