Wi fi[1]

Wi-Fi

Presented By:

Manish Srivastava

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.

US Frequency Bands

Band Frequency rangeUHF ISM 902-928 MHzS-Band 2-4 GHzS-Band ISM 2.4-2.5 GHzC-Band 4-8 GHzC-Band satellite downlink 3.7-4.2 GHzC-Band Radar (weather) 5.25-5.925 GHzC-Band ISM 5.725-5.875 GHzC-Band satellite uplink 5.925-6.425 GHzX-Band 8-12 GHzX-Band Radar (police/weather) 8.5-10.55 GHz

Wi-Fi Standards

Standard Speed Freq band 802.11 2 Mbps 2.4 GHz

802.11a 54 Mbps 5 GHz 802.11b 11 Mbps 2.4 GHz 802.11g 54 Mbps 2.4 GHz

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.

Wireless LAN Networks

Wi-Fi network services

Distribution and integrationAssociation, re-association, and

disassociationAuthentication and deauthenticationProviding privacy

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.

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.

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.

WLAN Architecture—Infrastructure Mode

To Wired Network

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.

Comparison of Two Structures

Infrastructure Ad hoc

Expansion X

Flexibility X

Control X

Routing X

Coverage X

Reliability X

Extended Service Area

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.

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.

Antennas, continued

Antennas come in all shapes and styles:Omni-directional:

Vertical WhipCeiling mount

Directional:Yagi (“Pringles can”) Wall mounted panelParabolic dish

How Can Several Users Communicate Simultaneously?

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.

For data exchange, many users can share one channel. A user sends information when no one else is sending.

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.

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).

RTS/CTS (Request-to-send/clear-to-send)Use Request-to-send/clear-to-send (RTS/CTS) mechanism

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.

After completion of the transmission, the destination station sends an ACK, opening contention for other users.

Spread spectrum in 802.11

It is a requirement imposed by the regulatory authorities for devices in ISM band in order to reduce interference.

There is also limitations on transmitted power.

We discuss two methods specified in 802.11, FHSS and DSSS.

DSSS in 802.11

Used by 802.11bSymbol transmission rate = 1MbpsMultipath spread of up to 1/1 Mbps = 1 µs does

not cause ISI. For indoor applications this ensures that the system does not suffer from ISI.

Chip rate = 11 McpsResolution is on the order of 1/11 Mcps = 90 ns.

Frequency Hopping in 802.11

The frequency can hop over 78 hopping channels each separated by 1 MHz. The first channel, Channel 0, starts at 2.402 GHz. Channel 1 is at 2.403 GHz, Channel 2, 2.404 GHz, and so on up to Channel 77 at 2.479 GHz (US, Canada, and Europe standards).

These frequencies are divided into three patterns of 26 hops each corresponding channel numbers (0, 3, 6, 9, …, 75), (1, 4, 7, 10, …, 76), (2, 5, 8, 11, …, 77)

Hop rate = 2.5 hops per second.

Frequency bands for DSSS

FHSS uses 1 MHz bandwidth (narrowband), but the center frequency hops over 76 MHz. DSSS uses a chip rate of 11 Mcps which occupies around 26 MHz of bandwidth (wideband).

The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced by 5 MHz

APs located close to each other can choose different channels to mitigate interference.

The coverage areas of two access points (Basic Service Sets, BSS) may overlap to increase capacity.

Thank you.