Chapter 3 Related Work
3.1 Existing Polling Schemes
3.1.3 Comparison of Existing Polling Schemes
We highlight the major differences among these existing polling schemes, including the proposed PEP scheme in Table 2. Except the RRP scheme, the ODP and the PEP schemes maintain a polling list dynamically. Therefore, the complexity of implementing of the ODP and the PEP schemes is higher than the RRP scheme. The PEP scheme consumes less power than the others, without reducing the throughput. In Chapter 4, we will describe the PEP scheme in detail.
Controlled Access Phase (CAP)
…..
Controlled Access Phase (CAP)
Table 2: Comparison of the three polling schemes.
Static Dynamic Dynamic
Complexity of implementation
Easy Medium Medium
Normalized power
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Chapter 4
Design Approach
4.1 Basic Idea
We propose a power-efficient polling (PEP) scheme to improve the ODP scheme. The IEEE 802.11e standard [6] defines the MAC frame format, as shown in Fig. 8. We will use the QoS control field for power saving purpose. The QoS control field is used to identify which traffic stream (TS) or traffic category (TC) a frame belongs to. A TS is defined as a set of
MAC service data units (MSDUs) to be delivered subject to the QoS parameter values provided to the MAC in a particular TSPEC. A TC is defined as a label for MSDUs that has a distinct user priority (UP). Each QoS control field contains five subfields that identify the sender frame type and subtype. These subfields are shown in Table 3. The TID subfield identifies a TC or TS to which the corresponding MSDU in the Frame Body field belongs.
The EOSP (end of service period) subfield is used by the HC to indicate the end of the current service period. The Ack policy identifies the acknowledgement policy.
Fig. 8: MAC frame format [6].
We will use the queue size subfield in the QoS control field. The queue size subfield indicates the amount of buffered traffic for a given TC or TS at the QSTA sending a MAC frame. A QSTA can request a TXOP by setting the queue size. If this field is set to zero, it represents that no buffered traffic in the QSTA’s queue. We suppose if this field is set to zero, a QSTA may have no frames to send when it enters the CAP again. When the QSTA have no
Octets:2 2 6 6 6 2 6 2 0-23124 4
Frame Control
Duration ID
Address 1 Address 2 Address 3
Sequence
frame to send or the size of the frame exceeds the given TXOP limit, the QSTA will send a Null frame to the QAP.
Applicable frame
(sub) types Bits 0-3 Bit 4 Bits 5-6 Bit 7 Bits 8-15 QoS (+) CF-Poll frames
sent by HC TID EOSP Ack policy Reserved TXOP limit QoS Data, QoS Null, and
QoS Data + CF-Ack frames sent by HC
TID EOSP Ack policy Reserved QAP PS buffer state
TID 0 Ack policy Reserved TXOP duration requested QoS data type frames
sent by non-AP QSTAs
TID 1 Ack policy Reserved Queue size
In our proposed scheme, as shown in Fig. 9, non-real time data traffic is only transmitted during EDCA. When a QAP accepts a new voice call from a QSTA, the QAP will add the QSTA to the polling list. Then the QAP in HCCA will periodically poll a QSTA according to the polling list and wait for transmission of uplink voice packets. The QAP will check the Null frame from the QSTA if the queue size field in the QoS control field is set to zero. The QAP will remove a QSTA from the polling list if this field is set to zero and the TXOP is not used up. When a removed QSTA starts to talk, it will use a higher access priority in EDCA to send a voice packet for joining the polling list. The proposed scheme makes sure that QSTAs in the polling list have frames to send. It avoids unnecessary waste of CF-Poll and Null frames and achieves the goal of power saving.
Table 3: QoS control field [6].
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Fig. 9: An example of the PEP scheme.
4.2 A Heuristic Method for Prediction Accuracy Enhancement
In order to predict silent QSTAs correctly, we add a heuristic method of allocated TXOP to the PEP polling scheme. According to the concept of six-state Brady’s speech model and the speech behavior in the real world, we set a criterion for removing QSTAs from the polling list. We first define the utilization of allocated TXOP for a QSTA.
where allocated TXOP means the TXOP assigned for a QSTA by the QAP.
Remaining TXOP means the portion of a given TXOP that is not used up by the QSTA.
By simulations, we derived the following rules:
allocated TXOP – remaining TXOP
Utilization = × 100%
allocated TXOP
PIFS SIFS SIFS SIFS SIFS PIFS
Beacon
Controlled Access Phase (CAP)
…..
(1). Utilization of allocated TXOP < 20%
In this case, we assume that it is in the downlink-only state which represents one station seldom talks. The QSTA will be removed from the polling list immediately. It represents that the QSTA seldom talks.
(2). 20% ≦ Utilization of allocated TXOP ≦ 70%
In this case, we assume that it is in the mutual-talk state which is between the uplink-only state and downlink-only state. The QSTA won’t be removed from the polling list at the moment. If this situation happens in two consecutive beacon intervals, the QSTA will be removed from the polling list.
(3). Utilization of allocated TXOP > 70%
In this case, we assume that it is in the uplink-only state which represents that one station always talks. The QSTA won’t be removed from the polling list at the moment. If this situation happens in three consecutive beacon intervals, the QSTA will be removed from the polling list.
4.3 The Operation of the PEP Scheme
Fig. 10 depicts the operation of the PEP scheme. When sending a Beacon frame by a QAP, the CAP begins. If it is not the end of the CAP, the QAP will send a CF-Poll to a QSTA in the polling list. The QSTA will send a QoS Null frame to the QAP after its transmission end.
The QAP will check if the queue size of the QoS Null frame is zero and calculate the utilization of allocated TXOP of this QSTA. By the three rules described in the last sec`tion, the QAP will make a decision whether or not to remove the QSTA from the polling list. When the CAP ends, the CP follows. If it is not the end of the Beacon interval, all QSTA can transmit data based on the CSMA/CA mechanism. If the QAP received a voice packet sent by a QSTA, the QSTA will be added to the polling list.
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Fig. 10: The flowchart of the PEP scheme.
20% ≦Utilization
≦ 70% ?
Remove the QSTA from the polling list and the counter set 0 Send QoS
Chapter 5
Simulation and Discussion
5.1 Simulation Model
For evaluation, we used the ns-2 simulator [20]. Simulation parameters are showed in Table 4 and the values of PHY-related parameters were from [9]. The length of a beacon interval is 20 ms. We used the G.723.1A codec with a payload of 20 bytes for our simulation [15]. Each station generates variable-bit-rate (VBR) traffic according to the two-state on-off speech model [11][12]. We also used the parameters specified in [12] to set time to
“talk-spurt” = 1 sec and time to “silence period” = 1.35 sec. In other words, the percentage of time spent in the talking state is 43% and the percentage of time spent in the silence state is 57%. Three performance metrics ─ normalized power consumption (%), aggregate throughput (Kb/sec) and average end to end delay (msec) ─ have used to evaluate the merits
of each scheme. We simulated and compared the round-robin polling scheme (RRP), the on-demand polling scheme (ODP), and the proposed power-efficient polling scheme (PEP).
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Table 4: Simulation parameters.
Parameter Value
Duration of the superframe 20 ms Voice coding rate in bps 5.3 K Transmission rate in bits/sec 11 M MAC header (QoS data type) in bits 30 x 8 Header overheads (IP+UDP+RTP) in bits 40 x 8
Physical overhead in seconds (including preamble length and header length)
192 µs
Beacon size in bit 40 x 8
SIFS 10 µs
PIFS 30 µs
Slot time 20 µs
Payload 20 bytes
5.2 Simulation Results and Discussion
We compare our PEP with the RRP and ODP quantitatively. Fig. 11 shows the normalized power consumption versus the number of voice stations. The normalized power consumption is defined as the percentage of a voice QSTA that is in active mode during a superframe [9]. We can see that the PEP scheme consumes less power than the RR and ODP schemes. The power consumption of the ODP and PEP schemes increased with the number of voice stations, which is due to the increased mean contention time. The PEP scheme outperforms the RR and ODP schemes by a margin of 24.5% to 37.1% and 12.9% to 15.1%, respectively.
Normalized power consumption (%)
RRP ODP PEP
Fig. 11: Normalized power consumption of voice stations.
In Fig. 12, we can see that the aggregate throughputs of three schemes are very close.
The aggregate throughput is computed by summarizing the throughput of all connection flows.
The aggregate throughput of the PEP scheme is slightly higher than that of the ODP scheme, but is slightly lower than that of the RRP scheme. This represents that the PEP scheme can reduce power consumption without sacrificing the aggregate throughput.
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Fig. 12: Aggregate throughput of voice stations.
We also measured the average end-to-end delay of voice stations. The average end to end delay is computed by summarizing the end to end delay of all connection flows and averaging it. If a removed QSTA has packets to send, it will be a penalty that the delay of this QSTA will increase. In Fig. 13, we observe that the RRP scheme has lower average end-to-end delay than the other two schemes, because the RRP scheme will not remove a QSTA from the polling list.
The average end-to-end delay of the PEP scheme is slightly higher than that of the RRP scheme, but is lower than that of the ODP scheme. This is because the prediction accuracy of the PEP scheme is higher than that of the ODP scheme.
0
Average end to end delay (msec)
RRP ODP PEP
Fig. 13: Average end-to-end delay of voice stations.
Chapter 6
Conclusions and Future Work
6.1 Concluding Remarks
In this thesis, we have presented a power-efficient polling (PEP) scheme for VoIP traffic over IEEE 802.11e HCF. A QAP can maintain its polling list dynamically. This scheme will reduce the unnecessary polling of silent QSTAs to achieve power saving by checking the queue size field in the Null frame that a QSTA sends to the QAP and the utilization of allocated TXOP. To increase the prediction accuracy of a QSTA entering the silence period, we have also added a heuristic method to evaluate the utilization of allocated TXOP in the PEP scheme. Simulation results have shown that the PEP scheme in terms of the normalized power consumption outperforms the RRP and ODP schemes from 24.5% to 37.1% and from 12.9% to 15.1%, respectively, without sacrificing the aggregate throughput.
6.2 Future Work
In our proposed PEP scheme, the thresholds of the utilization of allocated TXOP were derived from simulations. A more systematic way of deriving such thresholds deserves to further study. In addition to voice traffic, video traffic is also an important category of real time traffic, but the characteristics of these two types of traffic are different. Voice traffic is delay-sensitive, while video traffic can be buffered and then played. The future work is to consider both voice traffic and video traffic to further investigate power efficiency techniques for mobile handheld devices.
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