Wide-area wireless broadband systems are typically composed of end user subscriber stations (access devices), base stations (access nodes), packet switches and gateways.
Network Topology
Network topology is the physical and logical relationship between nodes in a network as well as the layout and structure of a network. The WiMAX system can be setup as a point to point (PTP), point to multipoint (PMP) or a mesh network.
Point to Point (PTP)
Point to point communication is the process of transferring information from one device (or point) to one other device (single receiving point). The WiMAX system can use PTP communication for high-speed communication links for backhaul (system interconnection) applications.
Point to Multipoint (PMP)
Point to multipoint communication is the process of transferring information from one device (or point) to multiple points or devices (multiple receiving points). The WiMAX system can use PMP to provide broadband access to multiple users per base station.
Mesh Network
A mesh network is a communication system where each communication device (typically a computer) is interconnected to multiple nodes (connection points) in the network allowing data packets to travel through alternate paths to reach their destination.
Some or all of the resources of a WiMAX system can be configured to provide mesh network services so that the need to interconnect base stations to access points (such as Internet gateways) can be reduced or eliminated. When a WiMAX system is setup as a mesh network, packets can hop across neighboring base stations to reach other points in the network.
WiMAX transceivers that are part of a mesh network are called nodes. Each mesh node is assigned a unique NodeID and each link between nodes is assigned a LinkID. Packets that enter into the mesh network contain their mesh network destination address (NodeID) and their current mesh link address (LinkID). A mesh node receives and forwards packets towards their destination (NodeID). As nodes in the network transfer packets towards their destination, they change the LinkID within the packet to reflect the next link that will be used in the mesh network.
Mesh nodes that can communicate with each other are part of a neighborhood. Neighborhoods can be small with neighbors that are directly adjacent to each other (immediate neighborhood) or they can be part of a larger neighborhood where nodes must communicate through other nodes so packets can reach their destinations (extended neighborhood).
Mesh node operation may be independent (distributed) or controlled ( scheduled) by another network device. When a mesh network is centrally coordinated, one of the nodes is designed as a master synchronization node. The master synchronization node receives requests from mesh nodes, analyzes the bandwidth and transmission requirements and distributes coordinating (scheduling) to other mesh nodes within the network. Mesh nodes that are part of a coordinated mesh system are called a mesh cell.
Mesh nodes regularly broadcast neighbor lists which contain information about available link connections that can be used as part of the mesh network and their associated scheduling times. These lists contain link quality, burst profile, RF power levels and control slot information that will be used to gain access and transmit on the link.
Mesh networks can be setup as either logical or physical connections. A logical mesh network is a system that uses existing WiMAX transceivers to receive and forward packets or data towards its destination. For logical mesh networks, links can be dynamically setup and removed as desired by the mesh network. Physical mesh nodes use transceivers that have their links preestablished so they are not addressable (cannot have their addresses dynamically assigned).
When a WiMAX device wants to attach to a mesh network (gain network entry), it must initially search to see if radio channels are available for a mesh network. If it finds an available radio channel, it needs to communicate with a mesh node that is willing to help it attach to the mesh network (called a sponsor node). The sponsor node will allow it to make a radio connection and negotiate basic communication parameters. It will then relay the request to join the mesh network to the node or part of the network that will authorize and assign resources to the new mesh node (e.g. the master synchronization node).
Figure 1 shows a WiMAX system configured as a mesh network. The base station (BS) devices in a WiMAX system are part of a neighborhood domain and the devices within this neighborhood can be configured as relay point nodes so they can help transfer packets from their neighbors towards their destination. A distant subscriber station that cannot directly communicate with the mesh base station can send packets to a neighbor that can relay the packets to the mesh base station that connects to Internet gateway. This example shows that a mesh network contains links between mesh nodes that are uniquely identified by link identifiers (LinkID) and that the destination point for packets traveling through the mesh network are identified by a node identifier (NodeID). Packets that travel through the mesh network contain both the destination address (NodeID) and the current link address (LinkID). As packets travel through mesh nodes, the link address changes.
A directed mesh network is communication system where each communication device (typically computers) is interconnected to multiple nodes ( connection points) through the use of highly directional connections (e.g. directional antennas). In directed mesh networks, data packets travel through the directed paths to reach their destination. WiMAX systems may use a directed mesh network to interconnect areas to a central facility or gateway (such as a gateway to the Internet).
Figure 2 depicts a WiMAX system that is setup as a directed mesh network. This diagram shows how a WiMAX operator can use directional antennas that interconnect to base stations to allow packets to be relayed from distant locations to a central location (such as an Internet gateway) using WiMAX frequencies. This example shows that the use of directional antennas results in highly focused transmission beams which have little impact on the overall radio coverage of the WiMAX system.
