System information

The ATM Adaptation Layer

Book Title

21-438

When ATM cells are transferred through a network, each cell is processed in isolation from all other

cells. All processing decisions are made based on the cell header; no processing of the data in the

payload ﬁeld occurs.

Figure 21-11 shows some examples of AAL processing.

Figure 21-11 AAL Processing Examples

Hosts A and C are connected to the network through ATM interfaces, so they do all their AAL

processing internally. The network does not do any processing for hosts A and C. Hosts B and D are

connected to native Ethernet interfaces on Nodes 1 and 2. Therefore, Node 2 does all the AAL

processing for Host D. Node 3 does no AAL processing.

Depending on the type of trafﬁc entering the ATM network, the AAL uses one of four different AAL

types to divide the trafﬁc into small segments. These types are classiﬁed according to the timing

relationship between the source and destination, the constant or variable bit rate, and the mode

(connection-oriented or connectionless). The AAL types deﬁned in the ATM standard are listed in

Table 21-3.

Table 21-3 AAL Types

AAL Type Examples of Trafﬁc Type

1 Circuit emulation, constant bit rate video

2 Variable bit rate video and audio

3/4 Connection-oriented or connectionless data transfer (AAL 3/4 has cell-by-cell error checking and

multiplexing)

5 Connectionless data transfer (AAL 5 has lower overhead than AAL 3/4)

Host B

Host A

Node 1

Node 2

Host D

Node 3

Host C

Ethernet

DS3 ATM

OC3 ATM