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Chapter 11 Cyber Physical systems and security 277

protocols are required to be implemented to allow a message to move from one
network to another
Layer 3 - Network: Routing of message packets.
Layer 4 - Transport layer: Splits the data into messages to be transported.
Layer 5 - Session layer: Establishes sessions between machines.
Layer 6 - Presentation layer: Converts data formats between application represen-
tations and network representations.
Layer 7 - Application layer: Provides a uniform layer that abstracts the behaviour of
the network.
In the model an application is taken as meaning any source of a request for network
access to the computer. An industrial control program would be considered an appli-
cation. Commands from the application to the network’s application services layer are
recognised and responded to by the applications layer. The presentation layer translates
data forms as required. The session layer at the sending computer works with the session
layer at the receiving computer to transmit data only as fast as it can be received. The
transport layer ensures that data is not lost or damaged in the lower layers. The four
application service layers must exist at each node.
The three network service layers of the specifications may be provided by telecom-
munication companies or may be built into a local area network. The network layer
handles routing of messages via shared networks, and multiplexes messages so that a
network can be shared by many applications. The data link layer adds addresses to
outgoing messages, examines all network data for messages addressed to this node, and
does basic error checking. The physical layer inserts messages onto the shared network
conductors and receives all message traffic, converting between binary data and what-
ever form data is in when being moved via the network.
There are situations where not all seven layers are required. Eliminating layers will
reduce the hardware and software cost. If, for example, both computers can use the data
in the same form, then the data translation function of the presentation layer is un-
necessary. If the node’s only function is to connect one type of network to another (e.g.,
token ring to carrier-sense multiple access with collision detection), then only the lower
2 or 3 layers of the model are necessary.
In the case on the Internet, the TCP/IP model, is used as it combines the
Transmission Control Protocol (TCP) and the Internet Protocol (IP). This model provides
end-to-end data communication specifying how data should be packeted, addressed,
transmitted, routed, and received. This functionality is organised into four abstraction
layers, which classify all related protocols according to the scope of networking involved.
From lowest to highest, the layers are the network layer, containing communication
methods for data that remains within a single network segment (link); the internet layer,
providing internet working between independent networks; the transport layer, handling
host-to-host communication; and the application layer, providing process-to-process