 and receive operations over SAPs. Figure 3.28 summarizes the different types of send and receive opera- tions that might exist in a SAP, and that are defined as part of AML. Note that each operation also exists in the four variants Info, Cn, Con and Cnl. Send-operations: Receive-operations: ssendREQ srecREQ discardSendORD arecREQ asendREQ recC_REP verifySendORD recU_REP sendORD recINV sendOFF sendINV Fig. 3.28 Send and receive operations defined in AML A short description of these operations follows: The operation ssendREQUEST is an operation that confirms if the remote SAP user has received the sent SDU. It is synchronous in the sense that the sending user cannot send anything to the remote user until the previous ssendRequest has been confirmed. The operation discardSendORDER can be used in combination with ssendREQUEST to break a blocking condition for the sending SAP user. As an alternative, the requesting user might set a timer value in the ssendRequest, which is the maximum time period the user is prepared to wait for the confirma- tion ssendConfirm. The operation asendREQUEST is an asynchronous variant of a confirmed send. It must be combined with the operation verifySendORDER, which is used by the sending user after asendRequest when he is prepared to receive the confirmation asendConfirm. The simplest form of send operation is the unconfirmed send, sendORDER. Since, in this case, the sending user does not get a confirmation that the remote user receives the SDU, it is best suited for the use of reliable CSP services. Since connectivity services of networks today tend to be more reliable than in the past, this type of send ha become more common. If the service is not reliable, the pro- tocol between the SAP users must define how transport errors are detected and handled. 3.2 The Communication Service Provider Two forms of polled send operations are possible, sendOFFER and sendINVITE. The CSP is the polling unit. The operation sendOFFER is not con- firming; sendINVITE is confirming. A sendOffer is the pol