◦ What is Wi-Fi ?
◦ Wi-Fi Standards
◦ Wi-Fi Technologies
◦ Wi-Fi Network Elements
◦ Channels and Association
◦ How a Wi-Fi Network Works?
◦ The 802.11 MAC Protocol
◦ The IEEE 802.11 Frame
◦ Mobility in the Same IP Subnet
◦ Wi-Fi NETWORK TOPOLOGIES
◦ Advantages and Limitations of Wi-Fi
◦ Wi-Fi Security
3. What is Wi-Fi?
Wi-Fi (Wireless *Fidelity) is a generic term that refers
to the IEEE 802.11 communications standard for
Wireless Local Area Networks (WLANs).
Wireless Technology is an alternative to Wired
Technology, which is commonly used, for connecting
devices in wireless using radio waves.
Allows you to access the Internet while on the move ;
you can remain online while moving from one area to
another, without a disconnection or loss in coverage.
4. IEEE 802.11 standard
In 1997, the Institute of Electrical and Electronic
Engineers (IEEE) drafted* the 802.11 standard for
wireless local area networking.
In 1999, networking hardware companies accepted the
standard and began manufacturing products using the
802.11b protocol which operated in the 2.4 GHz and
was capable of transmitting at speeds of 11 Mbps.
The 802.11a protocol was also released in 1999,
operating at 5 GHz with transmissions speeds of 54
Mbps, but its cost was high.
5. • IEEE 802.11 is a long and complicated standard. Despite the best efforts of the standards body,
there are bound to be areas that are ambiguous or not fully defined. Also there are a number of
features that are optional and different manufacturers might make different choices in their designs.
To avoid interoperability problems, the Wi-Fi Alliance was formed by a group of major manufacturers
and the logo "Wi-Fi" was created.
• To obtain Wi-Fi certification, a manufacturer must submit its product for testing against a set of "gold
standard" Wi-Fi products. The Wi-Fi Alliance created its own test plan based on IEEE 802.11. Some
features of IEEE 802.11 are not required for Wi-Fi certification. Conversely, there are some
requirements that are additional to the standard. Where there is ambiguity in the standard, the
correct behavior is defined by the way the gold standard products work. In this way interoperability is
ensured. In summary, Wi-Fi defines a subset of IEEE 802.11 with some extensions, as shown in
Relationship Between Wi-Fi and IEEE 802.11
6. The 802.11 Architecture
The next Figure illustrates the principal components of the 802.11 wireless LAN
The fundamental building block of the 802.11 architecture is the basic service set
(BSS). A BSS contains one or more wireless stations and a central base station,
known as an access point (AP) in 802.11 parlance. Figure shows the AP in each of
two BSSs connecting to an interconnection device (such as a switch or router), which
in turn leads to the Internet. In a typical home network, there is one AP and one router
(typically integrated together as one unit) that connects the BSS to the Internet.
10. The first three 802.11 standards share many characteristics
1. They all use the same medium access protocol, CSMA/CA
2. All three use the same frame structure for their link-layer
frames as well.
3. All three standards have the ability to reduce their transmission
rate in order to reach out over greater distances.
4. And all three standards allow for both “infrastructure mode”
and “ad hoc mode,”
11. The Three standards have some major
differences at the physical layer.
• The 802.11b wireless LAN has a data rate of 11 Mbps and operates in the
unlicensed frequency band of 2.4–2.485 GHz, competing for frequency
spectrum with 2.4 GHz phones and microwave ovens.
• 802.11a wireless LANs can run at significantly higher bit rates, but do so,
by highe operating at r frequencies.
• 802.11a LANs have a shorter transmission distance for a given power level
and suffer more from multipath propagation. yet with the higher-speed
transmission rates of 802.11a, allows users to have their cake and eat it
• 802.11g LANs, operating in the same lower-frequency band as
802.11b and being backwards compatible with 802.11b (so one
can upgrade 802.11b clients incrementally).
• 802.11n [IEEE 802.11n 2012], uses multiple-input & multiple-
output (MIMO) antennas; i.e., two or more antennas on the
sending side and two or more antennas on the receiving side
that are transmitting/receiving different signals. Depending on
the modulation scheme used, transmission rates of several
hundred megabits per second are possible with 802.11n.
A Hotspot is any location where Wi-Fi
Hotspots are equipped with a Broadband Internet
connection, and one or more Access Points that
allow users to access the Internet wirelessly.
Hotspots can be setup in any public location that
can support an Internet connection. All the locations
discussed below are examples of Hotspots.
What is a Hotspot ?
network access is made publicly available.
17. Elements of a Wi-Fi Network
Access Point (AP) - The AP is a wireless LAN transceiver
or “base station” that can connect one or many wireless
devices in the same time to the Internet.
Safeguards - Firewalls and anti-virus software protect
networks from uninvited users and keep information secure.
Wi-Fi cards (Adapters) - They accept the wireless signal
and relay information. They can be internal and external.
19. Channels and Association
• When a network administrator installs an AP, the administrator assigns a one or two-word
Service Set Identifier (SSID) to the access point. (When you “view available networks” in
Microsoft Windows XP, for example, a list is displayed showing the SSID of each AP in
• The administrator must also assign a channel number to the AP. To understand channel
numbers, recall that 802.11 operates in the frequency range of 2.4 GHz to 2.485 GHz. Within
this 85 MHz band, 802.11 defines 11 partially overlapping channels. Any two channels are
non-overlapping if and only if they are separated by four or more channels. In particular, the
set of channels 1, 6, and 11 is the only set of three non-overlapping channels.
• This means that an administrator could create a wireless LAN
with an aggregate* maximum transmission rate of 33 Mbps by
installing three 802.11b APs at the same physical location,
assigning channels 1, 6, and 11 to the APs, and interconnecting
each of the APs with a switch.
• Association :its the connection process between the wireless station and
the Access point in wireless network, wither it used passive scanning or
active scanning for connection between the tow entities.
21. How a Wi-Fi Network Works ?
• The 802.11 standard requires that an AP periodically# send *beacon frames, each of
which includes the AP’s SSID and MAC address. Your wireless station(e.g mobile
phone), knowing that APs are sending out beacon frames, scans the 11 channels,
seeking beacon frames from any APs that may be out there (some of which may be
transmitting on the same channel—it’s a jungle out there!). Having learned about
available APs from the beacon frames, you (or your wireless host) select one of the
APs for association.
22. • The process of scanning channels and listening for beacon frames is
known as passive scanning.
• A wireless host can also perform active scanning, by broadcasting a
*probe frame that will be received by all APs within the wireless
host’s range. APs respond to the probe request frame with a probe
response frame. The wireless host can then choose the AP with which to
associate from among the responding APs
24. The 802.11 MAC Protocol
• A station is either a wireless station or an AP.
• Because multiple stations may want to transmit data frames at the same
time over the same channel, a multiple access protocol is needed to
coordinate the transmissions.
• the designers of 802.11 chose a random access protocol & referred to as
CSMA/CA carrier sense multiple access with collision avoidance.
• meaning that each station senses the channel before transmitting, and
*refrains from transmitting when the channel is sensed busy.
25. • The 802.11 MAC protocol does not implement
There are two important reasons for this:-
• The ability to detect collisions requires the ability to send and receive
at the same time. Because the strength of the received signal is
typically very small compared to the strength of the transmitted signal
at the 802.11 adapter, & it is costly to build hardware that can detect
• More importantly, even if the adapter could transmit and listen at the
same time (and probably abort transmission when it senses a busy
channel), the adapter would still not be able to detect all collisions,
due to the hidden terminal problem and *fading.
(fading is deviation of the attenuation affecting a signal over
certain propagation media)
26. Hidden terminal problem
• In wireless networking, the hidden node problem or hidden
terminal problem occurs when a node is visible from a wireless
access point (AP), but not from other nodes communicating
with that AP. This leads to difficulties in media access control.
27. 802.11 CSMA/CA protocol.
Suppose that a station (wireless station or an AP) has a frame to transmit.
1. If initially the station senses the channel idle, it transmits its frame after a short period of time known as
the Distributed Inter-frame Space (DIFS); see
2. Otherwise, the station chooses a random backoff value using binary exponential backoff (as we
encountered in Section 5.3.2) and counts down this value when the channel is sensed idle. While the
channel is sensed busy, the counter value remains frozen.
3. When the counter reaches zero (note that this can only occur while the channel is sensed idle), the
station transmits the entire frame and then waits for an acknowledgment.
4. If an acknowledgment is received, the transmitting station knows that its frame has been correctly
received at the destination station. If the station has another frame to send, it begins the CSMA/CA
protocol at step 2. 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. 27
29. The IEEE 802.11 Frame
• Its contains a number of fields that are specific to its use for wireless links. The 802.11
frame is shown in Figure. The numbers above each of the fields in the frame represent
the lengths of the fields in bytes; the numbers above each of the subfields in the frame
control field represent the lengths of the subfields in bits.
30. Address Fields
The first three address fields. The 802.11 standard defines these fields as follows:
• Address 2 is the MAC address of the station that transmits the frame. Thus, if
a wireless station transmits the frame, that station’s MAC address is inserted
in the address 2 field. Similarly, if an AP transmits the frame, the AP’s MAC
address is inserted in the address 2 field.
• Address 1 is the MAC address of the wireless station that is to receive the
frame. Thus if a mobile wireless station transmits the frame, address 1
contains the MAC address of the destination AP. Similarly, if an AP
transmits the frame, address 1 contains the MAC address of the destination
• To understand address 3, recall that the BSS (consisting of the AP and wireless stations) is part of a
subnet, and that this subnet connects to other subnets via some router interface. Address 3 contains
the MAC address of this router interface.
The IEEE 802.11 Frame Types
• Control Frames
• Management Frames
– Probe Request/Response
– Association Request/Response
• Data Frames
32. Mobility in the Same IP Subnet
• In order to increase the physical range of a wireless LAN, companies and universities will often deploy
multiple BSSs within the same IP subnet. This naturally raises the issue of mobility among the BSSs.
• mobility can be handled in a relatively direct manner when the BSSs are part of the subnet. When
stations move between subnets, more sophisticated mobility management protocols will be needed.
33. • Let’s now look at a specific example of mobility between BSSs in the same subnet. Figure shows two
interconnected BSSs with a host, H1, moving from BSS1 to BSS2. Because in this example the
interconnection device that connects the two BSSs is not a router, all of the stations in the two BSSs,
including the APs, belong to the same IP subnet. Thus, when H1 moves from BSS1 to BSS2, it may
keep its IP address and all of its ongoing TCP connections.
• If the interconnection device were a router, then H1 would have to obtain a new IP address in the
subnet in which it was moving. This address change would disrupt (and eventually terminate) any
ongoing TCP connections at H1.
• But what specifically happens when H1 moves from BSS1 to BSS2? As H1 wanders
away from AP1, H1 detects a weakening signal from AP1 and starts to scan for a
stronger signal. H1 receives beacon frames from AP2 (which in many corporate and
university settings will have the same SSID as AP1). H1 then disassociates with AP1
and associates with AP2, while keeping its IP address and maintaining its ongoing
• This addresses the handoff problem from the host and AP viewpoint.
35. Wi-Fi NETWORK TOPOLOGIES
• Topology generally means network
shape or structure
AP-based topology(Infrastructure Mode)
Peer-to-peer topology(Ad-hoc Mode)
36. 1. AP BASED TOPOLOGY
• The client communicate through Access Point.
• BSA-RF coverage provided by an AP
37. 2. PEER-TO-PEER TOPOLOGY
• AP is not required.
• Client devices within a cell can communicate directly
with each other.
• It is useful for setting up of a wireless network quickly
38. Long-range Wi-Fi
• Long-range Wi-Fi is used for low-cost, unregulated point-to-point computer
network connections, as an alternative to other fixed wireless, cellular networks or
satellite Internet access.
• Wi-Fi networks have a range that's limited by the transmission power, antenna type,
the location they're used in, and the environment. A typical wireless router in an
indoor point-to-multipoint arrangement using 802.11b or 802.11g and a stock antenna
might have a range of 32 metres (105 ft). Outdoor point-to-point arrangements,
through use of directional antennas, can be extended with many kilometers between
39. WiFi Range
• Regardless of which setup you use, once you turn your Wireless
Access Point on, you will have a WiFi hotspot in your house.
• In a typical home, this hotspot will provide coverage for about 100 feet
(30 meters) in all directions, although walls and floors do cut down on
• Even so, you should get good coverage throughout a typical home. For
a large home, you can buy inexpensive signal boosters to increase the
range of the Hotspot.
40. Advantages of Wi-Fi
No Wires - A truly wireless networking solution.
No Waiting - Fast, easy deployments.
No Worries - A wireless networking system that is
secure, easy to manage, and built to grow with you.
Ease of Installation - Quick, easy setup.
Fast data transfer rates
41. Limitations of Wi-Fi
Data security risks :a huge challenge for Wi-Fi
Interference from other devices : such as
telephones, microwave ovens.
High power consumption :making battery life and
heat a concern .
42. Basic Wi-Fi Security Techniques
WEP(Wired Equivalent Privacy) : The original encryption technique specified
by the IEEE 802.11 standard.
WPA(Wi-Fi Protected Access ): A new standard that provides improved
encryption security over WEP.
WPA2 : is an improved version of WPA that uses Advanced Encryption
Standard (AES) technology.
• Wi-Fi is a simple, cost-effective way to connect to the Internet, without the
need to physically connecting wires.
• Hotspot is a geographic area setup in any public location, and has a readily
accessible wireless network.
• Security is a huge challenge for Wi-Fi Networks, many Security Techniques
are used to improve it.
• Wi-Fi Networks have a several limitations that should be concerned.
• The future of Wi-Fi is very bright. Its growing in
popularity because of decreasing costs and the freedom
it gives to users.