2.7 wlan ieee 802.11

Communication
Networks
Wireless LAN
– IEEE 802.11

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
Visit www.youtube.com/GURUKULA for Lecture Videos

In this session:
ARCHITECTURAL
COMPARISON
PROBLEMS IN
WLANs
ADDRESSING
MODES
ACCESS
METHOD
MAC LAYERS OF
WLAN
FRAME
FORMAT
ARCHITECTURE OF
WLAN

Architectural Comparison
• Obviously, its wired.
• Hosts are always connected
to the network
• Lack of mobility
• Point to point
communication
• Obviously, its wire free.
• Hosts need not be always
connected to the network
• High mobility
• Broad cast
communication
Similarities and connection to other NWs
Similarities:
Both uses PHYSICAL Addressing modes
Connecting to other NWs:
In wireless cases only the internal or limited area is wireless.
Example, The connection between your laptop and the
modem in your home is wireless. But still the connections
behind the modem are wired. (Remember ISP’s and Back
Bone Networks)

Problems in Wireless LANs (WLAN)
Unpredictable
Environment Reliability of message as
we cannot send it for
intended user
Power Requirements
for transmitting and
receiving
There are lots of chances
for Eavesdropping
➢ All these issues are common problems that arises
when we migrate from wired to wireless mode of
communication.
➢ As a communication engineer , we should
incorporate required mechanisms to overcome
these challengesVisit www.youtube.com/GURUKULA for Lecture Videos

Lets discuss in Engineering Perspective
Attenuation:
➢ The strength of electromagnetic signals decreases rapidly because
the signal disperses in all directions;
➢ The situation becomes worse with mobile senders that operate on
batteries and normally have small power supplies.
Interference:
➢ Another issue is that a receiver may receive signals not only from
the intended sender, but also from other senders if they are using
the same frequency band.
Multipath Propagation:
➢ A receiver may receive more than one signal from the
same sender because electromagnetic waves can be
reflected back from obstacles such as walls, the
ground, or objects. The result is that the receiver
receives some signals at different phases (because
they travel different paths). This makes the signal less
recognizable.

Access Control
Mechanism –
CSMA CA

Why not CSMA – CD in WLANs ?
1. Need for Full Duplex
To detect a collision, a host needs to send and receive at the same time, which means the host needs to work in a
duplex mode. Wireless hosts do not have enough power to do so. They can only send or receive at one time.
2. Hidden Node Problem:
• Any node present in their respective region can hear all of its respective
transmission
• From figure station C is outside the coverage of station A
• Station A is outside the coverage of station C
• Station B is covered by both A and C. Hence it can hear all communications
from A and C as well.
• A sends data to B. after some time C sends data to B. this leads in collision. (C
does not know that A is transmitting the data to B)
• In this case A and C are hidden Nodes to each other.
• This problem is overcome by using handshaking signals before transmission
3. Signal Fading
Due to large distance between the stations fading occurs. This prevents the station at one end from hearing
a collision at other end. Visit www.youtube.com/GURUKULA for Lecture Videos

Architecture of 802.11
• IEEE 802.11 standard defines 2 types of services they are
• Basic Service Set – BSS
• Extended Service Set – ESS

Basic Service Set (BSS)
• BSS is made up of stationary or
mobile stations and an optional
central base station known as Access
Point (AP)
• BSS without AP is called as Ad hoc
Networks
• BSS with AP is called as
Infrastructure Network

Extended Service Set
•ESS is made up of more BSS and
are connected to a “Distributed
Systems”
•The Distributed systems
connects the AP’s in the BSS
•There are 2 types of stations
➢Mobile Stations
➢Stationary Stations

Frame Format of IEEE 802.11 [WLANs]
• The MAC layer frame contains 9 fields.

Type of Frames
• There are 3 types of Frames
• Management Frame:
• These frames are used for initial communications between the
stations and access points
• Control Frame:
• These frames are used for accessing the channel and
acknowledging the frames.
• It can be an RTS frame, CTS Frame or ACK Frame
• Data Frames:
• These frames are used for carrying the data and control
information

Frame Control (FC) :
• Defines the type of the frame and some other
control information.
• It has 11 sub fields.
Protocol Version :
• It defines the version of the protocol.
Type :
• It defines the type of information.
• 00 – management frame
• 01 – control frame
• 10 – data frame
Sub Type :
• It defines the sub type of each frame.
• 1011 – RTS Frame
• 1100 – CTS Frame
• 1101 - Acknowledgement
More Flag :
• When this bit is “1” it indicated more fragments are to
be received
Retry :
• When this bit is “1” it indicates the retransmitted
frame
Pwr Mgt (Power Management) :
• When this bit is “1” it indicates station is in
power management Mode
More Data:
• When this bit is “1” it indicates station has more
data to send
WEP (Wired Equivalent Privacy):
• When this bit is “1” it indicates encryption is
implemented
RSVD (Reserved):
• This is reserved for future use
D (Duration):
• This field defines the duration of the
transmission used to set the NAV timers
• This is used as the ID for control frames
Addresses:
• There are 4 address fields
• The values in the address fields depends on the
values of “to DS” and “From DS” sub fields
SC (Sequence Control):
• This field defines the sequence number of the
frames for flow control.
Frame Body :
• This field can be between 0 bytes to 2312 bytes.
• This field contains the actual information based
on the values of the sub fields
FCS (Frame control Sequence):
• This field carries the 32 bit CRC for error
detection and Correction.
Visitwww.youtube.com/GURUKULAfor
LectureVideos

Addressing Mechanisms / Modes
• The values of the 4 Address fields will be decided based
on the values of “To DS” and “From DS”
Address 1:
• This field is always the address of next device
Address 2:
• This field is always the address of previous device
Address 3:
• This field is always the address of final destination if
it is not defined by address 1
Address 4:
• This field is always the address of Original Source if
it is not defined by address 2
ADDRESS REPRESENTS
1 Address of Next Device
2 Address of Previous device
3 Address of Final Destination
4 Address of Original Source

Understanding Addressing
1 2 3

Case 1
• In this case the frame is not
going to DS (To DS =0) also
not coming from DS (From
DS =0)
• The frame is moving from
one station to another
station of same BSS without
passing through DS

Case 2
• In this case the frame is not
coming from DS (From DS
=0) and the frame is going
to DS (To DS =1)
• The frame comes from a
station and going to the AP

Case 3
• In this case the frame is
coming from DS (From DS
=1) and the frame is going to
DS (To DS =1)
• The frame comes from AP
of one BSS and going to the
AP of another BSS

Case 4
• In this case the frame is
coming from a DS (From
DS =1) and the frame is not
going to DS (To DS =0)
• The frame comes from AP
and going to the station

MAC Layers of 802.11
• The MAC sublayer contains 2 protocols
• Distributed Coordination Function – DCF
• Point Coordination Function – PCF
• These 2 protocols are related with the access methods of the standard

Distributed Coordination Function

Point Coordination Function

Session Summary
ARCHITECTURAL
COMPARISON
PROBLEMS IN
WLANs
ADDRESSING
MODES
ACCESS
METHOD
MAC LAYERS OF
WLAN
FRAME
FORMAT
ARCHITECTURE OF
WLAN

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…

2.7 wlan ieee 802.11

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