iminds' course: preceding exercises

Department of Information Technology – Internet Based Communication Networks and Services (IBCN)

WLAN throughput:calculation exercises

Daan Pareit, Ph.D.

[email protected]

www.iminds.be

Ghent University - Department of Information Technology – Internet Based Communication Networks and Services (IBCN)

Overview

1. Short MAC theory recapitulation

2. Short PHY theory recapitulation

3. Example exercise explained

4. Other exercise assignments


Recapitulation of MAC layer theory


IEEE 802.11 – DFWMAC-DCF using CSMA/CA

DFWMAC-DCF using CSMA/CA station has to wait for DIFS (+ random back-off time if medium is busy)

before sending data receivers acknowledge at once (after waiting for SIFS) if the frame was

received correctly (CRC) automatic retransmission of data frames in case of transmission errors

(but new random back-off time)

t

SIFS

DIFS

data

ACK

waiting time

otherstations

receiver

sender data

DIFS

contention


IEEE 802.11 - MAC layer

Different IFS = different medium access priority

SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response length of SIFS determined by PHY

DIFS (DCF IFS) lowest priority, for asynchronous data service DIFS = SIFS + 2 time slots (length of time slot determined by PHY)

t

medium busy SIFS

DIFS

next framecontention


IEEE 802.11 – DFWMAC-DCF using CSMA/CA

Contention Window Back-off time = random value between 0 and CW Low CW many collisions High CW high delays Exponential back-off: adaptation to load of medium

If collision: CW doubles Example for HT PHY (802.11n): CWmin = 15 and CWmax= 1023then CW = (15, 31, 63, 127, 255, 511, 1023) depending on load

t

medium busy

DIFSDIFS

next frame

contention window(randomized back-off

mechanism)

slot time


IEEE 802.11 – DFWMAC-DCF with RTS/CTS

DFWMAC-DCF with RTS/CTS RTS/CTS: Request to Send / Clear to Send

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

sender data

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)


IEEE 802.11 – DFWMAC-DCF with RTS/CTS

Fragmentation

t

SIFS

data

ACK1

otherstations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

DIFS


MAC headers: data

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

payloadMSDU

CRC

2 2 6 6 6 62 40-2312bytes

MPDU = MAC Protocol Data Unit(= PSDU = PLCP Service Data Unit)

data

QoSControl

2

sender


IEEE 802.11 – MAC address format

DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address

Address 1: physical receiverAddress 2: physical transmitterAddress 3: logical receiver/sender/BSSIDAddress 4: logical sender

Filtering on address 1 ACK to address 2

scenario address 1 address 2 address 3 address 4

ad-hoc network DA SA BSSID -infrastructure network, from AP DA BSSID SA -

infrastructure network, to AP BSSID SA DA -

infrastructure network, within DS RA TA DA SA


MAC headers: ACK

receiver

ACK

FrameControl

DurationReceiverAddress

CRC

2 2 6 4bytes


MAC headers: RTS/CTS

sender

RTS

receiver

CTS

FrameControl


CRC

2 2 6 4bytes

FrameControl


TransmitterAddress

CRC

2 2 6 6 4bytes


IEEE 802.11n

Frame aggregation A-MSDU A-MPDU


IEEE 802.11e

Special control packets

Block ACK Request

Block ACK

FrameControl


CRC

2 2 6 4bytes

BlockAck Start Seq Control

2Transmitter

Address

6BlockAckReq

Control

2

FrameControl


CRC

2 2 6 4bytes

BlockAck Start Seq Control

2Transmitter

Address

6BlockAck Control

2BlockAck

bitmap

128


Recapitulation of PHY layer theory


IEEE 802.11a: PHY frame format

rate service PSDU

Variable [bits]

6 Mbit/s

PLCP preamble signal

symbols12 1 variable

reserved length tailparity tail pad

616611214 variable

6, 9, 12, 18, 24, 36, 48, 54 Mbit/s

PLCP header

16 µs

data

sender

PLCP preamble


PayloadMSDU

CRC

40-2312

IEEE 802.11a

SERVICE PSDU Tail PadPHY

MAC

LLC

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

2 2 6 6 6 62bytes

4 µs16 µs 16 bits 6 bits

LLC header

Payload

IP IP packet

8 bytes

@ PHY data rate

PLCP preamble signal


IEEE 802.11a: OFDM

Rate dependent parameters 250 000 OFDM symbols/s (symbol duration: 4 µs)

Data rate [Mbit/s] Modulation Coding rate

Coded bits per

subcarrier

Coded bits per OFDM

symbol

Data bits per OFDM

symbol

6 BPSK 1/2 1 48 24

9 BPSK 3/4 1 48 36

12 QPSK 1/2 2 96 48

18 QPSK 3/4 2 96 72

24 16-QAM 1/2 4 192 96

36 16-QAM 3/4 4 192 144

48 64-QAM 2/3 6 288 192

54 64-QAM 3/4 6 288 216


IEEE 802.11n: PHY frame format

PLCP preamble signal data

service PSDU

Variable [bits]

tail pad

616 variable


PayloadMSDU

CRC

40-2312QoS

Control

2

IEEE 802.11n

L-STF L-LTF L-SIG HT-SIG HT-STF HT-LTF HT-LTF SERVICE PSDU Tail PadPHY

MAC

LLC

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

2 2 6 6 6 62bytes

8 µs 8 µs 4 µs 4 µs 4 µs 4 µs8 µs 16 bits 6 bits

LLC header

Payload

IP IP packet

8 bytes

@ PHY data rate


IEEE 802.11n: data rate Modulation and Coding Schemes (MCS)

Symbol duration with long GI = 4 µs, with short GI = 3.6 µs after training fields


Example exercise explained


WLAN Throughput Exercise 1

What is the maximum throughput for IEEE 802.11a DFWMAC-DCF using CSMA/CA?

Assumptions: There is only one UDP sender (= STA) occupying the

wireless medium The UDP receiver (= AP) is close enough to the sender so

that data can be sent at maximum bit rate and no transmission errors occur

Propagation delay can be neglected

Parameters TSIFS = 16 s and Tslot = 9 s CWmin = 15 and CWmax = 1023 IP packet length = 1500 bytes & 500 bytes


Exercise 1: solution method Maximum throughput calculation (DFWMAC-DCF using CSMA/CA)

Throughput = 8 * payload (bytes) / (TDIFS + CWaverage + Tdata + TSIFS + TACK + 2 * )

TDIFS = TSIFS + 2 * Tslot = 16 s + 2 * 9 s = 34 s

CWaverage = CWmin/2 * Tslot if there is only 1 client (medium is always free)

Tdata = Tpreamble + Tphy + 1 / R * (bitsservice + bitstail + MAC-header + LLC-header + payload)

= 16 s + 4 s + 1 / R * (16 bits + 6 bits + MAC-header + LLC-header + payload)

= 20 s + 1 / R * (22 bits + 8 bits/byte * (28 byte + 8 byte + payload (byte))

TSIFS = 16 s

TACK = Tpreamble + Tphy + 1 / R * (bitsservice + bitstail + ACK-message)

= 16 s + 4 s + 1 / R * (16 bits + 6 bits + 8 bits/byte * 14 byte)

= 20 s + 1 / R * (22 bits + 112 bits)

= 20 s + 1 / R * 134 bits

: propagation delay R: physical data rate padding: granularity for Tdata and TACK is 1 symbol = 4 s roundup


Exercise 1: answer 1 Maximum throughput calculation (DFWMAC-DCF using CSMA/CA) for 1500 bytes

Throughput = 8 * payload (bytes) / (TDIFS + CWaverage + Tdata + TSIFS + TACK)


CWaverage = CWmin/2 * Tslot = 15/2 * 9 s = 67.5 s


= 16 s + 4 s + 1 / (54*106 bits/s) * (16 bits + 6 bits + 8 * 28 bits + 8 *8 bits + 8 *1500 bits)

= 20 s + 1 s / 54 bits * (22 bits + 8 * 28 bits + 8 * 8 bits + 8 * 1500 bits)

= 247.96 248 s

TSIFS = 16 s


= 16 s + 4 s + 1 s / 54 bits * (16 bits + 6 bits + 8 * 14 bits)

= 22.5 24 s

Throughput = 8 * 1500 bits / (34 s + 67.5 s + 248 s + 16 s + 24 s ) = 8 * 1500 / 389.5 Mbps = 30.8 Mbps


Exercise 1: answer 2 Maximum throughput calculation (DFWMAC-DCF using CSMA/CA) for 500 bytes

Throughput = 8 * payload (bytes) / (TDIFS + CWaverage + Tdata + TSIFS + TACK)


CWaverage = CWmin/2 * Tslot = 15/2 * 9 s = 67.5 s


= 16 s + 4 s + 1 / (54*106 bits/s) * (16 bits + 6 bits + 8 * 28 bits + 8 *8 bits + 8 *500 bits)

= 20 s + 1 s / 54 bits * (22 bits + 8 * 28 bits + 8 * 8 bits + 8 * 500 bits)

= 99.8 100 s

TSIFS = 16 s


= 16 s + 4 s + 1 s / 54 bits * (16 bits + 6 bits + 8 * 14 bits)

= 22.5 24 s

Throughput = 8 * 500 bits / (34 s + 67.5 s + 100 s + 16 s + 24 s ) = 8 * 500 / 241.5 Mbps = 16.6 Mbps


Other exercise assignments



What is the maximum throughput for IEEE 802.11a DFWMAC-DCF using RTS/CTS?

Assumptions: There is only one UDP sender (= STA) occupying the wireless

medium The UDP receiver (= AP) is close enough to the sender so that data

can be sent at maximum bit rate and no transmission errors occur Propagation delay can be neglected

Parameters TSIFS = 16 s and TSlot = 9 s CWmin = 15 and CWmax = 1023 IP packet length = 1500 bytes



What is the throughput for IEEE 802.11a DFWMAC-DCF using RTS/CTS?

Assumptions: There is only one UDP sender (= STA) occupying the

wireless medium The receiver (= AP) is far away from the sender: data can

still be sent at maximum bit rate, but packets are fragmentized (at MAC layer) to a maximum size of 500 bytes to limit transmission errors (which may be further neglected)

Propagation delay can be neglected




What is the throughput for IEEE 802.11a DFWMAC-DCF using RTS/CTS?

Assumptions: There is only one UDP sender (=STA) occupying the wireless

medium The receiver (= AP) is far away from the sender: data cannot be

sent at maximum bit rate, but at a lower bit rate of 36 Mbps Propagation delay can be neglected




What is the maximum throughput for IEEE 802.11n DFWMAC-DCF using CSMA/CA?

Assumptions: There is only one UDP sender (= STA) occupying the wireless medium,

sending best effort (BE) traffic The UDP receiver (= AP) is close enough to the sender so that data can be

sent at maximum bit rate and no transmission errors occur The network is set up for a mixed 802.11g/n environment and supports QoS The used hardware supports maximum 2 space-time streams Propagation delay can be neglected

Parameters TSIFS = 10 s, Tslot = 20 s Channel: 20 MHz @ 2.4 GHz, short guard interval AIFSN[BE] = 2, CWmin[BE] = 15 CWmax[BE] = 1023 IP packet length = 1500 bytes No frame aggregation



What is the maximum throughput for IEEE 802.11n DFWMAC-DCF using CSMA/CA with frame aggregation?

Assumptions: There is only one UDP sender (= STA) occupying the wireless medium,

sending best effort (BE) traffic The UDP receiver (= AP) is close enough to the sender so that data can be

sent at maximum bit rate and no transmission errors occur The network is set up for a mixed 802.11g/n environment and supports QoS The used hardware supports maximum 2 space-time streams Propagation delay can be neglected

Parameters TSIFS = 10 s, Tslot = 20 s Channel: 20 MHz @ 2.4 GHz, short guard interval AIFSN[BE] = 2, CWmin[BE] = 15 CWmax[BE] = 1023 IP packet length = 1500 bytes aggregation of 3 frames: A-MPDU versus A-MSDU

Department of Information Technology – Internet Based Communication Networks and Services (IBCN)

Contact

Daan Pareit, [email protected] Based Communication Networks and Services research group (IBCN)Department of Information Technology (INTEC)Ghent University - iMinds

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