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Spread Spectrum , , ( ) . - (Direct Sequence Spread Spectrum - DSSS) (Frequency Hopping Spread Spectrum - FHSS). 802.11 (Radio-Ethernet).
IEEE 802.11. (Working Group for Wireless Local Area Networks) (Institute of Electrical and Electronic Engineers, IEEE) (Vic Hayes) Lucent Technologies. ; , . .
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MAC- (PHY) .
MAC- , . :
ad-hoc, . .
, , , . . .
, Carrier Sense Multiple Access Collision Avoidance (CSMA/CA). (ready to send, RTS) . , . RTS (CTS), , . (ACK) . ACK , .
, . , 30 , 2048 . 4- CRC- . 400 FHSS 1500 2048 DSSS.
, ( , , , ) RC4 40- . : , , . , .
. , , , . .
Frequency Hopping Spread Spectrum (FHSS) Radio PHY. 1 / ( 2 /). 1 / (2GFSK), 2 / (4GFSK). 1 / , 1 , +170 . 170 . 2 / (+225, +75, 75, 225 ), () . 1 , . 79 2402 2480 1 . 400 .
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Direct Sequence Spread Spectrum (DSSS) Radio PHY. 1 2 /. 1 / Binary Phase Shift Keying (BPSK). 11- 11100010010, . , . , . 22 . 2 / QPSK. . , . , 90. , . , , . , . DSSS 14 , 83,5 .
(Infrared PHY) 1 / ( 2 /) - . , , , 20 .
, 802.11 (Direct Sequence Spread Spectrum, DSSS).
DSSS
. T , . , .
DSSS ( 0 1) . , , . , , . n , n- . n .
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, , (PN-), , .
, . , , () . , : ? , , . , , . . . , , . , . , , , , , , , , , (. 7.1).
. 7.1. . |
, , , , 802.11.
, .
802.11 11 (11100010010).
, .
1 /
802.11 : 1 2 /. DSSS 11- . 1 / 11×106 /, 22 . , 83,5 , , 3 . , , 11 22 , 5 . , 2400 2423 2412 . 2417 , , 11 , 2462 . , 11 3 . .
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(, ) (Differential Binary Phase Shift Key, DBPSK). . 0 π. ( 0), , π.
2 /
1 / IEEE 802.11 (Basic Access Rate), 2 / (Enhanced Access Rate). DSSS 11- , (Differential Quadrature Phase Shiftey). : 0, π/2, π 3π/2. , () . , 00 , 0; 01 , π/2; 11 , π; 10 , 3π/2.
802.11 , 2 / , 11×106 /, , .
7.2 7.2 802.11b/b+
IEEE 802.11b, 1999 , 802.11 1 2 / 5,5 11 /. 1 2 / , 5,5 11 / (Complementary Code Keying, CCK).
CCK-
CCK- , , , .
IEEE 802.11b 8- , .
, . , - . . .
{1, 1, j, j} , . , 8- CCK- : 1, 1, j, j, 1+j, 1j, 1+j, 1j. CCK- , , , , . , 8- , , 88=16777216 , , . CCK-. .
, CCK- 8 11 / 4 5,5 /. 1,385×106 (11/8 = 5,5/4 = 1,385), , 8- , , 11×106 . , 11 / 5,5 / 22 .
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5,5 11 / 802.11b, , 5,5 /, CCK- 11 /. , . , , , . , , , . , .