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Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer

Tian He

J. Sobrinho and A. krishnakumar

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Outline

Motivations

Possible approaches

A proposed solution in the paper

Evaluations

Conclusions & Comments

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Motivation

Real time applications are ever more popular– VOIP Market $5b by 2005.– Myriad of streaming services: VOD, Video Phone, D-Sharing.

Videoconferencing.

Major service under wireless network.– Currently data is dominate service in wired network, while Data is

“special service” in wireless network. real-time streaming is dominate market in wireless environment

IP networking towards wireless, mobile environment.

Inherently it is a Interesting research problem

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Required QoS

Real time traffic is not too sensitive to delay– ~400ms for VOIP, ~250ms for video conferencing

Very sensitive to jitter– As little as 150ms can be unpleasant. – VOIP specification require average e2e delay 145ms.

Effect of lost packets is strongly codec dependant.

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Even harder in wireless

Narrow bandwidth available – 802.11a:54Mbps 802.11b:11Mbps 802.11g/e:22Mbps– IEEE 802.3ae : 10Gbps 909 times faster

High control overhead– large synchronization fields– larger MAC headers 34B vs 14B in 802.3 – more management packets (AP registration)

Inherent contention media (open space)

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Why not 802.11 DCF

A wants to transmit

but channel is busy

B

A

C

Packet to Node C

Packet to Node CRTS

CTS

Contention slots

ACK

positive acknowledgment

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Existing solution

802.11 centralized approach: PCF to guaranteed QoS.

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Why not 802.11 PCF

Centralized PCF Scheme

– Single point of failure

– Single media, no space multiplexing

– High overhead. (Registration, Polling )

– In-compatible with multi-cell setting.

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Other Solutions for Real-time Traffic

Time Division Multiple Access (TDMA)– Fixed slotting: Inefficient – dynamic slotting: complex scheduling algorithm

Code Division Multiple Access (CDMA)– Fixed coding length: inefficient – Dynamic coding: dynamic code assignment

Token Ring Passing – Only suitable for single contention media

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Key ideas in this paper

DCF mode for data stations.

Special mode for real-time stations.

Real-time stations have priority over data station by using shorter IFS.

Real-time stations proactively send “black bursts”, of length proportional to waiting time.

Guarantee one and only one real-time station wins for each contention phase.

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Assumption

Every node can sense each other’s transmissions (no hidden/exposed terminal problem). No RTS/CTS is used.

Real-time stations periodically send out packets at same rate.

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Definitions

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Access procedures

1. Single data station access procedure

2. Interactions among data stations

3. Single real-time station access procedure

4. Interaction among real-time stations

5. Interaction between data stations and real-time stations

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1. Single data station access

CSMA/CA as access procedure.

Contention Window

Backoff-WindowBusy Medium

Tlong

Tshort

Tlong

TshortDATA ACK

DATA

t

A

A

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Difference from 802.11 Standard

Data stations keep sensing the channel even no packets ready for transmission. 802.11 only senses the channel when need.

It use the pervious channel status to decide whether back-off or transmit immediately.802.11 needs DIFS delay before make a decision.

This scheme consumes more energy , but has shorter delay.

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2. Interactions among data stations

CSMA/CA as access procedure.

Contention Window

Backoff-WindowBusy Medium

Tlong

TshortDATA

t

Contention Window

Backoff-WindowBusy Medium

Tlong

Tshort

A

B

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3. Single real-time station access

Contention Window

Tlong

Busy Medium

Tlong

Tshort

Tmed

Tmed

DATA

Tshort

DATA ACK

Tobs

Tobs

A

A

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4. Interaction among real-time stations

Round robin access among real-time stations

Busy

Busy

Tmed

Tmed

Tobs

Busy

Tobs

Data

Busy

A

B

Tmed

Tmed

Busy

Tobs

Data

Busy

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4.Interaction among real-time stations

How to set Tobs.

– Tobs must be shorter than a black burst slot, otherwise we station A will not back off.

– Tobs must be shorter than Tmed , so that no real-time station will access channel during observation.

TmedTlong

Busy Medium

Tobs

TmedTlong

Busy Medium

Tobs

Data

Schedule Time

A

B

Data

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5.Interaction between real-time and data stations

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Negative Acknowledgement

Positive acknowledgement has an efficiency penalty.

When receiver gets a packet from sender at time T, it expects another packet at time: T+ tsch

+tobs

.

When receiver do not receive the packet at expected time interval, it sends out a negative acknowledgement.

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Theoretical Analysis: Stability

Definition 1: – The system is stable if and only if whatever

the initial conditions is, there is an L >=0, such that the access delay for real-time station is zero after L rounds (converge)

Definition 2: – The system is unconditionally stable if and

only if it is stable no matter the magnitude of the perturbation T (Overshoot independent)

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Stability (contd)

DATA

Delay

Delay

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Stability conditions

The system is unconditionally stable if and only if following inequality holds

N is #real-time station (1)

In addition if , the system is stable if and only if following inequality holds

T is initial disturbance (2)

1

1

nt

t

unit

bslot

1

1

n

11

T

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Nominal values

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Simulation Result (1)

RD

R

RD

R

D

loadRealtime:

loadData:

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Simulation Result (2)

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Simulation Result (3)

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Conclusions

Distributed access Higher priority for real-time station to access

the channel Can be overlaid on 802.11 without changing

data stations Virtual TDMA structure for real-time stations

constant access rate. Under stable condition bounded access

delay

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Critical comments

Possible data contention between real-time stations,even assume no hidden & exposed channel problem. RTS/CTS is desired.

Fix channel access interval for real-time stations ( round robin), which is inefficient.

Only consider initial disturbance T. No stable analysis for periodic or sporadic disturbance.

Evaluations on how data stations impact the performance of real-time stations are more desired: converge(settling) time vs initial disturbance