ATM Layers

ATM is one of the three key layers of Broadband Integrated Services Digital Network (B-ISDN). The other two layers are the Physical Layer and the ATM Adaptation Layers (AALs).

Physical Layer

The Physical Layer is responsible for the electrical or optical transmission and receipt along the physical media between two devices. The physical layer supports different physical media and different media interface rates. The physical layer maps the cells into time-division multiplexed frames, which are then sent over the physical medium.

ATM layer

ATM layer is placed above the Physical Layer. The responsibility of ATM layer is to handle multiplexing and switching and maintains the Quality of Service (QoS). Whenever an ATM connection is made, a traffic contract is established between the user and the network. The characteristics of the user data are established by this traffic contract. Thus by determining the type of data, the network allocates the amount of resources (bandwidth) required to the user to ensure that the traffic can be supported. The user communicates such details as the average and burst traffic rates and the acceptable loss and delay levels. These parameters determine the QoS. The ATM layer is responsible for maintaining the QoS for each connection.

ATM Adaptation Layer (AALs)

The ATM Adaptation layer adapts user traffic into cell-format. AALs break data of various types, having different characteristics into 48 byte-cells. At the receiving end the cells are reassembled into the original form by the AAL. Different AALs exist for different types of traffic.
AALs are divided into 2 sub-layers namely Convergence Sub-layer (CS) and the Segmentation and Reassembly (SAR) sub-layers. The function of the CS layer is to describe the method in which non-ATM traffic is converted into ATM traffic. The function of SAR layer is to insert data into ATM cells and adds header information. There are different types of AALs, each supporting a different type of traffic or service.

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