SIGNALLING IN ATM:
#)The information necessary for traffic contracting is exchanged between the user and the network through these signaling messages .
#)The signaling information includes Qos parameters (like cell delay variation and cell loss ratio) and traffic descriptor (like PCR , SCR and MBS) .
#)Besides the UNI specification, ATM Forum has also come up with an elaborate recommendation for the Network-Network Interface(NNI).
ATM SIGNALLING REFERENCE MODEL:
#)The signaling layer sits on top of the Signaling Adaptation ATM Layer (SAAL).
#)The SAAL can be viewed as a special AAL used for the purpose of signaling.
#)SAAL is divided into two parts – Segmentation and Reassembly (SAR) and Convergence Sublayer (CS).
#)The CS is further divided into Common Part Convergence Sublayer (CPCS) and Service Specific Convergence Sublayer (SSCS).
#)Further, SSCS is divided into Service Specific Connection-Oriented Part (SSCOP) and service Specific Coordination Function (SCCF).
#)SSCOP is the core sublayer of SAAL and provides connection-oriented transfer of PDUs.
#)It may be noted that among different layers of ATM, SSCOP is the only layer where retransmission is done.
#)SAAL Provides an assured delivery of signaling messages on an otherwise unreliable ATM layer.
#)AAL5 forms a part of SAAL and thus, all nodes carrying the signaling message from source to destination must support AAL5.
#)Intermediate switches primarily concerned with switching of cells should support at least AAL5 in order to support signalling operations.
#)It is customary to refer to the ATM signalling layer as Layer 3.
#)This is because of a direct mapping between the functionality of SAAL and that of layer 2 of the OSI reference model.
#)Thus, SAAL is commonly referred to as Layer 2 and the ATM signalling layer as Layer 3.