2016 6th International Conference on Information Technology for Manufacturing Systems (ITMS 2016)

ISBN: 978-1-60595-353-3

1 INSTRUCTION

As wireless LANs become increasingly popular, people have produced higher requirements to the throughput. According to the IEEE 802.11n standardization[1], the rate of the physical layer can reach at most 600Mbps due to that the physical layer have introduced the OFDM and MIMO technologies. The MAC layer also have introduced frame aggregation and Block ACK mechanism to reduce the MAC overhead. For the physical layer rate, the range of selectable rates in 802.11n are larger than any prior 802.11 standard. So studying how to dynamically select transmission rate accordance with channel quality has more particular significance in the 802.11n.

Since IEEE 802.11 has no explicit provision on rate control, many articles have analyzed rate adaptation in-depth and proposed a series of classical rate adaptation algorithms [3]-[12] e.g. ARF[3], AARF[4], RRAA[5], SampleRate[7], ONOE[8] etc. These algorithms are all based on the basic ACK statistics. But Block ACK mechanism returns a Block ACK that includes a lot of MPDUs' transmission information after sending a number of MPDUs, the response time of Block ACK is longer than the basic ACK and one time statistics numbers are more than the basic ACK. So these algorithms are no longer suitable for Block ACK mechanism. The literature[13] proposed an algorithm for Block ACK mechanism, but it uses the SNR of BA frame to conduct rate selection and has no in-depth analysis

on the problems exist when the original algorithms are used under the Block ACK mechanism. We assess ARF under Block ACK mechanism and the results show that these classical algorithms exist serious problems about rate change. We analyze the cause of each problem and propose a new algorithm B-ARF to achieve higher throughput.

The contents of this article are organized as follows: Section 1 briefly introduces the Block ACK mechanism and the existing classical algorithms ARF, AARF; Section 2 assesses these classic algorithms under Block ACK mechanism; Section 3 analyzes simulation results of ARF in BA case; Section 4 proposes a new B-ARF algorithm and simulates it; and Section 5 summarizes this article.

2 RELATED WORKS

2.1 Block ACK mechanism

Block-ACK mechanism was firstly used in the IEEE 802.11e[2] and has been expanded in the IEEE 802.11n that put forward the compressed Block-ACK mechanism. When sender and receiver intend to use the Block-ACK mechanism, sender sends an add Block-ACK request frame (add BA-REQ) to the receiver, receiver which support Block-ACK mechanism will respond an add Block-ACK response frame (add BA-RSP), then they will begin to use Block-ACK mechanism for data transmission.

A New Rate Adaptation Algorithm over Block-ACK of 802.11 Protocol

Xueping Chen, Wenda Li, Hongyuan Chen School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, China

ABSTRACT: Rate Adaptation is one important way to improve the throughput in the 802.11 networks. However, the previously proposed rate adaptation algorithms were based on the Basic-ACK mechanism. In IEEE 802.11e and 802.11n protocols, the Block-ACK was introduced to reduce the overhead caused by the ACKs so as to improve the system throughput. In this paper, we focus on the rate adaptation under the Block-ACK. Through a lot of simulation results, we find that the conventional rate adaption algorithms (e.g. ARF) under Block-ACK may cause the worse throughput than under Basic-ACK. Furthermore, we reveal that the main reason is that the channel quality information can’t be feedback timely and accurately, especially over the time-varying channel. Finally, we proposed a new rate adaption (B-ARF) algorithm to improve the throughput under the Block ACK mechanism. The simulation results validate that the B-ARF algorithm outperforms those traditional algorithms.

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Channel access procedure of Block-ACK mechanism is presented in Fig.1. After the initialization, the sender sends a MPDU or RTS to the contend channel, the STA is able to send multiple MPDUs inside one SIFS interval after it receives the ACK frame or CTS frame correctly. When the STA finishes the data block transmission, it sends a Block-ACK request frame(BAR) to tell the receiver that data block send is finished and request for the BA(block ACK) frame. Receiver responses to a BA frame to the sender once it receives the BAR frame in the SIFS interval. BA frame is an aggregated frame that uses bitmap to store confirm information of these MPDUs. Due to the maximum number of MPDUs that a BA frame can be 64, a data block cannot contain more than 64 MPDUs.

Figure 1. An illustration of a burst transmission for the basic access and the RTS/CTS access [10].

2.2 Classical algorithm

In terms of rate adaptation algorithm, ARF algorithm was an early theory presented to improve the application layer throughput of 802.11 devices. In ARF algorithm, each STA tries to send a data packet with a higher rate when satisfied a successful transmission of ten packets continuously, but if the first higher rate packet transmission is error, the STA will go back to the previous transmission rate in the next transmission. When one STA sends two failed packets continuously, it decreases rate to the lower level. Each STA must starts a timer when it begins transmission and the timer is reset to the initial value in each increase rate or decrease rate. When the timer is timeout, the STA increase its default rate. In general, the timeout time is fifteen packets transmission time. However, many literatures have found the problem that ARF is less sensitive to fast-changing channel and in stable channel it increases the probably of retransmitting packets due to the fast increasing rate. To solve this, the [3] proposed AARF algorithm to deal with the

slow-changing channel. In AARF, if its first packet with higher rate is sent failed after increasing rate, it decreases rate back and at the same time double its rate increase threshold and timer threshold. The maximum rate increase threshold is 60 and the timer threshold has no limit. The STA decreases rate directly at two consecutively failed packets and the rate increase threshold and the timer threshold are reset to initial value. There are other good rate adaptation algorithms proposed, e.g. RRAA, CARA[6], Sample-Rate, ONOE, etc. These algorithms increase rate or decrease rate on different ACK statistics rules. Some of the previous algorithms differentiate collision and error by adding RTS/CTS frame, like RRAA algorithm.

All of these algorithms are based on basic ACK mechanism, when they are used in Block ACK mechanism, problems may occur; this paper is to find out the problems and give some substantive advices to help propose better rate adaptation algorithms under Block ACK mechanism.

3 SIMULATION AND ANALYSIS

Here we mainly simulate and analyze the classical algorithms ARF, because AARF does not perform well at time-varying channel. Our simulation tool is NS-3[15] which is an open source software and is very professional in network simulation aspect. NS-3's simulation modules are developing very rapidly which has supported A-MSDU mechanism and Block ACK mechanism and been implemented with a lot of mature rate adaptation algorithms, such as ARF, AARF, CARA etc.

3.1 Simulation environment

Our simulation scenario is multiple STAs which are mobile stations sending packets to an AP simultaneously in a saturated state. And there are 8 kinds of rate for choose, which are 6 Mbps, 9Mbps, 12 Mbps, 18 Mbps, 24Mbps, 36 Mbps, 48Mbps, 54 Mbps. Table 1 shows that the physical layer and MAC layer parameters set in NS-3 and they are consistent with 802.11n standards.

Table 1. PHY and MAC parameters in simulation. Parameters Value Slot time 9usec

SIFS 16usec Propagation delay 1usec

RTS frame length 20bytes

CTS frame length 14bytes ACK frame length 14bytes

MACsublayer overhead 26bytes FCS 4bytes

BlockAcknowledgeRequest 26bytes Block Acknowledge 32bytes PHY layer overhead 16usec

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Table 2. NS-3 configuration in the simulation.

Table 2 shows the configuration information

usedin NS-3 simulation. We employ YansWifiChannel model and YansWifiPhy model which are proposed by M.Lacage as the default configuration of the physical model. In MAC layer we choose QosWifiMac model proposed by M.Banchi of the LART lab.

The Block ACK threshold is set to 1 to enable Block ACK mechanism.

In order to make the system achieve saturation state, the rate of CBR application is set to 54Mbps and CBR is continuous. We employ YansErrorModel as the frame error model. In the model, the bit error rate relies on different modem, the BER in the BPSK and QAM is represented by the following formula:

0.5 ( / )BpskBER erfc snr signalSpread phyRate= ∗ ∗ (1)

2

2

2

/

1.5 log / ( 1)

1 (1 1/ ) ( )

2 1 (1 1)

2 / log

m

m

EbNo snr signalSpread phyRate

z EbNo m

z m erfc z

z z

QAMBER z

= ∗

= ∗ ∗ − = − ∗

= − −

=

(2)

Parameter m is QAM's constellation size. Hence, the BER is changing with the transmission rate.

About the propagation delay and propagation loss in the channel, we use the ConstantSpeed PropagationDelayModel and the LogDistance PropagationLossModel. The delay is set to a constant value which is calculated by the distance between STAs divided by the speed of light. And the receive power is calculated by the below formula:

tan10

( ) 10

exp log / tandis ce

rxPower txPowerDbm referenceLoss

onent referenceDis ce

= − − ∗

∗

(3)

In this simulation, we use the default settings in NS-3.

3.2 ARF simulation

The Block ACK mechanism is designed to improve the system's throughput compared to the ACK mechanism, in the way of reducing the overhead of MAC. However, when ARF is applied under Block ACK mechanism, the throughput is not higher than that of ACK mechanism. On the contrary, the throughput is much lower than that of ACK. As Fig.2 illustrates, the ARF achieves the throughput at different STAs under basic ACK mechanism and Block ACK mechanism with different number of MPDUs, the all simulations are at RTS/CTS access model for fairness.

In ARF algorithm simulation, we plot the rate variation figure with basic ACK and Block ACK with thirty MPDUs, as shown in Fig.3. From the Fig.3 we can see that the Block ACK's rate changes time is longer than that of basic ACK, and the rate changes are unstable in Block ACK mechanism. For instance, if a STA transmits 10 packets one time, the rate may change at the 20-th MPDU for the STA just transmit two data blocks and receives two BAs,

Figure 2. ARF throughput comparison under Block ACK and ACK at different STAs.

but the basic ACK have received 20 ACKs to decide the rate change. So the Block ACK mechanism reflects the channel state more slowly. The unstable rate change is due to the BA including large numbers of MPDUs' ACK. For example, if the STA transmits 64 MPDUs one time at 6 Mbps with no one failure, when receiving the BA, the STA starts to change rate according to the ARF rules. After counting the 64 packets' ACK information, the STA increases its rate six times, this means, its rate is changed to the 48 Mbps. To continue the discussion, the STA starts to send 64 MPDUs at 48 Mbps and we assume these MPDUs are all sent failed, because these MPDUs have a high error probability at such a high rate. Therefore the STA should decrease its rate 32 times until reaching the lowest rate after receiving the Block ACK, and the result is the STA decreases its transmission rate to the lowest rate 6 Mbps directly. From the above analysis, it is not difficult to

Parameters Values Channel YansWifiChannel

PHY YansWifiPhy MAC QosWifiMac

Remote Station Manager ArfWifiManager Data Mode OfdmRate

RTS CTS threshold 0 thatBytes Block ACK threshold 1

Mobility model RandomDirection2dM

obilityModel IP Address Ipv4

Socket UdpSocketFactory Application OnoffApplication

Data transmission rate 54 Mbps Packet Size 1500 Bytes

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understand the ARF algorithm is not suitable for Block ACK mechanism. So we should learn to make a STA increases or decreases its transmission rate one level a time as far as possible.

Figure 3. ARF rate change under Block ACK and ACK at 10 STAs.

3.3 Analysis And Adivces

In the RTS/CTS access model, the BA timeout can only be caused by the BAR error or BA error. Meanwhile, since the BAR and BA frames are strongly robust, their errors must be caused by the poor quality of channel. However, the ARF considers the BA frame timeout as a packet failure and one failure cannot lead to a change in the transmission rate. For the BAR or BA frames errors caused by the poor channel, we cannot simply consider BA frame timeout as one packet error like ARF, because this transmission process has led to many MPDUs retransmission due to high BER, and implies the next transmission process also have high BER at the current rate. So the rate should be decreased to reduce the BER when BA frame is timeout. Other rate adaptation algorithms based on basic ACK mechanism do not specially deal with the BA frame timeout, either. Also, here exists a problem of the timer setting. In ARF, the timer is set to 15, i.e., if the STA miss a BA frame, the STA may decrease its rate four times thanks to the timer timeout at the BA frame which includes 64 MPDUs acknowledge information. This also leads to the rate instability. So the ARF has a great influence on the system performance of throughput and delay under Block ACK mechanism. In the time-varying channel, we should design an algorithm to take into consideration the problem of Block ACK mechanism that has longer response time than basic ACK and let the transmission rate change as stable as possible.

4 PIMPROVED RATE ADAPTATION ALGORITHM ON BLOCK ACK

What can be learned from the above analysis is that the ARF algorithm performs badly when used in Block ACK mechanism. According to our observation, those algorithms based on basic ACK exist these problems: 1) rate change unstable; 2) handle Block ACK frame timeout and timer timeout unreasonable; 3) do not aware that the reflection channel status's time is longer in Block ACK mechanism. So we propose a new Block ACK rate adaptation algorithm (B-ARF) aiming at solving these problems. Our NS-3 simulation results prove that the proposed algorithm can achieve higher throughput than the ARF algorithm in Block ACK mechanism. The Fig.4 is the new algorithm B-ARF's flow chart.

The basic idea of the algorithm is the transmission rate can just be increased or decreased one level a time when counting the Block ACK frame. We specially handle the problems of the Block ACK frame timeout and timer timeout. In Block ACK mechanism, we can see the last packet transmission status have more influence than the front send packets in the next choice of transmission rate, i.e., the last packet receiving status reflect the quality of current channel most. So we choose the last ten packet receiving status (on condition BA includes more than ten MPDUs ACK information) to decide whether to increase or decrease transmission rate. When the number of Block ACK acknowledges is less than 10, we employ all packets for analyzing. In Fig.4, the MIBA is the MPDU's ACK index in the BA frame, NBA is the number of MPDUs in the BA frame. When the STA receives the BA frame that has two consecutive MPDU errors in the last ten MPDUs, it decreases its rate, at the same time the decrease flag is set to true so that the STA cannot decrease its rate again in this BA duration. When the STA receives the BA frame that has ten consecutive MPDUs transmit successfully, it increases its rate and set the increase flag to true to avoid the STA increases its rate again in this BA duration. We set the timer to the time of 15 MPDUs, when the STA 15 packets, and the rate has not been increased or decreased, the STA default increases its rate in next transmission. And the increase flag is set to true to avoid the rate changing again in this BA duration. As previously analyzed, BA frame timeout is a result of purely poor channel. We think this timeout cannot be simply regarded as a packet error, so the B-ARF algorithm directly decrease rate for BA timeout case. Then these MPDUs with retransmission have much chance to transmit successfully to reduce the delay of the system.

These parameters are obtained from massive experiments, and the simulation result with these parameters is far superior to the classical ARF

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algorithms under Block ACK mechanism. Fig.5 and Fig.6 are the comparison figures about B-ARF, ARF and AARF algorithm in throughput aspect with 5 and 20 STAs.

Figure 4. The flow chart of B-ARF algorithm.

Figure 5. ARF, AARF, B-ARF throughput under Block ACK mechanism with different MPDUs number at 5 STAs.

Figure 6. ARF, AARF, B-ARF throughput under Block ACK mechanism with different MPDUs number at 20 STAs.

5 CONCLUSIONS

In this paper, the classical rate adaptation algorithms ARF under Block ACK mechanism is analyzed by simulation results. Research indicates that the ARF algorithm exists some serious problems which influence system's throughput and delay. To solve these problems, we propose a new Block ACK rate adaptation algorithm (B-ARF), which counts last ten packets to accurately reflect the quality of channel, set rate change flag to keep the rate stable and handle Block ACK timeout and timer timeout specially. Simulation results show that our B-ARF algorithm has more stable and higher throughput than that of the classical ARF algorithms. The B-ARF algorithm may be improved by variation of these threshold parameters and a better method of processing Block ACK timeout and timer timeout. This will be our future work.

6 ACKNOWLEDGEMENT

This research is supported by Natural Science Foundation of Jiangsu Province (#BK20130773).

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