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Introduction

The most common wireless technologies for the provision of telecommunications services are those based on the use of radio spectrum (radio), such as mobile telephony, LMDS (Local Multipoint Distribution Service), the Wi-Fi networks (Wireless Fidelity), WiMAX networks (Worldwide Interoperability for Microwave Access), etc. [1].

A Wi-Fi network is actually a network that fulfills with the IEEE 802.11b standard, which is the most currently used []. It offers a maximum throughput of 11 Mbps and uses the frequency range of 2.4 GHz with three radio channels available. With this wireless network it’s able to create high speed LANs as long as the device to be connected is not far from the access point [].

The reach of the access points is usually about 20 meters within a local or home, although the presence of many different networks and thick walls may considerably reduce this range [].

Wi-Fi networks located with some proximity may cause interference, damaging the quality of the link, which means that the user experience is committed, even if there are few nodes connected to the network [].

In public sectors (airports, universities and shopping malls ) often operate multiple wireless Wi-Fi networks using access points that deliver a service to users, with little or no frequency planning []. The only way to avoid frequency overlap is performing a frequency planning between channels 1, 6 and 11, which leads to 3 frequencies of reuse, in the best case []. If repeated channel or adjacent channels are used, interference effects are produced by the overlapping of the frequency bands, deteriorating the signal quality and the service provided [].

In [] there is a review of deployments in real scenarios, which focuses its study in showing how the interference in 802.11 networks, deployed spontaneously, significantly affects the performance of the applications concerned. In [] is addressed the issue of a Wi-Fi network with a dense deployment of access points, making not possible to avoid interference between different access points and clients with a design of non-overlapping channels.

The aim of this study is to determine the performance of the network in an environment with co-channel and adjacent channel interferences depending on the distance.

For the study, intrusive methods are used by the traffic injector (D-ITG) to flood the communication channel. For this work an application of Voice over IP (Skype), due is used in [], the parameters required to perform the simulation such as the number of packets per second and its size are determined, these are presented with more detail in Section II.

This work will have two scenarios with three access points each and varying the distance between them: the first scenario recreates a Wi-Fi network with interference by adjacent channel as the access points are configured on channels 1, 2 and 6 of the IEEE 802.11b standard respectively. The distance variation is 2, 4 and 8 meters apart for nodes connected to the access point 1 due to the working area. The second scenario has two set points of access in channel 1 access and the third in channel 6 generating a co-channel interference, same way are evaluated the changes according to the distances set in scenario 1. All scenarios are realized under the UDP protocol with applications such as streaming audio, video and voice over IP (VoIP), it is preferable to use UDP [].

The detailed explanation of the configuration of the scenarios and used methodology are found in section II, the results obtained with their respective analysis is done in Section III to evaluate future work and the conclusions reached of the work in Section IV and V respectively.

Methodology

To accomplish the objective of this study, scenarios are implemented in an area of 8mx10m, the distribution of access points are shown in Figure 1

[pic 1]

Figure 1. Workspace

For the working scenarios two mobile devices and 8 laptops are used, the characteristics of each device is shown in Table 1.

Table 1. Characteristics of computers

[pic 2]

As mentioned in the introduction there will be two scenarios, the first generates an adjacent channel interference using channels 1, 2 and 6 respectively in each access point. In the second scenario it is required to generate a co-channel interference by which two access points are set on channel 1 and the other on channel 6. In Table II the parameters for scenario 1 are detailed.

Table II parameters configured on the access points of scenario 1

[pic 3]

For the second scenario the same configuration of Table II is used except from the SSID and frequency in the access point 2 for being a co-channel interference the value of the frequency is changed to be the same of the access Point 1 (2412MHz), its SSID is interference. The distribution of the nodes in the working area is presented in Figure 2. For both scenarios the variation of the distances is made only for nodes connected to the access point 1, due to the size of the workspace.

[pic 4]

Figure 2. Initial distribution of devices in the working area

After setting these parameters, the scanning process, authentication and association necessary for the initiation of a communication between the user and the AP with the IEEE802.11b protocol are performed []. An active scan is realized as the station tries to find an AP, the authentication is open because the access point does not have security as can be seen in Table II, finally the association process is started, at the end a station can transmit and receive data. To check for connectivity between devices of the network, the ping command is used (Packet Internet Groper).

When the connection exists, we proceed to configure the traffic injection tool D-ITG (Distributed Internet Traffic Generator), it is a platform which allows producing the packet-level traffic, verifying the QoS parameters} [], [].

In a previous study the maximum capacity of the communication channel in the working area is determined and it is 7.7 Mbps. On the basis of the table presented in [] we know the bandwidth Skype needs, depending to the call to be performed. According to the work [], packets with an approximate size of 120 bytes are sent, plus an average time of five minutes per call is set.

In Table III the general parameters which the equipment are set with, are mentioned, the metric used is One-Way-Delay as it is the travel time of a packet between two network points, for this it is required that equipment are synchronized in order to avoid further mistakes.

In the next Table the settings that simulate Skype calls are shown, the first makes a call in HD hence its description is Call HD1, in [] is recommended that the bandwidth is 1.5 Mbps . On the second computer a group call of 3 people is made, the call descriptor is G3P and the recommended bandwidth is 2Mbps.

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