A Mind of Its Own
Using wireless mesh networks for video surveillance
- By Umberto Malesci
- Feb 01, 2009
Wireless mesh, a network architecture that uses intelligent wireless transmitters, provides more reliability and flexibility compared to traditional point-to-point and point-to-multipoint solutions.
Every mesh unit in a wireless mesh network not only transmits data packets coming from devices directly attached to it but it also can forward packets coming from other mesh units through the optimal path. Compared to a point-to-multipoint topology, where there is only one possible link between a camera and the central access point, mesh networks provide greater reliability because every wireless unit can select from multiple available paths. The flexibility and reliability a mesh-capable transmitter offers simply can’t be rivaled by traditional wireless devices, and wireless mesh is quickly becoming the preferred network architecture for high-end wireless video surveillance systems.
The Basics of MESH
Nodes in a wireless mesh network are powerful routing devices that run advanced mesh routing algorithms designed to evaluate and select the optimal path for every transmitted packet in real time. The dynamic mesh routing algorithm allows the routing of packets and video streams around obstacles, sources of interference or low-quality links, increasing reliability and flexibility.
Each mesh unit performs a real-time evaluation of the quality of every link in the network. Consequently, the mesh unit can change the path when it notices a drop in the quality of the path it is using.
Wireless mesh devices are completely auto-configurable; the routing protocol takes care of finding other mesh devices nearby and the location of the base units attached to the wired network. If a link goes down, the nodes in the network will immediately notice the failure and change their preferred path to avoid the failed link.
Routing in a wireless environment is conceptually similar to routing in the Internet, but it presents several differences in terms of evaluating the quality of a particular path. Internet routing is based on the layered abstractions developed by the Internet’s progenitors in the 1980s and 90s. However, the traditional layered approach is not effective in a wireless environment where the loss rate tends to be fairly high. While Internet routing is based on minimal hop count, in a mesh network the quality of wireless links is a parameter of crucial importance. Researchers at the Massachusetts Institute of Technology discovered that minimum hop count is not effective for wireless routing. Instead, a metric involving link-quality measurement is required to create reliable wireless mesh networks.
Because wireless links tend to have a high packet-loss rate, transmitters often need to resend the lost packets multiple times. Therefore, as shown in Figure 1, in a mesh network, it is often preferable to take a longer yet more reliable path rather than the unreliable direct route to the end point. Often the shortest and most direct path requires multiple retransmissions per packet, wasting bandwidth and increasing delays.
By continuously probing the different wireless links, every mesh node builds and updates statistics regarding every link available. The routing protocol then employs these link-quality tables to compare every possible path and pick the optimal one in real time.
Technical Challenges
Video streaming and video surveillance have been the latest additions to the list of applications for mesh technology. However, high-resolution video streaming over a mesh network presents several technical challenges. Bandwidth is always scarce when one deals with high-resolution video, and a multihop transmission may decrease usable bandwidth and increase delay. For example, as Figure 2 shows, a video packet that goes through three hops absorbs three times the bandwidth compared to a video packet that is only one hop away from its destination.
However, advanced mesh products solve these challenges by operating multiple radios at the same time. This way, the video stream can use many different channels to create high-bandwidth and lowdelay paths. By changing transmission channels at every hop, every transmission absorbs bandwidth from a different channel, which limits any loss of performance along the path. Operating multiple radios allows bandwidth to be increased by spreading the transmissions on multiple non-overlapping channels while decreasing the delay of the network. At every hop the packets go through, the mesh transmitter can choose the best frequency to reach the following node, which optimizes not only the path but the frequency spectrum usage as well. Using multiple channels and designing the network in the proper way is, very often, the way to get reliable wireless mesh networks for CCTV applications.
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Origins of Wireless MESH networking
Wireless mesh networks have been the subject of much academic research in the last two decades. The Defense Advanced Research Projects Agency funded research regarding mesh networks because of their intrinsic reliability and flexibility, with the idea of applying a self-healing network to military applications. Mobile and nomadic communication in battlefield and other hostile environments was the first application of mesh networks. The absence of any single point of failure makes mesh architecture the ideal solution for multipoint-to-multipoint communications. In a mesh network, there is no hub; every node is a peer node. There is no higher layer or controlling node that can be destroyed to bring down the entire network.
Many defense-related applications of mesh networks involved highly mobile and battery- powered units, creating challenging technical problems whose solutions are still in their infancy. However, mesh networks for data transmission, video streaming or temporary communications in case of natural disasters are a reality today, with several different products focused on different types of applications. |
Routing in a multihop, multi-radio mesh network requires advanced routing algorithms and powerful processors embedded in the transmitter. The optimization work is computationally expensive, and the processor needs to continuously crunch and solve complex optimization problems in real time.
Surveillance Applications
Mesh architecture is the solution to several problems faced by security system integrators when they work on wireless video surveillance systems. Mesh devices are completely auto-configurable and can decide how to transmit packets based on the present conditions of the channels rather than on predetermined configuration values.
In a mesh network, line-of-sight is not required from a camera directly to the base station, just to the next node in the mesh network. Each node in turn relays the stream until the packet reaches the control room. Traditional line-of-sight, point-topoint systems must be installed on high towers or poles to avoid obstacles like buildings or hills. With a mesh-based approach, a series of line-of-sight nodes can be used to route traffic around obstacles.
Reliability and flexibility are intrinsic key benefits of mesh architecture. Unpredictable sources of interference do not need to be accounted for in system design. A mesh network will identify the position of the source of interference and start routing packets around low-quality links.
Airports and industrial harbors are typical locations where mesh architecture solves several problems at once. Airplanes taxiing on the ground are often unpredictable and moveable sources of interference. During the design and the installation of a mesh-based video surveillance system, it won’t be necessary to identify every possible position where an airplane can transmit interference. The mesh network itself will find alternative paths in case some of the links are temporarily unusable due to interference. Similarly, using traditional wireless architectures in commercial seaports also is challenging because loaded container ships can be as tall as some buildings. However, using a mesh-based architecture, video streams will change their path if a docked container ship is impeding transmission. The transmitter will always have an alternative to reach the base station.
Temporary Surveillance
The advantages in terms of flexibility and ease of installation of mesh systems create the opportunity for surveillance applications requiring mobility. Mesh networks fit the needs of temporary or nomadic video surveillance. A new node can be easily added to the network without changing the configuration or the settings of the rest of the system. The routing protocol detects the new node automatically without human intervention.
Mesh-based video surveillance enables several innovative applications for video control. Police departments can easily move or add cameras for sporting events and global summit meetings or large demonstrations and strikes. Mesh units can be installed on police cars or vans, creating mobile cameras for special events. Temporary networks can be quickly installed in case of emergency or natural disaster, creating a moveable and easily deployable wireless infrastructure to transmit data, video and voice.
Moreover, the large use of omnidirectional antennas in mesh networks allows quick installations with no need for timeconsuming antenna alignments. Units can be easily moved without the intervention of any specialist because the network will automatically reconfigure itself.
Monitoring Como
Many of these advantages came into play in the municipality of Como, Italy, which recently installed one of the largest meshbased video surveillance systems in Europe. The municipality had a special requirement that made any other wireless technology impossible to use: city officials wanted cameras and wireless transmitters to be easily moveable and repositionable within the city center to meet changing needs in public safety and security.
The historic city center in Como has old and narrow streets crossing each other. The focus of the project was to install cameras in two areas of the center—the lake shore and the two most important and luxurious streets in Como. The system, however, had to be moveable, and the cameras had to be easily repositionable to nearby streets in case the local police need to control particular areas.
Mesh architecture solved each challenge faced by the municipality. The mesh units can be installed on light poles at the same height of the cameras because they do not need to be in line-ofsight with the final node mounted on top of the control room. Each node requires line-of-sight only with the following one and not directly with the control room. The street grid itself can serve as a wireless plan as channels can be directed down streets and, using repeaters, around corners (Figure 3).
The network is entirely self-forming and self-healing. Cameras and transmitting units can be quickly moved and repositioned by the municipality staff with no need for radio engineering expertise. Because mesh units mount small omnidirectional antennas rather than large directional grids, the system is visually and aesthetically discreet. Usually, the transmitter goes completely unnoticed.
Mesh topology is a step forward in wireless networking applied to video surveillance applications. Reliability and flexibility unmatched by any other wireless solution are intrinsic in this innovative architecture, where every node in the network is an intelligent router able to forward packets received from nearby units, deciding in real time the optimal path based on the channel and network conditions.
Mesh networks also enable the installation of wireless video surveillance systems where there are permanent or temporary obstacles or regular sources of unpredictable interference. Mesh networks are allowing the design and installation of video systems with the reliability of a wired network but with flexibility superior to any point-to-point or point-to-multipoint wireless system.
This article originally appeared in the February 2009 issue of Network-Centric Security.