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1
Real-Time Traffic over the IEEE 802.11 Medium Access Control Layer
Tian He
J. Sobrinho and A. krishnakumar
2
Outline
Motivations
Possible approaches
A proposed solution in the paper
Evaluations
Conclusions & Comments
3
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
4
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.
5
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
7
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.
9
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
10
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.
12
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
21
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)
23
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
25
Nominal values
26
Simulation Result (1)
RD
R
RD
R
D
loadRealtime:
loadData:
27
Simulation Result (2)
28
Simulation Result (3)
29
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
30
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