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284 ISDN USER PARTForward Transfer Message (FOT). An outgoing operator, who is assisting asubscriber or ISDN user in a call setup, can request the help of an incoming operatorby sending a (forward) FOT message.Information Request Message (INR). This is a request for additional callrelatedinformation, usually sent from the terminating exchange to the originatingexchange of a connection (end-to-end signaling).Information Message (INF). This is a response to an INR message and containsthe requested information.Pass-Along Message (PAM). This message holds another message that is transferredend-to-end with the pass-along method. This message and method is fornational use only.Application Transport Message (APM). This message is used by applicationsto exchange application-specific information by means of the application transportmechanism (Section 11.6.10).11.2.4 Parameters in Call-Control MessagesThis section describes the most important parameters of ISUP call-control messages[5]. At this point, it is suggested that the descriptions are merely perused and referredback to when reading later sections of this chapter.To allow the reader to locate a parameter description quickly, each parameter hasa reference number (e.g., Par.1). In the descriptions of this section, the parametersappear in the order of their reference numbers. In Sections 11.3–11.7, a parameteris always identified by its name and reference number.The parameters, their reference numbers, and the messages in which they canappear are listed in Table 11.2-2.In this section, the focus is on the parameter contents, and only their Value fieldsare shown in the figures. The parameter Name codes are listed in Table 11.2-3.Par.1 Access Transport. This is an optional parameter in IAM that contains oneor more Q.931 information elements. The IEs are transferred transparently betweenthe ISDN users. The parameter can include IE.4 (called party subaddress), IE.5(calling party subaddress), IE.8 (high-layer compatibility), IE.10 (low-layer compatibility),and IE.11 (progress indicator)—see Section 10.3.5.Par.2 Application Transport (ATP). This is an optional parameter in ACM,ANM, APM, CON, CPG, and IAM messages (Fig. 11.2-3). It carries applicationspecificdata (Section 11.6.10). Ext is the extension indicator (0 ¼ last octet,1 ¼ more octets follow). ACI is the application context identifier, which identifies theapplication. SNI is the send notification indicator (0 ¼ do not send, 1 ¼ send). RCIis the release call indicator (0 ¼ do not release, 1 ¼ release). The SI and APM