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Networking of Sensors and Contr ol Systems in Manufacturing
layer (AL)7, and passes from there to presentation layer (PL)6, from 209
there to session layer (SL)5, and so forth until the message is actually
transmitted over the communication path. The message arrives at
physical layer (PhL)1 for User 2 and then proceeds from physical
layer (PhL)1 to data link layer (DL)2, to network layer (NL)3, and so
forth, until User 2 has received the message. In order for Users 1
and 2 to communicate with one another, every message must pass
though all the layers.
The layered approach provides a structure for the messaging pro-
cedure. Each time a message moves from User 1 [application layer
(AL)7 to physical layer (PhL)1], additional processing and addressing
information is added to the beginning or end of the message. As the
original message moves on, new information is added to ensure cor-
rect communication. Then, as the message moves to User 2 [from
physical layer (PhL)1 to application layer (AL)7], this additional
information is removed, as illustrated in Fig. 4.9.
The layers work together to achieve “peer” communication. Any
information or operation instruction that is added at session layer
(SL)5 for User 1 will be addressing session layer (SL)5 for User 2. The
layers thus work as peers; each layer has a given operational or
addressing task to make sure the message is correctly communicated
from User 1 to User 2. Each layer associates only with the layers above
and below itself. The layer receives messages from one direction, pro-
cesses the messages, and then passes them on to the next layer.

4.8.1 Application Layer Communication
Communication begins when User 1 requests that a message be trans-
ferred from location 1 to location 2. In developing this message, it
may be necessary to have application software that will provide sup-
porting services to the users. This type of service is provided by
application layer (AL)7. Common software application tools enable,
for example, the transfer of files or the arranging of messages in stan-
dard formats.

4.8.2 Presentation Layer Communication
The initial message is passed down from the application layer (AL)7
to presentation layer (PL)6, where any necessary translation is per-
formed to develop a common message syntax that User 2 will under-
stand. If the two different users apply different computer or equip-
ment languages, it will be necessary to define the differences in such
a way that the users can communicate with one another. The basic
message that began with User 1 is translated to a common syntax that
will result in understanding by User 2. Additional information is
added to the message at presentation layer (PL)6 to explain to User 2
the nature of the communication that is taking place. An extended
message begins to form.