Week 5 - Lecture 9 Flashcards
What is a WLAN?
Wireless Local Area Network
An information system that supports diverse location-independent network service access to portable devices utilising radio channels
Communication for WLAN is standardized across industry
Different modulation methods are employed across versions including
FHSS, DSSS, high rate DSSS
Orthogonal frequency division multiplexing (OFDM)
In what two ways do wireless devices located within a basic service area communicate?
Infrastructure Basic Service Set (BSS)
Communication takes two successive hops
- WD to AP and AP to WD
Advantage is no restriction place on the physical distance between wireless devices
Independent Basic Service Set (IBSS), also BSSs or ad-hoc networks
Wireless devices communicate directly
Involves a small number of devices for a specific task and short period of time
Advantage is preserving system capacity at the cost of increased physical and MAC layer complexity
If a device is in a power-saving (PS) mode, the access point buffers those frames destined to reach the device during the period it will be in PS status. When the device exits the PS mode, the AP forwards the cached data frames to the device one by one. Thus, the AP is important in enabling energy saving policies and mechanisms
What is the idea of an extended service set (ESS)?
Support an arbitrary network size
Essentially multiple APs covering a large area, linking back to the same backbone network
What are the nine services of IEEE 802.11 (WLAN) based networks?
Three services dedicated to data transfer, others are for management
Authentication - pre-req to association since only authenticated users are authorised to use the network resources
If the APs authenticate any station, the system is called an “open system” or an “open network”
Association
Enables the connection between wireless devices and AP such that MAC frame delivery to the associated terminals is possible. unassociated wireless terminals are not permitted to obtain any service from the whole system.
Distribution service is exploited in infrastructure networks to exchange data frames
The AP, upon receiving a MAC protocol data unit (MPDU), uses the distribution service to forward it to the intended destination device.
Integration enables connection with a non-802.11 network. not expressed technically by the standard, except in terms of the services it should offer.
Reassociation generally initiated by a wireless terminal once the signal strength indicates that a different association is necessary, This means that handoff and reassociation requests are never commenced by APs
Disassociation - terminates an existing association, initiated by wireless station, any mobility information stored in the distribution system corresponding to the request station is removed at once.
Deauthentication - terminates any existing association and, consequently, use of network resources from a wireless device
Privacy - Non-compulsory, called wired equivelant privacy (WEP) which is rather weak.
IEEE 802.11i uses WiFi-protected access version 2 (WPA2) specifies better security mechanisms for wireless networks
WPA2 uses advanced encryption standard (AES) block cipher
What are the address schemes in IEEE 802.11 MAC
Distributed Coordination Function (DCF)
- Applies a listen before talk (LBT) approach and multiple-access carrier sense / collision avoidance (CSMA/CA) mechanism
Point Coordination Function (PCF)
What is a IEEE802.11 superframe composed of
a contention free period (CFP) and a contention period (CP), which alternated periodically in time
- PCF is used during the CFP interval
- DCF is used during the CP interval
What is a beaconframe used by APs for?
To initiate a superframe, the preserve synchronisation of the local timers in the associated stations and deliver protocol-related parameters
What are the time instances at which APs transmit management frames called?
Target beacon transmission time (TBTT), they are announced in the previous beacon frame
How does PCF work?
The point coordinator (PC), which is typically an AP, polls a device to request delivery of a pending frame
The frame sent to the device is complemented by a poll frame, that is DATA+CF-Poll
The device acknowledges data reception and sends data (MAC frames) should there be any pending frame aimed for the AP/PC
What are the two most distinct issues suffered by the contention free period?
inaccurate beacon frame delay
Indefinite data transmission
How does DCF work?
First listens (poll) the channel if idle for DIFS (DCF inter-frame space) and a backoff period then transmit
Request to send (RTD) and Clear to send (CTS) frames contain information about the duration of the data transfer
ACK frame helps to avoid packet retransmission
How is DCF backoff determined
Binary exponential backoff (BEB) mechanism.
What is the NAV (network allocation vector) and what is it used for?
Corresponds to the time required for the data transmission/reception including the duration of the ACK frame, as provided by the RTC/CTS frames
Used to resolve the status of the radio channel and the use of the wireless medium. the NAV both at the physical and MAC (virtual) layer is utilised.
What are the two sublayers of the IEEE 802.11 physical layer?
The PLCP sublayer which receives incoming MSDUs from the MAC layer, adds its own designated header, and the hands them over to the PMD
The PMD (Physical medium dependent) sublayer is responsible for transmitting every received bit from the PLCP over the wireless medium
The delivered information must have a preamble, which has a pattern that depends on the modulation technique deployed in the physical layer
