2 WIRELESS NETWORKING CONCEPTS Answer 1 Channel reuse in WLAN Channel reuse in WLAN helps in increasing the capacity as well as the performance of the channel. When two radio transmitters use same frequency, then co-channel interference takes place (Zhu & Wang, 2012). Sometimes some access points are able to hear other access points (APs)onthesamemediumorchannelthenco-channelinterferenceoccurs.Co-channel interference can reduce the usage of channels. The throughput of the channels gets affected in an adverse manner as the other clients and APs would have to wait for transmitting their data. Channel reuse helps to avoid co-channel interference and maximize the throughput of the channel (Jamil, Cariou & Helard, 2014). Dynamic control over receive sensitivity can be enabled by channel reuse for improving the spatial reuse of channels (Matsumura et al., 2012). Channel reuse designs are based on multi-channel architecture. The features of channel reuse are configured for operating in one of the three modes: static, disable and dynamic mode. The modes of channel reuse can be configured via 802.11g or 802.11a radio profile (Bellalta, 2016). Depending on the transmission power-level that is configured on the access point, the CCA or Clear Channel Assessment can be adjusted in the static mode. In the static mode, with the increase in CCA threshold, the power of the AP transmits decreases and vice versa. In the dynamic mode, the CCA thresholds are completely dependent on the channel loads. The feature of the dynamic mode gets enabled in an automatic manner when the wireless channel or medium surrounding the access point is busy for more than half of the time. In this case, the CCA threshold gets adjusted for accommodating transmissions between the access point and the
3 WIRELESS NETWORKING CONCEPTS distant client. The CCA detect threshold does not get tuned in the disable mode of the channel reuse. Fig 1: Channel Reuse in WLAN (Source:Jamil, Cariou & Helard, 2014, p. 305) Frequency reuse in Mobile Phone networks In mobile phone networks, frequencies are allocated to different services and reused by following a regular pattern of the areas known as cells (Novlan et al., 2012). All the cells are covered by a base station. These cells are hexagonal and arranged in such a manner so that adjacent cells do not have same frequencies (Ghosh et al., 2012). The frequencies of the cells can be reused in an effective manner if the same frequency cells are not adjacent to each other. A typical plan of frequency reuse utilizes seven different frequencies in the hexagonal cells. In wireless communication the area that is needed to be covered are divided into several cells
4 WIRELESS NETWORKING CONCEPTS havingdifferentfrequenciesforavoidinginterferenceandfacilitatingreuseoftheradio frequencies (Zhang et al., 2013). Frequency reuse facilitates effective communication within a single cell and enables the re-use of frequencies in the nearby cells. Frequency reuse enables the use of same frequencies for carrying out multiple conversations (Feng et al., 2014). Consider a situation where N number of cells use same number of frequencies and K is the total number of frequencies. Then the cell frequency of each cell is given by K/N. Suppose K=397, N= 7. Then each cell frequencies will be 395/7=56. Frequency reuse helps in utilizing radio frequencies that are same within an area and separated by a certain distance. It helps in the increase of capacity. Fig 2: Frequency Reuse (Source:Novlan et al., 2012, p. 2033) Answer 2 WMANorWirelessMetropolitanAreaNetworkhelpsinestablishingwireless connections between various locations in a metropolitan area. Its area coverage is more than that
5 WIRELESS NETWORKING CONCEPTS of WLAN but less than that of WWAN (Jia, Cao & Liang, 2015).One of the best known WMAN technologies is WiMAX. It has several benefits that can be utilized for improving the WMAN. Security challenges of WiMAX are: 1) Authentication related threats: Masquerading attacks can take place where one system masquerades its identity by reprogramming its hardware address with another device’s hardware address (Sari & Rahnama, 2013). Attacks can also take place on PKM protocol where the attacker saves the messages received from a legal subscriber station and performs replay attacks against the base station (Dubey & Kumar, 2013). The process of authorization has several vulnerabilities as it does not have any mechanism for ensuring message integrity. 2) Jamming attacks: In this type of attack a strong source of noise is introduced for reducingthecapacityofthechannel(Dubey&Kumar,2013).Thiscantakeplace unintentionally as well as intentionally. Jamming attacks can be easily performed by an attacker. This security challenge takes place in the physical layer. 3)DenialofServiceattack:SeveraltypesofDoSattackssuchasunencrypted management of communication and unprotected entry in the network can take place in WiMAX technologies (Dadhich, Narang & Yadav, 2012). When a subscriber station sends several false requests of authorization to a base station, then the base station uses all the resources by carrying out calculations for checking the validity of the certificate (Sari & Rahnama, 2013). This will lead to a DoS attack as the base stations will be unable to serve the subscriber stations any further. In DoS attack, an attacker forges the RNG-RSP or Ranging Request/Response message for minimizing the power of the subscriber stations or SS and does not allow the SS to transmit
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