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310 ISDN USER PARToriginating exchange returns an Information (INF) message that includes a Par.15(information indicators) and a Par.8 (calling party number).11.6.3 Closed User Group ServiceIn this service, the local exchange of the called subscriber allows or disallows theincoming call, based on closed user group (CUG) information about the callingand called parties (Section 9.4.7). The CUG information of the calling party is transferredby the IAM. Par.11 (closed user group interlock code) specifies the caller’sCUG, and Par.19 (optional forward call indicators) indicates whether the callingparty is allowed to access a party outside his CUG. The CUG information aboutthe called party is available at the terminating local exchange.11.6.4 User-To-User SignalingIn countries that offer this service, an ISDN user can exchange user-to-user informationduring the setup and release of a connection, by including a user–user IEin a Q.931 SETUP, ALERT, CONN, and/or DISC message (Section 10.3.5).A local exchange maps the IE.14 received in a Q.931 message into Par.28(user-to-user information, UUI) and includes the parameter in a sent ISUPmessage (Fig. 11.6-1). The other local exchange maps a received Par.28 into anIE.14 and includes it in a Q.931 message.11.6.5 Call ForwardingFigure 11.6-2 is an example of call forwarding in a network with ISUP trunks.Customer C 1 (an analog subscriber or ISDN user) initiates a call to customer C 2 ,who is currently forwarding incoming calls to customer C 3 . In turn, this customeris forwarding incoming calls to C 4 . The numbers of customers C 2 ,C 3 , and C 4 areN 2 ,N 3 , and N 4 .We examine the parameters relating to call forwarding in the IAMs sent byexchanges A, B, and C. The called party number (Par.6) is the number as perceivedby the exchange sending the IAM. The original called party number (Par.20) is thenumber dialed by C 1 . The redirecting number (Par.21) is the number of the redirecting(forwarding) party. Finally, the number of redirections (an item in Par.22) showshow often the call has been redirected.This information is used by the exchange that receives a forwarded call to decidewhether to forward the call again. In the first place, the call should not be forwardedto the original called number or to the redirecting number. Also, the exchange maybe programmed to not forward the call if it already has undergone a predeterminedmaximum number of redirections.The forwarding exchange also sends a backward Call Progress (CPG) message,which is repeated to the originating exchange A. The message includes a Par.13(event information), which indicates the reason why the call has been forwarded,and a Par.21 (redirecting number), which represents the customer who is requesting