Differentiated Services MAC

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Differentiated Services MAC
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Differentiated Services MAC

• Challenge Solve the lack of interoperability
  between wired and wireless networks for digital
  video and multimedia by developing a set of
  protocols and coding schemes that allow video
  data streams to share the adaptive nature of
  TCP-based data streams.
• Tasks (Harris)
• Investigate and implement the necessary
  enhancements to the IEEE 802.11 Wireless LAN MAC
  protocol to add the DiffServ QoS mechanisms
  required for MII.
• Tasks (Sun)
• Interface DiffServ to 802.11
• Design, prototype, and test an adaptive video
  architecture and coding scheme to implement
  DiffServ over an 802.11 network.

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IEEE 802.11 Review

• Basic Service Set (BSS) the basic building block
  of an IEEE 802.11 Wireless LAN.

• Independent Basic Service Set (IBSS) a BSS that
  forms a self-contained network, where no access
  to a Distribution System is available.

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802.11 Review (Contd)

• Extended Service Set (ESS) consists of one or
  more BSSs connected to a distribution system via
  an Access Point (AP).

Infrastructure
Access Point
Distribution System (DS) (e.g. 100 Mbps Ethernet)
BSS
Access Point
BSS

• Distribution System can be wired or wireless.
• A station on an IBSS or an ESS appears the same
  to higher protocol layers.
• MII will use an ESS architecture.

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802.11 Review (Contd)

• Two access methods
• Distributed Coordination Function (DCF) Best
  Effort Service uses CSMA/CA to allow for
  contended access to the wireless media.
• Point Coordination Function (PCF) Time-Bounded
  Service provides for uncontended access to the
  wireless media via arbitration by a Point
  Coordinator, which resides in the Access Point.
  PCF operates only when theres an infrastructure
  in place.
• DCF and PCF coexist in a manner that permits both
  to operate simultaneously within the same BSS.
• When a Point Coordinator is operating in a BSS,
  the two access methods alternate, with a
  contention-free period followed by a contention
  period.
• A cycle of contention and contention-free periods
  is called a Superframe.

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802.11 and MII
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80
Proposed Ratios for MII
Point Coordinated Function (Contention-Free
Interval)
Distributed Coordination Function (Contention
Interval)
Superframe

• PCF is the ideal media access mechanism to use as
  a starting point for MII.
• Video and multimedia traffic require guarantees
  of packet throughput and end-to-end delay.
• The Point Coordinator maintains a Polling List of
  stations that it polls during the contention-free
  period for their transmission.

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The MAC Challenge

• However, PCF -- as specified in the 802.11
  standard -- is not adequate for MII
• Start of contention-free period (CFP) is not
  exactly periodic since Beacon (which starts the
  CFP) must wait until channel is idle PIFS (PCF
  Interframe Spacing) delay.
• PCF may be forced to end early and, as a result,
  not serve some members of the polling list.

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Challenge (Contd)

• Other PCF drawbacks
• Access during the contention-free period (CFP)
  may not be exactly periodic due to polled users
• entering/exiting polling list
• not having anything to send
• transmitting data of varying packet sizes
• If there is burstiness in the contention-free
  (CF) traffic, the current PCF does not change to
  accommodate it. The CFP ends as usual, and CF
  traffic must queue until the next CFP.

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802.11 Enhancements

• Ideally, the PCF should guarantee a maximum delay
  to CF packets and have little or no jitter in the
  end-to-end packet delay.
• The MII team used the following criteria when
  embarking on improving 802.11 PCF
• The solution must be cost-effective.
• The solution must be implementable within the
  time constraints imposed by the MII schedule.
• Four enhancements to PCF were proposed
• Early Beacon
• Extended PCF
• Fully Extended PCF
• Rigid Polling

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PCF Enhancement Results

• Enhanced IEEE 802.11 PCF Provides TDMA-like
  Structure

SUPERFRAME
PIFS
SIFS
SIFS
SIFS
SIFS
SIFS
DCF
CF-End
Stream1
Stream3
Stream2
Beacon

• Beacon which initiates PCF starts exactly on
  time.
• This precise periodicity is at the expense of
  some DCF BW.
• For AP-to-STA multicast streams used on MII, no
  Poll or ACK is needed.
• Jitter in Streams occurs only when
• Polling List changes
• Polling List members have nothing to send
• Other variations in packet arrival.

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Simulation/Analysis Tools

• OPNET is the primary simulation/analysis tool.
• Telelogics SDT was used to better understand
  PCF.
• OPNET comes with an 802.11 DCF model that we
  validated.
• We created an 802.11 PCF model for OPNET and
  added
• specialized statistics probes.
• 11Mbps Physical/RF/Channel effects
• We ran the model on
• Standard PCF
• Early Beacon Enhancement
• Extended PCF Enhancement
• Fully Extended PCF Enhancement

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OPNET User Interface
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Simulation Parameters

• Useful parameters for simulation/analysis
  include
• RMS Delay Spread (characterizes degree of
  multipath effects)
• 100 nsec worst-case office environment
• 200 nsec department store or airport
• 300 nsec factory setting
• Packet Error Rate PER (x) aebx, where a and b
  are constants, and x is the received Eb/No in dB.
• Packet Size (in octets)
• Throughput the rate of useful data received by
  the end user in bits/sec.
• Efficiency measures effective utilization of
  the channel.

E
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Parameters (Contd)

• End-to-end delay the elapsed time from the
  acquisition of data at a source to its display,
  sounding, or consumption by a sink.
• Simulations apply to the wireless segment only,
  and includes queueing, protocol, contention,
  transmission, propagation, and radio hardware
  delays.
• End-to-end jitter a measure of the variability
  in end-to-end delay experienced by data packets.
• Results from burstiness in the data stream
• Is measured in the simulations as the statistical
  variance of the end-to-end delay measurements.
• Superframe duration (in seconds)

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RMS Delay Spread

• Greater RMS Delay Spreads and larger packet sizes
  lead to higher packet error probabilities.
• An RMS Delay Spread of 200 nsec would cluster
  between the two groups shown.
• This information helps us better tune the MII
  system for a variety of environments.

300 nsec RMS Delay Spread
100 nsec RMS Delay Spread
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Throughput

• Chart illustrates optimal packet size for three
  different RMS Delay Spreads.

Office
Department Store
Airport
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PCF Efficiency (version 1)

• PCF is most efficient for one-way multicast
  traffic to stations (MII scenario) using larger
  packets.
• A 2-way video teleconference scenario would be
  half as efficient.

Conditions error-free environment, AP transmits
Multicast Packets, no ACKs.
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PCF Efficiency (version 2)
Example higher protocol layers must reserve 5
Mbps of channel bandwidth to carry a 4.3 Mbps
multimedia load.
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Wireless Packet Delay

• All enhancements improve on PCF, with Fully
  Extended providing the greatest improvement.

Fully Extended
Standard PCF

• Conditions
• 200 msec Superframe size
• 80 PCF, 20 DCF
• 1500 byte packets

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Superframe Duration

• Below 25 msec, throughput decreases rapidly due
  to the increasing ratio of beacon packets to data
  packets.

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Superframe (Contd)

• Larger Superframe sizes result in longer
  end-to-end delay.
• Objective choose the smallest Superframe size
  that will maximize the throughput.

End-to-End Delay
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Superframe (Contd)
Packet Delays vs. Superframe Size

• Packet delay is plotted against Superframe size
  for average and maximum end-to-end packet delay.
• Smaller Superframe sizes lead to shorter packet
  delays.

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PCF Enhancement Conclusions

• Implement the Fully Extended PCF enhancement
• includes early beacon
• Use a Superframe size of 25 msec
• Have the Point Coordinator transmit
  (unacknowledged) multicast packets to stations
  during PCF.
• Do not poll stations during PCF.
• Use the following package sizes for the specified
  environmental conditions

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Other Enhancements

• Implement four priority queues
• (PCF) Guaranteed QoS Bounded delay and
  guaranteed bandwidth
• (PCF) Controlled Load QoS Service when
  Guaranteed QoS queue is empty
• (DCF) Best Effort What is typically available
  on the internet today
• (DCF) Less Than Best Effort Service when Best
  Effort queue is empty

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Traffic Shaping

• Leaky Bucket depth affects probability of packet
  drop.
• For traffic with Gaussian distributed
  instantaneous bit rate, the packet drop rate can
  be determined.
• Packet drop occurs when traffic rate exceeds mean
  rate for a certain duration.

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Admission Control

• A proposed admission control algorithm

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Future Enhancements

• When eventually implementing two-way video
  conferencing, consider
• Allowing Point Coordinator to send more than one
  poll to a mobile station.
• Additional simulation/analysis work required for
  this.

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MAC Firmware

• PRISM MAC firmware modifications delayed
• Switch from SMAC to CMAC architecture
• Increased complexity of CMAC architecture
• Limited software development environment
• PCF functionality not currently available
• MII team taking on the task of developing minimum
  PCF functionality.
• Access Point software image not baselined
• MII team currently evaluating effort required to
  baseline code
• Software Development Plan in place
• PCF Software Requirements Document created
• PCF Draft Test plan available
• PCF Draft Software Development Plan in place.

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MAC Firmware (Contd)

• PCF Preliminary Design complete.
• Two software developers assigned to this task
• Received training on new architecture
• Working closely with Choice design team in San
  Antonio, TX
• Near term objectives
• Baseline Access Point code
• Add minimum PCF functionality for MII
• Incorporate MII-specific functionality into PCF.