CSMA/CA 802.11 and 802.15.4

Carrier Sense Multiple Access with Collision Avoidance

In wireless communication it is not possible to detect collisions during transmissions.

CSMA/CA vs. CSMA/CD In CSMA/CA, backoff before collision. In CSMA/CD, backoff after collision.If the channel is sensed "idle" then the node is

permitted for transmission else the node defers its transmission for a random period of time called backoff.

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IEEE 802.15.4

IEEE 802.15.4 standard defines the characteristics of the physical and MAC layers for LR WPANs

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IEEE 802.15.4 Main Characteristics

Low power consumptionCSMA/CA channel access

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802.15.4 Physical Layer Operating Frequency Bands

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IEEE 802.15.4 PHY Layer Packet Structure

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An octet is a unit of digital information that consists of eight bits.

802.15.4 Modulation Scheme

Typical devices (1mW) are expected to cover a 10 -20 m range.

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Component of IEEE 802.15.4 LR-WPAN

Coordinator : Coordinator controls and monitor the established network. PAN Coordinator : A coordinator that is the principle coordinator of PAN. Cluster : A cluster is a small section of a bigger network, which has its

own coordinator. Device/End Node : A device is either a reduced/full function device. Any

device that is not a co-ordinator is an end node Personal Operating Space(POS) : It is the operating range of a node in

all directions.

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IEEE 802.15.4 Network Devices

Full Function Device(FFD) : It is a device which supports all the 49 primitives (14 PHY+ 35 MAC) supported by the technology. It acts as a PAN coordinator, a Coordinator, or just as an end node (device).

Reduced Function Device(RFD) : It is a device with reduced functionality which can only function as an end device or node. It can only communicate with the coordinator.

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IEEE 802.15.4 Topologies

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IEEE 802.15.4 MAC Layer

• Features

Simple and Flexible protocolExtremely Low CostEase of ImplementationReliable Data TransferShort Range OperationVery Low Power Consumption

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IEEE 802.15.4 MAC Layer

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IEEE 802.15.4 MAC SuperFrame Structure

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IEEE 802.15.4 – MAC Transmission – UnSlotted CSMA

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IEEE 802.15.4 – MAC Transmission – Slotted CSMA

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Medium Access Control CSMA/CA 802.11

• CSMA/CA: Carrier Sensing Multiple Access with collision avoidance

• Carrier Sensing: Listen before talking.

Channel Contention 802.11 (Time Domain)

Random Backoff = 11 Random Backoff = 22

DIFSTime

Time

Node 1 Node 2 Node 3

DATA

WAIT

DIFS

RB1=11 RB1=0

RB2=11RB2=22

SIFS

ACK

Channel Contention 802.11 (Frequency Domain)

• 802.11 a/g/n PHY adopt OFDM

Souvik Sen, Romit Roy Choudhury, and Srihari Nelakuditi “No time to countdown: migrating backoff to the frequency domain” In Proceedings of the 17th annual international conference on Mobile computing and networking (MobiCom '11). ACM, New York, NY, USA,

Channel Contention 802.11 (Frequency Domain) cont…

• Backoff within a Collision Domain

Souvik Sen, Romit Roy Choudhury, and Srihari Nelakuditi “No time to countdown: migrating backoff to the frequency domain” In Proceedings of the 17th annual international conference on Mobile computing and networking (MobiCom '11). ACM, New York, NY, USA,