Title of Invention: MULTI-HOP WIRELESS BACKHAUL NETWORK AND METHOD Abstract: Some embodiments of the invention provide an implementation for a multi-hop wireless backhaul network. These embodiments can advantageously be deployed in dense urban areas and/or co-located with wireless access nodes, such as base-stations of a cellular wireless communication system. Preferably wireless links between constituent network nodes are set-up hierarchically. A basic result of this is that peer-to-peer (child-to-child) communication is generally prohibited and circuits are forced to conform to a topology. The multi-hop wireless backhaul network may be used to carry delay sensitive, high-density last mile circuit traffic over Non-Line-Of-Sight (NLOS) broadband radio links. Moreover, some embodiments of the invention provide a method of path-healing for re-routing of circuit traffic from circuits that have experienced catastrophic failures. Detailed Description: RELATED APPLICATIONS This application is a continuation of U.S. application Ser. No. 12/624,964, filed on Nov. 24, 2009, which is a divisional application of U.S. application Ser. No. 10/747,942 filed on Dec. 31, 2003 and claims the benefit thereof. FIELD OF THE INVENTION This invention relates generally to communication systems and, in particular, to backhaul networks. BACKGROUND 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. SUMMARY OF THE INVENTION According to one broad aspect, the invention provides a multi-hop wireless backhaul network comprising: at least one NAN (network access node); a plurality of BNs (base nodes); a plurality of AGNs (aggregation nodes) each performing a switching function in relaying traffic between at least one of the base nodes and at least one of the network access nodes; wherein a hierarchical topology of active wireless connections is established with the at least one network access node at the top of the topology, and the base nodes at the bottom of the topology. In some embodiments, the multi-hop wireless backhaul network in combination with an access network comprising a plurality of access network nodes for which the multi-hop wireless backhaul network is providing backhaul functionality. In some embodiments, at least some of the access network nodes are co-located and connected to or integrated with respective nodes of the multi-hop wireless backhaul network. In some embodiments, the access network is a cellular wireless access network, and each access network node is a base station transceiver. In some embodiments, the multi-hop wireless backhaul network adapts to establish a plurality of virtual circuits through said hierarchical topology, each virtual circuit having an allocated bandwidth for delay-sensitive traffic, at least some of the virtual circuits being multi-hop virtual circuits. In some embodiments, each virtual circuit has a first circuit end-point in one of the at least one NAN, and has a circuit second end-point in either a BN of said plurality of BNs or an AGN of said plurality of AGNs. In some embodiments, each virtual circuit passes through one of said at least one NAN and has a first circuit end-point external to the BNs, AGNs, and at least one NAN, and has a second circuit end-point in either a BN of said plurality of BNs or an AGN of said plurality of AGNs. In some embodiments, the multi-hop wireless backhaul network adapts to establish a plurality of virtual circuits through said hierarchical topology, each virtual circuit having an allocated bandwidth for delay-sensitive traffic, at least some of the virtual circuits being multi-hop virtual circuits, wherein each virtual circuit either has a first circuit end-point in one of the at least one NAN or passes through one of the at least one NAN to an external end-point, and has a second circuit end-point in either a BN of said plurality of BNs or an AGN of said plurality of AGNs, which second circuit end-point is combined with, co-located with or locally connected to an access network node of said access network and provides backhaul capacity for the access network node. In some embodiments, the multi-hop wireless backhaul network further comprises a scheduler which performs a scheduling operation for nodes in the topology and transmits at least one message containing resource assignment information. In some embodiments, the at least one message comprises identifiers of two nodes for an active connection, and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, a multi-hop wireless backhaul network further comprises a scheduler on each node in the network, the schedulers collectively performing a scheduling operation to determine for each connection which two nodes are involved in the connection; and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, at least one BN or AGN is a second end-point for two or more virtual circuits. In some embodiments, at least two or more virtual circuits as established on respective distinct active wireless connections from the BN or AGN. In some embodiments, each virtual circuit is an end-to-end layer 2 circuit emulation, and wherein for each multi-hop virtual circuit. In some embodiments, the multi-hop wireless backhaul network adapts to dynamically allocate bandwidth to each of the active connections. In some embodiments, each node maintains respective topology information identifying at least any active connections the node is participating in or any nodes with which the node has an active connection. In some embodiments, each AGN comprises a plurality of spatially switched antennas, and a transceiver operating in a TDM (time division multiplex) basis on the plurality of spatially switched antennas. In some embodiments, each BN comprises a plurality of spatially switched antennas, and a transceiver operating in a TDM (time division multiplex) basis on the plurality of spatially switched antennas. In some embodiments, each NAN comprises a plurality of antennas in a multi-sector antenna arrangement and a respective transceiver operating on each sector. In some embodiments, each active connection is established between a respective pair of said antennas on two nodes in the network, and wherein each connection is allocated respective active time slots during which the connection is active. In some embodiments, the multi-hop wireless backhaul network further adapts to establish a plurality of alternate connections in addition to the active connections in said hierarchical topology. In some embodiments, the multi-hop wireless backhaul network further adapts to perform automatic path healing upon failure of an active connection or a node in the network, using at least one of the plurality of alternate connections. In some embodiments, at least some of the BNs have respective alternate connections to either an AGN of said plurality of AGNs or to a NAN of said at least one NAN, and wherein at least some of the AGNs have respective alternate connections to either another AGN of said plurality of AGNs or to a NAN of said at least one NAN. In some embodiments, the multi-hop wireless backhaul network adapts to allocate at least a signalling and/or ranging bandwidth for each alternate connection. In some embodiments, the multi-hop wireless backhaul network adapts to detect when a failure has occurred affecting at least one of said active connections, and to maintain communication between the endpoints of the connection by using at least one alternate connection. In some embodiments, the multi-hop wireless backhaul network adapts to perform automatic path healing upon failure by: a node in the hierarchy directly below the failure establishing an active connection over an alternate connection from the node. In some embodiments, for a virtual circuit using the active connection or node which failed, performing automatic path healing comprises: moving the virtual circuit to use at least one alternate connection, and scheduling bandwidth for the virtual circuit along at least each connection to form part of the virtual circuit after moving which was not previously part of the virtual circuit. In some embodiments, the multi-hop wireless backhaul network further comprises a scheduler which performs a scheduling operation on the nodes in the network and transmits one or more resource assignment messages containing resource assignment information. In some embodiments, the resource assignment message comprises identifiers of two nodes for a connection, and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, the scheduler generates new resource assignment information as part of the automatic path healing. In some embodiments, the multi-hop wireless backhaul network further comprises a scheduler on each node in the network, the schedulers collectively performing a scheduling operation to determine for each connection which two nodes are involved in the connection; and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, after a failure, the schedulers collectively perform a new scheduling operation. In some embodiments, each AGN and each BN is adapted to perform a ranging function to identify network nodes with which the AGN/BN can establish a connection. In some embodiments, upon at least one of power up, initialization and command, each AGN and each BN is adapted to perform a ranging function to identify another node with which the AGN/BN can establish a connection. In some embodiments, upon at least one of power up, initialization and command, each AGN and each BN is adapted to perform a ranging function to identify another node with which the AGN/BN can establish an active connection as part of said hierarchical topology, and in an attempt to identify at least one alternate node with which the AGN/BN can establish an alternate connection. In some embodiments, each alternate connection is established between a respective pair of antennas, and wherein each alternate connection is allocated respective signalling time slots during which the alternate connection is available for ranging or signalling. In some embodiments, each NAN dynamically allocates bandwidth to each AGN and/or BN with which the NAN has an active connection, and each AGN allocates bandwidth to each AGN and/or BN with which the AGN has an active connection. In some embodiments, the multi-hop wireless backhaul network further adapts to establish a plurality of alternate connections in addition to the active connections in said hierarchical topology. In some embodiments, the multi-hop wireless backhaul network further adapts to perform automatic path healing upon failure of an active connection or a node in the network, using at least one of the plurality of alternate connections. In some embodiments, the multi-hop wireless backhaul network adapts to dynamically allocate a bandwidth at least for signalling and/or ranging each alternate connection, and to allocate bandwidth for traffic for each alternate connection employed in path healing. In some embodiments, the multi-hop wireless backhaul network in combination with an element management system adapts to provide management functions for the multi-hop wireless backhaul network. In some embodiments, the element management system is connected to the multi-hop wireless backhaul network via another transport network. In some embodiments, the element management system is connected to the multi-hop wireless backhaul network via a metro network. In some embodiments, the multi-hop wireless backhaul network in combination with an element management system adapts to perform management functions for the multi-hop wireless backhaul network connected to the multi-hop wireless backhaul network via a metro network, and further comprising a base station controller co-located with the element management system, the base station controller providing a control operation for the base station transceivers. In some embodiments, the multi-hop wireless backhaul network in combination with an element management system adapts to perform management functions for the multi-hop wireless backhaul network connected to the multi-hop wireless backhaul network via a metro network, and further comprising a base station controller co-located with or locally connected to the element management system, the base station controller providing a control operation for the base station transceivers, wherein each the first end-point of each virtual circuit is in the element management system. In some embodiments, each virtual circuit is an Ethernet virtual circuit. According to another broad aspect, the invention provides a NAN (network access node) for use in multi-hop wireless backhaul network the NAN comprising: an interface to another transport network; for each of at least two sectors, each transceiver and antenna establishing an active connection with another node in the network, a respective antenna and a respective transceiver; the NAN being adapted to act as a wireless end-point for a plurality of virtual circuits through said network using said connections, each virtual circuit having an allocated bandwidth for delay-sensitive traffic, at least some of the virtual circuits being multi-hop virtual circuits. In some embodiments, the NAN further adapts to act as a circuit emulation end-point for the plurality of virtual circuits. In some embodiments, the NAN adapts to communicate on each connection using a respective schedule. In some embodiments, the schedule is in accordance with a received resource assignment comprising, for each connection, and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, each virtual circuit is an end-to-end layer 2 circuit emulation. In some embodiments, the NAN further adapts to establish at least one alternate connection in addition to the active connections. In some embodiments, the NAN adapts to allocate at least a signalling and/or ranging bandwidth for each alternate connection. In some embodiments, for each antenna, time slots are scheduled for and/or ranging functions. In some embodiments, each alternate connection is scheduled respective signalling time slots during which the connection is available for ranging and/or signalling. In some embodiments, each NAN dynamically allocates bandwidth to each active connection. In some embodiments, the NAN further adapts to allocate a bandwidth for each alternate connection for ranging and/or signalling. According to another broad aspect, the invention provides an AGN (aggregation node) comprising: a plurality of spatially switched antennas; a transceiver adapted to operate on the plurality of spatially switched antennas in a TDM fashion to establish active connections with other network nodes in a hierarchical manner; the AGN being adapted to perform a bi-directional relay function for backhaul traffic using at least one virtual circuit having an allocated bandwidth for delay sensitive traffic. In some embodiments, the AGN in combination with an access network node of an access network. In some embodiments, at least one virtual circuit terminates in the AGN for carrying traffic of the access network node. In some embodiments, the AGN adapts to perform layer 2 switching on traffic on the at least one virtual circuit. In some embodiments, the AGN adapts to schedule transmission on the spatially switched antennas in accordance with a resource assignment specifying for each of at least two connections and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, the AGN adapts to dynamically allocate bandwidth to each of the active connections. In some embodiments, the AGN further adapts to establish at least one alternate connection in addition to the active connections. In some embodiments, the AGN further adapts to perform automatic path healing upon failure of an active connection or a node in the network, using the at least one alternate connection. In some embodiments, the AGN further adapts to allocate a bandwidth at least for signalling and/or ranging for each alternate connection. In some embodiments, the AGN further comprises a scheduler performing a scheduling operation to determine for each active connection and each alternate connection and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, the AGN adapts to schedule time slots for ranging and signalling functions. In some embodiments, upon at least one of power up, initialization and command, the AGN is adapted to perform a ranging function to identify another node with which the AGN can establish an active connection, and in an attempt to identify at least one alternate node with which the AGN can establish an alternate connection. According to another broad aspect, the invention provides a BN (base node) for use at a bottom of a multi-hop wireless backhaul network, the BN comprising: a plurality of spatially switched antennas; a transceiver adapted to operate on the plurality of spatially switched antennas in a TDM fashion to establish an active connection with at least one other network node; the BN being adapted to be an end-point for at least one virtual circuit through the backhaul network having an allocated bandwidth for delay sensitive traffic. In some embodiments, the BN in combination with an access network node of an access network, wherein at least one virtual circuit terminates in the BN for carrying traffic of the access network node. In some embodiments, the BN adapts to schedule transmission on the spatially switched antennas in accordance with a resource assignment specifying for each active connection and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, the BN further adapts to establish at least one alternate connection in addition to at least one active connection. In some embodiments, the BN adapts to detect a failure on the at least one active connection, and to attempt to establish a traffic connection on the at least one alternate connection. In some embodiments, the BN further comprises a scheduler performing a scheduling operation to determine for each active connection and for each alternate connection and at least one of: time slots of communications, an encoding format, a signaling format, a modulated format and transmission power. In some embodiments, the BN adapts to schedule time slots for ranging and/or signalling functions to identify and/or maintain nodes with which connections can be established and is adapted to perform ranging and/or signalling during the time slots for ranging and signalling. In some embodiments, upon at least one of power up, initialization and command, the BN is adapted to perform a ranging function to identify another node with which the BN establish an active connection, and in an attempt to identify at least one alternate node with which the BN can establish an alternate connection. In some embodiments, a NAN, an AGN and/or a BN are each adapted to maintain an Ethernet bridging table containing for each of a first at least one address a respective egress wireless link identifier, and for each of a second at least one destination address a respective egress wireline link identifier, and to perform switching of packets using the Ethernet bridging table. Other aspects and features of the present invention will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described in greater detail with reference to the accompanying diagrams, in which: FIG. 1 is a block diagram of an example of a multi-hop wireless backhaul network provided by an embodiment of the invention; FIGS. 2A, 2B, 2C and 2D are topology tables for the entire network, a Base Node (BN), an Aggregation Node (AGN) and an Network Access Node (NAN) shown in FIG. 1, respectively, in accordance with an embodiment of the invention; FIG. 3 is a block diagram of a communication system, including the multi-hop wireless backhaul network of FIG. 1, provided by another embodiment of the invention; FIG. 4 is a block diagram of an example of a multi-hop wireless backhaul network provided by an embodiment of the invention that illustrates examples of spatially switched antenna beam patterns for a number of constituent network nodes; FIG. 5 is an example of a resource (i.e. bandwidth) allocation bar-chart associated with part of the network of FIG. 1; FIG. 6 is a flow chart depicting a method of network topology discovery and maintenance provided by an embodiment of the invention; FIG. 7 is a flow chart depicting a method of network topology discovery and commencement of backhaul link use provided by an embodiment of the invention; FIG. 8 is a timing diagram depicting a method of periodic ranging for a node having active and alternate nodes above it in the hierarchy, provided by an embodiment of the invention; FIG. 9A is a flow chart depicting a method of path-healing provided by an embodiment of the invention; FIG. 9B is a signalling diagram illustrating an example of the method of path-healing depicted in the flow chart of FIG. 9A; FIG. 10 is a block diagram showing an example of how circuit emulation within the multi-hop wireless backhaul network can be performed; FIG. 11 is a diagram of an example physical layer structure based on MIMO OFDMA; and FIG. 12 is a system diagram of an example of spatial antenna processing using MIMO. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Conventional multi-hop wireless networks have typically been created for low-capacity, delay tolerant, short-haul data transport to/from consumer electronics and Local Area Network (LAN) and Wide Area Network (WAN) operations. As such, known multi-hop wireless network standards include a number of characteristics which prevent the use of such multi-hop wireless networks in backhaul network design carrying delay sensitive circuit switched traffic. For example, conventional multi-hop wireless networks are based on data packet routing. Consequently, each node in a multi-hop wireless network is effectively a router that parses each data packet it receives to determine the data packet's ultimate destination and then re-transmits the data packet as required. The routing functionality requirement alone makes each node relatively complex. Each node in a data packet's path delays the arrival time of the data packet to its ultimate destination, since each node parses the data packet before it is re-transmitted. Such delays are tolerated because the data carried on conventional multi-hop wireless networks is expected to be delay tolerant. When considering the applicability to backhaul network design, such delays are not acceptable since circuit traffic is sensitive to timing. Multi-Hop Wireless Backhaul Network A first embodiment of the invention provides a multi-hop wireless backhaul network, which preferably has a high capacity. The multi-hop wireless backhaul network is composed of a number of distinct types of network nodes which are hierarchically connectable. These different types of nodes are used to build emulated virtual circuits through the multi-hop wireless backhaul network, as described in detail below. These types of nodes consist of Network Access Nodes (NANs), Aggregation Nodes (AGNs) and Base Nodes (BNs). In its simplest sense, a NAN is the highest network node in the hierarchy. A NAN has functionality beyond this which is included in some embodiments. For example, in some embodiments, each NAN connects the multi-hop wireless backhaul network to another network, for example a metro network. Each NAN can form wireless connections to multiple BNs and/or AGNs. There may be one or multiple NANs in the multi-hop wireless backhaul network. Because the NAN is at the top of the hierarchy, it is a wireless end-point for any virtual circuits. The NAN may or may not be a circuit end point for a given virtual circuit. More specifically, in some embodiments, each NAN is a circuit emulation endpoint for virtual circuits formed within the multi-hop wireless backhaul network. For example, in embodiments in which the metro network is a circuit switching network, for example a TDM based network, the NAN behaves as a circuit emulation endpoint, and has virtual circuit termination capabilities built in. In some embodiments, a given NAN does not behave as a circuit endpoint for virtual circuits formed within the multi-hop wireless backhaul network, but instead incoming wireless traffic out a wireline port and vice versa on virtual circuits which pass through the NAN. For example, in embodiments in which the metro network is a packet switching network, for example an Ethernet based network, the virtual circuits have one circuit end-point point within the multi-hop wireless backhaul network, and a second circuit end-point outside the multi-hop wireless backhaul network. In its simplest sense, an AGN is an intermediate network node in the hierarchy. One or more AGNs can be interposed between a BN and a NAN. The closer, in terms of hops to a NAN, an AGN is the higher in the hierarchy of the multi-hop wireless backhaul network it is considered to be. The AGNs generally serve to aggregate and relay/amplify circuit traffic between network nodes in the up stream (BN/AGN towards NAN) and down stream (NAN towards BN/AGN) directions. In a system limited to a maximum of two hops, each AGN connects one or more BNs to a NAN. In the event a larger number of hops are permitted, an AGN connects one or more BNs or AGNs below it in the hierarchy to one or more AGNs or NANs above it in the hierarchy. An AGN is not a circuit endpoint for circuit traffic that it is relaying on a circuit between a NAN and a BN/AGN or for traffic that it is aggregating from multiple BNs/AGNs to a NAN. That is, if an AGN is interposed within a circuit between two other nodes (e.g. a NAN and a BN or another AGN), it is preferable that the AGN simply switch the circuit traffic between the end-nodes of the circuit. The AGNs can add (i.e. aggregate) additional traffic in the up-stream direction and drop traffic from in the down-stream direction. AGNs interposed between two network nodes inherently increase circuit lengths, which can be advantageous for emulating long circuits and delivering circuit traffic to between circuit end-points that are far away from one another (e.g. a NAN and a distant BN). In its simplest sense, a BN is a network node that is the lowest network node in the hierarchy. Preferably, a BN serves as a circuit emulation end-point. BNs are typically located at the periphery of the multi-hop wireless backhaul network. In operation, wireless connections between the various network node types are established in a hierarchical manner from each BN to a NAN. This implies that each BN is connected to an AGN or a NAN directly, and each AGN is also ultimately connected to a NAN; possibly through another AGN. As indicated above, in some embodiments, multiple AGN hops are implemented and in such cases an AGN may be connected to another AGN. In typical implementations, the distances between network nodes and available bandwidth will serve to limit the number of traffic carrying links any one network node can establish. For systems featuring multiple AGN hops to connect a BN to a NAN, the AGNs are also hierarchically arranged such that a given AGN is only connectable to AGNs above it and AGNs below it in the hierarchy. There is no ambiguity as to whether an AGN is upstream or downstream of another AGN because the hierarchical connectivity precludes an AGN being both upstream (for one traffic flow) and downstream (for another flow) of another AGN. There are not any BN→BN traffic connections nor are there any NAN→NAN traffic connections. It is noted that in some embodiments, network nodes may communicate using service and maintenance channels to exchange Operation and Management (OAM) information with one another, which is different from backhaul data traffic. The strict hierarchical topology does not necessarily need to be enforced for such communications. Bandwidth is allocated hierarchically with network nodes higher in the hierarchy providing capacity to serve all connections for nodes lower in the hierarchy that depend on network nodes higher in the hierarchy. The NAN downstream bandwidth of each NAN is divided between any AGNs and/or BNs to which it is connected. Multiple different NANs, in some instances, can provide bandwidth to the same set of AGNs and BNs. Similarly, the downstream bandwidth of each AGN is divided between any AGNs and/or BNs to which it is connected. In some embodiments, this bandwidth allocation is done dynamically by each NAN and each AGN. In such an embodiment, each NAN dynamically allocates its downstream bandwidth between its dependent AGNs and/or BNs, and similarly each AGN dynamically allocates its bandwidth between its dependent BNs and/or AGNs. An example of such an implementation is described in further detail below. Each AGN is responsible for dynamic bandwidth partitioning and priority scheduling in order to provide communication links (paths) for network nodes dependent on it (e.g. BNs or other AGNs). AGNs that have multiple dependent network nodes aggregate traffic from these network nodes onto at least one up-stream link. In a preferred embodiment, the entire multi-hop wireless backhaul network is “overlaid” over an access network, for example a cellular wireless access network. In such an embodiment, the multi-hop wireless backhaul network provides backhaul connectivity for the access network. In such an embodiment, each BN is co-located with and connected to or integrated with a respective access network node of the cellular wireless access network, a cellular base-station transceiver for example. Virtual circuits are established from each BN to or through a NAN to transport traffic for the respective access network node(s). Each BN is involved in circuit emulation for traffic entering and leaving the multi-hop wireless backhaul network through the access network node(s). In some embodiments, one or more AGNs may also be co-located or connected to a respective access network node(s) of the access network, such as a cellular base-station transceiver. Such AGNs also serve as circuit end-points. Such AGNs are involved in circuit emulation for traffic entering and leaving the multi-hop wireless backhaul network through the access network nodes. In some embodiments, a particular node may have two or more nodes connected above itself in the hierarchy. In such instances, each of the two or more nodes may each provide a portion of the total bandwidth allocated to the particular node. Each node that provides bandwidth to the particular node is considered an active node. Referring now to FIG. 1, shown is a block diagram of an example of a multi-hop wireless backhaul network, generally indicated by 200, provided by an embodiment of the invention. The multi-hop wireless backhaul network 200 is made-up of a number of Base Nodes (BNs) BN1 101, BN2 103, BN3 105, Aggregation Nodes (AGNs) AGN1 111, AGN2 113 and Network Access Nodes (NANs) NAN1 121, NAN2 123, where “BN1”, “BN2”, etc. are identifiers of the network nodes within the multi-hop wireless backhaul network. Together the BNs, the AGNs and the NANs are the constituent network nodes for multi-hop wireless backhaul network 200. Although a specific number and arrangement of BNs, AGNs and NANs are shown, it is to be understood that more generally any appropriate number of each type of node, appropriately arranged, could be present within a single multi-hop wireless backhaul system. As discussed above, some of the constituent network nodes may be co-located and connect to or integrated with access network nodes. With further reference to FIG. 1, as an example, BN3 105 is shown to be co-located with base-station transceiver BTS2 106. Similarly, AGN2 113 is shown to be co-located with a BTS1 104. It was also discussed above that NANs are used to connect the multi-hop wireless backhaul network to another network or networks. With further reference to FIG. 1, NAN1 121 and NAN2 123 are shown to have core network connections 61 and 63, respectively. These connections in the illustrated example are to a metro network. In a preferred embodiment, the NANs are connected directly or indirectly to network element(s) (such as a BSC/RNC) responsible for managing the underlying access network and/or to network element(s) responsible for managing the multi-hop wireless backhaul network. The constituent network nodes that make-up the multi-hop wireless backhaul network 200 shown in FIG. 1 set up backhaul connections according to the hierarchical topology described previously. The specific traffic carrying links (a link is the same as a connection) shown in FIG. 1 will now be described in detail. Active links are shown in FIG. 1 using solid lines. Circuits are shown in FIG. 1 using bold solid lines. Alternate links are shown in FIG. 1 using dashed lines. The alternate links will be described in detail below together with the path healing embodiment. BN1 101 has an active link 71 to the AGN1 111. BN2 103 has an active link 73 to the AGN2 113. BN3 105 has an active link 79 to the NAN2 123. AGN1 111 has an active link 75 to the NAN1 121. AGN2 113 has an active link 77 to the NAN2 123. The active links 71, 73, 75, 77, 79 are used to carry circuit traffic to and from the respective network nodes they are associated with. One specific example set of alternate links is also shown in FIG. 1. The BN1 101 is shown to have an alternate link 81 to the NAN1 121. The BN2 103 has an alternate link 83 to the AGN1 111. The BN3 105 has an alternate link 89 to the AGN2 113. The AGN1 111 has an alternate link 85 to the NAN2 123. The AGN2 113 has an alternate link 87 to the NAN1 121. The alternate links 81, 83, 85, 87, 89 will be described in detail below together with an embodiment that also includes a method for full/partial path-healing. Using the active links 71, 73, 75, 77, 79 circuits between pairs of constituent network nodes can be emulated. This means that dedicated virtual channels, within the active links 71, 73, 75, 77, 79, are set-up to carry traffic between the circuit end-nodes. For example, as shown in FIG. 1, a first circuit 95 is shown between circuit end-nodes AGN2 113 and NAN2 123. Circuit traffic can be sent up-stream from the AGN2 113 to the NAN2 123, and, down-stream from the NAN2 123 to the AGN2 113 co-located with the BTS1 104. Other network nodes (e.g. BN2 103, BN3 105, AGN1 111, and NAN1 121) that have active or alternate links with either AGN2 113 or NAN2 123 cannot infringe upon the bandwidth allocated for the dedicated virtual channel set up on active link 77 used in the emulation of circuit 95 during normal operation. In another example, a circuit 93 is shown between circuit end-nodes BN1 101 and NAN1 121. In this example, the circuit 93 requires that dedicated virtual channels be set-up on active links 71 and 75, each of which has the AGN1 111 as an end-node. The dedicated virtual channels within the active links 71 and 75 provide enough bandwidth to support the circuit traffic between the BN1 101 and the NAN1 121. With respect to the circuit 93, the AGN1 111 serves to relay/switch circuit traffic carried through the circuit 93. The AGN1 111 is not used in the emulation of the circuit 93, and preferably only acts as a relay/switching node for circuit 93. The AGN1 111 may simultaneously act as and add/drop node for forwarding/adding backhaul traffic upstream originating from a co-located base-station (not shown) and dropping/delivering backhaul traffic to the co-located base-station from the NAN1 121. As will be discussed in more detail further below, in order to enable the path-healing method, a given network node preferably maintains at least separate links above itself in the hierarchy. An active link with at least one of the two nodes hierarchically above is established and maintained. An active link may also be maintained to the second node in which case traffic up the hierarchy from the given node can be divided between the two nodes above. Alternatively, the second node can be an “alternate” node meaning that in nominal conditions, traffic is not sent to the second node. Connectivity with the alternate node is maintained on a “keep alive” basis to enable rapid re-configuration if the active node fails. The path healing method will be discussed in detail below. Topology Management In a preferred embodiment, at least some of the management of the interconnection between network nodes in a multi-hop wireless backhaul network is distributed to the network nodes themselves. This will be referred to as Topology Management or TM functionality that is provided on each NAN, AGN and BN. The TM functionality may be provided in the form of a suitable combination of at least one of software, hardware and firmware residing in each network node. In some embodiments topology information is maintained at each constituent network node, for example by the above described topology management functionality. In some embodiments, each network node keeps a record of the entire multi-hop backhaul network's connectivity, but at a minimum, the topology information contains an up-to-date record of a network node's local connectivity. This hierarchical topology information may be maintained in any suitable form by each network node. In an example implementation, the hierarchical topology information is stored in a topology table that maintains a hierarchical view of other nearby network nodes. Shown in FIG. 2A is a topology table containing an entire hierarchical topology view for the active links shown in the example provided in FIG. 1 that could be maintained at a Network Topology Manager. In some embodiments this type of hierarchical system view is also maintained within each network node. Alternatively, the minimum required by each node may be maintained as shown in FIGS. 2B, 2C and 2D which are example minimum active link topology information 40, 42, 44 for the BN1 101, the AGN1 111 and the NAN1 121, respectively, which are provided and maintained within the BN1 101, the AGN1 111 and the NAN1 121, respectively. As described above, each topology table maintains a hierarchical view of the multi-hop wireless backhaul network 200 from the perspective of the constituent network node on which the topology table resides. Also shown are alternate link topologies 41, 43, 45 used in embodiments featuring path healing as described in detail below. Element Management System In some embodiments an additional network management function is provided in the form of an Element Management System (EMS). The EMS may be implemented as a hardware/software component on a NAN in the multi-hop wireless backhaul network. Preferably, the EMS resides on a an element external to the BNs, AGNs and NANs of the multi-hop wireless backhaul network, for example on a NAN, a Network Topology Manager or Access Network Controller responsible for managing an underlying access network. In some embodiments, the EMS is maintained in a centralized location, since such a location typically also houses logic (hardware, software, firmware, etc.) for high-level control functions for an entire multi-hop wireless backhaul network. Such a central location is also where backhaul traffic typically terminates into other communication systems, such as voice networks in PSTN systems. In an alternative embodiment, the EMS is located away from a multi-hop wireless backhaul network but connected to the highest levels of the hierarchy in the multi-hop wireless backhaul network. In such an embodiment, the EMS still stores and maintains an up-to-date record of relevant network node connections and circuits in use within the multi-hop wireless backhaul network, similar to the topology table shown in FIG. 2A. The EMS can also be used for path reconfiguration. Path reconfiguration might be performed to deal with long-term load balancing or to replace failed node/links. The EMS is typically also responsible for high-level OA&M in the multi-hop wireless backhaul network, macroscopic radio resource management, interfacing to and configuration with core network nodes (e.g. BSC, RNA, metro connections, etc.), protection configuration, fault and alarm management, performance management and configuration management. Shown in FIG. 3 is an example of where and how the EMS might be located and configured in regard to the multi-hop wireless backhaul network 200 described previously with respect to FIG. 1. The NAN1 121 and the NAN2 123 of the multi-hop wireless backhaul network 200 are connected to a metro operator network 210 which is in turn connected to an access network controller 222 which also contains the EMS 150. The access network controller 222 performs control functions in respect of the underlying access network for which backhaul transport is being provided by the multi-hop wireless backhaul network. If the underlying access network is a cellular access network, the access network controller might be a Base-Station Controller/Radio Network Controller (BNC/RNC) 222, which belongs to a wireless operator 220. In the example of FIG. 3, the control function for the underlying access network is co-located with the high-level management function EMS for the multi-hop wireless backhaul network. In other embodiments, the EMS may be separately located. A communication network to which a multi-hop wireless backhaul network is connected to provides network access to various pre-determined amounts of bandwidth. In the example of FIG. 3, the communication network providing this bandwidth is a metro network 210, but other types of networks may alternatively be employed. In some embodiments the predetermined amounts of bandwidth can range from fractional amounts of a T1 link, to multiple T1 links. In other embodiments the predetermined amounts of bandwidth can provided through at least one of OC-3 and STM1 types of links. The bandwidth is first distributed among the NANs of a multi-hop wireless backhaul network. The NANs in turn dynamically allocate the bandwidth among AGNs and BNs dependent upon them. In the example of FIG. 3, the communications network is a metro network. This might for example implement a traditional circuit switched function, Ethernet private line or Ethernet virtual private line. In a preferred embodiment, Native Ethernet is employed on the multi-hop wireless backhaul network and the communications network to which it is connected. This would allow backhaul traffic to pass through as being natively Ethernet ready. Spatially Switched Antennas In some embodiments, the network nodes like the AGNs and the BNs described above have spatially switched antennas. Each such network node has an antenna arrangement allowing transmission on a plurality of antenna beams that are each oriented in a different direction. Any appropriate multi-beam configuration may be employed. These beams may be adaptively defined, statically defined, they may be statically shaped beams which are individually steerable, or formed by a beam steering array in which a fixed set of beams can be steered collectively to one of several discrete orientations. In a specific embodiment, each network node is implemented with a single switched transceiver design that employs Time Division Multiplexing (TDM) to cycle through a number of spatially switched antennas at one network node. In such embodiments, a single NLOS radio unit is required within each constituent network node, and this single radio unit enables dynamic bandwidth allocation at each constituent network node. The diagram provided in FIG. 4 shows a very specific example of a multi-hop wireless backhaul network, generally indicated by 300, for the purpose of illustrating how spatially switched antenna beam patterns can be employed in each of the constituent network nodes. The multi-hop wireless backhaul network 300 is made up of a NAN3 129, an AGN3 119, and BN4 107 and BN5 109. Again, although a specific number and arrangement of BNs, AGNs and NANs are shown, it is to be understood that more generally any appropriate number of each type of node, appropriately arranged, could be employed. The NAN3 129 has a fixed connection (not shown) to an external network (not shown) through which network access to the multi-hop wireless backhaul network 300 is provided. The NAN 129 has three antenna beams 301, 303, 305, which are constantly active (in this example), that project away from the NAN 129 in order to transmit and receive signals from the area around the NAN 129. The NAN 129 has multiple radios, for example, three that are connected to antennas that project antenna beams 301, 303, 305 on carrier frequency f1. The antenna beams 301, 303, 305 are appropriately chosen and configured to minimize interference between signals transmitted/received on each with respect to one another. The AGN3 119, the BN4 107 and BN5 109 each project six spatially switched antenna beam patterns away from their location using a single radio (per node). As for the NAN3 129, the AGN3 119, the BN4 107 and the BN5 109 also transmit and receive on the carrier frequency f1 in a TDM manner. Thus, only one of the six spatially switched antenna beam patterns for each of the AGN3 119, the BN4 107 and BN5 109 is active at one time. The BN5 109 is shown to have an active spatially switched antenna beam (lobe) 311 that overlaps the antenna beam 301 of the NAN3 129. When spatially switched antenna beams 311 and antenna beam 301 are both simultaneously active, backhaul link 51 is active, which means that backhaul traffic can flow between the BN5 109 and the NAN3 129 in the up and down-stream directions. Similarly, the AGN3 119 is shown to have active spatially switched antenna beams (lobes) 313 and 315. It is to be understood that if TDM is employed, the spatially switched antenna beams 313 and 315 would not be active simultaneously. The AGN3 119 uses spatially switched antenna beam 313 to establish backhaul link 53 to the NAN3 129 when both spatially switched antenna beam 313 and antenna beam 305 are active simultaneously. As for backhaul link 51, backhaul link 53 is used to transmit and receive backhaul traffic to and from the NAN3 129. The BN4 107 is shown to have an active spatially switched antenna beam 317. The BN4 107 is also shown to be outside the antenna beam patterns 301, 303, 305 of the NAN3 129. As such, the BN4 107 uses the AGN3 119 as a relay node to connect to the NAN3 129. When spatially switched antenna beam patterns 315 and 317 are simultaneously active, backhaul link 55 is established between the AGN3 119 and the BN4 107. For the example shown in FIG. 4, each of the network nodes (i.e. the AGN3 119, the BN4 107 and the BN5 109) synchronize the manner in which they cycle through their respective spatially switched antenna beams. This synchronization is done so that the backhaul circuit links 51, 53 and 55 can effectively and efficiently be used to transport backhaul traffic around multi-hop wireless backhaul network 300. Circuit Emulation In some embodiments, each circuit emulated within the multi-hop wireless backhaul network terminates at a NAN.
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