A Survery and Qualitative analysis of MAC Protocols for ... · PDF file(Time Division Multiple Access) It divides the medium into Information Frames (IF), which are used to send data,

A Survey and Qualitative Analysis of A Survey and Qualitative Analysis of

MAC ProtocolsMAC Protocols for Vehicular Ad hoc for Vehicular Ad hoc

NETworksNETworks (VANETs)(VANETs)

Menouar, H. and Filali, F. and Lenardi, M.IEEE wireless communications 2006

컴퓨터통계학과모빌임베디드

발표자 : 오용석 [email protected]

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� MANETs (Mobile Ad-hoc NETworks)

■ Nodes self-configure themselves and interact without using fixed infrastructures of centralized administration

■ Two neighboring terminals can not transmit simultaneously because a transmission collision occurs

■ So, in order to efficiently share the medium, Medium Access Control (MAC) protocols are proposed by the research community

IntroductionIntroduction

protocols are proposed by the research community

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� VANETs (Vehicular Ad-hoc NETworks)

■ Efficient medium share is even more difficult in the VANETs due to high node mobility and fast topology changes

■ The main targeted applications for VENETs

� Active Safety, i.e. the set of hw/sw tools able to prevent accidents instead of acting on accidents cars

� C2C (car to car) communications can inform drivers approaching intersections about other vehicles approaching from other directions or dangerously turning

IntroductionIntroduction

other vehicles approaching from other directions or dangerously turning

� In general, the amount of information to be transmitted is relatively small, but the transmission reliability as well as the latency and the packet dissemination are fundamental

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� In MANETs a transmission channel is a shared medium,

■ so, in order to avoid transmission collisions, when a node is transmitting on one channel, all nodes in its neighborhood (before transmitting) have to wait until it releases this channel.

■ Because MANETs do not have a fixed infrastructure, it is not easy for their nodes to know if the medium is in use or not.

A short overview of MAC protocols for MANETsA short overview of MAC protocols for MANETs

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� ALOHA

■ The first MAC protocol proposed for packet radio networks

■ Based on random access: when a node wants to use common channel

� First, it transmits on it

� Then, if a transmission collision occurs

� It waits for a random time before retransmitting again

■ The maximum throughput is 18.4% of the channel capacity for a fixed message


■ The maximum throughput is 18.4% of the channel capacity for a fixed message length

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� S-ALOHA (Slotted ALOHA)

■ The ALOHA random access causes an important throughput reduction

■ Slotted ALOHA was proposed

■ This protocol divides the medium into several time slots and a sender attempts to transmit at the beginning of a time slot

■ Compared to ALOHA, in S-ALOHA the vulnerable period of a transmission is


halved, so doubling the efficiency (maximum throughput) of the system

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� CSMA (Carrier Sense Multiple Access)

■ A node that has data to send senses the common channel at first

■ If it is idle, it transmits

■ Otherwise, it attempts again after a random time

� CSMA/CD (Collision Detection)

■ Collision detections was then added, to detect collisions during transmissions,


■ Collision detections was then added, to detect collisions during transmissions, stopping them and attempt again later.

■ It is still not optimal in case of charged network, when a lot of collisions can occur

■ The main weakness of CSMA/CD is that it does not solve the problem of the hidden and the exposed terminals

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� MACA (Multiple Access with Collision Avoidance)

■ Overcomes the hidden terminal problem by agreeing on transmission with the destination

� MACAW (MACA Wireless)

■ To let the exposed terminals be aware about the transmissions durations time in its neighborhood,


■ MACAW proposes

� Data Sending (DS)

� ACKnowledgment (ACK)

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Hidden terminal problemHidden terminal problem

C

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A B

Exposed terminal problemExposed terminal problem

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A DB C

� BTMA (Busy Tone Multiple Access) MAC protocol

■ To overcome the hidden terminal problem, i.e. splitting the channel transmission into two channels: data and control channel

� The first tone : data packet transmission

� The second tone : busy tone signal

A short overview of MAC protocols for MANETs A short overview of MAC protocols for MANETs

� DBTMA (Dual Busy Tone Multiple Access) extends BTMA by using two busy tones

■ The first is used by the sender to inform its neighbors that it is transmitting

■ The second is used by the receiver to inform its neighbors that it is receiving data packets

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� FPRP (Five Phase Reservation Protocol) was the first proposed TDMA(Time Division Multiple Access)

■ It divides the medium into Information Frames (IF), which are used to send data, and Reservation Frames (RF) used for IF reservations

� FDMA (Frequency Division Multiple Access)

■ The medium is slotted, so several nodes can transmit simultaneously


� CDMA (Code Division Multiple Access)

■ Several orthogonal codes are available, and each nodes uses one code to “encrypt” messages before transmitting them

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� VANETs (Vehicular Ad hoc NETworks) can be considered as a specific case of traditional MANETs.

■ Characteristics

� High mobility and speed

� The network topology changes frequently and very fast

� Vehicles move only on predetermined roads

� They do not have the problem of resources limitation in terms of data storage and

MAC Protocols for VANETsMAC Protocols for VANETs

� They do not have the problem of resources limitation in terms of data storage and power

� GPS or Galileo

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� IEEE 802.11 is a wireless communication standard

■ Centralized mode

■ Ad-hoc mode

� IEEE 802.11 is often used for VANETs prototyping implementations, tanks to the large availability in the market of cheap IEEE 802.11-based wireless devices

IEEE 802.11 STANDARDIEEE 802.11 STANDARD

� IEEE 802.11 addresses the MAC and the physical layer

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� DCF (Distributed Coordination Function) is responsible of the medium access based on CSMA/CA

� PCF (Point Coordination Function) is another method to access the medium designed for centralized networks and real-time services; it is out of scope in this paper

IEEE 802.11 MAC Layer (1)IEEE 802.11 MAC Layer (1)

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� Two ways can be used to determine if the medium is idle or not

■ Physical carrier sensing

� It cannot overcome the hidden terminal problem since the hidden terminal cannot be hear physically

■ Virtual carrier sensing

� Based on the NAV (Network Allocation Vector) to solve the hidden terminal problem

� The NAV is just a timer that indicates the duration for which the medium will be busy


� IFSs (Inter Frame Spacing)

■ Are set between two successive transmission frames in order to manage the medium access process

� When using IEEE 802.11 in ad-hoc mode for VANETs, before attempting to transmit, each vehicle has to first check the medium state.

■ If it is sensed to be idle for a certain duration time (DIFS), the vehicle can transmit.

■ Otherwise, it backs off and attempts again after an amount of time chosen within a contention window (CW)

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3. If the receiver is ready to receive the transmission, it waits for a Short IFS (SIFS) time and them replies by sending a CTS packet including the transmission durations time

6. The receiver vehicle, after successfully receiving data from, waits for another SIFS then it sends an ACK only to the sender.

1. It sends an RTS packet including its ID and the duration time of the whole transmission

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2. Set their NAV according to the transmission durations time indicated in the RTS packet

4. All neighbors receiving this CTS set their NAV according to the indicted transmission duration time

5. When receiving the CTS, the sender vehicle waits for SIFS before starting the data transmission

Inter Frame Spaces (IFS) are important

SIFS < DIFS


Sender RTS DATA

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Receiver

Neighbor 1

Neighbor 2

CTS ACK

NAV (RTS)

NAV (CTS)

Distributed IFS (DIFS)

Short IFS (SIFS)

Towards a IEEE 802.11 physical layer for VANETsTowards a IEEE 802.11 physical layer for VANETs

Name Year Band (GHz)Throughput (

Mbps)

802.11a 1999 5 54

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802.11a 1999 5 54

802.11b 1999 2.4 11

802.11g 2003 2.4 54

802.11n 2009 2.4/5 600

� The Wireless Access in Vehicular Environments (WAVE), which is referred as well as IEEE 802.11p.

■ Requirements are mostly coming from vehicular Active Safety concepts and applications, where reliability and low latency are extremely important

■ WAVE uses CSMA/CA

■ 5.850-5.925 GHz spectrum in North America

WAVE (IEEE 802.11P)WAVE (IEEE 802.11P)

■ OFDM

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� ADHOC MAC is a MAC protocol for VANETs

� It works in slotted frame structure

� It uses a dynamic TDMA that can be easily adapted to the UMTS Terrestrial Radio Access Time Division Duplex (UTRA-TDD)

ADHOC MACADHOC MAC

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� Proposed by extending the Reservation ALOHA (R-ALOHA)

� BCH (Basic Channel) is one time slot periodically repeated

� Each vehicle has to have a global view of the transmission in a two hops neighborhood to overcome hidden terminal problem

� In RRALOHA, each vehicle sends on its BCH its Frame Information (FI), which is a vector with N entries that indicate how were sensed the

Reliable ReservationReliable Reservation--ALOHA (RRALOHA (RR--ALOHA) ALOHA)

which is a vector with N entries that indicate how were sensed the status of the previous N time slots in the previous frame

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5 1 2 4 7 6 3 5 1 2 4 7 6

FI3FI5

FI1FI2

FI4FI7

FI6

� With directional antenna, terminals can transmit in a specific direction

■ Reducing the transmission collisions

■ Increasing the channel reuse possibility

� D-MAC (Directional MAC)

■ It requires that each terminal knows its geographic position and neighbors by using GPS or Galileo

Directional AntennaDirectional Antenna--based MAC protocolsbased MAC protocols

■ A sender initiates a handshake before transmitting based on RTS, CTS, and ACK packets transmission

■ The RTS is sent on directional or omni-directional according to the ongoing transmission in the neighborhood

■ A directional antenna that receives an RTS or a CTS becomes blocked and do not transmit during the neighbor’s transmission time

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� By using directional antennas

■ Transmission collisions can be reduced

■ The channel transmission reuse can be increased

� Unfortunately, directional antenna systems seem still too complex and hard to manage in real implementations

DD--MAC (Directional MAC)MAC (Directional MAC)

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Qualitative Comparison of VANET MAC protocols Qualitative Comparison of VANET MAC protocols

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� 802.11 (CSMA/CA)

� ADHOC MAC (RR-ALOHA)

� D-MAC

ConclusionsConclusions

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Documents

A Survery and Qualitative analysis of MAC Protocols for ... · PDF file(Time Division Multiple Access) It divides the medium into Information Frames (IF), which are used to send data,