Introduction

ATM AAL

• ATM AAL are Adaptation Layers. These connect the cell based services to digital bit-stream services. There are 5 layers: AAL1, AAL2, AAL3, AAL4, AAL5.
• Most implementations rely on AAL1, AAL2 and AAL5.
• AAL1 is used for constant bit rate (CBR) services.
• AAL2-5 are used for variable bit rate (VBR) services.
• AAL5 is used for data.

AAL1

• Used for constant bit rate services such as voice and video.
• Requires timing synchronization between between source and destination (SONET support clocking).
• AAL1 uses Sequence Numbers (SNs) and Sequence Number Protection (SNP) to verify correct order.

AAL2

• Not widely used.
• Designed for VBR.

AAL3/4

• Not widely used.

AAL5

• Used for variable bit rate with connectionless applications or connection oriented to send and receive data.
• Used to send IP over ATM or LAN Emulation (LANE).
• Cells are padded to fit the 48 bytes payload.

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