【文章內(nèi)容簡(jiǎn)介】 sers receive these packets through anotherNIC attached to its laptop or other putational hardware employed in the work is IEEE 1 pliant. This standarddefines the media access method and the physical layer specifications of a defines two modulation techniques: DSSS (Direct Sequence Spread Spectrum) and FHSS (Frequency Hopping Spread Spectrum). These modulation techniques are inpatible with each other and it should be observed when expanding the wireless work. Our wireless work operates at GHz (microwave band), provide a bandwidthof 2 Mbps with fallback to 1 Mbps, DSSS modulation and CSMNCA access method(Carrier Sense Multiple Access/Collision Avoidance) with ACK. The avoidance collision mechanism is needed in a wireless work as a host cannot detect a collisionafter it sends a packet, different from an Ether work. 3. The Installation Process In this section we present the phases involved in the installation process and the developedoptimization model. Design and Capacity Planning of Wireless Local Area Network 331 Mapping The Demand Area As a pilot experience and due to a limitation of having only three base stations available,we considered the third and fourth floors of our building as our demand 1 shows the demand area mapping of each floor. The lighter areas correspondto aisles. They were excluded from the demand area because they are in a circulation area and their signal levels are usually better than inner rooms (fewer obstacles). Thus,the impact caused by its removal is irrelevant. The dark areas correspond to faculty rooms and labs. They are, effectively, our demand areas. Figure 1: Demand area of 3rd e 4th floors Laboratory rooms mainly occupy the third floor while faculty rooms mainly occupythe fourth floor. The set of rooms forms a rectangular area with an inner freespace. The demand area mapping was obtained by dividing the total area into small quadrangularpieces of demand points. Due to the variable room size, we defined a squaresize unit so that the area of each room is an integer multiple of this square area. Theidea is to avoid situations where a unit belongs to two adjacent rooms. With this mapping,we got 1144 square units of 0,7O x 0,70 m2, being 592 on the 3rd floor and 552on the 4th floor. 338 Session Eighr Network Design and Planning Choosing Candidate Locations 3 In the next stage, we had to choose candidate locations to the BS. A good candidatesquare must offer low cost of installation and good attendance area. Questions likephysical security, available infrastructure and flexibility are also process of choosing candidate locations with different characteristics let usunderstand the BS behavior and its reach, trying to discover locations that could giveus a better coverage. Thus, we chose three candidate locations on laboratories at thethird floor, one on a faculty room of the 4th floor and two on the aisles (3rd and 4thfloors). Chosen locations are shown in Figure 1. Signal Measurement After choosing the candidate locations, we must calculate or measure the signal levelreceived from each candidate BS at each demand point. On outdoor environments, this is usually calculated through signal prediction algorithms. In our work, however, we preferred to measure the signal received at each demand great number of demand points (1 144) and the small area of each point (0,49m2) would make very difficult to measure the signal level at each demand point. The problem was solved by grouping the demand points into small but larger groups. Typically, we formed groups of four or six demand points. For each group, we performedonly one measurement and we assumed that the measured signal level has the same value for all group elements. With this simplification, the number of necessary measurements for each BS was reduced from 1144 to 253. The munication signal quality is measured in decibel (db). A higher valuemeans a better signal quality. Values over 20 db indicate excellent quality. Values between 1 1 and 20 db indicate acceptable signal quality and values under 10 db means poor or no munication capacity. The signal measurement was done using the software WaveManagerKlient IEEE,implemented by the WavePOINTI1 manufacturer. This software lets us register the received signal from each BS simultaneously and save the information in a log file for later treatment. During the measurement, however, we verified that the signal level at each point was very sensitive to obstacles and highly dependent of the mobile unit orientation. Given the same demand point as reference, if we turn the mobile unit to left or right by 90 degrees, we can get a pletely different signal level. Figure 2 shows the signal level variation from two distinct BS that arrives at the same demand point while the mobile unit is turned around by 360 signal level variability according to the mobile unit orientation introduced a new ponent to the problem. It is not sufficient to measure the signal level of the BS at each point, but it is also necessary to choose or calculate the signal value that best represents the signal quality at that point. We defined a method for measuring the signal that could provide the most representative signal level at each demand point. The data was collected while the mobile Design and Capacity Planning of Wireless Local Area Network 339 4 Signal Level Variation Along Time Figure 2: Signal level variation along time unit was turned around slowly until it pleted 360 degrees. Thus, we tried to represent all possible orientations of the mobile unit for that point. With this methodology, we obtained approximately 80 signal level values from each BS for each demand point. The next step was to analyze and treat the collected information. The BS signallevel at a demand point is a discrete function. This behavior is because the