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doc.: IEEE /065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 3 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
Citation preview
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 1
doc.: IEEE 802.11-02/065r1
Submission
802.11g MAC Analysis and Recommendations
Menzo Wentink ([email protected])
Ron Brockmann ([email protected])
Maarten Hoeben ([email protected])
Tim Godfrey ([email protected])
Mark Webster ([email protected])
Steve Halford ([email protected])
Carl Andren ([email protected])
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 2
doc.: IEEE 802.11-02/065r1
Submission
802.11g MAC Related Settings• The following parameters are used:
802.11b 802.11a 802.11gaSIFSTime 10 usec 16 usec 10 usecaSlotTime 20 usec 9 usec 20 usecaCWmin 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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 3
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 4
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 5
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 6
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 7
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 8
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 9
doc.: IEEE 802.11-02/065r1
Submission
DCF Performance
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 10
doc.: IEEE 802.11-02/065r1
Submission
Average Frame Tx Durations
*) RTS CTS OFDM features cheap collisions (cost of one RTS) and built-in hidden node protection
*
0 150 300 450 600 750 900 1050 1200 1350 1500 1650 1800
Transmission Time (usec)
cck (11b)
rts-cts ofdm 24
pbcc 22
cck-ofdm 24
ofdm 24
Durations for a 1500 Byte TCP frame transmission
rtsctsdataackav. backoff 15av. backoff 31
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 11
doc.: IEEE 802.11-02/065r1
Submission
Throughput Comparison for 24/22 Mbps
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Throuhput (Mbps)
cck11 pbcc22 ofdm rts/cts cck-ofdm ofdm
802.11g Performance (22/24 Mbps, CWmin = 15)
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 12
doc.: IEEE 802.11-02/065r1
Submission
Mixed b/g
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 13
doc.: IEEE 802.11-02/065r1
Submission
Performance in a mixed scenario, without RTS/CTS(802.11g / legacy)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (sec)
Thro
ughu
t (M
bps)
Node 1 (802.11g)Node 2 (802.11g)Node 3 (legacy)Node 4 (legacy)Aggregate
Mixed b/g – without RTS/CTS
the throughput of the legacynodes goes up
the aggregate throughput goes down
The throughput of OFDM nodes diminishes, because OFDM yields for CCK, but not v.v.
2 OFDM nodes without RTS/CTS+ 2 legacy nodes4 legacy nodes
The unprotected OFDM packets collide with legacy CCK. The OFDM TCP flows are starved.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 14
doc.: IEEE 802.11-02/065r1
Submission
Perormance in a mixed scenario with RTS/CTS(CWmin = 31 for 802.11g)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (sec)
Thro
ughp
ut (M
bps)
Node 1 (802.11g)Node 2 (802.11g)Node 3 (Legacy)Node 4 (Legacy)Aggregate
Mixed b/g – with RTS/CTS, CWmin 31
the aggregate throughput goes up
The throughput of OFDM and legacy goes up by same amount due to fairness of DCF. RTS/CTS-protected
2 OFDM nodes with RTS/CTS2 legacy nodes4 legacy nodes
Protected OFDM transmissions nicely mix with legacy
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 15
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 16
doc.: IEEE 802.11-02/065r1
Submission
Performance in a mixed scenario, with RTS/CTS(CWmin = 15 for 802.11g)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (sec)
Thro
ughp
ut (M
bps)
Node 1 (802.11g)Node 2 (802.11g)Node 3 (legacy)Node 4 (legacy)Aggregate
Mixed b/g – with RTS/CTS, CWmin 15
the legacy throughput levels
the throughput of OFDM nodes goes up, because of more efficient transmissions and smaller CWmin.
2 OFDM nodes with RTS/CTS+ 2 legacy nodes
4 legacy nodes
RTS/CTS-protected OFDM transmissions
nicely mix with legacy
the aggregate throughput goes up
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 17
doc.: IEEE 802.11-02/065r1
Submission
Migration from Legacy to 802.11g
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 18
doc.: IEEE 802.11-02/065r1
Submission
TCP performance during migration to 802.11g(CWmin = 15, OFDM 36 for 802.11g nodes)
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25
Time (sec)
Thro
ughp
ut (M
bps)
Node 1Node 2Node 3Node 4Aggregate
Migration to 802.11g from legacy
4 b
2 g-nodes2 b-nodes
3 g-nodes1 b-node
4 g-nodesw/o rts/cts
Individual throughputs
aggregate throughput
OFDM and legacy CCK transmissions are mixed.
4 g-nodes
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 19
doc.: IEEE 802.11-02/065r1
Submission
Pure OFDM UDP Performance Comparison
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 20
doc.: IEEE 802.11-02/065r1
Submission
Performance compared for CWmin = 15 and CWmin = 31
16
17
18
19
20
21
22
23
24
25
26
1 2 3 4
# backlogged contenders
Thro
ughp
ut (M
bps)
CWmin = 15CWmin = 31
Performance in relation with CWmin (1)
CWmin = 31
CWmin = 15
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 21
doc.: IEEE 802.11-02/065r1
Submission
Performance in relation with CWmin (3)CWmin 15 vs. 31
0
5
10
15
20
25
30
0 5 10 15
# backlogged contenders
Thro
ughp
ut (M
bps)
CWmin = 15CWmin = 31
CWmin = 31
CWmin = 15
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 22
doc.: IEEE 802.11-02/065r1
Submission
Pure OFDMTCP Performance Comparison
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 23
doc.: IEEE 802.11-02/065r1
Submission
Throughput comparison for TCP
0
5
10
15
20
25
Throughput (Mbps)
Rate = 36 Rate = 54
Contention Window comparison
CWmin = 31CWmin = 15
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 24
doc.: IEEE 802.11-02/065r1
Submission
802.11e QoS Scenarios
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 25
doc.: IEEE 802.11-02/065r1
Submission
802.11e/g migration scenario
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20sec
Mbp
s
Node 1Node 2Node 3Node 4Aggregate
Migration with 802.11e HCF Bursting
4 b-nodes
2 g-nodes(CFBs)
2 b-nodes
3 g-nodes(CFBs)
1 b-node
Individual throughputs
Aggregate throughput 4 g-nodes
Throughput for g-nodes rises sharply
Legacy throughput levels
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 26
doc.: IEEE 802.11-02/065r1
Submission
2x 12 Mbps Video over 802.11g, in legacy environment,with 802.11e HCF CAPs
0
5
10
15
20
25
30
0 5 10 15 20
Time (sec)
Thro
ughp
ut (M
bps)
AggregateNode 1 (.11g video)Node 2 (.11g video)Node 3 (.11b legacy)Node 4 (.11b legacy)
Streaming video with 802.11e/g
aggregate throughput
2x 12 Mbps video
no starvation of background
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 27
doc.: IEEE 802.11-02/065r1
Submission
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)
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 28
doc.: IEEE 802.11-02/065r1
Submission
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
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 29
doc.: IEEE 802.11-02/065r1
Submission
Element for Legacy Indication
• 802.11g introduces the need for a BSS to indicate the presence of legacy stations (either associated to, or in the vicinity of the BSS) so the 802.11g stations can make optimal decisions on whether RTS/CTS (or other protection mechanisms) are needed for OFDM frames.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 30
doc.: IEEE 802.11-02/065r1
Submission
Recommendations
In the form of Motions
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 31
doc.: IEEE 802.11-02/065r1
Submission
• 802.11g stations need to know if any legacy stations are associated in the BSS. If no legacy stations are associated, the 802.11g stations do not need to use protection mechanisms for OFDM frames.
• The AP keeps track of associated stations, and knows (by their capability information bits) whether they are 802.11g stations or legacy stations.
• Legacy stations will not understand this new element, and will ignore it.
Need for a new element
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 32
doc.: IEEE 802.11-02/065r1
Submission
Element Definition• A new element is defined, with one octet value.• The octet contains two 1-bit fields.
– B0 is set to 1 if any 802.11b stations are associated– B1 is optional. It is set to the same value as bit 0
unless optional, additional information is provided.• This bit may be used by “smart” APs that implement
techniques to provide additional information to stations.
– “r” bits are reserved.
Element ID Length =1 B0 B1 r r r r r r
B0 B7One OctetOne Octet One Octet
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 33
doc.: IEEE 802.11-02/065r1
Submission
Mandatory Functions• An 802.11g conformant AP must generate this
element.– The AP must set bit 0 to a “0” if no 802.11b stations
are associated. The AP must set bit 0 to a “1” if any 802.11b stations are associated.
– If the AP is not providing additional information, it must set bit 1 to the same value as bit 0.
• There is no mandatory behavior for a station. It may or may not make use of this element. – The recommended use of this information is to indicate
the need to use protection mechanisms (such as RTS / CTS) for OFDM frames.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 34
doc.: IEEE 802.11-02/065r1
Submission
Use of Bit 1• Bit 1 must be set to the same value as bit 0, unless
additional information is conveyed through the following encoding:
Bit 0 Bit 1 Meaning
0 1 No 802.11b legacy stations are associated, but the AP recommends the use of protection mechanisms (possibly because legacy frames from another BSS have been received by the AP)
1 0 802.11b legacy stations are associated, but the AP suggests that protection mechanisms are not necessary currently, possibly because the legacy stations have all been “quiet” (perhaps in power save).
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 35
doc.: IEEE 802.11-02/065r1
Submission
Add a new clause to 7.3.2 (7.3.2.last+1) containing the following text:– The legacy indication element provides 802.11 stations with an indication
of the presence of legacy stations in the BSS. See Figure xx. Stations may use this information to control their use of protection mechanisms (such as RTS / CTS) for OFDM frames. An Access Point shall generate this element in each Beacon Frame. The AP shall set bit 0 to a “0” if no 802.11b stations are associated. The AP shall set bit 0 to a “1” if any 802.11b stations are associated. The AP shall set bit 1 to the same value as bit 0 unless it is providing additional, optional information. If optional information is provided, it shall be according to this table:
• The editor is requested to assign a unique element ID.
Bit 0 Bit 1 Meaning0 0 No 802.11b legacy stations are associated, and the AP suggests that protection mechanisms are not currently
needed.0 1 No 802.11b legacy stations are associated, but the AP recommends the use of protection mechanisms
1 0 802.11b legacy stations are associated, but the AP suggests that protection mechanisms are not currently needed.
1 1 802.11b legacy stations are associated, and the AP recommends the use of protection mechanisms
Element ID Length =1 b0 b1 r r r r r rB0 B7One OctetOne Octet One Octet
Figure xx: Legacy Indication Element
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 36
doc.: IEEE 802.11-02/065r1
Submission
Motion on RTS/CTS usage for OFDM
• Instruct the editor to incorporate the text in the previous slide into the draft.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 37
doc.: IEEE 802.11-02/065r1
Submission
Background on Rate for ACK frames• IEEE 802.11-1999 Section 9.6:
– “All Control frames shall be transmitted at one of the rates in the BSSBasicRateSet (see 10.3.10.1), or at one of the rates in the PHY mandatory rate set so they will be understood by all STAs.”
– “In order to allow the transmitting STA to calculate the contents of the Duration/ID field, the responding STA shall transmit its Control Response frame (either CTS or ACK) at the same rate as the immediately previous frame in the frame exchange sequence (as defined in 9.7), if this rate belongs to the PHY mandatory rates, or else at the highest possible rate belonging to the PHY rates in the BSSBasicRateSet.”
• IEEE 802.11b modified this section to read:– “All Control frames shall be transmitted at one of the rates in the BSS
basic rate set so that they will be understood by all STAs in the BSS.”– “To allow the transmitting STA to calculate the contents of the
Duration/ID field, the responding STA shall transmit its Control Response and Management Response frames (either CTS or ACK) at the highest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immedi-ately previous frame in the frame exchange sequence (as defined in 9.7). In addition, the Control Response frame shall be sent using the same PHY options as the received frame. “
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 38
doc.: IEEE 802.11-02/065r1
Submission
Motion to instruct the editor to add text to section 9.6 as follows:
• “All Control frames shall be transmitted at one of the rates in the BSS basic rate set so that they will be understood by all STAs in the BSS. For the IEEE 802.11g PHY, Control Response frames shall be sent at one of the Extended Rate PHY (ERP) mandatory rates in response to an OFDM frame as described below.
• “To allow the transmitting STA to calculate the contents of the Duration/ID field, the responding STA shall transmit its Control Response and Management Response frames (either CTS or ACK) at the highest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immediately previous frame in the frame exchange sequence (as defined in 9.7). In addition, the Control Response frame shall be sent using the same PHY options as the received frame. For the IEEE 802.11g PHY, if the received frame was sent at an OFDM rate, the Control Response frame shall be sent at the highest mandatory ERP rate that is less than or equal to the rate of the received frame. “
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 39
doc.: IEEE 802.11-02/065r1
Submission
Motion on aCWmin• Instruct the editor to add a sub clause
19.4.3.8.5 specifying to use the table in sub clause 18.3.3 for the MAC timing calculation, with the following changes:– Use an aCWmin value of 15 unless in a 11b
legacy network which uses the value in 18.3.3– aMACProcessingDelay is < 2us
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 40
doc.: IEEE 802.11-02/065r1
Submission
Motion on the signal extension for ERP/OFDM
• Add a sub clause 19.4.3.8.6 to state that the packet is followed by a Signal Extension Field which is quiet time (no carrier) of 6 microseconds.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 41
doc.: IEEE 802.11-02/065r1
Submission
Motion on the signal extension for CCK-OFDM
• Change sub clause 19.6.2.4.1 to state that the Signal Extension is quiet time (no carrier).
• Change figure 19.6.2.4.1 to indicate that the Signal Extension is quiet time
• Change sub clause 19.6.2.4.5 to specify that the Signal Extension is quiet time.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 42
doc.: IEEE 802.11-02/065r1
Submission
Motion to instruct the editor to change the TXtime
equation for ERP/OFDM• Change the Txtime equation in 19.4.4.1 (which is
currently a copy of the .11a definition) to add the 6 us Signal extension. The new equation would be:
• TXTIME =T PREAMBLE +T SIGNAL +T SYM *Ceiling((16 + 8*LENGTH + 6 )/ N DBPS )+Signal Extension
• Where Signal Extension is defined as 6 microseconds.
January 2002
Brockmann, Hoeben, Wentink (Intersil)
Slide 43
doc.: IEEE 802.11-02/065r1
Submission
Motion on Adjacent channel rejection
• Instruct the editor to add the following text to Section 19.4.3.10.1:– While receiving legacy 802.11b signals (1, 2, 5.5, 11
Mbps), the adjacent channel rejection should conform to the specifications of Subclause 18.4.8.3. While receiving OFDM signals (6, 9, 12, 18, 24, 36, 48, and 54 Mbps), the adjacent channel rejection shall conform to Subclause 17.3.10.2 with a +/- 25 MHz spacing.