【正文】 調(diào)度策略成為當(dāng)前AOS系統(tǒng)的核心和研究的熱點(diǎn)。虛擬信道模型采用三條虛擬信道VCVCVC3，其靜態(tài)優(yōu)先級(jí)分別為，3(數(shù)字越小，優(yōu)先級(jí)越高)。類似的，在第個(gè)調(diào)度時(shí)刻，當(dāng)上個(gè)調(diào)度時(shí)刻剩余的包數(shù)和時(shí)間內(nèi)到達(dá)的新包數(shù)總和大于等于時(shí)，至少可形成一個(gè)完整幀，此時(shí)VC2即可占用物理信道，發(fā)送數(shù)據(jù)幀。 時(shí)間片輪轉(zhuǎn)調(diào)度算法所謂輪詢調(diào)度算法是指，對(duì)系統(tǒng)中參與調(diào)度的虛擬信道按照嚴(yán)格的順序，依次輪流選擇每個(gè)虛擬信道來(lái)占用物理信道并發(fā)送數(shù)據(jù)幀，即為系統(tǒng)中每個(gè)虛擬信道分配一個(gè)固定的時(shí)間片，在各自的時(shí)間片內(nèi)該虛擬信道占用物理信道并發(fā)送數(shù)據(jù)幀。對(duì)虛擬信道調(diào)度中經(jīng)典調(diào)度算法進(jìn)行了研究，做出了一些仿真實(shí)驗(yàn)，并對(duì)靜態(tài)優(yōu)先級(jí)虛擬信道調(diào)度算法進(jìn)行了深入研究。delay IntroductionIn deep space munication, information should be transmitted back to earth in downlink with high transmission rate with high plexity. Adoption of dynamic management and standards are the keys for evolution of space techniques [1–3]. The purpose of establishing consultative mittee for space data systems(CCSDS) is to standardize space data system in structure,munication protocol and services and helps to transfer and process data in a normalized approach[4]. CCSDS file delivery protocol (CFDP) is a kind of protocol for transmitting and receiving data by relaying of storage/retransmission in advanced orbiting system[5],which works for reliable data transmission under the circumstances of huge latency and large bit error rate (BER) [6]. CFDP includes four reliable transmission models: deferred negative acknowledge (ACK) model(DNM),immediate negative acknowledge model (INM),prompted negative acknowledge model (PNM) and asynchronous negative acknowledge model (ANM) [4]. CFDP reliable transmission mechanismsThe four kinds of negative acknowledge (NAK) models can be divided into two categories: the first category includes DNM only and the second one includes INM,PNM and ANM. The main difference between these two categories is that there is no retransmission of packet data units (PDUs) in the Tinc stage of the first category,whereas there does exist retransmission of PDUs in Tinc stage of the second category,but only once. The two categories both have Tinc and Tdef stages and the Tdef stage is for lost PDUs’ retransmission. Features of CFDP reliable transmission In CFDP,data are fragmented into pieces which are packaged into PDU,and there is a metadata PDU (MPDU) which holds filename,filesize,source and destination ID,etc and the MPDU needs no feedback acknowledge the same as most PDUs. That is to say,PDUs are definitely delivered after MPDU whether MPDU is received or not and there is no handshaking procedure of linkestablishment before file delivering. In CFDP,all PDUs (MPDU included) have a source ID in the PDUheads,so if a PDU with a new ID is received,which means the ining of a new file. Also there is a special domain in every PDU that indicates beginning and ending of every PDU so receiver can identify which PDU is missing by checking this domain. As we can see,retransmission is inevitable in transmission to guarantee the reliability of information. There is a field in NAK indicating serial information of retransmissionneeded PDUs,and the filed is empty if no PDUs lose.Comparing with traditional automatic repeat request(ARQ),most PDUs in CFDP need no ACK but NAK,andACK is only used for feedback of end of file (EOF) and finish (FIN). Transmission mechanism of the first category Transmission procedure of DNM is shown in . In Tinc stage,the transmitter delivers MPDU and PDUs successively,and no retransmission occurs even PDUs lose in Tinc stage. The transmitter delivers an EOF to a receiver right behind the last PDU,then if the EOF is received,an ACK EOF is fed back. At the same time the retransmission stage Tdef begins. If the receiver finds PDUs missing,a NAK is fed back and a naktimer is initialized at the same time. Then transmitter redelivers PDUs that were wanted by the receiver when it receives the NAK. If the NAK is lost in the feedback link,a naktimer is used to trigger a new NAK for lost PDUs,and set a new timer again,till all PDUs are received successfully. Finally,an FIN is sent back to transmitter and a ACK is delivered to receiver,which means link is closed by transmitter,and the receiver closes the link also when receiving the ACK. FigA1 Transmission procedure of DNM CFDP Transmission mechanism of the second category INM CFDP Transmission procedure of INM is shown in . Different from DNM,a retransmission mechanism is introduced into Tinc stage. A NAK is triggered by the first successfully received PDU after a lost PDU,and the transmitter redelivers the wanted PDU by the receiver. However,no matter this transmission is successful or not,every wanted PDU is redelivered once at most in Tinc stage. When the receiver receives EOF and all retransmissions finish,the Tdef stage is beginning for other lost PDUs. The Tdef stage of INM is the same as that of DNM.FigA2 Transmission procedure of INM CFDP PNM CFDP Transmission procedure of PNM is shown in in Tinc stage of PNM are triggered by external events. During file delivery,an external event at the transmitter side triggers a prompted signal (Prompt),and if the receiver receives this signal,a NAK is generated and a naktimer is initialized whether PDUs are lost or not (if no PDU is lost, NAK is empty). Every NAK only counts the lost PDUs between itself and the previous NAK,and EOF is also a default prompted signal. But EOF will not trigger NAK if no PDUs lost between EOF and the previous Prompt. After EOF is received or retransmissions triggered by Prompts are all terminated,the Tdef stage following procedures are the same as that of INM.FigA3 Transmission procedure of PNM CFDP ANM CFDP Transmission procedure of ANM is shown in . In ANM,retransmission in Tinc stage is triggered by an external event at the receiver side that is different from PNM,and other procedures are the same as that of PNM.FigA4 Transmission procedure of ANM CFDP Delay evaluation of CFDP in deep space In evaluation,the number of PDUs (MPDU included) is N,BER in the downlink is Pe, PePDU is the packet lost ratio, PeEOF is the probability of lost ratio of EOF,LPDU is the length of a dat