# An Efficient and QoS Supported Multichannel MAC Protocol for Vehicular Ad Hoc Networks

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## Abstract

**:**

## 1. Introduction

- (1)
- The EQM-MAC protocol uses less time to deliver safety messages and allocates more time to make time slot reservations and channel coordination for SCHs. Therefore, nodes have more opportunities to perform SCH reservation to deliver different service classes packets, and the number of successful reservations can be greatly increased.
- (2)
- The non-safety messages can be simultaneously transmitted on SCHs during the whole SI. Therefore, the saturation throughput and the utilization of SCHs can be further increased.
- (3)
- EQM-MAC protocol can offer sufficient QoS in terms of throughput and delay for non-safety messages through adjusting the minimum CW according to the vehicle density.

## 2. Related Works

## 3. Efficient and QoS Supported Multichannel MAC Protocol

#### 3.1. SCH Selection and Access Reservation Scheme

#### 3.2. Analysis of Differentiated Minimum Contention Window

**Assumption**

**1**(Poisson distribution of vehicles on load).

**Assumption**

**2**(Uniform distribution of vehicle speed).

**Assumption**

**3**(Ideal channel conditions).

**Assumption**

**4**(Ideal channel conditions).

**Assumption**

**5**(Only one access category in one node).

**Theorem**

**1.**

**Proof.**

- (1)
- When the channel is busy, the backoff timer is frozen;
- (2)
- When the channel is free, the backoff timer will subtract one;
- (3)
- Within ${m}^{\prime}$ backoff stage, collision makes backoff stage increase and CW double. Otherwise, collision makes CW remain ${2}^{{m}^{,}}{W}_{i,0}$;
- (4)
- When a WSA/RFS packet is successfully transmitted or reaches its maximum retransmission number m, the backoff timer is reset.

## 4. Performance Analysis

#### 4.1. Throughput Analysis

- (1)
- Let ${T}_{SI}$ and M denote the length of a SI and the number of lanes in each direction, respectively.The RSU estimates the number of newly entering vehicles, ${n}_{new}$, during a SI by [22]$${n}_{new}=2M\xb7{V}_{avg}\xb7\beta \xb7{T}_{SI}$$$${T}_{VII}={L}_{total}\xb7{T}_{rrts}+{m}^{\u2033}\xb7{T}_{cp}$$
- (2)
- Let ${T}_{SaSlot}$ denote the duration for transmitting a safety-related message. According to Figure 1, we have$${T}_{SI}={T}_{CFI}+{T}_{VII}+{T}_{WI}$$$${T}_{CFI}=N\xb7{T}_{SaSlot}+2{T}_{CLI}.$$$$\begin{array}{cc}\hfill {T}_{CLI}=& \frac{2\u2308{log}_{2}{N}_{max}\u2309{N}_{max}}{{R}_{d}}\hfill \\ \hfill \phantom{\rule{1.em}{0ex}}& +\frac{\left(\u2308{log}_{2}{N}_{AC}\u2309+\u2308{log}_{2}C{W}_{max}\u2309\right){N}_{AC}}{{R}_{d}}\hfill \\ \hfill \phantom{\rule{1.em}{0ex}}& +\frac{\u2308{log}_{2}{T}_{CFI}\u2309+\u2308{log}_{2}{T}_{VII}\u2309+\u2308{log}_{2}{T}_{WI}\u2309}{{R}_{d}}\hfill \\ \hfill =& \frac{\left[2\times 8\times 200+(2+10)\times 4+3\times 8\right]bits}{6Mbps}\hfill \\ \hfill =& 0.55\phantom{\rule{3.33333pt}{0ex}}\mathrm{ms}\hfill \end{array}$$
- (3)
- Let ${N}_{sch}$ represent the number of available SCHs in the VANETs.
- (4)
- Let ${G}_{cch}$ denote the total number of successful SCHs reservation on CCH during the WSA interval. Let ${G}_{sch}$ be the number of non-safety packets transmitted over all ${N}_{sch}$ SCHs during the whole SI. We have$${G}_{cch}=\frac{{T}_{WI}}{E\left[{T}_{reser}\right]}.$$$${G}_{sch}=\frac{{N}_{sch}\xb7{T}_{SI}}{{T}_{data}}$$$${T}_{data}=DIFS+{T}_{h}+{T}_{da}+SIFS+{T}_{ack}+2\delta $$

#### 4.2. Delay Analysis

## 5. Performance Evaluation

- The IEEE 1609.4 protocol [10]: This is the default multichannel protocol with fixed CCH interval (50 ms) and SCH interval (50 ms). All nodes use the CSMA/CA mechanism to perform channel access for the transmissions of safety-related and WSA messages on the CCH, and switch to the specific SCH to disseminate non-safety messages during the SCHI.
- The multichannel TDMA MAC protocol specifically for VANETs scenario (VeMAC) [17]: VeMAC protocol is considered to be the very beginning in research of TDMA MAC for V2V communication. Each node has two transceivers: Transceiver 1 is always tuned to CCH to transmit safety messages and make SCH reservations while transceiver 2 can be tuned to any SCH to transmit non-safety messages. The VeMAC protocol works in a distributed way, and thus each node requires to exchange additional information to obtain a time slot for transmitting safety-related messages [17]. According to the protocol, the length of a VeMAC (packet) is about 650 bytes (${N}_{max}=200$), and the duration for transmitting this packet is thus about 0.9 ms given ${R}_{d}$ = 6 Mbps [17]. In the following analysis, the length of each frame defined in VeMAC protocol is 200. For facilitation of the analysis, each node always makes a successful SCH reservation in a frame, and service provider can only transmit one service packet for a successful SCH reservation in a frame.
- The Coordinated multichannel MAC (C-MAC) protocol [16]: With the coordination of the RSU, C-MAC protocol provides contention-free broadcasting for safety-related messages, and thus lowers the collision probabilities for transmissions of safety-related messages. Through optimizing the SCH interval, the maximal saturation throughput of SCHs is obtained.
- The QoS supported Variable CCH Interval (Q-VCI) MAC protocol [5]: The Q-VCI protocol can support the QoS delivery in a multi-rate multichannel VANETs environments by adjusting the minimum CW for different service classes at each node. In addition, the Q-VCI protocol adaptively tunes the CCHI to ensure the transmissions of safety-related messages and to maximize the throughput of SCHs according to the traffic conditions. We set $\alpha $, data rate for two SCHs and data rate for the other SCHs, respectively, be 3, 6 Mbps and 9 Mbps in the Q-VCI MAC protocol.

#### 5.1. Simulation Scenario

#### 5.2. Simulation Results

## 6. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 5.**Saturation throughput on SCHs. (

**a**) Saturation throughput versus ${R}_{{S}_{0}/{S}_{1}}$ (${N}_{0}$ = 20, ${N}_{1}$ = 20);(

**b**) Saturation throughput versus ${R}_{{S}_{0}/{S}_{1}}$ (${N}_{0}$ = 40, ${N}_{1}$ = 60); (

**c**) Saturation throughput versus the number of nodes (${N}_{0}$ = ${N}_{1}$, ${R}_{{S}_{0}/{S}_{1}}$ = 2); (

**d**) Saturation throughput versus the number of nodes (${N}_{0}$ = ${N}_{1}$, ${R}_{{S}_{0}/{S}_{1}}$ = 6.)

**Figure 6.**Non-safety packet delay. (

**a**) Delay versus ${R}_{{S}_{0}/{S}_{1}}$ (${N}_{0}$ = 20, ${N}_{1}$ = 20); (

**b**) Delay versus ${R}_{{S}_{0}/{S}_{1}}$ (${N}_{0}$ = 40, ${N}_{1}$ = 60); (

**c**) Delay versus the number of nodes (${N}_{0}$ = ${N}_{1}$, ${R}_{{S}_{0}/{S}_{1}}$ = 2); (

**d**) Delay versus the number of nodes (${N}_{0}$ = ${N}_{1}$, ${R}_{{S}_{0}/{S}_{1}}$ = 6.)

**Figure 7.**Saturation throughput on SCHs versus the number of nodes for kinds of protocols (${N}_{0}$ = ${N}_{1}$). (

**a**) ${R}_{{S}_{0}/{S}_{1}}$ = 1, ${L}_{data}$ = 1000 bytes; (

**b**) ${R}_{{S}_{0}/{S}_{1}}$ = 1, ${L}_{data}$ = 3000 bytes; (

**c**) ${R}_{{S}_{0}/{S}_{1}}$ = 2, ${L}_{data}$ = 1000 bytes; (

**d**) ${R}_{{S}_{0}/{S}_{1}}$ = 2, ${L}_{data}$ = 3000 bytes; (

**e**) ${R}_{{S}_{0}/{S}_{1}}$ = 6, ${L}_{data}$ = 1000 bytes; (

**f**) ${R}_{{S}_{0}/{S}_{1}}$ = 6, ${L}_{data}$ = 3000 bytes.

**Figure 8.**Non-safety packet delay versus the number of nodes for kinds of protocols (${N}_{0}$ = ${N}_{1}$). (

**a**) ${R}_{{S}_{0}/{S}_{1}}$ = 1, ${L}_{data}$ = 1000 bytes; (

**b**) ${R}_{{S}_{0}/{S}_{1}}$ = 1, ${L}_{data}$ = 3000 bytes; (

**c**) ${R}_{{S}_{0}/{S}_{1}}$ = 2, ${L}_{data}$ = 1000 bytes; (

**d**) ${R}_{{S}_{0}/{S}_{1}}$ = 2, ${L}_{data}$ = 3000 bytes; (

**e**) ${R}_{{S}_{0}/{S}_{1}}$ = 6, ${L}_{data}$ = 1000 bytes; (

**f**) ${R}_{{S}_{0}/{S}_{1}}$ = 6, ${L}_{data}$ = 3000 bytes.

Notation | Definition |
---|---|

SI | Synchronization Interval |

CFI | Contention-Free Interval |

VII | Vehicle Identification Interval |

CLI | Coordination and Length Information |

CP | Coordination Packet |

RRTS | Reservation Request To Send |

RFS | Request For Service |

ACK/NACK | Acknowledgement / Non-acknowledgement |

SUL | SCH Usage List |

AC | Access Category |

N | The total number of nodes |

${N}_{i}$ | The number of nodes delivering non-safety packets with $A{C}_{i}$ |

$\beta $ | The vehicle density on the highway (vehicles/m) |

R | The RSU coverage |

$P(j,l)$ | The probability that j vehicles exist within length l of the highway |

${n}_{new}$ | The number of newly entering vehicles during an SI |

M | The number of lanes in each direction |

${P}_{i}$ | The collision probability that a node delivering a WSA/RFS packet with $A{C}_{i}$ collides with other nodes when access the channel |

${\tau}_{i}$ | The stationary probability that a node sends WSA/RFS packets belonging to $A{C}_{i}$ in a random time slot |

${P}_{i,suc}$ | The probability that a node transmits WSA/RFS packets belonging to $A{C}_{i}$ to make a successful reservation |

${P}_{suc}$ | The probability that a successful transmission of WSA/RFS packets occurs in a time slot |

${P}_{idle}$ | The probability that the channel is idle |

${P}_{col}$ | The probability that the channel collision occurs |

Z | The time interval between two consecutive free time slots before a reservation is successfully made |

${T}_{suc}$ | The duration of a successful reservation |

${T}_{col}$ | The duration for a transmission collision when the node is performing a reservation |

${T}_{wsa}/{T}_{rfs}$ | The duration for transmitting a WSA/RFS packet |

${T}_{ack}$ | The duration for transmitting an ACK packet |

$\delta $ | The duration of the propagation delay |

${T}_{reser}$ | The duration from the time instant when a WSA/RFS becomes the head of the MAC queue to the time instant when a reservation is successful made or dropped due to reaching the maximum retransmission number |

${T}_{SI}$ | The duration of an SI |

${T}_{CFI}$ | The duration of CFI |

${T}_{VII}$ | The duration of VII |

${T}_{WI}$ | The duration of WSA interval |

${T}_{rrts}$ | The duration of transmitting a RRTS packet |

${T}_{cp}$ | The duration of transmitting a CP |

${T}_{data}$ | The duration of a non-safety packet transmission on the SCH |

${T}_{h}$ | The cost of MAC-layer header and physical-layer header |

${T}_{da}$ | The duration of transmitting the payload of a non-safety packet |

$AIFS$ | The arbitration inter frame space |

$AIFSN$ | The AIFS number |

$SIFS$ | The duration of an Short Inter Frame Space (SIFS) |

$DIFS$ | The duration of a DIFS |

$\sigma $ | The duration of an idle time slot |

${m}^{\prime}$ | The maximum times that the CW can be doubled |

m | The maximum number of retransmissions |

${W}_{i,0}$ | The minimum CW size of $A{C}_{i}$ |

${W}_{i,j}$ | The CW size of $A{C}_{i}$ in the jth stage |

${b}_{i,j,k}$ | The stationary distribution of k state in jth stage for $A{C}_{i}$ |

${N}_{sch}$ | The number of available SCHs in VANETs |

${G}_{cch}$ | The total number of successful SCH reservations on CCH |

${G}_{sch}$ | The number of non-safety packets transmitted over all ${N}_{sch}$ SCHs during the whole SI |

${L}_{wsa}/{L}_{rfs}$ | The payload of a WSA/RFS packet |

${L}_{ack}$ | The payload of an ACK packet |

${L}_{data}$ | The payload of a non-safety packet |

${S}_{sch}$ | The total throughput obtained on ${N}_{sch}$ SCHs |

${S}_{i}$ | The throughput obtained on ${N}_{sch}$ SCHs of nodes delivering non-safety packets with $A{C}_{i}$ ($i=0,1,2,3$) |

${R}_{{S}_{0}/{S}_{1}}$ | The predefined throughput ratio of non-safety packets with $A{C}_{0}$ to $A{C}_{1}$ |

${\zeta}_{i}$ | The average number of successful transmissions belonging to $A{C}_{i}$ ($i=0,1,2,3$) on the SCHs of each node |

${n}_{i,suc}$ | The number of successful transmissions belonging to $A{C}_{i}$ (${}_{i}=0,1,2,3$) on the SCHs |

${T}_{i,delay}$ | The total transmission delay of non-safety packets belonging to $A{C}_{i}$ (${}_{i}=0,1,2,3$) |

${R}_{d}$ | The transmission data rate on both CCH and SCHs |

${N}_{max}$ | The maximum number of nodes exists in an RSU converge |

${N}_{masch}$ | The maximum number of SCHs in vehicular environments |

${N}_{AC}$ | The number of different ACs in vehicular networks |

$C{W}_{max}$ | The maximum CW size in vehicular networks |

${m}^{\u2033}$ | The number of rounds that a node has to be experienced before it is identified by RSU |

SCH | Available Slots |
---|---|

1 | 2, 7, 8 |

2 | 2, 4, 6 |

3 | 1, 3 |

4 | 1, 5 |

Parameters | Values |
---|---|

Number of SCHs (${N}_{sch}$) | 4 |

Number of CCH | 1 |

Date rate for each channel (${R}_{d}$) | 6 Mbps |

RSU coverage (R) | 300 m, 500 m |

Number of lanes (M) | 2-lane in each direction |

Channel bandwidth | 10 MHz |

Channel model | Rayleigh fading |

Pathloss exponent | 4 |

Noise power | −100 dBm |

Transmission power | 23 dBm |

Average of vehicle density ($\beta $) | 0.02 to 0.3 vehicles/m |

Vehicle velocity ($[{V}_{min},{V}_{max}]$) | [60, 100], [80, 120] km/h |

${W}_{0,0}$ | 32 |

${W}_{1,0}$ | 32∼1024 |

${m}^{\prime}$ | 5 |

m | 10 |

MAC header | 256 bits |

PHY header | 192 bits |

The payload of a WSA/RFS packet (${L}_{wsa}/{L}_{rfs}$) | 216 bits + PHY header |

The payload of an ACK packet (${L}_{ack}$) | 128 bits + PHY header |

AIFSN for $A{C}_{0}$ ($AIFSN\left[0\right]$) | 6 |

AIFSN for $A{C}_{1}$ ($AIFSN\left[1\right]$) | 9 |

The duration of an SIFS ($SIFS$) | 32 $\mathsf{\mu}$s |

The duration of an DIFS ($DIFS$) | 58 $\mathsf{\mu}$s |

a slot time ($\sigma $) | 13 $\mathsf{\mu}$s |

The propagation delay ($\delta $) | 1 $\mathsf{\mu}$s |

The duration of transmission an RRTS packet (${T}_{rrts}$) | 60 $\mathsf{\mu}$s |

The duration of transmission a Coordination Packet (${T}_{cp}$) | 100 $\mathsf{\mu}$s |

The duration of transmission a safety-related packet ${T}_{SaSlot}$ | 0.4 ms |

Sending frequency of safety messages | 10 Hz |

The payload of a non-safety packet (${L}_{data}$) | 1000, 3000 bytes |

Safety packet size | 200 bytes |

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**MDPI and ACS Style**

Song, C.; Tan, G.; Yu, C. An Efficient and QoS Supported Multichannel MAC Protocol for Vehicular Ad Hoc Networks. *Sensors* **2017**, *17*, 2293.
https://doi.org/10.3390/s17102293

**AMA Style**

Song C, Tan G, Yu C. An Efficient and QoS Supported Multichannel MAC Protocol for Vehicular Ad Hoc Networks. *Sensors*. 2017; 17(10):2293.
https://doi.org/10.3390/s17102293

**Chicago/Turabian Style**

Song, Caixia, Guozhen Tan, and Chao Yu. 2017. "An Efficient and QoS Supported Multichannel MAC Protocol for Vehicular Ad Hoc Networks" *Sensors* 17, no. 10: 2293.
https://doi.org/10.3390/s17102293