A Cognitive MAC Protocol for
QoS Provisioning in
Overlaying Ad Hoc Networks
Advisor：Wei-Yeh Chen
Student：王璽農
 L. C. Wang, A. Chen, and D. S. L Wei, “A Cognitive
MAC Protocol for QoS Provisioning in Overlaying Ad
Hoc Networks,” in Proc. 4th IEEE CCNC, Las Vegas,
Nevada, pp. 1139 -1143, January 2007.
1
Outline
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Abstract
Introduction
The cognition cycle(4-stages)
Simulation(NS-2) & Results
Conclusion
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Abstract
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One fundamental issue for a CR network is how
CR users establish an overlaying ad hoc link on
licensed and unlicensed bands.
On licensed band, the CR functions aim to
detect the presence of primary users and
vacate the occupied spectrum to avoid the
interference.
On the unlicensed band, the CR functions have
to improve the efficiency and fairness of
spectrum usage as well as support the QoS.
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Introduction
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The fixed spectrum allocation(靜態分配) may not
be always effectively used everywhere.
In the literature, most of studies for MAC protocol
design focus on the dynamic spectrum
selection(動態選擇).
The main contribution is to propose a CR device
while coexisting with the legacy users on both
licensed and unlicensed bands.
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The cognition cycle

Observe-stage
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Plan-stage
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Decide-stage
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Act-stage
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Establish Neighbor-List(1/2)
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we suggest a neighbor list establishment to
record the information of the primary and CR
users.
we partition the observed frames into three
categories and store the observed information
into three tables:
 Primary user information table (PIT)
 Reservation Information Table (RIT)
 Contention Information Table (CIT)
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Establish Neighbor-List(2/2)
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A CR user is designed to observe the status of
frame transmissions around its neighborhood for
a period of Tobv.
The duration of Tobv must be longer than the
maximum repetition period between two
successive delay-sensitive frame.
 rt-nodes = CR users sending delay-sensitive
 nrt-nodes = non-real-time frames
<BACK>
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Contention Resolution
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One of conditions is to prevent the CR users from
interfering the existing legacy system, and one is
to efficiently and fairly access the unused
spectrum during a short time.
we suggest three improved approaches as
follows:
 Gating mechanism(管控機制)
 Linear back-off algorithm(線性退避演算法)
 Stall avoidance scheme(攤位避免計劃)
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Gating mechanism(1/2)
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When a CR user is requested for transmission,
the gating mechanism first checks whether a
legacy user occupies the channel or not.
If so, the transmission of this CR user is
deferred.
Otherwise, the optimal transmission probability
p is calculated.
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Gating mechanism(2/2)
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Apply the p -persistent algorithm to determine
whether the frame can be transmitted or not.
If the frame is granted for transmission, the
CR user immediately sends the frame.
Otherwise, the frame will be deferred and
again contend for the channel access.
<BACK>
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Linear back-off algorithm
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To expedite the channel access for delaysensitive flows.
If the request of the delay-sensitive traffic flow is
collided, the CW size (CWrt) increases according
to the following principle:
 CWrt = min(CWmax, CWmin × (N_att − 1))
<BACK>
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Stall avoidance scheme(1/2)
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To improve the fairness for the access in short
time, we develop a scheme aiming to reduce the
transmission delay of the nrt-nodes with
excessive buffered frames.
But the specific goal is to minimize the variance
of the transmission delay.
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Stall avoidance scheme(2/2)
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The suggested stall avoidance scheme with
respect to the nrt-nodes is described as
follows.(預定起始值的訊框緩衝最大值=Qthreshold)
CWstall < CWmin

The request of a delay-sensitive traffic flow still
contends for the channel access with CWmin, the
stalled non-real-time frame with CWstall can have
a higher probability to win the channel contention.
<BACK>
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Invited
reservation procedure
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The invited reservation procedure is designed for
supporting the delay-sensitive flows.
接收器負責發送CTS用來控制訊框，讓訊框傳輸以
及對於隨後的訊框延遲來保留時槽，rt-CTS訊框用
來禁止其他用戶的傳輸，因隱藏節點的問題導致碰
撞緩和。由於CR接收器學習了在觀察階段中主要
用戶的頻譜使用時間，隨著動態的調整讓程序能夠
保留在沒有干擾主要使用者的情況下。
<BACK>
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Distributed frame
synchronization mechanism
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One important issue is to develop a distributed
approach to ensure the frame synchronization
among all the CR users.
The basic idea of the proposed distributed
synchronization algorithm (rt-CTS, ACK).
Therefore, all the stations can access the channel
in the designated period without influencing the
transmissions in the reserved time.
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CR MAC &
traditional CSMA/CA MAC
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CR MAC &
traditional CSMA/CA MAC
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CR MAC &
traditional CSMA/CA MAC
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CR MAC &
traditional CSMA/CA MAC
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Conclusion

此文提出了一個感知MAC協定，建立一個CR ad
hoc網絡中配置QoS的傳統無線系統。研究設計的
機制可以補充傳統CSMA / CA的MAC協定的不足，
以實現目標的感知無線網絡。
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