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22.4 INTRODUCTION TO ATM SIGNALING 733TABLE 22.3-1 SSCOP PDUsPDU Acronym FunctionBegin BGN Establish connectionBegin Acknowledge BGAK Accept connection requestEnd END Release connectionEnd Acknowledge ENDAK Confirm connection releaseSequenced Data SD Sequentially numbered PDUsUnnumbered Data UD Unsequenced PDUsFigure 22.3-4. SD and UD PDU formats.layer. The main PDU types are listed in Table 22.3-1. BGN, BGAK, END, andENAK are link management functions, used to establish and release a SSCOP (signaling)connection between two nodes. Once established, a SSCOP connection issemipermanent and is used to transfer messages for all calls managed through a signalingvirtual channel (SVC—see Section 22.4).SD is the PDU for acknowledged data transfer; UD is the PDU for unacknowledgeddata transfer. A signaling message is carried as the information field of aSD PDU. The formats of SD and UD PDUs are shown in Fig. 22.3-4. For otherPDUs, we refer the reader to [11].22.4 INTRODUCTION TO ATM SIGNALINGSignaling in an ATM network is used to establish, manage, and release VCCs andVPCs, as well as to negotiate QoS and other data transfer parameters that areused during the connection. Several ATM signaling protocols exist, and they canbe grouped into two classes based on the type of interface: UNI or NNI. They canalso be categorized in terms of sponsorship: ITU-T or ATM Forum. Table 22.4-1provides a summary of the most commonly used ATM signaling protocols.TABLE 22.4-1 ATM Signaling ProtocolsInterface Type ITU-T Protocol ATM Forum ProtocolUNI DSS2 UNI (V4)NNI B-ISUP PNNIAINIB-ICI