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Differentiated Services MAC

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Title: Differentiated Services MAC


1
Differentiated Services MAC
2
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.

3
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.

4
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.

5
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.

6
802.11 and MII
20
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.

7
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.

8
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.

9
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

10
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.

11
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

12
OPNET User Interface
13
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
14
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)

15
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
16
Throughput
  • Chart illustrates optimal packet size for three
    different RMS Delay Spreads.

Office
Department Store
Airport
17
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.
18
PCF Efficiency (version 2)
Example higher protocol layers must reserve 5
Mbps of channel bandwidth to carry a 4.3 Mbps
multimedia load.
19
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

20
Superframe Duration
  • Below 25 msec, throughput decreases rapidly due
    to the increasing ratio of beacon packets to data
    packets.

21
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
22
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.

23
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

24
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

25
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.

26
Admission Control
  • A proposed admission control algorithm

27
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.

28
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.

29
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.
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