WLAN Performance Aspects Mohammad Hossein Manshaei Jean-Pierre - - PowerPoint PPT Presentation
Mobile Networks Module C- Part 1 WLAN Performance Aspects Mohammad Hossein Manshaei Jean-Pierre Hubaux http://mobnet.epfl.ch 1 Performance Evaluation of IEEE 802.11(DCF) Real Experimentations HoE on IEEE 802.11b Analytical
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Mohammad Hossein Manshaei Jean-Pierre Hubaux Mobile Networks http://mobnet.epfl.ch
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– HoE on IEEE 802.11b
– Bianchi’s Model
– HoE on ns-2
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PHY Layer MAC Layer
P
= Probability of Transmission
= Probability of Collision = More than one transmission at the same time = 1 – (1- )N-1 1 2 3 N
AP
1 2 3 4 N N-1 N-2 We want to calculate the throughput of this network.
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– station has to wait for DIFS before sending data – receiver acknowledges at once (after waiting for SIFS) if the packet was received correctly (CRC) – automatic retransmission of data packets in case of transmission errors t SIFS DIFS data ACK waiting time
stations receiver sender data DIFS Contention window The ACK is sent right at the end of SIFS (no contention)
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– station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) – acknowledgement via CTS after SIFS by receiver (if ready to receive) – sender can now send data at once, acknowledgement via ACK – other stations store medium reservations distributed via RTS and CTS
t SIFS DIFS data ACK defer access
stations receiver sender data DIFS Contention window RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV: Net Allocation Vector RTS/CTS can be present for some packets and not for other
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channel sender1 sender2 sender3 One slot time sender4 Collision Idle data Busy data DIFS wait wait Idle DIFS Busy wait wait wait wait Idle wait wait wait wait Idle wait wait wait wait Idle data Idle Busy Busy wait wait wait wait data DIFS Busy wait wait Idle wait wait wait wait collision data DIFS Idle data Idle Busy Busy wait wait wait wait wait wait Busy wait wait
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(i,0) (i,1) (i,2) (i,CW i-2) (i,CW i-1) 1 1 1 1 (m,0) (m,1) (m,2)
(m,CW m-2) (m,CW m-1)1 1 1 1
(s(t), b(t))
(Backoff Stage, Backoff Timer)
(0,0) (0,1) (0,2) (0,CW0-2) (0,CW0-1) 1 1 1 1 (i-1,0) (m-1,0) 1-p p 1/CW0 p/CW 1 p/CWi p/Cw i+1 p/CWm 1/CW m
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channel sender1 sender2 sender3 One slot time sender4 collision Idle data Idle Busy Busy data DIFS Busy
(0, 8) (0, 7) (2, 2) (2, 3) (0, 2) (0, 3)
Idle
(0, 8) (7, 1) (2, 3) (0, 3)
Idle
(0, 9) (7, 2) (2, 4) (0, 4)
data DIFS Idle data Idle Busy Busy Busy
(7, 3) (7, 4) (2, 5) (2, 6) (0, 5) (0, 6)
Idle
(0, 6) (0, 1) (2, 1) (0, 7)
Collision Idle data Busy data DIFS Idle DIFS Busy
(0, 6) (0, 5) (0, 7) (0, 6) (1, 3) (3, 6) (0, 5) (0, 4)
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Probability of transmission:
,
lim ( ) , ( ) , (0, ), (0, 1)
i k t i
b P s t i b t k i m k CW
Stationary distribution:
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Successful Transmission
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(0,0) (0,1) (0,2) (0,CW0-2) (0,CW0-1)
(i,0)
(i,1) (i,2) (i,CWi-2) (i,CWi-1) (i-1,0) (m,0) (m,1) (m,2)
(m,CWm-2) (m,CWm-1)
(m-1,0) 1 1 1 1 1-p 1 1 1 1 1 1 1 1 p 1/CW0 p/CW1 p/CWi p/Cwi+1 p/CWm 1/CWm
Collision
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bi,0 = p bi-1,0
(i,0)
(i,1) (i,2) (i,CWi-2) (i,CW
i-1)(i-1,0) 1 1 1 p/CW
ibm,0 = p bm-1,0 + p bm,0
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After some derivations system of two nonlinear equations with two variables p and :
Can be solved numerically to obtain p and
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– Ptr: Probability of at least one transmission in slot time – Ps: Probability of successful transmission during a random time slot – L: Average packet payload size – Ts: Average time to transmit a packet of size L – Tc: Average time of collision – Tid: Duration of the idle period – tACK: ACK transmission time – tH: Header transmission time – tL: Payload transmission time
[ ] [ ] (1 ) (1 )
s tr i s tr s tr s c tr id
P P L E Payload Transmitted by user i in a slot time E Duration of slot time P P T P P T P T
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1 (1 ) (1 ) 1 (1 )
N tr N s N s H L ACK c H L
P N P T t t SIFS t DIFS T t t DIFS
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Basic Mode RTS/CTS Basic Mode RTS/CTS
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