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Among the tremendous vareity of different
wireless technology,V2X communication system can choose any of them based on established
state  of the art protocols and systems. Actually
selection of these communication technologies strongly depends on the particular
deployment scenario 49.Therefore lots of project has been undertaken in order
to creat a standard architecture. Finally, a unified architecture named
Communications Access for Land Mobiles (CALM) was formed 50.The best part of
this architecture is that it allows applications to use best of any wireless
communications that is available to a vehicle at any given moment.CALM system
recommended two very different wireless technologies as a basis both safety and
Comfort application 51.Recommended wireless communication are

o  
IEEE 802.11p based on Wireless Access
in Vehicular Environments (WAVE)

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o  
Cellular Communication based UMTS

 

 

1.1.1  
IEEE 802.11 and WAVE

IEEE 802.11p based on WAVE is designed in
order to support vehicular wireless communication.WAVE is used in such
environments where the physical layer are  changing very rapidly and shorter distance
communications are required. WAVE ensure interoperability between devices attempting
to communicate where transactions must be completed in time frames much shorter
than IEEE 802.11 networks.In this regards IEEE 802.11p adopted necessary
modifications based on 802.11 standard. The physical  and MAC layer protocols of IEEE 802.11p are
designed as a complete vehicular networking stack according to the
specification mentioned in the IEEE 1609 family of protocols 52-55.In
addition with this adaptation,it also maintain security,management and QoS
services.

 

 

                             Figure 11 : Overview of WAVE standard

 

Similar to IEEE 802.11,physical layar of IEEE
802.11p also addressed Orthogonal Frequency Division Multiplexing 56.But the
diffrence is that instead of using usual 20 MHz channel width,it uses 10 MHZ
channel width so that it can easily be completed by doubling all timing
parameters 57.The Control Channel (CCH) is responsible for control and safety
messages.It is surrounded by Service Channels. Service Channels consist of four
channels. Stations uses any of these channels to transmit non-safety messages.There
are two more channels which are reserved for special use.Link layer allow
stations to operate in a special WAVE mood to address different wifi
technologies. This allow any station to communicate with Basic Service Set (BSS) and permit to
process packets through BSS with which they are connected 58.There are two
mechanisms to met QoS requirements of the vehicular Networks.First one is achieved
by CCH.Each station are not equipped with severals transceivers.So it may
happens that any station can miss control or safety informations if all stations
are not tuned properly to the CCH.This problem can be solved by CCH
intervals.In this way,a station needs to tune to the CCH for the first 50ms of
100 ms time slot so that it can hear management and safety messages.The second
mechanism for enforcing QoS, Access Class (AC) will be assigned to each regular data during
transmission. This access class contain higher 
priority numbers.Because of transmission oppurtunity limit,contention
window and Arbitrary Inter frame-spacing (AIFS) delay may occur to get access to the
channel (59). In this situation,services contain higher priority number will be
able to access to the channel first 60.

 

 

 

 

 

 

1.1.2  
UMTS

Cellular Networks offer a convenient way of
avoiding additional infrastructure to support C2X communication. A third
generation mobile telecommunications technology, Universal Mobile
Telecommunication System (UMTS)
developed and standardized by the third generation partnership project (3GPP)
is already widespread use in C2X communications 606162. In UMTS data
transmission process,transport channels are mapped onto physical channels that are
multiplexed to form data strem.Three of these channels which are particularly
important for C2X communications are described below 63.

 

1.1.2.2   
Random Access Channel (RACH)

 

 

 

 

 

 

 

 

 

 

 

 

 

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