Az ATM interface - Reference model

Standards, recommendations:

• ITU-T I.363 - AAL
• ITU-T I.361 - ATM layer
• ITU-T I.371 - Traffic control
• ITU-T I.432 - Physical interfaces
• ITU-T I.571 - ATM management

                                      user-data
                                        |  A    G.702 PDH
                                        |  |    video, audio,
                                        v  |    IP (PPP, Ethernet)
     +----------------------------------|--|-----------------------------------+
     |  Control (Plane)           | User Layer (Plane) PPP, IP, Ethernet, .... |
     |       (Traffic contract)   | not ATM standard                           |
     |                                                                         |
     +------------------+--------+-----------+----------------------+----------+
     |                  |        |           |                      |          |
     |  Service classes:| AAL1   |   AAL2    |        AAL3/4        |  AAL5    |
     |                  |        |           |                      |          |
     |                  +--------+-----------+----------------------+----------+
     | Adaptation Layer (AAL)                                                  |
     |                    Convergence Sublayer (CS)                            |
     |                       Service Specific Convergence Sublayer (SSCS)      |
     |                       Common Part Convergence Sublayer (CPCS)           |
     |                  --------------------                                   |
     |                    Segmentation and Reassembly Sublayer (SAR)           |
     +-------------------------------------------------------------------------|
                        |                           A
     primitives         |-ATM_DATA request          |-ATM_DATA indication      (48 bytes segment)
                        v                           |
     +-------------------------------------------------------------------------|
     | ATM Layer  (Service independent, cell formatting, header generation)    |
     |            (Multiplexing/demultiplexing of cells, flow control with GFC |
     +-------------------------------------------------------------------------+
                        |                           A
     primitives:        |-PHY_DATA request          |-PHY_DATA indication      (53 bytes cells)
                        v                           |
     +-------------------------------------------------------------------------+
     | Physical         Transmission Convergence Sublayer (TC)                 |
     | Layer            ---------------------------------                      |
     | (PHY)            Physical Medium Dependent Sublayer (PMD)               |
     +-----------------------------------------|-------------------------------+
                                               |
                                             STM-1, STM4
                                             2048 kbit/s PCM, 1544 kbit/s PCM
                                             25,6 Mbit/s ATM, 51,84 Mbit/s ATM
                                             ADSL

Main function of AAL layer is that it breaks the dataflow to transmit into segments of 48 bytes and it assembles the dataflow sent by the peer entity from the received segments of 48 bytes.
The dataflow mapped into AAL frames at the user side entry of ATM. The relationship between types of AAL frames and service types of ATM is the next:

Functions of the ATM layer:

• Multiplexing to a continuous data flow the cells arriving from different sources (VPI/VCI) to an output port and vice versa demultiplexing the cells going to different destinations.
• In this layer are VPI/VCI fields changed and the appropriate output port selected at the switches.
• If a cell comes from the upper (ATM adaptation) layer, then it must be labeled with a header. If it goes to the upper layer header must be removed.
• On the user network interface flow control functions can be implemented.

Functions of TC:

• HEC generation/ check
• Cella scrambling/descrambling
• Line coding/decoding
• Cell delineation
• Cell rate decoupling

Functions of PMD:

• Bit synchron
• Connection of physical media