MIL-STD-188-176
the segmentation and reassembly (SAR) sublayer, and the common part convergence sublayer (CFCS). The CFCS functions include delineation and transparency of user information, and error detection. The SAR sublayer handles (1) the segmentation of data units so that they can be mapped into fixed-length payloads of
the ATM cells, and (2) the reassembly of data units from ATM cells. The SSCS provides the specific service-related functions of the AAL. Variable bit rate (VBR) services include data service for a range of applications from bursty data to
compressed video. Constant bit rate (CBR) services include voice and video (circuit emulation). AAL protocols AAL 1-5 for user services are described below.
5.2.1 AAL 1. AAL 1 shall be implemented in accordance with ANSI T1.630. AAL 1 provides a 47-octet user payload, with a 1- octet header to support timing and sequence integrity. AAL 1 supports CBR applications in which a timing relationship is required between source and destination, such as voice or video (circuit emulation). The CBR service clock may be synchronous or asynchronous to the B-ISDN network clock. The normal mode shall
be synchronous. Bit count integrity (BCI) shall be ensured in either mode, in conformance with ANSI T1.630, section 9. For the asynchronous mode, clock timing regeneration shall be by adaptive clock recovery as described in AF-SAA-0032.000, section 3.4.2. This method uses local procedures to compensate for jitter, with no timing information carried through the network. Adaptive
clock recovery methods are not subject to standardization. AAL-1 shall support circuit emulation for DS1 and E1 for both unstructured and structured data transfer, in accordance with
AF-SAA-0032.000. Adaptive clock recovery is not effective against wander (low frequency component to the cell delay variation). For the asynchronous mode, SRTS as defined in 3.4 of AF-SAA-0032.000 may be used where excessive cell delay variation may be experienced.
5.2.2 AAL 2. AAL 2 supports VBR applications in which a timing relationship is required to exist between source and destination, such as compressed video. This service has not yet been standardized.
5.2.3 AAL 3/4. AAL 3/4 supports VBR data transfer. AAL
3/4 shall comply with ANSI T1.629, which defines the SAR and
CFCS. AAL 3/4 shall provide message mode and streaming mode
service, in both assured and non-assured operations, in
accordance with ANSI T1.629, section 6. For non-assured
operations, optional error discard allows corrupted FDUs to be
delivered to the user. For assured operations, a reliable link
protocol is required in the SSCS. For non-assured operations the
SSCS is null. Specific AAL 3/4 requirements are given in 5.2.3.1
and 5.2.3.2.
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