L9

Question 1

What is WLAN?

Answer 1

WLAN (Wireless local area network) is an information system that supports diverse location-independent network service access to portable devices utilising radio channels

Question 2

How is WLAN standardized?

Answer 2

Standardized through IEEE.802.11

Question 3

What modulation techniques are used in WLAN?

Answer 3

Modulation techniques included FHSS, DSSS, high rate DSSS

Question 4

How do wireless devices communicate? (2 ways)

Answer 4

Infrastructure Basic Service Set (BSS): all communications must go through an access point (AP)

Independent Basic Service Set (IBSS): wireless devices communicate directly

Question 5

What is an advantage of an Access Point for wireless communication?

Answer 5

AP helps save battery power because it stays on but the devices connected to it can sleep, but has a central point of failure

Question 6

What are three services of IEEE 802.11 based networks?

Answer 6

Authentication: only authenticated users are authorised to use the network resources

Association: enables the connection between wireless devices and AP (no glitches)

Distribution: exploited in infrastructure networks to exchange data frames

Question 7

What is a superframe composed of in IEEE 802.11?

Answer 7

A superframe is composed of a contention-free period (CFP) and a contention period (CP) which alternate periodically in time

Question 8

What are the access schemes in IEEE 802.11?

Answer 8

Distributed Coordination Function (DCF): listen-before talk

Point Coordination Function (PCF)

Question 9

What is the difference between PCF and DCF in IEEE 802.11?

Answer 9

PCF Operation: Centralized control of access to the wireless medium, with a coordinator allocating specific time slots (CFPs) for device transmission.

DCF Operation: Distributed contention-based access where devices contend for access using CSMA/CA and random backoff if collisions occur.

Question 10

What two layers can be distinguished in IEEE 802.11?

Answer 10

PLCP sublayer: receives incoming MSDUs from the MAC layer, adds its own header, and then sends to PMD

PMD (Physical Medium Dependent): transmits every received bit from the PLCP over the wireless medium

Question 11

What are multiple access physical layer techniques supported in IEEE 802.11?

Answer 11

FHSS: transition from one frequency to another in a random pattern

DSS: used to transmit a narrowband signal over a much wider frequency

Question 12

How is DSS accomplished in IEEE 802.11

Answer 12

Accomplished by applying a chipping sequence (pseudorandom noise codes) to the information bit stream