Transcript A Rate-Adaptive MAC Protocol for Wireless Networks

A Rate-Adaptive MAC Protocol
for Multi-Hop Wireless Networks
By Gavin Holland, Nitin Vaidya and
Paramvir Bahl
Presented by: Helal chowdhury
Telecommunication Laboratory, university of oulu
Contents
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Introduction
Receiver Based Auto Rate (RBAR)
RBAR implementation
Simulation
Conclusions
References
Introduction
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IEEE802.11
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Supports DSSS, FHSS , and IRDA at the physical layer.
RTS/CTS hand-shake.
Transmission rate 10Mbits/s.
Rate Adaption
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Rate adaption is the process of dynamically switching data rates to match the channel
conditions. There are two aspects to rate adaption:
Channel quality estimation
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Rate Selection
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By Sender
By receiver-> RBAR(Receiver Based Auto rate)
By Sender ->ARF(Auto rate Fallback)
By Receiver -> RBAR(Receiver Based Auto rate)
Why receiver based rate adaption
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The goal of rate adaption is to provide optimum throughput. The motivations for RBAR
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Rate selection can be improved by proving more timely and more complete channel quality.
Channel quality information is best acquired at the receiver.
Transmitting channel quality information to the sender can be costly.
RBAR modified DCF Protocol
DCF: To coordinate the transfer of data packet.
NAV: To announce the duration of packet.
DRSH: Final reservation Time
DCTS: Reservation time
DRTS: Reservation time (IEEE 802.11)
DRTS: Tentative reservation time (RBAR)
RBAR EVENT FLOW
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S choose a data rate r1, using some heuristic,
and sends r1 and the size of the data packet n
in the RTS to R.
A, overhearing the RTS, uses r1 and n to
calculate the duration of the reservation,
marking it as tentative.
R, having received the RTS, uses some
channel quality estimation and rate selection
technique to select the best rate r2 for the
channel conditions, and sends r2 and n in the
CTS to S.
B, overhearing the CTS, calculates the
reservation using r2 and n.
S responds to the CTS by placing r2 into the
header of the data packet and transmitting the
packet at the selected rate. If r1≠r2, S uses a
unique header signaling the rate change.
A, overhearing the data packet, looks for the
unique header. If it exists, it recalculates the
reservation to replace the tentative reservation
it calculated earlier.
A
S
r1, n
R
r1, n
r2, n
r2, n
B
r2, n
ACK
r2, n
RBAR MAC Header
Framl
Duration
control
Dest.
Source
Address
Address
BSSID
Sequnce
Body
FCS
control
IEEE 802.11 MAC Header
Framl
control
Duration
Dest.
Source
Address
Address
RBAR Reservation SubHeader
RBAR MAC Header
HCS
BSSID
Sequnce
control
Body
FCS
RBAR RTS/CTS Implementation
Frame
Duration
Rate
&
Dest.
Source
control
Length
Address
Address
FCS
IEEE
802.11
RTS
RBAR
RTS
Frame
Duration
Rate
&
Dest.
control
Length
Address
FCS
IEEE
RBAR
802.11
CTSCTS
 In RBAR, instead of carrying the duration of the
reservation , the packets carry the modulation rate and the
size of the data packet.
If there is rate mismatch between sender and receiver
DRTS refer to as tentative reservation.
Final reservations are confirmed by the presence or
absence of Reservation SubHeader (RSH).
RBAR PLCP Header
Sync
SFD
Data
Signal RSH
Rate
Service
Length
CRC
Rate
RBAR PLCP
PLCP header
header
802.11
 In standard 802.11, the PLCP header contains an 8 bit signal field.
In RBAR, the PLCP header has been divided into two 4 bit rate
subfields.
 Thus, the PLCP transmission protocol is modified as follows: when
the MAC passes a packet down to the physical layer, it specifies two
rates, one for the subheader and one for the remainder of the
packet.
Simulation Model
• Error Model
– Fast fading Channel model
– Slow Fading Channel model
• Movement Model
– random waypoint mobility pattern
• Trafic Model
– CBR
– FTP
– ON/OFF Pareto source generating traffic
Slow fading Channel
Fast Fading Channel
Variable Traffic Source
Multi-Hop Performance
Conclusions
• Showed that RBAR improve network throughput.
• RBAR outperforms ARF.
References
• Gavin Holland, Nitin Vaidya, Paramvir Bahl, ” A Rate-Adaptive MAC
Protocol for Wireless Networks,” Technical Report TR00-019, Dept.
of Computer Science, Texas University.
• wsl.stanford.edu/~ee360/adaptiveMAC_Jie.ppt
• B. Sadeghi, V.Kanodia, A.Sabharwal, and E. Knightly,”Opportunistic
Media Access for Multirate Ad Hoc Networks”, Department of
Electrical and Computer Engineering, Rice University.