intelligent transportation systems - sti- · pdf filemaca prma reservation tdma. centralized....

Intelligent Transportation Systems

Medium Access Control

Prof. Dr. Thomas Strang

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Recap: Wireless InterconnectionsNetworking types

Broadcast+ Scalability+ Range– Delay– Individuality

Cellularo Scalabilityo Rangeo Delayo Individuality Ad-hoc

– Scalability– Range+ Delay+ Individuality

Relevance

M. Röckl and T. Strang, 2009

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0 Medium Access Control

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Medium Access ControlBasic functions

Medium Access Control is required to control the access of multiple nodes on a shared medium (e.g. wired or wireless channel)Multiple Access Control: During packet reception no other node within interference range should transmit simultaneously Prioritization: Packets with higher priority should get access to the medium first

Local: packet prioritization of a single nodeGlobal: packet prioritization of all nodes

Multi-channel operation: Packets should beassigned to the appropriate channel (e.g.channel with lowest load, fastest data rate,regulatively assigned)

Node 1

Node 2

Node n


Channel 1

Channel 2

Channel m

…

…

Packet 1

Packet i

Packet 1

Packet k

Packet 1

Packet l… … …

Shared Medium

Node 1

Node 2

Node 3

Node 5

Node 4


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Medium Access ControlMAC in VANETs

Static channel assignment structure, such as static TDMA, is not suitable for Vehicular Ad-hoc Networks (VANETs) due to their lack of a priori knowledge about transmission characteristics and synchronization

dynamic channel assignmentCentralized channel coordination, such as polling, is not feasible in VANETs due to the lack of a central infrastructure

decentralized channel coordination

Nodes get access to the channel only if required (event-triggered)Nodes have to compete with other nodes to gain access to the medium (contention)


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ChannelAssignment

ChannelAccess

ChannelCoordination

ChannelSeparation


Decentralized

Multiple access

(Slotted) AlohaCSMA

MACAPRMAReservation TDMA

Centralized

Reservation-based

SDMA TDMA FDMA CDMA

Random Access

Hybrid SolutionsCSMA/CA

DynamicStatic

SDMA = Space Division Multiple AccessTDMA = Time Division Multiple AccessFDMA = Frequency Division Multiple AccessCDMA = Code Division Multiple Access


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0 Random Access MAC

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Medium Access ControlAloha MAC

Part of the ALOHAnet developed by University of Hawaii in 1970If a node has data to transmit, it immediately accesses the channelIf the receiver is within interference range of two transmitters which simultaneously send data on the channel, both packets are lost due to collision (complete or partial collisions)Max. throughput: ~18% (82% lost due to packet collisions)

Sender A

Sender B

Sender C

Completecollision

t

Partialcollision

+ Simple– Bad channel utilization


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Medium Access ControlAloha Protocol

Any terminal is allowed to transmit without considering whether channel is idle or busy If packet is received correctly, the base station transmits an acknowledgement.If no acknowledgement is received by the sender,

1) it assumes the packet to be lost 2) it retransmits the packet after waiting a random time, usually with probability Pr in every slot.

[J-P

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Medium Access ControlSlotted Aloha

Time is divided in discrete intervals (=time slots)A node can only transmit at the beginning of a slotIf a receiver is within interference range of two transmitters which simultaneously send data, both packets are lost due to collisionPacket size restricted to slot sizeMax. throughput: ~36% (64% lost due to packet collisions)

Complete collisions only

t

+ Doubles channel utilization compared to pure Aloha– Synchronization required– Still suboptimal channel utilization

Sender A

Sender B

Sender C


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Collision probability vs. throughput

1 ThroughputCollisionrateOfferedTraffic

= −

[Ric

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Medium Access ControlCarrier Sense Multiple Access (CSMA)

Collisions cannot be detected in wireless communication as it is done in wired communication because of high power difference between sent and received signal

“Listen while talk” is not feasible in wireless networks

Idea: Listen when not sending …If someone else is transmitting, listen until transmission is finishedIf the channel is idle, stop listening and start transmitting

“Listen before talk”

No interruption of ongoing transmission (“polite Aloha”)


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CSMA with Collision Avoidance

CSMA: Sensing the medium before accessing it ( listen before talk)Physical carrier sensing: The medium is sensed to be busy if the received signal power exceeds a CS signal threshold valueVirtual carrier sensing: Every node maintains a network allocation vector (NAV) and updates it by means of the duration field of the currently sent message

Drawback: Collisions are likely to occur directly after current transmission ends if several nodes wait for the medium to become idle

Idea: Every node waits for additional random time (called backoff time) before transmission starts

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Medium Access ControlCSMA/CA in its simple form

Sensing

Sensing DATA

Sensing DATABusy Backoff

If a node wants to access the shared medium, at first it senses the medium for a certain time

If the medium has sensed to be free for that certain time, the node accesses it immediately and begins to send the packet

If the medium has sensed to be busy, the node chooses a random backoff time it has to wait after the medium is free again, before it starts transmitting

B. Kloiber and T. Strang, 2010

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Medium Access ControlCSMA Persistency

1-persistent CSMAIf channel is idle, node immediately accesses the channel If channel is busy, node persistently senses the channel until it becomes idleProblem: When channel becomes idle all waiting nodes access the channel in parallel

Collisions can occur infinitely

Non-persistent CSMAIf channel is idle, node immediately accesses the channel If channel is busy, node starts backoff and reschedules future sensingProblem: Long idle times in case of long backoff times

p-persistent CSMAIf the channel is idle:

node accesses the channel with probability p, ornode starts the backoff with probability (1-p)

If channel is busy, node persistently senses the channel until it becomes idleGood trade-off between 1-persistent CSMA and non-persistent CSMA

Busy?

Sense Carrier

Transmit

noyes

Busy?

Sense Carrier

Transmit

noyes

Wait

Busy?

Sense Carrier

Transmit with probability porStart backoff with p. (1-p)

noyes


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Medium Access ControlSlotted p-persistant CSMA

Sender A wants to transmit.It selects a backoff number of 3

t

Sender A

Sender B

Sender C

3 2 1

8 7

Sender B wants to transmit.It selects a backoff number of 8

6 5 4

2 1

Sender A’s backoff counter is zeroIt accesses the channel

3 2 1

3 2 1 Collision

Sender B detects busy channeland freezes backoff timer

1. If a node wants to transmit, it senses the channel2. If the channel is busy, it waits until it becomes idle3. When channel becomes idle, node generates a random

number for backoff counter4. As long as the channel is idle the backoff counter is

decremented with every slot5. If the channel becomes busy, the backoff counter is

frozen until the channel gets idle again6. When the backoff counter reaches zero, the node starts

transmitting in the next slot

Sender A defers transmission because channel is busy

Sender C wants to transmit.It selects a backoff number of 2


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Medium Access ControlTransmission Acknowledgements in CSMA

Successful transmission can not be detected by listening on the channel as it is done in CSMA/CD

Acknowledgements (ACK) sent by the receiver after successful reception of a data packet can be used to provide feedback to the senderAcknowledgements in CSMA:

Automatic retransmission triggered by missing acknowledgements Higher layer (e.g. TCP/IP) retransmission schedule usually not suitable for wireless communicationData link layer ACKs can be scheduled faster and can be tailored to respective PHY layer

normally used in wireless CSMA systemsProblem: Multiple ACKs in multicast or broadcast communication will result in the so called broadcast storm problem

collision of ACK packets

Based on Ni et al. (1999): The Broadcast Storm Problem in a Mobile Ad Hoc Network


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Medium Access ControlThroughput Performance Comparison

0 1 2 3 4 5 6 7 8 9Traffic Load

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

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AlohaSlotted Aloha

1-persistent CSMA

0.5-persistent CSMA0.1-persistent CSMA

0.01-persistent CSMA

Nonpersistent CSMA

Source: Dharma et al. (2003): Multiple Radio Access


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0 Ad-hoc Networking Problemsdirectly influencing MAC

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Medium Access ControlInterference

Transmission range:Range within a packet can be successfully received if there is no interferenceDetection range (=Carrier sense range):Range within communication is not possible but signal can be detected by carrier sensingInterference range:Range within signal can interfere with other signals, channel sensed to be free (signal adds to background noise)

Interference Range can be up to10x of the Communication Range!


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Medium Access ControlAd-hoc networking problems

Hidden-Terminal-Problem (HTP)Packet collision at the receiver (R1) due to simultaneous transmission of nodes (S1 & S2) that are not within each others detection range but in the interference range of the receiver

Packet collisionsExposed-Terminal-Problem (ETP)

Transmission blocking due to ongoing transmission between two nodes (S1 & R1) with a third node (S3) that is not in the interference range of the receiver (R1) but in detection range of the sender (S1) and thus suppresses its transmission to a fourth node (R2) which is not in the sender’s (S1) interference range communication between S3 and R2 would be possible without collisions but will be suppressed

Low bandwidth utilization

R1

S2S1

R2

S3

Collision occurs at the receiver (not at the transmitter)

Carrier Sensing cannot avoid collisions


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Closed-Open Network

Network diameter < range Network diameter > range

•An Open Network is a network where not all the nodes are visible.

•Hidden terminal problem

•A Closed Network is a network where all the nodes are visible.

•No hidden terminal problem

C. Rico-Garcia and T. Strang, 2010

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MANETs and their issues

Capture effect: There are at least two simultaneous transmissions within the communication range, but due to a sufficient signal-to-signal ratio one message can be received correctly (b)

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Positive aspect of capture effect

Sensrange = 5 km

km

dB

dB

Ideal Channel

km

dB

Sensrange = 5

Sensrange = 5

Hata Okumura Channel

Hata Ideal Channel

[Rico-Garcia 2010]

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0 Reservation Based MAC

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Medium Access ProtocolChannel Reservation

Reservation of an exclusive channel can be used to avoid packet collisionsChannel reservation has to be known to all nodes within two-hops from the senderImplicit vs. explicit reservation:

Implicit reservation:Reservation Aloha: Once acquired slot is implicitly assigned to the node for future usage until it is not used anymore

Explicit reservation:Busy Tone Multiple Access (BTMA) for multi-channel operation (control & data channel): busy tone transmitted by sender and receiver informs nodes in the vicinityRTS/CTS messages in MACA

Explicit channel reservation causes delays and increased traffic load

1 2 3 4 5 6 7 8

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F D

frame1

frame2

frame3

frame4

frame5


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Medium Access ControlMedium Access with Collision Avoidance (MACA)

Introduction of two-way handshake with two new messages exchanged prior to data transmission:

Request-To-Send (RTS): Announcement of the transmission initiator having data to transmit (Alice: “Hey, Bob?”)Clear-To-Send (CTS): Acknowledgement of the transmission responder being ready to receive data (Bob: “What do you have to say, Alice?”)

RTS and CTS include the amount of data to exchangeevery node that hears RTS and/or CTS is aware of the ongoing transmission

A node that receives …CTS and/or RTS suppresses its transmission for the specified time (avoids HTP)RTS and no CTS is allowed to transmit(avoids ETP)

If no CTS is received after sending an RTS (due toa collision or ongoing transmission), the initiator starts backoff algorithm and reschedules next RTSRTS/CTS is sometimes called virtual sensingin contrast to physical sensing in CSMA

Source: Karn (1990):MACA – A New Channel Access Method for Packet Radio

Neighbor Initiator Responder Neighbor

RTS RTS

DATA

CTS CTS

DATA

No transmission


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Hybrid Medium Access ControlCombining Random Access w/ Reservation Based MAC

CSMA/CA = CSMA (with ACKs) and MACAAs efficient as MACA in situation with HTP or ETP and as efficient as CSMA otherwisePhysical (Carrier Sensing) and virtual carrier sensing (RTS/CTS)

CSMA/CA is the standard medium access control techniques used in Wireless LAN systems (IEEE 802.11)


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Typical MAC Performance Metrics

Update Delay: Time interval between two consecutive messages from the same sender.

Delay / Latency: Time spending in the waiting queue, until the transmission is complete

Throughput: Average rate of successful message delivery over a communication channel

Stability: The system should be able to handle instantaneous loads that are greater than the maximum sustained load when long-term offered load is less than the maximum

Robustness: The wireless channel is time-varying and error-prone. This behaviourshould not cause an unstable system.

(Fairness, Power consumption, multimedia support) etc.


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Beaconing in VANETs

In Vehicular Ad-hoc Networks (VANETs), all nodes are broadcast transmitters and receivers, they are therefore sometimes referred to as Mobile Ad-hoc Multibroadcast Networks

Interesting to note: Obviously there cannot arise the exposed terminal problem

Base functionality, also for safety applications, is implemented using short periodic status update beacons (TCAS/ADS-B, AIS, Car2Car, RCAS, military applications)Beaconing is the core communication mode in many safety systems. The MAC layer has an important role in the successful transmission of beacons. The hidden terminal problem is a major issue.


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Scalability Challenges for Car2Car Networks


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Transmission Delay

Tx RxNetwork Classical networkbuffer

Network

Video, sound, data…

Tx Beaconing VANETShort status update

May I send?

MAC

MAC

Rx

NoYes![Rico-Garcia 2010]

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Update Delay

The Update Delay is the time between the reception of two consecutive status update messages coming from the same node.

TimeFrame

Status Update Receive

Status Update Receive Status

Update NOT

Receive

Status Update Receive

Status Update

NOT Receive

Update Delay Update Delay

Kno

wle

dge

certa

inty

B. Kloiber and C. Rico-Garcia, 2010

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Importance of the Update Delay

Actions (breaking, change in direction, etc.) are decided according to the status updates.

The larger the time between status updates, the lower the reaction time.

...

TABCTADAlertDBrakeDGuard

DComm

Train B Train A

RCAS Example:

Status update frequency = 1Hz ; 1 packet lost 2 s update delay.

At 160 km/h, 2s 90m


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General analysis of a MAC protocol

Update DelayCollision

Rate Throughput

The Update Delay depends on many parameters: Offered Traffic, Propagation Channel, Network Dynamics.


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Update delay medium access of Slotted Aloha

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0 Self Organized TDMA

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SOTDMA protocol as used in the Automatic Identification system (AIS)

1 1

Time

I want to send, which slots are free?

Node 2Node 1


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TimeObserving!

Present

MineNSS2 NSS2

SI=3 (20%)

Node 2Node 1

Among free slots I choose this as my central slot NSS

I send in this slot

And, in next frame I will choose and reserve 1 free slot between NSS and +-SI/2

1 1

Observing!

PresentReserve Reserve


SOTDMA protocol as used in the Automatic Identification system (AIS)

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Collision rate of SOTDMA.

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Update Delay SOTDMA

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Hidden terminal problem in SOTDMA

1 2 1 2 1 2 1

Time

Node 2Node 1 Node 3

Node 1

Node 2

Node 3

1 1 1 12 2 2

2 2 23 3 3

Reserve

3

3 3 3 3

2

2

2 1

1

Reserve

Reserve

3

If Node 1 and Node 3 would use different channels, Node 2 wouldn‘t see anycollision


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0 Cell-based Orientation-awareMulti Broadcast MAC

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COMB1 2 1 2 1 2 1

Time

Node 2Node 1 Node 3

Node 1

Node 2

Node 32 2 2

3 3 3 3

2 1

3 3 3 3

1 1 1 12 2 2 2 1

3

Reserve

Reserve

Reserve

3

2

Nodes in range can bein the same channel.

Nodes not in rangemust be in different channels.


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Number of channels and cell size in COMB

Node 1

All the nodes inside a channel see the same network closed SOTDMA network

12

3

4

5

6

7

8

9

10

11

12

At least 12 channels are needed

It is possible to reuse the same channels

12

3

4

5

10

11

3

4

5

6

7

810

11

12

12

3

4

5

6

7

8

9

10

11

12


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Protocols Comparison: Update Delay

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intelligent transportation systems - sti- · pdf filemaca prma reservation tdma. centralized....

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