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QoS of Voice over 802.11
with NS simulator
Presented by:
SaiKamal Neeli
Avinash Thota
VoIP-Introduction
VoIP-Voice over internet protocol.
- Transfers voice using IP packets over the internet.
How VoIP works- conversion of analog voice to digital signal bits using ADC.
- Compression of the digital bits in to a proper format for easy
transmission. Protocols like PCM etc are used.
- Inserting VoIP packets into data packets. VoIP data packets
are packed in real time transport protocol packets which are inside
UDP-IP packets.
-H323 is the signaling protocol used to call the users.
- In the receiver the received packets are converted in to analog
voice signals and then the signals are transferred in to the sound
card.
Network Simulator
Network simulator can be a software or hardware which can be used for
predicting the behavior of the network.
Different versions of the network simulators are ns-1, ns-2, ns-3. Ns-3
is the latest version of the network simulator.

Overview

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).

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.
Problem: No QoS
Overview
Both 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.
– 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.
802.11e
Goal
 Investigate QoS indications (for voice packets),
1.
2.
3.
Average Throughput (per voice connection).
Average Latency.
Packet Loss percentage.
Simulated Network
Setting Connections
Each
wireless station has a different wire line station it “talks” to.
There
There
are N + 4 connections: half from a wireless node to a wire line node
and half the other way (the connections are in one-direction)
are N voice connections. The parameters we investigate are taken only
from these connections
Average Throughput as function of VOIP Calls
9000
8000
7000
Average Throughput
6000
5000
4000
3000
2000
1000
0
0
5
10
15
20
25
Number of voice connections
----- 802.11e
----- 802.11b
Throughput is the average rate of successful message delivery
over a communication channel.
Average Latency as function of VOIP Calls
2
Average cbr Latency
1.5
----- 802.11e
----- 802.11b
1
0.5
0
0
5
10
15
20
25
Number of voice connections
Latency: It is the time between the moment a voice packet is
transmitted and the moment it reaches its destination
•It slows down your phone conversations
•Untimeliness can results in overlapping noises, with one speaker
interrupting the other
•Causes echo.
•Disturbs synchronization between voice and other data types,
especially during video conferencing
Percentage of dropped packets as function of VOIP Calls
70
Percentage of cbr packets that were dropped
60
50
40
30
20
10
0
0
5
10
15
Number of voice connections
----- 802.11e
----- 802.11b
20
25
Conclusions
 When the number of voice connection becomes large, the performance of 802.11b
and 802.11e is very much alike.
QoS
is not guaranteed by 802.11 standards because of the poor performance.
802.11e
is the only standard offering QoS to those applications which are time
sensitive like video and audio communications.
Thank You