IPX and IGX Switch Trunk Interfaces to ATM

Functional Description

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 2-6,the ATM Adaptation Layer consists of two sub-layers:

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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 2-11illustrates an example of the Adaptation Process.

Figure 2-11 SAR Adaptation Process

Variable Length

XXX Bytes

			48 Bytes					48 Bytes

Application Layer Information

CS – PDU

48 Bytes		SAR – PDU
		ATM Cells

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