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#)ATM provides elaborate signaling support for dynamic establishment and release of Switched Virtual Circuits (SVC).

#)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) .

#)ATM forum has also published recommendations for signaling at the UNI. Both point-to-point and point-to-multipoint signaling procedures are laid out in ATM forum’s UNI Signalling Specification Version 4.0

#)Besides the UNI specification, ATM Forum has also come up with an elaborate recommendation for the Network-Network Interface(NNI).


#)The signaling procedures for ATM are defined for both User-Network Interface (UNI) as well as Network-Network Interface (NNI) .

#)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.

#)This is because signaling messages require reliability, as against normal user cells which may not require it.



#)SSCF provides a mapping between the primitives provided by SSCOP to that required by specific-user applications.

#)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.

#)ATM provides support for both channel-associated signalling as well as channel, non-associated signalling. Thus, all signalling messages are carried on VPI = 0 and VCI = 5.