Wi-Fi

Wireless Communications

Sheldon Lou

www.powerpointpresentationon.blogspot.com

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.

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 Notes802.11 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

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

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.

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.

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?

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.

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).

How does CSMA/CA (Carrier Sensing Multiple Access with Collision Avoidance) Work? (p. 189, Example 4.18)

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.

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.Use Barker code (Example 3.16, p. 116).

Complementary code keying (CCK)

Used to increase the data rate to 11 MbpsExample 17, p. 119Sec. 11.3.4, p. 457

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), see p. 454, Fig. 11.5.

Three APs can coexist without any hop collision, that results 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 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 (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 Service Sets, BSS) may overlap to increase capacity. For example, up to 8 users can use VoIP simultaneously through one access point. With two overlapping APs, 16 users can talk simultaneously. But the two APs have to use non-overlapping channels.

Modulation

Gaussian frequency shift keying (GFSK) is used (Sec. 3.6.1, p. 97).

Wi-Fi network services

Distribution and integrationAssociation, re-association, and

disassociationAuthentication and deauthenticationProviding 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 the same 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 integration function is specific to the distribution system used and therefore is not specified by 802.11, except in terms of the services it 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 the registration information to determine which access point to use for any mobile station.

Reassociation

When a mobile station moves between basic service areas within a single extended service area, it must evaluate signal strength and perhaps switch the access point with which it is associated. Reassociations are initiated by mobile stations when signal conditions indicate that 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 the reachability of the mobile station through a different access point.

Disassociation

To terminate an existing association, stations may use the disassociation service. When stations invoke the disassociation service, any mobility data stored in the distribution system is removed. Once disassociation is complete, it is as if the station is no longer attached to the network. Disassociation is a polite task to do during the station shutdown process. The MAC is, however, designed to accommodate stations that leave the network without formally disassociating.

Authetication/deauthentication

Physical security is a major component of a wired LAN security solution. Wired network’s equipment can be locked inside offices. Wireless networks cannot offer the same level of physical security, however, and therefore must depend on additional authentication routines to ensure that users accessing the network are authorized to do so. Authentication is a necessary prerequisite to association because only authenticated users are authorized to use the network. (In practice, though, many access points are configured for "open-system" mode and will authenticate any station.)

Deauthentication terminates an authenticated relationship. Because authentication is needed before network use is authorized, a side effect of deauthentication is termination of any current association.

WiMax

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.

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.

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.

WiMax (Cont’d)

In congested cities, WiMax products could shift traffic to help relieve heavy demand on broadband networks.

WiMax will work with other shorter-range wireless standards, including Wi-Fi, which has taken off as an easy way to provide Internet access throughout a home or business.

Eventually, advocates hope to see the standard evolve into a mobile wireless Internet service similar to cellular data technologies. It may not ever be as wide-area as cellular but will offer higher data rates.

WiMax Protocols

The protocols that govern WiMax have been standardized. They are collectively referred to as 802.16.

Like Wi-Fi = 802.11, WiMax = 802.16.

Overall vision for 802.16 is that carriers (e.g., ISP) would set up base stations connected to a public (wired) network. This is like cellular.

Each base station would support hundreds of fixed subscriber stations. Fixed means that subscriber stations do not move. Plans to expand the standard to include mobile stations is in the working.

More on WiMax

Base stations will use the 802.16 protocols to dynamically allocated uplink/downlink bandwidth to subscriber stations based on their demand.

802.16 has been developed for several frequency bands (various licensed frequencies in 10-66 GHz, also licensed and unlicensed frequencies in 2-11 GHz).

In the unlicensed bands, 802.16 can be used as a backhaul for wi-fi systems or a longer-range alternative, i.e., replacing hotspots with hotzones.

Some Technical Specs on WiMax

The radio technology is based on OFDM.

802.16 standards incorporate use of adaptive antenna arrays, which can be used to create dynamic beams in desired directions.

Standards offer option for a mesh mode network topology.

Mesh Networking in WiMax

When a subscriber unit is not in line of sight with the base station (does not have a good signal strength), then it may be able to make a peer-to-peer connection to a neighbor, i.e., hop to a neighbor’s subscriber unit.

The neighbor’s unit may be in line of sight with the base station, in which case this neighbor would serve as a relay station (a repeater).

If the neighbor’s unit is not in line-of-sight then another hop can be made.

Mesh Mode

Trunk(Wired)Network

Trunk

Residential Business

WiMax Costs

Analysts estimate that subscriber stations for home access will initially cost up to $300.

Base stations will cost as little as $5,000 but will reach $100,000, depending on their range. Each base station may be able to support up to 60 T1 class subscriber lines.

In some cases, consumers would lease subscriber stations from carriers the way they do with cable set-top boxes as part of their service plans.

WiMax Predictions

Base stations will be able to connect to other base stations within a range of up to 30 miles with data transfer speeds of up to 75 megabits per second.

Subscriber stations, the set-top box-like devices, will connect to base stations with ranges of up to three miles and transfer speeds of up to 15 megabits per second.

WiMax T1 class lines may cost 10% of wired T1 costs.

Products will start being available 2005-2006.

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