E no sdu is lost duplicated or delivered out of

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Unformatted text preview: DU is lost, duplicated, or delivered out of sequence). However, there is no guarantee that Type-A SDUs supplied at multiple SAPs by multiple sending users will be delivered to the receiving users in the order initially supplied. 3.2.2.2.3 Expedited (Type-B) Service The Type-B Service is normally used either in exceptional operational circumstances, typically during spacecraft recovery operations, or when a higher layer protocol provides a retransmission capability. For the Type-B Service, SDUs supplied by the sending user are transmitted only once (i.e., no retransmission is performed). The service provider does not guarantee that all Type-B SDUs are delivered to the receiving user. However, the sequence of Type-B SDUs supplied by the sending user at a SAP is preserved through the transfer over the space link, although there may be gaps in the sequence of SDUs delivered to the receiving user. If SDUs of both service types are supplied at a SAP, the sequence of Type-A SDUs and the sequence of Type-B SDUs are both preserved through the transfer over the space link, but the entire sequence of both SDUs may not be preserved. 3.2.2.3 Reliability Provided by TM-SDLP and AOS-SDLP Neither the TM Space Data Link Protocol (TM-SDLP) nor the AOS Space Data Link Protocol (AOS-SDLP) guarantees completeness of the SDUs delivered to the receiving users, but some services may signal gaps in the sequence of delivered SDUs. These protocols do not provide the sending users with a confirmation that data has been received by the receiving users. The sequence of SDUs supplied by the sending user at a SAP is preserved through the transfer over the space link, although there may be gaps in the sequence of SDUs delivered to the receiving user. The order of SDUs submitted to different SAPs is not maintained. 3.2.3 PRIORITY AND LATENCY The mechanism used by the Space Data Link Protocols for transferring data with different priority and latency requirements is the use of ‘Virtual Channels’. The Virtual Channel facility allows one Physical Channel to be divided into multiple separate logical data channels, each known as a Virtual Channel (VC). Each Virtual Channel carries a separate sequence of SDUs, which may have different priority and latency requirements from those carried on the other Virtual Channels. By using an appropriate algorithm for multiplexing Virtual Channels onto the Physical Channel, the priority and latency requirements of SDUs can be met to some extent (see 4.2). CCSDS does not recommend any standard algorithm for multiplexing Virtual Channels. It is CCSDS 130.2-G-1 Page 3-3 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS the responsibility of the project to ensure that the multiplexing algorithm used for that project actually satisfies the requirements. The TC-SDLP optionally uses MAP (Multiplexer Access Point) Channels to multiplex different sequences of SDUs, each of which may have different priority and latency requirements, onto one Virtual Channel. If the SLE F...
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