Chapter 1 Introduction
1.3 Thesis Objective and Organization
In the thesis, we focus on the power management in fully-connected ad hoc networks.
We assume that time is divided into beacon intervals which begin and end approximately at the same time at all nodes. We propose a power saving efficient MAC protocol that integrates voice and Data traffic with the QOS method and the ATIM window adjustment to achieve a better trade-off between power consumption, network throughput and delay.
The remainder of this thesis is organized as follows. Chapter 2 provides a review of
IEEE 802.11 PSM in DCF and related works. Chapter 3 presents our proposed protocol. In Chapter 4, we describe our simulation model and discuss simulation results. Finally, a conclusion to this thesis is presented in Chapter 5.
Chapter 2
Overview of IEEE 802.11 Power Saving Mode and Related Works
In this chapter, firstly, we will review the power management as defined in IEEE 802.11, and then discuss some drawbacks in the current standard in section 2.1. Much research has been done in power saving for mobile devices in the past. We will also show the inefficiency of the other power-saving mechanisms and will be presented in section 2.2.
2.1 Overview of IEEE 802.11 PSM
We briefly review the main operation of the power saving mode in an IEEE 802.11 ad hoc network. According to the IEEE 802.11 power saving mode, all nodes are connected synchronously by waking up periodically to listen beacon messages. This scheme requires the synchronization among the stations. Therefore, a T S F ( time synchronization function) is defined in the standard. To save energy, stations go to the Power Saving (PS) mode when no incoming or outgoing traffic is present. In PS mode, the system is in a Doze state and its transceiver is shut down to save power. If a node acquires the medium, it will send an ATIM frame to the destination node based on the CSMA/CA access scheme. The ATIM frame is announced inside the ATIM window. If the destination node receives the ATIM frame, it will respond with an ATIM-ACK frame and stay active to receive data in the data window. After the ATIM window, the buffered data should be sent based on the CSMA/CA
access scheme. If a node fails to send its ATIM frame in the current ATIM window, it should retransmit the ATIM frame in the next ATIM window. If a node does not send or receive any ATIM frame during the ATIM window, it will switch to PS mode to decrease power consumption until the next beacon interval begins. T he IEEE 802.11’s power management is shown in Figure 2.1-1.
Figure 2.1: Power-saving mechanism in IEEE 802.11
Fig 2.1-1 shows an example. Initially, all nodes wake up at the beginning of the beacon interval. Since all nodes did not send or receive any ATIM frames in the ATIM window of the first beacon interval, all nodes will switch to sleep state in the Data transmission window.
In the ATIM window of the next beacon interval, node A has a packet destined for node B.
Therefore, nodes A sent ATIM frames to node B based on the CSMA/CA access scheme and
transmit buffered data to node B based on the CSMA/CA access scheme.
The performance of PSM is affected by the size of the ATIM window. When the ATIM window is too short with transmitting stations in the system, the contention in ATIM is high and only a few stations can send their ATIMs successfully. Then the channel utilization of data window of the same beacon interval is poor. If ATIM window is too long with transmitting stations in the system, many stations can enter the data transmission. This will cause not only higher contention in data window, but also the waste of energy for those who couldn’t get a chance to transfer data during the current beacon interval, since all stations who succeed in transferring their ATIMs are required to stay in Active Mode throughout the beacon interval. Both the ATIM window and the beacon interval are fixed length. It was shown in [7] that PSM performed well when the length of the ATIM window was approximately 1/4 of the beacon interval. Furthermore, the IEEE 802.11 PSM has two-contention transmissions (one for the ATIM frame and one for data packet). This will often cause a mismatch of the number stations in each contention period. For example, we may have many successful nodes in the ATIM window, but only a few nodes can transmit their data packet in the data transmission interval due to the high contention in the data window.
The power-saving mechanism suffers several problems with end-to-end delays and throughput. If a sender wants to immediately transmit packets to a receiver but has not transmitted an ATIM frame to the receiver, the packets cannot be sent at once. Therefore, PSM shall allow for longer larger delays as compared to the normal IEEE 802.11.
Furthermore, if the network traffic is heavy, then a sender in PSM cannot inform a receiver of its pending packets by the ATIM frames for an ATIM window, resulting in a declination of throughput. Ad hoc networks in PSM have large end-to-end delays and a degraded throughput.
As will be seen from this thesis, we improve the energy efficiency in PSM without degrading throughput.
2.2 Related Works
In the section, we review several existing power management protocols for IEEE 802.11ad hoc networks as following.
2.2.1 Power-Saving Mechanism in Emerging Standards for Wireless LANs: The MAC Level Perspective
Woesner et al. [7] presented simulation results for the power saving mechanisms of two wireless LAN standards, IEEE 802.11 and HIPERLAN. It showed the different sizes of beacon intervals and ATIM windows in IEEE 802.11 had a significant impact on throughput and energy consumption. The authors indicate that the ATIM window size should take approximately 1/4 of the beacon interval. However, the effects of load variety is not put much focus on. We can see here that it affects this optimal ratio between the ATIM window and the beacon interval quite significantly.
2.2.2 A power-saving scheduling for IEEE 802.11 mobile ad hoc networks
M. T Liu et al. [8] proposed an energy efficiency MAC protocol for IEEE 802.11 networks by scheduling transmission after the ATIM window and adjusting the Beacon
Interval dynamically to adapt to the traffic loads. All stations are required to be able to hear from each other directly. Nodes that overhear ATIM frames will generate a contention-free schedule for data transmission in the rest of the beacon interval, rather than let those nodes that have succeeded to announce in the ATIM window to contend again for the data transmission. With all the information received at each station during the ATIM window, a deterministic scheduling can be generated. This not only eliminates extra contention in the data transmission but also increases the efficiency of power saving.
2.2.3 An energy efficient MAC protocol for IEEE 802.11 WLANs
In [9], Wu et al. proposed an energy efficiency MAC protocol for IEEE 802.11 networks
by scheduling transmission after the ATIM window and adjusting the ATIM window dynamically to adapt to the traffic Loads. But they did not consider the energy consumption caused by overhearing among neighboring nodes. Nodes schedule those to-be-transmitted data frames after ATIM window. According to a buffered data frame’s duration, nodes determine the transmission order. Data transmission takes place to avoid unnecessary frame collision and backoff time. Besides, nodes adjust the ATIM window dynamically to adapt to the traffic loads.
Chapter 3
Power-Saving MAC Protocol 3.1 Overview
The proposed power-saving MAC is an energy-efficient dynamically-adjustable ATIM window protocol designed for real- and non-real time data broadcasting. Under the proposed protocol, data is transmitted according to a dynamically updated transmission schedule. Each node utilizes prioritized contention based on Enhanced Distributed Channel Access (EDCA) for periodical real-time (CBR Voice) and non-real time (Data) traffic. Each node with real-time (CBR Voice) traffic initializes access to medium is through contention, but once a node reserves transmission time, its reservation time in the subsequent Beacon Interval continues automatically as long as the node continues to broadcast a packet in each Beacon Interval. Thus, nodes need only contend for transmission time at the beginning of Real-time (CBR Voice) traffic bursts in the ATIM window.
Nodes transmitting Real-time (CBR Voice) traffic also require the Reservation Occupation Table maintenance scheme to ensure correct channel utilization. The CBR Voice nodes transmit Beacon messages which carry the Reservation Occupation Table to each network node at the beginning of the ATIM window. Beacon generator handover occurs during the CBR Voice transmission schedule by specifying the new
Figure 2.1 indicates that the fixed lengths of the ATIM window and the beacon interval of IEEE 802.11 ad hoc power saving mode (PSM) often consume excess energy.
The inefficiency of the fixed ATIM window size under different traffic loads requires dynamic adjustment of the ATIM window size.
Finally, in the power saving mode (PSM) of IEEE 802.11, nodes that successfully announce ATIM frames stay active during the entire beacon interval. After Data transmission, nodes need not remain active state. After a node finishes Data transmission, the proposed power-saving MAC protocol switches the node from an active state to a sleep state to minimize power consumption. The novel feature of the proposed MAC protocol is the decreased idle time in active state and the switchover times (active-to-sleep state or sleep-to-active state).
The following sub-sections describe these features in more detail.
3.2 Basic Operation
The proposed power-saving MAC protocol matches time-frame duration to the periodic rate of voice packets. Figure 3.1 presents the frame format. Each frame consists of two sub-frames: a ATIM sub-frame and a data transmission sub-frame. The ATIM sub-frame consists of a beacon generation window and a ATIM window. The data transmission sub-frame consists of a CBR Voice window and Data window.
Figure 3.1: Representation of Proposed power-saving MAC frame format
3.2.1 Definition for new management frames in ATIM window
Because the ATIM and ATIM_ACK frame defined in the standard has MAC header, Frame Check Sequence (FCS) and an empty frame body, the standard was slightly modified to include that information in the ATIM and ATIM_ACK frame. A frame body of several octets was added to deliver that information can be easily done and will not affect other operations defined in the standard. Besides, the beacon frame carries the ROC (Reserved OCcupation Table) to indicate Reserved CBR Voice information. The following definitions and frame formats are used in the proposed protocol.
ROC (Reserved OCcupation Table)
– Indicates Reserved CBR Voice information including Source address, Destination address, and Transmission Order
– Beacon Frame carries ROC used by residential nodes
Data
BGS (Beacon Generation Sequence)
– Nodes with reserved CBR Voice must send Beacon with ROC at TBTT in sequence
– The beacon transmission sequence with ROC
• Every CBR Voice responder attached to Every CBR Voice sender to form the BGS.
Figure 3.2: Beacon Frame with ROC
Figure 3.3: ATIM and ATIM_ACK Frame for Data Time
001~110 for CBR Voice 0/1
3.2.2 Beacon generation procedure
In the initial startup stage, nodes at TBTT that have no BGS within the Ad Hoc contend to transmit the beacon frame (Fig. 3.4). Each node calculates a random delay uniformly distributed within the range from 1/4 to 2*CWmin*SlotTime (CWmin is the minimum contention window used in the backoff mechanism), then waits for the duration of the delay using the backoff mechanism. When the delay counter expires, the beacon is transmitted with a timestamp, which is a copy of the sender’s local TSF timer of the sender, with some adjustment. Nodes cancel the beacon transmission if a beacon arrives before their random delay counter expires. When a beacon is successfully received, stations set the TSF timer according to the timestamp of the beacon if the value of the timestamp is later than the TSF timer of the station itself in PSM of 802.11.
Therefore, the TSF timer can only be adjusted forward. In the proposed protocol, Timers can be set both forward and backward to ensure that all nodes wake up simultaneously.
Figure 3.4: Beacon generation Window
As soon as nodes have BGS, those that have reserved bidirectional CBR Voice must TBTT
1/4*CWmin*SlotTime 2*CWmin*SlotTime
B B B
Beacon without ROC Beacon with ROC
send a Beacon with ROC at TBTT in turn. If the Beacon generator node fails, the remaining network nodes should be able to compensate for this situation and should be able to continue normal operation as quickly as possible.The BGS is a more natural and complete way of backing up the beacon generator and it provides a perfect beacon transmission order list of ROC after ATIM exchange. Every CBR Voice responder is attached to CBR Voice sender to form the BGS. Nodes that have reserved CBR Voice can listen for the beacon and become the beacon generator whenever the previous beacon generator fails. The first sender in the sequence is the first generator; the second sender is the second generator; and the senders append the responders. The backup nodes listen to the beacon, which is a part of normal network operation. If the second generator does not hear the beacon for a short interframe space (SIFS) time, then the first generator is assumed dead and the second generator modifies the ROC and transmits the beacon with ROC. If the beacon is not transmitted for two SIFS times, then the third generator understands that both generators are dead. It then modifies ROC and transmits the beacon. If after N SIFS times no beacon is transmitted, then the rest of the nodes understand that No CBR Voice is reserved, and each node calculates a random delay uniformly distributed within the range between 1/4 and 2*CWmin*SlotTime and the transmitted beacon. The Maintenance scheme of Reserved OCcupation Table can be summarized as follows:
• Initially, nodes send ATIM message piggyback Transmission Order (s1, s2) if No
Beacon Generation Sequence (Fig. 3.5).
• If reservation is successful, pair nodes continues to reserve the Transmission in future cycles (Fig. 3.6).
• Finally, nodes send ATIM message and release the piggyback ROC after the conversation is completed or if the partner dies (Fig. 3.7).
• If the pair Nodes suddenly shut down, the other nodes should know the pair nodes have died and modifies ROC.
Figure 3.5: Maintenance scheme of Reserved OCcupation Table (一)
21
Figure 3.6: Maintenance scheme of Reserved OCcupation Table (二)
22
Figure 3.7: Maintenance scheme of Reserved OCcupation Table(三)
3.2.3 ATIM Information Exchange procedure
After the beacon transmission window, nodes enter the ATIM window. For ATIM information exchange, the system piggybacks the number of pending packets of ATIM announcement for Data. The information about the sender, receiver, and length of the pending data are contained in ATIM frame denoted ATIM (Sender ID, Receiver ID, Data Length). The transmission Release/Reservation status of ATIM announcement for CBR Voice is also piggybacked. The information about the sender, receiver, and Release/Reservation status of the CBR Voice traffic is contained in the ATIM frame denoted as ATIM (Sender ID, Receiver ID, TX status). According to IEEE 802.11 regulations, all nodes are fully connected and all PS mode nodes can wake up at almost the same TBTT. At the TBTT, each node wakes up for an ATIM window interval. If a node with buffering Data to a PS mode node, it sends an ATIM frame to the PS mode node within the ATIM window period. Upon receiving the ATIM frame, the PS mode node responds to an ATIM ACK to the sender of the ATIM frame and completes the reservation of the data frame transmissions. If a node with buffering CBR Voice to a PS mode node, it sends an ATIM frame to the PS mode node within the ATIM window period. Upon receiving the ATIM frame, the PS mode node responds with an ATIM ACK to the sender of the ATIM frame and joins the Reservation OCcupation Table (ROC). These pair nodes are not required to send further ATIM frames during the
or if their pair nodes do not respond to with an ATIM ACK to the sender during CBR voice transmission. Due to the broadcast nature of the wireless medium, each node can overhear the ATIM exchange information and get all node transmission tables (Data Length and CBR Voice Transmission Order) as Table 3.1 shows.
Each node employs prioritized contention-based Enhanced Distributed Channel Access (EDCA) as defined in the 802.11e [10]. Each transmission queue has a different interframe space (AIFS) and a different contention window limit. Each node that intends to transmit data calculates its priority according to the collected data profiles. The proposed protocol uses Data and voice traffic only. Nodes adjust the contention window size and AIFS time. T he smaller i t s AIFS, the higher t he priority a node can have.
Similar protocols apply to the contention window size. Figure 3.8 illustrates the time diagram of EDCA [10]. The 802.11e suggests the use of different AIFS and different contention window limits according to different ACs. Table 3 . 2 shows the parameters for the maximum contention window (CWmax), the minimum contention window(CWmin)
TX RX Data Length TX Order A C 10B s1 C A 15B s2 D C 30B
B D 20B s3 D B 10B s4 E B 25B
C E 40B
Table 3.1:Transmission table during ATIM window
and AIFS for each AC.
Figure 3.8: The timing diagram of 802.11e EDCA[10]
AC CWmin CWmax A
I
AC_BK aCWmin aCWmax 7
AC_BE aCWmin aCWmax 3
AC_VI (aCWmin + 1)/2 aCWmin 2
AC_VO (aCWmin + 1)/2 (aCWmin + 1)/2 2 Table 3.2: The default EDCA parameters [10]
The value of AIFS is determined by the following equation (1)[10]:
AIFS = AIFSN × aSlotTime + SIFS (1)
where the value of AIFS Number (AIFSN) is an integer greater than zero and is dependent on each AC.
3.2.4 Traffic-Load oriented ATIM window adjustment
To conserve the power used by PS mode nodes and to improve network throughput, the relationship between the lengths of the ATIM window and the corresponding beacon
interval is discussed in [7] as mentioned above. The authors indicated that the ATIM window
size should take approximately 1/4 of the beacon interval. The proposed scheme can
dynamically adjust the ATIM window according to the channel idle time and the actual traffic load of the network. Each node senses how long the channel is continuously idle during the ATIM window. If nodes sense that the channel is idle longer than AIFS (AC_BK) + CWmin, the assumption is that no nodes have buffered frames to send.
Besides, if ATIM collision occurs and Remaining Beacon Interval ≧ TMAXMPDU,
nodes could stay awake until no further ATIM collision occurs or remaining Beacon Interval ≦ TMAXMPDU. Therefore, all nodes can close the ATIM window and enter the
data transmission window according to transmission table. The following Fig. 3.9 is the traffic Load-oriented ATIM window adjustment algorithm.
Figure 3.9: Traffic-Load oriented ATIM window adjustment In ATIM window
Uses ATIM and ATIM_ACK with EDCA scheme to reduce collision and generate a contention-free transmission table
Dynamically Adjust ATIM Window Size {
While(1) {
R=0 R( Retransmit counter)
IF (Remaining BI <= TSIFS +TFRAME Min) Then AW=(Tcurrent-TBTT) ,Break;
ELSE Do {
CW=random [0, Min (CWmin*2R﹐CWmax) ]．SlotTime R=R+1
}
While( ATIM collision occur and Remaining BI ≧TMAXMPDU) IF (Tchannel idle ≧AIFS(AC_BK) + CW)
then AW=(Tcurrent-TBTT)+ AIFS(AC_BK) +CW , Break;
}
Nodes immediately transmit CBR Voice and Data on schedule }
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
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