Local Estimators for 802.11 MAC Channel Quality
Domenico Giustiniano, David Malone, Douglas J. Leith and Konstantina Papagiannaki
Università degli Studi di Roma-Tor Vergata; Hamilton Institute, NUI Maynooth; Intel Research, Pitsburgh.
802.11 MAC Channel Quality
Idle/Busy Estimator
This is a passive estimator using carrier sense. Uses notion of slots.
802.11 Binary Exponential Backoff mechanism.
• Transmitters cannot detect collisions explicitly.
• Receivers send ACKs after successful reception.
• Missing ACK causes doubling of backoff and retransmission.
Tx_succ
1
2
Other
3
4
5
6
Tx_unsucc
7
8
9
Other
Tx_succ
10
Other
11 12 13
14 15 16
17 18
MAC Slots
We consider 4 slot types at sender.
Many reasons for packet loss:
Idle Slots Station has seen the medium as idle and would decrement counter if in backoff.
Collisions simultaneous transmission of packets by stations following MAC rules.
Insufficient signal strength resulting in failed decoding. Caused by fading, noise or low power.
Hidden nodes transmissions of other stations unable to follow MAC rules.
Other Transmissions Station has detected the medium as busy due to other nodes transmitting.
Successful Transmissions Station 1 has transmitted and received an ACK.
Unsuccessful Transmissions Station 1 has transmitted, timed-out while waiting for an ACK and is
about to resume its backoff.
Aim: Understand error causes
Understanding local environment in order to adapt:
Power/Channel If noise is too great.
Carrier Sense If MAC protocol is failing.
Backoff If network is too busy.
R−I
;
pc =
R
1 − (T − A)/T
pe = 1 −
1 − pc
(1)
Supposing:
Introduce two backwards-compatible estimators that give insight into local environment.
1. transmit T times and of these A are successful.
2. R other slots and that I are idle.
3. probability that others transmit is independent of station transmitting.
CRC-based Estimators
Hidden Nodes and Fragmentation
Errors seen by receiver:
We use 802.11’s fragmentation feature when we want to identify hidden node errors.
PHY error an error in the PLCP preamble or header.
• fragmentation cuts packets into several smaller units,
CRC32 error packet is decoded but CRC check fails.
• each an ordinary 802.11 frame and ACKed independently,
CRC32 error
MAC
Header
• first fragment contends for access,
MSDU
• subsequent fragments may be bursted as single busy slot,
CRC
• 802.11 NAV (virtual carrier sense) on data reserves medium at sender,
• NAV will reserve medium at receiver after ACK is sent.
packet 1
PLCP
• thus subsequent fragments get significant protection from hidden nodes,
PSDU
• some undesirable features of fragments might be avoided with 802.11e TXOP.
PLCP
PSDU
Estimators with a link with low SNR
80
2
3
4
5
6
7
PHY slots(20µsec)
If we split collisions into PHY and CRC32: pc = pc1 + pc2.
CRCerr
= pe + pc2 − pepc1 − pepc2 ≈ pe + pc2
R−I
50
tx1,err
tx2,err
rx1,err
rx2,err
70
Estimators Value (%)
1
Estimators with a Hidden node
Estimators Value (%)
packet 2 (Hidden)
60
50
40
tx1,err
tx2,err
rx1,err
rx2,err
40
30
20
10
(2)
30
0
10
20
30
40
50
60
70
80
90
100
10
20
30
40
Time (sec)
50
60
70
80
90
100
Time (sec)
Low SNR, no hidden node
Testbed Setup
Hidden node, high SNR
Performance of Estimator
• Soekris net4801 single board computers with miniPCI slot,
Estimators with a node with low SNR
• Intel 2915ABG mini PCI cards with hand-customised firmware/driver,
50
Interference Range
ST1
ST1
ST2
Data AP
AP
ST2
ST4
ACK
AP1
H
98
30
20
(a) Low SNR scenario
(b) Hidden node scenario.
30
40
50
60
Time (sec)
70
80
90
100
0
Convergence of the estimator in presence of a node with low SNR.
40
25
20
15
10
30
25
20
15
10
5
5
0
0
1
2
3
4
5
Number of Stations
6
7
Estimators for an interference-free station
2
3
4
5
6
Total Number of Stations (Hidden + Non-Hidden)
Results for one hidden node.
10
20
30
40
50
60
Time (sec)
70
80
90
CRC errors for a hidden node.
pc (Clean channel)
pc (Idle/Busy)
ph + pc2 (CRC)
ph (Idle/Busy)
35
Estimators Value (%)
30
90
Estimators with a Hidden Node
Exact Frame Error Prob
pc (Idle/Busy)
pe + pc2 (CRC)
pe (Idle/Busy)
35
92
86
20
Estimators in absence of Interference
40
94
88
10
ST3
96
10
0
Data STA
rxcrc/txerr
40
H−RX
ST3
Estimators Value (%)
100
pn + pc2 (CRCerr)
pc (Idle/Busy)
pn (Idle/Busy)
Percentage (%)
Transmission Range
Estimators Value (%)
• Atheros AR5213 mini PCI cards with hand-customised driver.
CRC errors for a Hidden node
7
100