Configuring the AAL and Encapsulation Type

This section covers configuring the AAL and encapsulation type options.

ATM Adaptation Layers

• AAL1

voice and video, uncompressed

• AAL2

voice and video, compressed

• AAL3/4

SMDS packets

• AAL5

data

• AAL5

SEAL—Simple and Efficient Adaptation

Layer

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. Cisco CCIE Prep vl.O—Module4-17

ATM Adaptation Layers: AAL1

AAL1, a connection-oriented service, is suitable for handling circuit-emulation applications, such as voice and video conferencing. Circuit-emulation service also accommodates the attachment of equipment currently using leased lines to an ATM backbone network. AAL1 requires timing synchronization between the source and destination.

ATM Adaptation Layers: AAL2

AAL2 is suitable for conveying packetized voice and video traffic.

ATM Adaptation Layers: AAL3/4

AAL3/4 supports both connection-oriented and connectionless data. AAL3/4 works primarily for network service providers and aligns closely with Switched Multimegabit Data Service (SMDS). You can use AAL3/4 to transmit SMDS packets over an ATM network.

ATM Adaptation Layers: AAL5

AAL5 is the primary AAL for data and supports both connection-oriented and connectionless data. You can use it to transfer most non-Switched Multimegabit Data Service (SMDS) data, such as classical IP over ATM and Local Area Network Emulation (LANE). Some technicians refer to AAL5 as the simple and efficient adaptation layer (SEAL), because the Segmentation and Reassembly (SAR) sublayer simply accepts the Convergence Sublayer Packet Data Unit (CS-PDU) and segments it into 48-octet SAR-PDUs without adding any additional fields.

AAL5 prepares a cell for transmission in three steps. First, the Carrier Selection (CS) sublayer appends a variable-length pad and an 8-byte trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte boundary of an ATM cell. The trailer includes the length of the frame and a 32-bit cyclic redundancy check (CRC) computed across the entire PDU. This allows the AAL5 receiving process to detect bit errors, lost cells, or cells that are out of sequence. Second, the SAR sublayer segments the CS-PDU into 48-byte blocks. The router does not add a header and trailer (as is in AAL3/4), so the router cannot interleave messages. Finally, the ATM layer places each block into the Payload field of an ATM cell. For all cells except the last, the ATM network sets a bit in the Payload Type (PT) field to zero to indicate that the cell is not the last cell in a series that represents a single frame. For the last cell, the ATM network sets the bit in the PT field to one.

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