ENGINEERING FACULTY,UNIVERSITYOF PORTO
Technical Report no. 1
Robson Costa
Supervisor: Paulo Portugal (Ph.D.) Co-supervisor: Francisco Vasques (Ph.D.) Co-supervisor: Ricardo Moraes (Ph.D.)
2010, September c Robson Costa, 2010 Contents
List of Figures ii
List of Tables iii
List of Abbreviations iv
1 Introduction1 1.1 Benefits ...... 2 1.2 Challenges ...... 2
2 IEEE 802.11 Standard4 2.1 IEEE 802.11 Medium Access Mechanisms ...... 5 2.1.1 DCF - Distributed Coordination Function ...... 6 2.1.2 PCF - Point Coordination Function ...... 7 2.1.3 EDCA - Enhanced Distributed Channel Access ...... 9 2.1.4 HCCA - HCF Controlled Channel Access ...... 11
3 IEEE 802.11n Amendment 14 3.1 PHY Enhancements ...... 15 3.1.1 MIMO - Multiple-Input Multiple-Output ...... 15 3.1.2 Channel-bonding ...... 17 3.2 MAC Enhancements ...... 18 3.2.1 Frame aggregation ...... 19 3.2.2 Block ACK ...... 21 3.2.3 Reverse Direction Protocol ...... 22
4 Review of Relevant Work 23 4.1 Real-Time communication in IEEE 802.11 ...... 23 4.1.1 CA - Collision Avoidance ...... 23 4.1.2 CS - Collision Solver ...... 26 4.1.3 CR - Collision Reducer ...... 27 4.2 Comparison of the solutions presented ...... 30
5 Conclusion 31
References 37
i List of Figures
2.1 Original IEEE 802.11 MAC architecture [1]...... 5 2.2 IEEE 802.11e MAC architecture [2]...... 5 2.3 Interframe spaces in the DCF and PCF mechanisms [1]...... 6 2.4 DCF service [2]...... 6 2.5 PCF service [2]...... 8 2.6 CFP foreshortening [2]...... 9 2.7 Interframe spaces in the EDCA mechanism [2]...... 10 2.8 HCCA service [2]...... 12
3.1 Receive diversity...... 16 3.2 Transmit diversity...... 16 3.3 Spatial Multiplexing...... 17 3.4 DCF basic operation...... 18 3.5 IEEE 802.11e TXOP and block ACK...... 19 3.6 Two-level aggregation in IEEE 802.11n...... 20 3.7 Block ACK in IEEE 802.11n...... 21 3.8 Reverse Direction in IEEE 802.11n...... 22
4.1 Comparative between approaches presented...... 30
ii List of Tables
2.1 IEEE 802.1D to IEEE 802.11e map...... 10 2.2 Access Category Medium Access Parameters...... 11
3.1 Comparison between IEEE 802.11 network standard...... 15
iii List of Abbreviations
AC Access Category
ACK Acknowledgement
AIFS Arbitration Interframe Space
BE Best-Effort Traffic
BK Background Traffic
CSMA Carrier Sense Multiple Access
CSMA/CA Carrier Sense Multiple Access with Collision Avoidance
CTS Clear-to-Send
CW Contention Window
DCF Distributed Coordination Function
DIFS Distributed Interframe Space
EDCA Controlled Channel Access
EIFS Extended Interframe Space
HCCA Enhanced Distributed Channel Access
HCF Hybrid Coordination Function
IEEE Institute of Electrical and Electronics Engineers
IFS Interframe Space
MAC Medium Access Mechanism
PCF Point Coordination Function
PHY Phyical Layer
PIFS PCF Interframe Space
QAP Quality of Service Access Point
QoS Quality of Service
RTS Request-to-Send
iv LIST OF TABLES v
SIFS Short Interframe Space
TXOP Transmission Opportunity
VI Video Traffic
VO Voice Traffic Chapter 1
Introduction
The industrial communication scenario is progressively evolving thanks to the availability of even more performing networks that may be employed at all the levels of factory automation systems. In particular, focusing on the lowest level (usually referred as factory floor), besides field-buses, which have been for long time the networks traditionally deployed, two other types of communication system are nowadays available:
• Real-Time Ethernet (RTE) Communication Systems: can be classified as Networked Con- trol System (NCS), derived from well known IEEE 802.3 Ethernet [3], are based on full duplex switched configurations and exploit the impressive increasing of the communication speed in order to provide the performance required at the device level.
• Wireless Communication Systems: can be classified as Wireless Networked Control Sys- tem (WNCS), available for industrial applications giving the chance to build on-the-fly con- trol systems allowing a greater flexibility, mobility and reduced costs when compared with the RTE-based solutions.
The major requirement for industrial wireless networks is the support of timely communica- tion services. Therefore, the demand for high performance wireless networking with real-time (RT) capabilities is one of the most relevant research challenges in this domain. How nowadays the IEEE 802.11 Wireless Local Area Networks (WLANs) [2] is a de facto standard for wireless networks, the focus of this technical report is turned to it. In this chapter the main benefits and challenges when WLANs are utilized as real-time com- munications networks in the factory floor are presented. In the chapter2, is presented a brief introduction of IEEE 802.11 standard and yours main drawbacks when utilized as real-time com- munication system. The chapter3 presents the main enhancements provided by the IEEE 802.11n amendment [4]. The chapter4 presents a survey of state-of-the-art about solutions for real-time communications systems operating on top of IEEE 802.11 standard. Finally, in chapter5, the conclusions are presented.
1 Introduction 2
1.1 Benefits
The main benefits on the use of wireless networks in real-time communication systems when compared with the RTE-based solutions are the greater flexibility, mobility and reduced costs that this kind of network can provide.
• Flexibility: supporting the trends towards reconfigurable production systems that need to be adaptable to changing need of the market [5]. Parallel to flexible automation concepts on application level, wireless networks provide utmost flexibility on the communication side.
• Mobility: which is advantageous in areas where nodes are widely scattered or where wires can be installed only with great difficulties due to hostile environments. Mobility in this respect does not necessarily mean that nodes may go wherever they want; they can as well stay within a strictly confined area (e.g., robotic arm).
• Reduced Costs: when flexibility and mobility of a line of production is increased, hence their costs will drop due to reduced downtime for changes that may be necessary and also the reduction of wiring used.
1.2 Challenges
The utilization of WLANs to industrial applications presents a number of additional chal- lenges compared to home or enterprise applications, the most severe being to strict requirements about transmission delays and frame loss ratio. In enterprise Voice-over-IP (VoIP) applications can tolerate transmissions latencies up to 150 ms and up to 1 percent [6] data corruption on the exchanged frames, thanks to adaptive play-out control and error concealment algorithms. By contrast, factory automation systems usually require shorter cycle times, that usually range (about) from 1 to 10 ms but could be well bellow 1 ms for specific applications (e.g., motion con- trol). In addition, minimizing communication jitters is also important for many control systems based on cyclic operation. Another important requirement in automated control systems are deterministic performance guarantees. In industrial networks, runtime performance degradation in not an option for mission- critical applications. This requirement is also enforced during device roaming, which leads to the requirements for real-time handover. Therefore, at present, wireless communications are used in industrial environments mainly to enable simpler and more cost-effective maintenance and diag- nostics functions. According to Cena et al. in [7], the wireless solutions and products today available on the market are generally considered unsuitable for implementing distributed control applications and systems, in particular when real-time is one of the key issues. This is mainly due to three reasons:
• communication over radio channels is very sensitive to electromagnetic interferences (often present in abundance in industrial environments), which may cause excessively high error rates; 1.2 Challenges 3
• interference may be generated as a consequence of overlapping wireless networks, that are out of the sphere-of-control1 of the real-time system;
• the nature of the random access scheme used by the WLANs, namely, the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), prevents that this kind of network presents a deterministic behavior.
As a consequence, in wireless networks there is not any guarantee that a frame will be delivered timely to the intended target(s), and not even that it will be delivered at all. Thus, no upper bound can be ensured on transmission latency, that is usually unacceptable in most control systems.
1The concept “inside/outside“ sphere-of-control was defined by Kopetz [8]. Whenever a real-time entity is in the sphere-of-control of a subsystem, it belongs to a subsystem that has the authority to change all the value of this real-time entity. Outside its sphere-of-control, the value of the entity can be observed, but cannot be modified. Chapter 2
IEEE 802.11 Standard
With the authorization of the use of three frequency bands in 1955 by the Federal Communi- cations Commission (FCC), U.S. agency responsible for regulating the use of frequency spectrum, in 1990 the Institute of Electrical and Electronics Engineers (IEEE) established a committee to define a standard for wireless connectivity which had its first standard approved in 1997 under the name of IEEE 802.11 [1] with nominal data rates of 1 and 2 Mbps; In 1999 it was approved the IEEE 802.11b [9] and 802.11a [10], which use the frequencies of 2.4 and 5 GHz and are capable of nominal rates of transmission of 11 and 54 Mbps, respectively. The 802.11b standard, while achieving lower transmission rates, gained a greater share of the market than 802.11a, the reasons for this were basically two: first, the 802.11b interfaces were less expensive than the 802.11a and, second, 802.11b implementations was introduced on the market before than the 802.11a implementations. In 2003, the standardization committee of the IEEE approved the IEEE 802.11g [11] standard that, like 802.11b, works in the 2.4GHz frequency, but reaches up to 54 Mbps of nominal rate of transmission. In 2005 it was approved the IEEE 802.11e [12] specification, adding differentiated levels of Quality-of-Service (QoS) to the supported applications, including the transport of voice and video over IEEE 802.11 networks. In 2007 it was published a revised version of IEEE 802.11 [2] standard, that incorporates the amendments from 1 through 8 including a corrigendum. Recently, in 2009 was approved the IEEE 802.11n [4] defined as Enhancements for Higher Throughput, which uses multiple antennas for transmission and reception, Multiple-Input Multiple- Output (MIMO), reaching nominal rate of transmitting up to 600 Mbps using frequency bands of 2.4 GHz and/or 5 GHz. This chapter presents a briefly introduction of IEEE 802.11 standard (more specifically the Medium Access Control MAC layer) and yours main drawbacks when utilized as real-time com- munication system.
4 2.1 IEEE 802.11 Medium Access Mechanisms 5
2.1 IEEE 802.11 Medium Access Mechanisms
The original IEEE 802.11 [1] standard define the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the medium access mechanism, referred as Distributed Coordination Function (DCF). More accurately, the IEEE 802.11 MAC sublayer introduces two MAC functions: the mandatory DCF and the optional Point Coordination Function (PCF) if a contention-free ser- vice is required (figure 2.1).
Figure 2.1: Original IEEE 802.11 MAC architecture [1] .
With the objective to provide Quality-of-Service (QoS) in IEEE 802.11 networks, in 2005 was published the IEEE 802.11e amendment that incorporates an additional coordination function called Hybrid Coordination Function (HCF) (figure 2.2). The HCF mechanism schedules the ac- cess to the channel by allocating Transmission Opportunities (TXOP) to each of the stations. Each TXOP is defined by a starting time and a maximum duration (i.e., a time interval during which the station keeps the medium access control) enabling that multiple frames may be transmitted within an acquired TXOP. It may be allocated through one of two access mechanisms specified by the HCF: the Enhanced Distributed Channel Access (EDCA) and the HCF Controlled Channel Access (HCCA).
Figure 2.2: IEEE 802.11e MAC architecture [2]. IEEE 802.11 Standard 6
2.1.1 DCF - Distributed Coordination Function
The Distributed Coordination Function (DCF)1 access method is the basic IEEE 802.11 mech- anism, where stations perform a so-called backoff procedure before initiating a transmission. For this, the DCF imposes an idle interval between consecutive frames, which is called the Interframe Space (IFS) (figure 2.3). Different IFSs are defined in order to impose different prior- ities to multiple frame types as following: SIFS (Short Interframe Space), PIFS (PCF Interframe Space), DIFS (Distributed Interframe Space) and EIFS (Extended Interframe Space). SIFS is the shortest of the interframe spaces and it is used for ACK frames. Only stations operating under the Point Coordination Function (PCF) will use PIFS. DIFS is used by stations operating under the DCF mechanism to transmit data frames and management frames. EIFS is used in communication- error conditions .
Figure 2.3: Interframe spaces in the DCF and PCF mechanisms [1].
In the DCF, when a station wants to transmit, it previously senses the medium with the Carrier Sense (CS) mechanism, implemented by the Physical Layer (PHY), and if the medium remains idle during one DIFS time interval, it immediately starts the transmission (figure 2.4).
Figure 2.4: DCF service [2].
1An additional mechanism, RTS/CTS, is defined in the IEEE 802.11 standard to solve the hidden terminal problem and to adequately handle the transmission of long messages. For further details, please refer to [2]. 2.1 IEEE 802.11 Medium Access Mechanisms 7
Otherwise, the station selects a random time called backoff time. The duration of this random interval is a multiple of the Slot Time (ST), which is a system parameter that depends on the characteristics of the PHY. The number of slots is an integer in the range of [0, CW], where CW
(Contention Window) is initially assigned as aCWmin.A backoff counter is used to maintain the current value of the backoff time. In this case, stations keep sensing the medium (listening) for this additional random time, after detecting the medium as idle for a DIFS interval. If the medium gets busy due to interference or other transmissions while a station is down-counting its backoff counter, the station stops down- counting and defers the medium access until the medium becomes idle for a DIFS interval again. A new independent random backoff value is selected for each new transmission attempt, where the CW value is increased by (oldCW ×2+1), with an upper bound given by aCWmax. As soon as the backoff counter reaches zero, the station can retry its transmission.
2.1.1.1 Drawbacks
The DCF mechanism have basically two main drawbacks. The first is that, it only provide a best-effort service, not supporting any QoS guarantees. Typically, time-bounded applications (e.g., real-time and VoIP) require specified bandwidth, low delay and jitter. In the DCF, all the stations contend for the medium with the same priorities, without any differentiation mechanism to guarantee high-priority for the time-bounded applications. The second drawback is the non-determinism of the probabilistic contention resolution algo- rithm that serializes the contending messages whenever a collision occurs. Therefore, the major challenge concerning the design of new RT architectures for CSMA- based networks is that the channel is a shared resource. As a consequence, there is the need to prioritize RT data messages (for both medium access and to resolve collisions), specially when the communication infrastructure is shared with uncontrolled traffic sources.
2.1.2 PCF - Point Coordination Function
The Point Coordination Function (PCF) is one of the main solutions intended to support QoS in IEEE 802.11 wireless networks. It has been proposed in the original IEEE 802.11 standard as an optional access mechanism with the objective to provide a contention-free service. It implements a centralized polling scheme to support synchronous data transmissions, where the Point Coordi- nator (PC) performs the role of polling master. The PCs are used to ensure that the medium is provided without contention restricting the access to the medium making that associated stations can transmit data only when they are allowed to do so by the PC. This PCs reside in Access Points (APs), so the PCF mechanism is restricted to infrastructured networks. The contention-free service is not provided full-time. Periods of contention-free service ar- bitrated by the PC alternate with the standard DCF-based service. When the PCF is used, time on the medium is divided into the Contention-Free Period (CFP) and the Contention Period (CP). Access to the medium in the first case is controlled by the PCF, while access to the medium in the IEEE 802.11 Standard 8 second case is controlled by the DCF. Although access is under the control of a central entity, all transmissions must be acknowledged. At the beginning of the CFP, the PC transmits a Beacon frame. One component of the bea- con announcement is the maximum duration of the CFP (CFPMaxDuration). All stations that receives this Beacon set the Network Allocator Vector (NAV) to the maximum duration to lock out DCF-based access to the wireless medium. As an additional safeguard to prevent interference, all contention-free transmissions are separated only by the SIFS and the PIFS interval time (figure 2.3). Both are shorter than the DIFS interval time (used by DCF), so no DCF-based stations can gain access to the medium using the DCF access method (figure 2.5).
Figure 2.5: PCF service [2].
After the PC has gained control of the wireless medium, it polls any associated stations on a polling list (that is the list of privileged stations solicited for frames during the CFP) for data transmissions. During the CFP, stations may transmit only if the PC solicits the transmission with a contention-free polling frame (CF-Poll). Each CF-Poll is a license to transmit one frame. Multiple frames can be transmitted only if the PC sends multiple poll requests. To ensure that the PC retains control of the medium, it may send a CP-Poll to the next station on its polling list if no response is received after one PIFS interval time. The minimum length of the CP is the time required to transmit and acknowledge one maximum- size frame. However, is possible that contention-based service overrun the end of the CP. When this occurs, the CFP is foreshortened (figure 2.6). When the CFP is foreshortened, the existing frame exchange is allowed to complete before the beacon announcing the start of contention-free operation is transmitted. The CFP is shortened by the amount of the delay. Contention-free service ends no later than the maximum duration from the expected beginning point, which is referred to as the Target Beacon Transmission Time (TBTT). The PC may also terminate the CFP prior to its maximum duration by transmitting a CF-End frame. It can base this decision on the size of the polling list, the traffic load, or any other factor that the PC considers important. 2.1 IEEE 802.11 Medium Access Mechanisms 9
Figure 2.6: CFP foreshortening [2].
2.1.2.1 Drawbacks
Despite mechanism being well suited to handle delay-sensitive applications, most part of the WLAN network cards never actually implemented the PCF scheme due to complexity rea- sons [13]. Furthermore, the cooperation between CFP and CP modes may lead to unpredictable beacon delay [14, 15] if in the instant scheduled to sent the next beacon (TBTT) the medium is busy. This delay defers the transmission of data frames during CFP, which may severely impact the QoS performance of real-time applications. Another problem of the PCF mechanism is the difficult to predict the transmission time of a polled station. A polled station is allowed to send a frame of any length between 0 and 2304 bytes, which may introduce variable transmission time. Furthermore, the PHY rate of the polled station can change according to the varying characteristics of the channel, so the AP is not able to predict in a precise manner the transmission time. This prevents the AP to provide guaranteed delay and jitter performance for other stations present in the polling list during the rest of the CFP.
2.1.3 EDCA - Enhanced Distributed Channel Access
The Enhanced Distributed Channel Access (EDCA) mechanism was designed to enhance the DCF mechanism providing differentiated transmission services with four access categories (ACs). Each frame arriving at the MAC layer with a defined priority will be mapped into one of the four ACs (background, best-effort, video and voice), where the background and voice are the lowest and highest ACs priorities, respectively. These ACs are based on the eight priority levels of IEEE 802.1D standard (table 2.1). IEEE 802.11 Standard 10
Table 2.1: IEEE 802.1D to IEEE 802.11e map.
802.1D Priority 802.1D Designation 802.11e Priority 802.11e Designation 1 BK - Background 0 BK - Background 2 Spare 0 BK - Background 0 BE - Best Effort 1 BE - Best Effort 3 EE - Excellent Effort 1 BE - Best Effort 4 CL - Controlled Load 2 VI - Video 5 VI - Video 2 VI - Video 6 VO - Voice 3 VO - Voice 7 NC - Network Control 3 VO - Voice
Different levels of service are provided to each of the ACs, based on three independent mech- anisms: (i) the Arbitration Interframe Space (AIFS); (ii) the Transmission Opportunity (TXOP) time interval; (iii) the Contention Window (CW) size. In figure 2.7 are showed the relationships between the multiple AIFSs in the EDCA scheme.
Figure 2.7: Interframe spaces in the EDCA mechanism [2].
For a station operating under EDCA, each frame will wait that the medium remains idle during an AIFS[AC] interval. The duration of the AIFS[AC] interval is given by:
AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime (2.1) where the AIFSN[AC] is a positive integer that must be greater than or equal to 2 for all stations, except for the QoS Access Points (QAPs), where it shall be greater than or equal to 1. The TXOP, that is the time interval during which the station keeps the medium access control, is used for that multiples frames can be transmitted within an acquired TXOP if there is more than 2.1 IEEE 802.11 Medium Access Mechanisms 11 one frame pending in the AC for which the channel has been acquired. If a station wants to transmit a frame while the channel is busy, or becomes busy before the expiration of the AIFS[AC], the backoff procedure is invoked (third traffic differentiation mecha- nism). The contention window is defined by the aCWmin[AC] and aCWmax[AC] attributes, in con- trast to the legacy DCF where the initial values were randomly select among the [0,CW] interval defined by the physical layer. In the EDCA mechanism, the backoff procedure selects a random number, in the range [0,CW], where the CW size is initialized at aCWmin[AC]. When a transmis- sion fails, CW is increased by [(oldCW)[AC]+1)×PF −1] upper bounded by aCWmax[AC] where PF is the persistence factor (its default value is PF = 2). On the other hand, the backoff counter decreases the backoff interval whenever the medium is detected to be idle for AIFS[AC]. The aCWmin, aCWmax and TXOP parameters depends of the default characteristics of the PHY layer (table 2.2).
Table 2.2: Access Category Medium Access Parameters.
AC CWmin CWmax AIFSN TXOPmaxa/g/n TXOPmaxb 0 aCWmin aCWmax 7 0 0 1 aCWmin aCWmax 3 0 0 2 (aCWmin + 1)/2 − 1 aCWmin 2 3008 µs 6016 µs 3 (aCWmin + 1)/4 − 1 (aCWmin + 1)/2 − 1 2 1504 µs 3264 µs
2.1.3.1 Drawbacks
Although the EDCA access method is an evolution compared to the traditional DCF, your main drawback remains the non-determinism of the probabilistic contention resolution algorithm. A common setup of real-time communication systems in EDCA-mode is the utilization of voice (VO) access category to transmit RT traffic. However, if the the network is overlapping by other using the same frequency channel and a VO traffic transmission begin, a interference will occurs in the RT traffic and a non-deterministic delay will be generated. In [16], Moraes et al. have assessed the behavior of this category when used to transfer periodic small sized packets in an open communication environment. Both the number of packet losses and the average size of the MAC queues forecast an unacceptable number of deadlines losses for RT messages streams, even for intermediate load cases.
2.1.4 HCCA - HCF Controlled Channel Access
The HCF Controlled Channel Access (HCCA) mechanism was proposed in the IEEE 802.11e amendment to improve the PCF scheme. It is based on a Round-Robin scheme and it is intended to guarantee bounded delay requirements. Similarly to the PCF scheme, the APs in HCCA contains a logical entity known as the Hybrid Coordinator (HC) that keeps tracks of HCCA client stations and schedules the polling intervals. Polled access as implemented in HCF allows a station to IEEE 802.11 Standard 12 request a TXOP, instead of just determining that one is available. The HC polls all the stations in the polling list, even though some stations may not have messages to transmit. When the HC polls a station that has no packets to transfer, the station will transmit a null frame. HCF operation combined with HCF admission control allows the HC to intelligently determine what resources are available on the wireless medium and accept or reject application traffic streams. HCF can operate in two modes, one coexisting with EDCA and the other using a contention-free period (CFP), similar to PCF. In the beginning of the HCCA operation, the AP send a beacon frame specifying the start time and duration of a CFP, after the HC offers a TXOP to HCF-capable stations by sending QoS CF- Polls to them. The stations must reply back within a SIFS time interval with data frames or with a QoS null frame, indicating the station has no traffic or the frame it desires to send is too large to do so in the time allocatted in the TXOP. The CFP ends when the HC sends a CF-End frame, or when the CFP duration expires (figure 2.8).
Figure 2.8: HCCA service [2].
What truly differentiates HCCA access method from EDCA is the admission-control mecha- nism. Unlike the EDCA, which use a Distributed Admission Control (DAC) relies on the stations to interpret and respect the transmit budget advertised in the QoS parameter, the HCCA requires that the station request particular reservation parameters for the application traffic stream, such as VoIP, from the HC. The HC can evaluate and determine whether there is enough budget available on the wireless medium to facilitate the requested traffic stream. The HC can then accept, reject, or even offer an alternative set of parameters to the station. This mechanism is far more robust and effective than DAC. However, this robustness does not come without a penalty. The HC must keep a strict schedule of traffic streams. This is because the admission control of HCCA centers on the Traffic Specification (TSPEC). The TSPEC allows the client station to specify parameters such as frame/stream 802.1D priority, frame size, frame and data rates and delay. This data is sufficient for the HC to determine whether the wireless medium can sustain exist- ing traffic streams and this newly requested stream without degrading any of the existing streams. The TSPEC also indicates to the HC how often the station is expecting to get polled. The station 2.1 IEEE 802.11 Medium Access Mechanisms 13 must generate a unique TSPEC for each traffic stream it wants to transmit and receive with priority and for each direction of the stream (e.g., a bidirectional VoIP call requires two traffic streams).
2.1.4.1 Drawbacks
Beyond the problem of the polling overhead, that is roughly equal to the time interval from sending the polling frame till the end of the ACK frame [17], some preliminary studies [18, 19], have already shown that HCCA may not be able to guarantee the expected RT communication requirements. Furthermore, it is still not clear if the HCCA mechanism will be implemented in next gen- eration of the WLAN networks cards, solving the availability problem of the PCF mechanism [13]. Therefore, it is expected that multiple applications will use the EDCA mode to support RT communications, due to the widespread availability of this access mode. Chapter 3
IEEE 802.11n Amendment
The development of the IEEE 802.11n amendment began in 2003 with the main objective to allow rates of at least 100 Mbps, essentially doubling the existing maximum rate of 54 Mbps for the IEEE 802.11 a/g specifications. Many companies submit proposals to the IEEE for this new amendment, but in February of 2005 only two proposals was selected, both with strong backing from various companies. The first proposal, presented by the WWiSE (World Wide Spectrum Efficiency) group, suggest the use of channels with similar bandwidth to the existing IEEE 802.11 b/g networks (20 MHz) as well as the use of multiple transmit and receive antennas, or MIMO (Multiple-Input Multiple- Output), to achieve throughput rates of around 135 Mbps in real-world conditions. The second proposal, presented by the TGn (Task Group) Sync, suggest doubling the band- width to 40 MHz, to essentially double the throughput. In addition, other, more sophisticated processing techniques allowed the TGn Sync devices to transmit data at rates up to 315 Mbps. The two proposals evolved over the next couple of months to form the main competing propos- als for IEEE 802.11n standard, but due to the significant support each proposal enjoyed, neither proposal was able to obtain the majority vote required for adoption toward the IEEE 802.11n standard. Finally, in July 2005, a group consisting of members of both proposal groups agreed to form a joint proposal group, which submitted a new proposal to the TGn workgroup in January 2006. This proposal, referred to as the TGn Joint proposal, combined the benefits of the other proposals, and formed the basis of the drafts for the IEEE 802.11n standard, resulting in the final version presented in October 2009 called “IEEE 802.11n - Amendment 5: Enhancements for Higher Throughput”. To achieve the increased throughput and range envisioned for IEEE 802.11, this amendment describes enhancements to both the Physical (PHY) and Medium Access Control (MAC) layers. This chapter presents an exposition on the techniques used in IEEE 802.11n to achieve the im- provements to throughput and range proposed by this amendment.
14 3.1 PHY Enhancements 15
3.1 PHY Enhancements
The main modifications in the PHY layer include:
• The use of multiple transmit and receive antennas, known as Multiple-Input Multiple-Output (MIMO);
• The use of channel bonding, offering possibility for using 40 MHz channel.
3.1.1 MIMO - Multiple-Input Multiple-Output
The MIMO system in IEEE 802.11n define that multiple antennas at the transmitters and/or receiver are used at the same time (and on the same frequency band). To enable this, transmitters and receivers must have multiple radio frequency (RF) processing chains to go with their multiple antennas; the techniques used are signal processing techniques implemented in the PHY layer hardware with some amount of high-level control available to the driver. The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. However, the individual radios often further limit the number of spatial streams that may carry unique data. The X × Y:Z notation helps identify what a given radio is capable of. The first number (X) is the maximum number of transmit antennas or RF chains that can be used by the radio. The second number (Y) is the maximum number of receive antennas or RF chains that can be used by the radio. The third number (Z) is the maximum number of data spatial streams the radio can use. For example, a radio that can transmit on two antennas and receive on three, but can only send or receive two data streams would be 2 × 3:2. The maximum theoretical throughput achieved by the IEEE 802.11n (600 Mbits/s) occurs only in 4 × 4:4 setup (table 3.1).
Table 3.1: Comparison between IEEE 802.11 network standard.
802.11 Frequency Bandwidth Allowable MIMO Data rate per stream Modulation Protocol (GHz) (MHz) streams (Mbits/s) - DSSS, FHSS 2.4 20 1 1, 2 a OFDM 5 20 1 6, 9, 12, 18, 24, 36, 48, 54 b DSSS 2.4 20 1 1, 2, 5.5, 11 g OFDM, DSSS 2.4 20 1 1, 2, 6, 9, 12, 18, 24, 36, 48, 54 20 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2 n OFDM 2.4, 5 4 40 15, 30, 45, 60, 90, 120, 135, 150
In the IEEE 802.11n two basic classes of multiple antenna techniques are used: i) Spatial Diversity, that provide more reliability and range and; ii) Spatial Multiplexing, that provide more performance. The Spatial Diversity technique increase the reliability and range by sending or receiving redundant streams of information in parallel along different spatial paths between transmit and receive antennas. The use of extra paths improves reliability because it is unlike that all of the IEEE 802.11n Amendment 16 paths will be degraded at the same time. The use of multiple antennas to gather a large amount of signal at the receiver can improve the range and some performance also. The spatial diversity technique can be subdivided in Receive Diversity and Transmit Diversity, making reference to the receive or transmit antenna. In figure 3.1 is shown one transmit antenna at a node sending to two receive antennas at a second node. This is known as 1x2 system. Real systems may have more than two receive antennas. With this setup, each receive antenna receive a copy of the transmitted signal modified by the channel between the transmitter and itself.
h x 11
y1 = h1 1 x h12 y2 = h1 2 x Tx Rx
Figure 3.1: Receive diversity.
In figure 3.2 is shown two transmit antennas at a node sending to one receive antenna at a second node. This is known as 2x1 system. This can be useful when the AP has more antennas than the client, so that it can use its multiple antennas to benefit a single antenna client.
h11 x y = (h1 1 + h2 1 )x x h21
Tx Rx
Figure 3.2: Transmit diversity.
The disadvantage of transmit diversity compared to receive diversity is that the transmitter must know the channel beforehand in order to select between antennas or to precode the signals. This require feedback from the receiver. In the IEEE 802.11n, there are antenna selection, rate selection, and channel state feedback packets that the receiver can use to send information to the 3.1 PHY Enhancements 17 transmitter. Another solution is, since the proprieties of RF channels are reciprocal, the transmitter learn the channel gains when it receives a packet from the target receiver. The Spatial Multiplexing technique increase performance by sending independent streams of information in parallel along the different spatial paths between transmit and receive antennas. This improves performance because, if we take care in how we construct and decode signals, adding an antenna and independent stream of information need not slow down the streams that are already being sent. In figure 3.3 is shown two transmit antennas at a node sending to two receive antennas at a second node. This is known as 2x2 system.
h y 11 1= h1 1 x 1 + h2 1 x x1 h12 h21 x2 h22 y 2= h1 2 x 1 + h2 2 x2 Tx Rx
2
Figure 3.3: Spatial Multiplexing.
3.1.2 Channel-bonding
The IEEE 802.11n can operate on both 2.4 GHz and 5 GHz bands using two separate channels to transmit data, thus doubling the rate in principle. The legacy IEEE 802.11 a/b/g systems use a single 20MHz channel, but IEEE 802.11n can operate in the 40MHz mode over two adjacent channels, one as the control and the other as the extension. However, all the 40 MHz channels are partially overlapping in the 2.4 GHz band (IEEE 802.11 b/g), as opposed to the 20 MHz band channels 1, 6 and 11 which are non-overlapping. Thus using 40 MHz channels can also lead to degradation in the throughput due to increased interference with neighboring channels. The use of 40 MHz channels is recommended only when all stations can operate in 5 GHz, like in environments that have only IEEE 802.11n stations or in hybrid environments with both IEEE 802.11n and IEEE 802.11a stations. With this at least 12 non-overlapping channels are provided. IEEE 802.11n Amendment 18
3.2 MAC Enhancements
The main modifications in the MAC layer include:
• The addition of frame aggregation scheme, allowing the sending of multiple MAC frames in one PHY layer packet to reduce overhead;
• The block ACK enhancements, acknowledging frames in blocks, also to reduce overhead;
• A Reverse-Direction (RD) protocol, that allows the transmit station currently holding the air channel to efficiently transfer control to another station, without the need for the other station to initiate a data transfer;
• A schemes for co-existence with legacy devices;
• Features of IEEE 802.11e, Quality of Service (QoS), to support delay-sensitive applications;
• A battery saving feature for WLAN in handheld devices called Power Save Multi-Poll (PSMP);
• Features of IEEE 802.11y, enabling a extended channel switch announcement, that allow an Access Point (AP) to switch between support of 20 MHz only, and 20 MHz / 40 MHz;
• Features of IEEE 802.11k, that improves the radio resource management, allowing the effi- cient use of multiple APs within a network;
• Features of IEEE 802.11r, allowing a support for fast roaming.
In order to understand the higher throughput enhancement in the MAC of IEEE 802.11n, a brief discuss of overhead in the legacy IEEE 802.11 MAC will be address. The widely used DCF is a distributed channel access mechanism based on CSMA/CA. A successful packet transmission in DCF is illustrated in figure 3.4. When a station has an MPDU to transmit, it waits a fixed time interval called DIFS before transmission.
Figure 3.4: DCF basic operation.
When MAC finds an idle channel, it enters a backoff procedure with a backoff timer, which is determined randomly by the contention window (CW). If the channel is still idle during and after the backoff procedure, the station immediately accesses the channel. The station wich successfully received the MPDU should send an acknowledge (ACK) frame back to the sending station. A SIFS 3.2 MAC Enhancements 19 waiting time is applied before replying the ACK message. The whole transmission procedure end when the sending station successfully receives the ACK frame. The overhead of the DCF mechanism results in the inefficiency of the channel utilization, and thus limits the data throughput. When the payload is small, the overhead is relatively large and is less efficient. The percentage of the overhead among all usable airtime increases as the physical transmission rate increases. The overhead limits the achievable data throughput. In the higher data rate scenario, although the frame transmission time is reduced, the other part of the overhead is unchanged due the backward compatibility issue. Thus, to achieve higher throughput in IEEE 802.11, a reduce in the percentage of overhead is fundamental. In IEEE 802.11e MAC protocol, the EDCA is introduced to enhance legacy IEEE 802.11 DCF operation. The support of QoS is provided with fourth access categories (ACs) like presented in chapter2. An important breakthrough in the IEEE 802.11e MAC mechanism is the introduction of the transmission opportunity (TXOP). This mechanism defines a period of time for a station accessing the channel to transmit multiple frames. During a TXOP period, the station can transmit multiple data frames without entering backoff procedure, which reduce the overhead due to contention and backoff and thus enhances the efficiency of channel utilization. A TXOP period can be obtained through a successful contention in EDCA. In pair with TXOP, the block acknowledgement (block ACK) mechanism can be used to fur- ther enhance the channel utilization efficiency. A station transmitting multiple data frames in TXOP can request one block ACK for all these frames instead of using legacy acknowledgement to each frame, and the resource of transmitting multiple acknowledgement and interframe spaces SIFSs is saved, as shown in figure 3.5.
Figure 3.5: IEEE 802.11e TXOP and block ACK.
3.2.1 Frame aggregation
Frame aggregation amortizes protocol overhead over multiple frames. It packs several data frames (MPDUs) into an aggregate frame (called A-MPDU). Overhead in multiple frame trans- missions is reduced since the header and interframe time is saved. Aggregation scheme achieves higher system gain for applications scenarios with small packets (e.g., Real-Time and VoIP appli- cations). In IEEE 802.11n MAC, the aggregation mechanism is designed as two-level aggregation scheme [20]. Two types of aggregation frame are defined: IEEE 802.11n Amendment 20
• A-MSDU: aggregate MAC protocol service unit;
• A-MPDU: aggregate MAC protocol data unit.
The aggregation mechanism can work with A-MPDU, A-MSDU, or using both of them to form two-level aggregation. A-MSDU is composed with multiple MSDUs and is created when MSDUs are received by the MAC layer. For ease in the de-aggregation process, the size of MSDU, including its own subframe header and padding, must be a multiple of 4 bytes. Two parameters are used for forming A-MSDUs: the maximum length of an A-MSDU, 3839 or 7935 bytes by default, and the maximum waiting time before creating an A-MSDU. These MSDUs must be in the same traffic flow (same TID) with the same destination and source. Broadcasting and multicasting packets are excluded. In the second level, multiple MPDUs are aggregated into an A-MPDU. A-MPDU are created before sending to PHY layer for transmission. Unlike the A-MSDU creation, MAC does not wait for additional time before the A-MPDU aggregation. MAC only use the MPDUs already in the queue upon creating A-MPDUs. The TID of each MPDU in the same A-MPDU might be different. The maximum size limit of A-MPDU is 65535 bytes. In A-MPDU, each MPDU has an MPDU delimiter at the beginning and padding bytes at the end. These bytes ensure that the size of each MPDU is multiple of 4 bytes. Delimiter is used to separate MPDUs in an A-MPDU. The de-aggregation process first checks the CRC integrity. If the CRC check is passed, the MPDU will be de-aggregated and sent to the upper layer.
Figure 3.6: Two-level aggregation in IEEE 802.11n. 3.2 MAC Enhancements 21
The two-level aggregation mechanism is shown in figure 3.6. In the first level, MSDUs re- ceived by MAC from the upper layer are buffered for a short time until A-MSDUs are formed according to their TIDs, destination, source, and the maximum size of A-MSDU. The complete A-MSDUs and other non-aggregate MSDUs then enter to the second level to form an A-MPDU. For compatibility reason, every MPDU in A-MPDU should not exceed 4095 bytes. Is important to pay attention to the fact that IEEE 802.11n aggregation does not support frame fragmentation. Only complete A-MSDUs or MSDUs could be contained in an A-MPDU. The whole aggregation mechanism completes when A-MPDU is created.
3.2.2 Block ACK
Originally, the block ACK operation incorporates the TXOP mechanism defined in IEEE 802.11e. In the IEEE 802.11n, this mechanism is further enhanced to be applied with the ag- gregation feature. Although a larger aggregation frame can significantly reduce the overhead in transmission, the frame error rate is higher as the size of the frame increases. Large frames in high bit-error-rate (BER) wireless environment have a higher error probability and may need more retransmission and with this the network performance might be degraded.
Figure 3.7: Block ACK in IEEE 802.11n.
To overcome this drawback in aggregation, the block ACK mechanism in IEEE 802.11n (figure 3.7) is modified to support multiple MPDUs in an A-MPDU. When an A-MPDU is re- ceived and errors are found in some of the aggregated MPDUs, the receiving node sends a block ACK only acknowledging those correct MPDUs. The sender only needs to retransmit those non- acknowledged MPDUs. Block ACK mechanism resolves the drawback of large aggregation in the error-prone wireless environment and further enhances the performance of IEEE 802.11n MAC. Note, block ACK mechanism only applies to A-MPDU, but not A-MSDU. That is, when an MSDU is found to be incorrect, the whole A-MSDU needs to be transmitted for error recovery. The maximum number of MPDUs in an A-MPDU is limited to 64 as one block ACK bitmap can only acknowledge at most 64. The original block ACK bitmap field with 64 × 2 bytes. These two bytes record the fragment number of the MSDUs to be acknowledged. However, fragmentation of MSDU is not allowed in IEEE 802.11n A-MPDU. Thus, those 2 bytes can be reduced to 1 byte, and the block ACK bitmap is compressed to 64 bytes. This is known as Compressed Block ACK. Compared with IEEE 802.11e, the overhead of block ACK bitmap in IEEE 802.11n is reduced. IEEE 802.11n Amendment 22
3.2.3 Reverse Direction Protocol
The Reverse Direction (RD) mechanism is a novel breakthrough to enhance the efficiency of TXOP. In conventional TXOP operation, the transmission is unidirectional from the station holding the TXOP, which is not applicable in some network services with bidirectional traffic (e.g., VoIP). The conventional TXOP operation only helps the forward direction transmission but not the reverse direction transmission. For application with bidirectional traffic, their performance de- grades by the random backoff and contention of the TXOP. Reverse direction mechanism allows the holder of TXOP to allocate the unused TXOP time to its receivers to enhance the channel utilization and performance of reverse direction traffic flows. The RD operation is illustrated in figure 3.8, where two types of stations are defined: RD initiator and RD responder. The RD initiator is the station which holds the TXOP and has the right to send Reverse Direction Grant (RDG) to the RD responder. RDG is marked in the IEEE 802.11n header and is sent with the data frame to the RD responder. When the RD responder receives the data frame with RDG, it responds with RDG acknowledgement if it has data to be sent, or without RDG if there is no data to be sent to the RD initiator. If the acknowledgement is marked with RDG, the RD initiator will wait for the transmission from the RD responder, which will start with SIFS or RIFS (Reduced Inter-Frame Space) time after the RDG acknowledgement is sent. The RIFS (that have 2 µs) can be used in the scheme when no packet is expected to be receiver after transmission. If there is still data to be sent from the RD responder, it can mark RDG in the data frame header to notify the initiator. The RD initiator still has the right to accept the request. To allocate the remaining TXOP, the initiator will mark the RDG in the acknowledge message or the next frame. To reject the new RDG request, the initiator just ignore it.
Figure 3.8: Reverse Direction in IEEE 802.11n.
The major enhancement in RD mechanism is the delay time reduction in reverse link traffic. These reverse direction data packets do not need to wait in queue until the station holds TXOP but can be transmitted immediately when the RD responder is allocated for the remaining TXOP. This feature can benefit a delay-sensitive service like VoIP. Chapter 4
Review of Relevant Work
This chapter reviews the state-of-the-art of real-time communication systems implemented on top of IEEE 802.11 networks. The main characteristics and drawbacks of each approach are pre- sented together with a classification into two levels. At the end of a comparative table is presented.
4.1 Real-Time communication in IEEE 802.11
The solutions presented in this survey use the classification defined by Moraes in [21] where approaches are classified according to how the collisions are dealt in order to provide a real-time (RT) communication service. Three types of classification are defined:
• Collision Avoidance (CA): used in traditional approaches to guarantee a RT communication service trying avoidance collisions;
• Collision Solver (CS): used replacing the traditional probabilistic collision resolution algo- rithm by an algorithm that ensures an adequate deterministic collision resolution;
• Collision Reducer (CR): used to try reduce the number of collisions through the use of adequate loosely-coupled distributed algorithms.
4.1.1 CA - Collision Avoidance
The Point Coordination Function (PCF) is one of the main solutions intended to support QoS in IEEE 802.11 wireless networks. It has been proposed in the original IEEE 802.11 standard as an optional access mechanism. It implements a centralized polling scheme to support synchronous data transmissions, where the Point Coordinator (PC) performs the role of polling master providing a contention-free service used to ensure that the medium is provided without contention restricting the access to the medium making that associated stations can transmit data only when they are allowed to do so by the PC. This PCs reside in access points (APs) restricting this mechanism to
23 Review of Relevant Work 24 infrastructured networks. The main drawback of PCF mechanism is the fact of most part of the WLAN network cards never was implemented due to complexity reasons [13]. The HCF Controlled Channel Access (HCCA) mechanism was proposed in the IEEE 802.11e amendment to improve the PCF scheme. It is based on a round-robin scheme and it is intended to guarantee bounded delay requirements. Similarly to the PCF scheme, the Hybrid Coordinator (HC) also polls all the stations in the polling list, even though some stations may not have messages to transmit. When the HC polls a station that has no packets to transfer, the station will transmit a null frame. As a consequence, the polling overhead is roughly equal to the time interval from sending the polling frame till the end of the ACK frame [17]. Furthermore, it is still not clear if the HCCA mechanism will be implemented in next generation of the WLAN networks cards, solving the availability problem of the PCF mechanism [13]; and some preliminary studies [18, 19] showing that HCCA may not be able to guarantee the expected RT communication requirements. A number of improvements have been proposed to reduce the HCCA polling overhead. For instance, Gao et al. [22] proposed a new admission control framework to replace the traditional CSMA mechanism. It uses the mean data rate and the mean packet size values to calculate the resource needed by each message flow. The main problem of this approach is the fact that is not taken into account the possibility of overlapped networks that can generate interferences among themselves. Son et al. [17] proposed a polling scheme where the HC punish the stations that have no packets to transmit. When a station transmits a null frame, this stations will not be polled again during a period of time. The main problem of this solution is that when the real-time traffic is larger then the Contention Free Period (CFP) the station will be punished. Moreover, if the RT traffic is aperiodic or sporadic, the RT stations can be punished as well. Lo, Lee and Chen [23] designed a multipolling mechanism called Contention Period Multipoll (CP-Multipoll), which incorporates the DCF access scheme into the polling scheme using different backoff values for the multiple message streams in the polling group, where each station executes the backoff procedure after receiving the CP-Multipoll frame. To avoids the interference from other stations performing the backoff procedures in the DCF mode, the first station in the polling list initializes its transmission immediately after receiving the CP-Multipoll frame. Moreover, in order to avoid the repeated collisions between stations that are operating on the same channel in the overlapping space, the values assigned in the CP-Multipoll frame among neighboring BSSs must be are different. However, in environments with many stations this mechanism can affect the scalability of the approach. Lee et al. [24] proposed a polling scheme based on a master slave solution. It uses a virtual polling list (VPL) that contains the MAC address of the wireless slaves to be polled, and a virtual polling period (VPP) that defines the duration of the polling cycle. When a slave receives a poll frame from the master, it can transmits a response frame to the master, or directly to another slave. Furthermore, after polling all the slaves registered in the VPL, the master invites other slaves into the network through the broadcast of an entry claim frame. The main problem of this approach is similar to the HCCA problem. The Master invite stations one by one, even stations that does not 4.1 Real-Time communication in IEEE 802.11 25 have anything to transmit needs response with a ACK frame. Other problem is the cycle created, that force the stations wait all the cycle to can begin a new transmission. Ergen et al. [25] presents the WTRP (Wireless Token Ring Protocol), which is a MAC pro- tocol that exchanges special tokens and uses multiple timers to maintain the nodes synchronized. Each station transmits during a specified time and if enough time is left, the station invites nodes outside the ring to join. This solution has been proposed to eliminate the backoff inefficiencies and the collision problems in a ring topology, however the network is underutilized like in the previous approach. Other problem of token-based approaches is the join/leave of station in the communication system, forcing a reconfiguration of all system. In [26], Cheng et al. presents a wireless token passing protocol, named Ripple that utilizes fixed-duration DATA frames. Basically, Ripple modifies the data transmission procedure of 802.11 DCF and employs request-to-send (RTS) and ready-to-receive (RTR) frames as tokens. A station can only send a DATA frame if it holds a token. The main problem of this approach is the possi- bility of lost the token (e.g., a fault on the node that have the token), forcing a reconfiguration of all the system. In [13, 27], Miorandi et al. presents a solution based on a Master-Slave architecture on top of IEEE 802.11. In that proposal, cyclic packets are exchanged by means of periodic queries sent by the master to the slaves. Three different techniques were proposed to handle acyclic traffic: the first technique queries the slaves that signaled the presence of acyclic data, at the end of the current polling cycle. The second technique allows a slave, when polled, to send directly acyclic data to the master. The third one exploits the decentralized nature of the IEEE 802.11 MAC protocol. When acyclic data is generated, it allows a slave to immediately try to send data to the master. The main problem of this approach is the fact that all the stations needs use the proposed protocol, not taking into consideration the existence of other IEEE 802.11 devices in the same environment. In [28], Willig presents the FTDMA (Flexible TDMA) MAC protocol. FTDMA is based on a polling scheme, where a base station polls all registered real-time stations in every frame. A frame is logically subdivided into phases: SYNC, Polling, Reservation, Register, Current Scheduler and Data Transfer. The main advantage of the FTDMA over traditional TDMA solutions is that unused slots can be used by other stations. This is a interesting idea, but how this approach is relatively old, does not take into account the use of different IFS and CW values implemented in the IEEE 802.11e amendment. Rashid et al. [29] have pointed out the performance deficiencies of the HCCA scheduler and proposed a new scheduling scheme. The prediction and optimization-based HCCA (PRO- HCCA) uses a prediction mechanism to account for the dynamic intensity of VBR traffic. This mechanism tries to find an optimal allocation of available transmission time among the competing traffic streams. Although it was made an improvement in scheduling algorithm, the basic HCCA deficiency remains. Requests are allocated to stations that no have anything to transmit and a sequential cycle needs be obeyed. In [30], Boggia et al. presents an experimental evaluation using a TDMA approach over IEEE 802.11 MAC based on the hard real-time networking framework RTnet [31] and on the Xenomai Review of Relevant Work 26
[32] real-time task scheduler. Was used the wireless RT2500 chipset [33] by the fact that is the only supported by the RTnet framework. This approach is based on Master-Slave approach where the master station manages the synchronization sending periodic frames to the slave stations. Based on this packet and on the identification number of slave stations, each station knows at which time its slot begins and ends. The main drawback is that this work is not taken into account the existence of third traffic that have been generating interference in the synchronization real-time stations.
4.1.2 CS - Collision Solver
Another approach to support QoS guarantees are those based on forcing the collision resolution schemes in favor of the RT stations. A relevant proposal has been made by Sobrinho and Krish- nakumar [34], who adapted the EQuB mechanism (Black-Burst)[35] to ad hoc CSMA wireless networks. This scheme requires the shutdown of the standard retransmission scheme. RT sta- tions implementing the EQuB approach contend for the channel access after a medium interframe spacing tmed, rather than after the long interframe spacing tlong, used by standard stations. A similar scheme is presented by Hwang and Cho in [36], where voice nodes (RT stations) use energy-burst (EB) periods (similar to BB) to prioritize RT packets over data packets. The AP can transmit a VoIP packet after PIFS without backoff or contention. On the other hand, each voice station has its own address (ID), referred as VID (virtual identification). The VID can be assigned during the traffic stream (TS) setup procedure. The VID is expressed as a binary value, which is determined by the voice packet resolution period (VPRP). The station with the highest VID wins the contention. In [37], Sheu et al. proposed a priority MAC protocol based on Sobrinho’s approach, comple- mented by a binary tree referred as contention tree. Basically, the black-burst scheme is adopted to distinguish the priorities of stations. Stations with the same priority send messages in a round robin scheme. The basic idea is that a station can obtain an unique ID number, which depends on its position in the contention tree. In [38], Bartolomeu et al. proposed the WFTT (Wireless Flexible Time Triggered) inspired in the FTT paradigm, which has been successfully applied in the Controller Area Network (FTT- CAN) [39] and Ethernet (FTT-Ethernet) [40] technologies. The WFTT is a master-slave approach that aims at harnessing both the bandjacking power in gaining prioritized channel access and FTT’s flexibility, timeliness and efficiency in supporting real-time communications in applications encompassing static and/or dynamic requirements. The common problem of all Collision Solver approaches presented above is the fact that to implement them, changes in the hardware specification are necessary. In [41], Moraes proposed a real-time communication approach based on a Virtual Token Pass- ing procedure called VTP-CSMA. It circulates a virtual token among a number of RT devices. This virtual token is complemented by an underlying traffic separation mechanism that prioritizes the RT traffic over the uncontrolled traffic [42]. How the token is implemented independently in each station, the problem of the lost of token not occurs. However, in a hidden terminal situation, 4.1 Real-Time communication in IEEE 802.11 27 even occurring during small time, the lost of synchronization of token index can occurs resulting in collision between two RT stations, forcing the use of the CTS/RTS protocol. Another problem is the need to wait a full cycle for a new transmission and the scalability i.e., the lower-bound of cycle increase with the number of the RT stations. In [43], Costa et al. proposed a two-tier mechanism intended to be used at the MAC layer allowing the coexistence of RT traffic together with uncontrolled traffic sources. At the lowest level, it is used the underlying traffic separation mechanism [42], which guarantees that whenever a RT station is contending for the medium access, it will win the contention prior any non-RT station managing the AIFS and CW of the RT stations. In the upper layer is implemented a TDMA scheme to serialize the transmissions of RT stations, allowing the coexistence of multiple RT stations operating in an open communication environment. The main problems is the need to wait a full cycle for begin a new transmission and the scalability, i.e. the lower-bound of cycle increase with the number of the RT stations. In [44], Friedrich et al. proposed a mechanism for controlling the medium access called Wireless Real Time Medium Access Control (WRTMAC), developed from the EDCA scheme. The handling of the AIFS has been modified and replaced by the Real-Time Inter-Frame Space (RIFS) in order to make the medium access deterministic. The main problem of this approach is the fact that although the name of new IFS lead to think in a really reduced IFS this not occurs (RIFS = DIFS + i × ST). Thus, if another IEEE 802.11e network overlap the communication system, a non-deterministic delay can occurs if any voice or video traffic is transmitted.
4.1.3 CR - Collision Reducer
The Enhanced Distributed Channel Access (EDCA) mechanism available in IEEE 802.11e amendment is specifically intended to reduce the number of occurring collisions. A possible so- lution to provide real-time communication under EDCA would be to use the highest access cat- egory (voice) to transfer real-time messages. However, using the EDCA mechanism to support real-time communications suffer from some severe limitations, specially when considering next generation communication environments characterized by an unknown number of communicating devices and an unpredictable network load like showed by Moraes et al. in [16]. Furthermore, the non-determinism of the probabilistic contention resolution algorithm is not suitable behavior for real-time applications. An interesting evaluation of EDCA mechanism was presented by Cena et al. in [45] where changing the internal management structure of queues in AP to use different buffers for different queues the frame rate loss was drastically reduced. This modification was be made by the fact that high priority frames could be discarded for buffer overflow when the buffer (only one in the original model) becomes full because of lowest-priority interfering traffic frames. Hamidian and Korner¨ [46] presented an interesting solution that provides QoS guarantees to the EDCA mechanism. The proposed solution allows stations with higher priority traffic to reserve time for collision-free access to the medium using the Traffic Specification (TSPEC) as base. Basically, it proposes to transfer the HCCA admission control and scheduling algorithms Review of Relevant Work 28 from the HCCA controller to the contending stations. The main problem of this approach is the high overhead that TSPEC can generate in the network. Villalon´ et al. [47] designed the B-EDCA mechanism. It is able to coexist with legacy DCF- based stations. Basically, it changes the AIFS value of the highest AC to SIFS + aSlotTime when stations are in the Backoff state. Moreover, in order to keep the compatibility with the HCCA mechanism, a station implementing the B-EDCA mechanism must wait for an additional SIFS interval when the backoff counter reaches zero, i.e. 2 × SIFS + aSlotTime. The problem is that the SIFS + aSlotTime value defined for highest AC in stations is the same value of PIFS and voice/video ACs transmitted from Access Point (using AIFSN = 1). Wang et al. [48] designed a new collision resolution mechanism, referred as gentle DCF or GDCF. The difference between GDCF and DCF is that GDCF takes a more conservative measure by halving the CW value only if there are c consecutive successful transmissions. Conversely, DCF resets its CW to the minimum value once there is a successful transmission. The GDCF needs to maintain a continuous successful transmission counter that is reset to zero after each collision. Then, when a collision occurs GDCF works similarly to DCF. However, even using this new collision resolution mechanism the real-time traffic is not prioritized, which can result in delays. Yang and Vaidya [49] proposed the Busy Tone Priority Scheduling (BTPS) protocol. BTPS works similarly to the IEEE 802.11 DCF, with the difference that high priority and low priority stations behave differently during IFS and backoff stages. The BTPS protocol uses DIFS as the IFS for high priority stations. However, during DIFS and backoff stages, high priority stations with queued packets send a energy pulse every M slots, where M is a constant. Between two consecutive busy tone pulse transmissions, there should be at least one empty Slot Time interval, as the station must have a chance to listen to the data channel. When DIFS is used, even with the BTPS, if some overlap BSS use the same channel and is configured to use IEEE 802.11e, the medium can be capture creating a non-deterministic delay in transmissions. In [50], Vaidya et al. was proposed a distributed algorithm intended to provide fair scheduling in a WLAN, referred as DFS (Distributed Fair Scheduling). The DFS protocol behaves quite similarly to IEEE 802.11 DCF, except in what concerns the backoff interval initially calculated, which is chosen proportional to the finish tag of the packet to be transmitted. The finish tag is calculated similarly to the SCFQ (Self-Clocked Fair Queueing) algorithm [51]. Know et al. [52] was modified the distributed SCFQ algorithm combined with the prioriti- zation schemes proposed in the EDCA mechanism and specify the RT-FCR (RT Fast Collision Resolution), where the priorities are implemented by assigning different backoff ranges based on the type of traffic. The main problem of two previous approaches is the fact that only modifications in the backoff mechanism are implemented to provide the prioritization of the real-time traffic. If the environ- ment have another IEEE 802.11e network overlapping the network communication system using the same communication channel, the traffic using an short IFS (PIFS, QAP-AIFS[VO] or QAP- AIFS[VI]) can capture the medium before the real-time stations and generate a non-deterministic 4.1 Real-Time communication in IEEE 802.11 29 delay. In [53], Lopez-Aguilera et al. evaluated the performance of the IEEE 802.11e EDCA when its working procedure is unsynchronized. The authors proposed the use of AIFS values whose differences are not multiple of the slot time showing results that solves the strangulation of low priority traffic, fact that occurs in IEEE 802.11e EDCA. In [54], Lo Bello et al. proposed a Wireless Traffic Smoother (WTS) to support soft RT traffic over IEEE 802.11 WLANs. The presented solution is similar to the traffic smoother scheme previously proposed by Lo Bello for Ethernet networks in [55]. Although this approach use the DCF to take results, the migration to EDCA can be easily implemented. However, the main problem is the fairness of RT traffic in relation to the non-RT traffic, what can may lead to large delays and packet loss. In [56], Chang et al. proposed a new classification of traffic classes for called High Perfor- mance EDCA (H-EDCA) for suppressing unnecessary collisions from the same class traffic and different class traffic generated by the same minimum window for different classes traffic and the transit to the minimum backoff stage when a successful transmission occurs. Even with this mod- ifications, if same traffic with PIFS time contend the medium with real-time traffic it will capture the medium and the interference can occurs. In [57], Lobello et al. was present a preliminary results of a new mechanism called Contention Window Adapter. It dynamically changes the contention window size of the different ACs of the EDCA mechanism, according to the workload conditions of the wireless network. Even with this variable contention window if, some traffic is transmitted in aPIFSTime and/or the environment have a big fragmentation of third traffic (that use a aSIFSTime for subsequent fragments), the interference in the real-time system can occurs. In [58], Vittorio et al. proposed the CWFC (Contention Window Fuzzy Controller) that tries to reduce the number of collisions through a dynamic adaptation of the backoff parameters CWmin and CWmax for the different access categories. Such an adaptive control is performed by a fuzzy- logic controller, that take into account both the throughput and frame retransmission count. This solution have the same limitations of the previous approach presented. Only the change of CW is not sufficient to ensure system reliability. In [59], Wu et al. also propose a modified EDCA mechanism, where soft real-time guarantees are provided by dynamically adjusting the priority level of a traffic flow based on the estimated per-hop delay, and generating a non-uniformly distributed backoff timer for retransmitted frames according to their individual end-to-end delay requirements. This mechanism does not prevent that traffic using aPIFSTime capture the medium before the real-time traffic using EDCA mode. Other problem is the possibility that an station with fragmented MPDU capture the medium a little bit instant before. How the subsequent fragments of an MPDU transmit in sSIFSTime, this can generate a bigger delay. Review of Relevant Work 30
4.2 Comparison of the solutions presented
In addition to the classification used above, Moraes [21] create a second level of classification that is related to the achieved compatibility degree. This classification highlights how the proposed RT communication solutions keep or alter the compatibility with IEEE 802.11 compliant devices. Three different compatibility levels have been defined:
• Level 1: impossibility of coexistence between enhanced (real-time) and default devices in the same network domain, unless that all communicating devices (both real-time and non real-time) are implementing the same enhancements;
• Level 2: real-time communication proposals ables to offer real-time guarantees in pres- ence of third devices. Requires the use of specific hardware, impairing the use of COTS (Commercial Off-the-Shelf ) hardware;
• Level 3: real-time communication proposals ables to offer real-time guarantees in presence of third devices. Can be implemented upon COTS hardware, requiring just modifications at the firmware/software level of the real-time communicating devices.
The figure 4.1 summarize all the approaches presented in this survey utilizing the two classifi- cation methods presented above. . l t a . . e l l s l t t l . . v r a l a l e . s o A
a A l u t r a . l t a .
p l
C l a n e C . i B t u e M t S l t a -
a a t - C e l A A
A e i B S a o R
r y a t
A t e
- l u i e H d C C z l t e C P M C e g l a F C e - i A T k A S t - M g t r u e e e M S D D T F p F c R T S
C C I F - s C O o s g P S n e e P m - D p b E E p a T F R T u a a D P D i o - W C o l a - T F T C o n T D R h T o o P H G S C N W R M F P B B E S W V C W E H B G B D R L W H L C W
Collision Avoidance
Collision Solver
Collision Reducer
Level 1 Level 2 Level 3
Figure 4.1: Comparative between approaches presented. Chapter 5
Conclusion
Traditionally, when supporting real-time communication in CSMA wired networks, the timing behavior was guaranteed through the tight control of every communicating device. The coexis- tence of RT controlled stations with non-RT stations was made possible by constraining the traffic behavior of the latter. Unfortunately, this approach cannot be enforced in wireless environments as, at any instant, external traffic sources may start sharing the same radio channel. Based on this argument, we entirely agree with Bianchi et al. [60] that in wireless archi- tectures, the service differentiation mechanism must be compulsory introduced as MAC layer extension. With this differentiation is possible prioritize both access the medium of the RT traffic and the collision resolution of this traffic. The last is a critical point of CSMA-based networks because your probabilistic algorithm defined in the IEEE 802.11 standard. Many approach propose change this probabilistic algorithm to another deterministic mecha- nism, leading to define a lower-bound delay for the communication system. What is a suitable characteristic for real-time applications. However, only the change of this two mechanisms is not sufficient to guarantee the correct function of the real-time communication system. Beyond the variable transmission rate; that can change in accordance with the characteristics of the channel changing the time needed for all transmissions; the possibility of overlapping networks transmitting in the same frequency channel should be taken into account. Another problem for achieving determinism in the transmissions is the fact that, in some ap- proach, the queue size in the pooling list is not taken into account. How your size is variable, the non-determinism is created. Besides the need to solve all the above problems, a good solution is one that only changes (without any change in hardware) in RT stations are needed, and the compatibility with the other IEEE 802.11 stations is maintained. Moreover, a good approach would be one that provides a admission control mechanism with the possibility of easily change the scheduling algorithm.
31 Conclusion 32
Analyzing all the approaches and taking into account the traffic used as reference for their assessments, we see the need to make a division of environments based on fairness of the access to the medium when a solution for real-time communication is proposed. We can define two environments:
• Industrial Environments: where RT traffic (which may have different levels) have the highest priority, and the external traffic is treated as noise, without caring about its transmis- sion. In this case, the fairness is referenced only to the RT traffic.
• Enterprise/Home Environments: where the RT traffic (which may have different levels) have the highest priority but the correct transmission of the external traffic is taken into account (e.g., in a smart-office where the sensors and actuators exchange messages among themselves, the employees are in a video-conference or talking using VoIP). In this case, the fairness is referenced to all traffic.
Another points that need taken into account when a new approach for real-time communication system is proposed are the new standards for enhancement the throughput of the communications. The IEEE 802.11n can achieve 600 Mbps with multiple streams, implement the frame aggregation and the reverse direction (RD), what is a very important enhancement for bi-directional streams. In the future, the IEEE 802.11ac and IEEE 802.11ad standards are expected. Both provides transmission rates of at least 1 Gbps. The main difference is that the first operates below of 6 GHz, while the second operates in 60 GHz. References
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