Doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 1...

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


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


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


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


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


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


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


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


Slide 9

doc.: IEEE 802.11-02/065r1

Submission

DCF Performance

January 2002


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


Slide 11

doc.: IEEE 802.11-02/065r1

Submission

Throughput Comparison for 24/22 Mbps

0

1

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3

4

5

6

7

8

9

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12

13

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15

16

Throuhput (Mbps)

cck11 pbcc22 ofdm rts/cts cck-ofdm ofdm

802.11g Performance (22/24 Mbps, CWmin = 15)

January 2002


Slide 12

doc.: IEEE 802.11-02/065r1

Submission

Mixed b/g

January 2002


Slide 13

doc.: IEEE 802.11-02/065r1

Submission

Performance in a mixed scenario, without RTS/CTS(802.11g / legacy)

0

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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


Slide 14

doc.: IEEE 802.11-02/065r1

Submission

Perormance in a mixed scenario with RTS/CTS(CWmin = 31 for 802.11g)

0

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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


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


Slide 16

doc.: IEEE 802.11-02/065r1

Submission

Performance in a mixed scenario, with RTS/CTS(CWmin = 15 for 802.11g)

0

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9

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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


Slide 17

doc.: IEEE 802.11-02/065r1

Submission

Migration from Legacy to 802.11g

January 2002


Slide 18

doc.: IEEE 802.11-02/065r1

Submission

TCP performance during migration to 802.11g(CWmin = 15, OFDM 36 for 802.11g nodes)

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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


Slide 19

doc.: IEEE 802.11-02/065r1

Submission

Pure OFDM UDP Performance Comparison

January 2002


Slide 20

doc.: IEEE 802.11-02/065r1

Submission

Performance compared for CWmin = 15 and CWmin = 31

16

17

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20

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22

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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


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 = 31

CWmin = 15

January 2002


Slide 22

doc.: IEEE 802.11-02/065r1

Submission

Pure OFDMTCP Performance Comparison

January 2002


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


January 2002


Slide 24

doc.: IEEE 802.11-02/065r1

Submission

802.11e QoS Scenarios

January 2002


Slide 25

doc.: IEEE 802.11-02/065r1

Submission

802.11e/g migration scenario

0

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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


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

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25

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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


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


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


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


Slide 30

doc.: IEEE 802.11-02/065r1

Submission

Recommendations

In the form of Motions

January 2002


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


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


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


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


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


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


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


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


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


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


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


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


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.

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