Wireless Video Surveillance Tutorial
While wireless can solve challenging problems, it is far riskier to deploy and use than wired networks. As such, it is critical to understand when to use wireless systems and the key risks in designing such systems. If you use wireless networks prudently for video surveillance systems, the financial benefits can be quite significant. However, miscalculation in choice and design can result in significant reliability and scalability problems.
Let's review these key elements:
- Network Topology
- Available Frequencies
- Antenna Types
[Note: For those looking for a deeper treatment on radio frequency issues, gain, loss, link budgets, path loss, etc., see our RF for Wireless Surveillance tutorial.]
There are essentially three wireless network topologies, all with uses depending on how cameras are deployed:
- Point-to-Point: First, and most common are point-to-point (PtP) wireless links. In PtP networks, a single radio at the device location is connected to a single radio connected to the surveillance network. Directional antennas are most often used in PtP links, with multi-mile ranges possible. Parking lot cameras are one of the most common applications for PtP radios.
- Point-to-Multipoint: In point-to-multipoint (PtMP) wireless links, a single radio acts as base station, connected to the central network, with multiple radios transmitting to it. The radios used in PtMP setups may be the same as PtP in many cases, though some manufacturers use special radios for the base station to handle higher data rates. Antennas used for PtMP base stations vary, depending on where cameras are located. Omnidirectional antennas are common where cameras are located in all directions from the base station. If cameras are in one general direction, wider angle directional antennas such as sectors, may be used. Directional antennas are normally used at camera locations. PtMP is used in various applications, when multiple camera locations are within range of a single point, such surveillance of parks, connecting cameras across a campus, or city surveillance applications.
- Mesh: In mesh networks, each wireless radio connects to multiple other radios, providing more than one path for network traffic. This means that should one link fail, data is rerouted to another path, potentially preventing outages. Mesh is far more expensive than PtP or PtMP systems. In surveillance, mesh is most often seen in city surveillance, as few other applications have either the requirements or larger budgets of these projects.
Once topology is decided, frequency is the next most important choice in selecting wireless for surveillance. There are a number of frequency ranges available, each with their own pros and cons:
- 2.4/5.8 GHz: 2.4 and 5.8 GHz frequencies are unlicensed, free for use by anyone, and most often used in 802.11a/b/g/n wireless networks, such as those found in many homes and businesses. Because of this, these frequency ranges are typically the most crowded, with the greatest potential for interference. Additionally, 2.5 and 5.8 GHz are less able to penetrate obstacles than lower frequencies, making line of sight (LOS) key when deploying radios in these bands. In professional video surveillance, 5.8Ghz is more frequently used than 2.4Ghz as it is relatively less crowded.
- 900 MHz: 900 MHz radios are most often used when line of sight is not guaranteed, as this lower frequency band is better able to penetrate foliage and other obstacles. This penetration comes with a tradeoff, however, as 900 MHz wireless links have smaller frequencies band and lower bandwidth than 2.4Ghz or 5.8Ghz radios.
- Licensed: For critical wireless deployments, some frequencies are reserved for public safety use. These frequencies vary by region, with 4.9 GHz reserved for use in the United States. Licensed frequencies are more immune to interference than unlicensed, due to their regulated nature.
- TV white space: A recent development in wireless, TV white space frequencies were first opened up to wireless network use in 2010, with very few products currently available. These radios use frequencies in the VHF/UHF range which were vacated in the switch from analog to digital broadcast TV. Since they use lower frequencies (between 54 and 806 MHz), white space radios are better able to penetrate obstacles, but throughput is lower than higher frequencies, topping out at about 16 Mb/s in currently available product options.
- >5 GHz: Wireless radios are also available in a number of frequency ranges above 5.8 GHz, such as 10, 60, or 80 GHz. These frequencies are much more susceptible to interference due to environmental conditions such as rain, snow, and fog, however, and are generally not used in surveillance because of this.
Bandwidth in wireless networks is generally more limited than wired networks. In a wired network, available throughput may easily be up to ~700 Mb/s when using gigabit. Wireless links, however, typically deliver far less bandwidth. However, speeds have improved in the past few years as MIMO radios have become available, with some single radios delivering of up to 150 Mb/s throughput. MIMO, short for Multiple In Multiple Out, spreads radio signal across two or more paths to increase bandwidth and resistance to interference. MIMO radios may use two or more distinct antennas, or more commonly a dual-polarized antenna, which transmits both of these signals at once, with the beamwidths rotated 90 degrees. This image illustrates single versus dual-polarized antennas:
Environmental and site conditions may impact bandwidth significantly, especially as frequencies increase. 5.8 GHz frequencies and below are generally not affected by any but the most severe weather, such as heavy snow or torrential rain. Frequencies above this, however, may be impacted greatly, and thus should not be used for critical surveillance links. Aside from weather, slight changes in site conditions, such as foliage growing into the path of transmission, or antennas shifting slightly may cause intermittent issues, decreased bandwidth, or complete loss of link.
Be careful about manufacturer bandwidth claims. As a general rule of thumb, discount specified bandwidth levels by 50% to 75% when estimating potential for real world surveillance use. The good news is that even with such caution, wireless bandwidth even for a single megapixel camera (~4Mb/s) is generally easy to deliver on a dedicated PtP link. However, as wireless video systems get bigger and more complex, more careful estimation and testing becomes critical.
There are no hard and fast rules for transmission range in wireless networks. Distances are affected by issues such as obstructions, frequency in use, transmission power, and antenna gain. In typical installations where line of sight is possible, such as parking lots, distance is not much of a challenge when using patch antennas in PtP or PtMP configurations. However, while Multi-mile wireless links are easily possible with the right equipment, many users will find the calculations required in these scenarios challenging. Additionally, the longer the link, the more precise antenna alignment must be, making installation more difficult. Multi-mile links even must take the curvature of the earth into account, as it may become an obstacle to wireless transmission at long ranges.
Products Being Used
In surveillance, specialist suppliers are more commonly used than consumer WiFi systems to deliver higher throughput, support of external antennas and, sometimes, optimizations for video streaming. Our integrator survey results show the most popular wireless manufacturers.
Because wireless links are sensitive to fluctuations in site conditions, routine maintenance is a key concern in any deployment. Antenna alignment should be checked, connectors should be checked for corrosion, foliage in the path of the link should be trimmed, and more. We examine these issues in-depth in our Wireless Surveillance Recommendations.