
Configuring ATM Services
1-4
ATM Layers
The B-ISDN protocol reference model, on which ATM is based, consists of four
layers. Each layer communicates only with the layers directly above it or directly
below it. Figure 1-3 shows the B-ISDN ATM protocol reference model.
Figure 1-3. B-ISDN ATM Protocol Reference Model
The following layers relate directly to how Bay Networks routers support ATM
DXI and ATM FRE2:
• ATM layer
• ATM adaptation layer (AAL)
The
ATM layer
defines how two nodes transmit information between them. It is
concerned with the format and size of the cells and the contents of the headers.
The addresses of the cells are meaningful only to the two adjacent local nodes
(that is, usually not to the end nodes).
The
ATM adaptation layer (AAL)
converts higher-layer protocol data into formats
that are compatible with the requirements of the ATM layer, enabling ATM to
handle different types of information all within the same format.
The AAL is divided into two sublayers: the
Convergence Sublayer (CS)
and the
Segmentation and Reassembly (SAR) Sublayer
. These two sublayers convert
variable-length messages into 48-byte segments, while ensuring the integrity of
the data.
Higher Protocol Layer
ATM
Adaptation
Layer
(AAL)
Convergence Sublayer (CS)
Segmentation and Reassembly Sublayer (SAR)
ATM Layer
Transmission Convergence Sublayer
Physical Medium Dependent Sublayer (PMD)
Physical
Layer
(PHY)
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