Carrier-sense multiple access with collision avoidance CSMA/CA Definition Why CSMA/CA ? CSMA/CA : FEATURES The hidden node problem RTS/CTS? IEEE 802.11 RTS/CTS Exchange Binary Exponential Backoff CSMA-CA : FLOWCHART ADVANTAGES DISADVANTAGES

2. CSMA/CA?
CSMA/CA (Carrier-sense multiple access with collision avoidance).
It is a network multiple access method.
Here carrier sensing is used.
Here nodes attempt to avoid collisions.
Sense channel beginning of transmission when the channel is sensed as ‘idle’.

3. CSMA/CA is particularly important for wireless networks.
Collision avoidance is used to improve the performance of the CSMA.
Divide the channel equally among all transmitting nodes within the
collision domain.
 In CSMA/CD to wireless transmitters defensing their receivers during
packet transmission.

4.  Unreliable due to the hidden node problem.
 It is a protocol that operates in the data link layer.
 Performance is based upon the modulation technique used to
transmit the data between nodes.

5. The hidden node problem
In wireless networks, an issue known as hidden node
problem.
 When a sending device checks whether the channel is
idle.
It can only check within its broadcast range.
A device can be out of the range from the broadcast of
another device.
But still be on the network connecting them (within the
range of the WAP of the wireless network).
Therefore, they could both be sending data to the WAP at
the same time.
But It would be unaware of the presence of another
device.
As a result the WAP would not be able to receive any data
due to the data collisions.
Wireless devices A and C
cannot see each other, but
can communicate with B.
A B
C

6. Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access
to the shared medium.
It is alleviating the problem of hidden nodes.
For instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a
time.
However, wireless 802.11 implementations do not implement RTS/CTS for all transmissions.

7. CSMA/CA can optionally be supplemented by the exchange of a Request to Send
(RTS).
Packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended
receiver R.
All nodes within range of the sender, receiver or both, to not transmit for the duration of
the main transmission.
It is known as the IEEE 802.11 RTS/CTS exchange.
Implementation of RTS/CTS helps to partially solve the hidden node problem that is often
found in wireless networking

8. Binary Exponential Backoff
 If the medium was identified as being clear or the node received a CTS to explicitly
indicate it can send the frame.
Unlike CSMA/CD, it is very challenging for a wireless node to listen at the same time as it
transmits.
 The node awaits receipt of an acknowledgement packet from the Access Point to indicate
the packet was received and check summed correctly.
Acknowledgement must be arrive in a timely manner.
 If not it assumes the packet collided with some other transmission.
It is causing the node to enter a period of binary exponential backoff prior to attempting to
re-transmit.

9. No
Yes
Yes Yes
Start
K=0
Idle
Channel
?
Wait IFS
times
Station
Idle
?
Choose a random
number R between
0 and (2^K)-1
No
Wait
R
slots
Send
Frame
Wait
Time
Out
K>Kmax
?
K=K+1
Ack
Received
?
Success Abort
No No

10. Prevents collision.
Data is not lost unnecessarily Due to acknowledgements.
It is very much suited for wireless transmissions.
It avoids wasteful transmission.

11. Calls for long waiting times.
High power consumption.
Causes additional traffic.

12. 802.11 RTS/CTS.
IEEE 802.15.4(Wireless PAN).
GNET.
NCR WaveLAN.
Apple's Local Talk.
HomePNA.
Bus Networks.

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