Understanding ATM Protocol Stack Architecture

The Asynchronous Transfer Mode (ATM) Protocol Stack is the backbone of high-speed communication networks, facilitating the efficient transfer of voice, video, and data. Its architecture is structured into multiple layers that work together to ensure seamless data transmission. This article delves into the core elements of the ATM Protocol Stack, focusing on the ATM Adaptation Layer (AAL) and the Data Link Layer, both of which are crucial for the functionality and reliability of the ATM system.

Figure 1 illustrates the ATM protocol layers at end host points and at an ATM switch. As shown, end systems (host-A and host-B) consist of the PHY layer, ATM layer, AAL layer, and upper layers. The ATM switch comprises only two layers: the ATM layer and the physical layer.

ATM Protocol Stack

Figure 2 depicts the ATM protocol stack, which consists of the ATM physical layer (similar to OSI layer-1), the ATM layer, and the AAL layer. Upper layers reside above the AAL layer. Let’s explore the functions of all the ATM protocol layers, including any sublayers.

The functionalities of all ATM protocol layers are categorized into the control plane, user plane, and management plane.

• User plane layers handle user information transfer and associated controls, such as error control and flow control.
• Control plane manages call and connection-related control signals.
• Management plane is divided into plane and layer management. Plane management oversees the entire system functionality, while layer management manages all resources and parameters of the protocol entities.

AAL: ATM Adaptation Layer

The AAL layer in the ATM protocol stack consists of two sublayers: the convergence sublayer and the SAR (Segmentation and Reassembly) sublayer. The ATM AAL layer performs the following functions:

• AAL encapsulates user data generated by upper layers.
• It segments data into small-sized ATM cells of 48 bytes at the transmitting host.
• It re-assembles the segmented data at the receiving host.

ATM Layer

The ATM layer in the ATM protocol layers performs the following functions:

• This layer incorporates a header (5 bytes) to segmented cells of 48 bytes each. The ATM layer generates this header at the transmit end and extracts it at the receive end.
• It is responsible for generic flow control.
• It performs VPI (Virtual Path Identifier) vs. VCI (Virtual Channel Identifier) translation.
• It handles cell multiplexing and demultiplexing, i.e., multiplexing of logical channels to one physical channel and vice versa.
• The ATM layer provides a variety of services for ATM cells from ATM virtual connections.

ATM Physical Layer

The ATM physical layer in the protocol stack consists of the Transmission Convergence sublayer and the Physical Medium Dependent Sublayer. It performs the following functions:

• Transmission convergence sublayer:

  • HEC (Header Error Control) header sequence generation and verification
  • Cell Delineation
  • Transmit frame generation and recovery

• Physical Medium Dependent sublayer:

  • Bit timing
  • Physical medium-related encoding and decoding of bits

Conclusion

Understanding the ATM Protocol architecture, particularly the ATM Adaptation and Data Link layers, is essential for leveraging the capabilities of ATM networks. The layered approach ensures that different types of data are handled efficiently, enhancing the overall performance of the communication system. Whether for broadband networks or high-speed data transmission, ATM remains a vital component in modern telecommunication.