This document discusses access control methods in data link layers for communication networks. It explains the need for access control when multiple nodes use a shared medium like wireless networks. Access control mechanisms are classified into random access protocols and scheduled access protocols. Random access protocols like ALOHA and CSMA are described. CSMA variants including 1-persistent, p-persistent and CSMA/CD used in Ethernet are summarized. The document concludes with an overview of access control methods and references for further reading.

1. Communication
Networks
Access Control
in datalink layer
[Random Access Methods]
Part - 1

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. Access Control - Understanding

4. Need for Access Control
• when nodes or stations are connected and use a common link,
called a multipoint or broadcast link, we need a multiple-access
protocol to coordinate access to the link.
• The problem of controlling the access to the medium is similar
to the rules of speaking in an assembly.
• The procedures guarantee that the right to speak
• Ensure that two people do not speak at the same time, do not interrupt each other,
• do not monopolize the discussion, and so on.
• Many protocols have been devised to handle access to a shared
link. All of these protocols belong to a sublayer in the data-link
layer called media access control (MAC)
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5. Classification of Access Control Mechanism
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6. 1. Random Access Protocols
• Why is it called Random Access?
• First, there is no scheduled time for a station to transmit. Transmission is
random among the stations
• Second, no rules specify which station should send next. Stations
compete with one another to access the medium
• In Random Access Protocol, Each station follows a procedure that
answers the following questions:
• When can the station access the medium?
• What can the station do if the medium is busy?
• How can the station determine the success or failure of the transmission?
• What can the station do if there is an access conflict?
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7. 1.A - ALOHA
ALOHA, the earliest random access method, was developed at the University of
Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on
any shared medium.
PURE ALOHA SLOTTED ALOHA
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8. 1.B – CSMA – Carrier Sense Multiple Access
The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense
multiple access (CSMA) requires that each station first listen to the medium (or check the state of the medium)
before sending. In other words, CSMA is based on the principle “sense before transmit” or “listen before talk.”
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9. CSMA - Behaviour
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10. 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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11. 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…