The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications
(PIMRC'07)
CAPACITY IMPROVEMENT OF WIRELESS LAN VOIP USING
DISTRIBUTED TRANSMISSION SCHEDULING
Kei Igarashi, Akira Yamada and Tomoyuki Ohya
Research Laboratories, NTT DoCoMo, Inc.
3-5, Hikarino-oka, Yokosuka-shi, Kanagawa, 239-8536, JAPAN
{igarashike, yamadaakir, ooyat}@nttdocomo.co.jp
ABSTRACT
This paper proposes a Medium Access Control
(MAC) protocol that provides Quality of Service (QoS)
in Voice over IP (VoIP) over Wireless Local Area
Networks (WLANs). Our proposed protocol minimizes
packet collision and improves voice communication
quality in both single-cell and multiple-cell cases.
Introduction of the proposed protocol requires only
VoIP STA software modification, i.e. replacement of
Access Points (APs) is not necessary. Our protocol
also keeps backward compatibility with the legacy
devices that do not implement the proposed method.
We evaluate the performance of the proposed protocol
and show that it can increase the number of
accommodated VoIP calls by approximately 50%.
I.
INTRODUCTION
The Wireless Local Area Network (WLAN) based on
IEEE802.11 [1] has become increasingly popular due
to its low cost, flexibly, and ease of use. WLAN
devices have recently been implemented in consumer
electronic devices as well as laptop computers. In
addition to non-real-time applications such as HTTP
and FTP, the WLAN is expected to support real-time
applications such as Voice over IP (VoIP), video
streaming and other delay-sensitive applications. For
real-time WLAN applications, Quality of Service
(QoS) technologies for the WLANs represent a key
challenge to supporting such kinds of applications.
Many performance analysis studies have examined
the application of IEEE802.11 WLAN to real-time
services such as VoIP and Video. Most of them have
shown the difficulties in QoS [2-4]. In order to solve
these problems, IEEE802.11e adopted Enhanced
Distributed Coordination Access (EDCA) [5]. EDCA
defines four access categories (ACs) that provide
packet prioritization for the delivery of traffic.
Priority control on Carrier Sense Multiple Access
with Collision Avoidance (CSMA / CA) is realized by
setting the EDCA parameters to meet the QoS
demands of application.
However EDCA provides only the prioritized QoS
for different ACs and does not take into consideration
the QoS of traffics which belong to the same AC.
Hence it is difficult to meet the QoS demands of
each application using only static EDCA parameters.
When only EDCA is applied to VoIP, packet collision
probability increases as the number of simultaneous
1-4244-1144-0/07/$25.00©2007 IEEE
calls increases and this results in voice quality
degradation [6-8]. Therefore, new solutions are
needed to realize adequate quality for real-time
applications.
This study proposes a Medium Access Control
(MAC) protocol that provides QoS guarantees for
VoIP over WLAN. Our proposed protocol is based on a
feasible and flexible scheduling method that takes
advantage of the feature that VoIP packets are
generated at intervals of voice codec period. The
proposed protocol can not only suppress packet
collision but also minimize the costs associated with
development and introduction. The paper is
organized as follows. Section II describes EDCA
schemes and their problems. We propose a MAC
protocol for VoIP over WLAN in Section III. Its
performance evaluations are shown in Section IV. We
conclude the paper in Section V.
II. IEEE802.11e EDCA OVERVIEW
A. Enhanced Distributed Coordination Access
Enhanced Distributed Coordination Access (EDCA)
supports a prioritized mechanism on CSMA/CA.
EDCA differentiates packets from an upper layer into
four different ACs according to the applications and
their QoS characteristics. An overview of the EDCA
mechanism is illustrated in Figure 1. A station (STA)
with packets to transmit and senses the medium
before initiating a transmission. If the medium is
sensed busy, the STA defers its transmission to a
later time. If the medium is sensed idle for a prespecified time, i.e., Arbitration Inter Frame Space
(AIFS), then the STA generates a random backoff
period for an additional deferral time before
initiating transmission. The AIFS is given as
AIFS = AIFSN × aSlotTime + SIFS
(1)
where aSlotTime is the value of a slot time and SIFS
is the value of a Short IFS. AIFSN is an integer value
which is set according to the AC. The higher
prioritized AC has the smaller AIFSN value as
described in Figure 2. The random backoff period is
derived as
BackoffTim e = Random () × aSlotTime
(2)
where Random() is the pseudo-random integer drawn
from a uniform distribution over the interval [0,CW].
CW (Contention Window) is an integer within the
range of values of the PHY characteristics, aCWmin ≤
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
AIFS
Transmitter
Backoff
Collision
AP
Data
AIFS
VoIP A
Busy
SlotTime
VoIP B
SIFS
Receiver
Ack
Busy
Backoff
Ack
RTP
Ack
Ack
RTP
RTP
RTP
VoIP C
RTP
Delay
Figure 1: Access scheme in EDCA
Figure 3: Problems on EDCA
Backoff[BK]
AIFS[BK]
Data
Backoff[BE]
AIFS[BE]
Data
Backoff[VI]
AIFS[VI]
Busy
Data
Backoff[VO]
AIFS[VO]
Data
BK: Background
BE: Best effort
VI: Video
VO: Voice
Figure 2: Prioritization in EDCA
CW ≤ aCWmax. The CW is set equal to aCWmin at
the first transmission attempt, and it increases in the
following manner, until it reaches aCWmax.
CW = 2 n × (CWmin + 1) − 1
(3)
where n is the number of retransmissions.
The CWmin/CWmax values also vary for different
ACs. For higher prioritized ACs, CWmin and CWmax
are set to smaller values.
B. Problems on EDCA
As described above, EDCA provides prioritized QoS
for different ACs. However EDCA does not take into
consideration the QoS of traffics which belong to the
same AC. Figure 3 shows the problems that occur in
EDCA. The packet collision probability increases as
the number of calls increases due to simultaneous
transmission. Additionally the delay tends to increase
if more than one STA are in backoff states. These
problems result in voice quality degradation.
Therefore, it is necessary to restrict the number of
VoIP calls over a WLAN system.
Instead of EDCA, IEEE802.11e also provides the
polling based MAC protocol called Hybrid
coordination function Controlled Channel Access
(HCCA). However HCCA proposes no concrete
scheduling algorithms. In addition, collision of polling
packets from more than one access point (AP) may
always occur in multiple-cell environment because
VoIP application generates packets to transmit at a
constant interval.
III. PROPOSED TECHNIQUES
A. Overview of Proposed Scheme
We propose a MAC protocol to provide effective QoS
guarantees for VoIP over WLAN. Our proposed
protocol follows three policies: (1) No modification of
access points, (2) No hardware (H/W) modification of
VoIP terminals, and (3) Maintain backward
compatibility. The first policy enables users to
implement the proposed method on existing APs. The
second one minimizes the impact on implementing
the proposed method in VoIP terminals. The third one
prevents the proposed method from blocking
communications among terminals that are equipped
with only conventional techniques.
The flow chart of behaviour of STAs equipped with
our proposed protocol is shown in Figure 4. After each
STA connects to AP, each STA decides its own
transmission
timing
using
the
distributed
transmission scheduling. We call this stage the
observation phase. After that, the scheduled
transmission phase follows. In the scheduled
transmission phase, each STA transmits packets
using the dynamic priority setting. It is also possible
to re-schedule transmission timing if necessary.
Our proposed techniques are premised on APs and
STAs which host Unscheduled Automatic Power Save
Delivery (U-APSD), which is defined in the
IEEE802.11e. The U-APSD is intermittent reception
mechanism that AP transmits a packet upon
receiving a packet from an STA.
Association
(conventional methods)
Proposed techniques
Observation phase:
Distributed Transmission Scheduling
Scheduled transmission phase:
Dynamic Priority Setting
Yes
Re-scheduling
No
Figure 4: Flow chart of STA equipped with our
proposed protocol
Table 1: An example of scheduling table
Order
MAC address
1
2
3
:
xx:xx:xx:01:01:03
xx:xx:xx:03:02:06
xx:xx:xx:07:09:02
:
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
B. Distributed Transmission Scheduling
Because VoIP application generates packets to
transmit at a constant interval, each STA has only to
decide a schedule in a certain VoIP codec period. Each
STA in single-cell environment proceeds as follows.
First of all, in order to detect other STAs, each STA
observes down-link packets from AP and reads the
destination address field in the MAC header. This
enables each STA to recognize others. Secondly, each
STA makes a list of MAC addresses of STAs which
belong to the same AP. The final step in this phase is
to sort the MAC addresses in ascending order and
make a scheduling table such as Table 1. These
enable all the STAs in the same cell to share a
scheduling table. During this phase, STAs transmit
packets using EDCA.
C. Dynamic Priority Setting
After the scheduling table is made, each STA
creates a virtual slot periodically. Each STA sets the
priority of its virtual slot according to the scheduling
table. Specifically, each STA dynamically changes its
AIFSN, CWmin and CWmax. These inverted
numbers correspond to the priority of proposed
technique and (solid line) and EDCA (dotted line) in
Figure 5. This makes it possible to realize the
scheduled sequences shown in Figure 6. The “Best
Effort” STA in Figure 5 and 6 is not equipped with
the proposed techniques. The virtual slot of a VoIP
STA is shifted to the next VoIP STA in the length of
the time required for transmission of up-link and
down-link packets. Moreover, the duration of a
virtual slot is longer than the length of the time
required for transmission of up-link and down-link
packets. Therefore, the virtual slot of a VoIP STA
partially overlaps with that of the next STA. This
leads to flexible scheduling which prevents the
Voice Codec Period
AP
Tx Priority
VoIP A
VoIP B
VoIP C
Best Effort
EDCA
Proposed
Figure 5: Dynamic priority setting
AP
VoIP A
VoIP B
scheduled sequences from collapsing due to
interrupts from the previous VoIP STA or STA
equipped only with conventional techniques. If the
scheduled sequences collapse due to join of new STA,
each STA adopts any of the following methods. One
way is for the new STA to transmit a re-scheduling
request packet in a broadcast manner. The other is
for any STA connected already to detect frequent
failures of reception of an acknowledgement (ACK)
from AP and then transmit a re-scheduling request
packet in a broadcast manner.
D. Application to Multiple-Cell Environment
The number of the available frequency channels is
very limited in IEEE802.11 WLAN. This is why it is
inevitable that more than one cell with the same
frequency channel will overlap. In this section we
extend the proposed techniques to implement the
distributed transmission scheduling for multiple-cell
environment.
We consider the scenario in Figure 7 as our
example. AP1 and AP2 use the same frequency. VoIP
A, B and C are connected to AP1 and VoIP D and E
are connected to AP2 and VoIP C lies in the
overlapping area of AP1 and AP2. If the VoIP STAs
are equipped with only the technique described in
Section III.B, there is a possibility that the virtual
slot of VoIP C will overlap with that of VoIP D and E.
This leads to packet collision between VoIP C and
VoIP D/E due to the transmission at the same time.
Hence the virtual slots of the VoIP STAs in an
overlapping area need to be set in different timing
from those of VoIP STAs in not only the same cell but
also neighbour cells.
The technique proposed to solve the problem is
described using Figure 7 and 8. Figure 8 shows the
transmission sequences in the proposed technique. At
the beginning, each VoIP STA behaves in the manner
of Section III.B and makes the scheduling table of
VoIP STAs which belong to the same AP. After that,
each VoIP STA determines if packets from other APs
are being received. This enables VoIP C to find itself
in an overlapping area. VoIP C searches the virtual
slots which are not occupied by the VoIP D/E in
neighbour cell. Then VoIP C transmits a slot request
packet including the virtual slot number which is not
occupied by the VoIP D/E in a broadcast manner. This
leads for VoIP A and B to receive the necessary
information and make the scheduling table under the
multiple-cell environment.
AP1
AP2
VoIP C
Best Effort
Figure 6: Sequences by proposed techniques
VoIP A
VoIP B
VoIP C
VoIP D
VoIP E
Figure 7: Multiple-cell environment
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
Virtual Slot
1
AP1
2
A
VoIP A
(xx:xx:xx:01:01:03)
3
4
5
B.C.
B
1
2
C
A
A
5 VoIP
STAs
B
A
VoIP B
B
(xx:xx:xx:03:02:06)
C
(xx:xx:xx:07:09:02)
VoIP D
C
collision
D
(xx:xx:xx:02:03:11)
5 VoIP STAs
VoIP E
xx:xx:xx:07:09:02
Request:slot1
E
(xx:xx:xx:03:01:12)
AP2
D
5
1
VoIP STAs &
Best Effort STA
B
B.C.
VoIP C
Virtual Slot
15 VoIP STAs
16 VoIP STAs
3
4
5
5 VoIP STAs
Overlapping area
BackoffTimeSTA and BackoffTimeAP are, respectively,
70 μs and 10μs . TVoIP is given as
E
D
3
8 VoIP STAs
Figure 9: Simulation scenarios
D
E
2
8 VoIP STAs
1
E
2
Fig. 8 Multiple-cell scheduling
TVoIP = PLCP preamble + PLCP header / PLCP rate +
( MAC header + LLC header + IP header + UDP header +
RTP header + Voice Payload + FCS ) / rate
= 268μs
IV. PERFORMANCE EVALUATION
T Ack = PLCP preamble + PLCP header / PLCP rate +
( Ack + FCS ) / Ack rate
A. Simulation Scenarios
We evaluated the proposed scheme using the
OPNET simulator. The simulation parameters are
listed in Table 2. Voice signals are encoded with the
G.711 speech codec [9], which generates 160 bytepackets every 20ms with constant bit rate. The
simulations were performed in three specific network
scenarios, (i) VoIP STAs with the proposed techniques
and a Best Effort traffic STA with only EDCA in a
single cell (left side of Figure 9), (ii) two cells with
eight VoIP STAs in each cell (middle of Figure 9) and
(iii) three cells with five VoIP STAs in each cell (right
side of Figure 9).
Let us consider the theoretical number of VoIP
STAs that a cell can accommodate if no packet
collisions occur. The length of the time required for a
set of successful up-link and down-link transmission
is given as
AIFS STA + BackoffTime STA + TVoIP + SIFS + T Ack
(4)
+ AIFS AP + BackoffTime AP + TVoIP + SIFS + T Ack
where TVoIP is the transmission duration of a VoIP
packet and TAck is the transmission duration of an
Ack packet. For IEEE802.11b, aSlotTime is 20 μs and
SIFS is 10μs . Therefore AIFSSTA and AIFSAP are,
respectively, 50μs and 30μs from Equation (1) and
Table 2. The average BackoffTime is derived from
CW min / 2 × aSlotTime , so the average values of
Table 2: Simulation condition
PHY model
IEEE802.11b
11Mbps
Short preamble
Voice Payload
160byte
Voice Codec Period
20ms
Retry Limit
7
AIFSN
AP: 1
STA (in virtual slot): 2
STA (others): 30
CWmin
AP: 1
STA (in virtual slot): 7
STA (others): 15
CWmax
1023
Distribution of Best Effort traffic
Exponential
MAC payload size of a Best Effort packet
1500
= 152 μs
Assigning the values above to Equation (4), the
length of the time required for a set of successful uplink and down-link transmission is 1009μs . Hence the
theoretical number of VoIP STAs which can be
accommodated in a voice codec period (20ms) is as
follows.
floor{20 × 10 3 / 1019} = 19
(5)
B. Simulation Results and Discussions
Figures 10 and 11 depict the packet loss rate and
the average delay of VoIP traffic, respectively, under
the environment that both VoIP STAs and Best Effort
STA (offered load = 0, 2 and 4Mbps) coexist in a
single cell. The packet loss rate and the delay of the
proposed techniques in solid lines are lower than
those of EDCA in dotted lines. The packet loss rate of
EDCA is higher than 3 % which is a target value
defined by ITU-T G.1010 [10] when the number of
VoIP STAs is higher than 10 STAs. The proposed
techniques can support approximately 16 VoIP STAs
that is near the theoretical number obtained by
Equation (5). This shows that the proposed
techniques can effectively suppress packet collision.
Figure 12 illustrates the throughput of Best Effort
traffic when the number of VoIP STAs is 5, 10 and 15.
As can be shown from these results, the throughput
of Best Effort STA, which is not equipped with the
proposed techniques, is also improved. Figure 13 and
14 shows the packet loss rate and the average delay
under two-cell environment of scenario (ii) and threecell environment of scenario (iii), respectively. As is
clear from these figures, both the packet loss rate and
the delay of the proposed scheduling techniques are
lower than those of EDCA. Moreover, the multiplecell scheduling is more effective than the single-cell
scheduling. Hence, the proposed techniques improve
the performance under the multiple-cell environment
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
0.8
1
EDCA
EDCA(O L=2M)
EDCA(O L=4M)
Proposed
Proposed(OL=2M)
Proposed(OL=4M)
0.8
0.6
0.02
EDC A(PLR)
Single-cell scheduling(PLR)
Multiple-cell scheduling(PLR)
EDC A(Delay)
Single-cell scheduling(Delay)
Multiple-cell scheduling(Delay)
0.6
0.015
0.4
0.01
0.2
0.005
0.4
0.2
0
0
5
10
15
Number of VoIP STAs
0
20
4
6
8
10
12
14
16
0
Figure 13: Packet Loss Rate and Average Delay in
scenario (ii)
EDCA
EDCA(O L=2M)
EDCA(O L=4M)
Proposed
Proposed(OL=2M)
Proposed(OL=4M)
0.015
2
Number of VoIP STAs in overlapping area
Figure 10: Packet Loss Rate of VoIP in scenario (i)
0.02
0
0.8
0.02
EDC A(PLR)
Single-cell schduling(PLR)
Multiple-cell scheduling(PLR)
EDC A(Delay)
Single-cell scheduling(Delay)
Multiple-cell scheduling(Delay)
0.6
0.01
0.015
0.4
0.01
0.2
0.005
0.005
0
0
5
10
15
Number of VoIP STAs
20
0
Figure 11: Average Delay of VoIP in scenario (i)
3
6
9
12
Number of VoIP STAs in overlapping area
15
Figure 14: Packet Loss Rate and Average Delay in
scenario (iii)
8
EDCA(VoIP=5STAs)
EDCA(VoIP=10STA s)
EDCA(VoIP=15STA s)
Proposed(VoIP=5STAs)
Proposed(VoIP=10STAs)
Proposed(VoIP=15STAs)
6
REFERENCES
4
[1]
2
0
0
0
0
2
4
Offered Load [Mbps]
6
8
Figure 12: Throughput of Best Effort in scenario (i)
as well.
V. CONCLUSIONS
This paper presented a MAC protocol that provides
effective QoS for VoIP over WLAN. The
characteristics of our proposed protocol are (1) No
modification of access points, (2) No H/W modification
of VoIP STAs and (3) Backward compatibility in order
to minimize the costs of development and
introduction. Simulation showed that our proposed
protocol can increase the number of accommodated
VoIP calls by approximately 50%.
As future works, we will implement our proposed
protocol in a testbed and evaluate its performance.
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