Overcoming the Challenges of Wireless Transmission

Most of the time, video surveillance systems use hardwired solutions to transmit video and control signals—copper or fiber for traditional CCTV or standard network infrastructure for networked, digital systems. However, it’s worthwhile to consider using wireless video transmission under certain circumstances.

  • If a long distance separates the camera location from the control site. In this situation, trenching and cable installation may be cost prohibitive, right of way may be unavailable, and a leased line may incur too many recurring expenses.
  • If your cable route is limited. In historical buildings it may be virtually impossible to provide an affordable and workable cable route.
  • If you need a flexible deployment. For museums or exhibition halls, cameras must periodically be relocated to suit changing needs.
  • If you require covert installation. Small, battery-operated, wireless covert units can be deployed quickly and easily.
  • If you need a mobile solution. Mobile cameras can improve incident response by allowing the incident command center to monitor first-responder activities and provide accurate assessment of the situation.

Transmission Capacities and Security
Analog. Virtually all wireless video surveillance transceivers operate in the unlicensed part of the radio spectrum. Under the provisions of FCC regulations (47CFR parts 15 and 18), the industrial, scientific and medical (ISM) bands are a part of the unlicensed radio spectrum set aside for anybody to use without a license. One can easily find analog wireless video transceivers operating in the 5.8GHz, 2.4 GHz and 900 MHz ISM bands. There are a few wireless video devices operating in the 1.2 GHz amateur band, and some lower-power, short-range consumer products operating in the UHF band.

These analog video transceivers do not offer any special measures to secure the transmitted video signal. The full-frame, real-time NTSC video signal is sent via either FM or AM modulation, and the transmitted signal is received with the right receiver. Since these wireless video links operate in the unlicensed public access band, it is extremely easy and affordable for criminals to put together a wireless video receiver with an external antenna and a signal booster to intercept the unencrypted video signals.

To safeguard these transmissions, a pair of encoder and decoder video encryption units can be used to apply a line cut and rotate scrambling technique. An encryption key produces a random sequence of line cut positions. The encoder selects a cut point in the NTSC video signal, cuts the signal into two segments, and presents the last segment to the output first, followed by the first segment. At the receiving end, the decoder reverses the process to rebuild the video signal.

WLAN. In the digital video world, the network infrastructure is standards based, with virtually all wireless local area network (WLAN) devices complying with IEEE802.11a, IEEE802.11b or IEEE802.11g with a bandwidth of up to 54Mbps. These devices operate in 2.4GHz and 5.8GHz.

Due to challenges like latency, bandwidth availability, wireless link quality, high bandwidth demand and variability of video applications, the quality of video over WLAN is still poor when multiple cameras are sharing the same access point. The traditional priority and buffering technique cannot provide the proper quality of service needed to deliver streaming video over WLAN. A wireless network may suffer from sudden and severe drops in bandwidth, resulting in stuttering and frozen video. Hopefully the Wireless Metropolitan Area Network (Wi-Max) standard expected in the near future will boost the allowable bandwidth to 280Mbps at the access point up to a distance of 20km, eliminating or mitigating some of the video quality issues.

WLAN devices have built-in security measures to protect the transmitted information. There are two encryption standards in use:

  • Wired Equivalent Privacy (WEP). Since the WEP encryption key is static, with the right tool and a little time (as little as 15 minutes) a hacker can use reverse engineering to derive the key and gain immediate access to the information.
  • Wi-Fi Protected Access (WPA). This uses Temporal Key Integrity Protocol (TKIP), changing keys with every data packet to make it more difficult to analyze the captured radio signal and derive the encryption key.

Bluetooth.

4G. Another wireless technology that may be promising is the fourth-generation (4G) mobile network, which is still in the incubator stage. At present, the 3G mobile network can only provide up to 2Mbps—definitely too limiting for video surveillance applications. On the other hand, the 4G mobile network is designed to provide multimedia services at low cost and up to 100Mbps bandwidth. Potentially, 4G camera phones can be used for mobile applications to provide immediate video assessment of an incident by first responders.

Wireless Limitations
Before implementing wireless video, we have to understand and accept its limitations so that procedures can be established to mitigate the adverse effects.

Wireless communication will not be 100 percent reliable. It will be greatly affected by environmental conditions. Electromagnetic wave transmissions are affected by solar flares, lightning, moisture in the environment—including heavy rain, snow, rain clouds or even steady evaporations from cooling towers—and high wind.

For acceptable reception quality, the distance allowable between the transceivers depends on the output power, the antenna height above grade, type of terrain and line of sight—that is, how clear the path is between the antennas. The major effect on distance is the height of the antennas above ground.

Signal strength =
(ht)2 x (hr)2
d4

Where ht =Height of transmitter antenna above ground
hr =Height of receiver antenna above ground
d =distance between the antennas

From this overly simplified relationship, it can be seen that as the distance doubles, the signal strength will decrease by 16 times. That means that increasing the transmitting power has only a very limited effect on the distance. However, if the aboveground height of the antenna is doubled, the signal strength will improve by four times. By increasing the height of both the transmitting and receiving antenna, a much longer distance can be achieved with the same signal strength. Note that the antenna type will also affect the effective reception distance.

Selecting the Right Transceiver
Before selecting transceiver system, ask an installer to analyze the wireless link. The evaluation should consider the following factors:

  • Transmitter cable attenuation
  • Transmitter antenna gain
  • Path loss—includes free space loss and losses through obstructions
  • Receiver antenna gain
  • Receiver cable attenuation
  • Allowance for multipath signal attenuation
  • Allowance for Fresnel Zone obstruction attenuation
  • Allowance for signal fading due to weather conditions
  • Allowance for signal loss due to vegetation growth

Before placing an order, ask the installer to submit all the power budget considerations to make sure the installer has taken care of all issues revealed at the planning and survey stages. Always be conservative during the planning stage so that unexpected obstacles will not result in a complete re-plan.

Prepare for Installation
For a successful implementation, the first step is to carry out a thorough planning session and preliminary site surveys. Walk the site, gather drawings and maps, and collect the following information:

  • Potential locations of transmitters and receivers. Estimate the distance, note line of sight issues, potential mounting height, etc.
  • Installation site grouping. Consider the pan/tilt/zoom control requirements also. Some manufacturers use separate transceivers (usually in the 900 MHz ISM band) to handle camera control data transmission.
  • Terrain. Hills and lakes.
  • Neighborhood type. Urban, metro, rural.
  • Buildings and structures in vicinity. Anything metallic can block the radio signal substantially—reinforced concrete, aluminum siding, foiled insulation, storage cage, mirrors, lead windows and some UV solar films. Other material like brick, concrete masonry units, drywall or wood will reduce the signal depending on the moisture content.
  • Potential interference source. Power lines, transmission towers. A group of high-voltage power transmission lines may look transparent to the eye; however, to the radio wave they are like a block made up of six-foot-diameter metal pipes.
  • Vegetation, trees and foliage. Both winter and summer seasons. Don’t forget that trees grow yearly.
  • Vehicles and traffic.
  • Other noticeable ISM devices. Include your own existing and planned devices.
  • Power availability and quality. Make sure that there is no ground loop issue.
  • Grounding and lightning protection.

It will be beneficial to have a portable battery-operated survey kit from the manufacturer or installer complete with appropriate transmitter, receiver, antennas, a small LCD monitor and a camera to simulate the operation of the installation. The interference average signal to max signal received should be aimed at -85dBm or less. If the signal received is weak, there will be snow or noise on the monitor. It will also be helpful to carry a pair of binoculars to view the installation sites from a distance.

Before field installation, carry out a bench test to make sure that the transmitter and receiver pair works. However, do not power up the transmitter without an appropriate antenna connected, and always power down the transmitter before disconnecting the antenna. Otherwise the transmitter will be damaged.

Installation Considerations
The following should be considered during installation.

  • Because the ISM bands are unlicensed, other transceivers may be operating on the same channel in the vicinity. Changing the operating channel, repositioning the receiver and transmitter and/or using a directional antenna may reduce the effect of interference from other equipment.
  • When multiple transceivers have to be installed in close proximity, use radio channels that are as wide apart as possible and try to arrange the orientation to allow a 40-degree angle.
  • Antennas should be installed as close to the transmitter as possible, because there will be substantial signal loss if the antenna cable is too long. For example, at 2.4GHz an RG214 antenna cable has a loss of about 0.6dB for every three feet. There will be approximately 0.2dB loss for each connector also. Thus, the antenna cable should not be longer than 10 feet, to limit the signal loss to about 2.2dB. A 3dB gain on the antenna will thus compensate the loss in the cable and connectors. In some installations, it may be advisable to mount the transmitter close to the antenna in a weatherproof cabinet even though it will be more difficult to access for maintenance.
  • For optimal operation, mount the antennas at least 15 feet aboveground and above any other obstructions like roofs, parked vehicles, metal fences and road traffic.
  • Omni-directional antennas should not be mounted next to a wall or a pole. They should be mounted above the wall or pole. If necessary, install the antenna above a small wall-mounting pole to clear the roof of a building structure.
  • Directional antennas (usually parabolic dishes or patch type) should be mounted on the edge of a building or roof and pointing away from the building with the same polarization as the transmitter.
  • For mounting directional Yagi antennas, provide adequate support to take care of the lateral wind load so that there is minimal lateral movement. Yagi antennas have a directional radiation pattern, with sharp drop-offs at about 20 degrees on either side of the center, so exact alignment will be very important.
  • Exact alignment is critical for long-distance installations with high-gain, directional antennas. For distances over one mile, the alignment tolerance will be within a few degrees in the vertical or horizontal direction. For long distances, do not just rely on eye judgment. Consider using a portable GPS unit to pinpoint the location of the transmitters and receivers.
  • Weatherproof all connections by covering connectors with a layer of self-amalgamating electrical tape.

Wireless video surveillance should be deployed to supplement more reliable hardwired solutions unless cable installation proves impractical. The wireless segment of a new network video installation should use WLAN or WI-Max instead analog wireless video transceivers. For existing surveillance systems, design the wireless segment based on a hybrid solution, using analog cameras with video servers to digitize the signal and transmit it via a WLAN access point. This will facilitate an upgrade to an IP solution in the future.

Heung-Lim Chan is a senior consultant for Sako & Associates Inc., a leading provider of security consulting, design and construction management services. He is based at the company’s headquarters in Chicago and can be reached by phone (312-879-7230) or e-mail (hchan@rjagroup.com). For more information, visit the Sako & Associates Web site at www.sakosecurity.com.

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