10 - Wireless LANs

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10 - Wireless LANs

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom


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Agenda

Differences B/W Ethernet LANs & Wireless LANs Radio Frequency Transmission

Organizations that Standardize WLANs

WLAN Standards Comparison

WLANs Topology Building Blocks

Ad hoc Mode

Infrastructure Mode

WLANs Layer 1

Frequency Bands

Frequency Encoding

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Agenda

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Interference Coverage

WLANs Layer 2

CSMA/CA Algorithm

Implementing a WLAN

Wireless LAN Security

WLAN Security Standards

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Differences B/W Ethernet LANs & Wireless LANs


The big difference b/w the two is that WLANs usesradio waves to transmit data while the Ethernet LANs

uses electrical signals or light

WLANs must meet country-specific RF regulations

Ethernet LANs uses the CSMA/CD while WLANs use the

CSMA/CA (Collision Avoidance) algorithm

Collision detection is not possible in WLANs, because a

sending station cannot receive at the same time that ittransmits and, therefore, cannot detect a collision.

Instead, WLANs use the Ready To Send (RTS) and Clear

To Send (CTS) protocols to avoid collisions

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Differences B/W Ethernet LANs & Wireless LANs


Ethernet LANs can support full-duplex (FDX)communications if switch is used while with WLANs

, if more than one device at a time sends radio

waves in the same space and at same frequency,

nether signal is clear so the half-duplex (HDX)mechanism must be used

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Radio Frequency Transmission


Radio frequencies range from the AM radio band tofrequencies used by cell phones

Radio frequencies are radiated into the air by

antennas that create radio waves

When radio waves are propagated through objects,

they might be:

Absorbed e.g. by walls

Scattered e.g. by striking with uneven surfaces

Reflected e.g. by metal or glass surfaces

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Organizations that Standardize WLANs


Regulatory agencies control the use of the RF bands Regulatory agencies include the Federal

Communications Commission (FCC) for the UnitedStates and the European Telecommunications

Standards Institute (ETSI) for Europe The Institute of Electrical and Electronic Engineers

(IEEE) defines standards for specific types of WLANi.e. 802.11

The Wi-Fi Alliance offers certification forinteroperability between vendors of 802.11products

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The IEEE introduced WLAN standards with the creation of the 1997

ratification of the 802.11 standard

The original 802.11 standard has been replaced by more-advanced

standards

Table compares the different WLAN standards

Standard 802.11(1997)

802.11a(1999)

802.11b(1999)

802.11g(2003)

802.11n(2009)

802.11ac(Draft;Nov-11)

Frequency Band 2.4GHz

5 GHz 2.4

GHz

2.4 GHz 2.4/5

GHz

5 GHz

No of Channels 20 23 11 11 20 5

Modulation DSSS,FHSS

OFDM DSSS OFDM,

DSSS

OFDM OFDM

Data Rates inMbps

1, 2 6, 9, 12,

18, 24,

36,48, 54

1, 2,

5.5, 11

6, 9, 12,

18, 24,

36, 48,54

7.2,

14.4,

21.7,28.9,

-

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Table below shows the maximum range limits fordifferent WLANs standards

Range (feet) 802.11(1997)

802.11a(1999)

802.11b

(1999)

802.11g(2003)

802.11n

(2009)

802.11ac(Draft;

Nov-11)

Indoor 66 115 115 125 230 -

Outdoor 330 390 460 460 820 820

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The standard 802.11 defines the followingtopologies:

Ad hoc Mode

Ado hoc is the Independent Basic Service Set (IBSS)

topology i.e mobile clients connect directly without anintermediate access point

Ad hoc mode acts as workgroup, therefore, a drawback

of peer-to-peer networks is that they are difficult to

secure

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Infrastructure Mode In infrastructure mode, clients connect through an

access point

There are two infrastructure modes:

Basic Service Set (BSS) The communication devices that create a BSS are mobile

clients using a single access point to connect to each other or

to wired network resources

The Basic Service Set Identifier (BSSID) is the Layer 2 MAC

address of the BSS access points radio card

AP

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Extended Services Set (ESS)

The wireless topology is extended with two or more BSSs

connected by a distribution system (DS) or a wired

infrastructure

An ESS generally includes a common SSID to allow roaming

from access point to access point without requiring clientconfiguration

AP 1AP 2

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WLANs Layer 1


WLANs transmit data at L1 by sending & receivingradio waves

WLAN radio waves have a repeating signal that canbe graphed over time

The radio wave has the following attributes: Frequency

The number of times the waveform repeats per second,measured in hertz (Hz)

Amplitude

Amplitude is the height of the waveform, representingsignal strength

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WLANs Layer 1


Phase

Phase is the particular point in the repeating waveform

Graph below shows the graph of an 8KHz signal

The FCC or other national regulatory agencies specify

some ranges of frequencies called frequency bands

Frequency band is the range of consecutive frequencies

Amplitude Phase

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WLANs Layer 1


Wider the range of frequencies in a frequencyband, the greater the amount of information thatcan be sent in that frequency band

Frequency Bands

Frequency bands can be categorized in: Licensed Bands The FCC or equivalent agencies in other countries, license some

frequency bands like AM, FM radio & mobile phones

Un-Licensed Bands Un-licensed frequencies can be used without any permission

from the regulatory agency, however; devices that use thesefrequencies must still conform to the rules set up by theregulatory agency

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WLANs Layer 1


FCC defines three un-licensed frequency bands, table below

shows these frequency bands

When WLAN NIC or AP sends data, it can modulate

the radio signals frequency, amplitude & phase toencode 0 or 1

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WLANs Layer 1


Frequency Encoding There are three general classes of encoding:

Frequency Hopping Spread Spectrum (FHSS)

It uses all the frequencies in the band, hopping to different ones

The original 802.11 WLAN standard used FHSS

Direct Sequence Spread Spectrum (DSSS)

It uses one of several separate channels or frequencies

Designed for 2.4 GHz un-licensed band and used with 802.11b

This band has bandwidth of 82 MHz with a range from 2.40 GHzto 2.483 GHz

FCC divides the band into 11 different overlapping DSSS

channels as shown on next slide

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WLANs Layer 1


The three shaded channels do not overlap enough, therefore;

these channels (1,6, and 11) can be used in the same space for

WLAN and they wont interfere with each other

The significance of the non-overlapping channels is that when

you design WLAN with more than one AP (ESS), APs with

overlapping coverage areas should be set to use different non-

overlapping channels; as shown on next slide

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WLANs Layer 1


In the above design all the three BSS can send at the same time

without interference with each other

Each cell is running at a maximum data rate of 11 Mbps, while

at a cumulative bandwidth of 33 Mbps which is called WANs

capacity

BSS 1 BSS 2 BSS 3

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WLANs Layer 1


Orthogonal Frequency Division Multiplexing (OFDM) Like DSSS, WLANs that uses OFDM can use multiple non-overlapping

channels

Its is used by 802.11a, 802.11g & 802.11n

Interference

WLANs can suffer from interference from many sourceslike walls, floors or even from other radio waves in thesame frequency range

The Signal-to-Noise Ration (SNR) calculation measure theWLAN signal as compared to other undesired signal(noise) in the same space

The higher the SNR, the better the WLAN devices cansend data

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WLANs Layer 1


Coverage WLAN coverage area is the space in which two WLAN

devices can successfully send data

Coverage area depends:

Frequency band used by WLAN standard

Obstruction b/w & near WLAN devices

Interference from other RF energy

Encoding technique like DSSS and OFDM Figure on next slide shows the concept of coverage

area with varying speed for 802.11b BSS

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WLANs Layer 1


WLAN standards supports the idea of multiple speeds

A device near the APmay have strong signal,

so it can transmit &

receive data with the AP

at higher rates; while a

device at the edge of the

coverage area , where the

signals are weak, may still

be able to send & receive

data but at a slower speed

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WLANs Layer 2


The following problems occurs with WLAN mediaaccess at L2

If two or more WLAN devices send at the same time,using overlapping frequency ranges, a collision occurs

Also the device that is transmitting data cannotconcurrently listen for received data

To avoid this problems the WLAN is to use theCarrier Sense Multiple Access with Collision

Avoidance (CSMA/CA) algorithm The following list summarizes the key points about

CSMA/CA algorithm:

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WLANs Layer 2


1. Listen to ensure that the medium is not busy i.e no

radio waves currently are being received at the

frequencies to be used

2. Set a random wait timer before sending a frame

3. When random timer passed, listen again to ensurethat the medium is not busy, If isnt, send the frame

4. After the entire frame is sent, wait for an

acknowledgment

5. If no acknowledgment is received, resent the frame,

using the CSMA/CA logic to wait for an appropriate

time to send again

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Implementing a WLAN


The following steps guide the installation of a new

BSS/ESS WLAN Step 1: Verify the existing wired network

Connect a laptop Ethernet NIC to the same Ethernet cablethat will be used for the AP, if the laptop can acquire an IP,mask & other info using DHCP, and can communicate withother hosts, it is ready to accept the AP

Step 2: Install & Configure the APs Wired & Details AP uses the straight-through Ethernet cable to connect to

the LAN switch

APs operate at L2 and dont need an IP address to performtheir main functions, but for management as we used inEthernet switch, APs should also have an IP address

AP needs an IP address, subnet mask, default gateway IPaddress & possibly the IP address of a DNS server

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Implementing a WLAN


The Ethernet switch ports to which the APs to be

attached should be in the same VLAN

The following figure shows the ESS WLAN with all APs in

Ethernet VLAN2

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Implementing a WLAN


Step 3: Configure APs Details

APs can be configured with variety of parameters like:

IEEE Standard; e.g. a, b, g or multiple

Wireless Channels; e.g. 1, 6, and 11

Transmit Power

Service Set Identifier (SSID); 32-bit character for WLAN which

allows for roaming b/w APs, but inside the same WLAN

Step 4: Install & Configure One Wireless Client

The clients WLAN NIC tries to discover all APs bylistening on all frequency channels for the WLAN

standard it supports and select the AP from which the

client receives the strongest signal

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Implementing a WLAN


With Microsoft OS, the WLAN NIC the Zero Configuration

Utility (ZCF) allows PCs to automatically discover the SSIDs of

all WLANs

Also some WLAN NIC manufactures provide software that

can control WLAN NIC instead of OS

Step 5: verify WLAN Works from the Client

If it does not work, perform the site survey as:

Is the AP at the center of the area?

Is the AP or client right next to a lot of metal?

Is the AP or client area source of interference e.g. oven etc

It can be done with laptop, using WLAN NICs tools (most

WLAN NIC software shows signal strength & quality), walk

around while looking at signal quality measurement

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Implementing a WLAN


Beside site survey you may also want to check the

following:

Check to make sure that the NIC & APs radio waves are enabled

Check the AP to make sure that it has latest firmware

Check AP configuration, particularly the channel configuration

to ensure that it does not use a overlapping channel

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WLANs introduce a number of vulnerabilities that donnot exist for wired Ethernet LANs

Following are the several categories of threats:

War Drivers

This type of attacker often just wants to gain Internet accessfor free

The attacker drives around, trying to find APs that have nosecurity or weak security

Hackers

The motivation for hackers is to either find information ordeny services

The end goal of hacker is to enter the wired network usingthe wireless network without having to go through Internetconnections that have firewalls

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Employees

Employees can help hackers gain access to the

Enterprise network

Rogue AP

The attacker captures the packets in WLAN, finding SSID& cracking security keys (if they are used)

Then the attacker can set up her own AP, with the same

setting and get the Enterprises clients to use it

To reduce the risk off such attacks, three main typesof tools can be used:

Manual Authentication b/w the Client & AP

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Encryption; uses a secret key & a mathematical

formula to scramble the contents of the WLAN frame

Intrusion Detection Systems (IDS), Intrusion

Prevention Systems (IPS), and WLAN-specific tools

like Ciscos Structured Wireless-Aware Network(SWAN)

Table lists key vulnerabilities along with the solution

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Table lists the four major WLAN security standards

Wired Equivalent Privacy (WEP)

WEP provided weak authentication & encryption

The main problems with WEP were as: Static Pre-Shared Keys (PSK)

64-Bits keys that can be easily cracked

WEP should not be used today

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Due to these problems, many vendors included acouple of features that are not part of WEP

SSID Cloaking An AP feature that tells the AP to stop sending periodic Beacon

frames

Beacon frames lists the APs SSID & other configurationinformation

MAC Filtering AP can be configured with a list of allowed WLAN MAC addresses

Cisco Interim Solution b/w WEP & 802.11i

Because of the problems with WEP, vendors such asCisco, and the Wi-Fi Alliance industry association,looked to solve the problem with their own standards

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The main features of Cisco enhancements includedthe following:

Dynamic key exchange (instead of static pre-sharedkeys)

User authentication using 802.1x; instead of

authenticating the device by checking to see if thedevice knows a correct key, the user must supply ausername and password

A new encryption key for each packet

Wi-Fi Protected Access (WPA) After Cisco integrated its proprietary WLAN security

standards into Cisco APs, the Wi-Fi Alliance created amultivendor WLAN security standard WPA

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WPA essentially performed the same functions as the

Cisco proprietary interim solution, but with differentdetails:

Use dynamic key exchange, using the Temporal Key IntegrityProtocol (TKIP)

Use of either IEEE 802.1X user authentication or simpledevice authentication using pre-shared keys

IEEE 802.11i (WPA-2)

IEEE ratified the 802.11i standard in 2005

Like Cisco-proprietary solution & the Wi-Fi Alliances WPA802.11i uses:

Dynamic key exchange

Stronger encryption

User Authentication

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However; the details are different and 802.11i is not

backwardcompatible

It uses the Advance Encryption Standard (AES)

Wi-Fi Alliance calls 802.11i WPA2, meaning second

version of WPA Table summarizes the key features of various WLAN

security standards

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10 - Wireless LANs

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