SCADA communication topologies

The devices in a SCADA system communicate with each other to operate the system
effectively, and the devices are connected to one another in many ways, depending on the
requirements. The topologies used for SCADA communications can be defined in two ways:
physical, how the wires are physically connected, and logical, how the information is
transmitted through the network.

2.1 Point to point and multi-drop

Two devices can be physically connected in two ways, the first of which is point to point
where a dedicated communication link connects the two devices. The whole capacity of the
link is used by the two devices
for communicating. In multi-drop (multi-point), a single communication link is shared by
more than two devices. The channel is shared by all the connected devices in two ways. In
time sharing, specific time slots are allotted for each device. In spatial sharing, the devices
use the channel simultaneously by sharing the channel capacity. Following fig presents point-
to- point and multipoint links.
When two or more links are used to connect devices (nodes), they form a network topology
based on the way in which the devices are connected geometrically. The commonly used
topologies are bus, ring, star,
and mesh or a combination of these. With the advent of smart grid and larger systems,
networks like LAN, WAN are also in use in power systems.

Bus topology

Bus topology is flexible, commonly used for the master station communication, and can cater to any
communication technique, master slave, peer to peer, and so on. Each node is connected to a single
or redundant bus that carries the message, the nodes pick up the messages that are intended for
each individual node, and if any message is left without being accepted by any node, it is terminated
electrically at the end of the bus. Bus topology is reliable, and any node failure will not affect the
communication in the bus, and at the same time, the number of nodes can be increased or
decreased easily. Node-to-node communication is possible, and this
topology is not dependent on the master. Bus topology has some inherent disadvantages, as the
failure of the bus is difficult to pinpoint, messages not picked up by a node are lost at the end as they
are not returned. The bus may be busy during heavy traffic conditions, and a node may not be able
to send messages on time.

Ring topology

In ring topology, all the nodes including the master form a ring, or closed loop, and the messages are transmitted
from node to node in one direction.The message, if not accepted by any node, returns to the sender
which suffices as an acknowledgement. Direct node-to- node communication
is feasible in this topology, and any node can be a master. The major disadvantage is that the failure of any one
node disrupts the whole network. Increasing and reducing the number of nodes is also a problem, as
communication has to be stopped, and fault detection and isolation are also difficult.

Star topology
The star topology has a master that is the central hub, connected to the nodes by links. This is an easy
configuration to develop, maintain, monitor, and trouble shoot. Adding and removing nodes is easy; however,
this does not support direct communication between the nodes. The major disadvantage is the fact that in case of
the failure of the master station, the entire network fails.

Mesh topology
Mesh topology is also used which is an improvement over the ring; redundant links make the network more
reliable. Partially connected and fully connected mesh are used, depending on the redundancy level required.

SCADA data communication techniques

Master-slave

In the master-slave mode of communication, one device acts as the master that controls the communication and
the timing. All other devices can communicate only if the master initiates and allows the communication.
Slaves cannot communicate with each other independently and can communicate only when permitted by the
master. This technique can be used on any topology, and priorities are assigned for collecting data in some
systems. This system uses the communication resources at a minimum, as the master has to initiate it which
slows the speed of communication. A SCADA master will initiate communication from the slave remote
terminal units (RTUs) and intelligent electronic devices (IEDs).

Peer-to-peer
In the peer-to- peer mode, when an event occurs, any device can initiate communication with any other device
in the network, and all devices are equal, although sometimes a bus administrator is used to control traffic.
When used in SCADA systems, the SCADA master station will still receive the majority of the data and initiate
control commands; however, other devices will also have the capability to start communication.
Network communication can still happen even if the master fails. Star topology does not support peer-to-
peer, as all the connections terminate at the master and inter-node communication is not feasible. Peer-to-
peer uses the communication resources in a better fashion; however, when the number of nodes rises, the
performance decreases.

Multi-peer (broadcast and multicast)

The multi-peer echnique allows communication of an active device with other devices in the group in two ways:
broadcast and multicast. In broadcast, an active device sends a message to all other stations, master and
slaves included, which is unacknowledged. In multicast, an active station sends messages to a group of devices,
which are predefined, and the message is unacknowledged.
Data communication in SCADA
The signals from the field, both analog and digital, are acquired by the sensors or transducers and reach the
RTU/IED, and the analog signals get converted to digital by the analog-to- digital converters. The
communication system of the SCADA has to transfer this binary data to appropriate monitoring centers, whether
substations or state or national utility control centers.

The data communication which is the exchange of data between two devices, one device at the remote power
system equipment or component to be monitored or controlled, and the other at the control center or substation,
via some form of transmission medium.

Components of a data communication system

1. Message: The message is the information (data) to be communicated, which could be values, switch
positions, numbers, pictures, sound, video, or a combination of these.

2. Sender: The one who sends the message—the RTU/IED/ substation computer, telephone, video camera, and
so forth.

3. Receiver: The one to whom the message is destined, the front-end processor (FEP)/communication front end
(CFE) of the master station, substation, and so on.

4. Medium: The physical path by which a message travels from sender to receiver (e.g., twisted pair wire,
coaxial cable, fiber-optic cable, microwave, radio wave, etc.).

5. Protocol: A protocol is a set of rules and conventions that govern data communications and represents an
agreement between the communicating devices. Two devices may be physically connected, but for
data communication between the two, the devices should agree upon or understand the same protocol. A
protocol defines what is communicated, how it is communicated, and when it is communicated.

Transmission of digital signals

The digital data from a device are to be communicated to another device via some physical medium, and for this
purpose, the digital data are first converted to a digital signal for transmission. This process is called encoding.
Line coding is a technique used for converting the digital data into a digital signal and at the receiving end, the
signal is decoded to retrieve the digital data. The digital signal is generally nonperiodic, as the digital data are
not in any specific pattern of zeros and ones.
The transmission of a digital signal is done in two ways: the first method is to transmit the digital signal directly,
which is referred to as baseband communication, which requires a low pass channel with wide bandwidth.

The other way of transmitting a digital signal is to convert it to an analog signal. In the frequency domain, a
periodic digital signal, which is rare,will have infinite bandwidth and discrete frequencies, whereas a non-
periodic digital signal will have infinite bandwidth and continuous frequencies. A digital signal is a composite
analog signal with frequencies varying from zero and infinity. The resulting analog signal, which represents the
digital signal, can be transmitted using broadband communication.

Baseband communication
Baseband communication is referred to as direct transmission of the digital bit stream. This method is generally
used with transmission over copper circuits for short distance and for optical fiber communication. The method
implemented is called on-off keying. In this technique a 1 is transmitted when a voltage or current signal is
applied on the communication media and 0 when no signal is applied. The effectiveness of this
method can be measured by the receiver’s ability to decode the signal or reconstruct it. This technique when
used with copper circuits gives lower rate distance product. Thus, to compensate, this distortion of the received
signal, either the distance between transmitter and receiver is kept small or a lower data rate should be used for
transmission. Repeaters or the equalization filters (matched with the characteristics of communication
media) could also be used to eliminate this distortion in the signal.

Broadband communication
Broadband communication changes the digital signal to a composite analog signal for transmission, by
modulation technique. A sine wave is used as a carrier to transmit the digital signal. For a sine wave, three
specific attributes, frequency, amplitude, and phase, can be defined, and by changing any one of the attributes, a
different wave is created. The digital signal, which carries the data, is used to change frequency, amplitude,
phase, or a combination of amplitude and phase of an electrical signal, and these are the mechanisms used in
broadband transmission. When the frequency is changed, the phase is frequency shift key (FSK); for amplitude
variation, it is amplitude shift key (ASK); and for phase shifting by the digital signal, it is phase shift key (PSK).
However, the most popular technique in use is the quadrature amplitude modulation (QAM) where the phase
and amplitude of the analog signal are varied appropriately by the digital signal

OSI MODEL
The OSI Reference model has seven layers, the details of which are given below. The different layers
are categorized based on the functions they perform

The actual functions within each layer are provided by entities such as programs,
functions, or protocols, and implement the services for a particular layer on a single
machine. Several entities, for example a protocol entity and a management entity, may
exist at a given layer. Entities in adjacent layers interact through the common upper and
lower boundaries by passing physical information through service access points (SAPs).

Application layer
The application layer is the topmost layer in the OSI reference model. This layer is
responsible for giving applications access to the network. Examples of application-layer
tasks include file transfer, electronic mail (e-mail) services, and network management.
Application-layer services are much more varied than the services in lower layers,
because the entire range of application possibilities is available here. Application
programs can get access to the application-layer services in software through application
service elements (ASEs). There is a variety of such application service elements; each
designed for a class of tasks. To accomplish its tasks, the application layer passes
program requests and data to the presentation layer, which is responsible for encoding
the application layer’s data in the appropriate form

Presentation layer
The presentation layer is responsible for presenting information in a manner suitable for
the applications or users dealing with the information. Functions such as data conversion
from EBCDIC to ASCII (or vice versa), use of special graphics or character sets, data
compression or expansion, and data encryption or decryption are carried out at this
layer. The presentation layer provides services for the application layer above it, and uses
the session layer below it. In practice, the presentation layer rarely appears in pure form,
and it is the least well defined of the OSI layers. Application- or session-layer programs
will often encompass some or all of the presentation layer functions.

Session layer
The session layer is responsible for synchronizing and sequencing the dialogue and
packets in a network connection. This layer is also responsible for making sure that the
connection is maintained until the transmission is complete, and ensuring that
appropriate security measures are taken during a ‘session’ (that is, a connection). The
session layer is used by the presentation layer above it, and uses the transport layer
below it.

Transport layer
In the OSI reference model, the transport layer is responsible for providing data transfer
at an agreed-upon level of quality, such as at specified transmission speeds and error
rates.
To ensure delivery, outgoing packets are sometimes assigned numbers in sequence.
These
numbers are then included in the packets that are transmitted by lower layers. The
transport
layer at the receiving end subsequently checks the packet numbers to make sure all
have been delivered and to put the packet contents into the proper sequence for the
recipient. The transport layer provides services for the session layer above it, and uses
the
network layer below it to find a route between source and destination. The transport
layer
is crucial in many ways, because it sits between the upper layers (which are strongly
application-dependent) and the lower ones (which are network-based). The layers below
the transport layer are collectively known as the ‘subnet’ layers. Depending on how well
(or not) they perform their function, the transport layer has to interfere less (or more) in
order to maintain a reliable connection.

Network layer
The network layer is the third lowest layer, or the uppermost subnet layer. It is
responsible
for the following tasks:
 Determining addresses or translating from hardware to network addresses.
These addresses may be on a local network or they may refer to networks
located elsewhere on an internetwork. One of the functions of the network
layer is, in fact, to provide capabilities needed to communicate on an
internetwork
 Finding a route between a source and a destination node or between two
intermediate
devices
 Fragmentation of large packets of data into frames which are small enough
to be transmitted by the underlying data link layer (fragmentation). The
corresponding network layer at the receiving node undertakes re-assembly of
the packet

Data link layer

The data link layer is responsible for creating, transmitting, and receiving data packets.It
provides services for the various protocols at the network layer, and uses the physical
layer to transmit or receive material. The data link layer creates packets appropriate for
the network architecture being used. Requests and data from the network layer are part
of the data in these packets (or frames, as they are often called at this layer). These
packets re passed down to the physical layer and from there, the data is transmitted to
the physical layer on the destination machine. Network architectures (such as Ethernet,
ARCnet, Token Ring, and FDDI) encompass the data link and physical layers, which is why
these architectures support services at the data link level. These architectures also
represent the most common protocols used at the data link level. The IEEE (802.x)
networking working groups have refined the data link layer into two sub-layers: the
logical-link control (LLC) sub-layer at the top and the media-access control (MAC) sub-
layer at the bottom. The LLC sub-layer must provide an interface for the network layer
protocols, and control the logical communication with its peer at the receiving side. The
MAC sub-layer must provide access to a particular physical encoding and transport
scheme.

Physical layer
The physical layer is the lowest layer in the OSI reference model. This layer gets data
packets from the data link layer above it, and converts the contents of these packets into
a series of electrical signals that represent 0 and 1 values in a digital transmission. These
signals are sent across a transmission medium to the physical layer at the receiving end.
At the destination, the physical layer converts the electrical signals into a series of bit
values. These values are grouped into packets and passed up to the data link layer. The
mechanical and electrical properties of the transmission medium are defined at this
level. These include the following:
 The type of cable and connectors used. Cable may be coaxial, twisted pair, or
fiber optic. The types of connectors depend on the type of cable
 The pin assignments for the cable and connectors.
 Format for the electrical signals. The encoding scheme used to signal 0 and 1
values in a digital transmission or particular values in an analog transmission
depend on the network architecture being used.

Interoperability and open standards

Historically, SCADA system communication protocols have been developed as proprietary

protocols, each created by a manufacturer as part of a proprietary system, to meet the
specific needs of a particular industry. This was a matter of necessity, as suitable
standards had not hitherto existed. However, proprietary protocols have disadvantages
for the user. As a system is developed over time, the owner is either locked in to
expansion using the same proprietary system, or is compelled to replace substantial
parts of the system to change to another manufacturer’s protocol.
Arising from this underlying disadvantage and the increasing use of SCADA systems
generally, the need for open standards became recognized. This recognition has
translated into efforts by a number of organizations in a number of countries. However,
the emergence of standards that have wide acceptance has been a slow process.
The key benefit of an open standard is that it provides for interoperability between
equipment from different manufacturers. This means for example that a user can
purchase system equipment such as a master station from one manufacturer, and be
able to add RTU equipment sourced from another manufacturer. The RTU in turn may
have a number of control relays connected to it which are intelligent electronic devices
and also use the protocol. All of this equipment may be sourced from different
manufacturers, either in an initial installation, or progressively as the system is developed
over time. Some of the different benefits arising from the use of open protocols are
listed below, grouped into immediate and long-term effects.

Benefits from the use of open protocols

 Interoperability between multi-vendor devices

 Fewer protocols to support in the field
 Reduced software costs
 No protocol translators needed
 Shorter delivery schedules
 Less testing, maintenance and training
 Independent conformance testing may be provided
 Easy system expansion
 Long product life
 More value-added products from vendors
 Faster adoption of new technology
 Major operations savings