Cisco BPX-BXM-155-8DX Installation And Configuration Manual page 596

Table C-1
Traffic Class
End-to-End
Timing
Relationship
Bit Rate
Examples
Initially, four different adaptation layers (AAL1 through AAL4) were envisioned for the four classes of
traffic. However, since AAL3 and AAL4 both could carry Class C as well as Class D traffic and since
the differences between AAL3 and AAL4 were so slight, the two have been combined into one AAL3/4.
AAL3/4 is quite complex and carries a considerable overhead. Therefore, a fifth adaptation layer,
AAL5, has been adopted for carrying Class C traffic, which is simpler and eliminates much of the
overhead of the proposed AAL3/4. AAL5 is referred to as the Simple and Efficient Adaptation Layer,
or SEAL, and is used for Frame Relay data.
Since ATM is inherently a connection-oriented transport mechanism and since the early applications of
ATM will be heavily oriented towards LAN traffic, many of the initial ATM products are implemented
supporting the Class C Adaptation Layer with AAL5 Adaptation Layer processing for carrying Frame
Relay traffic.
Referring back to Figure C-1, the ATM Adaptation Layer consists of two sub-layers:
• Convergence Sub-Layer (CS)
Segmentation and Reassembly Sub-Layer (SAR)
•
Data is received from the various applications layers by the Convergence Sub-Layer and mapped into
the Segmentation and Reassembly Sub-Layer. User information, typically of variable length, is
packetized into data packets called Convergence Sublayer Protocol Data Units (CS-PDUs). Depending
on the Adaptation Layer, these variable length CS-PDUs will have a short header, trailer, a small
amount of padding, and may have a checksum.
The Segmentation and Reassembly Sub-Layer receives the CS-PDUs from the Convergence Sub-Layer
and segments them into one or more 48-byte SAR-PDUs, which can be carried in the 48-byte ATM
information payload bucket. The SAR-PDU maps directly into the 48-byte payload of the ATM cell
transmitted by the Physical Layer. Figure C-6 illustrates an example of the Adaptation Process.
Cisco BPX 8600 Series Installation and Configuration
C-6
Classes of Traffic and Associated AAL Layers (continued)
Class A
Yes
Constant
Uncompressed
voice, constant
bit-rate video
Class B Class C
Yes No
Variable Variable
Compressed Frame relay,
voice and video SNA, TCP-IP,
E-mail
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Appendix C ATM: An Overview
Class D
No
Variable
SMDS