Choices in Wireless Topologies

Three options are popular today for design of wireless networks - Point-to-Point, Point-to-Multipoint, and Mesh topologies.

Point-to-Point
As the name implies, a point-to-point wireless network is a direct link between two distinct locations. In the diagram, point-to-point connections are represented by the red lines. These connections are commonly used in cellular backhaul (from the BTS site towards the network operations center) and for building-to-building extensions of IP and circuit-switched services (i.e. analog PBX). Fiber optics and leased copper connections are examples of "wired" point-to-point networks.

Advantages of point-to-point networks include a dedicated link to a specific location and offering higher data capacities than on a shared topology like multipoint or mesh. Point-to-point networks typically include a matched pair of radios, which can be installed professionally or by a seasoned IT technician. For cellular backhaul, microwave point-to-point radios have been called into action during several natural disasters, such as wildfires, where cabled copper and fiber optics are inoperable. Most cellular operators have vehicle-mounted base stations, called a COW (cellular on wheels), which provide incremental or recovery network coverage and utilize a wireless point-to-point connection from the COW back to the network operations center.

Point-to-Multipoint

WiMAX is most often associated with the point-to-multipoint topology. Multipoint networks can be thought of as a hub and spoke combination. Multipoint networks use a base station as the "hub" and subscriber units as the connection points at the other end of the spokes connecting to the base station. In the diagram, a multipoint base station would be the red circles and the subscriber units are the black circles attached via black lines. Cellular phone networks are another example of a multipoint network, where the subscriber stations are the mobile phones.

A multipoint network can address broadband access requirements to more users than can be afforded with a point-to-point solution. In its simplest form, one base station and one subscriber would literally be a point-to-point network, although not a cost-effective one. Broadband capacities are shared amongst all of the subscribers connected to that base station.

Mesh Networks
A more recent wireless network topology that has been implemented for municipal Wi-Fi networks is mesh networking. Mesh offers a combination of point-to-point and point-to-multipoint capability by having each of the subscriber nodes able to communicate directly with other nodes. This obviates the base station element in the network, though one or more of the nodes are designated "off ramps" to connect the mesh with the Internet.

The most successful implementations of mesh have been when a single entity has access to the nodes (i.e. an operator in a municipal network). Early mesh vendors discovered that providing broadband wireless access over mesh nodes were challenging because one user taking down his node would impact the performance and operation of other nodes relying on that device for connectivity. Opponents to mesh topologies point out that latency (in the form of signal delay) can become pronounced over multiple hops to an "offramp", making video and audio streaming application performance unreliable.

WiMAX trivia: the original 802.16 specification incorporates hooks for mesh topologies, which were submitted by Nokia. It was envisioned that equipment could be developed to support either multipoint or mesh topologies. After Nokia decided to cancel its RoofTop mesh program and leave WiMAX Forum (they re-joined about a year later with renewed interest), no further work was done on the extensions for mesh. A variation of mesh, called MultiHop Relays, are being designed within the IEEE's 802.16j working group currently.

By Jeff Orr, ORR Technolgy

Thursday, January 17, 2008  | Permalink |  Comments (3)