AAL (ATM Adaptation Layer)

The ATM Adaptation Layer (AAL) is a layer in the ATM protocol stack responsible for adapting user data to the ATM cell format. The AAL protocol provides a common interface for different types of data to be transported over the ATM network. It takes care of segmentation and reassembly, as well as handling various data types and quality of service (QoS) requirements. In this article, we will explain the different aspects of AAL, including its types, functions, and protocols.

Overview of ATM

Before diving into the details of AAL, it is essential to have a brief understanding of the ATM network. Asynchronous Transfer Mode (ATM) is a high-speed, cell-based network technology used to transmit data, voice, and video over a network. ATM is a connection-oriented protocol that operates at the physical and data link layers of the OSI model. It uses fixed-size cells of 53 bytes to transfer data. The ATM network can provide different classes of service, depending on the QoS requirements of the applications.

Overview of AAL

The AAL protocol is a crucial part of the ATM network, responsible for adapting different types of data to the ATM cell format. AAL operates at the higher layers of the ATM protocol stack and provides a common interface for different types of data. AAL takes care of segmentation and reassembly of data, as well as handling various data types and QoS requirements.

AAL provides an efficient way to transmit various types of data over the ATM network. The ATM cell format is not suitable for all types of data, such as variable-length data or real-time data. AAL provides a mechanism to adapt these data types to the ATM cell format.

Types of AAL

There are two types of AAL protocols: AAL1 and AAL5.

AAL1

AAL1 is used for real-time, constant bit rate (CBR) data, such as voice and video. It is connection-oriented and provides a fixed bit rate to the user. AAL1 ensures that the data is delivered in sequence and with minimal delay. AAL1 uses a technique called time-division multiplexing (TDM) to transmit the data. TDM involves dividing the data into fixed-length time slots and transmitting them in a round-robin fashion.

AAL1 adds a header to each cell to identify the user data and ensure that the cells are delivered in sequence. It also adds a trailer to each cell to provide error correction and detection.

AAL5

AAL5 is used for variable-length, non-real-time data, such as email and file transfers. It is connection-oriented and provides a variable bit rate to the user. AAL5 uses a technique called segmentation and reassembly (SAR) to transmit the data. SAR involves breaking the data into fixed-size segments and transmitting them as separate ATM cells.

AAL5 adds a header to each cell to identify the user data and the sequence number of the segment. It also adds a trailer to each cell to provide error correction and detection.

Functions of AAL

The main functions of AAL are segmentation and reassembly, handling of different data types, and QoS management.

Segmentation and reassembly

The ATM cell size is fixed at 53 bytes, which is not suitable for all types of data. AAL provides a mechanism to adapt variable-length data to the fixed-length ATM cell format. AAL5 uses segmentation and reassembly (SAR) to break the data into fixed-size segments and transmit them as separate ATM cells. AAL1 uses time-division multiplexing (TDM) to transmit the data in fixed-length time slots.

Handling of different data types

AAL provides a mechanism to handle different types of data over the ATM network. AAL1 is used for real-time, constant bit rate (CBR) data, such as voice and video, while AAL5 is used for variable-length, non-real-time data, such as email and file transfers. AAL also supports other data types, such as circuit emulation services (CES), LAN emulation (LANE), and MPEG-2 transport.

Circuit emulation services (CES) allow legacy equipment that uses TDM to be transported over an ATM network. AAL1 is used for CES to ensure that the data is delivered in sequence and with minimal delay.

LAN emulation (LANE) allows Ethernet frames to be transported over an ATM network. AAL5 is used for LANE to segment and reassemble the Ethernet frames into ATM cells.

MPEG-2 transport is used for transporting digital television signals over an ATM network. AAL5 is used for MPEG-2 transport to segment and reassemble the digital TV signal into ATM cells.

QoS management

AAL provides a mechanism to manage the QoS requirements of different applications. The ATM network can provide different classes of service, such as constant bit rate (CBR), variable bit rate (VBR), available bit rate (ABR), and unspecified bit rate (UBR). AAL ensures that the user data is delivered with the required QoS, such as bandwidth, delay, and jitter.

Protocols of AAL

The AAL protocol is divided into two sub-layers: the convergence sub-layer (CS) and the segmentation and reassembly sub-layer (SAR). The CS is responsible for mapping the user data to the ATM cell format, while the SAR is responsible for segmenting and reassembling the user data.

AAL1 protocol

The AAL1 protocol uses the CS and SAR sub-layers to transmit real-time, CBR data. The CS maps the user data to the ATM cell format and adds a header to each cell to identify the user data and ensure that the cells are delivered in sequence. The SAR uses time-division multiplexing (TDM) to transmit the data in fixed-length time slots.

The AAL1 protocol also provides a mechanism for error correction and detection. It adds a trailer to each cell to provide cyclic redundancy check (CRC) error detection and correction. The AAL1 protocol can detect and correct errors up to three consecutive cells.

AAL5 protocol

The AAL5 protocol uses the CS and SAR sub-layers to transmit variable-length, non-real-time data. The CS maps the user data to the ATM cell format and adds a header to each cell to identify the user data and the sequence number of the segment. The SAR uses segmentation and reassembly (SAR) to break the data into fixed-size segments and transmit them as separate ATM cells.

The AAL5 protocol also provides a mechanism for error correction and detection. It adds a trailer to each cell to provide CRC error detection and correction. The AAL5 protocol can detect and correct errors up to four consecutive cells.

Conclusion

The ATM Adaptation Layer (AAL) is a crucial part of the ATM protocol stack responsible for adapting user data to the ATM cell format. AAL provides a common interface for different types of data to be transported over the ATM network. AAL takes care of segmentation and reassembly, as well as handling various data types and QoS requirements. There are two types of AAL protocols: AAL1 and AAL5, which are used for real-time, constant bit rate data, and variable-length, non-real-time data, respectively. AAL provides a mechanism to manage the QoS requirements of different applications, such as bandwidth, delay, and jitter.