ATM AAL Types: AAL1 vs AAL2 vs AAL3/4 vs AAL5

This article delves into the various ATM Adaptation Layers (AALs), specifically AAL1, AAL2, AAL3, AAL4, and AAL5. We’ll explore their differences and how they cater to different service requirements within the ATM protocol stack.

Understanding ATM Adaptation Layers (AALs)

ATM Adaptation Layers are designed to support diverse service classes, characterized by:

• Timing: Whether timing synchronization is required between the source and destination.
• Bit Rate: Whether the bit rate is constant (CBR) or variable (VBR).
• Connection Mode: Whether the communication is connection-oriented or connectionless.
• AAL Service Classes: (A, B, C, D)

Fig-1: ATM AAL Layers

AAL Sublayers: CS and SAR

The AAL layer in the ATM protocol stack is further divided into two sublayers based on their functionality:

• Convergence Sublayer (CS):

  • Manages lost and wrongly delivered ATM cells.
  • Handles timing recovery.
  • Performs interleaving.

• Segmentation and Reassembly Sublayer (SAR):

  • Fragments data streams from upper layers into ATM cell-sized units for transmission.
  • Reassembles ATM cells back into the original data stream for delivery to upper layers.

Fig-2: ATM Protocol Stack

AAL Type 1 (AAL1)

AAL1 is designed for Constant Bit Rate (CBR) services, such as voice and video, where consistent timing and data delivery are crucial.

Fig-3: ATM AAL Type-1 (AAL1)

The figure above illustrates the fields within the AAL1 protocol layer.

AAL Type 2 (AAL2)

AAL2 caters to Variable Bit Rate (VBR) services, particularly those with stringent timing requirements, like low bit rate video and audio. It’s more efficient than AAL1 for services that don’t always require a constant stream of data.

Fig-4: ATM AAL Type-2 (AAL2)

The figure above illustrates the fields within the AAL2 protocol layer.

AAL Type 3/4 (AAL3/4)

AAL3 and AAL4 were initially designed to support both connection-oriented (AAL3) and connectionless (AAL4) Variable Bit Rate (VBR) data services. However, they are often combined due to similarities. They are less commonly used than AAL5.

Fig-5: ATM AAL Type-3/4 (AAL3/AAL4)

The figure above illustrates the fields within the AAL3/AAL4 protocol layer.

AAL Type 5 (AAL5)

AAL5 is the most commonly used AAL for data transfer. It’s a simplified AAL designed for connection-oriented VBR services, offering lower overhead and simpler implementation compared to AAL3/4. It is often preferred for carrying data traffic.

Fig-6: ATM AAL Type-5 (AAL5)

The figure above illustrates the fields within the AAL5 protocol layer.