2.3b access control random access methods - part 3 - csma ca2.3b access control random access methods - part 3 - csma ca2.3b access control random access methods - part 3 - csma ca

1. Communication
Networks
Access Control
in datalink layer
[Random Access Methods]
Part 3 – CSMA – CA

2. Overview of Module - 2
• Framing
• What is Framing
• Types of Framing
• Methods for identifying the
boundaries
• Flow Control
• What is flow control
• Types of flow control
• Access Control
• What is access control
• Types of access control
• Ethernet – IEEE 802.3
• Evolution of Ethernet
• Frame Format of Ethernet
• Access Control for Ethernet
• Performance Evaluation of Ethernet
• WLAN – IEEE 802.11
• Architecture
• Bluetooth
• WIFI
• Zigbee
• 6LowPan
• Network Layer Services
• IPv4
• Packet switching
• Network Layer Protocols
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3. Classification of Access Control Mechanism
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4. 1.D – CSMA – CA [Collision Avoidance]
• In CSMA – CD the station senses the channel only for that
instant and decides the idleness of the channel.
• In this protocol the station, before sending the data senses that ,
is there any other transmissions from other stations for a
particular PERIOD.
• These periods are measured by a special timers called as
Network Allocation Vector (NAV)
• The operation remains as same as the CSMA – CD except the
waiting time. There are 2 timers involved in this process.
➢DIFS – Distributed Coordinated Function Inter Frame Space.
➢SIFS – Short Inter Frame Space.

5. Important Timers in CSMA - CA
• If station finds that the medium is continuously idle for
Distributed Coordinated Function Inter Frame Space (DIFS)
duration, it is then permitted to transmit a frame. If the
channel is found busy during the DIFS interval, the station
should defer its transmission.
• Short Inter Frame Space (SIFS), is the amount of time in micro
seconds required for a wireless interface to process a
received frame and to respond with a response frame
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6. Flow diagram of CSMA - CA
• Before sending a frame the source station
senses the channel for a period of DIFS.
• If the channel is found idle for the entire period
of DIFS then the sender sends the control
frame RTS “Request To Send”
• The receiver takes some time for processing the
received frame and to respond this time is
referred to as SIFS. Then the receiver responds
with the control frame CTS “Clear To Send”
• Other stations are not allowed to sense the
channel during this period

7. CSMA – CA – Work Flow
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8. Session Summary
• Access Control [Similar to people accessing the cell tower]
• Random Access Methods [Checks the channel randomly,
Idle – Transmit, Busy – Check again]
• ALOHA [Pure ALOHA, Slotted ALOHA]
• CSMA [ 1 Persistent, n Persistent, p Persistent]
• CSMA – CD [widely used in Ethernet]
• CSMA – CA [widely used in WLAN]
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9. References:
• Behrouz A. Forouzan, ―Data communication and Networking, Fifth
Edition, Tata McGraw – Hill, 2013
• Larry L. Peterson, Bruce S. Davie, ―Computer Networks: A Systems
Approach, Fifth Edition, Morgan Kaufmann Publishers, 2011.
• Few online References (Will be Mentioned in the description Section)
Thank You…