Calculo de enlace de microondas

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C H A P T E R

2

Site Planning Considerations
Introduction
The installation of a wireless network requires much the same basic planning as any wired network. The main difference is that the wireless signal requires some additional planning. This planning includes RF path planning, site preparation, and installation of outdoor components such as outdoor units, antennas, lightning protectiondevices, and cabling suitable for outdoor conditions. Usually, you also need to investigate the zoning laws as well as Federal Communications Commission (FCC) and Federal Aviation Administration (FAA) regulations. Although the technology implemented in Cisco’s broadband fixed wireless system can make use of multipath signals, reducing the effect of obstructions in the path, it is important thatthe characteristics of the path be carefully examined. With this knowledge, components and network requirements can be correctly planned for your specific application. This chapter provides insight into the planning necessary to prepare your site for your broadband fixed wireless system.

General Considerations
A basic consideration is the physical location of the sites at each end of the link.Because microwave signals travel in a straight line, a clear line of sight between antennas is ideal. Frequently, however, the locations of the desired links are fixed. When a clear line of sight cannot be achieved, you must plan accordingly. Other general site considerations include:
• • • •

Ability to install one or more antennas—Is the roof adequate to support the antenna(s) or will itrequire structural reinforcement? Will a tower have to be constructed? Are permits required? Possibility of future obstructions—Will trees grow high enough to interfere with the signal? Are there plans to erect buildings between the sites that may obstruct the path? Availability of grounding—Good grounding is important in all areas of the world, but in areas prone to lightning, it is especiallycritical. Availability of power—Are redundant power systems available if the area is prone to outages?

The planning of a wireless link involves collecting information and making decisions. The following sections will help you determine which information is critical to the site and will be an aid in the decision-making process.

Cisco Broadband Fixed Wireless Site Planning Guide OL-0312-02

2-1 Chapter 2 Weather

Site Planning Considerations

Weather
It is important to research any unusual weather conditions that are common to the site location. These conditions can include excessive amounts of rain or fog, wind velocity, or extreme temperature ranges. If extreme conditions exist that may affect the integrity of the radio link, Cisco recommends that these conditions be taken intoconsideration early in the planning process.

Rain and Fog
Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When microwave frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density andlong duration. If this is the case, shorter paths may be required. The systems discussed in this guide operate at frequencies below 6 GHz, so rain is not a concern. In most cases, the effects of fog are considered to be much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as temperature inversion, or very still air accompaniedby stratification. Temperature inversion can negate clearances, and still air along with stratification can cause severe refractive or reflective conditions, with unpredictable results. Temperature inversions and stratification can also cause ducting, which may increase the potential for interference between systems that do not normally interfere with each other. Where these conditions exist,...
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