Page 251 - Sensors and Control Systems in Manufacturing

Networking of Sensors and Contr ol Systems in Manufacturing
then converts the electronic signal back to the original frame that was 211
placed on the medium. This extended message is passed up to the
data link layer, which confirms that error-free communication has
taken place and that the frame has been received at the correct loca-
tion. Data link layer (DL)2 directs the frame routing in transit. When
the full frame is received at data link layer (DL)2, the routing infor-
mation is removed and the remaining information is transferred to
network layer (NL)3. Network layer (NL)3 confirms the appropriate
routing and assembles the packets. Afterward, the routing informa-
tion is stripped from the message. The message is then passed to
transport layer (TL)4, which controls the transport of the individual
messages. Transport layer (TL)4 confirms that the correct connection
has been achieved between User 1 and User 2 and receives the infor-
mation necessary to achieve the appropriate connection for this par-
ticular exchange.
The remaining information is often passed to session layer (SL)5,
which interprets whether the message is part of a continuing com-
munication and, if so, identifies it as part of an ongoing conversation.
The information is then passed to presentation layer (PL)6, which
performs any necessary translation to make sure that User 2 can
understand the message as it has been presented. The message is then
passed to application layer (AL)7, which identifies the necessary sup-
port programs and software necessary for interpretation of the mes-
sage by User 2. Finally, User 2 receives the message and understands
its meaning.

4.8.8 Adding and Removing Information in Computer
Networks Based on Open System Interconnect (OSI)
This step-by-step passing of the message from User 1 “down” to the
actual communication medium and “up” to User 2 involves adding
information to the original message prior to the transfer of informa-
tion and removing this extra information on arrival (Fig. 4.10). At
User 1, additional information is added step by step until the com-
munication medium is reached, forming an extended message. When
this information arrives at User 2, the additional information is
removed step by step until User 2 receives the original message. As
noted, a peer relationship exists between the various levels. Each
level communicates only with the level above or below it, and the
levels perform a matching service. Whatever information is added to
a message by a given level at User 1 is removed from the message by
the matching level associated with User 2. Communication is made
possible by having User 1’s physical layer (PhL)1 communicate with
User 2’s physical layer (PhL)1, data link layer (DL)2 communicate
with data link (DL)2, network layer (NL)3 communicate with net-
work layer (NL)3, and so forth to achieve an exchange between User
1 and User 2.