ATM standards

    BISDN Protocol Reference Model(PRM) for ATM:

    BISDN PRM consists of 3 planes.

    There is no defined(or standardized) relationship between OSI layers and BISDN ATM protocol model layers. But the following relations can be found. The Physical layer of ATM is almost equivalent to layer 1 of the OSI model and it performs bit level functions. The ATM layer can be equivalent of the lower edge of the layer 2 of the OSI model. The ATM Adaptation Layer performs the adaptation of OSI higher layer protocols.

    The BISDN Protocol Reference Model sublayers and functions can be shown by the following figure.

      Physical layerFunctions

      As shown in the above figure, Physical Layer is divided into two sub-layers. 1.Physical Medium(PM) sub-layer 2.Transmission Convergence(TC) sub-layer

      The PM sub-layer contains only the Physical Medium dependent functions. It provides bit transmission capability including bit alignment.It performs Line coding and also electrical/optical conversion if necessary.Optical fiber will be the physical medium and in some cases, coaxial and twisted pair cables are also used.

      It includes bit timing functions such as the generation and reception of waveforms suitable for the medium and also insertion and extraction of bit timing information.

      The TC sub-layer mainly does five functions as shown in the figure.The lowest function is generation and recovery of the transmission frame.

      The next function i.e. transmission frame adaptation takes care of all actions to adapt the cell flow according to the used payload structure of the transmission system in the sending direction. It extracts the cell flow from the transmission frame in the receiving direction. The frame can be a synchronous digital hierarchy (SDH) envelope or an envelope according to ITU-T Recommendation G.703.

      Cell delineation function enables the receiver to recover the cell boundaries.Scrambling and Descrambling are to be done in the information field of a cell before the transmission and reception respectively to protect the cell delineation mechanism.

      The HEC sequence generation is done in the transmit direction and its value is recalculated and compared with the received value and thus used in correcting the header errors. If the header errors can not be corrected, the cell will be discarded.

      Cell rate decoupling inserts the idle cells in the transmitting direction in order to adapt the rate of the ATM cells to the payload capacity of the transmission system. It suppresses all idle cells in the receiving direction. Only assigned and unassigned cells are passed to the ATM layer.

      ATM layer functions

      ATM layer is the layer above the physical layer. As shown in the figure, it does the 4 functions which can be explained as follows.

      Cell header generation/extraction: This function adds the appropriate ATM cell header(except for the HEC value) to the received cell information field from the AAL in the transmit direction. VPI/VCI values are obtained by translation from the SAP identifier. It does opposite i.e. removes cell header in the receive direction. Only cell information field is passed to the AAL.

      Cell multiplex and demultiplex: This function multiplexes cells from indiv- idual VPs and VCs into one resulting cell stream in the transmit direction. It divides the arriving cell stream into individual cell flows w.r.t VC or VP in the receive direction.

      VPI and VCI translation:This function is performed at the ATM switching and/or cross-connect nodes. At the VP switch, the value of the VPI field of each incoming cell is translated into a new VPI value of the outgoing cell. The values of VPI and VCI are translated into new values at a VC switch.

      Generic Flow Control(GFC): This function supports control of the ATM traffic flow in a customer network. This is defined at the B-ISDN User-to-network interface (UNI).

      ATM Adaptation Layer Functions(AAL):

      AAL is divided into two sub-layers as shown in the figure. 1.Segmentation and reassembly(SAR) 2.Convergence sublayer(CS)

      SAR sublayer: This layer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and at the receive side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers.

      CS sublayer: This layer performs functions like message identification and time/clock recovery. This layer is further divided into Common part conver- gence sublayer(CPCS) and a Service specific convergence sublayer(SSCS) to support data transport over ATM. AAL service data units are transported from one AAL serv- ice access point(SAP) to one or more others through the ATM network. The AAL users can select a given AAL-SAP associated with the QOS required to transport the AAL-SDU. There are 5 AALs have been defined, one for each class of service. The service classification for AAL can be shown by the following figure.

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