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56 Cards in this Set
- Front
- Back
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Who defines standards for Wireless LANs?
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Standards are defined by the IEEE.
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What is the name of a device that allows hosts to access a WLAN?
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Access Point (AP).
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What media do wireless LANs use to communicate data?
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WLANs use radiate energy waves, generally called radio waves to transmit data.
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WLANs use full duplex communication.
(True or false) |
False. WLANs can only use half duplex (HDX) mechanisms. If more than one device at a time sends radio waves in the same space at the same frequency, neither signal is intelligible. So only half duplex can be used.
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What type of carrier access do WLANs use?
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WLANs use the carrier sense multiple access with collision avoidance (CSMA/CA) algorithms to enforce HDX logic and avoid as many collisions as possible.
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802.11a
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WLAN standard designed in 1999. It operates at the 5 GHz band, with a maximum speed of 54 Mbps using OFDM. It has 23 overlapping channels.
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802.11b
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WLAN standard designed in 1999. It operates in 2.4GHz band, with a maximum speed of 11 Mbps using DSSS. It has 3 overlapping channels.
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802.11g
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WLAN standard designed in 2003. It operates in the 2.4 GHs band. It has a maximum speed of 11 Mbps using DSSS, and 54 Mbps using OFDM. It has 3 overlapping channels.
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802.11n
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WLAN standard passed in 2009. It operates in both the 2.4 and 5 GHz bands. It has a maximum speed of 150 Mbps. It has 9 overlapping channels.
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Ad Hoc Mode
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This mode is used when a wireless device only wants to communicate with only one of a few other devices directly, usually for a short period of time. In these cases, the device send WLAN frames directly to each other.
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Basic Service Set (BSS)
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Basic Service Set (BSS) uses a single AP to create the wireless LAN.
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Extended Service Set (ESS)
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Extended Service Set (ESS) uses more than one AP, often with overlapping calls to allow roaming in a larger area.
The ESS WLANs allow roaming, which means that users can move around inside the coverage area and stay connected to the same WLAN. When the device finds that the current AP is getting weaker, it finds a new, better AP with a stronger or better signal, and starts using that AP. |
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Independent Basic Service Set (IBSS)
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The IBSS is another name for an ad hoc network. It allows two devices to communicate directly, with no need for an AP.
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Wireless antenna
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WLAN devices use a radio and its antenna to send and receive the radio waves, making small changes to the waves to encode data.
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Wireless Encoding definition
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Wireless antennas make changes to the the frequency, amplitude, and phase of the of the waveform. These changes allow the devices to transmit data.
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Waveform
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A waveform is a graph of the wireless wave. WLAN radio waves have a repeating signal, allowing wireless transmissions to be graphed over time.
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Frequency
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The frequency is the number of times the waveform repeats per second. This is measured in hertz (Hz). This is the most important factor in discussions of WLANs.
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Amplitude
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Amplitude is the height of the waveform, representing signal strength.
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Phase
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Phase represents the particular point in the repeating waveform.
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Who regulates wireless frequencies in the US.
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Wireless frequencies are regulated by the Federal Communication Commission (FCC).
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Frequency Band
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A frequency band is actually a range of consecutive frequencies.
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Frequency Hopping Spread Spectrum (FHSS)
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Frequency Hopping Spread Spectrum (FHSS) uses all frequencies in a band, hopping to different ones. By using slightly different frequencies for consecutive transmissions, a device can hopefully avoid interference from other devices that use the same band, succeeding at sending data at some frequencies.
FHSS was used by the original 802.11 WAN standards, but current standards do not. |
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Direct Sequence Spread Spectrum (DSSS)
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Direct Sequence Spread Spectrum (DSSS) uses one of several separate channels or frequencies. This band has a bandwidth of 82 MHs, with a rage from 2.402 GHz to 2.482 GHz. This band can have 11 different overlapping DSSS channels.
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Orthogonal Frequency Division Multiplexing (OFDM)
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A wireless encoding method that can use multiple non overlapping channels.
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What wireless standards use FHSS?
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802.11 (original)
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What wireless standards use DSSS?
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802.11b, 802.11g
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What wireless standards use OFDM?
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802.11a, 802.11g, and 802.11n
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Signal to Noise Ratio (SNR)
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Signal to Noise Ration (SNR) is one key measure of interference in a WLAN. It measures the WLAN signal as compared to the other undesired signals (noise) in the same space. The higher the SNR, the better the WLAN devices can send data successfully.
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Effective Isotropic Radiated Power (EIRP) calculation
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The Effective Isotropic Power Power (EIPR) measured the power of an AP. This is the radio's power output, plus the increase in power caused by the antenna, minus any power lost in the cabling.
In effect, it's the power of the signal as it leaves the antenna. |
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Wireless signals are transmitted at the same speed, regardless of the strength of the signal.
(true or false) |
False. Weaker wireless signals cannot pass data at higher speeds, but they can pass data at lower speeds. For example, a device at the edge of the coverage area, might have a weak signal, but still be able to send and receive data, although at a slower speed.
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To double the coverage area of an AP, you need to double the original gain.
(true or false) |
False. In order to double the coverage area, you would need to quadruple the original gain.
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Generally speaking, WLAN standards that use higher frequencies have larger coverage areas and can send data faster.
(true or false) |
False. WLANs that use higher frequencies can send data faster, but generally have smaller coverage areas.
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How does the number of overlapping channels affect available bandwidth?
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The maximum data speed determines the maximum amount of data that can be transmitted. However, this is for each channel. So a device that operates on non-overlapping channels can have a higher effective bandwidth.
For example, a device that has a maximum speed of 54 Mbps with three overlapping channels would be able to communicate with three devices at 54 Mbps on each channel. This would mean an effective bandwidth of 162 Mbps. |
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How do collisions occur in a WLAN?
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Collisions occur when two device send data at the same time, using overlapping frequency ranges, a collision occurs, and none of the transmitted signals can be understood by those receiving the signal.
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CMSA/CA makes it impossible for collisions to occur within a WLAN.
(true or false) |
False. CSMA/CA means that the WLAN will do what it can to statistically minimize the chance of collisions, but they still occur.
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CSMA/CA requires that each frame is acknowledged.
(True or False) |
True. CSMA/CA require that each frame is acknowledged. If no acknowledgement is received, the sending device assumes that the frame was lost or collided, and the frame is resent.
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How does CSMA/CA attempt to limit collisions?
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First, a device listens to the medium to see if any devices are transmitting. If there are none, then it waits for a random period of time and listens again. If there is nothing detected, then the frame is sent.
After the frame is sent, then the device waits for an acknowledgement. If no acknowledgement is received, then the data is resent using the CSMA/CA logic to wait for the appropriate time to retransmit. |
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Wireless APs always have an IP address.
(True or False) |
False. Wireless access points are normally used at Layer 2, so no IP address is needed.
However, they may be assigned one for ease of management. |
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Service Set Identifier (SSID)
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The Service Set Identifier (SSID) is a 32 character text identifier for the WLAN.
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Why do all APs in an ESS WLAN need to have the same SSID?
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All APs need the same SSID so that when a user is moving between AP within the same ESS, they will be able to roam within the WLAN and connect to different APs.
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Microsoft Auto Configuration Module (ACM)
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The Auto Configuration Module (ACM) allows the PC to automatically discover the SSIDs of all WLANs whose APs are within range the NIC.
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Mutual Authentication
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Mutual Authentication uses a secret password, called a key, on both the client and the AP.
Using algorithms, the AP can confirm that the client does indeed know the right key value. The client can also confirm that the AP also has the right key value. |
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An attacker can learn a WLAN's key by listening to the frames being broadcast by the network.
(true or false) |
False. The key is never sent through the air, so even if the attacker is using a network analysis tool to copy every frame inside the WLAN, the attacker cannot learn the key value.
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WLAN Encryption
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Encryption uses a secret key and a mathematical formula to scramble the contents of the WLAN frame. The receiving device then uses another formula to decrypt the data. Without the secret encryption key, an attacker may be able to intercept the frame, but cannot read the contents.
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Intrusion Detection Systems (IDS)
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IDS toools typically receive a copy of packets via a monitoring port. The IDS can rate and report on each potential threat, and potentially ask other devices, such as firewalls and routers to help prevent the attack.
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Intrusion Prevention Systems (IPS)
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IPS tools often sit in the packet's forwarding path, giving the IPS the capability to perform the same functions as the IDS, but also to react and filter the traffic.
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Wired Equivalent Privacy (WEP)
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WEP was the original 802.11 security standard, providing authentication and encryption services. However WEP only provides weak authentication and encryption, so it is no longer used today.
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WEP Key Shortcomings
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WEP used static preshared keys (PSK). The key value had to be configured on each client and each AP, with no dynamic way to exchange the keys without human intervention. The key values are also short and easily cracked.
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SSID Cloaking
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SSID cloaking tells the AP to stop sending periodic Beacon frames. The AP stops advertising their SSIDs, so the user much know the SSID before they connect. However, it is easy to get an AP to broadcast its SSID, so it is a weak security feature.
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Beacon Process
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The Beacon Process helps each client learn about the available APs. It broadcast its SSID at periodic intervals, so that hosts are able to see it.
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MAC address filtering
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APs can be configured with a list of allowed WLAN MAC addresses, and filter out frames sent by clients whose MAC address is not on the list.
It is a weak security feature, as MAC addresses can be easily spoofed. |
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Dynamic Key Exchange
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A dynamic key exchange process helps security because the clients and the AP can change keys more often, without human intervention.
When key information is exchanged dynamically, a new key can be delivered for each packet, allowing encryption to use a different key each time. This way, even if an attacker managed to discover a key used for a particular packet, they would only be able to decrypt that one packet, minimizing the exposure. |
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WPA Security
(Wi-FI Protected Access) |
WPA was the standard that replaced WEP, and was a great security improvement. WPA includes the option to use dynamic key exchange, using the Temporal Key Integrity Protocol (TKIP)
The encryption algorithm uses the Message Integrity Check (MIC) algorithm. |
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Advanced Encryption Standard (AES)
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AES is an advanced encryption algorithm that was introduced with the WPA2 (802.11i) standard. It has longer keys and more secure algorithms.
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WPA2 Security Protocol
(802.11i) (Wi-Fi Alliance version 2) |
WPA2 is the more advanced version of the WPA protocol. It uses AES encryption for better security and longer keys. Has both static or dynamic key distribution.
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To ensure that users can move around within a coverage area, what is the recommended cell overlap?
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15 - 20%
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