An SSCOP-based Link
Layer Protocol for Wireless
LANs
Haoli Wang and Aravind Velayutham
IEEE Global Telecommunications Conference
1-5 December, 2003
San Francisco, USA
Outline
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Introduction
Problems
Solutions
Proposed Protocol
Analysis and Performance
Conclusions
Q&A
2
Introduction
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The IEEE 802.11 MAC protocol
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It’s the standard for medium access in
wireless LANs.
It provides fast recovery from frame losses
using a rapid link level positive
acknowledgement scheme, generally an
ACK frame.
3
Positive acknowledgement scheme
Receiver
Sender
ACK
ACK Timeout
Retransmission
4
Problems
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At a low error rate, the most of the
frame transmissions are correct and the
positive ACK scheme is inefficient. Since
each data frame requires on ACK, the
large traffic overhead of ACK
transmission wastes the scarce
bandwidth.
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Solutions
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To use the negative ACK scheme, in
order to reduce the amount of ACK
traffic by the receiver only requests the
retransmission of the lost frames.
The proposed protocol is an adaptation
of Service Specific Connection Oriented
Protocol (SSCOP) that has already been
implemented in ATM networks.
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Proposed Protocol
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Basic operation
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Timeout mechanisms
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USTAT operation
POLL/STAT operation
POLL Timeout
Data Timeout
Determination of  value
Buffer management
Modified NAV value
Frame formats
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USTAT operation
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A receiver can detect a frame loss by
checking the sequence number in the
frame header.
When a receiver detect a frame loss, it
sends an Unsolicited Status (USTAT)
control frame to the transmitter
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Example of USTAT operation
Receiver
Sender
USTAT
Out-of-order frame
Retransmission
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POLL/STAT operation
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After a transmitter sends out a specified
number of data frames without
acknowledgement, say , a POLL frame
is sent out to the receiver.
The receiver will report the status of the
last  frames to the transmitter using a
Status (STAT) frame.
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Example of POLL/STAT operation
Receiver
Sender
=3
POLL
STAT
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Buffer management
Receiver
Sender
3
4
5
POLL
Empty buffer
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4
5
STAT
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USTAT
Retransmission
8
7
8
Remove 8 from buffer
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Analysis and Performance
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Advantages of the proposed protocol
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To reduce the acknowledgement traffic because of
is the removal of ACK frame for each data frame
transmissions.
To eliminate of ACK timeout from 802.11
Simulation result
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To compared the proposed protocol with IEEE
802.11
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The overhead
The throughput
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To reduce the ACK traffic
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The occurrence of frame loss
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The POLL/STAT overhead with
increasing a  value
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Total throughput with increase number of
flows at frame loss rate = 0
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Total throughput with increase number of
flows at frame loss rate = 10%
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Total throughput with increase number of
flows at frame loss rate = 20%
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Conclusions
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The performance improvement of the
proposed protocol increases with
increasing bit error rate.
The other issues that will be improved
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buffer management for more link between
transmitter and receiver
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Q&A
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