Performance and QoS Issues in

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1 Performance and QoS Issues in Shashi Kumar Chinthakindi Clemson University,Clemson,SC ABSTRACT is the IEEE standard for wireless LANs. The paper discusses issues related to performance and QoS in networks, mainly b networks as it is the most widely used standard. The IEEE standard defines two operational modes for WLANS: infrastructure-based and infrastructure-less or ad hoc mode. In this paper, we present performance related issues of b standard in both the modes and some of the ongoing research for MAC level optimization. QoS enhancements to the standard being defined in e standard are presented. I.INTRODUCTION Wireless LANs are one of the fasted growing wireless access technologies. This technology is being increasingly deployed for wireless LAN communications. The IEEE standards specify an "over-the-air" interface consisting of radio frequency (RF) technology to transmit and receive data. It defines the physical layer and media access control (MAC) layer, each operating at a different frequency range and hence three variants of , a, b and g are defined for different physical layers b based wireless technology is the most popular and widely used wireless technology. The b protocol provides best effort services similar to Ethernet. In wired Ethernet, data rate is high and the physical layer s error rate is very low and hence QoS issues have been neglected. This does not apply for as most of the available data rate is consumed inter frame spacing, packet fragmentation, and MAC-level acknowledgment. For wired networks, IETF s integrated services and Differentiated services architectures guarantee QoS above the link layer. The e task group has proposed a number of enhancement to incorporate QoS for networks. This paper is organized as follows. In section II and introduction to b protocol is given. We then present some of the issues related to b networks in infrastructure based and ad hoc modes. The ongoing research for MAC level optimization is discussed in the next section. In the next section, QoS issues are presented along with the discussion on e standard. II IEE b The fundamental building block of a architecture is the cell known as basic service set (BSS). In infrastructure mode, one or more wireless stations connect to a central base station known as access point (AP). Multiple base stations can be connected together to form a distribution system (DS). In ad-hoc mode, multiple stations connect to each other. IEEE employs a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) medium access control protocol with binary exponential back-off algorithm, called distributed coordination function (DCF), and an optional Point Coordination function (PCF) which is a centralized MAC algorithm used to provide contention free service. PCF is built on top of DCF and exploits features of DCF to assure access to its users. A. Distributed Coordination Function DCF is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Before a data frame is sent, the station senses medium. If it is idle for al least a DIFS (DCF interframe space) period of time, the frame is transmitted. Otherwise a backoff time B (measured in timeslots is chosen randomly in the interval [0,CW), where CW is called Contention Window. Initially CW is set to CWmin. After the medium has been detected idle for at least a DIFS, the backoff timer is decremented by one for each time slot the medium remains idle. If the medium becomes busy during the backoff process, the backoff timer is paused, and is restarted when the medium has been sensed idle for a DIFS again. Once the backoff timer reaches zero, the station can initiate its frame transmission. Upon detection of collision (which is detected by the absence of an acknowledgement frame from the destination), the contention window is doubled according to the formula CWi = 2k+i 1 1, where i is the number of attempts (including the current one) to transmit the frame that has been done, and k is a constant defining the minimum contention window, CWmin = 2k 1. A new backoff time is then chosen and the backoff procedure starts over. The backoff mechanism is also used after a successful transmission before sending the next frame. After a successful transmission, the contention window is reset to CWmin[10]. In DCF all stations have equal probability to access the channel and share it according to equal frame rate and not according to equal throughput. This offers no support for priority access to the channel for time-sensitive traffic. Figure 1 shows the DCF message exchanges.

2 Figure 1: DCF Message Exchanges. RTS and CTS messages precedes each data packet. On receiving RTS, the neighboring nodes set their NAV s to the duration mentioned in RTS.On receiving CTS, the neighboring nodes of receiver set their NAVs to the duration mentioned in CTS. This way, the channel is reserved till the sender receives the ACK message. B. Point Coordination Function PCF is a centralized, polling-based access mechanism which requires the presence of a base station that acts as Point Coordinator (PC). If PCF is supported, both PCF and DCF coexist and in this case, time is divided into superframes as shown in figure 2. Each superframe consists of a contention period where DCF is used, and a contention free period (CFP) where PCF is used. The Point Coordinator (PC) periodically sends a beacon frame to broadcast network identification and management parameters specific to the wireless network. Since the beacon is sent using ordinary DCF access method, the base station has to contend for the medium, and therefore the CFP may be shortened. The PC keeps a list of mobile stations that have requested to be polled to send data. During the CFP, it sends poll frames to the stations when they are clear to access the medium. Upon reception of a poll frame, the station sends a data packet if it has any packet queued. To ensure that no DCF stations are able to interrupt this mode of operation, the IFS between PCF data frames is shorter than the usual DIFS. This space is called a PCF interframe space (PIFS). To prevent starvation of stations that are not allowed to send during the CFP, there must always be room for at least one maximum length frame to be sent during the contention period. The Point Coordinator (PC) periodically sends a beacon frame to broadcast network identification and management parameters specific to the wireless network. PCF splits the time into a contention-free period (CFP) and a contention period (CP). Only stations polled by the PC may transmit during the CFP. The CFP ends after the time announced by the beacon frame or by a CF-End Frame.[10] Even though the PCF can offer some sort of priority to an overloaded station, it cannot differentiate between traffic types or sources. Therefore, it cannot tell which stations have long queues of time-sensitive traffic, and which only hold best-effort traffic. Another problem with PCF is that the PC has to contend with other stations to gain control of the wireless medium. Therefore, the starting time and length of the CFP can vary. These downfalls led to the establishment of the IEEE e Work Group. Figure2: IEEE b superframe III.PERFORMANCE ISSUES OF b NETWORKS Wireless medium is an unreliable mode of data transmission and hence is never possible to achieve theoretical available bandwidth at the application level. The throughput that can be obtained over the b WLAN networks is much smaller than the normal bit rate of 11Mb/s. If there is not collisions, it is

3 shown that the maximum throughput achieved is 7.74 Mb/s [4]. The proportion of the useful throughput strongly depends on the number of competing hosts. In this section we discuss some of the problems that can arise in the wireless networks both in infrastructure mode and infrastructure-less modes (ad-hoc mode). The characteristics of the wireless medium make wireless networks fundamentally different from wired networks. The loss in performance can be attributed to various factors at various levels of protocol stack. The following discusses various issues that effect the performance of b networks. 1) The first performance loss is at physical radio link caused by noise. The theoretical limit of data rate R for a channel with bandwidth B with a Signal to Noise Ratio(SNR) is given by Shannon s equation R = B log2(1 + SNR) Additional loss at this level is caused because of multipath fading, Doppler shifts and bit errors. [1] A multipath is a phenomenon occurred in a wireless propagation channel where in the transmitted signal arrivals at the receiver from various directions over a multiplicity of paths because there are obstacles and reflectors in the wireless propagation channe. When the waves of the multipath signals are out of phase reduction in signal strength can occur and hence delay in transmission. This phenomenon is called Multipath fading. Doppler-frequency-shift is a deviation in the signal frequency due to a change in the path length between the transmitter and receiver. This can be due to movement of any or all of the following: the transmitter, the receiver, or reflective surfaces along the path. Lots of bit errors occur at this level because of the noise, Doppler shift phenomenon, multipath fading etc.. and hence accounts to the delay in transmission. 2) At physical layer, there is a significant amount of performance loss due to low data rate of the header for compatibility and reachability with all nodes in the network. This causes a reduced performance throughput [1]. Figure 3 gives the b frame format. The header PLCP Preamble and PLCP Header are transmitted at 1Mbps as shown in Figure3. Figure 3 : b frame format 3) TCP/IP over wireless networks: TCP/IP is the most popular in wired networks. TCP protocol is specifically adapted to wireless networks. The TCP/IP protocols can account for problems associated with the nature of the wireless network environment. The interaction of some features of MAC protocol (hidden/exposed problem, exponential back-off algorithm etc.. ) with the TCP protocol mechanisms ( Congestion control mechanisms) may lead to several unexpected serious problems. The performance of TCP/IP over wireless network is significantly affected when some of the mobile hosts use lower bit rate than the others. The reason being the assumption by TCP all losses in the network is due to congestion. 4) TCP over wireless also experience Self Collision, a scenario where the TCP data packets contend for the channel with the ACKs for the previous packets. This self collision is caused because of the half duplex nature of wireless networks. [1]. However [11] proposes a modification to the MAC protocol distributed coordination function to alleviate this problem. 5) Use of WEP ( Wired Equivalent Privacy ) encryption algorithm for every packets incur overhead and cause performance loss. 6) Station mobility may severely degrade the performance of the TCP protocol in mobile ad hoc networks. This is due to the inability of the TCP protocol to manage efficiently the effects of mobility. Station movements may cause route failures and route changes and, hence, packet losses and delayed ACKs. 7) Figure 4 shows a typical hidden station scenario. Let us assume that station B is in the transmitting range of both A and C, but A and C cannot hear each other. Let us also assume that A is transmitting to B. If C has a frame to be transmitted to B, according to the DFC protocol, it senses the medium and finds it free because it is not able to hear A s transmissions. Therefore, it starts transmitting the frame but this transmission will results in a collision at the destination Station B. [3]

4 Figure4: Hidden Station Problem the behavior of neighboring stations (a station must sense the medium before start transmitting) and by stations in its interfering range (interferences may cause collisions at the destination station).[3]. For example, it can be shown that in a string (or chain) topology, like the one shown in Figure 6, maximum expected bandwidth utilization is only This discrepancy is due to MAC inability to find the optimum schedule of transmissions by itself. In particular, in a chain topology it happens that stations early in the chain starve later stations (similar considerations applies to other network topologies). In general, the MAC protocol appears to be more efficient in case of local traffic patterns, i.e., when the destination is close to the sender[3] The use of four way handshake protocol is used to solve hidden node problem but causes loss in performance throughput due to overhead in additional management frames ( RTS/CTS/ACK). The performance issues for the IEEE b Ad-Hoc networks 1) The short-term unfairness caused due to hidden terminals configuration with the RTS-CTS exchange [3] 2) The presence of slow terminals in one-hop network slows down every other terminal that sends data at higher rates 3) The Exposed Station Problem: Let us assume that both Station A and Station C can hear transmissions from B, but Station A can not hear transmissions from C. Let us also assume that Station B is transmitting to Station A and Station C receives a frame to be transmitted to D. According to the DCF protocol, C senses the medium and finds it busy because of B s transmission. Therefore, it refrains from transmitting to C although this transmission would not cause a collision at A. The exposed station problem may thus result in a throughput reduction.[3] Figure 5: Exposed Station Problem 4) Influence of the network topology: Even in a static environment, the performances of an ad hoc network are strongly limited by the interaction between neighboring stations. Stations activity is limited by Figure 6: A String network topology IV. RELATED RESEARCH The following are some of the schemes seeking to modify the DCF channel access protocol to bring in performance enhancements. Multi-Rate b: Multi-Rate is the ability of a wireless card to automatically operate at several different bit-rates. IEEE b supports data transmission facility at multiple rates which are possible because of different modulation techniques optimized for different channel conditions. Auto Rate Fallback Scheme (ARF): As the multi-rate IEEE enhancements are physical layer protocols.,mac mechanisms are required to exploit this capability. The Auto Rate Fallback (ARF) protocol was the first commercial implementation of a MAC that utilizes this feature. It is a mechanism which gauges the changing channel conditions based on success or failure of previous transmissions. With ARF, senders attempt to use higher transmission rates after consecutive transmission successes (which indicate high channel quality) and revert to lower rates after failures. Under most channel conditions, ARF provides a performance gain over pure single-rate.[12] Receiver Based Auto Rate Scheme (RBAR): The core idea of RBAR is for receivers to measure the channel quality using physical-layer analysis of the request-tosend RTS) message. Receivers then set the transmission rate for each packet according to the highest feasible value allowed by the channel conditions. As the RTS message is sent shortly before data transmission, the estimation of the channel condition is quite accurate, so that RBAR yields significant throughput gains as compared to ARF (as well as compared to single-rate IEEE ). Moreover, as request- and clear-to-send messages are necessarily sent at the base rate so that all nodes can

5 overhear them, overhearing nodes are informed of the modified data transmission times so that they can set their backoff timers accordingly. [12] Hence rate adaptive scheme uses feedback information from the receiver to sense the channel condition rather than estimating it at the sender. Opportunistic Auto Rate Scheme (OAR): Opportunistic media access scheme (OAR) which extends RBAR and tries to utilize the period of good channel conditions to maximize the network throughput. The key idea of OAR is to opportunistically exploit high quality channels when they occur via transmission of multiple back-to-back packets. In particular, when the multi-rate MAC indicates that the channel quality allows transmission above the base rate, OAR grants channel access for multiple packet transmissions in proportion to the ratio of the achievable data rate over the base rate. Consequently, OAR nodes transmit more packets under high quality channels than under low quality channels.[12] DCF+: DCF+ tries to reduce the contention between data and TCP ACKs and hence solve self collision problem. In DCF, every node starts a contention after sensing the channel to be idle for DIFS period. DCF+ aims at removing the contention for TCP ACKs by living them preferential treatment and hence avoid the self collisions. V.QUALITY OF SERVICE QoS provides mechanisms to control access and usage of the medium based on the application. Each application has different needs in terms of latency, bandwidth and packet-error rate and, therefore, QoS must cater to each of these needs. IEEE b provides best effort service ie.., every data packet handed over to b interfaces receives similar treatment as other packets in terms of delivery guarantees. From application perspective, it receives no quality of service guarantees from the network in terms of available bandwidth, latency, jitters etc b provides a very rudimentary support for quality of service in its infrastructure mode of operation. The support is provided by MAC layer in terms of Point Coordinated Function. It is used to differentiate between traffic flows from different nodes. However, a mechanism Rether for IEEE b[1] provides bandwidth guarantees to individual flows.[1]. Rether is a software solution residing on the network stack of individual nodes and does not require any changes at MAC layer. The following figures shows the protocol layers and architecture of Wireless Rether[1] QoS limitation of the original MAC : DCF( Basic access method for the original MAC, Distributed Co-ordination function) does not have any provision to support QoS. All data traffic is treated in a first come first serve, best effort manner. It does not guarantee bandwidth, packet delay and jitter moreover throughput degradation occurs in heavy load. PCF (Point Coordination function) is an optional channel access function in the standard, which was design to support time bounded services. Contention free access to the wireless medium is controlled by a Point Coordinator (PC) collocated with the AP. It is regarded as an inefficient central polling scheme. Often unpredictable beacon frame delay occurs due to incompatible cooperation between CP and CFP modes. Another problem with PCF is the unknown transmission time of polled stations. The above are some of the problems with the PCF that led to the current activities to enhance to b protocol. To provide support for QoS the IEEE Task Group E (802.11e) defines enhancements to the original MAC described earlier. The e draft standard presents two new MAC schemes; EDCF is an extension of DCF, while the HCF is an extension of PCF. Unlike the original , where it was not compulsory to have a PC, both EDCF and HCF must boast a centralized Hybrid Controller (HC). With e, there may still be the two phases of operation within the superframes, i.e., a CP and a CFP, which alternate over time continuously. The EDCF is used in the CP only, while the HCF is used in both phases, which makes this new coordination function hybrid.[13] EDCF ( Enhanced Distributed Coordination function) : EDCF in e is the basis for the HCF.The EDCF is a contention-based channel access mechanism and introduces the concept of Traffic Categories (TCs). Within a station there is a maximum of eight TCs, each having its own MAC queue and backoff counter. The following four Key parameters are used for differentiation of priorities. 1)Minimum contention window size (CWMin). AC with higher priority is assigned a shorter CWMin. 2)Maximum contention window size (CWMax). 3)TXOP limit Specifies the maximum duration a QSTA can transmit and is specified per AC. The TXOP limit can be used to ensure that high-bandwidth traffic gets greater access to the medium. TXOP limit also makes the channelaccess protocol significantly more efficient. 4)Arbitration Inter-Frame Space (AIFS) specifies the time interval between the wireless medium becoming idle and the start of channel-access negotiation. Each AC is assigned a different AIFS[AC] based on the AC to further provide QoS differentiation. In the CP, each TC within the stations contends for a TXOP and independently starts a backoff after detecting the channel being idle for an Arbitration Interframe Space (AIFS); the AIFS is at least DIFS, and can be enlarged individually for each TC. After waiting for AIFS, each backoff sets a counter to a random number drawn from the interval [1,CW+1]. The minimum size (CWmin[TC]) of the CW is another parameter dependent on the TC. Priority over

6 legacy stations is provided by setting CWmin[TC]<15 (in case of a PHY) and AIFS=DIFS. Figure 8 illustrates the EDCF parameters. As in legacy DCF, when the medium is determined busy before the counter reaches zero, the backoff has to wait for the medium being idle for AIFS again, before continuing to count down the counter. A big difference from the legacy DCF is that when the medium is determined as being idle for the period of AIFS, the backoff counter is reduced by one beginning the last slot interval of the AIFS period. Note that with the legacy DCF, the backoff counter is reduced by one beginning the first slot interval after the DIFS period. After any unsuccessful transmission attempt a new CW is calculated with the help of the persistence factor PF[TC] and another uniformly distributed backoff counter out of this new, enlarged CW is drawn, to reduce the probability of a new collision. Whereas in legacy CW is always doubled after any unsuccessful transmission (equivalent to PF=2), e uses the PF to increase the CW different for each TC: newcw [TC]>=((oldCW[TC]+1)*PF)-1 The CW never exceeds the parameter CWmax[TC],which is the maximum possible value for CW. [13] [13] Figure 8: Multiple back off of MSDU(MAC Service Data Unit) streams with different priorities. A single station may implement up to eight transmission queues realized as virtual stations inside a station, with QoS parameters that determine their priorities. Figure 9: Virtual backoff of eight traffic categories Hybrid Coordination Function : HCF extends the EDCF access rules. The HCF has a notion of Hybrid Coordinator (HC) similar to the PC in PCF. The key differences between HCCA and PCF are that HCCA can poll the stations during CP and that it supports scheduling of packets based on QSTA s specific traffic-flow requirements. The traffic-flow requirements of the QSTAs are specified using Traffic Specifications (TSPECs). The HC can allocate TxOP (transmission opportunity, interval during which a node has the right to initiate transmissions)to itself or any other node at any time but after sensing the channel to be idle for a duration of PIFS, which is shorter than DIFS. This ensures that the HC has the highest priority over all other nodes at any given time. The HC allocates TxOP to pollable nodes in contention free periods and sometimes even during contention periods.[1]. Thus stations can be guaranteed predictable opportunity times. Using the distributed MAC approach, a prioritized service of heterogeneous traffic flows is provided. The e standard provides qualitative throughput, delay guarantees, MAC admission control mechanism and other QoS guarantees. References: [1] Srikant Sharma Analysis of b MAC: A QoS, Fairness, and Performance [2] Claude Chaudet, Dominique Dhoutaut, Isabelle Guérin Lassous Experiments of some performance issues with IEEE b in ad hoc networks [3] Giuseppe Anastasi, Marco Conti, Enrico Gregori IEEE Ad Hoc Networks: Protocols, Performance and Open Issues [4] Martin Heusse, Franck Rousseau, - Performance Anomaly of b [5] Stefan Mangold- IEEE e Wireless LAN for Quality of Service

7 [6] Shirish Karande, - ANALYSIS AND MODELING OF ERRORS AT THE b LINK LAYER [7] Yang Xiao, Jon Rosdahl Performance Analysis and Enhancement for the Current and Future IEEE MAC Protocols [8] Zuyuan Fang..- Performance Evaluation of a Fair Backoff Algorithm for IEEE DFWMAC [9] J. ANTONIO GARCIA-MACIAS..- Quality of Service and Mobility for thewireless Internet [10] ANDERS LINDGREN, ANDREAS ALMQUIST and OLOV SCHELÉN- Quality of Service Schemes for IEEE Wireless LANs An Evaluation [11] Haitao Wu, Yong Peng, Keping Long, Shiduan Cheng, and Jian Ma. Performance of Reliable Transport Protocol over IEEE Wireless LAN: Analysis and Enhancement. In Proceedings of the IEEE INFOCOM 02, June [12] B. Sadeghi, V. Kanodia, A. Sabharwal, and E. Knightly OAR: An Opportunistic Autorate Media Access Protocol for Ad Hoc Networks [13] Stefan Mangold1, Sunghyun Choi2, Peter May3, Ole Klein1, Guido Hiertz1, Lothar Stibor1 IEEE e Wireless LAN for Quality of Service