DCMAC: Data-oriented Cluster-based Media Access

Control Protocol for Vehicular Networks

Jinho Lee∗, Jaehoon (Paul) Jeong†, Tae (Tom) Oh‡, Junghyun Jun§, and Sang Hyuk Son¶

∗ Department of Computer Science & Engineering, Sungkyunkwan University, Republic of Korea† Department of Interaction Science, Sungkyunkwan University, Republic of Korea

‡ Department of Information Sciences & Technologies, Rochester Institute of Technology, USA.§ Department of Computer Science & Engineering, Indian Institute of Technology at Ropar, India

¶ Department of Information & Communication Engineering, DGIST, Republic of Korea

Email: {jinholee, pauljeong}@skku.edu, tom.oh@rit.edu, peterjun@iitrpr.ac.in, son@dgist.ac.kr

Abstract—In order to support efficient data services on vehic-ular networks, we propose a Data-oriented Cluster-based MediaAccess Control Protocol (DCMAC). For Intelligent Transporta-tion Systems (ITS), the demand for data services is increasing.Furthermore, most of vehicles will be equipped with DedicatedShort-Range Communications (DSRC) devices in the near future.Thus, it is necessary to offer scalable data services on vehicularnetworks. We propose a cluster-based MAC protocol for maxi-mizing the throughput of data services. Through clustering, wecan reduce contention period for time slot allocation and improvechannel utilization. Furthermore, we can significantly reduce thepower consumption for transmissions.

I. INTRODUCTION

Recently Internet of Things (IoT) has been widely re-

searched. As a part of it, many researchers are working on

Vehicular Ad Hoc Networks (VANETs). As Intelligent Trans-

portation Systems (ITS) [1] and autonomous vehicle technolo-

gies advance, the importance of VANETs is getting higher. In

order to support VANETs, 802.11p [2] is standardized. This is

currently used for vehicle safety, toll collection, and vehicle-

based communications. U.S. Department of Transportation is

trying to enact to mandate Dedicated Short-Range Communi-

cations (DSRC) [3] to all light vehicles [4]. Thus, it is timely

to study about large scaled VANETs for various applications

of ITS.

In addition to the 802.11p standard, many MAC protocols

are devised for VANETs [5]–[8]. Most of them are designed

for safety services. Vehicles regularly exchange Basic Safety

Messages(BSMs) [9], for safety services. For data services, ve-

hicles need to access RSUs. For each MAC protocol, vehicles

register themselves to RSUs through a Control Channel (CCH)

with a contention based protocol. Contention based protocol is

not efficient in large networks. Our Data-oriented Cluster-based

MAC protocol solve this problem with clustering. To the best

of our knowledge, our work is the first cluster-based MAC

protocol for data services in infrastructure to vehicle (I2V)

communications.

In this paper, we introduce our DCMAC. We reduce con-

tention period for vehicle registration for time slot allocation as

we cluster adjacent vehicles. Through this physical adjacency,

we also suggest a data transmission scheme so that we can

fully utilize Service Channel (SCH) and save energy. Besides,

we can proactively handover using position and mobility

information gathered by RSUs.

The rest of this paper is organized as follows. Section II is

the literature review of MAC protocols on VANETs. Section III

explains the design of our DCMAC. Several research issues are

proposed in Section IV. Section V concludes this paper along

with future work.

II. RELATED WORK

Much research has been done about MAC protocols on

VANETs [5]–[8]. 802.11p is a contention-based MAC protocol.

Contention-based channel access is based on proper inter-

mittent and short message sharing. Thus, for safety message

sharing, it seems reasonable to use contention-based channel

access. However, 802.11p standard uses about 10MHz of

bandwidth. This is overkill only for safety messages. If DSRC

is also used for data services, drivers can get real time naviga-

tion service, road information, video streaming, web surfing,

or e-mail service. For these kinds of data services, contention-

based 802.11p standard is not proper. Thus, we propose our

Data-oriented Cluster-based MAC protocol which is a hybrid

of contention-based protocol and contention-free protocol. Our

protocol is a hybrid MAC protocol of a contention-based

protocol for registration and contention-free protocol for data

transmission.

VeMAC [5] is a contention-free MAC protocol. VeMAC

allocates time slots based on vehicle’s position and moving

direction in order to reduce collisions. However, this MAC

protocol assumes that every vehicle is equipped with two

transceivers and one of them is dedicated to the control

channel. SOFT MAC [6] divides roads into several cells and

allocates frequency band to each cell. In this case, for a sparse

road, we can not fully utilize frequency band. TC-MAC [7]

is TDMA Cluster-based MAC. TC-MAC does not consider an

RSU. Thus, it is not proper for data services. As we use an RSU

as a coordinator, we also get rid of communications overheads

for clustering.

WPCF [8] is designed as a hybrid of contention-based

protocol and contention-free protocol. The basic idea of WPCF

is similar with our work as it uses contention-based protocol

for registration and contention-free protocol for data transmis-

sion. Due to contention-based registration, contention period

2016 30th International Conference on Advanced Information Networking and Applications Workshops

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DOI 10.1109/WAINA.2016.94

258

linearly increases as the number of vehicles increases. Through

clustering, we limit the number of vehicles which compete in

registration phase. This results in a constant contention period

regardless of the number of vehicles.

III. DCMAC

In this section, we introduce Data-oriented Cluster-based

MAC Protocol (DCMAC). DCMAC is designed to offer data

services efficiently. For efficient data services, we suggest using

two-tier clustering. For each layer, we categorize vehicles into

two types. One is Cluster Head (CH) which is responsible for

aggregating data of its cluster and transfering them to the RSU.

The other type is Cluster Member (CM). CMs transmit their

data to CHs. Fig. 1 shows the structure of DCMAC.

Fig. 1. DCMAC structure

In order to take advantage of this two-tier structure, we

propose a vehicle registering and clustering procedure first.

The procedure is as follows:

• Step 1: An RSU broadcasts an advertisement message.

• Step 2: CMs which get an advertisement message send

registration messages containing their positions and mo-

tion vector (i.e., speed and direction) to Cluster Heads.

• Step 3: CHs send gathered registration messages to the

RSU. Newly arrived vehicles which do not belong to any

cluster send register messages to the RSU directly. The

RSU send ACKs when it receives registration messages.

• Step 4: The RSU divides the vehicles into several clusters

and chooses CH, based on positions and motion vectors

of vehicles. Then, the RSU broadcasts this clustering

information to the vehicles.

Vehicles and the RSU need long contention period. The

required contention period length increases exponentially, as

the number of vehicles increases. For simplicity, if we assume

that vehicles try to register themselves into the RSU using the

Slotted-Aloha with transmission probability p, the probability

for a transmitting vehicle to be successful(Ps) is:

Ps = n× p(1− p)n−1. (1)

Then, from the detailed derivation in Appendix A, the expected

number of transmission (Es) for one successful transmission

is:

Es =∞∑

i=1

i(1− p)i−1p,

=1

np(1− p)n−1.

(2)

Thus, the sum of expected registration time for all vehicles:

n−1∑

k=0

Es,k =

n−1∑

k=0

1

(n− k)p(1− p)n−k−1. (3)

Number of Vehicles0 10 20 30 40 50 60 70

Reg

istr

atio

n T

ime

(Uni

t: A

loha

Slo

t)

×106

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Fig. 2. Number of Delay vs. Registration Time

We set p := 0.2. As seen in Fig. 2, the registration time

exponentially increases. On the contrary, in DCMAC case,

only CHs and newly arrived vehicles registers to the RSU.

Thus, DCMAC needs only constant length contention period

for registration. In Step 4, with the clustering information,

the RSU allocates time slots to each cluster. Time slots are

allocated proportionally to the number of vehicles.

After the registration phase, CHs gather data from CMs.

Then, CHs transmit data to the RSU one by one. The detailed

transmission procedure is as follows:

• Step 1: CMs transmit data to their corresponding CHs

simultaneously.

• Step 2: Following the given sequence, CHs send the

gathered data to the RSU.

• Step 3: After all CHs transmit data, RSU broadcasts an

advertisement message and begin registering and cluster-

ing again.

In Step 1, CHs gather data from CMs. If we use multiple

Service Channels (SCHs), CMs can transmit their frames to

CHs without interference as we allocate different SCHs to

adjacent clusters. Otherwise, clusters can gather data simulta-

neously when they are out of communication range of currently

transmitting CH. For this intra-cluster communications, CH

schedules the transmission sequence of CMs. CMs transmit

data according to the schedule. If there are vehicles which do

not have data to send, they stay idle. Then, the next scheduled

vehicle transmits in this time slot.

For example, in Fig. 3, among 8 CMs, only 4 CMs have

data. Then, if the original schedule is in Fig. 4(a), the actual

259

Fig. 3. Intra-cluster Communications

(a) Scheduled Sequence

(b) Actual Transmission Sequence

Fig. 4. Intra-cluster Transmission Sequence

transmission sequence is in Fig. 4(b). This cluster based data

transmission has advantages in the view of channel utilization

and energy efficiency. WPCF suggests that if a vehicle does not

transmit anything during its time slot, the RSU reports that the

time slot is empty. Then, the next scheduled vehicle transmits a

frame at the time slot. In this case, WPCF needs coordination

of the RSU in order to avoid the hidden node problem. On

the contrary, we can avoid the hidden node problem without

coordination of the RSU as we cluster adjacent vehicles.

Since CMs are adjacent, they can sense each other. If a CM

is silent at the allocated time slot, the next scheduled CM

can immediately transmit. Clustering also improves energy

efficiency. Vehicles consumes energy when they transmit data.

In Step 2, CMs transmit data to their adjacent CHs. In this

adjacent case, vehicles can save energy as they transmit with

weaker transmission power. Transmission power is inversely

proportional to the square of the distance. If the distances

between CH and CMs are shorter than the half of RSU’s

communication range, CMs only consumes less than 1/4 of

the transmission power to transmit to the RSU directly like

WPCF. Assume that transmission of CM consumes unit power

1, transmission of CH consumes power 4, and vehicles are

divided into 10 clusters. Then, the transmission power for

WPCF (Txw) and DCMAC (Txd) are:

Txw = 4× Number of Vehicles,

T xd = 4× Number of CHs + 1× Number of CMs

= 4× 10 + 1× (Number of Vehicles

10− 1)× 10

= Number of Vehicles + 30.

(4)

Number of Vehicles10 20 30 40 50 60 70 80 90 100

Tra

nsm

issi

on P

ower

Con

sum

ptio

n

0

50

100

150

200

250

300

350

400

WPCFDCMAC

Fig. 5. Number of Vehicles vs. Transmission Power

We can check these values in Fig. 5.

IV. RESEARCH-ISSUE

In this section, we discuss further research issues for DC-

MAC. We have following research issues:

• We need a clustering method to fully take advantage of

positions and motion vectors.

• In order to minimize energy consumption and interference

among clusters, we need a cluster head selection method.

• For road networks, RSUs can proactively handover with

the position and mobility information of vehicles. We need

to consider a handover method.

• Since CHs broadcast data to CMs, CHs can simultane-

ously transmit to multiple CMs which request the same

data. For example, road statistics are frequently requested

for navigation services. In this case, we can save time

slots as we consider a time slot sharing method.

V. CONCLUSION

In this paper, we proposed a Data-oriented Cluster-based

MAC protocol called DCMAC. Through clustering, we can

reduce the contention period as constant regardless of the

number of vehicles. Clustered vehicles are physically adjacent

with each other. With this characteristic, DCMAC maximizes

channel utilization without the hidden node problem. Besides,

clustered vehicles can transmit to each other with lower power.

This results in energy efficiency. As future work, we will

specify our data frame format and validate our DCMAC

through simulations.

260

APPENDIX A

EXPECTED NUMBER OF TRANSMISSION

Let Es be the expected number of transmission for success

and Pw be the probability that a slot is wasted. Then, from 1,

Pw = 1− Ps. Therefore,

Es =

∞∑

i=1

i · P i−1

w · PS

= Ps(1 + 2Pw + 3P 2

w + ...+ kP k−1

w + ...)

⇒Pw · Es = Ps(Pw + 2P 2

w + ...+ kP kw + ...)

⇒(1− Pw)Es = Ps ·

∞∑

i=0

P iw =

Ps

1− Pw

∴ Es =Ps

(1 − P2)2=

1

Ps

=1

np(1− p)n−1

(5)

B ACKNOWLEDGMENT

This research was supported by Basic Science Research

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Science, ICT & Future

Planning (2014006438). This research was supported in part

by the ICT R&D programs of MSIP/IITP [14-824-09-013,

Resilient Cyber-Physical Systems Research] and by the DGIST

Research and Development Program (CPS Global Center)

funded by the Ministry of Science, ICT & Future Planning.

Note that Jaehoon (Paul) Jeong is the corresponding author.

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