Specifications
C-6
Cisco BPX 8600 Series Installation and Configuration
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Appendix C ATM: An Overview
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.
End-to-End
Timing
Relationship
Yes Yes No No
Bit Rate Constant Variable Variable Variable
Examples Uncompressed
voice, constant
bit-rate video
Compressed
voice and video
Frame relay,
SNA, TCP-IP,
E-mail
SMDS
Table C-1 Classes of Traffic and Associated AAL Layers (continued)
Traffic Class Class A Class B Class C Class D