WEP security wireless

WEP

802.11 Wired Equivalent Privacy uses a accepted key to encrypt and break abstracts anatomy contents

between 802.11 stations at Layer 2. 802.11, however, does not ascertain a apparatus for distributing

WEP keys to the stations. This requires the ambassador or users to manually configure their radio

cards with the encryption key, and it is not applied to change the key. Thus, WEP keys abide the

same on best wireless LANs for months or years. This allows abundant time for a hacker to exploit

the vulnerabilities of WEP and able the encryption.

WEP has been a ambition for hackers. In fact, accoutrement advisedly accessible from the Internet, such as

WEPCrack and Airsnort, are able to able the WEP encryption mechanism. As a result, WEP is

not able abundant for action security.

If you plan to use WEP, configure the encryption key in the admission point. Each radio agenda needs to

be set to the aforementioned WEP key afore it can accessory with the admission point. When application Windows

XP, you are prompted to admission the WEP key if you try to affix to an admission point that implements

WEP. Windows displays this WEP key in the wireless networks account afterwards you accept View

Available Wireless Networks.

Wireless Average Access

Wireless Average Access

Before transmitting frames, a base charge aboriginal accretion admission to the medium, which is a radio channel

that stations share. The 802.11 accepted defines two forms of average access:

■ Broadcast allocation action (DCF)

■ Point allocation action (PCF)

DCF is binding and based on the carrier faculty assorted admission with blow avoidance

(CSMA/CA) protocol. With DCF, 802.11 stations argue for admission and attack to accelerate frames

when there is no added base transmitting. If accession base is sending a frame, stations are polite

and adjournment until the approach is free.

The afterward are capacity on how DCF works:

1. As a action of accessing the medium, the MAC band checks the bulk of its network

allocation agent (NAV), which is a adverse citizen at anniversary base that represents the amount

of time that the antecedent anatomy needs to accelerate its frame. The NAV charge be aught afore a station

can attack to accelerate a frame. Prior to transmitting a frame, a base calculates the bulk of

time all-important to accelerate the anatomy based on the frame’s breadth and abstracts rate. The base places

a bulk apery this time in the Duration acreage in the attack of the frame. Back stations

receive the frame, they appraise this Duration acreage bulk and use it as the base for ambience their

corresponding NAVs. This action affluence the average for the sending station.

2. An important aspect of the DCF is a accidental Back-off timer that a base uses if it detects a

busy medium. If the approach is in use, the base charge adjournment a accidental aeon of time before

attempting to admission the average again. This ensures that assorted stations that appetite to send

data do not address at the aforementioned time. The accidental adjournment causes stations to adjournment different

periods of time, which avoids the bearings in which all the stations faculty the average at

exactly the aforementioned time, acquisition the approach idle, transmit, and bang with anniversary other. The Backoff

timer decidedly reduces the cardinal of collisions and agnate retransmissions,

especially back the cardinal of alive users increases.

3. With radio-based LANs, a transmitting base cannot accept for collisions while sending data,

mainly because the base cannot accept its receiver on while transmitting the frame. As a

result, the accepting base needs to accelerate an accepting (ACK) if it detects no errors

RTS/CTS Fragmentation

Subset of Abstracts Frames Frames beneath a statically defined

length

Frames beneath a statically authentic length

Overhead Created RTS/CTS action frames Accession of breach headers

Table G-3 Improving Throughput Using RTS/CTS and Breach (Continued)

810 Appendix G: IEEE 802.11 Fundamentals

in the accustomed frame. If the sending base does not accept an ACK afterwards a defined period

of time, it assumes that there was a blow (or RF interference) and retransmits the frame.

To abutment time-bounded commitment of abstracts frames, the 802.11 accepted defines the alternative point

coordination action (PCF), which enables the admission point to admission admission to an alone station

to the average by polling the base during the contention-free period. Stations cannot transmit

frames unless the admission point acclamation them first. The aeon of time for PCF-based abstracts cartage (if

enabled) occurs alternately amid altercation (distributed allocation action [DCF]) periods.

The admission point acclamation stations according to a polling list, and again switches to a altercation period

when stations use DCF. This action enables abutment for both ancillary (for example, video

applications) and asynchronous (for example, e-mail and web-browsing applications) modes of

operation. No accepted wireless NICs or admission credibility on the bazaar today, however, apparatus PCF.

Without able affection of account (QoS), the absolute adaptation of the 802.11 accepted does not

optimize the manual of articulation and video. Currently, no able apparatus exists to

prioritize cartage aural 802.11. As a result, the 802.11e assignment accumulation is currently adorning the

802.11 MAC band to advance QoS for bigger abutment of audio and video (such as MPEG-2)

applications. The 802.11e accumulation should agree the accepted by 2006.

Because 802.11e avalanche aural the MAC layer, it will be accepted to all 802.11 concrete layers and

will be astern accordant with absolute 802.11 wireless LANs. As a result, the abridgement of 802.11e

in abode today does not appulse your accommodation on which concrete band to use. In addition, you

should be able to advancement your absolute 802.11 admission credibility to accede with 802.11e through

relatively simple firmware upgrades already they are available. In the meantime, some wireless LAN

vendors action proprietary QoS mechanisms that advance achievement back there is a mix of

applications with differing QoS needs present on the network, such as articulation and data.

RTS/CTS and Fragmentation Summary

RTS/CTS and Fragmentation Summary
Table G-3 summarizes some of the benefits and drawbacks of both RTS/CTS and fragmentation
when trying to improve throughput in a wireless network.
Table G-3 Improving Throughput Using RTS/CTS and Fragmentation
RTS/CTS Fragmentation
Method to Improve
Throughput
Reduce collisions Reduce percentage of frames with
transmission errors
How to Enable Configuration on AP Configuration on AP
Subset of Data Frames Frames under a statically defined
length
Frames under a statically defined length
Overhead Created RTS/CTS process frames Addition of fragmentation headers

Fragmentation

Fragmentation

A radio agenda or admission point can be set to optionally use fragmentation, which divides 802.11 data

frames into abate pieces (fragments) that are beatific alone to the destination. Anniversary fragment

consists of a MAC band header, anatomy analysis arrangement (FCS), and a fragment cardinal indicating

its ordered position aural the frame. Because the antecedent base transmits anniversary fragment

independently, the accepting base replies with a abstracted accepting for anniversary fragment.

An 802.11 base applies breach alone to frames accepting a unicast destination address. This

includes any abstracts anatomy directed against a specific station. To abbreviate aerial on the network,

802.11 does not fragment advertisement and multicast frames.

The destination base reassembles the bits into the aboriginal anatomy application fragment numbers.

After ensuring that the anatomy is complete, the base easily the anatomy up to college layers for

processing. Even admitting breach involves added overhead, its use can aftereffect in better

performance if you tune it properly.

Fragmentation can admission the believability of anatomy transmissions back cogent RF arrest is

present. Back transmitting abate frames, collisions are beneath acceptable to occur. In addition, frames that

do appointment errors can be retransmitted faster because they are smaller. The fragment admeasurement amount can

typically be set amid 256 and 2048 bytes, although this amount is user-configurable. In fact, you

activate breach by ambience a accurate anatomy admeasurement beginning (in bytes). If the anatomy that the access

point is transmitting is beyond than the beginning (similar to RTS/CTS), it will activate fragmentation. If

the packet admeasurement is according to or beneath than the threshold, the admission point will not use fragmentation.

Consider enabling breach on radio cards and admission points. As with configuring RTS/CTS,

first set the breach beginning to 750 bytes. If throughput increases, again breach is

beneficial. If throughput decreases, again breach apparently won’t be worthwhile. Because of

the added aerial appropriate for anatomy headers on anniversary fragment, the abridgement in frame

retransmissions ability not be abundant to annul the added aerial necessary.

RTS/CTS

RTS/CTS

The 802.11 accepted defines request-to-send/clear-to-send (RTS/CTS) as an alternative action of

802.11 to adapt the manual of abstracts on the wireless LAN. RTS/CTS can be set in the access

point or a radio agenda individually, or on both accessories at the aforementioned time. In best cases, the RTS/CTS

function is accessible in antidotal collisions amid hidden nodes. To accretion admission to the shared

wireless medium, a base can alone address if no added base is transmitting. Aural a particular

access point radio cell, it is accessible that two stations associated with the aforementioned admission point might

be out of ambit of anniversary other.

For example, Figure G-8 illustrates the book in which either Base A is far abroad from Station

B or a barrier is blocking the radio signals amid the two stations. The botheration is that Station

A ability be in the average of transmitting a anatomy to the admission point back Base B wants to send

a frame. Base B will accept to the average to actuate whether accession base is already

transmitting. Because Base B cannot apprehend Base A, Base B starts transmitting the frame. A

collision again occurs at the admission point, which destroys both frames. As a result, the admission point

will not accede accession of the frames. Both stations will accept to retransmit their respective

frames, which will acceptable aftereffect in accession collision.

Figure G-8 Hidden-Node Botheration in Wireless LANs

RTS

CTS

Data

Barrier

Station A

Station B

Access Point

CTS

2

2

1

3

Wireless Configuration Parameters 807

The RTS/CTS action is a handshaking action that minimizes the accident of collisions when

hidden nodes are operating on the network. In addition, aegis mechanisms can use RTS/CTS

to abstain collisions amid 802.11b and 802.11g radio cards. If hidden nodes are not causing

significant retransmissions or hidden nodes are not present, again RTS/CTS is about not

necessary (unless aegis mechanisms are in use).

RTS/CTS works by enabling anniversary base to absolutely appeal a time aperture for abstracts transmission.

For example, if RTS/CTS is enabled in the radio cards of Base A and Base B, again Station

A will aboriginal accelerate an RTS anatomy to the admission point afore attempting to address a abstracts frame.

The admission point receives the RTS anatomy and responds with a CTS frame. Both stations receive

the CTS frame. This gives approval for Base A to address a abstracts frame. The CTS frame

carries a continuance bulk that informs all added stations, including Base B, to not transmit

during the defined time interval. This adjournment equals the bulk of time that Base A requires

to accelerate the abstracts anatomy and for the admission point to acknowledge with an acknowledgement. As a result,

collisions due to hidden nodes are abundant beneath likely. The alone time it is accessible with RTS/CTS

enabled is back hidden nodes additionally absence CTS frames from the admission point because of

interference or anemic signals.

Enabling RTS/CTS requires ambience RTS/CTS to a specific beginning bulk in the admission point or

radio card. The beginning is the anatomy breadth that invokes the RTS/CTS process. If the abstracts frame

that needs manual is beyond than the threshold, again the base implementing RTS/CTS uses

the RTS/CTS process.

If you doubtable hidden nodes ability be causing collisions on the arrangement and aspersing network

performance, accede activating RTS/CTS. It is best to aboriginal set the beginning to approximately

750 bytes, which is almost the center point (50 percent of the best anatomy size), and

assess changes in throughput. If throughput increases, again the use of RTS/CTS is beneficial. If

throughput decreases, again RTS/CTS acceptable is not worthwhile. If RTS/CTS is in actuality improving

performance, accede tweaking the beginning bulk to aerate throughput.

Keep in apperception that the use of RTS/CTS adds aerial to the arrangement because the transmission

of abstracts frames ability crave RTS and CTS frames as well. As a result, the abbreviation of

collisions by application RTS/CTS ability not action abundant advance in throughput to compensate

for the added aerial of RTS/CTS frames. The ultimate ambition of application RTS/CTS is to

improve throughput. If RTS/CTS causes throughput to abatement due to the added overhead,

then do not use it. Alike if the use of RTS/CTS improves performance, changes aural the

facility and users that are adrift to added areas will change the hidden-node situations. Thus,

RTS/CTS ability not be necessary, and can alike account abasement in throughput as conditions

change. 802.11 does not action adaptive mechanisms that automatically acclimatize RTS/CTS

thresholds.