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Analiza utjecaja interferencije na rad WLAN mreža

WLAN (engl. Wireless Local Area Network) mreže se temelje na komunikaciji bežičnim putem pomoću elektromagnetskih valova. Kod WLAN mreža, signali se u obliku elektromagnetskih valova rasprostiru na frekvencijama u pojasu od 2.4 GHz i od 5 GHz. Pojas od 2.4 GHz sadrži 14 kanala od kojih su tri nepreklapajuća kanala i dio je nelicenciranog ISM (engl. Industrial, Scientific and Medical) pojasa te ga zbog toga koriste mnoge druge tehnologije što rezultira njegovom pretrpanosti. S druge strane, pojas od 5 GHz sadrži 25 nepreklapajućih kanala što ga čini mnogo pogodnijim za korištenje i manje zatrpanim. Rad WLAN mreža se temelji na IEEE (engl. Institute of Electrical and Electronics Engineers) 802.11 standardima čiji je razvoj od prvotnih 802.11a i 802.11b standarda, pa sve do nadolazećeg 802.11ax, doveo do poboljšanja u propusnosti, efikasnosti i brzini samih mreža. Međutim, problem interferencije je i dalje ostao prisutan. Postoje dva tipa interferencije: istokanalna interferencija i interferencija susjednih kanala. Istokanalna interferencija se događa kao posljedica utjecaja više signala koji se odašilju na istom kanalu, dok se interferencija susjednih kanala događa pri emitiranju signala na kanalima koji su susjedni ili se djelomično preklapaju. Usporedba tih dvaju vrsta interferencija je pokazala da je štetnija interferencija susjednih kanala jer dovodi do većeg gubitka paketa u WLAN mrežama. Uzroci interferencije mogu biti razni uređaji od mikrovalnih pećnica i bežičnih telefona, preko bluetooth uređaja, pa sve do zidova i susjednih pristupnih točaka. Od navedenih, susjedne pristupne točke predstavljaju najveći problem jer je njihov broj u praksi stalno rastući. Neki od načina kako se utjecaj interferencije može umanjiti su biranje najmanje okupiranog kanala, korištenje prikladnih razina odašiljačke snage, korištenje usmjerenih antena i primjena fizičkih prepreka za umanjenje signala od neželjenih pristupnih točaka. Pri analiziranju utjecaja interferencije na stadionu Poljud uočen je jako mali postotak iskoristivosti radio-frekvencijskog spektra u 2.4 GHz pojasu zbog interferencije sa mnogo susjednih pristupnih točaka koje se nalaze na istom ili susjednom (djelomično preklapajući kanal). Rezultati analize pokazuju da iskoristivost radio-frekvencijskog kanala opada s povećanjem utjecaja interferencije. Najkvalitetnije rješenje za navedeni problem je odabiranje kanala na kojem će raditi WLAN sustav a za kojega je istokaalna i interferencija susjednih kanala najmanja.// WLAN (Wireless Local Area Network) networks are based on wireless transmission over electromagnetic waves. In the WLAN networks, the signals are transmitted in the form of electromagnetic waves in frequency bands of 2.4 GHz and 5 GHz. The 2.4 GHz frequency band contains 14 channels, three of which are non-overlapping. This frequency band is a part of an unlicensed ISM (Industrial, Scientific and Medical) band which is used by a lot of other wireless technologies what makes this frequency band very congested in practice. On the other hand, 5 GHz frequency band contains 25 non-overlapping channels what makes it more suitable for practical usage and less congested. The WLAN operation is based on the 802.11 standards of IEEE (Institute of Electrical and Electronics Engineers), whose development from the original 802.11a and 802.11b standards up to the upcoming 802.11ax resulted in improved bandwidth, efficiency and speed of the WLAN networks. However, the problem of interference has remained present. There are two types of interference: co-channel interference and interference of adjacent channels. Co-channel interference occurs as a result of the influence of multiple signals emitted on the same channel while the interference of adjacent channels occurs when the signal is transmitted on channels that are adjacent or partially overlapping. The comparison of these two types of interference has shown that adjacent channels interference is more harmful because it results in greater loss of packets in WLAN networks. The cause of the interference may be different devices such as microwave ovens, cordless phones, Bluetooth devices and even walls and adjacent access points. Of all these interference causes, neighbouring (adjacent) access points are the biggest problem because their number is constantly growing in practice. Some of the ways in which the impact of interference can be mitigated are the selection of the least occupied 2 GHz or 5 GHz channel, the use of appropriate levels of transmitting power, the use of directed antennas and the use of physical impediments for reducing the signal from unwanted access points. Results of analyses show that the impact of interference on the football stadium Poljud, shows a low percentage of radio frequency spectrum utilization in the 2.4 GHz band. These results were observed due to interference with many adjacent access points on the same or adjacent (partially overlapping) channel. The results of the analysis show that the usability of the radio frequency channel decreases with increasing interference effects. The best solution for the aforementioned problem is to select the channel on which WLAN system will work and for which the interference of the adjacent channels and co-channel interference will be the lowest.

WLAN ; interferencija ; IEEE 802.11 ; frekvencija ; 2.4 GHz ; 5 GHz ; istokanalna interferencija ; interferencija susjednih kanala

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