QoS of Voice over 802.11 with NS simulator Prepared by: Yoshpa Benny Shraer Alexander Vainer Albert Instructors: Prof. Reuven Cohen Mr. Itai Dabran

QoS of Voice over 802.11QoS of Voice over 802.11with NS simulatorwith NS simulator

Prepared by:

Yoshpa Benny

Shraer Alexander

Vainer Albert

Instructors:

Prof. Reuven Cohen

Mr. Itai Dabran

OverviewOverview

802.11 - applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).

Problem: No QoS

802.11a - an extension to 802.11 that provides theoretically up to 54 Mbps in the 5GHz band, but realistically achieves 20-25 Mbps under normal conditions. 802.11a uses an orthogonal frequency division multiplexing encoding scheme (OFDM)

802.11b (802.11 High Rate or Wi-Fi) - an extension to 802.11 that provides 11 Mbps transmission in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.

OverviewOverviewBoth 802.11a and 802.11b have two channel

accessing mechanisms:

PCF - Point Coordination Function Based on polling technique – each station is polled in turn and stations with a packet pending for transmission sends the packet upon being polled. A dynamic list of stations may be maintained by the AP in order to increase efficiency.

DCF - Distributed Coordination Function Based on CSMA/CA – stations contend for the channel. Two basic schemes are used:

– Two way handshake: Acknowledgement is sent by the receiver to the sender upon successful reception of a packet. The acknowledgement is needed since the sender cannot determine whether its transmission was successful only by listening to it.

– Four way handshake: RTS/CTS mechanism, and then proceed with ACKs as above.

DCF – An ExampleDCF – An Example

If the first station senses channel idle for DIFS it sends a DATA packet. Otherwise it waits for the channel to be idle for DIFS and than selects random backoff – like the second source station above.

Backoff is selected from the range [0, CW-1]. At start, CW = CWmin.

After collision, CWnew = min {CWold * 2 , CWmax}.

CW is reset to CWmin after successful transmission

Destination station senses the channel idle for SIFS and then sends ACK. NAV is used for virtual carrier sensing – info is sent in data packets to

indicate how long the source intends to occupy the channel.

QoSQoS 802.11a, 802.11b MAC don’t support service differentiation. Relevant parameters for service differentiation:

PF – Persistence Factor. Determines how to increase CW after collision. Frame Size – For Voice packets it is determined by

the voice codec used.

QoS is very important for applications such as voice.

DIFS CWmin CWmax

Explained in previous slide

802.11e – An extension of 802.11 designed to improve its

medium access mechanism and to add support for service

differentiation. Uses the HCF – hybrid coordination function

which is queue based service differentiation scheme that uses

both DCF and PCF enhancements - EDCF and EPCF.

Project GoalsProject Goals Improvement of NS2 network simulator in order to

simulate statistical QoS in 802.11.

Show that Uplink/Downlink problem exists

in EDCF and suggest different solutions using

QoS parameters.

Investigation of QoS for Voice streams in 802.11e,

EDCF and comparison to 802.11b.

QoS Parameters ImplementedQoS Parameters Implemented DIFS –

In 802.11 DIFS is defined as a function of SIFS and is the same for all mobile nodes. In EDCF DIFS is different for every service class (Voice/Video/Data/etc.).

CWmin and Persistence Factor (PF) –

In 802.11 the same for all mobile nodes.

In EDCF it is different for every service class.

EDCF computes the CW differently then DCF:

CWnew = ( (CWold + 1) * PF ) – 1 (up to CWmax)

The lower these parameters are, the higher priority

the service - class gets in accessing the channel.

Investigate QoS indications (for voice packets), 1. Average Throughput (per voice connection).2. Average Latency.3. Packet Loss percentage.

A function of number of voice streams

A function of different QoS parameters (DIFS, CWmin, PF)

Goal 1Goal 1

Show improvement of our QoS implementation over the regular 802.11 MAC

Simulated NetworkSimulated Network

Actually, all wireline network is not simulated, since it is not interesting for our goal. Instead we let the access point to produce packets on all connections that go from wireline to wireless.

Setting ConnectionsSetting Connections

There are N + 4 connections: half from a wireless node to a wireline node

and half the other way (the connections are in one-direction)

Each wireless station has a different wireline station it “talks” to.

Voice Connections: - CBR over UDP.

- Packet size (with all overhead) : 180 bytes

as in G711 voice codec

- Packet inter-arrival time – 20ms

Best Effort Connections: DATA - FTP over TCP .

- Packet size (with all overhead) : 1560 bytes

There are N voice connections. The parameters we investigate are taken only from these connections

Parameters:

Voice: DIFS - 0.000020, CWMin - 15, PF - 2

Parameters:

Voice: DIFS - 0.000030, CWMin - 31,PF - 2

Parameters:

Voice: DIFS - 0.000040,CWMin - 3, PF - 2

Average Throughput as function of VOIP Calls

----- 802.11e

----- 802.11b

Other Parameters: Data: DIFS - 0.000080, CWMin - 63, PF – 2CWMin of 802.11b – 63CWMax – 1023

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 5 10 15 20 25

Number of voice connections

Ave

rage

Thr

ough

put

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 5 10 15 20 25


Ave

rag

e T

hro

ug

hp

ut

0

1000

2000

3000

4000

5000

6000

7000

8000

0 5 10 15 20 25


Ave

rag

e T

hro

ug

hp

ut

Average cbr Latency as function of VOIP CallsParameters:

Voice: DIFS - 0.000020,CWMin - 15,PF - 2

Parameters:


Parameters:


----- 802.11e

----- 802.11b


0

0.5

1

1.5

2

0 5 10 15 20 25


Avera

ge c

br

Late

ncy

0

0.5

1

1.5

2

0 5 10 15 20 25


Ave

rage

cbr

Lat

ency

0

0.5

1

1.5

2

0 5 10 15 20 25


Avera

ge c

br

Late

ncy

Percentage of dropped cbr packets as function of VOIP CallsParameters:

Voice: DIFS - 0.000020,CWMin - 15,PF - 2Parameters:


Parameters:

Voice: DIFS - 0.000040,CWMin - 3, PF - 2

----- 802.11e

----- 802.11b


0

10

20

30

40

50

60

70

0 5 10 15 20 25


Per

cent

age

of c

br p

acke

ts th

at w

ere

drop

ped

0

10

20

30

40

50

60

70

0 5 10 15 20 25


Per

cent

age

of c

br p

acke

ts th

at w

ere

drop

ped

0

10

20

30

40

50

60

70

0 5 10 15 20 25


Perc

enta

ge o

f cbr

pac

kets

that

wer

e dr

oppe

d

ConclusionsConclusions Our changes do work!

The improvement of service to Voice could be clearly seen when number of

voice connections is relatively small (up to 3 times more than number

of background connections).

The Voice class, to which we gave lower parameters (higher priority) got better service than in 802.11b => QoS

When the number of voice connection becomes large, the performance of 802.11b

and 802.11e (EDCF) is very much alike, since giving priority has meaning only

when there are many connections with lower priority. If there are many voice

connections, they compete mainly among them and not so much with the Data

connections.

Channel capacity limits the number of possible connections through

the channel. The channel contention method doesn’t help either.

Uplink / Downlink ProblemUplink / Downlink Problem

EPCF (Polling) – The average throughput is the same for uplink and downlink connection, since the AP gets access to the channel at least as often as any other station (its up to the AP to decide how often).

Downlink – Link from AP to the mobile station

Uplink – Link from the mobile station to the AP.

EDCF (CSMA/CA) – The AP is heavily loaded with traffic from the wireline network side and it needs to contend the channel with other mobile stations. This will introduce larger uplink throughput then the average downlink throughput.

Show that Uplink Downlink problem exists

in EDCF.

Goal 2Goal 2

Check dependence on number of mobile nodes

Suggest solutions using QoS parameters:– DIFS– CWmin– Persistence Factor

Uplink/Downlink Throughput as function of DIFSUplink/Downlink Throughput as function of DIFS

Other Parameters:

CWMin of AP = 15, PF of AP = 2

DIFS of Voice = 50,CWMin of Voice = 31, PF of Voice = 2

DIFS of BG = 80,CWMin of BG = 63, PF of BG = 2CWMax – 1023

----- Uplink Throughput

----- Downlink Throughput

0

5000

10000

15000

20000

25000

30000

35000

0 10 20 30 40 50 60 70 80 90

DIFS_AP

Tota

l Thr

ough

put

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 10 20 30 40 50 60 70 80 90

DIFS_AP

To

tal T

hro

ug

hp

ut

Num of Stations = 8 Num of Stations = 16

Num of Stations = 26

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 10 20 30 40 50 60 70 80 90

DIFS_AP

Tota

l Thr

ough

put

Uplink/Downlink Throughput as function of CWMinUplink/Downlink Throughput as function of CWMin



Other Parameters:

DIFS of AP = 40, PF of AP = 2



Num of Stations = 8



0

5000

10000

15000

20000

25000

30000

35000

0 5 10 15 20 25 30 35

CWMin_AP

Tota

l Thr

ough

put

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 5 10 15 20 25 30 35

CWMin_AP

Tota

l Thr

ough

put

0

10000

20000

30000

40000

50000

60000

0 5 10 15 20 25 30 35

CWMin_AP

To

tal T

hro

ug

hp

ut

Uplink/Downlink Throughput as function of PFUplink/Downlink Throughput as function of PFNum of Stations = 8 Num of Stations = 16




Other Parameters:

DIFS of AP =40,CWMin of AP = 15,



0

5000

10000

15000

20000

25000

30000

0 1 2 3 4 5 6 7 8 9

PF_AP

To

tal T

hro

ug

hp

ut

0

5000

10000

15000

20000

25000

30000

35000

0 1 2 3 4 5 6 7 8 9

PF_AP

Tota

l Thr

ough

put

0

5000

10000

15000

20000

25000

30000

35000

0 1 2 3 4 5 6 7 8 9

PF_AP

Tota

l Thr

ough

put

Conclusions – contConclusions – cont..

Access Point should dynamicallydynamically adjust its medium access parameters

(DIFS , CWmin and PF) depending on number of mobile stations and

network load. It should aspire to get at least equal service as all the mobile

stations it serves (together).

AP should also consider the type of data (priority classes) that passes

through it when adjusting its own medium access parameters. When adjusting AP’s priority, it is better to use DIFS and CWMin rather

than PF, and when PF is used, it should be accompanied with changes

in CWMin, since sometimes the changes PF inflicts are too strong and not

very predictable unlike the other two parameters.

Each one of the three parameters investigated allows to overcome the

uplink/downlink problem described earlier.

We have shown that as the number of stations (and thus network load) increases,

the AP has to get higher priority (lower QoS parameters) to reach the

uplink = downlink point.

BibliographyBibliography

IEEE 802.11e Wireless LAN for Quality of Service Scheduling of Voice Packets in a Low-Bandwidth Shared Medium

Access Network, Reuven Cohen, Liran Katzir (Technion, Israel)

Achieving Service Differentiation and High Utilization in 802.11

Vasilios A. Siris Supporting VBR VoIP Traffic in IEEE 802.11 WLAN in PCF Mode

Dongyan Chen, Sachin Garg, Martin Kappes and Kishor S. Trivedi Supporting VoIP Traffic in IEEE 802.11 WLAN with Enhanced MAC

for QoS - Dongyan Chen, Sachin Garg, Martin Kappes and Kishor S. Trivedi

Documents

QoS of Voice over 802.11 with NS simulator Prepared by: Yoshpa Benny Shraer Alexander Vainer Albert Instructors: Prof. Reuven Cohen Mr. Itai Dabran