1. Field of the Invention
The present invention relates to wireless mesh networks and in particular mesh networks used for mobile applications where continuity of operation is critical. In particular, the invention relates to clusters of mesh nodes that may become isolated from a wired or grounded network, but where communication remains possible within the isolated cluster, resolving issues pertaining to mesh topology, channel management and address management.
2. Background of the Invention
The instant invention relates to wireless networks comprising a set of wireless access points, commonly referred to as a mesh network.
In mesh architectures, mesh nodes act as central structural elements providing means of connectivity to a broad range of client devices. In some embodiments of the invention, mesh nodes are wireless access points. Each client device communicates with an available mesh node or access point. The communication between client devices and access points may occur using any means.
A “mesh” forms when a set of access points establishes communication with one another. The communication between the access points forms the strands of an ethereal mesh. Client devices sit in the spaces between the strands and establish communication with the access points which are found at the intersections of the mesh strands.
While wireless mesh networks provide additional functionality not available using other network topologies, certain difficulties are created by wireless mesh networks described in the prior art.
A serious problem for mesh networks is created by the wireless medium used for communication within the mesh. Radio is a shared medium where only one transmission may occur on each frequency at a time. The RF spectrum is divided into frequency ranges, or channels, to allow more concurrent transmissions. However, the channels cannot be made too small so as to interfere with one another. Consequently, useable communication frequencies are generally divided into no more than a dozen channels, not all of which may be used concurrently.
In simplest mesh networks, known as ad-hoc mesh networks, all communication occurs on a single frequency or channel. In such simple networks, each access point (or AP) comprises a single radio and antenna (forming a single node) which provides communication means with client devices and other nearby mesh nodes. The benefit inherent in this mesh is that the mesh coverage may be easily increased by introducing an additional wireless access point. The sole configuration step for a simple ad-hoc mesh is the selection of the communication channel. One drawback of ad-hoc mesh is that each access point and client device contends with other access points to use the single communication frequency. Another drawback is that each access point or node in a simple ad-hoc mesh carries the entire routing table for the mesh network as a whole, and must traverse the table each time a packet arrives in order to know how to process the packet. As a result of both of these characteristics, when the size of the mesh and the distance between access points increases, performance characteristics of the network decrease dramatically.
As networks grow, each access point's basic service set (BSS) increases to the point where it can become unmanageable. The ability to subdivide the network into smaller groups is an approach to prevent scaling problems, and one approach to subdividing the network is to introduce two radios into each access point.
In a prior art dual radio mesh, as is depicted in FIG. 1(A), access points include two radios: a relay radio 120 and a client-service radio 110. The relay radio 120 operates at a first frequency, or channel ChQ 130, to perform duplex (two-way) communication between the access points forming the mesh. Within this prior art mesh, all relay radios operate on the same channel, and each access point contains the routing table for the entire mesh network. Each access point in turn provides access to one or more clients using the client-service radio 110 operating on a second frequency. Essentially, these are one radio ad-hoc mesh nodes with a client-service radio added.
This prior art system, while simple to implement, does not scale beyond a limited number of radios. Rather, throughput drops exponentially as the number of access points increases, especially in instances where downstream access points attempt to connect to an exterior network 108. In essence, the access points in this conventional mesh form the wireless equivalent of a hub. Like hubs, single radio mesh backhauls do not scale as well as multi-radio backhauls in addressing high bandwidth requirements for mission critical mesh networks, especially when the single radio solution carries the entire routing table for the mesh in each node.
In contrast with the hub-like operation of FIG. 1(A) is the wired network switch presented in FIG. 2. As shown in FIG. 2, each network switch includes uplinks 210 and one or more downlinks 220 forming a tree-like structure. The traffic within each switch comprising this conventional wired switch stack operates within specified sub-domains. Therefore, the size of any one sub domain is limited and ensures that local traffic inside a sub domain, or between multiple sub domains does not slow down the entire network. This multi-domain switch architecture is more efficient than hub configurations in that it allows for scalable networking. Contributing to this scalability is the distributed routing table methodology that is typically implemented in a conventional wired network switch. However, drawbacks exist. Namely, the prior-art switch architecture has been implemented using physical wireline connections between discrete switches and not a flexible mesh architecture employing wireless connections.
The present invention is designed to overcome the challenges inherent in the prior art solutions. In one embodiment, the instant invention is based on a two-radio mesh network, where each mesh node includes one radio for the uplink backhaul and another servicing clients and providing the downlink backhaul to other nodes (descendent nodes) of the network. Mesh nodes implemented with such a multi-radio backhaul form a hierarchical tree-like network topology called a “Structured Mesh”, when they connect to each other, and as described in the referenced application Ser. Nos. 11/084,330, 10/434,948, and 12/352,457, operate in a manner similar to a wired switch stack where the routing table is distributed, thus aiding network scalability. A distributed routing table is constructed such that each node only contains information related to its descendant nodes and its parent node, but no other nodes in the network hierarchy tree above its parent. This way, the processing load for each node to process a packet is reduced. Although the highest performance for a “Structured Mesh” network occurs when separate radios are utilized for uplink and downlink connections, performance may also be enhanced when only a single relay radio is used as long as a distributed routing table methodology is utilized thus simplifying the routing computational task and thereby increasing the performance of the processor within the mesh node. For mission critical mesh applications such as military or first responder, it is also advantageous to include both single and multi-radio nodes whereby they can all communicate with a consistent routing protocol.
In some embodiments of the invention, the root of the hierarchical tree structure includes an external network connection to a server, to a WAN (Wide Area Network), to the Internet, or to any combination of these options. When a group of nodes implemented as such become separated or isolated such that the sub-network does not include a root connection as described above, challenges exist in maintaining a the required tree-like structure.
Unlike prior art mesh networks, a need exists in the art for a system capable of maintaining communication within a cluster during physical realignment of cluster components, as occurs during movement of wireless nodes in a mobile mesh network implemented as a tree-like structured mesh network.