A power-controlled MAC supporting service differentiation in mobile ad hoc networks

Abstract

The original power controlled multiple access (PCMA) protocol does not support service differentiation. In this paper, we extend PCMA to form a new media access control protocol supporting service differentiation in mobile ad hoc networks. To support QoS, we first introduce the in-station access category concept in 802.11e to PCMA. For service differentiation between access categories, our major contribution is to propose a sender-initiated busy tone based mechanism that allows a user to gain quick channel access. This quick access mechanism is only performed when the number of access failures exceeds a threshold. An access category with higher priority is assigned a lower threshold for easier channel access, and vice versa. Through analysis and simulation, we demonstrate that our protocol can provide better quality of service than 802.11e in terms of throughput, delay, loss, and fairness

Full text

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Title
A power-controlled MAC supporting service differentiation
in mobile ad hoc networks
Author(s) Shao, W; Shen, D; Yang, D; Li, VOK
Citation
The 16th IEEE International Symposium Personal, Indoor
and Mobile Radio Communications, Berlin, Germany, 11-
14 September 2005, v. 4, p. 2742-2746
Issue Date 2005
URLhttp://hdl.handle.net/10722/45870
RightsCreative Commons: Attribution 3.0 Hong Kong License

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2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications
A Power-Controlled MAC Supporting Service
Differentiation in Mobile Ad Hoc Networks
Wenjian Shao, Dongxu Shen, Daiqin Yang and Victor 0. K. Li
Department of Electrical and Electronic Engineering
The University of Hong Kong
Pokfulam, Hong Kong, China
{wjshao, dxshen, dqyang, vlii @eee.hku.hk
Abstract- The original power controlled multiple access
(PCMA) protocol does not support service differentiation. In this
paper, we extend PCMA to form a new media access control pro-
tocol supporting service differentiation in mobile ad hoc networks.
To support QoS, we first introduce the in-station access category
concept in 802.11e to PCMA. For service differentiation between
access categories, our major contribution is to propose a sender-
initiated busy tone based mechanism that allows a user to gain
quick channel access. This quick access mechanism is only per-
formed when the number of access failures exceeds a threshold.
An access category with higher priority is assigned a lower thresh-
old for easier channel access, and vice versa. Through analysis and
simulation, we demonstrate that our protocol can provide better
quality of service than 802.1le in terms of throughput, delay, loss,
and fairness.
I. INTRODUCTION
The provisioning of quality of service (QoS) is central to the
success of mobile ad hoc networks (MANET). Basically, there
are two strategies for QoS support. Integrated services (IntServ)
guarantees QoS requirements such as bandwidth and delay, but
it requires complicated schemes for resource reservation and
allocation. A simpler approach is differentiated services (Diff-
Serv), which prioritizes one type of traffic over another, with
no "hard" guarantees. In the dynamic environment ofMANET,
DiffServ is easy to implement, because each hop operates in-
dependently without end-to-end resource reservation.
paper, we discuss the design of power-controlled media access
control (MAC) protocol that supports DiffServ in MANET.
The achievement ofDiffServ in MANET relies on the design
of the MAC protocol. High priority traffic should have prece-
dence over low priority traffic in accessing the channel. Using
this approach, [2], [3], [41 and [5] have been proposed to pro-
vide service differentiation in IEEE 802.1 1 [1] based MANET.
Also, the developing IEEE 802.1 le [61 MAC protocol supports
DiffServ in this style. A major drawback of 802.1 1 based pro-
tocols is that the transmission power is fixed, thus leading to
low channel reuse ratio.
Power-controlled MAC protocols, such as PCMA [7], can
effectively increase channel reuse by adjusting the transmission
power to an optimal value instead of a fixed one. However,
PCMA does not consider QoS. In this paper, we provide Diff-
Serv support in PCMA, called PCMA-DS(DiffServ).
First, we introduce the access category (AC) concept in
802.1 le to PCMA. Our major contribution is to incorporate a
In this
sender-initiated busy tone with PCMA to differentiate the chan-
nel access probabilities between different access categories. In
our protocol, the sender-initiated busy tone is used to assist in
channel access. When a user is in urgent need of channel ac-
cess, for example, it has a delay-sensitive packet to send, it can
turn on the busy tone as a declaration. Overhearing the busy
tone, nearby nodes will refrain from transmission to yield the
access right to the urgent user. A high priority traffic is al-
lowed to apply the above busy tone mechanism earlier than low
priority traffic. Using computer simulation, we compare our
protocol with IEEE 802.1 le and PCMA. We find our protocol
perform better than 802.1 le and PCMA in terrns of throughput,
delay, packet loss, and fairness. Since we focus on extending
PCMA to support DiffServ, we do not consider the energy con-
sumption. This issue has been discussed in many papers, such
as [8] and [10].
This paper is organized as follows. In Section II, we review
the related work. In Section III, we describe the MAC protocol.
Simulation results are provided in Section IV. Conclusion is
given in Section V.
II. RELATED WORK
A. Differentiated Services in 802.11 Based Protocols
The basic mode of 802.11 [1] is the Distributed Coordina-
tion Function (DCF), which uses CSMA/CA as the fundamen-
tal channel access scheme. There are two basic mechanisms in
CSMA/CA for collision avoidance: inter-frame space (IFS) and
backoff. When a station wants to transmit a packet, it senses
the channel first and defers the transmission for predefined IFS
even if the channel is sensed idle.
busy, a backoff procedure is invoked, with the backoff time
decided as rand(0, CW) aSlot_time, where aSlot_time is
the slot time. The function rand(0, CW) picks a value ran-
domly from the range [0, CW], where CW is the contention
window size. QoS is not supported in 802.11 DCF. Although
the optional polling-based scheme (PCF) of 802.11 is designed
for time-bounded services in wireless LAN, it has many limita-
tions. More importantly, PCF only works for the infrastructure
configuration, not for the ad hoc mode.
Service differentiation can be achieved in 802.11 DCF by
modifying the IFS (based on different IFS values [2][3]) and
backoff times (based on the control of CW [3][4] or based on
the selection of different backoff times directly [5]). Smaller
If the channel is detected
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2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications
IFS and backoff time result in higher priority. These methods
provide only one priority for one station, which is not thecase
in actual networks.
IEEE 802.1 le extends the 802.11 MAC with a new Hybrid
Coordination Function (HCF). HCF combines two methods,
i.e. the enhanced distributed channel access scheme(EDCA)
and the polling-based HCF controlled channel access scheme
(HCCA). HCCA is the enhancement of PCF and not suitable
for infrastructureless MANET. In the environment ofMANET,
we consider the distributed EDCA. EDCA is the extension of
DCF to support DiffServ. By introducing in-station accesscat-
egories (AC), a QoS station can support four delivery priori-
ties, and eight user-defined traffic categories (TC) aremapped
to four ACs. Each AC has its independent queue andbehaves
as an independent MAC to contend for the channel. Sinceeach
AC is controlled by predefined parameters, such as arbitration
IFS (AIFS), CWmin and CWmaIx, priorities are implemented
between ACs.
B. Power-Controlled MAC Protocol
A major drawback of IEEE 802.11 based MAC protocols is
that the transmission power is always fixed, regardless ofcom-
munication distance. This feature makes 802.11 based proto-
cols inefficient in channel reuse. To improve the channel uti-
lization, a group of protocols using the power control mecha-
nism are proposed. The transmission power in these protocols
is adjusted to an optimal value instead of a fixed value, thus
reducing interference and encouraging channel reuse.
Among power-controlled MAC protocols, PCMA[7] is a no-
table one. In PCMA, each node needs to calculate the maxi-
mum allowable power for its transmission without interfering
with other ongoing receptions. This calculation technique is
called power bounding.
The power bound(Ptboh0,d) is de-
signed such that after propagation loss, the received power will
not exceed the tolerable additional noise (Na) of all neighbor-
ing receivers. To announce its tolerable additional noise, each
receiver adopts a receiver-initiated power-controlled busy tone
(BT,),whose transmission power (PtBTr) is inversely propor-
tional to its Na. Periodical pulses instead of continuous signals
are used for busy tones so that tones from different nodes can
be distinguished. According to the maximal received power on
BTr channel (PrBTP), each node can identify the most vul-
nerable receiver around it and decide its Ptbound. In PCMA, a
transmitter sends a request using its Ptbound. After receiving
the request, the receiver calculates the desired power (Ptdes)
and Na for the transmission according to the channel gain and
local noise level. If Ptbound > Ptdes, the receiver accepts the
request and data are sent with Ptdes-
PCMA uses power-controlled receiver busy tone and power
bounding algorithm to solve the hidden/exposed terminal prob-
lems and increase channel reuse. The power-controlled receiver
busy tone signifies an ongoing reception, thus solving the hid-
den terminal problem. IEEE 802.11 [1] makes access decision
based on the channel status, i.e., whether the channel is free or
busy. Unlike the "on/off model" used by IEEE 802.1 1, PCMA
allows transmitters to access the channel even when the chan-
nel is busy, as long as the transmission power is lower than
Ptbound,and the exposed terminal problem is implicitly solved.
Therefore, PCMA can achieve muchhigher throughput than
802.1 l based MACprotocol, especiallyathighload.
III. SUPPORTING DIFFSERV IN PCMA
A.DesignMotivations
TosupportDiffServ inPCMA, we can borrow theconceptof
accesscategoriesfrom 802.1 le to PCMA. For different ACs, a
mechanism is needed to differentiate the channel accessprob-
ability.In 802.1 le, this differentiation is achievedthroughdif-
ferent IFSs and backoff times. The AC with smaller AIFS and
backoff time (controlled by CWmin andCWUmax ) is more
likelyto win the contention. It should be noticed that theopera-
tions ofIFS and backoff time are conditioned on channel status,
i.e., whether the channel isbusyor free.
However, as discussedpreviously,the transmission decision
of PCMA is not related to whether the channel isbusy: a sta-
tion is allowed to transmit aslongas the desired transmission
powersatisfies the transmissionpower bound, regardlessof the
channel status. InPCMA, a station will refrain from transmis-
sion when itspowerbound is less than its desired transmission
power, i.e., PtBound < Ptd,,. With small IFS and backoff
time, a station can have aquick retry. However, itspowerbound
maystill be less than its desired transmissionpoweron theretry.
On thecontrary,a station withlargeIFS and backoff timemay
havePtBound >Ptdes, thussuccessfully accessing the chan-
nel. Therefore, control of IFS and backoff time is not an effec-
tivewaytoimplementDiffServ in PCMA.
Obviously,the channel accessprobability is determinedby
Pr{PtBOund > PtdeS}.
powerbound of a station to meet therequirement of desired
powerthan other stations, this station is more like to access
the channel and hashigher priority. SincePtd,,is determined
bythe communicationdistance, SNRrequirement, and receiver
noise level, we canonly manipulate PtBound to achieve Diff-
Serv. The valueofPtBoundisdependenton theon-going trans-
missions within thevicinity.
around, PtB,und will becomesmaller, and vice versa. Differ-
ent stations have differentPtBound. Even for asingle station,
its PtBoundis timevarying.
Ideally,ifPtBaundcanmonotonically increase after a station
has decided totransmit, channel access can bequickly obtained.
This ispossibleif theneighboring nodes cooperate in the fol-
lowing way: neighboring nodes that are in the idle state do
notcompetefortransmission, andneighboring nodes in trans-
mission will not initiate new transmission after they finish. In
thisway,interference to theintending node will subside and its
PtBound will increase. Once PtBound rises above Ptdes, the
node can start transmission.
The above isonly possible when the neighboring nodes are
aware of the transmission intention of that station. In ourpro-
tocol, we devise a declaration mechanism assistedby a sender-
initiatedpower-controlled busy tone forquick channel access.
In such ascheme, when a node fails to access the channel since
itspowerbound is smaller than the desired transmissionpower,
it will turn on the sender-initiated busy tone using aproperly
selectedpower. Overhearing thebusy tone, nearby nodes will
refrain from transmissions toyield the access right to theurgent
If it is easier for the transmission
If there are more transmissions
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2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications
node. For service differentiation, high priority traffic is allowed
to apply the sender-initiated busy tone mechanism earlier than
low priority traffic.
B. System Assumptions
In this paper, we make the same assumptions on the channels
as in PCMA [7]. They are:
* The channel gain remains unchanged during the transmis-
sion of RTS and DATA.
. The channel gain from node A to node B equals the chan-
nel gain from node B to node A.
* The gain of the data channel equals the gain of the busy
tone channel.
For the same considerations, periodical busy tone pulses in-
stead of a solid tone are used in the busy tone channel as in
PCMA. According to [9], only 1-2% of bandwidth is needed
for busy tones to achieve the best performance of the system.
Thus, we assume the overhead caused by busy tones is negligi-
ble.
We also assume nodes know the required transmission power
to the desired location (denoted as Ptdes). To determine Ptdes,
a sender must have the distance information between itself and
the receiver. Many location-aware MAC protocols are proposed
to help senders determine the distance to the desired destina-
tion, for example, through global positioning system (GPS).
Distance also can be estimated through the previous transmis-
sions, as discussed in [10].
C. Protocol Description
1) Quick Channel Access by Busy Tone Declaration:
basic idea ofthe quick channel access is to use a sender-initiated
power-controlled busy tone as a mechanism to announce the ac-
cess request. We denote this busy tone asBTt,and the receiver-
initiated busy tone in PCMA as BTr. An intending node will
set up BTt using power Ptdes to notify its neighbors that it
wants to send packets using this power. Nodes overhearing the
busy tone will update their local noise level accordingly, and
those that may otherwise transmit will refrain from transmit-
ting. Gradually, with the completion ofon-going transmissions,
PtBoundwill become larger than Ptdes, after which the node
can transmit.
The busy tone declaration steps are summarized as follows.
1) When node A fails to access the channel for more than
a given threshold (represented as F_Thresh) number of
times, it tums on BTt using Ptdes until it can access the
channel.
2) If a node senses the BTt channel with maximal re-
ceived power (PrBTt) larger than the threshold for cor-
rect packet decoding (RX_Thresh), i.e., PrBTt
RX_Thresh, it will both backoff its transmission and
reject any RTS request until this periodical pulse disap-
pears.
3) Else, a node will add the received pulse powerto its noise
level on the data channel to account for the interference
from node A when node A transmits later.
4) After node A turns on its BTt, it continues sensing the
BT, channel and updating its Ptbound. When Ptbound >
The
>
Ptdes, it turns off itsBTtand tries to access the channel
using the steps in PCMA.
Obviously, this scheme assures prompt channel access. In
our protocol, since nodes sensing PrBTt > RX_Thresh are
potential receivers of node A, they will refrain from new com-
munications (both send and receive) and wait for A's connection
request. With the completions of ongoing receptions around
node A,Ptb0,ndof node A gradually increases. When it be-
comes greater than Ptdes, node A is eligible to access the
channel. On the other hand, because nodes sensing PrBTt <
RX_Thresh will not be the receivers of node A, they can con-
tinue setting up new connections. Their transmissions will not
collide with the later transmission from node A, since interfer-
ence from node A has been considered.
We incorporate the sender-initiated busy tone with PCMA to
generate a dual busy tone MAC protocol. So, a node in idle
status not only monitors the BTr channel to update its power
bound, it also monitors the BTt channel to check whether it
is blocked by the BTt signal. Usually, a node will access the
channel the same way as in PCMA, except it needs to monitor
the BTt channel. If it fails, it will backoff and access again.
Only when the number of failures reaches a predefined thresh-
old, F_Thresh, will the node use the BTt as an assistance to
access the channel.
2) Integration ofAccess Categories in PCMA:
tocol, we use the method similar to that in 802.1 le to support
multiple access categories inside one station. There are mul-
tiple independent queues in our MAC protocol, and each AC
maintains its own queue. The traffic will be mapped to these
queues according to the traffic type. Each AC independently
contends for the channel using the steps defined in PCMA as a
virtual station. If more than two ACs within a station happen to
contend the channel at the same time, a virtual collision occurs.
In this situation, the AC with higher priority will win the virtual
contention. When an AC fails to access the channel, including
virtual collision, it will backoff and try again.
Note that all ACs have the same CWmni, and CWmax in our
protocol. We do not differentiate the ACs according to backoff
times. We differentiate ACs by assigning different F_Thresh
values to different ACs. A smaller F_Thresh favors higher
priority AC, which will use the busy tone declaration mecha-
nism earlier and have fewer access attempts than lower priority
AC.
In our pro-
IV. SIMULATION RESULTS
In our simulation, we study the DiffServ capability of our
protocol using NS2.27, and compare it with that of both
802.1 le EDCA and PCMA.
A. Simulation Setup
In the test environment, we use the default settings of wire-
less channel and physical layer in NS2.27. The antenna gain
is 1, the height of each antenna is 1.5 m, the system loss is 1
and the carrier frequency is 916 MHz. The RX_Thresh is
-64 dBm, CS_Thresh is -78 dBm, SIR_Thresh is 10 dB
and the maximal transmission poweris 24.5 dBm. Forpropaga-
tion model, two-ray-ground model is used. With these settings,
the maximal transmission range is 250 m.
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2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications
We construct a network with an area of 1000 m x 1000 m,
within which 100 nodes are randomly distributed. The band-
width of the data channel is 2 Mbps. The maximal queue length
for each AC is 50 packets. In this paper, since we focus on the
performance of the MAC layer protocol, only one hop neigh-
bors are randomly selected to form communication pairs so as
to remove the influence of routing. In future research, we will
study the performance of our protocol under multi-hop environ-
ment.
Three types of traffic are transmitted between each commu-
nication pair. The first traffic type is the background traffic fol-
lowing the Poisson traffic model.
Kbps. It has the lowest priority and is mapped to AC_BK. The
second type is MPEG1 video traffic with an intermediate prior-
ity. It is VBR with an average rate of 32 Kbps and mapped to
AC_VI. The third type is real time voice traffic, which has the
highest priority. The voice traffic is CBR at 8 Kbps and mapped
to AC_VO. The parameters of ACs are configured in Table 1,
whereCWm.in, CWmax and AIFSN are the parameters for
802.1 le and F_Thresh is for our protocol. The F_Thresh
value ofAC_BK is "N/A" means that the quick channel access
method is not available for AC_BK. When increasing the num-
ber of communication pairs from 10 to 100, the normalized (by
the channel bandwidth) traffic load G is from 0.4 to 4.
Table 1: Configuration of access categories
AC
CWmin
CWmax
AC_VO
3
7
ACVI
7
15
AC_BK
15
1023
Its average data rate is 40
AIFSN
2
2
7
F_Thresh
1
2
N/A
B. Performance Comparison
1) Throughput:
PCMA-SD protocol with 802.1 le EDCA and the original
PCMA. Fig. 1 shows the average throughput of three types of
We first compare the throughput of our
Offered Traffic Load (G)
Fig.
EDCA. PCMA and PCMA-SD.
1.
Throughput comparisons of three types of traffic among 802.1 le
traffic for one communication pair.
throughput a user can obtain for each type of traffic.
It represents the average
Firstly,
we observe that ourprotocolcanprovide DiffServsupport. For
high priority voice and video traffic, ourprotocol canprovide
much higher throughput than PCMA. While theperformance
of low priority background traffic is worse than PCMA since
theaggregated throughput ofourprotocol is almost the same as
that of PCMA.
Secondly, we observe that our protocol can provide better
differentiated services than 802.1 le EDCA. As loadincreasing,
thethroughput of the three types of traffic decline with differ-
ent ratesaccording to theirpriorities. Since ourprotocol uses a
powercontrolmechanism, thusenjoying ahigher channel reuse
than 802.1 le, the degradation is much less severe. More impor-
tantly, voice traffic, with the highest priority, exhibits a con-
stant throughput in our protocol. In contrast, voicethroughput
in 802.1 le reduces by 30% when the traffic load G = 4.
Thirdly, we observe that PCMA has the worst performance in
voice traffic throughput, since it has no DiffServ support. How-
ever, since PCMA hashigherchannel reuse ratio and higher ag-
gregated throughput than 802.1 le, it outperforms 802.1 le with
medium priority video and low priority background traffic. We
can say that the capacity of a MAC protocol is a key to QoS
support.
.-
(a
0)
0
-J
O
Voce
PCMA
8D0
OdI
(a
4O40
0
0
-J
20,
0.4
0.8
1.2
Offered Traffic Load(G)
1.6
2
2.4
289
3.2
36
4
0.8
1.2
Offered Traffic Load(G)
1.6
2
2.4
2.8
3.23.6
Fig. 2.
PCMA and PCMA-SD.
Loss rate comparisons of three types of traffic among 802.1 le EDCA.
2) Loss Rate:
portant QoS measure. From Fig. 2, we can see that three types
of traffic experience different loss rates in our protocol, while
the loss rates are almost the same in PCMA, which is another
indication of DiffServ capability ofour protocol.
We also find that our protocol achieves lower loss rates than
802.1 le EDCA for all three traffic types. Especially, the loss
rate of the highest priority voice traffic is near zero in our pro-
tocol.
3) Delay: Fig. 3 presents the distribution of end-to-end de-
lay under different traffic loads. For delay-sensitive applica-
tions, such as voice and video, both 802.11e EDCA and our
Fig. 2 compares the loss rates, another im-
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