Backhaul networks serve to efficiently transport large amounts of data across and between communication systems. The data carried in a backhaul network typically originates from (or is destined to) multiple sources in one geographic area and is destined for (or originates from) multiple other sources located in other geographic areas. As such, a backhaul network must provide aggregation points for data in each geographic area and efficiently transport aggregations of data between different geographic areas.
Knowledge about the ultimate destinations (e.g. particular end-user devices, web-servers, etc.) of data is not required since backhaul networks do not provide direct access to individual end-users. An aggregation point, provided by backhaul network, is typically co-located with an end-user distribution and access system (e.g. a cellular base-station) that provides access to end-users. The distribution and access system is responsible for parsing aggregations of data and delivering respective portions of the aggregate data to respective end-users.
Backhaul networks have been implemented using a number of technologies, such as wireline, optical links, Point-To-Point (PTP) microwave links, and Point-to-Multi-Point (PMP) broadcast radios. Each of these technologies has a number of flaws that make deployment restrictive and expensive in dense urban areas.
Wireline and optical links require the expensive installation of a physical medium (e.g. twisted pair copper lines, optical fibers, etc.) to carry signals between backhaul network nodes. In dense urban areas, where real estate is relatively expensive and the disruption of daily life is unwelcome, the placement of supporting equipment and the physical medium is a problem that network planners and operators would like to avoid. Wireline and optical links become even less cost-effective for the amount of capacity provided beyond 3 to 4 T1's of capacity.
PTP microwave links appear to be less expensive than wireline and optical links. However, PTP microwave based systems are more expensive to integrate into wireless access systems (e.g. cellular wireless networks). PTP microwave based systems also require absolute Line-Of-Sight (LOS) between any two nodes intended to have a communication link and a dedicated frequency channel for each such link. The absolute LOS requirement is an impractical requirement in dense urban areas. The need for a dedicated frequency channel for each communication link introduces a number of problems discussed below.
When employing PTP microwave links in a backhaul network of any size frequency channel planning/assignment is essential to ensure frequency channel overlap and the communication collisions this would cause do not occur. This in turn leads to a duplication of equipment in each backhaul network node, in that each backhaul node must have a unique radio unit for each communication link it is intended to support. Consequently, PTP microwave based systems are difficult to expand because frequency assignments must be adjusted and new radio equipment added to existing backhaul network nodes every time there is a requirement to increase capacity.
Point-to-Multi-Point (PMP) broadcast radios, such as PMP LMDS (Local Multipoint Distribution Service), are typically only useful in a high-density subset of deployments that satisfy strict LOS requirements. In fact, employing PMP broadcast radios actually enforces a less flexible network topology than the use of PTP links in that PMP broadcast radios do not easily lend themselves to circuit emulation and typically do not provide very high capacity.