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January 2001 doc.: IEEE 802.11-02/065r0
802.11g MAC Analysis
Menzo Wentink
mwentink@intersil.com
Ron Brockmann
rbrockma@intersil.com
Maarten Hoeben
mhoeben@intersil.com
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
802.11g MAC Related Settings
• The following parameters are used:
802.11b 802.11a 802.11g
aSIFSTime 10 usec 16 usec 10 usec
aSlotTime 20 usec 9 usec 20 usec
aCWmin 31 slots 15 slots 15 slots
• A 6 usec silence period is added to OFDM frames, to
mitigate for the 16 usec OFDM SIFS
• ACK frames shall be sent at a Basic Rate or PHY
mandatory rate
• The RTS Threshold can be dynamically set by a link
optimization algorithm, or by an information element in
the beacon
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Recommendation: SIFS 10 usec
• OFDM requires a 16, not 10 usec RX-TX
turnaround
• This is solved in CCK-OFDM by adding a 6 usec
postamble to the packet, effectively extending the
SIFS for the receiver
• The transmitter is active longer than necessary,
and the TX-RX turnaround time available is
significantly reduced
• Recommendation: add a 6 usec silence period is
added to each OFDM frame, with the same
function as the CCK-OFDM postamble
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Recommendation: Slot Time 20 us
• When 802.11 DS was defined, a 20 us slot was
equivalent to 5 bytes at the highest rate of 2 Mbit/s
• Today, 20 us can transfer 135 bytes at 54 Mbit/s !
• Backoff slots are very expensive – this favors
bursting techniques in PCF and TGe HCF
• Slot time is part of the definition of PIFS and
DIFS affecting core MAC/TGe behaviours, and
cannot be changed without significant coexistence
issues
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Recommendation: CWmin 15
• High cost of slot time calls for shorter backoff
window
• 802.11a uses CWmin 15
• Extensive simulations show CWmin 15 gives
markedly higher overall performance in all typical
scenarios than CWmin 31
• 802.11g nodes operating in full 802.11b backward
compatibility mode (not using the 802.11g rates)
should comply with 802.11b and use CWmin 31
• For .11g+e products, CWmin can be overruled
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
ACK Rates
• It is desired to transmit OFDM ACK frames in
response to OFDM DATA frames because they
are substantially more efficient
• Section 9.6 of 802.11-1999 and 802.11b contradict
on whether this is required/forbidden when the
Basic Rates do not include OFDM rates in a
mixed environment
• Recommendation: clarify section 9.6 to support
the use of OFDM Mandatory rates in response
to OFDM frames even if they are not part of
the Basic Rate Set as described in 02/xxx
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
RTS Threshold
• RTS/CTS is used to protect OFDM frames in a
mixed b/g environment
• Can either be enabled/disabled statically by MIB
variable, or a dynamic link optimization algorithm
can be used
• Perhaps, a Recommended Practice can be defined
• Legacy 802.11b STAs do not have to use
RTS/CTS, unless required to optimize the link for
hidden nodes or excessive collision scenarios
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Analysis of MAC Performance
• DCF Performance
• Mixed b/g – without RTS/CTS
• Mixed b/g – with RTS/CTS, Cwmin 31
• Mixed b/g – with RTS/CTS, Cwmin 15
• Migration rom Legacy to Pure OFDM
• Pure OFDM, TCP DCF Efficiency, CWmin 15/31
• Pure OFDM, UDP DCF Efficiency, CWmin 15/31
• TGe QoS Bursting
• TGe QoS Video Scenario
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
DCF Performance
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Average Frame Tx Durations
Durations for a 1500 Byte TCP frame transmission
ofdm 24
rts
cck-ofdm 24
cts
data
pbcc 22
ack
av. backoff 15
rts-cts ofdm 24
* av. backoff 31
cck (11b)
0 150 300 450 600 750 900 1050 1200 1350 1500 1650 1800
Transmission Time (usec)
*) RTS CTS OFDM features cheap collisions (cost of one RTS) and built-in hidden node protection
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Throughput Comparison for 24/22 Mbps
802.11g Performance (22/24 Mbps, CWmin = 15)
16
15
14
13
12
11
10
9
Throuhput
8
(Mbps)
7
6
5
4
3
2
1
0
cck11 pbcc22 ofdm rts/cts cck-ofdm ofdm
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Mixed b/g
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Mixed b/g – without RTS/CTS
Performance in a mixed scenario, without RTS/CTS
(802.11g / legacy)
10
The unprotected OFDM
9 packets collide with
8 legacy CCK. The OFDM
the aggregate throughput goes down TCP flows are starved.
Throughut (Mbps)
7
6 Node 1 (802.11g)
2 OFDM nodes without RTS/CTS Node 2 (802.11g)
5
+ 2 legacy nodes Node 3 (legacy)
4 legacy nodes
4 Node 4 (legacy)
Aggregate
3
2 the throughput of the legacy
nodes goes up
1
The throughput of OFDM
0
nodes diminishes, because
0 1 2 3 4 5 6 7 8 9 10 11 12
OFDM yields for CCK,
Time (sec) but not v.v.
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Mixed b/g – with RTS/CTS, CWmin 31
Perormance in a mixed scenario with RTS/CTS
(CWmin = 31 for 802.11g)
10
the aggregate throughput goes up
9
8 Protected OFDM
Throughput (Mbps)
transmissions nicely
7
mix with legacy
6 Node 1 (802.11g)
2 OFDM nodes with RTS/CTS Node 2 (802.11g)
5
4 legacy nodes 2 legacy nodes Node 3 (Legacy)
4 Node 4 (Legacy)
Aggregate
3
2 The throughput of OFDM
and legacy goes up by same
1
amount due to fairness of
0 DCF. RTS/CTS-protected
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (sec)
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
DCF Fairness
• For equal CWmin, throughput increase is
distributed over all nodes!
– DCF gives each node equal number of transmit
opportunities, regardless of their data rate
– Legacy 802.11b frame transmissions are longer and
they hog media time with their inefficient modulations
– Aggregate throughput increases but less than expected
• By using a smaller CWmin, TGg nodes can get
higher priority
– Since their transmissions are shorter, total time spent on
the media is comparable to legacy nodes
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Mixed b/g – with RTS/CTS, CWmin 15
Performance in a mixed scenario, with RTS/CTS
(CWmin = 15 for 802.11g)
10
the aggregate throughput goes up
9
RTS/CTS-protected
8 OFDM transmissions
nicely mix with legacy
Throughput (Mbps)
7
6
2 OFDM nodes with RTS/CTS Node 1 (802.11g)
+ 2 legacy nodes Node 2 (802.11g)
5
Node 3 (legacy)
4 legacy nodes
4 Node 4 (legacy)
Aggregate
3
the throughput of OFDM
2
nodes goes up, because of
1 more efficient transmissions
and smaller CWmin.
0
0 1 2 3 4 5 6 7 8 9 10 11 12 the legacy throughput
Time (sec) levels
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Migration from Legacy to
802.11g
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Migration to 802.11g from legacy
TCP performance during migration to 802.11g
(CWmin = 15, OFDM 36 for 802.11g nodes)
20
18
aggregate throughput
16
Throughput (Mbps)
4 g-nodes
14
w/o rts/cts
Node 1
12
Node 2
4 g-nodes
10 Node 3
3 g-nodes Node 4
8
1 b-node Aggregate
2 g-nodes
6
2 b-nodes
4b
4
2 Individual
0
throughputs
0 5 10 15 20 25
OFDM and legacy CCK Time (sec)
transmissions are mixed.
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Pure OFDM
UDP Performance Comparison
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Performance in relation with CWmin (1)
Performance compared for CWmin = 15 and CWmin = 31
26
25
CWmin = 15
24
23
Throughput (Mbps)
CWmin = 31
22
CWmin = 15
21
CWmin = 31
20
19
18
17
16
1 2 3 4
# backlogged contenders
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Performance in relation with CWmin (3)
CWmin 15 vs. 31
30
CWmin = 15
25
Throughput (Mbps)
CWmin = 31
20
15 CWmin = 15
CWmin = 31
10
5
0
0 5 10 15
# backlogged contenders
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Pure OFDM
TCP Performance Comparison
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Throughput comparison for TCP
Contention Window comparison
25
20
15
Throughput CWmin = 31
(Mbps) CWmin = 15
10
5
0
Rate = 36 Rate = 54
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
802.11e QoS Scenarios
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Migration with 802.11e HCF Bursting
802.11e/g migration scenario
20
18
Aggregate throughput 4 g-nodes
16
3 g-nodes
14 (CFBs)
2 g-nodes
Node 1
1 b-node
12 (CFBs) Node 2
Mbps
2 b-nodes
10 Node 3
8 Node 4
6 Aggregate
4 b-nodes
4
2 Individual
0 throughputs
0 5 10 15 20
sec Legacy throughput levels
Throughput for g-nodes rises sharply
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Streaming video with 802.11e/g
2x 12 Mbps Video over 802.11g, in legacy environment,
with 802.11e HCF CAPs
30
25
aggregate throughput
Throughput (Mbps)
20
Aggregate
Node 1 (.11g video)
15
Node 2 (.11g video)
Node 3 (.11b legacy)
10 Node 4 (.11b legacy)
5 2x 12 Mbps video
0
0 5 10 15 20
Time (sec) no starvation of background
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Simulation Environment
• Network Simulator (NS)
– from University of California
– 802.11 added by Carnegie Mellon
– 802.11e EDCF added by Atheros
• We added
– 802.11g PHY (next to 11b PHY)
– Dynamic Rate selection and duration calculation
– 802.11e Contention Free Bursting
• Typical simulation setup
– 4 stations (b or g) and 1 AP (g)
Submission Brockmann, Hoeben, Wentink (Intersil)
January 2001 doc.: IEEE 802.11-02/065r0
Conclusions
• Mixed 802.11b/g operation increases
network throughput
• Pure 802.11g operation is efficient
• TGe enhancements work for mixed and
pure g networks; provide greater MAC
efficiency
• Recommendations to be adopted
Submission Brockmann, Hoeben, Wentink (Intersil)
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