Mesh technology boosts wireless performance

As the popularity of wireless networking grows, users are demanding higher bandwidth, greater coverage, and improved reliability. The standard point-to-point or point-to-multipoint technologies, such as 802.11 and Bluetooth, are short range, line-of-sight, wireless networks with some inherent limitations. The bandwidth of these networks decreases as additional nodes join the network and share the access-point data stream. Bandwidth also suffers as the range increases, due to link noise and transmission errors. Most standard wireless networks have "dead spots" within their effective range that exhibit low signal quality due to intervening objects or low signal strength. Depending on its location, a single failure may easily bring down an entire network. Designers are dusting off mesh-networking technology to overcome many of these shortcomings and at the same time extend wireless coverage over a neighborhood or an entire metropolitan area.

Wireless-mesh networks are low-power, multihop systems that process messages by passing packets from node to node until they reach their destination. Unlike a point-to-point network node that filters out all packets except its own, a mesh-network node receives and retransmits packets addressed to other nodes. A multihop network operates much like the Internet and provides redundant communications paths from source to destination. If a path stops working due to hardware failure or interference, a mesh network automatically reroutes packets through an alternative path. Figure 1a is a schematic representation of a traditional point-to-multipoint network with a single base station or access point communicating with many user nodes. Figure 1b shows a mesh network with multiple connection paths between any two nodes.

One of the major advantages of mesh technology over traditional point-to-point networks is the drastic reduction in power required at each node. A multihop-network node requires only enough transmitted power to reach neighboring nodes; single-hop nodes must assume the maximum specified range. In some applications, such as distributed sensor networks, individual nodes may be spaced quite closely, resulting in extremely low transmitter power and long battery life.

Low-power nodes also effectively increase the aggregate data capacity and bandwidth of the network. When two devices try to transmit simultaneously in a single-hop network, the contention protocol shares the data capacity, thus giving each device a portion of the available bandwidth. Because individual node power is low in a mesh network, devices at different parts of the network may actually simultaneously transmit on the same frequency without interference. This reuse of the spectrum improves the spatial capacity of the network.

The shorter distance between nodes also yields a higher network bandwidth. In any fixed-power-level radio-frequency transmission, reception errors due to noise increase as the distance between transmitter and receiver increases. Most networking protocols use variable error-correcting schemes that sacrifice bandwidth to continue operating with high noise levels. Mesh networks eliminate this problem by transmitting over many short, noise-free datapaths instead of one large, potentially noisy hop.

Another big benefit of mesh networks is the redundant datapaths that yield improved reliability and higher data rates than single-hop networks. Temporary local interference, such as another radio signal or an attenuating object, may block or reduce the data rate in a traditional network; mesh technology simply routes the data through an alternative, unaffected path. Likewise, a hardware failure in any one of the mesh nodes will not completely disable a mesh network. The redundancy of mesh technology also boosts the effective bandwidth, because multiple data streams may be transferred simultaneously. For example, a home-networking environment may transfer a video-data stream from the living-room DVD player to a bedroom television while audio data streams from the home computer to a portable, poolside MP3 player.

Notwithstanding the benefits of mesh technology, network experts have concerns about the security issues in a multihop environment. For example, in a wireless-mesh network of computer systems, secure data may pass through many unauthorized nodes before reaching its destination. This data exposure may require sophisticated encryption or node authentication to prevent disclosure. A mesh network may also present opportunities for hackers to substitute erroneous routing information or to introduce unauthorized traffic. These security issues become more apparent when you use mesh technology to span a wide area, such as a campus or community.

Potential mesh-network users and developers have a range of hardware and software requirements to define and implement before deploying a new multihop system. For example, mesh node transceivers should be very low power to allow reuse of spectrum in other parts of the network. Although some interesting work is under way in low-power nodes, most developers choose to start with existing, widely available hardware, such as 802.11, and create the software protocol to enable a multihop system.

Mesh networks represent low-cost, high-bandwidth technology for providing the "last-mile" or neighborhood-level communications infrastructure. Mesh-Networks, for example, uses multihop routing between nodes installed on light poles, buildings, vehicles, and end-user devices to provide community-wide Internet access for subscribers. Its mesh-enabled architecture supports fixed and mobile broadband connections by combining an ad hoc peer-to-peer routing technology with a proprietary QDMA (quadrature-division multiple-access) radio protocol. QDMA radio technology uses direct-sequence spread spectrum and operates in the 2.4-GHz ISM (industrial, scientific, and medical) band. MeshNetworks' software discovers and shares routing information and reconfigures datapaths when you add a device to or subtract one from the network. MeshNetworks offers its technology to OEMs in the form of an ASIC chip with built-in routing technology, QOS (quality-of-service) management, and precision position location. In addition to silicon, MeshNetworks offers a Wireless Modem Card in a PCMCIA form factor and fixed location routers. The MWR-6300 is a light-pole-mounted router used to guarantee wireless coverage in large geographic areas, campuses, or in-building applications while the client population is growing (Figure 2).

LocustWorld offers a free software package to enable mesh networks that cover a neighborhood or community. Its MeshAP is an open-source-software project that runs on the Linux operating system and turns any x86-compatible PC into a mesh-network client, repeater, gateway, and access point. The LocustWorld MeshAP provides remote management, security, and access algorithms to support the mesh-routing functions. As adjacent nodes establish communication, they verify the identity of the other node through a signed digital certificate. Then, they set up a 2048-bit key-encrypted data stream. The software includes a range of functions that let operators decide who they are going to let onto the network. Authorized users could be subscribers who pay for a permanent service or casual users who log in with a ticket that gives a fixed amount of online time. The program also maintains a bandwidth profile for each user to allocate a data quota and priority on the network. As users consume Internet bandwidth, they use up their quota, and, as they consume their quota, their priority falls. In this way, fresh users take priority over those that have been hogging the service, and an equitable share is made of the available bandwidth resources. Once a user exceeds his or her quota, the user is not barred, but he or she can no longer get priority access to the service. To simplify mesh-network setup, LocustWorld also offers consulting services and hardware, such as the $400 MeshBox, a dedicated router device preloaded with the MeshAP software (Figure 3).

Tropos Networks offers a broadband cellular Wi-Fi (Wireless Fidelity) mesh-networking system that enables metro-scale, broadband, wireless data coverage for carriers, network operators, and service providers. Its wireless-mesh system allows residents and tourists to access the Internet from anywhere in the service region, as long as they have a client device equipped with a standard Wi-Fi network connection. A network operating system embedded in each Wi-Fi cell provides the key intelligence. It enables the self-organizing and -healing of mesh technology. As providers add new cells, autodiscovery and optimal-path selection seamlessly integrate them into the network. Service providers can expand aggregate capacity by simply connecting more wired backhaul links anywhere in the network. Tropos Networks provides both indoor and outdoor Wi-Fi cell devices along with the operating system and network-management software (Figure 4). The cellular Wi-Fi system, service, and support options are available now and typically cost $20,000 to $50,000 per square mile, depending on the geography and RF environment.

Some of the original mesh-network research was funded by and in support of military applications. For example, mobile battlefield networks require high data rates, low probability of detection, and resistance to jamming. Mitre Corp has been working with mobile mesh, an open source, ad hoc network system that allows users to exchange information in a wireless environment without the need for a fixed infrastructure. Each user is free to move about while communicating with others. The communications path between any two users can traverse multiple wireless links, and the radios can be heterogeneous, allowing different types of links to be part of the same ad hoc network. Because nodes are mobile, the topology is dynamic, and the network must constantly scan for links and build communication paths. Within the ad hoc network, each node acts as a router and forwards packets on behalf of others. Mitre hosts a Web page from which you can download the mobile mesh software at www.mitre.org/work/tech_transfer/mobilemesh/index.html.

Mesh networks may get a long-term boost from a new RF initiative at Intel. "Radio Free Intel" is the company's vision of integrating low-cost radio capability into every silicon product it makes. These radios will form the technological foundation for the convergence of communications and computing. The company's vision is to extend communications capability from today's PDAs, notebooks, and cell phones to consumer electronics, appliances, furniture, and even clothing. Intel has also teamed with the University of California—Berkeley for research into a specialized type of mesh network. Early work at the Intel Research Berkeley Lab has produced small sensor nodes that form self-configuring, low-cost adaptive networks. The company has combined silicon advances with communications-network research to allow thousands of tiny, embedded sensing devices, called motes, to self-assemble and wirelessly connect while using very little power.

Expanding on the motes theme, Crossbow Technology offers a family of sensors and tools that enables rapid development of wireless-sensor networks for monitoring and detecting a variety of targets, such as enemy personnel or chemical threats. Crossbow Technology is producing the Mica family in collaboration with researchers at the University of California—Berkeley's computer-science department. The Mica architecture includes sensor modules and wireless-communication processor modules. Mica can detect ultrasmall vibrations, acoustic noise, and magnetic disturbances, as well as conventional light, temperature, and proximity. It also includes a sensor-interface port that enables incorporation of chemical, biological, and other specialized sensors. The processor modules run a very small operating system, called TinyOS, and communicate bidirectionally with other Mica sensor nodes or a radio base station. Crossbow has developed several generations of the Mica family, including the coin-sized MICA2DOT (Figure 5). Surveillance systems are built by combining modules that cost $150 to $300, depending on sensor type and volume.

Similarly, Ember Corp provides radio chips, mesh-networking software, and support tools to OEMs and system integrators for sensing and control applications. The EmberNet Protocol Stack includes self-healing mesh-routing and discovery software, along with publishing and subscription services. The stack requires as little as 20 kbytes of program memory and 4 kbytes of RAM, including packet buffers. Using the EmberNet API, developers can manage encryption keys and modes and turn encryption on and off. For customers concerned about interference in harsh radio-frequency environments, Ember offers several radio options, including spread-spectrum modules that operate in the license-free ISM frequency bands. Ember also provides an evaluation kit to demonstrate the performance of the radio chips and the EmberNet embedded networking software. The kit includes sample hardware, applications, and a PC-based evaluation platform. The $4950 EM1020 evaluation kit includes 12 plug-and-play, battery-operated mesh nodes, network-management software, and sample applications.

Mesh technology offers the network user extended coverage, higher throughput, and better failure recovery than do traditional, fixed-infrastructure, wireless networks. Although passing secure traffic through neighboring nodes is a potential threat, sophisticated encryption techniques should provide the necessary security. Mesh networking is simply a miniature version of the Internet, with all of its benefits and dangers, and you can expect to see steady growth in this multihop technology.

Author Information

You can reach Technical Editor Warren Webb at 1-858-513-3713, fax 1-858-486-3646, e-mail wwebb@edn.com.