Title: Differentiated Services MAC
1Differentiated Services MAC
2Differentiated 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.
3IEEE 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.
4802.11 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.
5802.11 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.
6802.11 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.
7The 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.
8Challenge (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.
9802.11 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
10PCF 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.
11Simulation/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
12OPNET User Interface
13Simulation 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
14Parameters (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)
15RMS 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
16Throughput
- Chart illustrates optimal packet size for three
different RMS Delay Spreads.
Office
Department Store
Airport
17PCF 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.
18PCF Efficiency (version 2)
Example higher protocol layers must reserve 5
Mbps of channel bandwidth to carry a 4.3 Mbps
multimedia load.
19Wireless 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
20Superframe Duration
- Below 25 msec, throughput decreases rapidly due
to the increasing ratio of beacon packets to data
packets.
21Superframe (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
22Superframe (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.
23PCF 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
24Other 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
25Traffic 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.
26Admission Control
- A proposed admission control algorithm
27Future 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.
28MAC 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.
29MAC 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.