2.3a access control random access methods - part 2 - csma cd 2.3a access control random access methods - part 2 - csma cd

1. Communication
Networks
Access Control
in datalink layer
[Random Access Methods]
Part – 2 – CSMA CD

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.C – CSMA – CD [Collision Detection]
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5. What if channel is Free ?
• Assume station A in Figure has a frame to send to station D.
• Station A first should check whether any other station is sending
(carrier sense).
• Station A measures the level of energy on the medium (for a short
period of time, normally less than 100 μs).
• If there is no signal energy on the medium, it means that no station is
sending (or the signal has not reached station A).
• Station A interprets this situation as idle medium. It starts sending
its frame.
What if channel is Busy ?
• On the other hand, if the signal energy level
is not zero, it means that the medium is
being used by another station.
• Station A continuously monitors the
medium until it becomes idle for 100 μs.
• It then starts sending the frame. However,
station A needs to keep a copy of the frame
in its buffer until it is sure that there is no
collision.
When to stop Sensing the Channel?
The medium sensing does not stop after station A has started sending the frame.
Station A needs to send and receive continuously. Two cases may occur:
Case 1: Station A has sent 512 bits and no collision is sensed (the energy level
did not go above the regular energy level), the station then is sure that the frame
will go through and stops sensing the medium.
Case 2: Station A has sensed a collision before sending 512 bits. In this case both
stations should refrain from sending and keep the frame in their buffer for
resending.
However, to inform other stations that there is a collision in the network, the
station sends a 48-bit jam signal. The jam signal alert other stations about the
collision.
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6. 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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7. 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…