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Tuesday, January 3, 2012

PowerPoint Presentation On WI-FI

WI-FI Technology PPT


1. Wi-Fi Wireless Communications
Sheldon Lou

2. What is Wi-Fi?
The standard for wireless local area networks (WLANs). It’s like a common language that all the devices use to communicate to each other. If you have a standard, people can make all sorts of devices that can work with each other.
It’s actually IEEE 802.11, a family of standards. The IEEE (Eye-triple-E, Institute of Electrical and Electronics Engineers Inc.) is a non-profit, technical professional association of more than 360,000 individual members in approximately 175 countries. The Wireless Ethernet Compatibility Alliance started the Wi-Fi--wireless fidelity--certification program to ensure that equipment claiming 802.11 compliance was genuinely interoperable.

3. US Frequency Bands
Band Frequency range
UHF ISM 902-928 MHz
S-Band 2-4 GHz
S-Band ISM 2.4-2.5 GHz
C-Band 4-8 GHz
C-Band satellite downlink 3.7-4.2 GHz
C-Band Radar (weather) 5.25-5.925 GHz
C-Band ISM 5.725-5.875 GHz
C-Band satellite uplink 5.925-6.425 GHz
X-Band 8-12 GHz
X-Band Radar (police/weather) 8.5-10.55 GHz

4. Standard Speed Freq band Notes
802.11 2 Mbps 2.4 GHz (1997) 802.11a 54 Mbps 5 GHz (1999) 802.11b 11 Mbps 2.4 GHz 802.11g 54 Mbps 2.4 GHz

5. ISM Band
ISM stands for industrial, scientific, and medical. ISM bands are set aside for equipment that is related to industrial or scientific processes or is used by medical equipment. Perhaps the most familiar ISM-band device is the microwave oven, which operates in the 2.4-GHz ISM band. The ISM bands are license-free, provided that devices are low-power. You don't need a license to set up and operate a wireless network.

6. WLAN Architecture—Ad Hoc Mode
Ad-Hoc mode: Peer-to-peer setup where clients can connect to each other directly. Generally not used for business networks.

7. Ad Hoc Structure
Mobile stations communicate to each other directly.
It’s set up for a special purpose and for a short period of time. For example, the participants of a meeting in a conference room may create an ad hoc network at the beginning of the meeting and dissolve it when the meeting ends.

8. WLAN Architecture--Mesh
Mesh: Every client in the network also acts as an access or relay point, creating a “self-healing” and (in theory) infinitely extensible network.
Not yet in widespread use, unlikely to be in homes.

9. Infrastructure network
There is an Access Point (AP), which becomes the hub of a “star topology.”
Any communication has to go through AP. If a Mobile Station (MS), like a computer, a PDA, or a phone, wants to communicate with another MS, it needs to send the information to AP first, then AP sends it to the destination MS
Multiple APs can be connected together and handle a large number of clients.
Used by the majority of WLANs in homes and businesses.

10. Roaming
In an extended service area, a mobile station (MS) can roam from one BSS (Basic Service Set) to another.
Roughly speaking, the MS keeps checking the beacon signal sent by each AP and select the strongest one and connect to that AP.
If the BSSs overlap, the connection will not be interrupted when an MS moves from one set to another. If not, the service will be interrupted.
Two BSSs coverage areas can largely overlap to increase the capacity for a particular area. If so, the two access points will use different channels, as we will explain later.

11. Antennas
All WLAN equipment comes with a built-in omni-directional antenna, but some select products will let you attach secondary antennas that will significantly boost range.

12. Antennas, continued
Antennas come in all shapes and styles:
Vertical Whip
Ceiling mount
Yagi (“Pringles can”)
Wall mounted panel
Parabolic dish

13. How Can Several Users Communicate Simultaneously?
As we have discussed, there is a difference between a network designed for voice conversation and one for data exchange.
For voice conversations, like telephone and cell phone calls, each person has a dedicated channel during the entire conversation. (3G and 4G cell phones are somewhat different, as we will explain later.)
For data exchange, many users can share one channel. A user sends information when no one else is sending.
New technologies try to accommodate both voice and data transmissions, as we will discuss in this course.

14. Share one channel in data communication
In data communication, data are grouped into packets/frames. Each packet/frame contains a number of bits of information.
Devices (phones, computers, etc.) don’t communicate simultaneously. It’s like they are sharing one single cable (the air in this case), only one person can use it at one time.
Before an MS (mobile station) sends its packets, it checks to see if someone else is sending information. Only when the medium is free can an MS sends packets.
If some station is sending or receiving signal, the MS that intends to send will generate a random waiting time and wait for its turn. If several MSs are all waiting for their turns, since their waiting times are randomly generated and thus not equal, they will not start sending simultaneously. Thus collision (two or more MSs sending signals simultaneously) is avoided.
It’s called Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA).

15. RTS/CTS (Request-to-send/clear-to-send)
Use Request-to-send/clear-to-send (RTS/CTS) mechanism (p. 191-192, Fig. 4.17 & p. 462, Fig. 11.14) to avoid collision when two MSs cannot hear each other (blocked by a wall …).
A terminal ready for transmission sends an RTS packet identifying the source address, destination address, and the length of the data to be sent.
The destination station responds with CTS packet.
The source terminal receives the CTS and sends the data.
Other terminals go to the virtual carrier-sensing mode (NAV signal on), therefore the source terminal sends its packet with no contention.
After completion of the transmission, the destination station sends an ACK, opening contention for other users.

16. WiMax

17. What is WiMax?
WiMax is a radio technology that promises to deliver two-way Internet access at speeds of up to 75 Mbps at long range.

Its backers claim that WiMax can transmit data up to 30 miles between broadcast towers and can blanket areas more than a mile in radius with bandwidth that exceeds current DSL and cable broadband capabilities.

So, some believe that it could slash the cost of bringing broadband to remote areas.

18. WiMax (Cont’d)
WiMax, short for Worldwide Interoperability for Microwave Access, is the latest of the wireless "last mile" broadband technologies.

ISP see WiMax as a means of connecting rural or remote areas with broadband service, something that would be technically, physically or economically difficult to do by burying wire for DSL or cable connections.

Laying wires is especially difficult in hilly areas like Susquehanna.

19. Benefits over Satellite
In rural areas, the real competition to WiMax would be satellite data services.

The benefit that WiMax offers over satellite is that satellite offers limited uplink bandwidth (upload data rates are not as high as download data rates).

Further, satellite suffers with high latency.

20. Thanks

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