Pulling Video Out of Thin Air

When we last visited the concept of wireless in this column (see “Fly Through Air With Greatest of Ease,” January 2008) we spoke primarily about the protocols and standards that define the wireless networking environment.

This time around, I want to delve more into the implementation of a video network, focusing on hardware best practices, as well as looking at some of the issues you need to watch out for when trying to put video on a wireless link.

Wireless network topologies fall into three basic topologies: point-to-point, point-to-multipoint and mesh.

3 Types of Wireless Networks
Point-to-point and -multipoint networks are pretty straightforward. They consist of a single transmitter and receiver pair, or in the case of multipoint, a single receiver and multiple transmitters. For getting a single video stream from point A to B, a simple point-to-point system is the easiest way to go.

If you have a multiple camera system, point-to-point may not make sense. If your cameras all have a clear line of sight to a central point, maybe the roof of a building where a physical network can be accessed, then point-to-multipoint, where you have a transmitter at each camera and a single receiver on the building, would be most logical. This is provided you have enough signal coverage and available bandwidth at the receiving end to handle all the video streams at the same time.

But what if you have many cameras spread out across a geographically diverse area, such as a university campus, or even a city? Then it’s time to mesh it up.

One of the most prolific technologies in recent years has been city-wide mesh wireless networks. A large motivating factor in that is the use of video for both policing and traffic monitoring situations. Also, when dual radio devices are used, the mesh network can be shared by two different sets of users, combining the video policing and public Wi-Fi, or maybe traffic monitoring and police department mobile data networks.

The Monster Mesh
In a mesh network, each radio device is considered a node, and acts as both a transmitter and receiver. You generally have three types of mesh nodes: access points, bridges and backhauls.

Access point (AP) nodes are usually those in the field talking directly to edge devices like cameras or police cars. Each AP node has a wireless connection to adjacent APs. Often, if a system is small enough, it may be made up of just AP nodes, all connected together, forming a self-healing mesh. If a node in a self-healing mesh goes offline, other nodes in the area can bypass the bad node and redirect network traffic around it, maintaining the integrity of the network.

Once the surrounding nodes detect that the bad node has been repaired, traffic resumes its normal path. Self-healing mesh networks can also reroute traffic depending on other conditions, such as noise from interference or anything that slows down the data paths.

For some larger systems, there may be too many edge devices for a single mesh. One of the problems with mesh networks is that each hop (when a signal gets passed from one node to the next) uses up some amount of bandwidth. Because video streams use a relatively large amount of bandwidth, after four or five hops there may not be enough available to carry the streams any further, causing extreme latency or even data loss.

In this case, a backhaul must be used. Backhaul nodes are radios that are usually more powerful and can provide a larger pipe than regular AP nodes. These backhaul nodes would pull signals off of multiple APs and relay that data back to a central point. Backhaul links are generally point-to-point.

While more powerful radios and faster data-forwarding mechanisms provide this larger pipe, another important aspect is the antenna.

Another very important aspect of wireless network design and implementation is the antenna. There are many different types of antennas, each with their own important function. Incorrect antenna selection can often mean the difference between a functioning system and a nonfunctioning one, or even more frustrating, an intermittent system.

3 Types of Wi-Fi Antennas
The antennas themselves are passive devices, meaning they do not provide any amplification of an incoming or outgoing signal. They simply take one type of energy and turn it into another (electrical signals into radio frequencies, and vice versa). We do, however, say that some antennas have a higher or lower gain (measured in dB) than other antennas. This means some antennas are more directional than others, and can either radiate or receive signals better from one direction than others. The higher the gain, the better the antenna receives signals from a given direction (and rejects signals from other directions).

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