ATM Adaptation Layer:
ATM adaptation layer provides following services are divided into categories as real time and non-real time. Real time meaning has stringent delay and timing requirements. Out of five categories explained below first two are real time while other three are non-real time:
Constant Bit Rate: The CBR service category is intended for traffic with rigorous timing requirements such as voice, circuit emulation and certain types of video, that require a constant cell transmission rate for entire duration of a connection. The traffic rate is descried by peak cell rate (PCR).
- Real-time Variable Bit Rate: This service category is intended for variable bit traffic such as curtain types of videos, with rigorous timing requirement. Traffic is described by Peak Cell Rate (PCR), Maximum Burst Size (MBS), and Sustainable Cell Rate (SCR).
- Non-Real Time Variable Bit Rate: This ATM service category addresses burst sources, such as data transfer, that do not have rigorous timing requirement. Traffic is described by PCR, SCR and MBS.
- Available Bit Rate: This ATM service category is intended for sources that can dynamically adapt the rate at which they transmit cells in response to feedback from the network. This service allows the sources to exploit the bandwidth that is available in the network at a given point in the time. The sources adjust the rate at which they transmit into the network by implementing the congestion control algorithm that provides the rate flow information.
- Unspecified Bit Rate: This ATM service category does not provide any quality of service guarantees. The peak cell rate may not be specified. This service is appropriate for non-critical applications that can tolerate or readily adjust to the loss of cell.
- The AAL is divided into two sub layers: Segmentation and reassembly sublayer is to segment the PDUs of higher layer into blocks that are suitable for insertion into ATM cell payload at the source and to reassemble the higher layer PDUs from the sequence of received ATM cell payloads at destination. The Convergence Sublayer is divided into Common Part (CPCS) and Service Specific Part (SSCS). CPCS deals with packet framing and error detection functions that all AAL users require. The SSCS provides function that depends on the requirement of specific classes of AAL users.