One-hop vs. multi-hop broadcast protocol for

DSRC safety applications

Hien P. Luong, Suong H. Nguyen, Hai L. Vu and Bao Quoc Vo

Centre for Advanced Internet Architectures, School of Software and Electrical Engineering

Swinburne University of Technology, Hawthorn, VIC 3122, Australia

Email: {hluong, hsnguyen, hvu, bvo}@swin.edu.au

Abstract—In vehicle-to-vehicle communication, safety mes-sages could be broadcasted over one-hop or multi-hop usingdifferent transmission ranges to warn each other of changingconditions or dangers ahead. We investigate the broadcast per-formance considering one-hop and multi-hop transmissions andstudy the effect of different transmission ranges on the multi-hop broadcasting performance. Our results show that multi-hoptransmission can provide significant performance improvementwhen the transmission range is chosen appropriately.

I. INTRODUCTION

Dedicated Short Range Communication (DSRC) refers to

the use of wireless communication among vehicles (V2V) or

communication between vehicles and infrastructure to improve

the safety and efficiency of road traffic. The current IEEE stan-

dard for DSRC, 802.11p [1], defines the specifications of the

medium access control layer (MAC) and physical layer. At the

MAC layer, two main frames broadcasted by each vehicle are

beacon frames and data frames. Beacon frames are periodically

sent by a vehicle to inform others of its own information or

its neighbors information such as vehicles’ position or speed.

Meanwhile, a data frame contains information relating to the

application. In this paper we will consider safety applications

where data frames are safety messages.

Safety message is initiated by a vehicle and broadcasted

over one-hop or multi-hop to warn surrounding vehicles on

the road of changing conditions or dangers ahead. There have

been a number of protocols proposed for safety messages

broadcasting in the literature. The simplest approach is to

rebroadcast the message every time it is received by a vehicle

(or a node). However, this easily leads to a serious problem,

referred to as a broadcast storm [2], where a successful

reception of safety message is prevented by high packet/frame

collisions caused by many redundant rebroadcasting messages.

The broadcast storm problem has been tackled in later work

by reducing the number of vehicles that will rebroadcast the

safety message upon its reception. One approach is to assign

a certain forwarding probability for each vehicle, based on

certain criteria. For example, a scheme in [3] assigns for-

warding probability for every receivers based on their number

of neighbors, while scheme in [4] gives higher probability

for nodes that have further distance from the source. A so-

called irresponsible forwarding (IF) scheme in [5] computes

the forwarding probability based on not only the distance

from the source but also the density of vehicles resulting in

a superior performance when compared with similar existing

schemes in [6].

Another promising approach is using a so-called dominating

set (DS) [7] to reduce the number of forwarding vehicles while

retaining the broadcasting coverage. Lim et al. [6] shows that

DS is a potential solution to solve broadcast storm problem.

There existed several DS-based broadcast protocols [8], [9],

[10]. Among those, [8] purely chooses forwarding vehicles to

be those in DS. In [9], [10] the set of forwarding vehicles can

extend beyond DS to include nodes with high ranking based

on their geographical location.

In this paper, we will investigate the performance of the

single-hop broadcast and that via multi-hop transmissions us-

ing the IF [5] and DS [8] broadcasting schemes. In particular,

a vehicle in the IF scheme will rebroadcast (or forward) the

safety message after its first successful reception with the

following probability

p = e−ps(Tx−d)/c,

where ps is a network density, Tx is a transmission range, d is

a distance between the receiving vehicle and the source, and

c ≥ 1 is a coefficient used to adjust the forwarding probability.In the DS scheme, a vehicle will rebroadcast the message with

probability one if (and only if) it belongs to the dominating

set DS. A set is a DS if every vehicle (or node) in the network

belongs to this set or has a neighbour belonging to this set,

which is determined through three steps below.

1) A node is in DS if it has at least two neighbours that

are not direct neighbours themselves (i.e. are not within

the transmission range of each other).

2) After Step 1, let A,B ∈ DS. Let N(A) be a set

of all neighbours of node A. Remove A from DS if

N(A) ⊆ N(B) and any of the element of the tuple

key(A) = (degreeA, xA, yA) is less than the corre-

sponding element of B, where degreeA is the number

of neighbours of A, and xA, yA are its two coordinates.

3) After Step 2, let B,D,E ∈ DS. Remove B from

DS if N(B) ⊆ N(D) ∪ N(E) and key(B) =min{key(B), key(D), key(E)}.

The performance evaluation is in terms of the packet delivery

ratio (PDR) and average delay, where PDR is defined as the

probability of successfully delivering the safety message to all

intended vehicles within the vicinity of a source node.

II. SIMULATION RESULT AND DISCUSSION

In this section we describe our simulation setup, present the

results and discuss our observations on the performance of the

two studied broadcasting schemes (i.e. DS and IF schemes)

using various transmission ranges.

A. Simulation Setup

Consider a three-kilometer high way where vehicles are

moving in the same direction with speed chosen randomly

in the range of [60, 80] km/h. Vehicles enter into the network

following a Poisson process with a fixed parameter λ vehi-

cles/second. The value of λ is chosen such that the average

density on the considered link belongs to the following set

{25, 40, 75, 130} vehicles/km. The transmission range (Tx) isset to 200 meters for single-hop, while for multi-hop it can

be 100, 130, 150 or 200 meters. To adjust the number of

retransmissions, the coefficient c in the IF scheme [5] is taken

from the set {3, 7, 10, 20}.

Beacon and safety messages with the same packet size of

400 bytes are simulated, but only the performance for safety

messages is evaluated. Each vehicle in the network broadcasts

a beacon message every 100 milliseconds. Safety messages

occur only occasionally in emergency situations, but require

to be received promptly by most (or all) of the nodes within

its vicinity, while beacon messages may tolerate more losses.

The MAC parameters of the IEEE 802.11p protocol are set as

follows. Contention window (W ) is equal to 32, distributed

interframe space (DIFS) and short interframe space (SIFS)

are 64 µs and 32 µs, respectively. Data rate and basic rate

have the same value of 6Mbps. We conduct the simulation

using network simulator (ns-2, version 2.33) [11] with a total

of 5000 safety messages for each scenario.

B. Result and Discussion

1) Packet Delivery Ratio for DS scheme: Figure 1a shows

the PDR obtained using different transmission ranges for the

DS scheme. Observe that when the density is low (e.g. less

than 80 vehicles/km), using multi-hop in DS scheme with

large transmission range (i.e. Tx=200m) is desirable, which

outperforms the single-hop performance using the same Tx.

It is because at low density, the network is sparse with low

collision probability, and hence allowing retransmission can

greatly improve the broadcasting performance.

As the density increases, the PDR of the single-hop de-

creases rapidly due to the increase in collisions caused by

hidden terminals [12]. In contrast, the performance of the

multi-hop DS scheme decreases at a much slower rate due to

the improvement in successful reception via retransmissions,

and the ease of negative impact caused by hidden nodes

since forwarding nodes have a different set of hidden nodes

compared to the original source. At very high densities (e.g.

130 vehicles/km), reducing Tx (e.g. from 200m to 100m) can

be beneficial as a smaller Tx results in less number of hidden

nodes in each transmission.

2) Packet Delivery Ratio for IF scheme: Figure 1b shows

the PDR for the IF scheme using the 200m transmission

range with different values for the parameter c. We also plot

the PDR for the single-hop and DS scheme using the same

transmission range for comparison. The increase in c value

results in an increase in the average number of retransmissions

(or forwarding) in the network. As seen in Figure 1b, the

optimal value for c is between 7 and 10. For low densities,

the value of c would not play an important role as small c is

already enough to create a sufficient number of retransmissions

to achieve a maximum PDR. At a high density, choosing a very

high c value to substantially increase the number of forwarding

does not improve the performance. That is because when the

network is dense, the high number of forwarding nodes causes

more collisions which in turn reduces the PDR performance.

Figure 1c shows the PDR for the IF scheme with optimal

value c = 7 using different transmission ranges which followsa similar trend to that of DS. At high densities, however, the

gaps between PDRs of different ranges in the IF scheme are

smaller than those of DS.

TABLE I: The average number of retransmitting nodes for DS

and IF schemes with different Txand density.

DS

Density Tx = 100 Tx = 130 Tx = 150 Tx = 200

25 1.99 2.57 2.82 3.53

40 3.25 4.01 4.76 6.1

75 6.47 8.83 9.88 12.07

130 13.35 17.72 23.16 33.83

IF

Density Tx = 100 Tx = 130 Tx = 150 Tx = 200

25 4.14 5.14 5.68 6.88

40 6.14 7.21 7.91 9.16

75 9.49 10.72 11.38 11.85

130 13.79 13.69 13.40 12.38

3) Impact of the number of retransmissions on PDR: As

seen in Table I together with Figure 1a and Figure 1c, in

general, increasing the number of retransmissions does help

to improve the performance, although there is an upper limit

to this increase because beyond that limit, the performance

degrades as the number of retransmissions increases. Based

on our simulation results for the network described in this

paper, that upper limit appears to be between 13 and 14.

4) Impact of transmission range on the number of colli-

sions: Recall that reducing Tx will result in decreasing hidden

collision probability for each single transmission. However,

when the density is low (i.e. <75 vehicles/km), it does not help

to improve the PDR as the total number of hidden collisions

increases over many retransmissions. This explains why the

PDR decreases when Tx decreases at low densities.

On the other hand, at very high densities (i.e. 130 vehi-

cles/km), decreasing Tx does help to improve the PDR. It

is because smaller Tx in high density reduces the overall

number of collisions due to the reduction in the number of

retransmissions.

5) End-to-end Delay: The corresponding end-to-end delay

for the three schemes DS, IF and single-hop is shown in

25 40 75 130

0.5

0.6

0.7

0.8

0.9

1

Vehicle Density (vehicles/km)

Packet

Deliv

ery

Ratio

Single−hop with Tx=200

CDS with Tx=100

CDS with Tx=130

CDS with Tx=150

CDS with Tx=200

(a) PDR for Single-hop, DS with different Tx.

25 40 75 130

0.5

0.6

0.7

0.8

0.9

1

Vehicle Density (vehicles/km)

Packet

Deliv

ery

Ratio

Single−hop with Tx=200

IF with c=3

IF with c=7

IF with c=10

IF with c=20

CDS with Tx=200

(b) PDR for IF (different c), DS and Single-hop.

25 40 75 130

0.5

0.6

0.7

0.8

0.9

1

Vehicle Density (vehicles/km)

Packet

Deliv

ery

Ratio

IF with Tx = 100

IF with Tx = 130

IF with Tx = 150

IF with Tx = 200

(c) PDR for IF scheme with different Tx.

25 40 75 1300

2

4

6

8

10

12

14

16

18

20

Vehicle Density (vehicles/km)

End−

to−

end D

ela

y (

ms)

Single−hop with Tx=200

DS with Tx=100

DS with Tx=200

IF with Tx=100

IF with Tx=200

(d) Average Delay for Single-hop, DS and IF.

Fig. 1: Performance Metrics for Single-hop, DS and IF schemes

Figure 1d. As we can see from this figure, single-hop scheme

takes less time than IF and DS in all the scenarios studied.

However, the delay for all schemes is still less than 20ms

which is sufficient for safety applications [13].

III. CONCLUSION

We provide a number of observations that could help in the

design of V2V communication protocol for safety applications.

Firstly, a simple protocol based on multi-hop transmission with

a sufficiently large transmission range can work reasonably

well for low density networks. Secondly, it is crucial to obtain

an appropriate number of nodes retransmitting the message.

Thirdly, there is an impact of transmission range on the

number of retransmission, that is, increasing Tx leads to an

increase in the number of retransmitting nodes. However, in

high-density networks (e.g. 130 vehicles/km), this trend may

be slightly reverse. Finding the optimal parameters requires

better understanding via modeling the above relationship and

will be conducted in our future research.

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