Motorola V186 - Cell Phone - GSM System Planner Manual page 100

Communications
The lower three layers of the MDLC protocol stack are commonly known as Network
Services. These layers only are used when communicating with intermediary sites which
make it possible to pass any data through the system and not require the total system to
know the details of the data. Each layer adds (removes) data to what was received and
thereby communicates with equivalent layers in the destination (source) site—see figure
above.
RTU-to-RTU communications suppress the Presentation, Session, and Transport layers;
all layers are present for SCADA Manager-to-RTU communication and for
communications with the STS.
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