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120 Chapter 4: ATM Solutions
into cells, packaging cells into frames, and controlling the flow of bits between transmitting and
receiving devices. The ATM layer establishes virtual connections and passes ATM cells through
the ATM network. The ATM layer delivers the 48-byte payload on established connections. The
AAL is responsible for allowing data conversions from multiple applications to and from the ATM
cell. AAL translates higher-layer services, such as voice and video, into the size and format of an
ATM cell. AAL also supports packet-based services, such as IP and Frame Relay. Figure 4-5
illustrates the relationship between the OSI model and the ATM model.
Figure 4-5
OSI Model and ATM Model
ATM Adaptation Layers (AAL)
The following sections discuss the five AALs. In addition to its other capabilities, ATM services
provides functionality at five levels of support.
AAL1
The AAL1 protocol can transfer service data units received at a constant bit rate from the source
and can deliver them at the same bit rate to the destination. AAL1 supports connection-oriented
services that require constant bit rates and have specific timing and delay requirements. Voice
and video are examples of applications that use AAL1.
AAL2
The AAL2 protocol supports ATM transport of connection-oriented variable-bit-rate
applications that do not require constant bit rates.
AAL3/4
ATM combines AAL3 and AAL4 into a single unit called AAL3/4. AAL3/4 supports both
connection-oriented and connectionless variable-bit-rate traffic, such as SMDS.
AAL
ATM
Physical
Network
Data link
Physical
Upper
layers
ATM
model
OSI
model
87200333.book Page 120 Wednesday, August 22, 2001 2:18 PM