ATM Technology
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intercalated between the high layers (TCP/IP, LAN Emulation, etc) and the physical layer, which manages the
phenomena related to medium of transmission.
The ATM layer deals with the commutation and the multiplexing of the cells. It delivers ATM cells to the
physical layer for transport through the network.
The AAL layer adapts high layers and ATM world. It contains the process of segmentation and re-assembly
(SAR) and the Convergence Sub layer (CS).
The SAR segments each packet coming from high layers into ATM cells for the ATM layer. Conversely the
SAR reassembles incoming ATM cells into packets in a form expected by high layers.
The CS maintains for the high layers the quality of service (QoS) that is defined for its traffic class. The high
layers are assured that the network will deliver that QoS for the life of the connection.
ATM Adaptation Layer (AAL)
ATM Layer
Physical Layer
High Layers
Figure 6: ATM Model Layers
Traffic Classes
The concept of traffic classes comes from the world of telecommunications. As in telephone networks, with
each connection is associated a sort of contract that defines the behavior that the network must respect (QoS).
The idea is to make the network able to transport different traffic types for high-bandwidth applications such
as voice, data, and video.
To meet the user's needs for high-bandwidth networks, ATM standards define four traffic classes associated
with five AAL protocols (AAL1 with AAL5 (Table 2).
Traffic Class
Traffic Description and Example
A
(AAL1)
Constant Bit Rate (CBR), Connection-oriented,
Synchronous traffic (uncompressed voice and video)
B
(AAL2)
Variable Bit Rate (VBR), Connection-oriented,
Synchronous Traffic (compressed voice and video)
C
(AAL3/4)
Variable Bit Rate (VBR), Connection-oriented
Asynchronous Traffic (X25, TCP/IP, Frame Relay, …)
D
(AAL5)
Variable Bit Rate (VBR), Available Bit Rate (ABR),
Connectionless, Asynchronous Traffic (LAN, TCP/IP,
Frame Relay, …)
Table 2: Traffic Classes
ATM with Traditional LANs
Taking into account the enormous base of installed Ethernet and Token-Ring LANs, it was important for
ATM networks to be able to support the existing applications. Several standards make it possible today.
CLASSICAL IP OVER ATM worked out by the IETF (Internet Engineering Task force)
