l from the CSP, which is responded to by the SAP user with a sendOrder. In case of the poll sendInvite, the user responds with either asendRequest or ssendRequest. The srecREQ operation is an operation that is used by the SAP user when it needs a new SDU from the remote user, before it can proceed with the dialogue between users. The operation is synchronous, which means that if the CSP has not received an SDU, it will wait to respond to the srecRequest until an SDU arrives. When an SDU comes in, the CSP responds with srecConfirm, which includes the SDU. The srecRefuse might be used to indicate that a corrupted SDU arrived, or it might be omitted. In order to not be blocked indefinitely wait- ing for incoming SDUs, the requesting user might set a timer value in the srec- Request, which is the maximum time period the user is prepared to wait for the next SDU from the remote user. arecREQUEST is an alternative to the srecREQUEST. The operation arecRE- QUEST is suitable for non-polling procedures. The SAP user calls the CSP with a arecRequest when it needs an SDU from the remote SAP user. The requesting SAP user is not blocked if there are no SDUs, and it can repeat the operation at any time, e.g. at regular intervals. Thus, the important difference from srecREQUEST is that the CSP does not wait to respond until an SDU arrives-it responds immediately. If a received SDU exists, the CSP responds with arecCo- nfirm(SDU,...). If not, the CSP responds with arecRefuse("noSDU"). A more passive form of reception is the recC_REPORT. In this case, the CSP sends, in a recReportConfirm, correctly received SDUs to the SAP user as soon as they arrive. The reception of erroneous SDUs might be reported to the user in a recReportRefuse. An alternative to recC_REPORT is recU_REPORT. It comprises a single ASP, recRepOrder. This ASP delivers received SDUs without any guarantee that they are correct. A very different form of receive operation is recINVITE. In this case, the CSP polls the user for reception with a recInvite. If the user 