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The ATM Adaptation Layer
The ATM Adaptation Layer (AAL) provides the translation between the larger service data units of the upper
layers of the OSI Reference Model and ATM cells. It works by receiving packets from the upper−level
protocols and breaking them into 48−byte segments to be dumped into the payload of an ATM cell. The AAL
has two sublayers: segmentation and reassembly (SAR) and the convergence sublayer (CS). The CS has
further sublayers: the common part (CP) and the service specific (SS). Like protocols specified in the OSI
Reference Model, Protocol Data Units (PDUs) are used to pass information between these layers.
The AAL translates between the different types of network traffic—such as video streams, data packets, and
voice packets—of upper−layer processes and ATM cells. In other words, the AAL receives packets from
upper−level protocols and breaks them into the 48−byte segments that form the payload field of an ATM cell.
Several types of AAL standards are defined for this layer. Which AAL you use will largely depend on the
type of traffic and what you are trying to do with the traffic. The characteristics of each AAL are as follows:
AAL1 (Class A)—This layer is a connection−oriented service that provides end−to−end timing
provisions. It maintains a constant data transfer rate, which is used for transporting telephone traffic
and uncompressed video traffic. This is known as a constant bit rate (CBR) service. It is appropriate
to use AAL1 to transport voice and video traffic or another type of timing−sensitive data.
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AAL2 (Class B)—This layer is reserved for data traffic that requires variable bit rates (VBR) and
timing sensitivity, such as multimedia. It multiplexes short packets from multiple sources into a single
cell with end−to−end timing and connection orientation.
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AAL3/4 (Class C)—This layer was designed for network service providers; it closely aligns with
SMDS. This layer uses no VBR and has no timing requirements. It supports both connection−oriented
and connectionless data for WAN links using Frame Relay or X.25. This layer is perfectly suited for
use in environments that need to send or receive large files. AAL3 is identical to AAL4, with the
exception that the AAL3 layer is connection−oriented only, whereas AAL4 is both
connection−oriented and connectionless.
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AAL5 (Class D)—This layer is the primary AAL used to transfer non−SMDS data. It supports both
connection−oriented and connectionless data. This layer is used for such applications as classical IP
(CLIP) over ATM and LANE. Catalyst switches use this layer to provide LANE services for ATM.
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ATM networks provide the transport method for several different independent emulated LANs. When a
device is attached to one of these emulated LANs, its physical location no longer matters to the administrator
or implementation. This process allows you to connect several LANs in different locations with switches to
create one large emulated LAN. This arrangement can make a big difference, because attached devices can
now be moved easily between emulated LANs. Thus, an engineering group can belong to one ELAN and a
design group can belong to another ELAN, without the groups ever residing in the same location.
LANE also provides translation between multiple−media environments, allowing data sharing. Thus, Token
Ring or FDDI networks can share data with Ethernet networks as if they were part of the same network.
Specifying ATM Connections
ATM networks manage traffic by establishing and configuring each connection. When establishing the
connection, the connection type and the resources required to support the connection are specified as a class of
service. This class is used to provide users with a guaranteed QoS.
The classes of service for QoS are defined by the ATM Forum and are as follows:
Available bit rate (ABR)—Supports variable−rate data transmissions without preserving any timing
relationships between the source and destination nodes. This connection type provides for a best effort
service above a specified minimum cell rate (MCR).
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Constant bit rate (CBR)—Typically used to carry constant rate traffic and represented by fixed
timing. CBR is typically used for circuit emulation, uncompressed voice, and multimedia data traffic.
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