VoIP is an application of multimedia, which requires in time ... 7th WSEAS Int. Conf. on Electronics, Hardware, Wireless and Optical Communications, Cambridge, UK, February 20-22, 2008 .... through extension of PCF called HCF Controlled.
7th WSEAS Int. Conf. on Electronics, Hardware, Wireless and Optical Communications, Cambridge, UK, February 20-22, 2008
Quality of Service in Wireless LAN Fozia Hameed, Imran Ahmad COMSATS Institute Of Information Technology, Islamabad, Pakistan
Abstract The current Quality of Service (QoS) standard for wireless networks from the widely used 802.11e family is IEEE 802.11e. The scope of this standard is to enhance the existing 802.11 Media Access Control (MAC) so as to improve and manage QoS, to expand support for LAN applications with QoS requirements and provide classes of service. This standard also provides improvements in the capabilities and efficiency of the protocol. This paper presents a review on QoS in Wireless LAN. In this paper original 802.11 was discussed from QoS point of view. Methods, which are used to enhance QoS, are elaborated. Other features of 802.11e are pointed and QoS related standards are discussed. Keywords: Wireless LAN, IEEE 802.11, IEEE 802.11e, QoS
1. Introduction: WLANs are used in different environments and different types of information are transmitted. Convergence, i.e. the use of the same network for multiple purposes, such as communicating data, telephony, video conferencing, is an important trend in the field of ICT. Since, in wireless networks the single medium is used by the applications of all the users, hence the network quality is more degraded as compared to the wired networks. In WLANs where more access points are simultaneously active, another issue is roaming. When a node moves or reception conditions change, it will usually select the access point in its range that has the highest signal strength. Roaming is the event of switching from one access point to another, which causes a significant amount of delay. This delay is unacceptable in real time applications. Every transmitting and receiving node in the network
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typically buffers incoming and outgoing packets. This causes latency and jitter. The overhead of roaming, security mechanisms, retransmissions, as well as data and voice convergence add latency and jitter in WLAN. VoIP is an application of multimedia, which requires in time and quick service of voice. When we want to achieve this task one would face many challenges especially when enforcing QoS. VOIP improves bandwidth efficiency by using advance voice compression techniques and bandwidth sharing in packet switched networks. Second, it facilitates the creation of new services that combine voice communication with other media and data applications like video, white boarding and file sharing. WLAN environment is very dynamic so static solutions like increasing port count or port speed as in case of Ethernet segment saturation do not work. This paper is a review of the current state of the art in wireless networks from QoS point of view. Due to the nature of wireless networks and its characteristics, specific issues appear, are addressed .In Section 2, Original 802.11 is overviewed from QoS point of view. In section 3, 802.11 with QoS is addressed. Other features of 802.11e are discussed in section 4. Section 5 summarizes the standard for QoS in wireless LAN. In section 6, conclusion is drawn and future work is mentioned.
2. Original 802.11and QoS 802.11 is an evolving family of specifications for wireless local area networks (WLANs) developed by a working group of the Institute of Electrical and Electronics Engineers (IEEE). There are two modes of communications for wireless stations in MAC protocol. ISBN: 978-960-6766-40-4
7th WSEAS Int. Conf. on Electronics, Hardware, Wireless and Optical Communications, Cambridge, UK, February 20-22, 2008
Distributed Coordination Function (DCF), based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). DCF provides coordination, but not provide priority access neither to the wireless medium nor to real time multimedia traffic compared to data traffic. Point Coordination Function (PCF), supports time-sensitive traffic flows. Wireless access points periodically send beacon frames to communicate network identification and management parameters specific to the wireless network [1]. It assists a station to use free pooling periods for sending delay sensitive data. The main drawback of PCF is not to provide QOS based on the different requirements of different applications. There is delay in beacon transmission due to its dependency on contention which affects all the rest of the frames resulting in a great delay. It may also results in uncontrolled transmission time. It lacks differentiation and cannot handle the various requirements of different types of traffic [2]. A common QOS problem for PCF is that the 802.11 MAC doesn't specify any admission control mechanism and under high loads performance will deteriorate.
3. 802.11 with QoS support The current Quality of Service (QOS) standard for wireless networks is used from the widely used 802.11e family. This standard aims to enhance the existing 802.11 Media Access Control (MAC) to provide QoS support for Wireless LAN applications and provide classes of service. It also aims at improving efficiency of protocols. It might be used for transport of voice, video conferencing, and media stream distribution. IEEE developed enhancements to original coordination modes of 802.11 MAC protocol to facilitate QOS. In 802.11e a new MAC layer function Hybrid Coordination Function (HCF) is proposed. It uses a contention-based channel access method called Enhanced Distributed Channel Access (EDCA) that operates concurrently with a polling-based HCFControlled Channel Access (HCCA) method. One important new feature of HCF called transmission opportunity (TXOP), which helps
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to solve the problem of unpredictable transmission time of a polled station in PCF. IEEE 802.11e distinguishes two approaches to QOS, prioritized and parameterized QOS.
3.1 Prioritized QoS An enhancement to DCF known as the Enhanced Distributed Channel Access (EDCA) introduces the concept of traffic categories thus enabling prioritized QoS (a problem faced in PCF and DCF). There are 4 priority levels in every channel and 4 queues for every priority level thus more applications could be handled according to their requirement. When using EDCA, stations try to send data after detecting the medium is idle and after waiting a period of time defined by the corresponding traffic category called the Arbitration Inter-Frame Space (AIFS). Each service class can use specific AIFS values to differentiate the QOS received by the corresponding traffic. A higher-priority traffic category will have a shorter AIFS than a lowerpriority traffic category and will have to wait for short interval thus receiving better QoS. For avoiding the collision a station waits for contention window (CW), a random number of time slots before transmission of data. If other stations send data before end of CW, the station waits for another idle time. Better service is guaranteed to stations by use of small CW min and CW max ECDA prioritize stations by allowing multiple frames without contending again called Contention-Free Bursting (CFB), which is limited by TXOP Limit. CFB fails to provide any priority in the period of contention. A queue is maintained for each Access Category (AC). Collisions are handled virtually by transmitting the frames from highest priority queue to access medium. Results from different simulations confirm that differentiation mechanisms of EDCA can protect higher-priority class from a lowerpriority class, but cannot reduce the contention between different traffic flows within the same priority class.
ISBN: 978-960-6766-40-4
7th WSEAS Int. Conf. on Electronics, Hardware, Wireless and Optical Communications, Cambridge, UK, February 20-22, 2008
EDCA is not effective or efficient in supporting the delay-sensitive voice traffic. There is no guarantee of successful transmission of voice packet due to contention-based nature. Time allocated for transmission of payload is very small relative to total transmission time of packet In contention it may happen that count down timer of CW of low priority station becomes very smaller than high priority stations and it can grab the medium resulting in a long waiting time for high priority channel.
3.2 Admission Control and ECDA QoS may be provided at APs by using Admission Control Algorithm along with ECDA. The admission control algorithm for VoIP flow is implemented at the AP, and determines whether a given traffic flow can be accommodated in the BSS without jeopardizing the QoS requirements of other active flows [3]. These algorithms are implemented by vendors according to their customer requirements.
shows impact of wireless network metrics on QoS of an application in wireless environment.
4. Other Features of 802.11e The centralized scheduler used in 802.11e guarantees collision avoidance and, therefore, improved ability to deliver time-critical payloads. The ability to honor critical QoS contracts such as delay, jitter and bandwidth is much improved and system reliability is achieved. Improvements in channel robustness are achieved through forward error correction (FEC) and selective retransmission. Channel throughput is further improved through the possibility of dynamic channel change. Wireless Multimedia Enhancements (WME) provides an interim QoS solution for 802.11 networks. WME was intended to provide a well defined and accepted 802.11 QoS mechanism, which prevent the spread of non-interoperable methods.
3.3 Parameterized QoS
5. Related QoS standards
Parameterized QoS is provided by IEEE through extension of PCF called HCF Controlled Channel Access (HCCA). A hybrid controller polls stations during an initial period called contention-free period (CFP). Specific start time and a maximum transmit duration is granted polling station. It results in time-scheduled transmissions. Multi class algorithms are also supported by HCAA. Delay variation is eliminated by forbidding any station to finish before next beacon.
Even after 802.11e is fully implemented in purchasable products, it probably still won’t be enough to ensure quality and reliability for realtime applications in a large enterprise environment, depending on how large an installation is, and on the particular Wi-Fi system vendor’s architecture.
An HCCA admission control algorithm checks whether the TXOP of the request summed with the current TXOP allocations exceed or not the maximum fraction of time that can be used by HCCA. Simulations shows that HCCA behaves appropriately when used with Constant Bit Rate traffic streams, but fails to deliver consistent performance for streams with Variable Bit Rate, such as video applications.Table1 on next page
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In order to improve performance other extensions are used to provide quality and reliability for real time applications such as VoIP. QOS parameters, which affect the performance in Wireless environment, are given in the table below. It may involve speeding up roaming times among wireless Basic Service Sets (BSS) so that sessions aren’t interrupted, packets aren’t dropped, and quality doesn’t degrade. Here are the extensions that should potentially play an important role in QoS enforcement over WLANs:
ISBN: 978-960-6766-40-4
7th WSEAS Int. Conf. on Electronics, Hardware, Wireless and Optical Communications, Cambridge, UK, February 20-22, 2008
WIRELESS NETWORK METRICS
IMPACT OF AN UNSUSTAINABLE ENVIRONMENT Delay High Jitter High Impact of rate adaptation on delay and jitter High Impact of roaming on delay and jitter High Impact of overlapping BSSs on delay and jitter High Impact of RTS/CTS impact on delay and jitter High Impact of power management on delay and jitter High Impact of MAC layer fragmentation on delay and jitter High Impact of encryption on delay and jitter High WME relative priority forwarding rate High WSM stream bandwidth allocation High Table1. Impact of wireless network metrics on QoS of an application in wireless environment
•802.11r – This is the fast-roaming protocol (in development) that speeds session hand-off times as a client device moves from one access point (AP) to another, while keeping the user’s authentication credentials and real-time session intact. The current working goal is to keep this hand-off time under 50 milliseconds, a value acceptable for VoIP. [1] •802.11k – This is the Radio Resource Management protocol (on which 802.11r relies), which aims to hasten a client’s roaming decisions by pre-discovering all neighboring APs, the distances to them and their available call capacity.[1] •802.11i – The pre-authentication component of the security standard reduces roaming time by enabling the client to authenticate with neighboring APs before roaming effectively.[1]
As future work, we will evaluate performance of different approaches for providing QoS in wirless LAN and will propose and test new mechanism for improving QoS in wireless LAN.
References: [1] Răzvan Beuran ,“VoIP over Wireless LAN Survey”, http://www.jaist.ac.jp last visited on 20th september,2007 [2] Ping Wang, Hai Jiang, and Weihua Zhuang, ” Capacity Improvement and Analysis for Voice/Data Traffic over WLAN”, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS,IEEE Transaction on Wireless Communication, volume 6, issue 4, April 2007, 1530-1541 [3] Ajay Bakre Intel Corporation, “Intel VoIP over WLAN Architecture”, WICON’06, The 2nd Annual International Wireless Internet Conference, August 25, 2006,
6. Conclusion: In this paper a comprehensive review is done on how QoS is achieved in wireless LAN. Original MAC protocol 802.11 is elaborated and shortcomings are addressed. How PCF and DCF have been enhanced and what the enhanced provide regarding QoS are4 discussed. More features other than QoS features are mentioned. QoS standards are overviewed. The most important metrics on QOS of an application in Wireless environment are tabulated.
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ISBN: 978-960-6766-40-4
