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Wireless mesh Networking Architectures

figureStrengths. The beauty of the basic nonmesh point-to-multipoint network is simplicity. Communication, unless traffic is very heavy, is relatively deterministic because there are no hops and minimal or managed collisions. It also allows for maximum throughput because there is no added routing and no added route discovery. Finally, it is easy to understand and easy to manage. Because of this simplicity, it also tends to be the lowest cost for its specific size and function.

Limitations. Unfortunately, the simplicity also causes a number of limitations. The networks will tend to be small—large networks only work if polled from a central point and this requires very specific message management. There are also single points of failure and no ways to route around changing conditions. The network follows the belief that if it worked the first time, it will always work. Therefore, you must be sure of good RF conditions.

ZigBee PRO (ZigBee 2007)
Key Characteristics. ZigBee is built on top of 802.15.4 using DSSS in the 2.4 GHz band. End points sleep, routers don't sleep, and a coordinator is needed to start the network and to allow points to join the network. ZigBee has had three different versions of the standard—2004, 2006, and ZigBee PRO (ZigBee 2007). ZigBee 2004 is no longer used and ZigBee 2006 had significant limitations. ZigBee PRO includes key features for frequency agility, message fragmentation, and enhanced security associated with key management. The routing of messages follows the previously described cluster-tree methodology where routes to all points are maintained at each cluster. This allows a very short routing time, but requires lots of routes. Discovery of routes uses the AODV algorithm where paths are explored between clusters.

figureNetwork Architecture. The network (Figure 5) consists of three specific types of points. A ZigBee coordinator (ZC) is required for each network and initiates network formation; the coordinator may act as a router once the network is formed. The ZigBee router (ZR) is actually an optional network component, although a network without routers becomes a point-to-multipoint network; the router participates in multihop routing of messages. Finally, the ZigBee end device (ZED) does not allow association and does not participate in routing. As such it is often referred to as a child because it doesn't really have any responsibilities.

Figure 5. ZigBee network architecture

Strengths. End devices are very low power because they are subservient to parental routers. Cluster-tree routing provides quick knowledge of routes and thereby efficient routing.

With ZigBee PRO, frequency agility switches away from problem channels automatically in a sort of on-demand frequency hopping. Message fragmentation support allows for long messages and security is flexible with support for separated keys. Finally, the network can scale to be very large.

figureLimitations. The biggest limitation tends to be in terms of power in the routers. Routers must be powered; they can never go to sleep. In addition, cluster-tree routing means that network changes require a lot of route discovery traffic and heavy traffic volume means lots of collisions and potential message loss. Finally, a coordinator is needed to start and manage the network, so if the coordinator goes down, no one can join and the network can't start.

Wireless HART
Key Characteristics. Gaining in popularity, Wireless HART uses the time synchronized mesh protocol (TSMP) created by Dust Networks. Unlike other networks, the time-based system uses TDMA (time slots) for an access method. The network is optimized for low power, all nodes can be sleeping routers, and every node is a router. A gateway is required to keep the network synchronized due to the critical time synchronization of sleeping and waking functions. Like ZigBee, Wireless HART is built on top of 802.15.4 DSSS, but it adds a more deliberate frequency-hopping algorithm. Security includes encryption and authentication.

Network Architecture. Figure 6 illustrates a typical network topology. Note that all the nodes are routers. The illustrated routes change dynamically based on visibility within specific time slots as the message hops through the different DSSS channels. The relationship between any two nodes is negotiated to be in a specific time slot, thereby minimizing the probability of any collisions. When sleeping, nodes awaken during their time slot and listen to see if there is any traffic. Clocks are kept synchronized by the gateway.

Figure 6. WirelessHART network architecture

Strengths. Every node is a router at very low power consumption and most of its time is spent listening. Since transmissions occur only within the allocated time slot, retransmissions are minimized. Communications are very reliable, with every message acknowledged. Networks are able to scale to a moderate level or around 1000 nodes. Frequency hopping minimizes the probability of interference and security includes encryption and appropriate authentication.

Limitations. Because of the time slot approach, latency is long and nondeterministic. It takes a while for a network to form and for all of the nodes to negotiate their individual time slots. Because communications are slotted, the available 802.15.4 bandwidth is split up, meaning that throughput is minimized for bursty traffic. A powered gateway (coordinator) is required for the network to stay functioning, opening up a single point of failure if the gateway is unavailable for an extended period of time. Finally, the radios are very expensive compared to the other available solutions.