wireless LAN

Question 1

2 categories of wireless LAN & components

Answer 1

Ad hoc WLANs: wireless devices communicate directly with each other.

Infrastructure WLANs: wireless devices communicate with other wireless or wired devices through a wireless router or wireless access point.

Question 2

Wireless routers combine which features into a single device?

Answer 2

Cable/DSL modem, WAP, DHCP server & NAT, switch, firewall.

Question 3

Wireless access point does what?

Answer 3

Interconnects a wired LAN with a WLAN. It does NOT interconnect 2 networks. The access point connects to the wired LAN. The wireless devices that connect to the wired LAN via the access point are on the same subnet as the access point.

Question 4

Factors that affect wireless antenna effectiveness.

Answer 4

Distance, coverage pattern, environment (indoor or outdoor), interference with other access points.

Question 5

2 types of antenna.

Answer 5

Omnidirectional: radiate power at relatively equal power levels in all directions.

Unidirectional: focus their power in a specific direction. This avoids interference with other wireless devices and reaches greater distances.

Question 6

Wireless standard (IEEE …?)

Answer 6

802.11

Question 7

2 bands commonly used for WLAN

Answer 7

2.4-2.5 GHz & 5.75-5.875

Question 8

Non-overlapping channels

Answer 8

1 (2.412 GHz), 6 (2.437 GHz), 11 (2.462 GHz), 14 (2.484 GHz not supported in US)

Question 9

What cannot be detected wirelessly?

Answer 9

Collision.

Question 10

CSMA/CA - what is CA?

Answer 10

Collision avoidance. A transmitting device notifies other wireless devices that it is ready to send and it waits for acknowledgement. If it doesn’t receive acknowledgement, it starts the random backoff timer and tries again later.

Question 11

FDMA

Answer 11

Frequency division multiple access.

Question 12

TDMA

Answer 12

Time division multiple access.

Question 13

CDMA

Answer 13

Code division multiple access. Only one channel occupies the entire bandwidth of the link. All stations can send data at same time (no time sharing). Based on coding theory - each sender is assigned a code which is a sequence of numbers called chips.

Question 14

3 transmission methods

Answer 14

DSSS, FHSS, OFDM

Question 15

DSSS

Answer 15

Direct-sequence spread spectrum. Modulates data over an entire range of frequencies. The chip rate is much higher than the information signal bit rate.

Question 16

FHSS

Answer 16

Frequency-hopping spread spectrum. Devices hop between predetermined frequencies or channels.

Question 17

OFDM

Answer 17

Orthogonal frequency-division multiplexing. Uses relatively slow modulation rate combined with the simultaneous transmission of data over 52 data streams.

Question 18

IBSS

Answer 18

Independent basic service set (aka ad hoc WLAN). Contains no access points, just wireless devices.

Question 19

BSS

Answer 19

Basic service set. Typical WLAN with one AP connected to the network.

Question 20

ESS

Answer 20

Extended service set. Contains more than one AP working together to provide more coverage than a single AP.

Question 21

RFI

Answer 21

Radio frequency interference. Caused by other devices using similar frequencies to the WLAN device.

Question 22

Common sources of RFI

Answer 22

Other WLAN devices, cordless phones, baby monitors, microwave ovens, wireless security systems, physical obstacles, signal strength.

Question 23

SSID

Answer 23

Service set identifier. Human-readable network name. Beacons are periodically broadcast by access points to advertise the presence of the access point to clients.

Question 24

Active scanning

Answer 24

When a mobile device wants to join a network or moves around in & out of coverage, it sends a probe message with an SSID. AP’s that receive the probe will respond. Mobile selects one of the AP’s (strongest signal) and sends an association request. The selected AP responds with association response.

Question 25

Passive scanning.

Answer 25

AP sends out beacons which carry information about the access point, including the SSID. Mobiles hear these beacons, and if they find a better AP they may switch to it.

Question 26

Why no CSMA/CD in WLAN?

Answer 26

answer

Question 27

WLAN standards and difference.

Answer 27

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