Evaluation Analysis of the Performance of IEEE b and IEEE g Standards
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1 Evaluation Analysis of the Performance of IEEE and IEEE Standards Antonis Athanasopoulos Research Fellow Evangelos Topalis PostDoc Researcher Christos Antonopoulos Research Fellow Stavros Koubias Professor Applied Electronics Laboratory, Department of Electrical & Computer Engineering, University of Patras, Greece Abstract The IEEE Wireless LAN standard is one of the most popular wireless standards in the market today. Since 1997 when the first version of the IEEE was launched in the market, a lot of different versions has been announced and developed. In this paper, a comprehensive evaluation analysis of the IEEE and IEEE has been carried out, examining the performance of both standards at the MAC sub-layer, in terms of QoS, using two different simulation tools. Finally, the comparison for both cases is discussed. 1. Introduction During the last few years, wireless communications have gained a great part in the market of communications, offering very important development perspectives in mobile telephony, wireless Internet and generally in wireless LANs. Nowadays, WLANs can support a variety of industrial and home automation applications. Wireless LANs can be categorized according to the extent of their covering area into: Wireless Local Area Networks-WLANs Wireless Wide Area Networks-WWANs Wireless Personal Area Networks-WPANs. A wireless LAN is a very flexible structure for data communications, which might be implemented either as an alternative of a wired LAN (within a building or a geographical region) or as an extension providing some extra coverage area between a wired backbone network and a mobile user carrying some wireless apparatus. Also, WLANs are divided into two main categories depending on the topology architecture: The AP (Access Point) infrastructured WLANs and the adhoc WLANs. The AP-infrastructured WLANs architecture, is based on at least one AP providing a server function. All kind of communication between all wireless nodes should pass through this AP. This AP might be connected to a wired backbone network as well. Mobile Ad hoc Networks (MANETs) are autonomous networks consisting of routing nodes (or some routing nodes with other nodes that do not route) that are free to move about. They may be connected to a larger network e.g. the Internet, or operate as an isolated intra-network. Summarizing, the term wireless networking refers to technology that enables two or more computers/devices to communicate using standard network protocols, but without network cabling. Strictly speaking, any technology that does this could be called wireless networking. The current buzzword however generally refers to wireless LANs. This technology, fuelled by the emergence of cross-vendor industry, has produced a number of affordable wireless solutions, such as IEEE [1], [2], Bluetooth and HomeRF standards. A major obstacle for deployment of wireless networks is the existence of multiple standards. Currently, WLANs are mostly based on the IEEE standard. More specific, most of the applications, products and research today, regarding wireless solutions, are making use of or protocols. During the past years, a lot of research has been carried out, in order to evaluate the performance, regarding the MAC access methods, of the wireless standards developed [3], [4]. In [5], the impact of various key parameters (i.e.rts/cts) on the actual performance of wireless LAN protocol was verified. Furthermore, a lot of effort has been put aiming useful enhancements for the published standards [6] [9]. In general, the outcome in all cases turns to be that, depending on the application, a specific MAC protocol has to be used.
2 In this article, several simulation tests were carried out, examining the performance of both protocols. The performed simulations were undergone using two different simulation tools: a) the wireless C++ simulator [10] and b) the OPNET v.11.0 [11]. A comprehensive analysis of the results is given below. Furthermore, a comparison of the results obtained, between the two simulation tools has been carried out. The structure of the paper is organized as follows: Chapter 2 is an overview about the several IEEE versions. Also, a small description of the MAC layer of both IEEE and IEEE is given. The next section, Chapter 3, includes the simulations as well as the results obtained. Moreover, discussion on the results is provided. Finally, Chapter 4 has the general conclusions of the work. 2. IEEE Wireless Standards In 1997, the Institute of Electrical and Electronics Engineers (IEEE) created the first WLAN standard. They called it after the name of the group formed to oversee its development. Unfortunately, only supported a maximum bandwidth of 2 Mbps - too slow for most applications. For this reason, ordinary wireless products are no longer being manufactured. Since then, many versions of the initial standard have been launched. A very brief description of each version is given below: IEEE a: This standard supports higher data rates, compared to the other versions, by using OFDM modulation in the frequency band of the 5.7 GHz. The data rates vary from 6 to 54 Mbps. IEEE : This is the most popular standard of all versions. It was published in September An analytical description lies in the next paragraph. IEEE c: Offers frame bridging functionality. IEEE d: Further extensions, in order the standard to work in other frequency bands. IEEE e: It provides QoS support at the MAC layer, even with the basic/mandatory access mechanism (EDCF, Enhanced DCF HCF, Hybrid Coordination Function) IEEE f: Suggested practice for IAPP (Inter Access Point) Protocol. IEEE : A extension, supporting higher data rates. It is described in more detail in paragraph 2.2. IEEE h: a s spectrum management (DCS, Dynamic Channel Selection and TPC, Transmit Power Control) IEEE i: MAC enhancements for better data security. 2.1 The : An overview The IEEE standard defines 3 different Physical layers providing different data rates. The three PHY kinds are: a) the FHSS, b) the DSSS and c) the IR. Both FHSS and DSSS use the ISM 2,4GHz frequency band. The FHSS uses 79 channels separated with a 1MHz from each other, adopting the GMSK. The offered bandwidth is 1 or 2Mbps depending upon the GMSK. The DSSS uses exactly the same frequency spectrum with DBPSK coding for the 1Mbps and DQPSK for 2Mbps. Finally, the IR designed mostly for indoor applications. Recently, a proposal for higher data rates (i.e. 5,5Mbps and 11Mbps) was adopted. This was achieved by using a different coding technique, the Direct Sequence/Pulse Position Modulation (DS/PPM) The IEEE MAC architecture is shown in Fig.1. The MAC sub-layer defines two access coordination mechanisms, the basic Distributed Coordination Function (DCF), which is mandatory and the optional Point Coordination Function (PCF). Networks that are making use of both DCF and PCF functions support two kinds of transmissions: asynchronous and synchronous. MAC Extent Point Coordination Function (PCF) Distributed Coordination Function (DCF) Figure 1. IEEE MAC architecture Asynchronous transmission is provided by DCF, which is basically a CSMA/CA access method, while synchronous transmission provided by PCF follows a round-robin polling-based access mechanism. Figure 2. RTS/CTS access mechanism
3 The basic DCF is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). Carrier sensing is performed using physical carrier sensing (by air interface) as well as virtual carrier sensing. Virtual carrier sensing uses the duration of the packet transmission, which is included in the header of RTS, CTS, and DATA frames, Fig.2. The channel is considered to be busy if either physical or virtual carrier sensing indicates that the channel is busy. When a station wants to transmit a packet, it needs to sense the channel. If the channel is idle in DIFS interval. Then, the station sends a RTS. After it receives a CTS from the receiver, the sender will send a data frame after waiting SIFS. If the sender receives an ACK from the receiver, the transmission is successful. In the meantime, other stations just wait a NAV (Network Allocation Vector) time, which indicates the remaining time of the on-going transmission sessions. Using the duration information in RTS, CTS and Data frames, stations update their NAVs whenever they receive a frame. When the sender finds the medium is busy, the sender waits a back-off window. The length of the back-off window is considered to be a counter. The station will try to retransmit when the counter reaches zero. This technique aims to the elimination of the hidden node problem. 2.2 The : An overview The is a kind of hybrid of a and. The new features of the standard are dealing with the release of a new Physical layer, the support of preamble type as well as the MAC CTS-toself new protection mechanism. The uses the same transmission technology found in a, which is called Orthogonal Frequency Division Multiplexing (OFDM). This increases the amount of data transmitted in a given time slice (i.e. higher data rates). However, unlike a, which operates in a 5GHz band, uses carrier frequency bands that are around 's 2.4GHz primary carrier frequency. More precisely, this standard supports the following PHY layers: ERP-DSSS/CCK (mandatory) ERP-OFDM (mandatory) ERP-DSSS/PBCC (optional) DSSS-OFDM (optional) The ERP-OFDM is the new mandatory physical layer introduced by. With the OFDM technique IEEE a s data rates are provided at the 2.4 GHz ISM frequency band. As it was stated above, at the MAC layer the CTS-to-self mechanism is used. The main goal of this mechanism is to reduce the overhead caused in the network due to RTS/CTS signaling sequence. Unlike the RTS/CTS, CTS-to-self cannot eliminate the hidden node problem the wireless networks suffer, unless all stations are within the transmitting node s range. A short description of the CTS-to-self technique is given below: Assume that Wireless Station 1 has data to send to Station 2, Fig. 3. Wireless Station 1 sends a CTS frame. This frame is received from all stations lie within WS1 coverage area (i.e WS2 and WS3). If all stations are inside WS1 coverage area then no collision will occur, but if a number of stations are outside WS1 coverage a collision is most likely to occur. Figure 3. CTS-to-self mechanism 2.3 Working in harmony The also offers backward compatibility with, so that somebody hopefully won't have to toss all the gear he has accumulated. Computers or terminals set up for can fall back to speeds of 11 Mbps. This feature makes and devices compatible within a single network. Modification of an access point to compliance usually involves only a firmware upgrade. But compatibility still remains a large question mark across the gamut of products. The promise of is that it will deliver a-like data rates, with 's better transmit distances, and handling of reflections and occlusions. And because it works in the same 2.4GHz frequency, it ought to allow easier interoperability between and. 3. Simulation Results The purpose of our simulations is to examine and evaluate the performance of both standards under the same topology and traffic scenarios, in terms of MAC
4 layer s QoS parameters, such as Throughput, Bandwidth Utilization and Media Access Delay. This statistic contains the delay of a packet from the time it is picked up from the transmitter until it is successfully received from the receiver. The following lines specify the parameters set, regarding the configuration of the topology and traffic scenarios for the performed simulations. The simulation time was set be 150 seconds. All cases involved ten (10) wireless nodes. When using the RTS/CTS mechanism was used while for the case the CTS-to-self technique. The packet length was 8000bits (~1024bytes) and the RTS threshold 2000bits (a quarter of the packet length). Several distributions dealing with the packet generation rate (i.e. constant, Poisson etc) and length (constant, exponential etc) were used. The Data Rate was set to be equal for both standards because the scope of this paper is to examine the performance of the MAC layer under the same data rate conditions (11Mbps). The following sets of figures depict the simulation results obtained from the wireless C++ simulator. All nodes are within the coverage area of each other, hence there are no hidden nodes. BW Utilisation Delay (msec) Network Utilisation Figure 4. Network utilization Media Access Delay Wireless Nodes Figure 5. Media access delay Throughput (bps) Network Throughput Figure 6. Mean network throughput Observing Fig. 4, it is more than obvious that the network capacity in terms of BW Utilization is higher when the protocol was used. This can be easily explained due to the RTS/CTS exchange signaling, which adds some extra overhead to the network. Hence, the difference between the two protocols in terms of network utilization is approximately 19.6%. Taking into account the above condition, degradation to the system throughput, when the network uses the MAC layer is expected. Figure 6 confirms that estimation. Arithmetically speaking, the calculated difference of the mean network throughput among the two protocols is approximately 30.5%. Furthermore, Fig. 5 displays the average media access delay of each node. There is no doubt that performs better due to shorter delays. Summing up, performs better at all three metrics chosen for the MAC layer evaluation. Thus, better QoS can be delivered compared to the. The next sequence of simulations, involves hidden nodes. The network topology is shown in Fig. 7. It has to be mentioned that the coverage area of each node was set to be 300m. All the other simulation parameters remained as described previously. Table 1 shows which nodes are hidden from each wireless station. This table arises from the calculated distances between the nodes, in accordance to the topology pattern. Table 1. Hidden nodes Wireless Nodes Hidden Nodes 1 3, 4, 5, 6, , 5 3 1, 8 4 1, 2, 7, 8, 9 5 1, 2, 7, 8, 9 6 1, 8 7 4, 5 8 3, 4, 5, 6, , 5, , 8, 9
5 Figure 7. Topology pattern The next figure presents the network capacity measured in bandwidth utilization units. The mean difference between the two standards for this scenario is approximately 30.02%. The following set of figures, features the results obtained from the simulation tests undergone by the simulator OPNET v The QoS metrics compared for the purposes of our study are network throughput and media access delay. Commenting on Fig. 10 and Fig. 11, where the network topology deals with nodes in LOS exclusively, it can be observed that the network using the characteristics performs better than the one using the. This outcome confirms the observation concluded by the results obtained during the first part of our simulations. 0.6 Network Utilisation BW Utilisation Figure 8. Network utilization Examining the above figure it can be seen that the hidden node problem has degraded the network performance in terms of bandwidth utilization. The performs better when the hidden node problem occurs. This is because the RTS/CTS signaling exchange avoids in a great degree possible packet collisions compared to CTS-to-self MAC mechanism used by. In order to be more precise the difference between the two cases is approximately 29.8%. The above situation results to a poor IEEE throughput performance compared to IEEE. This can be also seen in Fig. 9. Network Throughput Figure 10. Network throughput & delay Figure 11. Network throughput & delay The two following figures (Fig. 12 and Fig. 13) are the results produced by OPNET when the topology pattern of Fig. 7 and Table 1 was used. At this case the hidden node problem has to be taken into the account. Throughput (kbps) Figure 9. Network throughput Figure 12. Network throughput & delay
6 5. References [1] IEEE WG, Reference number ISO/IEC :1999(E) IEEE Std , 1999 edition, International Standard [for] Information Technology - Telecommunications and information exchange between systems-local and metropolitan area networks-specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, 1999 Figure 13. Network throughput & delay Comparing Fig. 10 to Fig. 12, it is clearly seen that the s network performance is not affected by the hidden node problem due to the RTS/CTS MAC protection mechanism. On the other hand, s network performance, in terms of QoS characteristics such as throughput and media access delay, seems to degrade when hidden nodes exist. Even a quick view at Fig. 11 and Fig. 13 reveals a lot higher media access delay and lower system throughput. The reason of those poor characteristics is the CTS-to-self mechanism implemented at MAC level. As it was mentioned before, the comparison of the standards was carried out using two different simulation tools: the C++ Wireless Simulator and the OPNET v In general, the results derived from both tools were the same, under the same topology patterns and almost same traffic conditions. The small deviation observed, must be ought to possible different implementations of the protocols for each simulation tool. 4. Conclusions By making a detail study analysis of the results obtained from the wireless C++ simulator and OPNET, some comments can be made. The seems to have the same performance with both topology and traffic scenarios, while the seems to perform poorly when hidden nodes are involved into the topology scenario. This observation matches with the expectations arise from the theoretical part of this paper and the descriptions of the MAC layer protection mechanisms described above. Thus, generally speaking, networks would have both more stable performance and QoS characteristics, independent of both network topology and data traffic, while networks seem to be vulnerable when exposed to hidden node problem. [2] IEEE TM 2003 [amendment to IEEE Std IEEE TM, 1999 edition (Reaff 2003)] [3] Shreyas Sadalgi, A Performance Analysis of the Basic Access IEEE Wireless LAN MAC Protocol (CSMA/CA) [Online].Available: algi/network.pdf [4] Dimitris Vassis, George Kormentzas, Angelos Rouskas and Ilias Maglogiannis, The IEEE Standard for High Data Rate WLANs, IEEE Network May/June 2005 [5] Y. Zahur, M. Doctor, S. Davari, T.A. Yang Performance Evaluation, Communications, Internet, and Information Technology, CIIT 2003, 11/17/ /19/2003, Scottsdale, AZ, USA [6] Qiang Ni, et al. A Survey of QoS Enhancements for IEEE Wireless LAN, Journal of Wireless Communications & Mobile Computing, Wiley 2004, Issue 5 [7] Hossam Hassanein et al. Infrastructure-based MAC in wireless mobile ad-hoc networks, Elsevier B.V, Journal Ad Hoc Networks 2004 [8] Anders Lindgren et al. QoS Schemes for IEEE A Simulation Study, Mobile Nets & Applicattions 8, , 2003 Kluwer Academic Publishers [9] Kil-Woong Jang, Sung-Ho Hwang, and Ki-Jun Han A Dynamic Backoff Scheme to Guarantee QoS over IEEE Wireless Local Area Networks EurAsia-ICT 2002, LNCS 2510, pp , 2002 Springer-Verlag Berlin Heidelberg 2002 [10] Pythagor Simulation Tool, http// [11] OPNET v.11.0 PL1, http//
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