Motorola PowerQUICC II MPC8280 Series Reference Manual page 1170

Introduction
AAL2 is subdivided into two sublayers, as shown in Figure 33-2:
• Common part sublayer (CPS)—In the CPS sublayer, variable length packets coming
from multiple users are assembled into CPS-PDUs belonging to a single ATM VC.
• Service-specific convergence sublayer (SSCS)—The SSCS sublayer handles the
mapping of user data to the CPS sublayer. The SSCS segments large data frames into
smaller CPS packets and also provides different services to the user, such as
transmission error detection. The SSCS sublayer is further divided into three
service-specific layers:
— Service-specific segmentation and reassembly sublayer (SSSAR)
— Service-specific transmission error detection sublayer (SSTED)
— Service-specific assured data transfer sublayer (SSADT)
User
SAP
The AAL2 microcode implements the CPS and SSSAR sublayers. (The SSADT and
SSTED sublayers are not implemented.) As shown in Figure 33-2, the user can access the
CPS sublayer directly or through the SSSAR sublayer. The SSSAR sublayer is used mainly
for transferring large data frames.
The AAL2 microcode also enables switching from one PHY | VP | VC | CID combination
to another; an example is shown in Figure 33-3.
33-2
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User User User
SAP
Service specific assured data transfer
SAP
Service specific transmission error detection
SAP
Service specific segmentation & reassembly
Common part sublayer
(SAP): Service access point
Figure 33-2. AAL2 Sublayer Structure
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SSADT
SSTED
SSCS
SSSAR
CPS
CPS
AAL2
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