Abstract:
A method includes receiving a schedule of transmission start slots on a network node, the slots representing reserved and/or nonreserved opportunities for initiating data transmission by at least one network device in a network, the reserved opportunities associated with specific network devices, and the non-reserved opportunities available for non-reserved use by any network device on the network, and adjusting the schedule in accordance with successful transmissions by other network nodes. Another method includes generating a schedule of transmission start slots on a master node, where the slots represent reserved and/or nonreserved transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, the reserved opportunities being associated with specific network nodes, and the nonreserved opportunities being available for nonreserved use by any the network devices on the network, and distributing the schedule to the network devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit from U.S. Provisional Patent Application No. 60/983,615, filed Oct. 30, 2007, and 60/989,658, filed Nov. 21, 2008 which are hereby incorporated in their entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to data networks generally and to media access allocation in data networks in particular. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are many different types of data networks, of which Ethernet is perhaps the best known. Some data networks have resource reservation schemes. One such network is HomePNA (Home Phoneline Network Alliance) v3.1 which is designed to work over existing telephone lines to create a home/small office network. U.S. patent application Ser. No. 11/000,524, filed Dec. 1, 2004 and assigned to the common assignee of the present invention, describes generally how to extend the HomePNA v3.1 standard to operate over a hybrid network of telephone and coax lines. 
         [0004]    HPNA v3.1 and other such resource reservation networks have a scheduler, described hereinbelow, to guarantee media resources to network devices, to prevent collision between multiple network devices using the same line and to ensure quality of service. In coax networks, preventive collision detection limits the dynamic range of the network devices, which may impose physical limitations on the size of the network, so it is preferable to use collision avoidance methods for media access in coax networks. 
         [0005]    Such a collision avoidance method is disclosed in U.S. patent application Ser. No. 11/218,708 entitled ‘Collision Avoidance Media Access Method for Shared Networks’, filed Sep. 6, 2005 and assigned to the common assignee of the present invention. This application is incorporated herein by reference. The collision avoidance/ carrier sensing media access (CA/CSMA) method disclosed in the application employs a media access plan (MAP) having sub-burst slots. Each sub-burst slot has a shorter duration than a minimal transmission burst duration (e.g., 8-32 μsecs), is associated with a particular one or group of network participants, and represents a reserved opportunity for the initiation of a data transmission by its associated network participants. 
         [0006]    The MAP for a transmission cycle dictates a schedule of sub-burst slots, wherein numbered sub-burst slots are scheduled in a particular order.  FIG. 1A , reference to which is now made, shows an exemplary sub-burst slot schedule  10 , in which five sub-burst slots numbered  0  through  4  are scheduled in sequential order. Sub-burst slot schedule  10  may also be seen as a grid of transmission opportunity start times. The start time STN for each sub-burst slot N is the moment at which the network participant associated with sub-burst slot N may begin to transmit. 
         [0007]    In the initial grid of transmission opportunity start times (before any transmissions occur), the start time of each sub-burst slot N, ST N , occurs after the sum of the durations of the sub-burst slots preceding sub-burst slot N. For example, as shown in  FIG. 1A , the initial start times STi 0 , STi 1 , STi 2 , STi 3 , and STi 4  of sub-burst slots  0 - 4  respectively, occur at (t=0), (t=d 0 ), (t=d 0 +d 1 ), (t=d 0 +d 1 +d 2 ), and (t=d 0 +d 1 +d 2 +d 3 ) respectively, where d 0 , d 1 , d 2 , and d 3  are the durations of sub-burst slots  0 - 4  respectively. 
         [0008]    The principal advantage of sub-burst slots over regular sized time slots is that when a network participant does not use its transmission opportunity, minimal time is wasted before the opportunity to transmit is passed to the next network participant in the queue. On the other hand, when a network participant opts to transmit when its turn arrives, the allowable transmission duration is not limited by the short duration of the sub-burst slot. Rather, the sub-burst slot expands to encompass the required transmission burst duration. Accordingly, the start times of the succeeding sub-burst slots are delayed by an amount of time equal to the portion of the transmission duration which exceeds the original sub-burst slot duration. In effect, the entire grid of transmission opportunity start times shifts by this amount. 
         [0009]    For example, as shown in  FIG. 1B , reference to which is now made, timing diagram  15  for an exemplary transmission cycle operating in accordance with sub-burst slot schedule  10  shows how a transmission during sub-burst slot ‘1’ alters the initial grid of transmission opportunity start times for the sub-burst slots following sub-burst slot ‘1’. As shown in  FIG. 1B , start times STb 2 , STb 3  and STb 4  are incremented by x, the portion of the transmission transmitted during sub-burst slot ‘1’ which exceeds the original sub-burst slot duration d 1 . 
         [0010]    In a network employing the CA/CSMA method described hereinabove, all of the participating network nodes receive the MAP and extract from it their relative transmission opportunities. They then employ physical carrier sensing (PCS) to monitor transmissions occurring over the network so that, subsequent to each transmission, they can synchronize to an updated transmission opportunities (TXOPs) schedule accounting for transmission-induced shifts in the sub-burst slot start time grid. 
       SUMMARY OF THE PRESENT INVENTION 
       [0011]    An object of the present invention is to improve upon the prior art. 
         [0012]    There is therefore provided, in accordance with a preferred embodiment of the present invention, a method including receiving a schedule of transmission start slots on a network node, where the transmission start slots represent at least one of reserved and non reserved transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, the reserved opportunities being associated with specific network devices, and the non reserved opportunities being available for non reserved use by any network device on the network, and adjusting the schedule in accordance with successful transmissions by other network nodes. 
         [0013]    Further, in accordance with a preferred embodiment of the present invention, the method also includes attempting to transmit in accordance with a transmission start slot representing a non reserved transmission opportunity. 
         [0014]    Still further, in accordance with a preferred embodiment of the present invention,the method also includes detecting at least an unsuccessful attempt to transmit, backing off in accordance with a random number of the non scheduled transmission initiation opportunities as per the schedule, reattempting to transmit in accordance with a transmission start slot representing a subsequent non scheduled transmission opportunity as per the schedule. 
         [0015]    Additionally, in accordance with a preferred embodiment of the present invention, the detecting includes transmitting a request to transmit (RTS), and waiting a predefined period of time to receive a “clear to send” (CTS) in reply to the RTS. 
         [0016]    Moreover, in accordance with a preferred embodiment of the present invention, the detecting includes using physical collision detection based on echo cancellation. 
         [0017]    Further, in accordance with a preferred embodiment of the present invention, the adjusting includes detecting transmissions by other the network nodes in accordance with the schedule, and delaying the transmission start slots in accordance with a transmission length of the detected transmissions. 
         [0018]    Still further, in accordance with a preferred embodiment of the present invention, the network uses a powerline medium. 
         [0019]    Additionally, in accordance with a preferred embodiment of the present invention, the method also includes transmitting an in use signal, expanding the transmission start slot to a standard sized backoff window, backing off within the backoff window in accordance with a random duration, transmitting data, and resolving any ensuing detected collisions within the backoff window as necessary. 
         [0020]    There is also provided, in accordance with a preferred embodiment of the present invention, a method including generating a schedule of transmission start slots on a master node, where the transmission start slots represent at least one of reserved and non reserved transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, the reserved opportunities being associated with specific network nodes, and the non reserved opportunities being available for non reserved use by any network device on the network, and distributing the schedule to the network devices. 
         [0021]    Further, in accordance with a preferred embodiment of the present invention, the method also includes tracking transmissions by the network devices in accordance with the schedule, and adjusting the schedule by rescheduling the transmission start slots in accordance with at least a length of the tracked transmissions. 
         [0022]    Still further, in accordance with a preferred embodiment of the present invention, the tracking also includes detecting non reserved transmissions from among the tracked transmissions, and adding scheduled transmission start opportunities for the network devices associated with the detected non reserved transmissions to a new the schedule to be distributed to the network devices. 
         [0023]    Additionally, in accordance with a preferred embodiment of the present invention, the tracking also includes detecting a lack of reserved transmissions from among the tracked transmissions, and removing scheduled transmission start opportunities for the network nodes associated with the lack of scheduled transmissions from a new schedule to be distributed to the network devices. 
         [0024]    Moreover, in accordance with a preferred embodiment of the present invention, the network uses a powerline medium. 
         [0025]    There is also provided, in accordance with a preferred embodiment of the present invention, a master node including a scheduler to generate a schedule of transmission start slots, where the transmission start slots represent at least one of reserved and non reserved transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, the reserved opportunities being associated with specific the network devices, and the non reserved opportunities being available for non reserved use by any the network devices on the network, and means to distribute the schedule to the network devices. 
         [0026]    Further, in accordance with a preferred embodiment of the present invention, the master node also includes means to track transmissions by the network devices in accordance with the schedule, and a schedule adjuster to adjust the schedule by rescheduling the transmission start slots in accordance with at least a length of the tracked transmissions. 
         [0027]    Still further, in accordance with a preferred embodiment of the present invention, the schedule adjuster also includes means to add the scheduled transmission start opportunities for the network nodes associated with non scheduled transmissions to a new schedule to be distributed to the network devices. 
         [0028]    Additionally, in accordance with a preferred embodiment of the present invention, the schedule adjuster also includes means to remove the scheduled transmission start opportunities from a new schedule to be distributed to the network nodes, where the transmission start opportunities are associated with the network nodes for which there are no associated scheduled transmissions detected. 
         [0029]    Moreover, in accordance with a preferred embodiment of the present invention, the network uses a powerline medium. 
         [0030]    There is also provided, in accordance with a preferred embodiment of the present invention, a network node including means to receive a schedule of transmission start slots on a network node, where the transmission start slots represent at least one of reserved and non reserved transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, the scheduled opportunities being associated with specific network nodes, and the non scheduled opportunities being available for non reserved use by any network device on the network, and a schedule adjuster to adjust the schedule in accordance with successful transmissions by other network nodes. 
         [0031]    Further, in accordance with a preferred embodiment of the present invention, the node also includes means to transmit in accordance with a transmission start slot representing a the non reserved transmission opportunity. 
         [0032]    Still further, in accordance with a preferred embodiment of the present invention, the node also includes means to detect at least an unsuccessful attempt to transmit, and a back off mechanism to reattempt to transmit in accordance with a random “backing off” in accordance with the non reserved transmission initiation opportunities as per the schedule. 
         [0033]    Additionally, in accordance with a preferred embodiment of the present invention, the means to detect includes means to transmit a RTS, and means to process an expected CTS to be received in reply to the RTS. 
         [0034]    Moreover, in accordance with a preferred embodiment of the present invention, the means to detect includes a physical collision detector to use echo cancellation to detect collisions. 
         [0035]    Further, in accordance with a preferred embodiment of the present invention, the adjustor includes means to detecting transmissions by other the network nodes in accordance with the schedule, and a rescheduler to delay the transmission start slots in accordance with a transmission length of the detected transmissions. 
         [0036]    Still further, in accordance with a preferred embodiment of the present invention, the network uses a powerline medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
           [0038]      FIG. 1A  is an illustration of a schedule of sub-burst slots according to an exemplary media access plan (MAP) operative in a network employing a collision avoidance/ carrier sensing media access (CA/CSMA) method; 
           [0039]      FIG. 1B  is a timing diagram illustration for an exemplary transmission cycle for the schedule of  FIG. 1 ; 
           [0040]      FIG. 2  is an illustration of an exemplary contention based TXOP schedule; 
           [0041]      FIG. 3A  is an illustration of a novel TXOP schedule combining an overall contention free sub burst slots model with additional contention based sub burst slots, constructed and operative in accordance with a preferred embodiment of the present invention; 
           [0042]      FIG. 3B  is an illustration of a timing diagram for an exemplary transmission cycle operating in accordance with the TXOP schedule of  FIG. 3A ; 
           [0043]      FIG. 4  is an illustration of an exemplary transmission session transmitted in accordance with the TXOP schedule of  FIG. 3A ; and 
           [0044]      FIG. 5  is an illustration of an exemplary series of sub-burst slot assignments as they may appear in successive MAPs issued in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the present invention. 
         [0046]    A network employing the CA/CSMA method as described in the Background may incur an inherent overhead and loss in network throughput whenever a sub-burst slot is not used by the device to which it is assigned. It will be appreciated that as the number of unused sub-burst slots increases, so does the amount of wasted media access time resulting in a decrease in effective network throughput. 
         [0047]    Network throughput may be improved by assigning sub-burst slots exclusively to devices on a contention free basis only if and when such devices are actively transmitting. Such a method may improve network throughput by preventing the “waste” of unused sub-burst slots. Wasted media access time may be minimized, since unused sub-burst slots may largely be avoided. Once a device ceases to transmit, its associated sub-burst slots may be reassigned to another actively transmitting device. However, implementing such a method may also prevent previously inactive devices from transmitting. If sub-burst slots may be assigned solely to actively transmitting devices, then new or currently inactive devices may not have the opportunity to register for assignment of sub-burst slots. 
         [0048]    Applicants have realized that by adding a number of contention based slots to a TXOP comprising contention free sub bursts slots, network throughput may still be improved while providing for the registration of new or inactive devices. The media access rules for the added contention based slots may use a contention-based media access method typically based on collision detection/inference and the use of a backoff mechanism. An example of such a method may be the IEEE 802.11 Distributed Coordination Function (DCF). The backoff mechanism may run on top of the underlying sub-burst slot mechanism and may effectively select a random contention-based sub-burst slot from among those available in which to retry a transmission. Collision detection may be performed using physical collision detection (e.g. based on echo cancellation), or logical collision detection based on acknowledgement, such as RTS-CTS (Request To Send-Cleared To Send) and ACK mechanisms. 
         [0049]    Reference is now made to  FIG. 2 .  FIG. 2  shows an exemplary contention based TXOP schedule  50  with a backoff mechanism such as DCF. Nodes  60  may compete for transmission opportunities  55  which may be “raffled off” by a media access controller (not shown). For example, node  60 A may transmit transmission  65 . Once transmission  65  may be completed, other nodes, such as nodes  60 B and  60 C may compete for a transmission opportunity  55 . If a node  60  is the only one to attempt to transmit according to a given opportunity  55 , then it may continue to transmit until its transmission is completed. In the example of  FIG. 2  it may be assumed that in such manner Node  60 A “received permission” to transmit transmission  65 . 
         [0050]    However, if two nodes both attempt to transmit as per the same opportunity  55 , the resulting collision may preclude either one of them from completing a transmission  65 . For example, as shown in  FIG. 2 , both node  60 B and node  60 C may have attempted to transmit as per the transmission opportunity  50  labeled “1”. The resulting collision may be resolved by a backoff mechanism such as DCF. Each of the involved nodes may “back off” in accordance with a randomized function and attempt to transmit as per a different opportunity  55 . For example, as shown in  FIG. 2 , node  60 B may reattempt to transmit as per the transmission opportunity labeled “7” and node  60 C may attempt to do so as per the opportunity  55  labeled “5”. In such manner, the collision between nodes  60 B and  60 C may be resolved. 
         [0051]    It will be appreciated that such backing off may continue for as many additional attempts as needed. Furthermore, other nodes  60  may also be attempting to transmit at the same time. The time required to resolve the collision may be a function of the overall traffic on the network medium. 
         [0052]      FIG. 3A , to which reference is now made, shows a novel TXOP schedule  100  combining an overall contention free sub burst slots model with additional contention based sub burst slots, constructed and operative in accordance with a preferred embodiment of the present invention. By adding contention based slots to a prior art contention free TXOP schedule, the present invention may increase network throughput while enabling registration of new or previously inactive devices wishing to begin transmission. It will be appreciated that TXOP schedule  100  may be generated and updated as necessary by a scheduler on a master node on the network as per the prior art. 
         [0053]    TXOP schedule  100  may comprise allocated sub burst slots  110  and “wildcard” sub burst slots  115 . Allocated sub burst slots  110  may be contention free in nature; each slot  110  may be reserved for transmission by a single node  60 . Furthermore, in accordance with a preferred embodiment of the present invention, allocated sub burst slots  110  may only be allocated to nodes  60  that may have already begun transmitting. For example, as shown in  FIG. 3A , there may be three such nodes  60  as represented by allocated slots  110  labeled  1 ,  2  and  3 . Each of the scheduled allocated slots  110  may be reserved for one of these three nodes  60 . In contrast, wildcard slots  115  may be available for transmissions by nodes  60  on a contention basis, where such nodes  60  may not have associated allocated slots  110 . Accordingly, it will be appreciated that schedule  100  may comprise a mix of both reserved transmission start slots (allocated slots  110 ) and non reserved transmission start slots (wildcard slots  115 ). 
         [0054]    For example, nodes  60 D and  60 E may be currently inactive or may not have previously transmitted. Accordingly they may not have associated allocated slots  110 . Accordingly, they may attempt to transmit as per a wildcard slot  115 . If only one node  60  may attempt to transmit according to a wildcard slot  115 , then it may continue to transmit until its transmission is completed as in the prior art. 
         [0055]    However, it is possible that more than one node  60  may attempt to transmit as per a given slot  115 . For example, if node  60 D and node  60 E may both attempt to transmit as per wildcard slot  115 A, then the resulting collision may prevent either one from continuing the transmission. 
         [0056]    Nodes  60 D and  60 E may each comprise means to calculate a random backoff from wildcard slot  115 A. For example, as shown in  FIG. 3A , node  60 D may reattempt to transmit as per wildcard slot  115 D; whereas node  60 E may reattempt to retransmit as per wildcard slot  115 C. It will be appreciated that as in the prior art, such backing off may persist until the collision is resolved. 
         [0057]    When a node  60  is able to successfully transmit in a wildcard slot  115 , it may provide sufficient identifying information to a master node (using signaling protocols or other methods, e.g. information in the frame control) that may enable the master to assign a unique allocated slot  110  to the device. 
         [0058]      FIG. 3B , to which reference is now made, illustrates a timing diagram  105  for an exemplary transmission cycle operating in accordance with TXOP schedule  100 . Timing diagram  105  may comprise unused sub burst slots  110 ′, unused wildcard slots  115 ′, scheduled transmission sessions  210  and wildcard transmission sessions  215 . Unused sub burst slots  110 ′ and scheduled transmission sessions  210  may together correlate to allocated sub burst slots  110  as represented in  FIG. 3A , and may therefore be associated with nodes  60 A,  60 B and  60 C. Similarly, unused wildcard slots  115 ′ and wildcard transmission sessions  215  may together correlate to wildcard slots  115  as represented in  FIG. 3A , and may therefore be associated with nodes  60 D and  60 E. 
         [0059]    Scheduled transmission sessions  210  may represent transmissions by nodes  60 A,  60 B and  60 C taking advantage of allocated sub burst slots  110 . It will be appreciated that each transmission session  210  may have impacted on the original schedule  100  in  FIG. 3A  by delaying the start of subsequent sub burst slots  110  and wildcard slots  115 . However, other than a delayed start due to a transmission session  210 , the remaining portion of schedule  100  after each such transmission session  210  may have remained generally the same. 
         [0060]    Similarly, wildcard transmission sessions  215  may represent transmissions by nodes  60 D and  60 E taking advantage of wildcard slots  115 . As discussed in the context of  FIG. 3A , nodes  60 D and  60 E may both have attempted to transmit during unused wildcard slot  115 ′A. After detecting a collision, both nodes  60  may have “backed off” from wildcard slot  115 A; neither node  60 D, nor node  60 E may have continued to attempt to transmit during wildcard slot  115 A. Instead each may have attempted to transmit in accordance with a random subsequent wildcard slot  115 . As shown in  FIG. 3B , in accordance with the exemplary embodiment of  FIG. 3A , node  60 D may have transmitted during wildcard transmission slot  115 D, and node  60 E may have transmitted during wildcard transmission slot  115 C. These transmissions may be respectively represented in  FIG. 3B  as wildcard transmission sessions  215 D and  215 C. 
         [0061]    It will be appreciated that wildcard transmission sessions  215  may have impacted on the original schedule  100  in a similar manner as scheduled transmission sessions  210 . Each wildcard transmission session  215  may have delayed the start of subsequent sub burst slots  110  and wildcard slots  115 . However, other than a delayed start due to a transmission session  215 , the remaining portion of schedule  100  after each such transmission session  215  may have remained generally the same. 
         [0062]    It will be appreciated that a transmission session  210  or  215  may comprise more than a single one-way transmission of a frame control and data payload. For example, as shown in  FIG. 4 , to which reference is now made, in addition to a data payload  160 , a transmission session  210  or  215  may also comprise an RTS  150  from a transmitting node  60  and an answering CTS  155  from a node  60  that may receive the transmission. Similarly, a transmission session  210  or  215  may also comprise a “received transmission”  170  or “not received transmission  175 ” response from a receiving node  60 . U.S. Patent Application #(C-25-US), assigned to the common assignees of the present application and hereby incorporated in its entirety by reference, may disclose a method for using such “received” and “not received” responses to detect collisions and/or otherwise unsuccessful transmission sessions. A typical session  210  or  215  may also comprise an inter-frame gap (IFG)  180  which may delineate the end of a session  210 / 215 . 
         [0063]    In accordance with a preferred embodiment of the present invention, it may be possible to implement different media access schemes within different slot times in a dynamic manner over the same network medium. For example, a TXOP schedule  100  may comprise only allocated slots  110 , thus implementing the prior art media access method used by G.9954 (HPNA 3.1) within a shared TXOP. Alternatively, only wildcard slots  115  may be defined, thus implementing a pure backoff based media access method such as IEEE 802.11 DCF. The present invention may also therefore include any hybrid media access scheme of contention-free and contention-based sub-burst slots. 
         [0064]    Reference is now to  FIG. 5  which represents an exemplary series of sub-burst slot assignments as they may appear in successive MAPs  200 . In MAP  200 A all of the relevant nodes (e.g. nodes  1 - 4 ) may be transmitting and may have been assigned allocated sub-burst slots  110 . In MAP  200 B node  4  may have stopped transmitting and its sub-burst slot may have been reassigned as a wildcard slot  115 . In MAP  200 C node  3  may also have stopped transmitting and its sub-burst slot may also have been reassigned among the remaining transmitters. The trend may continue until finally in MAP  200 E only a contention-based wildcard sub-burst slot  115  may remain. Accordingly, MAP  200 E may represent a media access scheme that is pure contention-based media access with backoff. 
         [0065]    In accordance with a preferred embodiment of the present invention, a “backoff window” maybe defined as a target area of schedule  100  in which nodes  60  may reattempt to transmit. As determined by a randomizing function, a node  60  may reattempt to transmit as per any wildcard slot in the backoff window. For example, in  FIG. 3  a backoff window may be defined as starting with wildcard slot  115  B and ending with wildcard slot  115 E. 
         [0066]    It will be appreciated that there may be a correlation between the size of the backoff window and the likelihood that subsequent retransmissions may or may not succeed without recurring collisions. The larger the window, the more likely that a subsequent retransmission may succeed without collision. However, it will be similarly appreciated, that the larger the window, the longer it may take for a retransmission to actually complete. Applicants have therefore realized that it maybe beneficial to adjust the size of the backoff window in accordance with network traffic. 
         [0067]    In accordance with an alternative preferred embodiment of the present invention, multiple backoff windows may be defined as per 802.11. For example, a relatively small backoff window may be defined for a first retransmission attempt. If the first retransmission attempt may fail, the original backoff window may be progressively replaced with increasingly larger backoff windows until a retransmission may be successful. In such manner, a node  60  may attempt to complete its transmission as quickly as possible in accordance with the prevailing conditions of a network medium. 
         [0068]    In the event of collisions in networks without RTS-CTS and/or ACK mechanisms for collision detection, there may be exposure to loss of synchronization with the grid. Contending nodes may continue to transmit while listening nodes may be unable to discern an expected duration for the ongoing transmission(s), thus rendering the current transmission grid unusable. In accordance with an alternative preferred embodiment of the present invention, such loss of synchronization may be prevented by expanding a wildcard slot  115  to a standard sized backoff window as soon as the beginning of a transmission is detected. 
         [0069]    A node  60  wishing to transmit in a wildcard slot  115  may indicate an intention to use wildcard slot  115  by first transmitting an INUSE signal. Listening nodes  60  may then adjust their associated schedules  110  to indicate that the duration of that particular wildcard slot  115  may now be that of a standard backoff window. It will be appreciated that wildcard slot  115  may be expanded regardless of whether or not an actual collision is detected. A node  60  transmitting during any wildcard slot may always proceed as if a collision had indeed been detected; “backing off” a random duration of time within the backoff window before transmitting. If a collision does indeed occur, then the contending nodes  60  may resolve the contention within the backoff window without affecting the other listening nodes  60 . If there is no collision, then the transmitting node  60  may complete its transmission unaffected. In either case, by expanding the wildcard slot  115  into a backoff window, the chances of colliding transmissions throwing off the synchronization within the network may be greatly reduced. 
         [0070]    It will be appreciated that this method may increase the duration overhead for transmission during wildcard slots  115 . However, the additional overhead may be incurred only when a transmission actually takes place. When slots  115  may be unused, no additional overhead may be incurred. 
         [0071]    As per the prior art, an expected duration may be included in a transmission to enable listening nodes  60  to adjust schedule  100  accordingly. It will be appreciated that in the event of a collision during a wildcard slot  115 , it may not be possible to for a listening node  60  to receive this expected duration, which may in turn lead to a loss of synchronization as the colliding transmission(s) may continue. Because there may be no way to know how long a colliding transmission may continue, listening node  60  may have to use physical carrier sensing until the collision subsides, and only then may it attempt to resynchronize with the network. In accordance with a preferred alternative embodiment of the present invention, the impact of collisions during wildcard slots  115  may be lessened by a priori limiting the length of transmissions that may be initiated during slots  115 . 
         [0072]    It will be appreciated that the specification of HPNA v3.1 may be exemplary. The present invention may be implemented in accordance with other standards such as 802.11, HPAV, and G,hn. It will be appreciated, however, that support for the present invention must be included the current specifications for a standard in order for such implementation to be successful. 
         [0073]    While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.