AAL1 prepares a cell for transmission. The payload data consists of a synchronous sample (for example, one byte of data generated at a sampling rate of 125 microseconds). The sequence number field (SN) and sequence number protection (SNP) fields provide the information that the receiving AAL1 needs to verify that it has received the cells in the correct order. The rest of the payload field is filled with enough single bytes to equal 48 bytes.
AAL1 is appropriate for transporting telephone traffic and uncompressed video traffic. It requires timing synchronization between the source and destination and, for that reason, depends on a media that supports clocking, such as SONET. The standards for supporting clock recovery are currently being defined.
AAL3/4 is designed for network service providers and is aligned with Switched Multimegabit Data Service (SMDS) because AAL3/4 is used to transmit SMDS packets over an ATM network. The convergence sublayer (CS) creates a protocol data unit (PDU) by pre-pending a beginning/end tag header to the frame and appending a length field as a trailer.
AAL5 is the adaptation layer used to transfer most non-SMDS data, such as classical IP over ATM and LANE. NOTE
Service classes enable ATM to statistically multiplex varied traffic types. Based on the requirements of each type of stream (its burstiness, tolerance to delay, and so on), packet and circuit traffic are converted to the appropriate cell structure dictated by the AAL. The ATM endpoint equipment performs this conversion, but the equipment within the core of the WAN does not perform or use the conversion.
Several ATM adaptation layers are available:
• ATM adaptation layer type 1 (AAL1)— This layer is designed for CBR, time-dependent traffic, such as voice and video.
• ATM adaptation layer type 2 (AAL2)— This layer is designed for VBR traffic, such as compressed voice.
• ATM adaptation layer type 3/4 (AAL3/4)— This layer is intended for VBR, delay-tolerant data traffic requiring some sequencing or error-detection support.
• ATM adaptation layer type 5 (AAL5)— This layer is used for VBR, delay-tolerant connection-oriented data traffic requiring minimal sequencing or error-detection support. LAN traffic is typically encapsulated with AAL5, and many compressed voice implementations use AAL5 as well.
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