5. Ad Hoc Network
• This mode is a collection of
computers that are
associated so that they
can directly send frames
to each other.
• There is no access point.
6. Infrastructure Network
• Each client is associated with
an Access Point that is in turn
connected to the other
network.
• The client sends and receives
its packets via the AP.
WHAT IS ACCESS POINT ?
8. Data Link
Layer
MAC Sublayer
In 802.11, the MAC (Medium Access Control) sublayer determines
how the channel is allocated, that is, who gets to transmit next.
Logical Link Layer
Above it is the LLC (Logical Link Control) sublayer, whose job it is to
hide the differences between the different 802 variants and make
them indistinguishable as far as the network layer is concerned.
12. 802.11 Physical Layer
802.11a
• upto 54 Mbps
• OFDM (Orthogonal Frequency Division Multiplexing)
• 5 GHz
• Formed first but was approved later than 802.11b.
18. • IEEE 802.11 defines two MAC sublayers: 1) Distributed coordination function (DCF)
2) Point coordination function (PCF)
• 802.11 tries to avoid collisions with a protocol called CSMA/CA (CSMA with Collision Avoidance).
• This mode of operation is called DCF (Distributed Coordination Function) because each station
acts independently, without any kind of central control.
19. • Initially before sending station senses whether
channel is free or not.
• Otherwise, the station chooses a random backoff
value using binary exponential backoff.
• When the counter reaches zero the station transmits
the entire frame and then waits for an
acknowledgment.
• If an acknowledgment is received, the transmitting
station knows that its frame has been correctly
received at the destination station.
• If the acknowledgment isn’t received, the transmitting
station reenters the backoff phase in step 2, with the
random value chosen from a larger interval.
20. • PCF (Point Coordination Function) in which the access point controls all activity in its cell, just
like a cellular base station. However, PCF is not used in practice because there is normally no
way to prevent stations in another nearby network from transmitting competing traffic.
• There are two main reasons why 802.11 MAC does not implement collision detection:
⚬ Station must be able to send data and receive collision signals at the same time.
⚬ Hidden terminal problem
21. • From the perspective of node 1, node 2 is a
"hidden" node. If a simple transmit-and-pray
protocol was used, it would be easy for node 1
and node 3 to transmit simultaneously, thus
rendering AP unable to make sense of anything.
• An optional RTS/CTS mechanism uses the NAV to
prevent terminals from sending frames at the
same time as hidden terminals.
Hidden Terminal Problem
Img source:
22. • The network allocation vector is a virtual carrier-sensing mechanism used with wireless
network.
• While RTS/CTS sounds good in theory, it is one of those designs that has proved to be of little
value in practice.
• CSMA/CA with physical and virtual sensing is the core of the 802.11 protocol.
30. • Association
• Re association
• Dissociation
• Authentication
• Distribution
• Integration
• Data Delivery
• Privacy
• QOS traffic scheduling
• Transmit power control
• Dynamic Frequency Selection
802.11 Services
31. Future Prospects & Conclusion
• IEEE 802.11ax™, or Wi-Fi 6, is the most recent standard in the
series, published in 2021.
• Next to take center stage will be Wi-Fi 7.
• Wi-Fi 7 is based on features defined in the IEEE P802.11be™
draft amendment.
• The evolution of IEEE 802.11-based Wi-Fi standards continues
today, providing much faster data transmission rates, longer
ranges, and more reliable and secure connections.
32. References
• 802.11 Wireles Networks The Definitive Guide; Matthew S. Gast;
ISBN:0-596-00183-5
• Tanenbaum, Andrew S., 1944-Computer networks / Andrew S.
Tanenbaum, David J. Wetherall. ISBN-13: 978-0-13-212695-3
(alk. paper)
• Computer networking: a top-down approach / James F. Kurose,
University of Massachusetts, Amherst, Keith W. Ross, NYU and
NYU Shanghai. ISBN 9780133594140
• Forouzan, Behrouz A. Data communications and networking I
Behrouz A Forouzan. ISBN 978-0-07-296775-3