IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x

1

PAPER

Cooperation between channel Access control and TCP
Rate Adaptation in Multi-hop Ad hoc Networks
Pham Thanh GIANG†a) , Nonmember and Kenji NAKAGAWA††b) , Member

SUMMARY
In this paper, we propose a new cross-layer
scheme Cooperation between channel Access control and TCP
Rate Adaptation (CATRA) aiming to manage TCP ﬂow contention in multi-hop ad hoc network. In CATRA scheme, wecollect useful information from MAC and physical layers to estimate channel utilization of the station. Based on this information, we adjust Contention Window (CW ) size to control thecontention between stations. Then, we can achieve the fair channel access of each station and the eﬃcient spatial channel usage.
Moreover, the fair value of bandwidth allocation for each ﬂow
is calculated and sent to the Transport layer. Then, we adjust
the sending rate of TCP ﬂow to solve the contention between
ﬂows and the throughput of each ﬂow becomes fairer. The performance of CATRA scheme is examined on various multi-hop
network topologies by using Network Simulator (NS-2).
key words: Cross-layer method, per-ﬂow/per-station fairness,
ﬂow/station contention, throughput, TCP rate control, channel
access control, IEEE 802.11 back-oﬀ algorithm, multi-hop ad
hoc.

1.

Introduction

Binary Exponential Back-oﬀ (BEB) mechanism [1] inIEEE 802.11 [2] seems to provide all the contending
stations the same opportunity of access to the sharedchannel. However, in multi-hop topologies, BEB mechanism often suﬀers from the unfairness problem and low
throughput [3]. Moreover, BEB mechanism determines
the CW size corresponding only to the congestion condition, so it does not consider other conditions aboutneighbouring stations or higher layer, e.g., the numberof ﬂows in the channel or the number of users in thesystem. Thus, the CW size after some congestion may
not be the optimal value for fairness.
TCP employs a window-based Additive Increase
Multiplicative Decrease (AIMD) congestion control
scheme to adjust the transmission rate [4]. Thus TCP
performance in multi-hop wireless network depends
critically on the congestion window in use. If the window grows too large, there are too many packets to compete for the same medium. That increases the network
congestion and degrades the throughput and fairness
performance [5], [6]. In this paper, we propose a new
cross-layer method for determining better CW size in†Institute of Information Technology, Vietnam Academyof Science and Technology††
Department of Electrical Engineering, Nagaoka University of Technologya) E-mail: ptgiang@ioit.ac.vnb) E-mail: nakagawa@nagaokaut.ac.jp

the MAC layer and TCP rate to achieve TCP fairness
performance in multi-hop ad hoc networks.
The rest of the paper is organized as follows: Section 2 reviews related work. Section 3 describes ourcross-layer scheme. Section 4 evaluates our CATRA
method. Finally, Section 5 concludes the paper.
2.

Related WorkTCP challenges in 802.11 ad-hoc networks has been
deeply investigated in the past several years as thereport in [8]. Many factors may cause losses and affect TCP performance in multi-hop wireless networks
such as: route failures caused by node mobility, random wireless loss, medium access contention. To improve the network performance, some studies tried tosolve the problems at each layer independently as alayered design method, and some studies tried to cooperate some layers to exchange important information asa cross-layer design method.
In the layered design method, some studies focused on the malfunctions of IEEE 802.11 MAC
layer. Li [9] investigated Extended Inter-Frame Spacing (EIFS) problem, i.e., the ﬁxed EIFS value leads tounfair bandwidth allocation for each station. They proposed ﬂexible EIFS values based on a measurement of
the length of Sensing Range (SR) frame. However, thelength of SR frame cannot be always recognized becauseof the spatial reuse of the bandwidth. The three-pair
problem is introduced by Chaudet [10]. The analysis of
the three-pair problem in [10] is based on Markov chain
and gives some accurate results.
Some other studies considered to modify the BEB
mechanism to improve performance of IEEE 802.11.
Fang [11] reviewed unfairness problem in the BEB
mechanism. Each station estimates the channel utilization of itself and that of neighboring stations thenadjusts the back-oﬀ algorithm to give each station somevalue of bandwidth, which they call the statistical fair
access. The statistical fair access must be predeﬁned,
but it is diﬃcult in general because the value depends
on the topology. There are some other studies on thecontrol of CW size to achieve per-ﬂow fairness [12],[13], but almost all of them consider only single-hop
networks.
The others focused on the wrong behavior of TCP
mechanism in multi-hop ad hoc networks. Xu [14] pro-IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x2

posed the Neighborhood RED (NRED) scheme on network layer to enhance TCP fairness. By considering
neighborhood channel usage, intermediate nodes detect
neighborhood congestion and drop packets via NRED
scheme according to ﬂow’s channel usage. Chen [15]and Fu [6] showed that the conventional TCP window
grows too large, so there are too many packets competing for the channel. Their studies showed that there is
the optimal value of TCP rate at which the throughput is maximum by improving spatial channel reuse.
Their idea is adjusting TCP window size to achieve theoptimal value.
Recently, some studies applied cross-layer design
method to improve TCP performance. They tried to investigate information from MAC layer to adjust TCP
rate to achieve a good TCP performance in terms offairness and throughput. Cheng [16] proposed TCPCL, which slightly modiﬁes the legacy IEEE 802.11
MAC and TCP protocols. The standard IEEE 802.11
MAC layer provides a reliable operation over the communication channel by deﬁning a retry limit parameter(RETL). Based on RETL value, link failure is reported
to the TCP layer. Nevertheless, if the link experiences ahigh degree of contention, the MAC layer may mistakenly infer a link failure. Nahm [7] proposed a fractional
window increment scheme for TCP (TCP-FEW) to prevent unnecessary network contention by limiting the
growth rate of TCP’s congestion window. Zhang [17]and Natalizio [18] considered to control the transmission rate of TCP by utilizing the MAC information by across-layer method. In [17], TCP adjusts the transmission rate by determining a better congestion window
value based on the real MAC channel eﬃciency. However, they did not focus on the wireless MAC layer’s
contention and also can not exactly identify which partof MAC channel eﬃciency in the intermediate node.
In this paper, we propose a new cross-layer schemeand focus on TCP performance in multi-hop wirelessnetwork. First, we solve the unfairness problem at
MAC layer by determining a suitable CW size in IEEE
802.11 for each station. Second, we control TCP sending rate to achieve fair throughput for each ﬂow.
3.

Cooperation between channel Access control and TCP Rate Adaptation

In multi-hop ad hoc networks, the unfairness problem in TCP traﬃc is due to both a wrong behavior
of Distributed Coordination Function (DCF) mechanism in the channel access mechanism and a wrong behavior of TCP mechanism in traﬃc rate control. The
wrong behavior of DCF mechanism makes it diﬃcult
for some stations to access the channel. In this paper,
we will propose a new cross-layer scheme Cooperation
between channel Access control and TCP Rate Adaptation (CATRA). In CATRA scheme, we collect useful
information from MAC and physical layers, then adjust

the CW size based on these information. By using thisﬂexible CW size in the back-oﬀ state, the behavior of
DCF mechanism will be improved and disadvantaged
stations will get more chance to access the channel.
In general, the wrong behavior of TCP mechanism
gives some TCP ﬂows very large bandwidth, while itgives other ﬂows a few bandwidth. We will propose a
new method to investigate information from MAC andphysical layers to adjust TCP rate dynamically. Thus
the improvement of the behavior of TCP mechanism
will achieve the fair share for each ﬂow.
3.1

Channel Access control

Binary Exponential Back-oﬀ (BEB) mechanism [1]does not necessarily ensure the per-station fairness in
multi-hop ad hoc networks [3]. Here, the per-station
fairness means fair bandwidth allocation to every station. Moreover, the per-station fairness is not necessarily good for the per-ﬂow fairness. For ideal per-ﬂow
fairness, we must investigate the number of ﬂows which
are transmitted from the station.
We deﬁne the ideal per-ﬂow fairness as that all
ﬂows get the same throughput. The Fair Bandwidth
Ratio (F BR) for per-station fairness, which we call
F BRS is deﬁned by
F BRS =nSEN D,
ntotal(1)where nSEN D is the number of ﬂows which are transmitted from the station. We call such ﬂows as SEND
ﬂows. ntotal is nSEN D plus the number of ﬂows which
use the channel with SEND ﬂows.
The SEND ﬂows at the station include the directﬂow, which is generated by that station, and forwarding
ﬂows, which are required to forward from the neighboring stations. The ﬂows are identiﬁed by MAC and IP
addresses of both source and destination by decoding
the header of packets. We can easily get nSEN D from
MAC layer.
ntotal is the total of the number of two kinds ofﬂows at the examined station’s view. The ﬁrst kind ofﬂows, denoted as TX ﬂows, includes all ﬂows from stations in the transmission range of the examined station
and also ﬂows generated from the examined station itself. The number of TX ﬂows in the transmission range,
denoted as nT X , also can be known from MAC layer.
The second kind of ﬂows, denoted as CS ﬂows, include
ﬂows from stations which are out of the transmission
range but in the carrier sensing range of the examinedstation. We cannot distinguish the CS ﬂows by the local information of the examined station. However, thestation can sense the existence of CS ﬂows by the physical power. We consider the number of CS ﬂows whichshare the channel with the examined station as one if
there exist some CS ﬂows, so we deﬁne

GIANG and NAKAGAWA: COOPERATION BETWEEN CHANNEL ACCESS CONTROL AND TCP RATE ADAPTATION

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{
nCS =

0, if there is no CS ﬂow,
(2)
1, if there are one or more CS ﬂows,

and deﬁne the total number of ﬂows ntotal as(3)ntotal = nT X + nCS .

In complex models, we cannot exactly identify thevalue of ntotal due to the lack of information of hidden stations. In that case, the assumption of nCS = 1
makes the value of F BRS not accurate, but by addingnCS = 1 to nT X we can aware that some hidden stations require sharing the channel.

S2

S1

R
Flow 1

Flow 2

Fig. 1

A basic multi-hop wireless network model

Algorithm 1 Active time estimation
Initialization:
T Active = 0
t=0
Begin
for each packet p do
if p → destID == localID thenif (p → M acHeader → T ype == ACK) and (p →T cpHeader → T ype == T CPDAT A ) thent = t+0.5∗CW ∗ST +TRT S +SIF S +TCT S +SIF S +TT CPDAT A + SIF S + TACK + DIF S
else if (p → M acHeader → T ype == DAT A) and(p → T cpHeader → T ype == T CPACK ) thent = t+0.5∗CW ∗ST +TRT S +SIF S +TCT S +SIF S +TT CPACK + SIF S + TACK + DIF S
end ifend ifend forfor each interval time EP do
T Active = 0.8 ∗ T Active + 0.2 ∗ t
t=0
end forEnd

We also can obtain the Real Bandwidth Ratio(RBR) of the examined station, which we call RBRS .
The RBRS is calculated by measuring the T Active of
the station in a predeﬁned Estimation Period (EP ).
The T Active of the station is deﬁned as the average time
for transmitting packets from the station during each
EP . Algorithm 1 shows how to estimate the T Active bysensing packets.
The RBRS is deﬁned as the ratio of the T Active tothe EP , i.e.,
T Active.
(4)
EP
Based on the original CW value of the back-oﬀ algorithm of the IEEE 802.11 and the collected information above, we will determine a better CW size in the
back-oﬀ state. We propose a new CW size as follows;
(
)
RBRS
′
CW = min(5)
CW, CWmax .
F BRS
RBRS =In (5), the F BRS value is used as a threshold ofchannel access priority. If the station recognizes thatits RBRS is smaller than its F BRS , CW ′ is smaller
than the original CW . Then, the station can increase
its chance to access the channel and then the bandwidth allocation will be increased. On the other hand,
if the station recognizes that its RBRS is lager thanits F BRS , CW ′ is larger than CW . Thus, the station
decreases its chance to access channel, so other disadvantaged stations will get more chance to access thechannel. The channel bandwidth is used more eﬀectively and the fair bandwidth allocation between stations is improved.
In our mechanism, the disadvantaged stations havemore chance to access channel. Throughput and fairness are often trade-oﬀ performances. However, in our
proposed method, even in case that, some stations require only small oﬀered load ﬂows, they will accesschannel easily, and the remaining bandwidth is sharedby other stations. Thus, the throughput performance
is not degraded.
3.2

As an example of a basic multi-hop wireless network model, see Fig. 1. Stations S1 and S2 are in atransmission range, and also stations S1 and R are inanother transmission range. Stations S2 and R are outof the transmission range but in carrier-sensing range.
The values of nSEN D , nT X , nCS , ntotal and F BRS foreach station will be shown in Table 1.
Table 1
Station S1
Station S2
Station R

An example of Fair Bandwidth RationSEN D
2
1
0

nT X
3
3
2

nCS
0
0
1

ntotal
3
3
3

F BRS
2/3
1/3
0

TCP Rate Adaptation

The channel access control mechanism in Sec. 3.1 canimprove the Fair Bandwidth Ratio between stations but
it does not necessarily ensure the per-ﬂow fairness. In
this subsection we will propose an algorithm to achieve
the per-ﬂow fairness by controlling the TCP rate. In
TCP mechanism, the congestion window often grows
too large in ad hoc networks [5], [6], and there are too
many packets to compete for the same medium, then
the throughput and fairness performance are degraded.
Our idea to achieve fair share bandwidth allocation for
ﬂows is adjusting TCP rate. Let us consider the TCP
ﬂow generated from the examined station and control
the rate of the ﬂow by collecting information from MAC

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x

4

and physical layers. Remember we deﬁned ntotal in (3),which is the number of ﬂows sharing with the TCP ﬂow.
Thus, we deﬁne the Fair Bandwidth Ratio F BRf to the
TCP ﬂow as
1
F BRf =,
(6)
ntotal

1/3 by (6). Suppose RBRf = 1/2, i.e., Flow 1 obtained
more than the Fair Bandwidth Ratio F BRf = 1/3.
Then, in order to achieve the fairness, we will reduce the obtained bandwidth by giving the delay∆f =where the subscript f stands for f low.
The Real Bandwidth Ratio RBRf for the TCP
ﬂow is deﬁned as
RBRf =

Active
Tf

EP

,

RBRf
3
9
tr
Tf = Tf = ∗ win ∗ T f .
F BRf
2
2

RBR’ f = 2/9(12)

RBR f = 1/2(7)

Activewhere the T f
is the average time for transmitting
packets of the TCP ﬂow at the examined station during
EP .
If the TCP ﬂow has more ratio of bandwidth than(6), then we will reduce the bandwidth by giving somedelay before generating new packets. In TCP mechanism, at a certain time, the number of packets which
are ready to send in a TCP ﬂow is
win = cwnd + highest ack − cur seqno,(8)where cwnd is TCP congestion window size, highest ack
is the highest sequence number of received TCP ACK
packet, cur seqno is the last sequence number of sent
TCP DATA packet. Assuming the TCP ﬂow achieves
the Fair Bandwidth Ratio F BRf of (6), the time Tf
for transmitting win packets is calculated asTf =win ∗ sf
ntotal ∗ win ∗ sf=
F BRf ∗ BBtr
= ntotal ∗ win ∗ T f ,win packets

Tf tr

∆f
current time
Fig. 2

TCP rate adaptation of the Flow 1

Therefore only win packets will be sent in ∆f , thecurrent bandwidth ratio RBRf = 1/2 is reduced to
tr

RBRf′

win ∗ T f
2
=
= ,
∆f
9

(13)

thus we can achieve the fairness approximately by
1 1 2
1
13
(RBRf + RBRf′ ) = ( + ) =
2
2 2 9
36
1
≈ = F BRf .
3

(14)

(9)
tr

where sf is the average packet size and T f is the avtr

erage time for transmitting one packet by the link. T f
is calculated as
Active

tr
Tf =

Tf
Nf

,

(10)

where Nf is the number of packets from the TCP ﬂow
Activeand Nf canwhich are transmitted in EP . T f
be easily obtained from MAC layer by monitoring thenumber of packets from the TCP ﬂow in an interval
time EP .
Then, if the TCP ﬂow has more than the fair bandwidth ratio (6), we will give a delay ∆f deﬁned by
∆f =

RBRf
Tf
F BRf

(11)

to the TCP ﬂow before generating new packets.
Let us consider, for example, the Flow 1 in Fig. 1.
The examined station is S1. In the transmission rangeof S1, there are 3 ﬂows, i.e, Flow 1, Flow 2 (S2-S1) and
Flow 2 (S1-R). So, we have ntotal = 3, hence F BRf =Algorithm 2 TCP Rate Adaptation
Beginfor each new received ACK packet do
RBRf
if> Highth then
F BRf
RBRf
∆f =
Tf {Give the delay ∆f before generating
F BRf
the next packet after the completion of win packets transmission.}
cwnd = max(cwnd − 1, 1)
RBRfelse if< Lowth thenF BRf
∆f = 0cwnd = cwnd + 1
else
Call original TCP mechanism
end ifend forEnd

Algorithm 2 shows our modiﬁcation in TCP mechRBRfanism. When
> Highth with Highth ≥ 1, TCP
F BRf
sending rate will be decreased by letting the TCP ﬂow

GIANG and NAKAGAWA: COOPERATION BETWEEN CHANNEL ACCESS CONTROL AND TCP RATE ADAPTATION

5

delay a longer time than their required time to transmit next generating packets and slightly reduce cwnd.
RBRf
When
< Lowth with Lowth ≤ 1, TCP sending
F BRfrate will be increased by letting the TCP ﬂow generate next packet without delay and have larger cwnd.
We let TCP operate as original TCP mechanism in theother case. Thus, per-ﬂow fairness is achieved by Algorithm 2.
The small delay time ∆f before generating new
packets will give chance for the ﬂows of neighbouring
stations can increase their throughput. Therefore, thediﬀerence of total throughput is negligible.

shown in Fig. 4, where n is the number of stations in
Fig. 3 including R, S1 and S2, TCP means originalscheme and CATRA is our proposed method. Also in
Fig. 4, Flow 1 and Flow 2 denote the throughput of
Flow 1 and Flow 2, respectively, and T otal is sum ofthroughputs which are received at all stations. They
include destination and relay stations. If no packet is
dropped in Scenario-1, we have:
T otal ≈ F low1 + (n − 1)F low2.

3

Flow 1

Flow 2

(15)

Total

4.

Performance Evaluation

We now evaluate the performance of our cross-layer
scheme by comparing with the original scheme on various asymmetric topologies of multi-hop wireless ad hoc
networks. We use Network Simulator (NS-2) [19] forevaluation. The simulation parameters are shown in
Table 2. In CATRA scheme, we set a high thresholdas Highth = 1.05, a low threshold as Lowth = 0.7 andestimation period as EP = 2[s] for all simulations.

Goodput [Mbps]

2.5
2
1.5
1
0.5
0
TCP

CATRA
n=3

4.1

Scenario-1: Long vs. Short hop ﬂows topology

This topology includes a station chain as Fig. 3. Distance between each station is 200 [m]. In chain topology, let the second and the last stations S1, S2 generate
TCP traﬃc to the station R. Because the long hop ﬂow
must travel through many relay stations, and it must
contend with the short hop ﬂow, it is diﬃcult for Flow 2
to reach the destination.

...

S2

S1

R
Flow 1

Flow 2

Fig. 3

Scenario-1: Long vs. Short hop ﬂows

The simulation results in terms of throughput are

CATRA
n=4

TCP

CATRA
n=5

TCP

CATRA
n=6

Number of stations

Fig. 4
Table 2 Parameters in the simulation
Channel data rate
11[Mbps]
Antenna type
Omni direction
Radio Propagation
Two-ray ground
Transmission range
250[m]
Carrier Sensing range
550[m]
MAC protocol
IEEE 802.11b (RTS/CTS is enable)
Contention Window
CWmin = 32, CWmax = 1024
Connection type
TCP/FTPBuﬀer size
100[packet]
Packet size
1[KB]
Simulation time
300[s]TCP

Throughput in Scenario-1

Fig. 4 shows that the larger n, the more diﬃcult
the long hop ﬂow Flow 2 reaches the destination in original scheme. While it gives fair throughput for long andshort hop ﬂows in CATRA scheme. Flow 2 in CATRA
scheme achieves much better throughput than the original scheme and we still keep good total throughput of
all ﬂows in the topology.
4.2

Scenario-2: The three-pair topology

Fig. 5 shows the topology of Scenario-2. The problem inthis scenario is also known as three-pair problem which
was ﬁrst investigated in [10]. In this scenario, stations
S1 and S3 are out of the carrier sensing range, hence
the two external pairs S1-R1 and S3-R3 are completely
independent, i.e., they can send packets simultaneously
without interference to each other. Thus, two external
pairs contend bandwidth only with the central pair S2R2, while the central pair contends with both external
pairs. In this topology, the central pair cannot accessthe medium in the original scheme.
In Fig. 6, throughput of Flow 2 of the central pairis zero in the original scheme. In CATRA scheme, theMAC layer contention is solved hence the bandwidth allocation of S2 is increased. Thus, throughput of Flow 2
is much improved. The total throughput in CATRA
scheme is smaller than the original scheme due to thechannel reuse of the original scheme. If the central paircannot access the channel bandwidth in the originalscheme, then two external pairs can access channel independently and use the whole channel capacity. Thus,

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x

6(600,200)(300,200)(0,200)(400,800)

R3

R2

R1

S2

Flow 1S1(600,0)(300,0)(0,0)(800,400)(600,400)

Flow 2

Flow 2S2

Y-AxisFlow 3S3(400,600)(400,400)(200,400)(0,400)

S1

R1

X-Axis(400,200)

Flow 1

Scenario-2: Three-pair problem

Fig. 5

Y-Axis(400,0)X-Axis5

R2

4.5

Goodput [Mbps]

4
3.5

Flow 1

3

Flow 2

2.5

Flow 3

2

Fig. 7

TCP: Flow 1
TCP: Flow 2
TCP: Total E2E

Total

1.5

0.4

0

TCP

Fig. 6

CATRA

Throughput in Scenario-2

Goodput [Mbps]

1
0.5

Scenario-3: The cross-chain topology

CATRA: Flow 1
CATRA: Flow 2
CATRA: Total E2E

0.3

0.2

0.1

0
0

the total throughput can be twice of the channel capacity. While in CATRA scheme, two external pairs must
share the channel with the central pair. Thus, the central pair achieves a half of the channel capacity, then
the two external pairs also have only a half of the channel capacity. Then, the maximum total throughput is
one and a half of the channel capacity.
4.3

Scenario-3: The cross-chain topology

100

150

200

250

300

Time [s]Fig. 8

4.4

Throughput in Scenario-3

Scenario-4: A TCP ﬂow contents with a various
speed UDP ﬂow

In Scenario-4, the throughput of ﬂows are compared
between the original and our CATRA schemes. The
topology is shown in Fig. 9. One UDP ﬂow contends
with one TCP ﬂow. We evaluate the throughput of theﬂows by changing the oﬀered load of the UDP ﬂow from0 to 6[Mbps].
(400,0)(200,0)

S2

S1

(0,0)
R
TCP
Y-Axis

The cross-chain topology, which is discussed in [20],is shown in Fig. 7. We will examine the contention
between two ﬂows that leads to short-term unfairness
problem. In this topology, if one ﬂow wins to occupy
the channel, it will have an opportunity to get the whole
bandwidth for long time until timeout by the intra-ﬂow
contention. Then the other ﬂow is diﬃcult in accessing
the channel. In this simulation, we let Flow 1 be moreadvantageous than Flow 2 by starting Flow 2 in one
second later than Flow 1.
The simulation result is shown in Fig. 8. In theoriginal TCP scheme, the throughput of Flow 1 andFlow 2 are much unstable while in CATRA scheme theshort-term fairness between two ﬂows is quite better.
Moreover, we still keep good result of the sum of two
ﬂows’ throughput in CATRA scheme.

50

UDP
X-AxisFig. 9Scenario-4: TCP vs. UDP ﬂow

In the original scheme, let us call the TCP and
UDP ﬂows as “original TCP ﬂow” and “original UDP

GIANG and NAKAGAWA: COOPERATION BETWEEN CHANNEL ACCESS CONTROL AND TCP RATE ADAPTATION

7ﬂow”, respectively. Similarly, in CATRA scheme, wecall them as “CATRA TCP ﬂow” and “CATRA UDP
ﬂow”, respectively. In CATRA scheme, a UDP ﬂow isapplied the channel access control in Sec. 3.1, but notapplied the TCP rate control in Sec. 3.2.

3

Original TCP flowwhere n is the number of ﬂows, xi is the end-to-end
throughput of f low i.
As Fig. 11, CATRA scheme achieves much better
fairness performance than the original scheme.

2

Original UDP flow

1.5

Original Total
throughput
CATRA TCP flow

1

CATRA UDP flow

0.9

CATRA Total
throughput

0.8

0.5

0
0

1

2

3

4

5

6

Offered load [Mbps]

Fig. 10

1

Fairness Index

Throughput [Mbps]

2.5

in Fig. 11. In there, we use Fairness Index as deﬁned
by [21] as follows;∑n( i=1 xi )2∑nFairness Index =,(16)n · i=1 x2i

Throughput in Scenario-4

Original

CATRA

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

4.5

Scenario-5: The random topology

Scenario-5 is a random topology. We make topologies with 50 stations at random position in 1000[m] ×1000[m] area. Among those 50 stations, n stations arechosen randomly and these n stations generate TCP
traﬃc to one destination station. The average of Fairness Index and total end-to-end throughput are usedas the metrics to compare the throughput and fairness
performances, respectively. These terms of the network
performance are examined versus the number of ﬂows.
Each data point is the average over 20 simulations.
The simulation result in Fairness Index is shown

2

4

6

8

10

12

14

16

18

20

Number of Flows

Fig. 11

Fairness Index in Scenario-5

The total end-to-end throughput is sum of all
throughputs which are received at the destination station and Total throughput is sum of throughputs which
are received at all stations. As Fig. 12, our total endto-end throughput is smaller than the original scheme.
The degradation of end-to-end throughput is due tosharing channel between each ﬂow and also its forwarding ﬂow. However, regardless of the reason of channel
reuse, our total throughput Fig. 13 is similar to theoriginal one.

2
1.8

Total E2E Goodput [Mbps]

The results of the throughput of each ﬂow andthe total throughput are shown in Fig. 10. For offered loads of UDP larger than 1[Mbps], the throughputof the original TCP ﬂow is about 1.5[Mbps] and thatof the original UDP ﬂow is 0.5[Mbps], which meansvery unfair. While in CATRA scheme, the throughput
of CATRA TCP and CATRA UDP ﬂows are almost
the same. In the original scheme, at the station S1,
the sending bandwidth is much more than the receiving bandwidth, so the throughput of the original TCP
ﬂow is much larger than that of the original UDP ﬂow.
While in our CATRA scheme, due to the channel access control for both UDP and TCP ﬂows and the TCP
rate control for TCP ﬂow, the unfairness of the originalscheme was successfully dissolved.
As for the total throughput, both original and
CATRA schemes achieve almost the same values. We
can see that the throughput of the CATRA TCP isslightly smaller than that of the CATRA UDP ﬂow because of the overheads of TCP mechanism such as ACK
packets, and so on.

Original

1.6

CATRA
1.4
1.2
1
0.8
0.6
0.4
0.2
0
2

4

6

8

10

12

14

16

18

Number of Flows

Fig. 12

Total end-to-end throughput in Scenario-5

20

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x8

4.5Original

Total Goodput [Mbps]

4

CATRA

3.5
3
2.5
2
1.5
1
0.5
0
2

4

6

8

10

12

14

16

18

20

Number of Flows

Fig. 13

5.

Total throughput in Scenario-5

Conclusion

We proposed a new cross-layer scheme to enhance TCP
performance in multi-hop wireless ad hoc network. We
measured the channel utilization and deﬁned the Fair
Bandwidth Ratio by the number of ﬂows in the carrier sensing range of the examined station. The IEEE
802.11 CW size is modiﬁed for achieving per-station
fairness. The information about channel utilization was
sent to Transport layer to adjust TCP sending rate toachieve the per-ﬂow fairness. The simulations on various topologies have proved the eﬀectiveness of CATRA
scheme. In addition to fairness, CATRA scheme alsoachieves quite good performance in terms of throughput.
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