This service only guarantees complete delivery of

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Unformatted text preview: plete end-to-end delivery through the entire network. Neither TM-SDLP nor AOS-SDLP has a retransmission capability, so retransmission must be done by a higher-layer protocol if complete delivery of data is required. 4.4.2 4.4.2.1 COMMUNICATIONS OPERATION PROCEDURE-1 (COP-1) General The TC-SDLP relies on forward error correction/detection mechanisms provided by the Synchronization and Channel Coding Sublayer to support the error-free delivery of SDUs. However, to improve protection against errors undetected by the Synchronization and Channel Coding Sublayer, an error-control mechanism is also available in TC-SDLP. This mechanism, in connection with the underlying services of the Synchronization and Channel Coding Sublayer, provides a very high level of protection against errors introduced on the space link. It is also advisable to use the Frame Error Control Field defined in reference [1] when the forward error correction/detection capability provided by the Synchronization and Channel Coding Sublayer is not sufficient. For information on the performance of the various error protection options provided by the Channel Coding Sublayer and the TCSDLP, refer to reference [21]. This error-control mechanism of the TC-SDLP is provided by the Communications Operation Procedure-1 (COP-1) when the Sequence-Controlled (Type-A) Service is used. It is a closed-loop procedure executed by the sending and receiving ends of TC-SDLP, and CCSDS 130.2-G-1 Page 4-5 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS utilizes an automatic request for retransmission (ARQ) procedure of the ‘go-back-n’ type to retransmit Transfer Frames that were rejected by the receiving end of the protocol. It allows Transfer Frames to be accepted by the receiving end only if they are received in strict sequential order, thus enabling the SDUs to be delivered to the layer above at the receiving end, correct and without omission or duplication, and in the same sequential order in which they were received from the layer above at the sending end. COP-1 consists of a pair of synchronized procedures for each Virtual Channel: a Frame Operation Procedure-1 (FOP-1) that executes within the sending entity; and a Frame Acceptance and Reporting Mechanism-1 (FARM-1) that executes within the receiving entity. The FOP-1 transmits Transfer Frames of a particular Virtual Channel to the FARM-1 of the same Virtual Channel. The FARM-1 returns reports of the status of Transfer Frame acceptance to the FOP-1 using data units called the Communications Link Control Words (CLCWs), which are transmitted by a Space Data Link Protocol used on the other direction. COP-1 provides two services (Sequence-Controlled Service and Expedited Service) that determine how reliably SDUs supplied by the sending user are delivered to the receiving user. 4.4.2.2 Sequence-Controlled Service The Sequence-Controlled Service (or the Type-A Service) is realized by an automatic request for retransmission (ARQ) procedure of the ‘go-back-n...
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