small contention window size. This may increase the
collision probability in the network when multiple
vehicles are simultaneously contending for a channel.
To address the above limitations of the IEEE 802.11p
access mechanism for broadcast service, many MAC pro-
tocols have been proposed in the literature [3 – 14]. The
ADHOC MAC protocol [3, 4] is a classical one proposed to
provide a prompt access and reliable channel for broadcast
services in VANETs. Based on the ADHOC MAC proto-
col, a variety of MAC protocols have been subsequently
proposed to support reliable broadcast services in various
network scenarios [5–14]. The ADHOC MAC protocol
operates in a time division structure, where timeslots are
grouped into frames and the frame length, i.e., the number
of timeslots contained in a frame, has a big impact on the
access performance of ADHOC MAC protocol. To inves-
tigate the impact of the frame length on the access per-
formance of the ADHOC MAC protocol, it is useful to
develop an analytical model that describes the relationship
between the frame length and the access performance.
However, Borgonovo et al. did not study the performance
modeling in [3, 4] and, to the best of our knowledge, nor
does other relevant work in the literature, which motivated
us to conduct this work.
In this paper, we develop an analytical model for ana-
lyzing the access performance of the ADHOC MA C pro-
tocol. A Markov chain is first constructed to describe the
number of vehicles which have acquired a timeslot suc-
cessfully at the end of a frame when using the ADHOC
MAC protocol. Based on the Markov model, an analytical
model is then derived to describe the relationship between
the fra me length and the channel utilization. Based on the
derived analytical model, an optimal frame length that
maximizes the channel utilization is further obtained.
Simulation experiments are conducted to verify the effec-
tiveness of the analysis model.
The remainder of this paper is organized as follows.
Section 2 reviews the related work. Section 3 derives the
analytical model of the ADHOC MAC protocol. Section 4
shows simulation results to verify the effectiveness of the
analytical model. Section 5 concludes the paper.
2 Related work
In the context of VANETs, a variety of MAC protocols
have been proposed to provide efficient broadcast services
in VAN ETs. In [3, 4], Borgonovo et al. proposed an
ADHOC MAC protocol for V2V communications to pro-
vide efficient and reliable broadcast service. Based on the
ADHOC MAC protocol, several MAC protocols have been
subsequently proposed to support rel iable broadcast
services in various network scenarios. In [5, 6], Omar et al.
proposed a VeMAC protocol which supports efficient one-
hop and multi-hop broadcast service on multiple channels,
and derived an analytical model to investigate how fast the
contending nodes can acquire a timeslot using the VeMAC
protocol. In [7], Miao et al. proposed an A-ADHOC pro-
tocol, which is an adaptive real-time distributed ADHOC
MAC protocol for VANETs. In [8], Liu et al. provided a
quantitative analysis of the successful probability for con-
tending nodes when using the A-ADHOC protocol, and
proved that an adaptive frame length is necessary. In [9],
Bharati and Zhuang proposed a cooperative scheme for
medium access control in VANETs, referred to as Coop-
erative ADHOC MAC (CAH-MAC). In [10], Bharati et al.
proposed an enhanced CAH-MAC (eCAH-MAC) that is
able to avoid cooperative collision and thus efficiently
utilize a time slot. In [11], Zhang et al. proposed a real-time
reservation protocol (RRP) to support reliable and timely
broadcast in VANETs. In [12], Guo et al. proposed an
adaptive collision-free MAC (ACFM) protocol tailored to
vehicular networ ks. In [13], Xie et al. proposed an access
scheme for emergency forecas ting in dynamic vehicular
networks. In [14], Huang and Chiu proposed an access
scheme to reduce merging collisions in VANETs. There-
fore, the ADHOC MAC protocol is a classical MAC pro-
tocol which has a big impact on many MAC protocols for
broadcast services in VANETs.
3 Analytical model for the ADHOC mac protocol
In this section, we first give a brief overview of the
ADHOC MAC protocol and then derive an analytical
model for the ADHOC MAC protocol.
3.1 Overview of the ADHOC MAC Protocol
The ADHOC MAC protocol is a prompt and reliable
broadcast MAC protocol proposed for VANETs [3]. It is
suitable for an environment in which the vehicles can be
grouped into a one-hop set (OHS) and all vehicles in an
OHS can communicate with each other in one hop, as
shown in Fig. 1. Although the ADHOC MAC protocol can
be used in a multi-hop environment, we only consider its
performance modeling and analysis in a one-hop environ-
ment in this paper, which can be used as a basis for the
performance modeling and analysis in a multi-hop
environment.
The ADH OC MAC protocol operates in a time division
structure, where timeslots are grouped into frames. Let
N denote the frame length, i.e., a frame contains N slots.
Initially, each vehicle within an OHS rando mly attempts to
acquire one of the N timeslots in a frame, and broadcasts a
800 Wireless Netw (2016) 22:799–812
123