The value w shall be set to a value between the

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Unformatted text preview: s ‘PW’; c) the width of its negative part, referred to as ‘NW’. The three variables are specified in the 6.1.8.2 and 6.1.8.3, as are the main related actions. Figure 6-1 illustrates the FARM Sliding Window concept with its different sections and actions. CCSDS 232.1-B-2 Page 6-3 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 PO Ð = P DISCARD FRAME & GO TO LOCKOUT STATE )+ V(R = 2 W/ S) N( = DISCARD FRAME & SET RETRANSMIT FLAG 1 1 ACCEPT FRAME & SET V(R) = V(R) + 1 N(S) = V(R) N(S) = V(R) Ð 1 W /2 PW E WINDOW AR EA W V( R) + IV SIT AREA = 256 Ð UT O W CK LO N( S) = N W = DISCARD FRAME N )= (S )Ð (R V N W NE G AT IV E W I N DO W AR EA = Figure 6-1: FARM Sliding Window Concept 6.1.8.2 FARM_Sliding_Window_Width (W) The FARM_Sliding_Window_Width is referred to as ‘W’ and gives the range over which the Frame Sequence Numbers of received Type-AD Transfer Frames may vary without lockout occurring. The value ‘W’ shall be set to a value between the following limits: 2 ≤ W ≤ 254 where ‘W’ is always an EVEN integer. Unless otherwise specified, the value ‘W’ shall be fixed for the entire duration of the mission. In particular, there are no COP-1 control commands for changing the value. CCSDS 232.1-B-2 Page 6-4 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 6.1.8.3 FARM_Positive_Window_Width (PW) and FARM_Negative_Window_Width (NW) As shown in figure 6-1: a) the FARM Positive Window area starts with V(R) and extends PW Transfer Frames in the positive direction; b) the FARM Negative Window area starts at V(R) – 1 (the number of the last accepted Transfer Frame) and extends NW Transfer Frames in the negative direction. The widths of both parts of the FARM Sliding Window are specified as follows: PW = NW = W/2 A Frame Sequence Number, N(S), falls outside the FARM Sliding Window (e.g., into the lockout area of width 256 – W) when: N(S) > V(R) + PW – 1 and N(S) < V(R) – NW When the Transfer Frame is in the lockout area, FARM-1 will discard the Transfer...
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