In this case these two saps share the same sap

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Unformatted text preview: ing user. In this case, these two SAPs share the same SAP address because these SAPs provide the same service to the sending and receiving users. The sending user of a service passes Service Data Units (SDUs) and Service Control Information (SCI) to the service provider. The SDUs are the data that are delivered to the receiving user. The service control information is used to specify how the SDUs should be handled by the service provider. In the case of TC-SDLP, the sending user may receive from the service provider information on the status of the delivery of SDUs to the receiving user. Receiving User Sending User SCI SDU SDU Space Data Link Protocol (Service Provider) Figure 3-1: A Service Provided by a Space Data Link Protocol CCSDS 130.2-G-1 Page 3-1 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS The interactions between a service user and a service provider are defined in the form of ‘primitives’. Primitives present an abstract model of the logical exchange of SDUs and control information between the service provider and the service user, but they do not specify how the interactions should be implemented. 3.2 SERVICE FEATURES 3.2.1 DIRECTIONALITY All the services provided by the Space Data Link Protocols are unidirectional. The sending user of a service can send data to the receiving user using a service provider (i.e., an entity of a Space Data Link Protocol), but it cannot receive data from the receiving user using the same service provider (see figure 3-1). If the sending user also requires to receive data from some user, it must use a service of a different service provider (of the same or a different Space Data Link Protocol) that provides services in the other direction. 3.2.2 RELIABILITY General The reliability guaranteed by the services depends on the protocol used and, in some cases, the service type selected by the user. Reliability Provided by TC-SDLP General Most of the services provided by the TC Space Data Link Protocol (TC-SDLP) have two service types: the Sequence-Controlled Service (or the Type-A Service) and the Expedited Service (or the Type-B Service). The service types determine how reliably the SDUs supplied by the sending user are delivered to the receiving user. Sending users of a service that has two service types specify for each SDU whether Type-A or Type-B should be applied. Both service types are provided by the Communications Operation Procedure-1 (COP-1) specified in reference [4]. Sequence-Controlled (Type-A) Service The Type-A Service utilizes an Automatic Repeat Request (ARQ) procedure of the ‘go-back-n’ type. It is implemented by sequence-control mechanisms, which include retransmissions of lost data, at both sending and receiving ends of the space link. It also uses a standard report returned from the receiving end to the sending end using a service of the protocol used in the other direction on the space link. CCSDS 130.2-G-1 Page 3-2 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS The service provider guarantees, with a high probability of success, complete, in-sequence delivery of Type-A SDUs supplied by a sending user at a single SAP (i.e., no S...
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