Chapter 3 Proposed Power-Saving MAC Protocol
3.2 Basic Operation
3.2.5 Transmission Window operation
At the end of ATIM window, each PS mode node that successfully have finished ATIM exchange during an ATIM window wakes up to transmit its Voice/data packets and then enters a doze state according to its individual transmission order. Each node transmits CBR Voice according to the ROC and transmits data according to the Data transmission scheme.
To minimize the switchover penalty of data transmission, the heuristic algorithm developed by Prof .Tsern-Huei Lee et al. is used. The sub-optimum heuristic algorithm that minimizes the number of switchovers from the Doze state to the Awake state is determined based on graph theory. Details of the heuristic algorithm are shown below.
Heuristic Algorithm
Step 1. Set Schedule_List = ∅.
Step 2. Select a node n of graph G with minimum degree.
Step 3. Assume that Neighbor(n) = {n1, n2, …, n } and deg(j n )i ≤ deg(ni−1) for all i, 2≤ i ≤ j. Append the list of edges (n, n1), (n, n2), …, (n, n ) to Schedule_List j and then remove these edges from graph G.
Step 4. Remove a node if all edges incident on it are removed.
Step 5. Let G denote the resulting graph. Stop the algorithm if graph G becomes empty.
Step 6. While node n is removed, set n = j nj−1 till n = n1.
Step 7. If node n is removed, go to Step 2. Otherwise, go to Step 3.
The scenario in Fig 3.10 below is an example of bidirectional CBR Voice and data transmission operation.
Beacon
Figure 3.10: Example of our proposed protocol omitted the beacon generation procedure procedure
To simply the presentation, the beacon generation procedure in the previous section is omitted. Five PS mode nodes involved in the ATIM frame transmission are the following:
A,B,C,D and E. During the ATIM window, ATIM frames are announced successfully as follows, i.e., ATIM(A, C, 1),ATIM(B, D, 1), ATIM(A, C, 10), ACK(C,A, 15) ,ATIM(D, C, 30),ATIM(B, D, 20), ATIM(E, B, 25),ATIM(C, E, 40). Therefore, at the end of the ATIM window, all network nodes should maintain the transmission table in Tables 3.3
and 3.4 if no transmission errors occur.
Table 3.3: Example of bidirectional CBR Voice Table
After ATIM
Transmission Table
Table 3.4: Example of Data Transmission Table
After the ATIM window, firstly, nodes with bidirectional CBR Voice transmit voice packet according to ROC. The A and C wait a SIFS time, and A transmits a Voice packet according to ROC and C response ACK frame to A. At the same time, B and D wake up according to ROC. After a SIFS time, C transmits a Voice packet to A according to ROC.
The A then sends a response to ACK frame to C. After completing the exchange of bidirectional CBR Voice traffic, A and C go into a doze state. Then B continues to transmit bidirectional CBR Voice to D according to ROC. The D follows the same rule as A and C. After completing the bidirectional CBR Voice transmission, the nodes transmit data based on Data transmission order. At the same time, A and C wake up at the CBR Voice transmission time of D to B and A transmit data frames to C follows the Data transmission order. Other nodes then follow the same rule as A and C. Finally, B and D wake up according to Data transmission order at the transmission time of E to B and
transmit their data frames. After completing the Data transmission, B and D enter a doze state until the end of the beacon interval since there is no entry in their transmission table.
This chapter proposes a new protocol for improving throughput and power saving by dynamically adjusting ATIM window length. Nodes are also allowed to stay awake for only a fraction of the beacon interval following the ATIM window. Consequently, more stations can go into sleep mode in the middle of the beacon interval and stay in the sleep mode for a longer duration. Chapter 4 presents the results obtained by simulating the PSM scheme in IEEE 802.11, the proposed protocol, and power-saving mechanism proposed by the M. T Liu et al. [8] and the energy efficiency MAC protocol proposed by Wu et al. [9] in order to evaluate the performance of the proposed protocol.
Chapter 4
Simulation and Discussion
This chapter describes computer simulations to evaluate the energy conservation performance of the proposed protocol in the last section for an ad hoc wireless LAN consisting of ten to twenty hosts. The assumptions are that all hosts are fully-connected, and no transmission errors have occurred, implying that no packet is lost due to poor quality of the channel, and all protocol can avoid overhearing in data transmission window.
4.1 Simulation Model
For our simulation, C++ i s u s e d to implement a simulator The proposed MAC protocol is compared with the original IEEE 802.11 power-saving operation and the M.
T Liu et al. [8] proposed power-saving mechanism, and the Wu et al. [9] proposed energy efficiency MAC protocol. Two traffic types are modeled.
– Only Pure Data
– Pure Data and CBR Voice (6pairs in 10 nodes)
Each simulation was performed 100 times. A 2Mbps channel rate is assumed. To investigate non-real time and real-time traffic over an ad hoc network, two traffic types
are considered.
z Pure Data traffic: The arrival rate of data frames from each station is smaller than 10 kbits within 0.1sec. The frame size is 256 bits to 1024 bits long.
z Voice traffic: Voice traffic is usually considered constant bit rate (CBR) traffic.
After referring to other voice traffic models [11] , the data rate of voice traffic is set to 64Kbps. The value of maximum tolerance is 20 ms. Voice traffic is assumed to have highest priority, and pure data traffic has the lowest priority. Voice belongs to real-time traffic and has the maximum tolerance delay time. According to the traffic models defined above, Table 4.1 summarizes the traffic parameters of the protocol.
Table 4.1: Simulation Parameters
AIFSN=7,CWmin=15 Beacon Period (Voice Delay Bound )
ATIM window size
4.2 Performance Measurements
From many papers analysis on energy saving issues, some conditions are required be covered for each performance measurement:
1. Energy goodput: We used this energy goodput define in [12] to evaluate power efficiency.
Eergy goodput =
yConsumed TotalEnerg
ransmitted TotalBitsT
The unit of energy goodput is bits/Joule, this metric measures the amount of data delivered per joule of energy. The higher the energy goodput is, the lower the energy it consumes.
2. Throughput: The throughput is defined the channel rates that can be used to transmit pure frame by all traffics types of stations. The contention cost is excluded from the throughput.
Throughput =
TotalTime
lPackets fSuccessfu
theAmounto
* th Frame_Leng
3. Average Frame Delay: A period of the time from a frame arrives the system to it transmits completely.
4.3 Simulation Results
Figures 4.1 and 4.2 show the Energy goodput (bits/joule) for pure data and voice &
pure data traffic, respectively. Figure 4.1 shows the Energy goodput for four power-saving protocols with data traffic. When the number of nodes increases, the proposed protocol always achieves better energy goodput than any other protocol because it minimizes switchover time in data transmission window. The protocol consumes less power than other protocols do.
Figure 4.2 illustrates the Energy goodput for power saving protocols with voice &
pure data traffic. When the number of voice pairs increases, a node spends less time in doze state because of voice transmission. Because the proposed protocol provides a Voice reserved scheme to reduce contention among the nodes in ATIM window.
Figure 4.1: Number of Nodes v.s Energy goodput with pure data traffic
Figure. 4.2: Number of Voice pair v.s Energy goodput with pure data &Voice traffic
E n e r g y g o o d p u t
Figures 4.3 and 4.4 illustrate the average throughput (bits/sec/node) for pure data and voice & pure data traffic, respectively. Figure 4.3 shows the average throughput for four power saving protocols with data traffic. In the simulation results, PSM with ATIM window size of 5 ms may suffer severe throughput degradation . If the ATIM is too small in PSM, time is inadequate to announce ATIM. The Wu protocol and the proposed protocol can achieve higher throughput by choosing a suitable ATIM window with traffic load. Liu also used variable beacon interval to accommodate data announced in ATIM window.
Figure 4.4 illustrates the average throughput for four power saving protocols with voice & pure data traffic. When the number of voice pair increases, the proposed protocol has fine average throughput because it spends less time on contention. Voice traffic can again be reserved to reduce contention among the nodes in ATIM window.
The other protocols have no such reserved scheme to avoid contention in ATIM window.
Pure
Figure 4.3: Number of Nodes v.s Average throughput with pure data traffic
Figure 4.4: Number of Voice pair v.s Average throughput with pure data &Voice traffic
A verage throughput
bits per sec per node
PSM
Figures 4.5 and 4.6 illustrate the average delay (msec per packet) for pure data and voice & pure data traffic, respectively. Figure 4.5 shows the average delay for four power saving protocols with data traffic. According to this figure, the PSM reveals a large average delay because it does not have sufficient time to announce the ATIM frame in the current ATIM window and contention in data transmission window. Nodes must retransmit ATIM frames in the next ATIM window, incurring a long average delay.
Figure 4.6 illustrates the average delay for four power saving protocols with voice
& pure data traffic. When the number of voice pairs increases, the protocol has better average delay because real time frames have the higher priority than pure data frame and has a reserved scheme to transmit continuously. The other protocols have no such reserved scheme to transmit data continuously.
Figure 4.5: Number of Nodes v.s Average Delay with pure data traffic
Figure 4.6: Number of Voice pair v.s Average Delay with pure data &Voice traffic
Average delay
0 200 400 600 800 1000
10 12 14 16 18 20
N um ber of N odes
msec per packet
PSM Liu W u p ro p o sed
A v e ra g e d e la y
0 1 00 2 00 3 00 4 00 5 00
1 2 3 4 5 6
N u m b er o f V o ice p air
msec per packet
PSM L iu W u p ro p o se d
Chapter 5 Conclusion
This thesis presents an energy-efficient MAC protocol to support voice/Data traffics over ad hoc networks. The simulation results also demonstrate that the protocol performs much better than the other protocols with real-time and non-real time traffic.
Data transmission is assumed to be perfect and fully connected. Future studies should consider the transmission error and apply the proposed protocols in a multihop network.
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