Page 250 - Sensors and Control Systems in Manufacturing

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Cha p te r
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4.8.3 Session Layer Communication
The message now passes from presentation layer (PL)6 to session
layer (SL)5. The objective of the session layer is to set up the ability for
the two users to converse, instead of just passing unrelated messages
back and forth. The session layer will remember there is an ongoing
dialog and will provide the necessary linkages between the individ-
ual messages so that an extended transfer of information can take
place.

4.8.4 Transport Layer Communication
The message then passes from session layer (SL)5 to transport layer
(TL)4, which controls the individual messages as part of the commu-
nication sequence. The purpose of the transport layer is to make sure
individual messages are transferred from User 1 to User 2 as part of
the overall communication session that is defined by session layer
(SL)5. Additional information is added to the message so that the
transport of this particular portion of the communication exchange
results.

4.8.5 Network Layer Communication
The message is then passed to network layer (NL)3, divided into
packets, and guided to the correct destination. The network layer
operates so that the message (and all accompanying information) is
tracked and routed correctly so it ends up at User 2. This includes
making sure addresses are correct and that any intermediate stops
between User 1 and User 2 are completely defined.

4.8.6 Data Link Layer Communication by Fiber Optics
or Coaxial Cable
The system now passes a message to data link layer (DL)2, which
directs each frame of the message routing in transit. Each frame is
loaded into the communication system in preparation for transmis-
sion. The message is prepared to leave the User 1 environment and
move into the communication medium. The next step is from data
link layer (DL)2 to physical layer (PhL)1. At this point, the frame is
converted into a series of digital or analog electronic signals that can
be placed on the communication medium itself—wire, fiber optics,
coaxial cables, or other means—to achieve the transmission of the
message from User 1 to User 2.

4.8.7 Physical Layer Communication
The electronic signal arrives at the correctly addressed location for
User 2 and is received by physical layer (PhL)1. The physical layer