04What is Wi-Fi

7/28/2019 04What is Wi-Fi

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Wi-Fi

Wireless Communications

Sheldon Lou



What is Wi-Fi?

The standard for wireless local area networks(WLANs). It’s like a common language that allthe devices use to communicate to each other.If you have a standard, people can make allsorts 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 ElectronicsEngineers Inc.) is a non-profit, technical professional association of

more than 360,000 individual members in approximately 175countries. The Wireless Ethernet Compatibility Alliance started theWi-Fi--wireless fidelity--certification program to ensure that

equipment claiming 802.11 compliance was genuinely interoperable.



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



Wi-Fi Standards

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

802.11n 600 Mbps 2.4/5 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 mostfamiliar 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 licenseto set up and operate a wireless network.



U-NII (Unlicensed National

Information Infrastructure) Bands

802.11n can operate at the 5G U-NII bands:

U-NII Low (U-NII-1): 5.15-5.25 GHz.

U-NII Mid (U-NII-2): 5.25-5.35 GHz.

U-NII Worldwide: 5.47-5.725 GHz.

U-NII Upper (U-NII-3): 5.725 to 5.825GHz.

http://en.wikipedia.org/wiki/U-NII%23cite_note-cisco1-2


http://en.wikipedia.org/wiki/U-NII%23cite_note-autogenerated1-3



http://en.wikipedia.org/wiki/U-NII%23cite_note-autogenerated1-3





U-NII bands



Wireless LAN Networks



WLAN Architecture— Ad Hoc Mode

Ad-Hoc mode: Peer-to-peer setup whereclients 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 itwhen 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 widespreaduse, unlikely to be in

homes.



WLAN Architecture—Infrastructure Mode

To Wired Network



Infrastructure network

There is an Access Point (AP), which becomes thehub of a “star topology.”

Any communication has to go through AP. If aMobile Station (MS), like a computer, a PDA, or a

phone, wants to communicate with another MS, itneeds to send the information to AP first, then APsends 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 XControl X

Routing X

Coverage XReliability X



Extended Service Area



Roaming

In an extended service area, a mobile station (MS) canroam from one BSS (Basic Service Set) to another.

Roughly speaking, the MS keeps checking the beaconsignal 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 toincrease the capacity for a particular area. If so, the twoaccess points will use different channels, as we willexplain later.



Antennas

All WLAN equipment comes with a built-inomni-directional antenna, but some select

products will let you attach secondary

antennas that will significantly boostrange.



Antennas, continued

Antennas come in all

shapes and styles:

Omni-directional:

Vertical WhipCeiling mount

Directional:

Yagi (“Pringles can”)

Wall mounted panel

Parabolic dish



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 phonecalls, 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.



Share one channel in data

communication

In data communication, data are grouped into packets/frames. Eachpacket/frame contains a number of bits of information.

Devices (phones, computers, etc.) don’t communicatesimultaneously. It’s like they are sharing one single cable (the air inthis 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 freecan an MS sends packets.

If some station is sending or receiving signal, the MS that intends tosend will generate a random waiting time and wait for its turn. If several MSs are all waiting for their turns, since their waiting timesare randomly generated and thus not equal, they will not start

sending simultaneously. Thus collision (two or more MSs sendingsignals 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 packetidentifying the source address, destination address, andthe length of the data to be sent.

The destination station responds with CTS packet.

The source terminal receives the CTS and sends thedata.

Other terminals go to the virtual carrier-sensing mode(NAV signal on), therefore the source terminal sends itspacket with no contention.

After completion of the transmission, the destinationstation 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 802.11b

Symbol transmission rate = 1Mbps

Multipath spread of up to 1/1 Mbps = 1 µs does

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

Chip rate = 11 Mcps

Resolution is on the order of 1/11 Mcps = 90 ns.Use Barker code.



Frequency Hopping in 802.11

The frequency can hop over 78 hopping channels eachseparated 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), see p.454, Fig. 11.5 .

Three APs can coexist without any hop collision, thatresults in a threefold increase in the capacity of the cell.

Hop rate = 2.5 hops per second.



Frequency bands for DSSS

FHSS uses 1 MHz bandwidth (narrowband), but thecenter frequency hops over 76 MHz. DSSS uses a chiprate 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 (see Fig. 11.6, P. 455). APs located close to each other can choose different

channels to mitigate interference.

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

example, up to 8 users can use VoIP simultaneouslythrough one access point. With two overlapping APs, 16users can talk simultaneously. But the two APs have touse non-overlapping channels.











Modulation

Gaussian frequency shift keying (GFSK) is

used.



Wi-Fi network services

Distribution and integration

Association, re-association, and

disassociation

Authentication and deauthentication

Providing privacy



Distribution

This service is used by mobile stations in an

infrastructure network every time they send data.

Once a frame has been accepted by an access

point, it uses the distribution service to deliver the frame to its destination. Any communication

that uses an access point travels through the

distribution service, including communications

between two mobile stations associated with thesame access point.



Integration

Integration is a service provided by the

distribution system; it allows the

connection of the distribution system to a

non-IEEE 802.11 network. The integrationfunction is specific to the distribution

system used and therefore is not specified

by 802.11, except in terms of the servicesit must offer.



Association

Delivery of frames to mobile stations is

made possible because mobile stations

register, or associate, with access points.

The distribution system can then use theregistration information to determine which

access point to use for any mobile station.



Reassociation

When a mobile station moves between basicservice areas within a single extended servicearea, it must evaluate signal strength andperhaps switch the access point with which it is

associated. Reassociations are initiated bymobile stations when signal conditions indicatethat a different association would be beneficial;they are never initiated by the access point. After the reassociation is complete, the distribution

system updates its location records to reflect thereachability of the mobile station through adifferent access point.



Disassociation

To terminate an existing association, stationsmay use the disassociation service. Whenstations invoke the disassociation service, anymobility data stored in the distribution system is

removed. Once disassociation is complete, it isas if the station is no longer attached to thenetwork. Disassociation is a polite task to doduring the station shutdown process. The MAC

is, however, designed to accommodate stationsthat leave the network without formallydisassociating.



Authetication/deauthentication

Physical security is a major component of a wired LANsecurity solution. Wired network’s equipment can belocked inside offices. Wireless networks cannot offer thesame level of physical security, however, and thereforemust depend on additional authentication routines to

ensure that users accessing the network are authorizedto do so. Authentication is a necessary prerequisite toassociation because only authenticated users areauthorized to use the network. (In practice, though, manyaccess points are configured for "open-system" modeand will authenticate any station.)

Deauthentication terminates an authenticatedrelationship. Because authentication is needed beforenetwork use is authorized, a side effect of deauthentication is termination of any currentassociation.

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04What is Wi-Fi