Reliable Networking
Wireless mesh — a network architecture for video surveillance systems 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 also forwards 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 the camera and the central access point, mesh networks provide greater reliability because every wireless unit can select from multiple available paths. Because the flexibility and reliability a mesh-capable transmitter offers cannot be rivaled by traditional wireless devices, wireless mesh is quickly becoming the preferred network architecture for high-end wireless video surveillance systems.
The origin of wireless mesh
Wireless mesh networks have been the subject of much academic research in the last two decades. The Defense Advanced Research Projects Agency (DARPA) funded research on mesh networks because of their intrinsic reliability and flexibility and with the idea of applying this type of self-healing network to military applications. Mobile and nomadic communication in the battlefield or in hostile environments was the first application of mesh networks. The absence of any single point of failure makes mesh architecture the best network topology where many communications are possible but when one doesn’t want to rely on layered and stellar architecture. In a mesh network, every node is a peer node, and there is no higher layer or controlling node that can be destroyed in order to bring down the entire network.
Many defense-related applications of mesh networks involve highly mobile and battery-powered units, thus creating challenging technical problems, for which solutions are still in their infancy. However, mesh networks for data transmission, video streaming or temporary communication in case of natural disasters are a reality today, with various products focused on different types of applications.
Self-forming and self-healing mesh networks
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, thereby increasing reliability and flexibility.
Every mesh unit performs an evaluation of the quality of every link in the network in real-time. Consequently, the mesh unit can change path in case it notices a drop in the quality of the currently used link.
Wireless mesh devices are completely auto-configurable. The routing protocol finds other mesh devices nearby and the location of the base units attached to the wired network. In case 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 of packets in a wireless mesh network
Routing in a wireless environment is conceptually similar to routing on the Internet, but it presents some differences in terms of evaluating the quality of a particular path. Internet-routing is based on the layered abstractions developed by the “inventors” of the Internet in the 1980s and 1990s. 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 a 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 (MIT) recently discovered that minimum hop count is not effective for wireless routing. Rather, a metric involving link quality measurement is required to ensure 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, wasting valuable bandwidth. In a mesh network, it is often preferable to take a longer yet more reliable path rather than the unreliable direct route to the endpoint (see diagram, above right). Often the shortest and most direct path would require 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.
Challenges in streaming video over wireless mesh networks
Video streaming and video surveillance have been the latest additions to the list of possible applications for mesh technology. However, high-resolution video streaming over a mesh network presents several technical challenges. One challenge is that bandwidth is scarce when one deals with high-resolution video. In addition, multi-hop transmission may decrease usable bandwidth and increase delay. For example, 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 (see diagram, above, left).
However, advanced mesh products solve these challenges by operating multiple radios at the same time. In this way the video stream can use many different channels to create high-bandwidth and low-delay paths. By changing the transmission channel at every hop, every transmission absorbs bandwidth from a different channel, thereby limiting any loss of performance during the path. Operating multiple radios allows bandwidth to be increased by spreading the transmissions on multiple non-overlapping channels while also decreasing the delay of the network. At every hop that the packets go through, the mesh transmitter can choose the best frequency to reach the following node, thereby optimizing not only the path but the frequency spectrum usage as well. Using multiple channels and designing the network in the proper way are often the best approach to get reliable wireless mesh networks for video surveillance applications.
Routing in a multi-hop, 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.
Advantages of using wireless mesh networks for video 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 the camera directly to the base station but necessary only to the next node in the mesh network that will then relay the stream to the following one and so on until the packet reaches the control room. Using a traditional point-to-point solution, obstacles such as trees, buildings or hills require the installation of high towers or poles in order to create line-of-sight between the antennas. Using a mesh-based approach, video streams do not have to overcome these obstacles — the traffic can simply be routed around them.
Reliability and flexibility are intrinsic benefits of mesh architecture. Sources of interference do not need to be predicted. The 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 movable sources of interference. During the design and the installation of a mesh-based video surveillance system, it is not necessary to identify every possible position where an airplane can interfere. The mesh network itself will find alternative paths in case some of the links are not usable due to interference.
Similarly, using traditional wireless technology in commercial seaports is also very challenging because loaded container ships — comparable to moving buildings — make the design of a traditional static wireless network almost impossible. However, using a mesh-based architecture, video streams will change their path in case a container ship docks in the middle of a wireless link impeding transmission through that particular path. The transmitter will always have an alternative to reach the base station.
Temporary and mobile video surveillance
The flexibility and ease of installation of mesh-based video surveillance systems create the opportunity for new applications for video surveillance when mobility and speed are required during the installation process. A new node can easily be added to the network with no need for changing the configuration or the settings of the rest of the system. The routing protocol detects the new node automatically with no need for human intervention.
Mesh-based video surveillance enables several innovative applications for video control. Police departments can easily move or add new cameras for temporary events such as summit meetings, demonstrations, strikes and sporting events, among others. 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 movable and easily deployable wireless infrastructure to transmit data, video and voice.
Moreover, the large use of omni-directional antennas in mesh networks allows quick installations with no need for time-consuming antenna alignments. Units can be easily moved without the intervention of any specialist because the network will automatically reconfigure itself.
Case study: Como, Italy
The Municipality of Como, Italy, recently ordered and installed a wireless mesh system to meet its need for an easily movable video surveillance system.
The historical city center in Como has old and narrow streets crossing each other perpendicularly. 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 movable, and the cameras had to be easily repositionable to nearby streets in case the local police need to control particular areas.
Mesh architecture easily solves every challenge faced by the municipality using other technologies. 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-of-sight with the final node. The streets can be used as channels where the video streams can flow, and any corner can be overcome by placing a mesh unit acting as a repeater on the corner itself to route packets around.
Because mesh units mount small omni-directional antennas rather than large directional grids, the visual impact of the system is very low, and usually, the transmitter passes completely unnoticed. The network is entirely self-forming and self-healing. Cameras and transmitting units can be quickly moved and repositioned by municipality staff with no need for the intervention of an expert.
Stepping in the right direction
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 the innovative architecture. Every node in the network acts as an intelligent router able to forward packets received from nearby units and decide 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 other wireless solutions can fail, such as those installed in the presence of movable obstacles sources of interference or in any case where frequent repositioning of the cameras is necessary. Mesh networks are allowing the design and installation of video surveillance systems with the reliability of a wired network, but with flexibility higher than any point-to-point or point-to-multipoint wireless system.
Umberto Malesci is president and co-founder of Fluidmesh Networks, Boston. He has extensive experience in academic research in the field of wireless networking, working as a researcher at Massachusetts Institute of Technology’s Computer Science and Artificial Intelligence Laboratory (CSAIL), and at USC’s Integrated Media Systems Center (IMSC) in Los Angeles. Malesci received a Bachelor of Science and a master of engineering in electrical engineering and computer science from MIT in Cambridge, Mass.
