Vehicular communication networks were allocated a special frequency band
of 75 MHz at 5.9 GHz by the U.S. Federal Communication Commission (FCC)
in 1999 to facilitate a short to medium (300 m to 1000 m) range
communications between vehicles and the roadside infrastructures [23], [24].
efficiency of traffic flow. This initiative has been well followed by commendable
efforts by the government, academia and industries to standardise the
technologies and services of vehicular networks.
2.5.1 Dedicated Short Range Communications (DSRC)
DSRC is a short to medium range communications service developed
primarily to support communications-based active safety applications of ITS.
It operates over a dedicated 75 MHz spectrum band at 5.9 GHz allocated in
1999 by United States FCC to provide the tightly controlled spectrum
requirement by Communications-based active safety systems for reliable
service. Following the spectrum band allocation, the American Society for
Testing and Materials (ASTM) in 2003, approved the ASTM-DSRC standard
which was based on the IEEE802.11a physical layer and 802.11 MAC layer
[7], [23]. This standard was later published as ASTM E2213-03. The service
and licensing rules that govern the use of the DSRC band were established in
February 2004 by the report issued by the FCC. The choice of DSRC over W i-
Fi is for the purpose of avoiding an intolerable and uncontrollable level of
interference that could hamper the reliability and effectiveness of active safety
applications based on the proliferation of Wi-Fi hand-held and hands-free
devices that occupy the 2.4 GHz and 5 GHz bands, along with the projected
increase in Wi-Fi hot spots and wireless mesh extensions [25]. DSRC is free
but operates in a licensed frequency band. It supports both V2V and V2I
communications by providing a secure wireless interface required by active
and low communication latency in small communication zones. It has
desirable qualities such as low latency, fast network acquisition provisioning,
high reliability, priority service provisioning for safety applications,
interoperability and provisioning of safety message authentication and privacy
[26]. The range of applications covered by these communication services
includes V2V safety messages, traffic information, toll collection,
infotainments, and several others. The DSRC spectrum is divided into 7
channels with each having a bandwidth of 10 MHz as opposed to the 20 MHz
IEEE 802.11a channel bandwidth. The smaller bandwidth channels of DSRC
offer improved wireless channel propagation with respect to multi-path delay
spread and Doppler effects in roadway environments. One of the channels is
dedicated solely for safety communications while two others are reserved for
future critical safety applications. The remaining channels which are service
channels are used for either safety or non-safety applications with safety
applications accorded higher priority [27]. Europe and Japan also have their
standards for DSRC with slight differences in frequency allocations.
2.5.2 Wireless Access in Vehicular Environments (WAVE)
In order to define the architecture and standardised set of services and
interfaces that ensure a secured V2V and V2I wireless communications, the
ASTM 2313 working group moved to the IEEE 802.11 standard group and
renamed the DSRC as IEEE 802.11p Wireless Access in Vehicular
Environments (WAVE) [28]. By the incorporation of DSRC into IEEE 802.11,
contrast to the different regional standards of DSRC by Europe, Japan and
America. The RSU and the OBU which are fixed and mobile devices
respectively are the two types of devices whose functionalities are defined by
WAVE standards. The W AVE standard stack consists of IEEE 802.11p
standard which deals with the physical and MAC layers of vehicular
communication, and IEEE 1609 standards which stipulate other higher-layer
protocols [29].
2.5.2.1 IEEE 802.11p Standards for WAVE
The IEEE 802.11p draft is an amendment of 802.11 standard intended
for new classes of applications to be used in a vehicular environment. These
include road safety and emergency services which require high reliability and
low latency. The PHY layer of 802.11p adopts 802.11a with the modification
of certain parameters such as symbol clock frequency tolerance, transmit
centre frequency tolerance, operating temperature, adjacent/non-adjacent
channel rejection, receiver minimum input sensitivity etc. The 802.11p PHY
uses 10 MHz channels with transfer rates of 3, 4.5, 6, 9, 12, 18, 24, and 27
Mbps compared to 20 MHz channels used by 802.11a. In 802.11p PHY,
Orthogonal Frequency Division Multiplexing (OFDM) is used as transmission
technique to divide the available frequency spectrum into narrower sub-
channels (subcarriers). The high-rate data stream is split into a number of
lower-rate data streams transmitted simultaneously over a number of
subcarriers, where each subcarrier is narrow banded. OFDM offers the benefit
hence, avoiding the situation of a single fade or interferer breaking an entire
link [30].
The MAC layer of 802.11p uses the enhanced distributed channel
access (EDCA) derived from the IEEE 802.11e. This provides prioritised
access to the channel by using queues with different arbitration inter-frame
spaces (AIFS). Each terminal in an 802.11p network has queues with different
priorities with the queue having the highest priority waiting for the shortest
period of time (shortest AIFS) before starting transmission. By this way,
different priorities are enforced, and stations having low priority traffic lose the
race for the channel when competing with stations with higher priority traffic
[7].
2.5.2.2 IEEE 1609 Standards for WAVE
It is worth noting that IEEE 802.11p is limited by the scope of IEEE 802.11
which strictly works at the media access control and physical layers [7]. The
operational functions and complexity related to DSRC are handled by the
upper layers of the IEEE 1609 standards. These standards define how
applications that utilise WAVE will function in the WAVE environment. They
reside above 802.11p and support the operation of higher layers without the
need to deal with the physical channel access parameters.
IEEE 1609.1 Resource Manager describes the wireless access method in WAVE environment and allows remote applications located outside
the vehicular environment to establish connection with WAVE enabled
vehicles [31].
IEEE 1609.2 Security Services describe various secure message patterns to process messages for WAVE systems. This standard
addresses security methods for WAVE management messages and
application messages that ensure security against eavesdropping,
spoofing and information linkage to unauthorised parties [32].
IEEE 1609.3 Networking Services define network and transport layer services such as addressing and routing to enhance secure WAVE
data exchange. It supports both W ave Short Messages (WSM)
services and to IPv6 (Internet Protocol version 6) [33].
IEEE 1609.4 Multi-Channel Operations offers enhancements to the IEEE 802.11 MAC to support WAVE operation by describing the
various standard message formats for DSRC applications [34].