Abstract:
Scheduling techniques for communication applications using receiver directed broadcast schemes are disclosed. The invention includes techniques for the implicit reservation of receive slots and techniques for the optimization of receiver directed broadcast schedules. The implicit reservation scheme includes the mechanics of implicit reservation. Implicitly reserved slots can be reclaimed by their owners. Further, simultaneous implicit reservations can be identified and resolved. Procedures are also provided for determining the number of channels and slots required to optimize a receiver directed broadcast schedule for a neighborhood of a given number of nodes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Cross-reference is made to related U.S. application Ser. No. 09/340,585, filed Jun. 28, 1999; U.S. application Ser. No. 09/552,144, filed Apr. 19, 2000; and an application entitled “Maintaining an Adaptive Broadcast Channel Using Both Transmitter Directed and Receiver Directed Broadcasts” by C. David Young filed on even date herewith. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to an improved dynamic assignment protocol, more particularly relates to improvements to receiver directed broadcast schemes, and even more particularly relates to implicit reservation and optimized scheduling techniques for receiver directed broadcast schemes. 
     BACKGROUND OF THE INVENTION 
     Mobile multi-hop packet radio networks are known for rapid and convenient deployment, self-organization, mobility, and survivability. Such a network is illustrated in FIG. 1. A transmission from one node, from node N 1  of FIG. 1 for example, can be broadcast to all nodes in its “neighborhood.” Ultra-high frequency (UHF) systems generally have a neighborhood defined by nodes located within line of sight of the transmitting node. The nodes of such a neighborhood are said to be located within one “hop” of the transmitting node. In FIG. 1, for example, nodes N 1 , N 3 , N 4 , N 5 , N 6 , N 7  and N 8  are members of the neighborhood surrounding node N 1 . 
     Nodes N 2 , N 9 , N 10 , N 11  and N 12  are each located two hops away from node N 1  and node N 13  can be said to be three hops away from node N 1 . When data transmitted from node N 1  is to be propagated multiple hops, the data must be relayed by one or more of node N 1 &#39;s neighbors. For example, data transmitted by node N 1  can be relayed by its neighbor node N 8  to a node such as node N 12  that is located two hops from node N 1 . 
     Receivers are generally capable of processing only one transmission at a time. When using such receivers, simultaneous transmissions (also known as collisions, contentions or conflicts) can be avoided by assigning a specific transmission time slot to each communicating node. Several approaches have been developed for assigning slots to nodes. The approach chosen for a particular application is generally a consequence of the type of network application (broadcast, multicast, unicast, datagrams, virtual circuits, etc.) at issue. Since the problem of optimally assigning slots is mathematically intractable, a heuristic approach has been applied. This approach resulted in the development of an integrated protocol that both chooses the number of slots to assign to each neighboring node and coordinates their activation in the network. 
     Many applications require self-organizing, wireless networks that can operate in dynamic environments and provide peer-to-peer, multi-hop, multi-media communications. Key to this technology is the ability of neighboring nodes to transmit without interference. Neighboring nodes transmit without interference by choosing time slots and channels that do not cause collisions at the intended unicast or multicast receivers. 
     The Unifying Slot Assignment Protocol (USAP), which is disclosed in U.S. Pat. No. 5,719,868, provides a protocol establishing such a communication system. USAP is a dynamic assignment protocol that monitors the RF environment and allocates channel resources on demand. It automatically detects and resolves contention between nodes for time slots, such contention arising for example from changes in connectivity. U.S. Pat. No. 5,719,868, issued Feb. 17, 1998, is hereby incorporated herein by reference in its entirety, including all drawings and appendices. 
     USAP permits a node to assign itself transmit slots based on information it has regarding when it is assigned to transmit and receive and when a neighboring node is scheduled to transmit. In one embodiment of USAP, receiver directed broadcast (RDB), a node can assign itself a receive slot instead of a transmit slot. For example, a node may need to assign itself a receive slot wherein it can receive from one, some or even all of its neighbors. This receiver directed type of assignment is disclosed in application Ser. No. 09/552,144, filed Apr. 19, 2000. The specification, including all drawing figures, of application Ser. No. 09/552,144, filed Apr. 19, 2000, is hereby incorporated herein by reference in its entirety. 
     There exists a need for further refinements and improvements to dynamic assignment protocol based systems using RDB. In particular, there exists a need for a technique that can implicitly reserve slots, for use in virtual circuits for example. In explicit reservation techniques, slots are taken from idle transmitters and are explicitly reassigned to nodes needing additional transmit capacity. In various applications, however, the use of explicit reservations can lead to an inefficient use of communication resources. Voice or video transmission applications, for example, can often tolerate a degree of initial packet loss and delay. 
     There is also a need to maximize throughput in systems using RDB schemes. For example, there exists a need for an optimized RDB scheduling technique that is capable, under heavy uniform traffic and for a given number of nodes and channels, of keeping each node busy all of the time. These needs are addressed and fulfilled by the detailed description provided below. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved technique for reserving slots in a receiver directed broadcast environment. 
     It is a feature of the present invention to utilize an implicit reservation of communication slots. 
     It is an advantage of the present invention to bring about a significant reduction of overhead by reducing the size of the control packet. 
     It is another object of the present invention to provide an improved technique for scheduling transmissions in a receiver directed broadcast environment. 
     It is another feature of the present invention to utilize an optimized RDB scheduling technique. 
     It is another advantage of the present invention to enable attainment of maximized throughput by keeping substantially every node busy transmitting or receiving nearly all of the time. 
     Thus, the present invention involves improved receiver directed broadcast systems and methods. The improvements can be carried out in an efficient, “waste-less” manner in the sense that throughput can be maximized or control overhead can be minimized. As noted herein, and as will be appreciated, the invention has several useful and valuable applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the invention, in conjunction with the appended drawings wherein: 
     FIG. 1 is a diagram of a network of nodes, the diagram also illustrating the neighborhood associated with three of the nodes. 
     FIG. 2 is a diagram illustrating a node activation form of communication. 
     FIG. 3 is a diagram illustrating a link activation form of communication. 
     FIG. 4 is a diagram depicting an example of a time division multiple access structure that can be utilized with a dynamic assignment protocol such as USAP. 
     FIG. 5 is a diagram depicting a receiver directed form of communication. 
     FIG. 6 is a flowchart depicting the steps related to an implicit reservation scheme. 
     FIG. 7 depicts a four node neighborhood. 
     FIG. 8 depicts an eight node neighborhood. 
     FIG. 9 depicts a frame suitable for use with the optimized eight node schedule of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     Prior to the development of USAP, a heuristic approach was typically taken to design an application specific protocol that both chose the number of time slots to assign to each neighboring node and coordinated their activation. The incorporated USAP approach separates the slot assignment mechanism from the heuristic and creates a single generalized protocol for reliably choosing the slots and coordinating their activation. A dynamic assignment protocol such as USAP can be used to support higher level heuristics. 
     A node can transmit to its neighbors via essentially one of two different methods. One method, node activation, allows only one active transmitter in a neighborhood in a given time slot. The other method, link activation, can permit more than one simultaneous transmission in the same time slot. 
     In the node activation technique, a single transmitting node communicates data to all of its neighbors simultaneously rather than on an individual basis. Node activation, also known as multicast or broadcast communication, is especially well suited for applications like address resolution and conferencing. The node activation technique is illustrated in FIG.  2 . In FIG. 2, a transmitting node N 15  is simultaneously sending  200 ,  202 ,  204 ,  206 ,  208  the same broadcast communication to each of its neighbor nodes N 14 , N 16 , N 17 , N 18  and N 19 . 
     In the link activation technique, the transmitting node has only one intended receiver. Link activation, also known as unicast communication, better serves high volume point-to-point traffic. The link activation technique is illustrated in FIG.  3 . In FIG. 3, three unicast transmissions are occurring simultaneously. Node N 15  is transmitting a unicast message  300  to node N 14 , node N 17  is transmitting a unicast message  302  to node N 16  and node N 19  is transmitting a unicast message  304  to node N 18 . 
     Referring to FIG. 4, a time multiplex structure  400  suitable for use with a USAP system is depicted. The time multiplex structure  400  of FIG. 4 is a time division multiple access structure (TDMA). One cycle  402  of the structure  400  includes “N” frames  404 . The number of frames  404  required for a particular embodiment is determined by the specifics of the underlying application. 
     FIG. 4 also illustrates the structure of a representative frame of the cycle. The time allocated to the representative frame is shown divided into “M” distinct time slots  406 . It will be appreciated that different numbers of time slots can be used in the various embodiments of the invention. The first slot of each frame  404  of the cycle  402  is a broadcast slot  408  for network management control packets. One of the N broadcast slots  408  is assigned to each node in the network. Therefore, for a network having N nodes, each node can transmit control packets once during each cycle  402 . More than one broadcast slot per frame can be used if it is desired that each node have more broadcast capacity per cycle  402  or that more nodes can broadcast per cycle  402 . Further, the broadcast slots can be dynamically assigned using the USAP approach described herein. 
     Each frame  404  can also include multiple frequency channels  410 . In FIG. 4, “F” different frequency channels are illustrated in the representative frame. Different embodiments of the invention include different numbers of time slots  406 , channels  410  and/or frames  404 . To determine whether a slot and a channel are available for allocation, the USAP methodology of incorporated U.S. Pat. No. 5,719,868 may be applied. Other alternative methodologies exist, however, and can be used in lieu of the USAP methodology. 
     Dynamic assignment protocols, USAP for example, can include techniques for establishing and maintaining an adaptive broadcast channel using a transmitter directed broadcast or a receiver directed broadcast. Both types of broadcast can be based on a distance-2 vertex coloring of the network graph that provides contention-free transmissions by allowing reuse of any slot only after two hops. As noted in the incorporated material, other schemes can be employed as well. Another scheme, for example, may permit reuse of a slot only if separated by three (or more) hops. 
     FIG. 5 illustrates a receiver directed assignment of a receive slot. The map coloring is the same as that used in FIGS. 2 and 3. In FIG. 5, node N 15  has five neighbor nodes N 14 , N 16 , N 17 , N 18 , N 19 . Node N 15  has assigned itself a receive slot. In one embodiment, a node such as node N 15  may assign to itself a receive slot that is designated for reception of a transmission from one neighbor node. In this embodiment, only one communication, the communication  500  from node N 14  to node N 15  for example, would take place during the receiver assigned receive slot. 
     In another embodiment, a node assigns itself a receive slot designated for use by two of its neighbors. Referring to FIG. 5, for example, such an embodiment could involve receive slot transmissions  500 ,  504  from node N 14  and node N 17 . Other embodiments can permit three, four or more neighboring nodes to use the receive slot. In yet another embodiment, a node assigns itself a broadcast receive slot in which it can receive transmissions from all of its neighbors. FIG. 5 provides an example of a receiver directed broadcast slot wherein all five of node N 15 &#39;s neighbors are transmitting  500 ,  502 ,  504 ,  506 ,  508 . It need not be required, however, that all neighbor nodes transmit during a receiver directed broadcast slot. In many applications, only one or a subset of the neighbors will transmit during a given RDB slot. 
     Although an RDB allocation is managed as a broadcast transmission at the receiver, it is in effect a unicast transmission in embodiments wherein only one transmitter can use it at a time. The RDB unicast scheme can be a time-multiplexed unicast scheme where a receiving node&#39;s neighbors take turns transmitting to it. This is in contrast to a dedicated unicast allocation scheme wherein a slot is reserved for one transmitter/receiver pair. 
     FIGS. 2,  3  and  5  are representative of just one of many different network configurations that can use RDB. It will be appreciated that network configurations of various sizes, shapes, proportions and connectivity can use RDB and the present invention. Further, nodes having a greater or a fewer number of neighboring nodes, for example any of the nodes of FIG. 5, can use RDB and the present invention. 
     Advantages of RDB unicast over dedicated unicast include less management overhead and more flexible sharing of channel resources. In addition, since transmission parameters can be adjusted for each individual receiver instead of for multiple receivers, RDB unicast enables a more efficient transmission parameter adaptation than does standard broadcast. The result is a significant improvement over standby broadcast techniques that must set the transmission rate to accommodate the slowest neighbor. 
     patent application Ser. No. 09/340,585, filed on Jun. 28, 1999, discloses a USAP Multiple Access system that is based on USAP. The entire specification, including any and all drawing figures and appendices, of application Ser. No. 09/340,585, filed Jun. 28, 1999, is hereby incorporated herein by this reference. In USAP Multiple Access, the transmitter based broadcast occurs in the broadcast slots and standby slots of the TDMA structure. Dedicated unicast reservations replace and use the standby slots as needed. The use of the standby slots is keyed to the broadcast slots so that a node that assigns itself a particular broadcast slot is entitled to the use of the corresponding standby slots. 
     The patent application Ser. No. 09/649,802 filed on even date herewith, by C. David Young, and entitled “Maintaining an Adaptive Broadcast Channel Using Both Transmitter Directed and Receiver Directed Broadcasts” discloses a related system, USAP Multiple Broadcast Access (USAP MBA). The entire specification, including any and all drawing figures and appendices, of the patent application Ser. No. 09/649,802 filed on even date herewith, by C. David Young, and entitled “Maintaining an Adaptive Broadcast Channel Using Both Transmitter Directed and Receiver Directed Broadcasts,” is hereby incorporated herein by this reference. USAP MBA maintains the USAP MA relationship between the broadcast and standby slots, but replaces the standby transmitter based broadcast with standby RDB. In addition, the reservations are also based on RDB instead of dedicated unicast. The USAP MBA disclosure provides an example of a system within which the present inventions can be used. The present inventions can also be used with other systems. For example, they can be used with RDB systems having communication slots that are static and pre-assigned to a node, as well as with other types of dynamically allocated RDB systems. 
     An explicit reservation takes place whenever a node must take an affirmative step to secure a slot before transmitting it. This can occur, for example, in dynamic RDB schemes such as those noted above, or in RDB systems wherein communication slots are static and are pre-assigned to a node. By way of further example, an explicit reservation may be required when a node wishes to borrow a slot that has been previously, dynamically or statically, assigned to another node. 
     In applications wherein some amount of initial packet loss and delay is tolerable, such as in voice or video applications, the explicit reservation technique is wasteful and inefficient. In such applications, an implicit reservations scheme, as explained below, has distinct advantages. Besides the synergism of allowing traffic to be sent on demand and without prior reservation, implicit reservations save a significant amount of room in the bootstrap by removing the field used for indicating the explicit reservation. An embodiment of an implicit reservation scheme is depicted in FIG.  6 . 
     The implicit reservation scheme is based on the fact that, in a RDB environment, each of the nodes in the neighborhood knows that a non-transmitting node is always ready to receive on a particular channel. Therefore, if a node has excess traffic for a particular neighbor (beyond that of its normal broadcast and RDB slots) it can determine  600 , via its stored information for example, which slots are not currently assigned to, reserved by, or otherwise blocked by other nodes. Upon identifying such slots, the node with excess communications can just start transmitting  602 , without making an explicit reservation, in the unused slot or slots assigned to the intended receiver by the RDB scheme. 
     When implicit reservations are enabled, the receiving node assumes  604 , upon receiving traffic in a slot, that the transmitting node has made an implicit reservation. The receiving node then announces  606  that it is busy receiving in the slot. In a USAP system, for example, it can start announcing a ‘self receive’ in its standby/reservation receive bootstrap record. As a result of the announcement, other transmitters are prohibited from implicitly reserving the slot. 
     Upon determining  608  that the traffic in the implicitly reserved slot has ceased, after waiting for a brief time out period for example, the receiving node can announce  610  that the slot is once again unassigned. Further, if desired, the neighbor node owning the slot can be permitted to reclaim  612  the slot for its own transmissions to the receiver associated with that RDB slot. In a USAP system, for example, a neighbor node can accomplish such a result by announcing a ‘self transmit’ in its standby transmit slot field. After determining that the owner is reclaiming the slot, the receiving node can announce  614  a ‘conflict’ and thereby cause any other neighbor node to abandon  616  its implicit or explicit reservation. 
     Similarly, if numerous packets containing errors are received  618 , the receiving node can be allowed to assume that more than one neighbor is trying to implicitly reserve the same slot. In such an event, the receiving node can announce  620  a conflict in the slot. Upon receiving notification of the conflict, the node or nodes implicitly reserving the slot will abandon it. 
     The savings obtained via the use of implicit reservations can be seen by considering the USAP MBA bootstrap for example. In one embodiment of USAP MBA, explicit reservations are encoded as two maps of one bit fields. These fields correspond to up to two neighbors&#39; receive slots that reserving node would like to use. The size of the two fields are calculated as follows: 
     
       
         3 bits*2 neighbors=6 bits, 
       
     
     and 
     
       
         1 bit*8 slots*2 neighbors=16 bits. 
       
     
     The value of the bit indicates whether the slot is an unassigned slot or a self transmit slot. 
     Overall, the USAP MBA bootstrap requires a total of 50 bits, about 9 bytes. Use of an implicit reservation scheme instead of the explicit reservation scheme, however, permits the reservation transmit slot fields to be omitted from the bootstrap. Consequently, in this example embodiment, the size of the bootstrap can be reduced 44 percent to 28 bits. 
     FIGS. 2 and 5 respectively illustrate transmitter based broadcast and receiver directed broadcast. In broadcast slots, a node such as node N 15  can use its slot to transmit, as in FIG. 2, to all of its neighbors at once. Likewise, in the context of FIG. 5, a node such as node N 15  can receive from any of its neighbors (if, for example, only one neighbor transmits at a time). 
     To prevent RDB collisions, neighbors needing to transmit data to a node can be scheduled unique communication slots, for example unique slot and channel combinations, which are keyed to the broadcast slot each is assigned. When desired, an “optimal RDB schedule” can be implemented. As noted, the optimal RDB schedule can also be used with other systems, such as RDB systems having communication slots that are static and pre-assigned to a node, as well as other types of dynamically allocated RDB systems. An optimal schedule can be designed, for example, to fulfill the following objectives: 
     1. Use as many channels as possible to allow the maximum number of simultaneous transmissions to occur. 
     2. If a node wishes to use another node&#39;s transmission opportunity, it has an equal probability of effecting its own transmit and receive opportunities. 
     3. The schedules for sparser neighborhoods are imbedded within the schedules for denser neighborhoods. 
     The justification for the first objective is that, under a uniform traffic load, maximum throughput can be achieved if all nodes are kept busy 100% of the time either transmitting or receiving. The number of channels (C) required to achieve this end is related to the number of nodes in the neighborhood (N), by the equation C=N/2. Likewise the number of slots (S) required, derives from the fact that every node must both transmit and receive from N−1 other nodes. Thus, the relation S=2(N−1) is obtained. 
     The second objective is subtler and will be explained with the help of the following four node schedule. FIG. 7 depicts a four node neighborhood. The table below relates two channels (C 1  and C 2 ) to six slots (S 0 , S 1 , S 2 , S 3 , S 4  and S 5 ) for four nodes (N 0 , N 1 , N 2  and N 3 ) of FIG.  7 . 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Optimized Four-Node Schedule 
               
             
          
           
               
                 Txer → Rxer 
                 C0 
                 C1 
               
               
                   
               
               
                 S0 
                 N 0  → N 1   
                 N 3  → N 2   
               
               
                 S1 
                 N 1  → N 0   
                 N 2  → N 3   
               
               
                 S2 
                 N 0  → N 2   
                 N 1  → N 3   
               
               
                 S3 
                 N 2  → N 0   
                 N 3  → N 1   
               
               
                 S4 
                 N 0  → N 3   
                 N 2  → N 1   
               
               
                 S5 
                 N 3  → N 0   
                 N 1  → N 2   
               
               
                   
               
             
          
         
       
     
     In this schedule, node N 0  uses (S 0 , C 0 ) as its primary slot for transmitting to node N 1 . However, if node N 2  or node N 3  currently have no traffic for node N 1 , then node N 0  can use their slots, (S 3 , C 1 ) and (S 4 , C 1 ), on a reservation basis to communicate with node N 1 . Node N 0 &#39;s use, however, of either one of these slots will reduce its own transmit and receive opportunities, (S 3 , C 0 ) and (S 4 , C 0 ). Consequently, node N 0  must weigh its decision about which additional slots to reserve for transmissions to node N 1 , against its needs to communicate with the other nodes in its neighborhood. Presumably this decision is easier if all nodes have an equal choice between receive and transmit opportunities. 
     The optimal schedule for two nodes is (S 0 , C 0 ) and (S 1 , C 0 ). Notice that the two node schedule is imbedded in the four node schedule. A scheduling embodiment having the smaller schedules imbedded in the larger schedule has the added benefit of permitting a smooth transition between neighborhoods of different densities (such as according to the USAP MA Adaptive Broadcast Cycles for example). A schedule satisfying all three objectives is considered “optimal.” 
     FIG. 8 depicts an eight node neighborhood. An optimal eight node schedule for the neighborhood of FIG. 8 is listed below. Note that, in this embodiment, the optimal four node schedule is imbedded within this optimal eight node schedule. Four channels (C=N/2) and fourteen time slots (S=2(N−1)) are required for the optimal schedule for this eight node neighborhood. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Optimized Eight-Node Schedule 
               
             
          
           
               
                 Txer → Rxer 
                 C0 
                 C1 
                 C2 
                 C3 
               
               
                   
               
               
                 S0 
                 N 0  → N 1   
                 N 3  → N 2   
                 N 4  → N 5   
                 N 7  → N 6   
               
               
                 S1 
                 N 1  → N 0   
                 N 2  → N 3   
                 N 5  → N 4   
                 N 6  → N 7   
               
               
                 S2 
                 N 0  → N 2   
                 N 1  → N 3   
                 N 4  → N 6   
                 N 5  → N 7   
               
               
                 S3 
                 N 2  → N 0   
                 N 3  → N 1   
                 N 6  → N 4   
                 N 7  → N 5   
               
               
                 S4 
                 N 0  → N 3   
                 N 2  → N 1   
                 N 4  → N 7   
                 N 6  → N 5   
               
               
                 S5 
                 N 3  → N 0   
                 N 1  → N 2   
                 N 5  → N 6   
                 N 7  → N 4   
               
               
                 S6 
                 N 0  → N 4   
                 N 1  → N 6   
                 N 2  → N 5   
                 N 3  → N 7   
               
               
                 S7 
                 N 0  → N 5   
                 N 1  → N 4   
                 N 6  → N 2   
                 N 7  → N 3   
               
               
                 S8 
                 N 0  → N 6   
                 N 2  → N 4   
                 N 5  → N 3   
                 N 7  → N 1   
               
               
                 S9 
                 N 0  → N 7   
                 N 3  → N 4   
                 N 5  → N 2   
                 N 6  → N 1   
               
               
                 S10  
                 N 1  → N 5   
                 N 2  → N 6   
                 N 4  → N 3   
                 N 7  → N 0   
               
               
                 S11  
                 N 1  → N 7   
                 N 3  → N 5   
                 N 4  → N 2   
                 N 6  → N 0   
               
               
                 S12  
                 N 2  → N 7   
                 N 3  → N 6   
                 N 4  → N 0   
                 N 5  → N 1   
               
               
                 S13  
                 N 4  → N 1   
                 N 5  → N 0   
                 N 6  → N 3   
                 N 7  → N 2   
               
               
                   
               
             
          
         
       
     
     Thus, two, four and eight node RDB cycles can be handled by five channels. To scale to sixteen nodes optimally requires eight channels and thirty slots. An acceptable schedule can be achieved, however, by using five channels and 48 slots (i.e. there are 120 ways of taking two of eight nodes, multiplied by two for transmit and receive, and divided by five channels). In general,        S   ≡       (         N           2         )     ×     2   C       ≡         N   !         2   !          (     N   -   2     )         ×     2   C       ≡         N        (     N   -   1     )       C     .                            
     Accordingly, a 32 node schedule with five channels requires 199 slots. It will be appreciated that the present invention can be further expanded to include neighborhoods having more than 32 nodes. 
     FIG. 9 presents a frame optimized for the eight node schedule. In this schedule, an eight node neighborhood would have a maximum latency of 125 ms since every node can transmit to every other node in every frame. Note that this is the same latency as the current USAP MA eight node standby broadcast schedule, the difference being that each node has seven transmit slots instead of one. 
     Even though the RDB slots of the embodiment of FIG. 9 are 50% shorter (due to the need for fourteen slots instead of eight), throughput is still tripled when most of the traffic is point-to-point. Having only five channels begins to be detrimental in the 16 and 32 node schedules, however, where latencies are about two and 3.5 times that of the standby broadcast schedule. In such environments, for example, a sixteen channel structure can be implemented for optimally handling neighborhoods of up to 32 nodes. 
     It is thought that the method and apparatus of the present invention will be understood from the description provided throughout this specification and the appended claims, and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The forms herein described are merely exemplary embodiments thereof.