Civil Engineering Reference
In-Depth Information
the concepts and functions defi ned in OSI model. Under OSI model, a typical
networking system is defi ned to be built from seven major layers. These layers in
order from bottom to top are (1) physical layer, (2) data link layer, (3) network layer,
(4) transport layer, (5) session layer, (6) presentation layer, and (7) application layer.
Network structure is built in such manner that each layer and its components serve
the layers above it and being served by layers below it. First three layers (physical,
data link, and network) are known as media layers and their data units are, bit, frame
and packet (datagram) respectively. Last four layers are host layers (transport, ses-
sion, presentation, and application) and the types of their data units are segments
(transport), and data (session, presentation, and application). Starting from bottom
level, function of the fi rst layer is signal and binary transmission, while the second
layer function is physical addressing or what it usually is known as switching. The
physical layer defi nes the electrical and physical specifi cations for devices such as
fi ber optical cables that are responsible for transmission of data through this physi-
cal medium to the destination physical layer. The data link layer defi nes the proce-
dural method of data transferring between elements in the (same) network. This
layer (data link layer) is the layer that transfers data between nodes in a wide area
network (WAN) or local area network (LAN). This layer is subdivided to two sub-
layers of logic link control (LLC) and media access control (MAC). In building
controls operation the network address of each sensor or actuator operating in the
building resides in (MAC) layer, and therefore this layer is responsible for receiving
and sending signals from and to the sensors and actuators in the system with indi-
vidual network addresses.
Function of the third layer (network layer) is logical addressing. This layer
provides the means of transferring data between a host on one network to destina-
tion host on a different network. Function of the fourth layer (transport layer) is the
data fl ow control. The main responsibility of this layer is controlling reliability and
providing an error-free data transfer between the end users. Responsibility of the
fi fth layer (session layer) is managing sessions between the applications, and
responsibility of the sixth layer (presentation) is data presentation and converting
machine dependent information to machine independent information. Finally the
seventh layer (application) interacts with software applications to defi ne functions
such as identifying and synchronizing communication.
As an example the data exchange between two network systems (network one
and network two) each made of its own OSI defi ned seven layers should basically
follow the proceeding path. The operator at network one via a key board and inter-
face at application layer generates a data that will travel down the seven layers on
network one and at each layer the required formatting and addressing data will be
added to the data until at the lowest level or layer one of network one it is ready to
be delivered to network two. Respectively, network two will receive the data through
its lowest layer and send the data up through its OSI defi ned seven layers in which
at each layer formatting and addressing data added in network one will be read and
as a result directs the fi nal data correctly to the top layer (application layer) at net-
work two. At this time the operator at network two is able to see the data as it was
sent by the operator at network one, and then provide his own response through a
similar path.
