Chapter 8 - Wireless LANs

Question 1

Basic service set (BSS)

Answer 1

WLANs that have just one AP are called BSS WLANs. BSS WLANs are said to run in infrastructure mode because wireless clients connect to an AP, which is typically connected to a wired network infrastructure. A BSS network is often used in residential and SOHO locations, where the signal strength provided by a single AP is sufficient to service all of the WLAN’s wireless clients.

Question 2

Carrier sense multiple access collision avoidance (CSMA/CA)

Answer 2

Just as CSMA/CD is needed for half-duplex Ethernet connections, CSMA/CA is needed for WLAN connections because of their half-duplex operation. Similar to how an Ethernet device listens to an Ethernet segment to determine whether a frame exists on the segment, a WLAN device listens for a transmission on a wireless channel to determine whether it is safe to transmit. In addition, the collision-avoidance part of the CSMA/CA algorithm causes wireless devices to wait for a random backoff time before transmitting.

Question 3

Channel bonding

Answer 3

With channel bonding, two wireless bands can be logically bonded together, forming a band with twice the bandwidth of an individual band. Some literature refers to channel bonding as 40-MHz mode, which refers to the bonding of two adjacent 20-MHz bands into a 40-MHz band.

Question 4

Decibel (dB)

Answer 4

A ratio of radiated power to a reference value. In the case of dBi, the reference value is the signal strength (that is, the power) radiated from an isotropic antenna, which represents a theoretical antenna that radiates an equal amount of power in all directions (in a spherical pattern). An isotropic antenna is considered to have gain of 0 dBi.

Question 5

Direct-sequence spread spectrum (DSSS)

Answer 5

Modulates data over an entire range of frequencies using a series of symbols called chips. A chip is shorter in duration than a bit, meaning that chips are transmitted at a higher rate than the actual data. These chips not only represent encoded data to be transmitted, but also what appears to be random data. Because both parties involved in a DSSS communication know which chips represent actual data and which chips do not, if a third-party intercepted a DSSS transmission, it would be difficult for that party to eavesdrop on the data because he would not easily know which chips represented valid bits. DSSS is more subject to environmental factors, as opposed to FHSS and OFDM, because it uses an entire frequency spectrum.

Question 6

Enterprise mode

Answer 6

In the context of wireless networking, this refers to using a centralized authentication server such as RADIUS for authentication, instead of a pre-shared key (PSK).

Question 7

Extended service set (ESS)

Answer 7

WLANs containing more than one AP are called ESS WLANs. Like BSS WLANs, ESS WLANs operate in infrastructure mode. When you have more than one AP, take care to prevent one AP from interfering with another. Specifically, nonoverlapping channels (that is, channels 1, 6, and 11 for the 2.4-GHz band) should be selected for adjacent wireless coverage areas.

Question 8

Frequency-hopping spread spectrum (FHSS)

Answer 8

Allows the participants in a communication to hop between predetermined frequencies. Security is enhanced because the participants can predict the next frequency to be used but a third party cannot easily predict the next frequency. FHSS can also provision extra bandwidth by simultaneously using more than one frequency.

Question 9

Independent basic service set (IBSS)

Answer 9

A WLAN can be created without the use of an AP. Such a configuration, called an IBSS, is said to work in an ad-hoc fashion. An ad hoc WLAN is useful for temporary connections between wireless devices. For example, you might temporarily interconnect two laptop computers to transfer a few files.

Question 10

Multiple input multiple output (MIMO)

Answer 10

MIMO uses multiple antennas for transmission and reception. These antennas do not interfere with one another, thanks to MIMO’s use of spatial multiplexing, which encodes data based on the antenna from which the data will be transmitted. Both reliability and throughput can be increased with MIMO’s simultaneous use of multiple antennas.

Question 11

Omnidirectional antenna

Answer 11

Radiates power at relatively equal power levels in all directions (somewhat similar to the theoretical isotropic antenna). Omnidirectional antennas are popular in residential WLANs and SOHO locations.

Question 12

Orthogonal frequency-division multiplexing (OFDM)

Answer 12

Whereas DSSS uses a high modulation rate for the symbols it sends, OFDM uses a relatively slow modulation rate for symbols. This slower modulation rate, combined with the simultaneous transmission of data over 52 data streams, helps OFDM support high data rates while resisting crosstalk between the various data streams.

Question 13

Personal mode

Answer 13

In the context of wireless networking, this refers to using a pre-shared key (PSK) instead of a centralized server, such as RADIUS, for authentication.

Question 14

Unidirectional antenna

Answer 14

Unidirectional antennas can focus their power in a specific direction, thus avoiding potential interference with other wireless devices and perhaps reaching greater distances than those possible with omnidirectional antennas. One application for unidirectional antennas is interconnecting two nearby buildings.

Question 15

War chalking

Answer 15

If an open WLAN (or a WLAN whose SSID and authentication credentials are known) is found in a public place, a user might write a symbol on a wall (or some other nearby structure) to let others know the characteristics of the discovered network. This practice, which is a variant of the decades-old practice of hobos leaving symbols as messages to fellow hobos, is called warchalking.

Question 16

Wi-Fi Protected Access (WPA)

Answer 16

The Wi-Fi Alliance (a nonprofit organization formed to certify interoperability of wireless devices) developed its own security standard to address the weaknesses of Wired Equivalent Privacy (WEP). This new security standard was called Wi-Fi Protected Access (WPA) Version 1.

Question 17

Wi-Fi Protected Access Version 2 (WPA2)

Answer 17

Uses Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) for integrity checking and Advanced Encryption Standard (AES) for encryption. These algorithms enhance the security offered by WPA.

Question 18

Wired Equivalent Privacy (WEP)

Answer 18

A security standard for WLANs. With WEP, an AP is configured with a static WEP key. Wireless clients needing to associate with an AP are configured with an identical key (making this a preshared key [PSK] approach to security). The IEEE 802.11 standard specifies a 40-bit WEP key, which is considered to be a relatively weak security measure.

Question 19

Wireless router

Answer 19

Attaches to a wired network and provides access to that wired network for wirelessly attached clients, like a wireless AP. However, a wireless router is configured such that the wired interface that connects to the rest of the network (or to the Internet) is on a different IP network than the wireless clients. Typically, a wireless router performs NATing between these two IP address spaces.