WAVE Media Access Control Layer

WAVE Media Access Control

Layer

WAVE Physical Layer

•

7.1 MAC Layer Operations

•

7.2 MAC Layer Amendments

MAC Layer

•

IEEE 802.11 defines MAC layer in DSRC

•

It is channel access with Carrier

Sensing Multiple Access with Collision

Avoidance (CSMA/CA)

•

Uses back off timer to reduce frame

collision

•

MAC filters out frames not meant for

CSMA/CA

802.11 sender

1 Initially: it senses channel idle

for short DIFS

(Distributed Inter-Frame Space) time

then

- transmit entire frame

2 Otherwise,

choose random backoff time, timer

counts down while channel idle and is frozen if

channel is busy.

3 When timer reaches zero (if channel is sensed

idle), transmit entire frame

sender

receiver

DIFS

data

•

Transmitter must sense the medium before making a decision to

send a frame

•

If the medium is busy transmitter waits

•

Transmitter only send a frame if the medium is continuously idle

for some time.

•

This waiting time is the sum of Distributed coordination function

InterFrame Space (DIFS) + randomly calculated back-off time

CSMA/CA

•

Does not require coordination among nodes

•

It is simple to implement

•

Offers good performance over moderately

loaded channels

•

IEEE 802.11 adopts enhanced distributed

channel access (EDCA)

•

Four different priority classes achieved by

picking different values of Arbitration

InterFrame Spacing (AIFS)

CSMA/CA Example

Busy

Idle

Transmit request from upper layer

Back-off timer is loaded with a random

value (0-CW) e.g. 6 AIFS Wait for AIFS

time Decrement back-off

timer Back-off timer is zero, start transmitting

Busy

TX2

TX1

Time Time Transmitter 1 Transmitter 2 (AIFS=4) 6 5 4 3 2 1 0 Transmit request from upper layer

Back-off timer is loaded with a random

value (0-CW) e.g. 5

AIFS Wait for AIFS

time

Decrement back-off

timer

Back-off timer is zero, start transmitting Time Transmitter 3 (AIFS=8) 5 4 3 _{2 1 0}

Idle

TX3

Defer counting down

Hidden Terminal Effect

•

B hears A and C, but A and C do not

hear each other

•

Messages from A and C can collide over

the air

A

B

•

WiFi addresses Hidden Terminal with RTS/CTS messages

•

V2V DSRC is a broadcast and there is not RTS/CTS

Basic Service Set

•

In IEEE 802.11 (WiFi) standard

•

BSS is a group of radios configured to talk to

each other over the air

•

All other communication is filtered out

•

Infrastructure BSS contains a station acting

as Access Point (AP)

•

48 bit BSSID is the MAC address of the AP

•

Ad Hoc mode does not require Access Point,

Independent BSS (IBSS)

Basic Service Set

Internet

Access

Point

BSS

BSSID: AP MAC

Ad Hoc

IBSS

IBSSID:

0xFFFFFFFFFFFF

Outside of the Context of a BSS

•

802.11p (MAC layer amendment) for DSRC: ability

to send Outside of a Context of a BSS (OCB)

•

Vehicles can immediately communicate with each

other and with road side equipment with out any

additional overhead

Exercise

Busy

Transmit request from upper layer

Back-off timer is loaded with 5 AIFS

_TX2

TX1

Time Time Transmitter 1 Transmitter 2 (AIFS=4) 5 4 3 2 1 0 Transmit request from upper layer

Back-off timer is loaded with 8 Time Transmitter 3 (AIFS=6) Medium State 8 AIFS 7 6 5

Idle

Busy

4 3 2 1 0

TX3

Idle

Busy

defered

Complete

MAC Layer Modeling in ns-2

Transmission Coordination Transmission Reception Coordination Reception Channel State Manager Back-Off Manager

Upper Layers

PHY State Manager Power Monitor

MAC

PHY

RF Model

Mobile

Node

Channel State Manager SM

IFS – Inter Frame Space

Cs1 – Carrier Sensing (physical) indicating medium is busy

Cs0 – Carrier Sensing (physical) indicating medium is idle

Nav1 – Network Allocation Vector (virtual) indicating medium is busy

Nav0 – Network Allocation Vector (virtual) indicating medium is idle

CS_IDLE, CS_BUSY – signals to the Back-Off Manager

Cs0Nav0 Cs0Nav1 Cs1Nav0 Cs1Nav1 Wait IFS CS_BUSY CS_BUSY Start new NAV Timer CS_IDLE CS_BUSY NAV timer expired Start new NAV timer NAV received NAV timer expired

CS_IDLE CS_BUSY Update NAV timer Update NAV timer CS_IDLE

Back-Off Manager SM

No Back-Off Back-Off Running Back-Off Paused Initialize Back-off counter (CS_BUSY)

Signal “back-off zero” to TC Initialize Back-off counter (CS_IDLE) CSM signals CS_BUSY CSM signals CS_IDLE

CSM - Channel State

Manager

TC – Transmission

Coordination

Transmission

Reception

Transmission Coordination SM

TC_IDLE Data queued? Wait for CTS Wait SIFS Transmit RTS RTS Pending Data Pending Transmit Data Back-off needed RTS needed Retry Limit Reached Back-off needed Yes No No No No Back-off zero Yes Yes Yes

Frame transmission request from upper layers

CTS timer expired

Start Back-off counter RTS – Request To Send

CTS – Clear To Send SIFS – Short IFS

Reception Coordinator SM

RC Idle

Wait TX Wait SIFS

No NAV or Unicast data

frame received Transmission complete