Carrier sense multiple access with collision avoidance (CSMA/CA) in 802.11 and 802.15.4

1. CSMA/CA 802.11 and 802.15.4

2. 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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3. 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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4. IEEE 802.15.4 Main Characteristics
Low power consumption
CSMA/CA channel access
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5. 802.15.4 Physical Layer Operating
Frequency Bands
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6. IEEE 802.15.4 PHY Layer
Packet Structure
An octet is a unit of digital information that consists of eight bits.
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7. 802.15.4 Modulation Scheme
Typical devices (1mW) are expected to cover a
10--20 m range.
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8. Component of IEEE 802.15.4 LRWPAN
 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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9. 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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10. IEEE 802.15.4 Topologies
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11. IEEE 802.15.4 MAC Layer
• Features
Simple and Flexible protocol
Extremely Low Cost
Ease of Implementation
Reliable Data Transfer
Short Range Operation
Very Low Power Consumption
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12. IEEE 802.15.4 MAC Layer
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13. IEEE 802.15.4 MAC SuperFrame
Structure
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14. IEEE 802.15.4 – MAC Transmission –
UnSlotted CSMA
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15. IEEE 802.15.4 – MAC Transmission –
Slotted CSMA
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16. Medium Access Control CSMA/CA
802.11
• CSMA/CA: Carrier Sensing Multiple Access
with collision avoidance
• Carrier Sensing: Listen before talking.

17. Channel Contention 802.11 (Time
Domain)
Random Backoff = 11
Node 3
Node 2
Node 1
RB1=11
Random Backoff = 22
RB1=0
SIFS
DIFS
DATA
ACK
Time
DIFS
WAIT
Time
RB2=22
RB2=11

18. 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,

19. 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,