ATM Option System Reference - Comtech EF Data

Product Overview
and higher capacity are a result of:
• Uniform cell size, which facilitates both transmission and switching,
• Reduced flow control and error handling requirements.
1.1.2 ATM Layers and Sublayers
ATM works through three distinct layers as it processes and transmits data from one end of
the user application to the other:
• ATM Adaptation Layer (AAL), where data from a user application is segmented
to proper length and some control and housekeeping features are added to
make a 48-octet payload.
• ATM Layer, where a 5-octet header is attached to the payload.
• Physical Layer, which handles the interface to the transmission media.
The steps are reversed at the receiving unit:
• The cell is passed upward from the physical layer to the ATM layer, which
removes the 5-octet header.
• From the ATM layer the cell goes to the AAL layer, where the payload is reassembled
and passed up to the user layer.
The AAL has two sublayers:
• Convergence Sublayer (CS), which processes the user data according to the
required AAL service class.
• Segmentation and Reassembly Sublayer (SAR), which fragments the data
units at the transmitting end so that they can be packaged as a fixed-sized payload.
At the receiving end, the SAR reassembles the payloads from several cells
back into the high-level protocol units.
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