ATM Adaptation Layer AAL

The AAL translates between the larger service data units (SDUs) (for example, video streams and data packets) of upper-layer processes and ATM cells. Specifically, the AAL receives packets from upper-level protocols (such as AppleTalk, Internet Protocols [IPs], and NetWare) and breaks them into the 48-byte segments that form the payload field of an ATM cell. Several ATM adaptation layers are currently specified. Table 18-3 summarizes the characteristics of each AAL.

Table 18-3. ATM Adaptation Layers

Characteristics

AAL1

AAL3/4

AAL4

AAL5

Requires timing between source and destination

Yes

No

No

No

Data rate

Constant

Variable

Variable

Variable

Connection mode

Connection-oriented

Connection-oriented

Connectionless

Connection-oriented

Traffic types

Voice and circuit emulation

Data

Data

Data

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