WLAN QoS

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WLAN QoS

• Ronald Lucas

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Introduction

• With the emergence of Voice Over IP, requirements
  to support Voice Over IP over Wireless LANs
  without degradation of its quality of service
  has become very important.
• A quality of service focused MAC Layer standard,
  IEEE 802.11e was developed to meet these
  requirements.

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Introduction cont.

• Voice over IP over Wireless LANs is more
  compelling since you can install these Wireless
  LANs in public spaces, which are backed up by
  the Internet.
• This allows for users to have telephone service
  without wires anywhere that the Wireless LAN is
  located.

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Introduction cont.

• IEEE 802.11e enables frames from quality of
  service sensitive applications to be transmitted
  sooner than other frames, which minimizes
  latency.
• IEEE 802.11e also has new power management
  features that will help with the life of the
  mobile device being used.
• IEEE 802.11e has channel-use efficiency gains,
  which allow for privileged treatment to any
  type.
• IEEE 802.11e also prioritizes time-critical data.
• IEEE 802.11e will work with 802.11-compliant
  devices.

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Channel Access

• IEEE 802.11e provides contention-based and polled
  access mechanisms, these are both enhancements to
  the 802.11 mechanisms.
• This leads to the reduction of access delay and
  jitter in delivering QoS-sensitive frames from
  the source to the destination.
• WLAN served by an AP, allow stations to
  communicate directly with one another.
• Transmit Opportunity (TXOP), allows a station to
  transmit a sequence of frames without having to
  contend for a channel, following a successful
  channel access attempt.

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Legacy Contention-Based Channel Access

• The legacy contention-based access mechanism,
  distributed coordination function, each station
  listens to the channel and if it is busy,
  postpones the transmission and enters the backoff
  procedure.
• This is done by deferring transmissions by a
  random time, which avoids collision between
  multiple stations, which would otherwise attempt
  to transmit after the completion of the current
  transmission.

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Contention-Based Channel Access

• This extends the 802.11 standard to provide frame
  prioritization, frames of higher priority will
  access the channel sooner.
• MAC protocol for 802.11e contention-based channel
  access is CSMA/CA, which use the parameters
  contention window min and contention window max,
  which the values assigned cause the window to be
  shorter or longer.
• 802.11e will allow a station consecutive
  transmissions of frames from the same access
  category without the need for backoff by using
  spacing between consecutive frames.
• 802.11e will drop frames once the expiration of
  the frames based on the time queued has been
  meet.

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TCMA MAC Protocol

• CSMA/CA protocol, a station engaged in backoff
  countdown must wait while the channel is idle for
  time equal to DIFS before decrementing its
  backoff immediately following a busy period, or
  before attempting transmission.
• TCMA (Tiered-Contention Multiple Access)
  protocol, variable lengths of this time interval,
  which is called Arbitration-Time Inter-Frame
  Space (AIFS), lead to varying degree of
  accessibility to the channel.
• A shorter AIFS will give a station an advantage
  in contending for channel access.

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802.11e Polled Channel Access

• The improved PCF polled channel access mechanism
  is the 802.11e HCF controlled channel access.
• It resembles the PCF, but polling is not limited
  to the contention-free period, the polling
  schedule is tailored to the time profile of the
  individual traffic streams which reduce overhead,
  delay, and jitter, and uplink transmit
  opportunity (TXOPs) cause frames to be
  transmitted sooner.
• 802.11e polled access mechanism results in a
  polling schedule that better matches the
  generation of frames in a periodic traffic
  stream, allowing for superior delay/jitter
  performance and better channel use efficiency.

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Admission Control

• Admission control provides bandwidth management
  to ensure that QoS-sensitive applications, such
  as voice and video, will have satisfactory QoS.
• Overloading WLANs with excessive number of users
  with high-priority access would lower the QoS,
  therefore requests are submitted by stations for
  the admission to specific traffic streams to the
  AP, which accepts or declines.

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Admission Control for Contention-Based Channel
Access

• This is an optional feature for a station and AP,
  the AP decides to allow stations that employ
  contention-based access in the WLAN to transmit
  traffic using the parameters of an access
  category.
• The AP then can track and manage bandwidth use
  and it is also not necessary to impose admission
  control on all access categories.
• The AP advertises to the WLAN the access
  categories that are protected by the admission
  control.
• A stations request, submitted by and add Traffic
  Specification (ADDTS) frame, describes the
  traffic stream to be admitted. The response to
  the ADDTS, if accepted, furnishes in the Medium
  Time field the channel time the station is
  allocated for uplink transmissions.

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Admission Control for Polled Channel Access

• Admission Control is automatic when using a
  polled channel access, the AP will reject an add
  Traffic Specification (ADDTS) request if it
  cannot meet the requirements for a service period
  schedule requested by a station for a traffic
  stream.
• The priority of a traffic stream may be
  considered in admission control, an admission
  control request from traffic stream with a higher
  priority may cause an admitted stream to be
  dropped.

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Power Management

• 802.11e standard amendment offers new mechanisms
  to help battery-powered devices conserve power by
  enabling them to power down their receivers and
  transmitters intermittently without loosing
  connectivity or data.
• A station will inform the AP of being in either
  power saving mode, or active.
• This is done by changing the Power Management bit
  in the frame control field, when the frame is
  being sent by the station.

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Legacy Power-Save Mechanism

• Frames buffered at the AP for a power-saving
  station employing contention-based access are
  delivered when the station sends a special
  control frame, the power save poll.
• The AP sends a single buffered frames to a
  station after receiving a power save poll,
  immediately or soon after.
• More power save polls are required in order ro
  retrieve additional buffered frames.
• Stations using legacy power-save mode rely on
  traffic indication map (TIM) to learn if the AP
  holds buffered data for it.
• A power-saving station that supports legacy
  polled access need not send PS-polls in order to
  retrieve its buffered frames.

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Automatic Power Save Delivery

• This is the delivery of unicast frames from the
  SP to a power-saving station, this is done to
  reduce the signaling traffic caused by power save
  poll and their acknowledgements.
• The AP may deliver buffered frames to their
  destination power-saving stations either on a
  previously negotiated time or in response to
  receiving transmissions from the station that
  trigger such delivery.

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Scheduled Automatic Power Save Delivery

• This mechanism is good for voice, and
  audio/video, because the downlink transmissions
  to power-saving devices will occur at a schedule
  that is known in advance, thus obtaining the need
  for special signaling between the station and AP.
• The AP and the station negotiate a time in
  advance in which the station will be fully
  powered and will be able to receive any frames
  that are buffered for it at the AP.

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Full Unscheduled Automatic Power Save Delivery

• This was introduced for stations accessing the
  channel by contention, to enhance the efficiency
  of legacy power save.
• A power-saving station may use a PS-Poll, any
  data, and Null frame (trigger frames) to notify
  the AP that it is fully powered and ready to
  receive transmissions.
• The same as legacy power-save mechanism, a
  station can learn about its buffer status by
  listening to the beacons for its TIM.

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Hybrid Unscheduled Automatic Power Save Delivery

• This allows a station to choose between legacy
  power save delivery and automatic power save
  delivery based on access category.
• Trigger frames are used to initiate the delivery
  of buffered frames associated with access
  categories that have been designated delivery
  enabled.
• The station also designates in advance the access
  categories of the frames that may serve as
  trigger frames.

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QoS in Wireless Mesh Networks

• Wireless mesh takes on many forms, the most
  common is a collection of nodes that form an ad
  hoc network and are capable of serving as WLAN
  APs.
• These nodes are called mesh points, can forward
  traffic received from 802.11 stations to other
  mesh points with ultimate destinations that
  include WLAN stations attached to other mesh
  points somewhere on the wired networks.
• Protocols for forwarding, routing, and channel
  access must be specified for the mesh points,
  which requires an ad hoc networking standard with
  multi-hop capability.
• A wireless mesh network can be used to enable
  WLAN service when wiring for APs is not readily
  available in an enterprise, or for a temporary
  network that can be easily set up and torn down.
• There are challenges in routing and security,
  especially when mobility is contemplated.
• It is not clear what choices will ultimately be
  made for the IEEE 802.11 mesh standard.