An overview of IEEE 802.11 (wireless network standard) - Essay Example

AN OVERVIEW OF IEEE 802.11 The leading ity involved in the specification and ratification of technology related standards is the 'Institute ofElectrical and Electronics Engineers' (IEEE) (BECTA, 2005). Presently there are three wireless standards originated from IEEE. They are IEEE 802.11a, 802.11b and 802.11g. IEEE 802.11b It is the most widely used wireless network standard. It is used by most public wireless 'hotspots' (Khayat, 2002). 802.11b standard was ratified by IEEE in 1999. The main features of this standard are as given below. It operates in 2.4 GHz spectrum. It has nominal data transfer rate of 11 Mbps and a practical rate of about 4-7 Mbps. It offers three non-overlapping channels. These features are adequate for most data transfer applications and for accessing internet but might be inadequate for multimedia access. It might also face problems when users in large number access the network from a single access point. The frequency of operation, i.e., 2.4 GHz, coincides with the spectrum used by cordless phones, microwave ovens, etc. Therefore the interference problems are more likely to occur. IEEE 802.11a It was rated by IEEE in 1999. The main features of this standard are as given below. It uses 5 GHz frequency spectrum. It has a nominal data transfer rate of 54 Mbps with a practical rate of 17-28 Mbps. It offers 8 non-overlapping channels. It has a signal range of about 50 metres with data rates tending to drop at 10-15 meter range. It uses 'Orthogonal Frequency Division Multiplexing' (OFDM) which is a technology which when used prevents multi path distortion and has resilience to RF interference. 802.11a suits to the conditions of multiple users accessing the network with high data rates. It is best suited for classrooms where students can access multimedia, digital video or database packages (BECTA, 2005). IEEE 802.11g It was rated in June 2003. It combines the features of both 802.11b and 802.11a and operates in 2.4 GHz spectrum with a speed of 54 Mbps. Its main features are as given below. It operates in 2.4 GHz spectrum. It has a nominal speed of 54Mbps with practical speed of 19-30 Mbps. However in presence of 802.11b equipments the speeds drop to around 60%. It offers three non-overlapping channels. It is less power efficient than 802.11b. Interference with 2.4GHz frequency device is possible. It uses OFDM. MAC Layer IEEE accepts the 802 standard logical link control (LLC) protocol. It also offers, for the purpose of wireless communication, physical layer (PHY) and medium access control (MAC) sub layers. There are two physical layers offered by 802.11 namely, direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). According to the transmission methods and frequencies 802.11 is categorised into three main groups; 802.11b, 802.11a, 802.11g. The main features of these three categories have already been stated above. The main drawback of 802.11a is that it is not backward compatible with 802.11b as both of them use a different frequency spectrum. This results in lesser interest of users towards 802.11a. The advantage of 802.11a, however, is that it operates in 5 GHz spectrum which can be used unlicensed. The 2.4 GHz spectrum used by other two standards interferes with that of other devices such as cordless phones and microwave ovens. The 802.11 MAC supports two basic medium access protocols: contention-based distributed coordination function (DCF) and optional point coordination function (PCF). (Zhu, Hua, Li, Ming, Chlamtac, Imrich & Prabhakaran, B., 2004). PCF causes the wireless channel to be divided into super frames. Super frames in turn provides two periods, a contention-free period (CFP) for PCF and a contention period (CP) for DCF. There is a device known as point co-ordinator which is usually an access point (AP). The point co-ordinator polls for grant of access to the wireless channel at the start of CFP. On obtaining the channel, it checks the stations regularly and sets priorities to these stations as per their importance, i.e., their sensitivity towards delay. According to their respective importance, the point co-ordinator allows them to transmit data. Although the optional PCF is designed for delay-bounded services, it is centralized and can only be used in the network of infrastructure mode (Zhu, Hua, Li, Ming, Chlamtac, Imrich & Prabhakaran, B., 2004). In case of DCF another scheme known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used. In this case an instruction is transmitted by the MAC layer to the receiver in order to check whether other carriers are transmitting or whether the channel is available. If there is none other then it goes on to transmit its data after a certain time interval and waits for acceptance signal. This signal determines whether the sent information has reached the other end or not. In case it does not receive it, which means the data has not been delivered, another attempt after checking the channel is made. Thus there are a number of methods at the disposal of the transmitter to check for the usage of channel which includes observing and looking for real signals and checking whether any signals are expected. In order to accomplish this all data packets contain a value which indicates the period of time for which transmitting station will occupy the channel. All the stations are provided with this information and they send the data only when the indicated amount of time expires. Once the channel appears to be idle the next transmitting station will have to wait for a period of time called DCF Inter-Frame Space (DIFS). However, if the channel was previously active it has to wait for DIFS time and in addition to that a random number of back off slot times. This time gap is necessary in order to avoid two stations, waiting to transmit data, transmit together. The time determined by the random number of back off slots is known as Contention Window (CW). If a transmitter which wants to transmit data senses that the channel has become active, it will have to wait for some time till the channel is again free. This type of co-ordination ensures foolproof data transmission but it obviously has a disadvantage regarding the lower speed of transmission. This is because of the time required by the stations to wait while the channel is busy. Hence, this causes the networks to become slower. In networks of higher usage this problem becomes even more prominent. In view of this WLANs may not provide a suitable QoS in their current form for systems where real time data transfer is required (Radioelectronics.com, 2004). QoS Enhancements The shortcomings of the above schemes can be overcome by employing certain Quality of Service (QoS) parameters. This ensures to set priorities to transmissions by tagging them and allowing the applications requiring higher quality of service to move first followed by others according to their own priorities. This will help in diminishing the delay level and jitter on certain data such as that used for video and VoIP purposes. A new standard has been introduced for the purpose of developing a new MAC layer. This is 802.11e standard. Here a priority level is assigned to every transmission by the user known as User Priority (UP) levels which are presently eight in number. Having done this, the transmitter then prioritises all the data it has to waiting to be sent by assigning it one of four Access Categories (AC). (Radioelectronics.com, 2004) For successful deployment of the new MAC layer the features of both DCF and PCF are employed but in a modified form. This is then termed as Hybrid Co-ordination Function (HCF). Here the names given to the modified elements of DCF and PCF are Enhanced Distributed Channel Access (EDCA) and HCF Controlled Channel Access (HCCA) respectively. EDCA Among the two EDCA offers a method employing which the prioritisation of traffic is achieved but it is still a contention based system and hence it is difficult to guarantee QoS. Even now there is a possibility that the lower valued data transmitted can prevent the data of higher importance from another transmitter to take priority. EDCA involves the use of a new class of inter frame space called an Arbitration Inter Frame Space (AIFS). In this case the higher priority data is provided with the shorter AIFS which in turn associates with it a proportionally shorter contention window (CW). The gaining of access to the channel by the transmitter is then by normal procedure. However, due to the shorter AIFS and CW it is but natural that the possibility of gaining access to the channel is increased. Although, statistically a higher priority message will usually gain the channel, this will not always be the case. (Radioelectronics.com, 2004) HCCA The HCCA implements a technique which is unlike EDCA and involves a polling mechanism. Thereby there is a guarantee about the Quality of service it provides. Here in this case the transmitter is allowed to gain access to the channel for a specified number of packets. On completion of sending of the packets the channel is then released. The control station in HCCA which is as before normally the Access Point is known as the Hybrid Coordinator (HC) and responsible for taking control of the channel. Here also there is an inter frame space (IFS) which is termed as a Point Coordination IFS. However this is shorter than the DIFS discussed earlier and it is thus always in control of the channel. After gaining the control all the stations or transmitters in the network are polled. The polling is done to determine the priorities to each station resulting in the higher priority transmitters to transmit the data. However, this may result in longer than expected delays for lower priority data packets. QOS AND MOBILITY MANAGEMENT IN HYBRID WIRELESS NETWORKS In addition to roaming and horizontal handoff among 802.11 WLANs, supporting QoS anytime, anywhere, and by any media requires seamless vertical handoffs between different wireless networks such as WLAN, mobile ad hoc network (MANET), Bluetooth, Universal Mobile Telecommunications System (UMTS), and wideband code-division multiple access (WCDMA) (Zhu, Hua, Li, Ming, Chlamtac, Imrich & Prabhakaran, B., 2004). There have been certain new architectures/schemes which have been planned for seamless integration of WLAN and various wireless network interfaces. All of them are discussed as under. Integration of WLAN and MANET Lamont and Wang (Lamont and Wang, 2003), in their investigation of problem of session connectivity worked for providing connectivity while the users roamed continuously across multiple WLANs and MANETs. In the proposed network architecture, the Optimized Link State Routing (OLSR) protocol is given the task of routing within MANETs whereas handoff between WLANs and MANETs is supported through automatic mode detection and node switching capabilities of the mobiles. Integration of WLAN and Bluetooth Conti and Dardari (Conti and Dardari, 2003) in their proposal proposed for the purpose of the evaluation of the interference between IEEE 802.11 and Bluetooth an integrated analytical. The model considers both PHY and MAC layers and the implementation of the model is quite easy. The performance is evaluated by packet error probability in terms of the relative distances between the two systems for different conditions (Zhu, Hua, Li, Ming, Chlamtac, Imrich & Prabhakaran, B., 2004). Integration of WLAN and 3G wireless networks: Jaseemuddin (M. Jaseemuddin, 2003) proposed another architecture for the purpose of integrating UMTS and IEEE 802.11 WLANs. Due to the use of 802.11 primarily for high-speed best effort service, a mobile node can be designed to maintain two connections in parallel (a data connection through WLAN and voice connection through UMTS). Conclusion 802.11 DCF and PCF access methods do offer a best-effort service for wireless users. There are continuously researches going on to improve 802.11 WLAN performances. The QoS techniques improve the DCF and PCF access method and provide differentiated services as a result of assignment of high priorities to certain important stations. EDCA and HCCA are the two main access methods which provide, respectively, differentiated services and guaranteed services as basis for QoS support in WLAN (Gannoune, Lassad, Robert, Stephan, and Rodellar, Daniel (n.d.). However there are certain drawbacks and limitations of IEEE 802.11e also. There are still some issues to be solved and much more research work has to undergo for deploying proposed QoS mechanisms for IEEE 802.11e standard, i.e., EDCF and HCF. It is worth mentioning that 802.11e ha snot got the approval yet and therefore there is further testing to be done in this regard. Bibliography British Educational Communications And Technology Agency, BECTA January 2005, Wireless Local Area Networks (WLAN), Viewed February, 24, 2006, Khayat, Mike M. March 12, 2002, Wireless Local Area Network (WLAN), Viewed February 24, 2006, < http://faculty.ed.umuc.edu/meinkej/inss690/khayat.pdf > Zhu, Hua, Li, Ming, , Chlamtac, Imrich & Prabhakaran, B. August 2004, A SURVEY OF QUALITY OF SERVICE IN IEEE 802.11 NETWORKS, Viewed April 21, 2006, < www.cse.iitb.ac.in/varsha/allpapers/ wireless/kiran/surveyofQoSin802_11.pdf > Ni, Qiang, Romdhani, Lamia & Turletti, Thierry 2004, A Survey of QoS Enhancements for IEEE 802.11 Wireless LAN, Viewed April 21, 2006, < www.hamilton.ie/Qiang_Ni/papers/JWCMC_Qiang.pdf > Radioelectronics.com 2004, 802.11e for QoS-the new standard to provide quality of service, QoS for 802.11 Wi-Fi applications, Viewed April 21, 2006, < http://www.radio-electronics.com/info/wireless/802_11e/QoS.php > Gannoune, Lassad, Robert, Stephan, and Rodellar, Daniel (n.d.), A Survey of QoS Techniques and Enhancements for IEEE 802.11 Wireless LANs, Viewed April 21, 2006, < team.iict.ch/LGE/IEEE-QoS-Survey.pdf > M. Jaseemuddin, "An Architecture for Integrating UMTS and 802.11 WLAN Networks," IEEE ISCC '03, June 2003, pp. 716-23. A. Conti et al., "Bluetooth and IEEE 802.11b Coexistence: Analytical Performance Evaluation in Fading Channels," IEEE JSAC, vol. 21, Feb. 2003, pp. 259-69. L. Lamont et al., "Integrating WLANs & MANETs to the IPv6 Based Internet," IEEE ICC '03, vol. 2, May 2003, pp. 1090-95. Read More