The communication loop is only between two stations. Either station can initiate communication, or it can be configured so that one station controls the other.
Topology is the geometric arrangement of nodes and loops that form a network. The following topologies are the most commonly used by a SCADA system:
A) POINT TO POINT
The communication loop is only between two stations. Either station can initiate communication, or it can be configured so that one station controls the other.
Stations can be connected using permanent cables or a dedicated public line. The dedicated line can be analog or digital. Temporary connections such as dial-up lines, microwave, radio, or satellite transmissions can also be used.
Select this topology if you need a direct connection to the remote station. In these cases, redundancy systems must be considered as part of the system design.
Two-wire means that the transmission medium uses two wires for both transmitting and receiving the signal. For example, a public telephone line uses two wires to establish a connection between two users.
B) POINT TO MULTIPOINT (MULTIDROP)
The communication loop involves three or more stations, and the master station, which controls communication with the remote stations, initiates the communication.
Communication can be established using a public analog or digital line. Atmospheric media such as microwaves, radio, or satellite can also be used.
This topology is the most commonly used by SCADA. Four lines means that the transmission medium uses two lines for transmitting and two lines for receiving the signal. Public service providers typically enable the use of all four communication lines.
C) MULTIPOINT TO MULTIPOINT
The communication loop is between three or more stations where the master station does not necessarily initiate communication with the other stations.
This is a special topology used by some radio modem providers. It facilitates point-to-point communication between stations.
TRANSMISSION MODE SELECTION
The transmission mode is the way information is sent and received between network devices. For SCADA systems, the network topology generally determines the data transmission mode.
When using 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:
In a multipoint-to-multipoint topology, the transmission mode is full-duplex between stations and modems, and half-duplex between modems. This means:
SELECTION OF A MEDIA OUTLET
Select a media outlet taking the following into consideration:
• Application transmission needs.
• Control center locations and remote sites.
• Distances between sites.
• Available services.
• Budget allocated to the project.
Types of media available:
a) PUBLIC SWITCHED TELEPHONE NETWORK (PSTN)
The dial-up network is provided by a telephone company. We use the telephone line every day, and it allows for the transmission of voice and data.
In Peru the provider is Telefónica del Perú, but currently we have other providers such as BellSouth, TIM, etc.
Advantages and capabilities:
• It is useful if little data collection is required.
• Access to a public phone line is available.
• The telephone company charges a monthly fee for the service. The cost depends on the number of connections and the connection time.
• The communications network achieves a speed of up to 56,000 bps.
• The network is a two-wire cable that supports a half-duplex and full-duplex connection if the connection is point-to-point.
Disadvantages:
• Transmission is expensive if larger volumes of information are required from remote sites.
• The modem connection may cause errors; the transmission rate is 1 error per 1,000,000 bits.
• The medium cannot be used in locations where this service is not available.
• Time is required for connection (dialing) and connection establishment.
• Additional logic is required for automatic connection.
Use Bell Standard or Consultative Committee for International Telephones and Modems (CCITT).
b) PRIVATE LEASED LINE (PLL)
It is a dedicated telephone line that maintains a constant connection between two or more locations using analog transmission. The line is available 24 hours a day and is typically used for voice and data transmission.
c) DIGITAL DATA SERVICES (DDS)
It's a private line with a special bandwidth that uses digital technology to transfer data at high speeds and with low error rates using a dedicated private line. The line is available 24 hours a day.
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