Wireless bridge for IPVS video surveillance systems
Many schools (used interchangeably with businesses and organizations) have installed wireless equipment to connect students and teachers to the wired network. This type of installation is called an Access Point. An access point offers two functions: to convert the medium for the transmission of signals from wireless to cable and vice versa, and to serve several clients at the same time as a hub. It is a popular use of wireless technology. What is not so well known is the subject of this article: the wireless bridge and our legal obligations. We raised these issues for the first time as a report of the research and development efforts made by our IPVS team. The findings are helpful for schools when considering an extension of the school’s data network for various purposes, including video surveillance. The applications are not limited to IPVS.
A wireless bridge consists of a pair of wireless equipment as well as the two pillars that support the span of a bridge that crosses the port. Each wireless device consists of a radio and an antenna. The radio is used to convert wire-to-air and air-to-wire signals. The antenna serves to amplify the strength of the signals so that they can reach a great distance or run fast over the airspace (empty also if applicable). A bridge is different from an access point in many ways. We have learned a few things from our bridge project and the most important is the issue of legitimacy.
The airspace has been full of signals from day one, whether we know them or not. Scientists have created a numerical scale called the electromagnetic spectrum to classify the type of signals, and this scale covers human speech, visible light, microwaves, X-rays, and all levels of radiation. The electromagnetic scale is commonly expressed in frequency in KHz, MHz, and GHz (Kilo, Mega, or Giga Hertz) and in a logarithmic progression. Human speech exists at the KHz level. Radio operates at MHz and GHz levels. Light and radiation occupy the higher GHz regions. The energy content increases correspondingly on the scale as frequency.
The use of airspace is regulated. For example, the police will knock on our door if we shout loudly after 11pm. Certainly, we are not allowed to shine a strong search light on a neighboring house at any time. Radio is a different story because it requires a receiver to see or detect the signals. Due to this invisible nature, spectrum use is expressly regulated by governments. As for New Zealand, there is a Radio Spectrum Management (RSM) group in the Ministry of Economic Development dedicated to regulated uses of wireless technology. Spectrum requires licenses to operate with some exceptions and these free bands are normally very small. Our daily use of wireless equipment can have 2.4 GHz or 5 GHz as license-free airspace for transmission. RSM has specified the maximum level of power transmitted by each wireless equipment so that the same limited airspace can accommodate more users. This legal limit has practically imposed a ceiling on transmission speed and distance, as both performance indicators depend on the signal power level.
We have noticed that many wireless products on the market only have 2 operating parameters. One is the spectrum of use, such as 2.4GHz or 5GHz. The other is the transmission protocol promulgated by the IEEE (Institute of Electrical and Electronics Engineering) in the US as 802.11a, b, g and n. Some of these products are capable of generating radio signals well above the legal limits in force as allowed in each country. International suppliers and possibly local product distributors are not liable for any infringement, but users are.
Most wireless access points are installed within the school campus for use by teachers and students on campus. Even if the signs are above the legal limits, neighboring houses may not be bothered because the signs would have weakened along the distance from the facility to the campus boundary. A bridge is a different story. A bridge crosses territories of various properties and the strength of the signals is easily detectable. RSM has established legal limits in units of EIRP – Equivalent Isotropic Radiation Power. This leads us to learn more about antenna technology.
The antenna amplifies the radio signals and the amplification is expressed in dBi, which stands for Decibel Isotropic. In simple words, it is the relationship between the vector sum of the signal strength in all directions and a simple reference design. A bridge appears as a point-to-point arrangement and we would think that there is only one flow of radio signals. This is not true and depends on the shape of the antenna. Satellite dishes would be close to a single, pointed stream, but they may not be economical to use. Flat panels are a good compromise as the physical size can be smaller and the signs are quite in focus. Obviously, a rod-shaped antenna is cheaper to install, but a rod radiates in all directions. We normally use parabolic or flat panel for bridges and rod form for access points.
If you have visited Taipei before, you would have known that the Grand Hotel there is an icon because the Grand Hotel looks like a former residence of the Chinese emperor. The Grand Hotel sits on a hill and offers a panoramic view of a large part of Taipei City, including the Dashui River. Our wireless consultant has commissioned a test of the wireless bridge that crosses the Grand Hotel and a riverside park. The straight-line spatial distance is 1.7 km and the bridge achieved a throughput of 81 Mbps (megabits per second). The bridge can be compared to a standard local area network cable, but a standard 100Mbps cable connection can only deliver about 60Mbps over 100 meters.
While the cost of a pair of wireless bridges is substantially less than running a cable over 1.7 km of terrain regardless of legitimacy and technical issues, maintenance considerations will make the difference even greater. In addition to the cost advantage, the wireless approach also achieves a higher level of performance. This sounds too ideal. Be careful with the catches. This article has only brought the legal issue to your attention and there are technical challenges to a smooth and successful installation.