Abstract:
A method implemented on a network node includes receiving an adjustable schedule of transmission start slots, where the transmission start slots represent transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices on a network, and transmitting a current position of the node within all groups of the schedule suitable for full resynchronization to the adjustable schedule by at least one other the network node.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 12/261,170, filed Oct. 30, 2008, which claims benefit from U.S. Provisional Patent Application Nos. 60/983,615, filed Oct. 30, 2007, and 60/989,658, filed Nov. 21, 2007, which are hereby incorporated in their entirety by reference. This application also claims benefit from U.S. Provisional Patent Application No. 61/152,702, filed Feb. 15, 2009, which is hereby incorporated in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to data networks generally and to media access allocation in data networks in particular. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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 an opportunity for the initiation of a data transmission by its associated network participants. 
     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 ST N  for each sub-burst slot N is the moment at which the network participant associated with sub-burst slot N may begin to transmit. 
     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. 
     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. 
     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 . 
     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. Then they 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. 
     Successful implementation of PCS is important for optimal operation of collision avoidance as described hereinabove. The carrier sensors in all of the network nodes must receive the same information regarding transmissions occurring over the network in order to guarantee synchronization of all nodes to the same timing and transmission opportunities schedule. 
     SUMMARY OF THE PRESENT INVENTION 
     There is provided, in accordance with a preferred embodiment of the present invention, a method implemented on a network node including receiving an adjustable schedule of transmission start slots, where the transmission start slots represent transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices on a network, and transmitting a current position of the node within all groups of the schedule suitable for full resynchronization to the adjustable schedule by at least one other the network node. 
     Further, in accordance with a preferred embodiment of the present invention, the transmission start slots in the adjustable schedule are scheduled in transmission groups, each transmission group associated with a group of the network devices, and the current position contains at least an indication of a current the transmission group and a current the transmission start slot within the current transmission group. 
     Still further, in accordance with a preferred embodiment of the present invention, the transmitting also includes transmitting the current position together with a data payload. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmitting also includes transmitting the current position together with a null transmission. 
     Moreover, in accordance with a preferred embodiment of the present invention, the current position also contains indications of each other transmission group and current transmission start slots with the transmission groups. 
     Further, in accordance with a preferred embodiment of the present invention, the transmitting is in response to the network node losing synchronization with the adjustable schedule. 
     Still further, in accordance with a preferred embodiment of the present invention, the transmitting is scheduled without relation to a loss of synchronization with the adjustable schedule by the network node. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmitting also includes transmitting the current position within at least one of an acknowledgement (ACK) or request to send (RTS). 
     There is also provided, in accordance with a preferred embodiment of the present invention, a method implemented on a network node including receiving an adjustable schedule of transmission start slots, where the transmission start slots represent transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, listening for transmissions by other the network nodes according to the received schedule, the transmissions at least indicating a current position within all transmission groups of the adjustable schedule, and adjusting the received schedule in accordance with the indicating. 
     Further, in accordance with a preferred embodiment of the present invention, the transmission start slots in the adjustable schedule are scheduled in the transmission groups, each transmission group associated with a group of the network devices, and the current position contains at least an indication of a current the transmission group and a current the transmission start slot within the current transmission group. 
     Still further, in accordance with a preferred embodiment of the present invention, the transmissions include the current position and a data payload. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmissions comprise the current position and a null transmission. 
     Moreover, in accordance with a preferred embodiment of the present invention, the adjusting also includes synchronizing the current transmission group in the adjustable schedule in accordance with the current transmission group in the current position, and synchronizing the current transmission slot in the adjustable schedule in accordance with the current transmission slot in the current position. 
     Further, in accordance with a preferred embodiment of the present invention, the current position also contains indications of each other the transmission groups and current transmission start slots within the transmission groups. 
     Still further, in accordance with a preferred embodiment of the present invention, the adjusting also includes synchronizing each the transmission group in the adjustable schedule in accordance with each the transmission group in the current position, and synchronizing each current transmission slot in each transmission group in the adjustable schedule in accordance with the current position 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmitting also includes transmitting the current position and at least one of an ACK or RTS. 
     There is also provided, in accordance with a preferred embodiment of the present invention, a network node including a receiver to receive an adjustable schedule of transmission start slots, where the transmission start slots represent transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices on a network, and a transmitter to transmit a current position representing all current transmission groups for resynchronization to the adjustable schedule by at least one other the network node. 
     Further, in accordance with a preferred embodiment of the present invention, the transmission start slots in the adjustable schedule are scheduled in the transmission groups, each transmission group associated with a group of the network devices, and the current position contains at least an indication of a current the transmission group and a current the transmission start slot within the current transmission group. 
     Still further, in accordance with a preferred embodiment of the present invention, the transmitter also includes means to transmit the current position with a data payload. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmitter also includes means to transmit the current position with a null transmission. 
     Moreover, in accordance with a preferred embodiment of the present invention, the current position also contains indications of each other the transmission groups and current transmission start slots with the transmission groups. 
     Further, in accordance with a preferred embodiment of the present invention, the transmitter also includes means for transmitting the current position and at least one of an ACK or RTS. 
     There is also provided, in accordance with a preferred embodiment of the present invention, a network node including a receiver to receive an adjustable schedule of transmission start slots, where the transmission start slots represent transmission initiation opportunities for the initiation of data transmission by at least one of a plurality of network devices in a network, a listening to listen for transmissions by other the network nodes according to the received schedule, the transmissions at least indicating a current position for all transmission groups within the adjustable schedule, and an adjuster to adjust the received schedule in accordance with the current position. 
     Further, in accordance with a preferred embodiment of the present invention, the transmission start slots in the adjustable schedule are scheduled in the transmission groups, each transmission group associated with a group of the network devices, and the current position contains at least an indication of a current the transmission group and a current the transmission start slot within the current transmission group. 
     Still further, in accordance with a preferred embodiment of the present invention, the transmissions include the current position and a data payload. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmissions include the current position and a null transmission. 
     Moreover, in accordance with a preferred embodiment of the present invention, the adjuster also includes means for synchronizing the current transmission group in the adjustable schedule in accordance with the current transmission group in the current position, and means for synchronizing the current transmission slot in the adjustable schedule in accordance with the current transmission slot in the current position. 
     Further, in accordance with a preferred embodiment of the present invention, the current position also contains indications of each other the transmission groups and current transmission start slots within the transmission groups. 
     Still further, in accordance with a preferred embodiment of the present invention, the adjusting also includes means for synchronizing each the transmission group in the adjustable schedule in accordance with each the transmission group in the current position, and means for synchronizing each the current transmission slot in the each transmission group in the adjustable schedule in accordance with the current position. 
     Additionally, in accordance with a preferred embodiment of the present invention, the transmitter also includes means for transmitting the current position and at least one of an ACK or RTS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         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; 
         FIG. 1B  is a timing diagram illustration for an exemplary transmission cycle for the schedule of  FIG. 1 ; 
         FIG. 2A  is an illustration of a novel transmission start slot TDMA (time division multiple access) contention TXOP schedule, designed and operative in accordance with a preferred embodiment of the present invention; 
         FIGS. 2B-E  are illustrations of timing diagram for exemplary transmission cycles operating in accordance with the schedule of  FIG. 2A ; 
         FIG. 3A  is an illustration of a novel transmission start slot TDMA (time division multiple access) contention TXOP schedule, designed and operative in accordance with a preferred embodiment of the present invention; and 
         FIG. 3B  is an illustration of a timing diagram for an exemplary transmission cycle operating in accordance with the schedule of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     While the HPNA network described in the Background is designed to run on telephone lines, other implementations are also possible. For example, HPNA 3.1 may also be implemented on home power lines. It will be appreciated that power lines are not ideal data carriers. They are designed to provide electrical current as needed, and accordingly, they are subject to frequent surges and other possible causes of interference. As media for data transmission they are inherently noisier than telephone lines and/or dedicated data lines. 
     In such a noisy environment, network synchronization may be lost if one or more of the network nodes miss a transmission over the network due to the intermittent noise that may be typical on power lines. In another scenario, loss of synchronization may occur when noise on the line is incorrectly interpreted by a node as a valid data transmission. An unsynchronized node may then mistakenly identify a transmission opportunity associated with a different node as its own opportunity to transmit, and collisions may occur, increasing the packet error rate (PER). This undesirable situation could continue for a relatively long period of time until a new media access plan (MAP) is publicized and the network nodes are resynchronized. 
     In the presence of such interference, PCS be problematic when used for powerline media and other methods may be required to maintain synchronization. In accordance with a preferred embodiment of the present invention, timed-reception may be implemented as an alternative to PCS, and the CSMA sub-burst slots media access method may be modified accordingly to achieve a “virtual carrier sensing” (VCS) mode. 
     Reference is now made to  FIGS. 2A and 2B .  FIG. 2A  illustrates a novel transmission start slot TDMA (time division multiple access) contention TXOP schedule  110 , designed and operative in accordance with a preferred embodiment of the present invention.  FIG. 2B  illustrates timing diagram  115  for an exemplary transmission cycle operating in accordance with transmission start slot schedule  110 . 
     As shown in  FIG. 2A , schedule  110  may comprise a multiplicity of transmission start slots  120 , herein labeled from 0 to 4. Each transmission start slot  120  has a scheduled minimum duration of d′. It will be appreciated that as will be described hereinbelow, d′ may be slightly longer in duration than duration d in  FIG. 1A . Unlike the prior art, transmission start slots may have a longer duration than a minimal transmission burst duration 
     In accordance with a preferred embodiment of the present invention, nodes with nothing to transmit may not remain silent as in the prior art. Instead, each such node may transmit a short frame with a null indication when it has no data to transmit.  FIG. 2B  illustrates a timing diagram  115  for an exemplary transmission cycle based on transmission start slot schedule  110 . 
     Transmission start slots  120  from  FIG. 2A  may be replaced by transmission frames  220  in  FIG. 2B . Each transmission frame  220  may comprise a preamble  222  and a gap  224 . A preamble  222  may represent an analog signal transmitted by a node at the beginning of a transmission frame  220 . Gaps  224  may represent a period of non transmission “silence” at the end of a transmission frame. Gaps  224  may serve to demark the end of a frame  224  and make it easier for nodes to detect a subsequent preamble  222 . 
     Transmission frame  220 C (corresponding to transmission start slot  120 C) represents a transmission of a node with a data payload to transmit. When a node has data to transmit, a duration  240  may be added as a part of a frame control (FC) with preamble  222 . The FC may then be followed by data payload  250  and gap  224 . Duration  240  may specify a new d′ for the associated transmission start slot  120 . For example, in  FIG. 2A , transmission start slot  120 C may have a scheduled duration of d′. However, duration  240  may indicate that the expected duration for transmission frame  220 C may now be d′ plus the expected duration of a transmission of data payload  250 . A grid scheduler on the receiving node may use this information to update the grid of transmission opportunity start times originally represented by schedule  110 . 
     Null symbols  230  may be used to indicate that a node has no data payload to transmit. Null symbols  230  may be included as a type field in, or in place of an FC. As shown for exemplary transmission frames  220 A,  220 B,  220 D and  220 E corresponding to transmission start slots  120 A,  120 B,  120 D and  120 E from  FIG. 2A , such nodes may indicate the transmission of “null frames” by transmitting a null  230  between preamble  222  and gap  224 . It will be appreciated that other nodes may interpret a null frame as an indication that the transmitting node may have no data payload to transmit. In such a case it may be expected that a current transmission frame  220  may be of a standard length of d′ and no other signal processing may be necessary until d′ duration may have passed. In accordance with an alternative preferred embodiment of the present invention, a null frame may also comprise a duration  240 . 
     Nodes on the network may therefore effectively “ignore” any ensuing transmissions until d′ duration may have passed. Any transmissions received may be assumed to be random interference on the line, and accordingly may be ignored without requiring any interpretation or processing. In accordance with an alternative preferred embodiment of the present invention, during this time the nodes may enter an energy conservation mode by turning off their receivers. 
     It will be appreciated that the present invention may enable a receiving node to calculate when a next transmission frame  220  may be expected without having to rely on PCS. VCS may be used instead to determine exactly when a next frame may be expected. 
     It will be appreciated that transmission frames  220  as shown in  FIG. 2B  may represent a simplified representation of the components of a typical transmission frame  220 . For example, in accordance with a preferred embodiment of the present invention, the FC for transmissions with data payloads such as transmission frame  220 C may also comprise fields indicating the “current position” of the transmission within the grid of timeslot opportunities as represented in schedule  110 . Such information on “current position” may be used by a node to resynchronize to the grid as necessary. 
     Reference is now made to  FIG. 2C  which illustrates exemplary timing diagram  125 , a more detailed view of the exemplary timing diagram  115  of  FIG. 2B . Null transmission frames  220 A, B, D and E may generally be the same as in timing diagram  115 . However, transmission frame  220 C may comprise two additional elements: group  242  and slot  245 . It will be appreciated that group  242  and slot  245  may together indicate a “current position” for the transmitting node. 
     For example, if line interference causes a node to lose synchronization, it may listen for a next transmission frame  220  with a data payload  250  such as frame  220 C. When frame  220 C may be received, a “lost” node may resynchronize to the grid using the information in the current position fields. “Current position” information may include, as shown in exemplary timing diagram  125 , group and slot information as per the current schedule  110 . Accordingly, the lost node may resynchronize within a given group when a data payload transmission (such as represented by frame  220 C) is received from a node in that given group. It will be appreciated that the elements shown as part of the “current position” may be exemplary; the present invention may be implemented with other such additional fields in the FC as necessary. 
     It will be appreciated, however, that the amount of “current position” information in a transmission frame  220  may be limited. For example, the FC for transmission frame  220 C may comprise group  242  and slot  245  values for the currently transmitted frame  220 , but may not include information regarding other groups. Accordingly, while a non-synchronized node may use such current position information to resynchronize with the schedule for the group of nodes currently transmitting, it may not be able to synchronize with the schedule for the other groups that may follow as per schedule  110 . 
     In accordance with a preferred embodiment of the present invention, null transmissions may be expanded to include group and slot information in an FC. Reference is now made to  FIG. 2D , which illustrates an exemplary timing diagram  135 , also per schedule  110 , in accordance with another preferred embodiment of the present invention. As shown, any transmission frame  220  may also include current position information, regardless of whether it maybe a null transmission, such as represented by frames  220 A, B, D and E, or a data payload transmission such as represented by  220 C. Accordingly, a non synchronized node may synchronize to a current group and slot position by detecting any single transmission frame  220  from that group, regardless of whether or not the detected transmission may be full payload transmission. A non synchronized node may therefore resynchronize with a current group even if none of the nodes in the group may be currently transmitting data payloads. Accordingly, the present invention may enable resynchronization to all groups in a relatively short period of time, regardless of the level of data traffic on the network. 
     It will be appreciated that timing diagram  135  may be exemplary; the current invention may also be configured in such a manner that only some, but not all, null transmissions may include current position information. For example, in accordance with another exemplary preferred embodiment, only null transmissions in the first slot  120  in a given group may include current position information. Alternatively, a percentage of nodes transmitting null transmissions with current position information may be set to be a function of the reliability and quality of the available bandwidth 
     Reference is now made to  FIG. 2E  which illustrates exemplary timing diagram  145 . Timing diagram  145  may be generally similar to timing diagram  135 . However, in accordance with another preferred embodiment of the present invention, transmission frame  220 A which may represent a null transmission, may comprise expanded current position information which may represent the node&#39;s current view of the current position of all scheduled transmission groups. For example, the current MAP may include time slots  120  scheduled for three groups. The node transmitting frame  220 A may provide its own current position information in group  242 A and slot  245 , thus enabling a non synchronized node to synchronize with the current group. Frame  220 A may also comprise groups  242 B and C as well as slots  245 B and C which may represent the current position for the other two groups, thus enabling a non synchronized node to synchronize with the entire schedule based on a single received transmission. 
     It will be appreciated that the number of groups  242  represented in frame  220 A may be exemplary; the present invention may include any number of scheduled groups. Furthermore, it will also be appreciated that the present invention may also include other configurations for the percentage and/or order of null transmissions with expanded current position information. 
     In accordance with an exemplary preferred embodiment of the present invention, other non null transmission frames  220  without data payloads may also include current position information. Such non null transmission frames  220  without data payloads may include, for example, acknowledgement (ACK) and request to send (RTS) frames. Such frames may also be sufficiently short in length to facilitate the addition of current position and/or expanded current position information as necessary. Accordingly it will be appreciated the embodiments of  FIGS. 2D and 2E  may include frame types such as ACK and RTS in addition to, or even instead of null transmission frames. 
     It will be appreciated that TDMA transmission start slots media access may be less efficient than the prior art. The overhead required to transmit a null  230  along with preamble  22  and gap  224  may increase the duration of a null transmission frame  220  vis-à-vis a silent frame as represented in  FIG. 1B . Such increased duration may reduce the bandwidth available for the transmission of data. 
     In accordance with an alternative preferred embodiment of the present invention, the duration of transmission cycles  220  may be reduced by removing nulls  230 . Reference is now made to  FIGS. 3A and 3B .  FIG. 3A  illustrates a novel transmission start slot TDMA (time division multiple access) contention TXOP schedule  310 , designed and operative in accordance with a preferred embodiment of the present invention.  FIG. 3B  illustrates timing diagram  315  for an exemplary transmission cycle operating in accordance with transmission start slot schedule  310 . 
     As shown in  FIG. 3A , schedule  310  may comprise a multiplicity of transmission start slots  320 , analogous to transmission start slots  120  in the embodiment of  FIG. 2A . However, each transmission start slot  320  may have a scheduled minimum duration of t. As will be described hereinbelow, t may be shorter in duration than duration d′ in  FIG. 2A . 
     Transmission frames  420  in  FIG. 3B  may generally correspond to transmission start slots  320  in  FIG. 3A . As in the previous embodiment, each transmission frame  420  may comprise a preamble  222 . Durations  340  and data payloads  350  may also be used as in the previous embodiment. For example, transmission frame  420 C, including a preamble  322 , a duration  340  and gap  324 , may represent a transmission from a node with data to transmit. 
     However, transmission frames  420  from nodes with no data payloads to transmit may be different than in the previous embodiment. Instead of transmitting a null  230  ( FIG. 2B ) to indicate a “null” transmission, such nodes may instead transmit a preamble  325 . Preamble  325  may be an analog signal sufficiently different from preamble  322  to be recognized by receiving nodes as a different type of preamble. When a preamble  325  may be received, receiving nodes may interpret it to signal a null transmission without explicitly requiring a null  230  to be actually transmitted as well. 
     It will be appreciated that by eliminating the transmission of nulls  230 , the duration of transmission frames  420  may be generally shorter than transmission frames  220 . It will further be appreciated that transmission start slots  320  may be of shorter duration than transmission start slots  120 . Duration t may be expected to be shorter than duration d′. An exemplary value of t may be equal to d, as in the prior art. Therefore, in accordance with a preferred embodiment of the present invention, schedule  310  may be of generally the same duration as schedule  10 . Transmission start slots  320  may be sub-burst slots with a shorter duration than a minimal transmission burst duration 
     It will be appreciated that by eliminating the transmission of nulls  230 , it may not be possible for non synchronized nodes to resynchronize using “current position” included in such transmissions. In accordance with an alternative preferred embodiment of the present invention, if a node loses synchronization, it may use PCS to detect a transmission frame from its own group. Once synchronization with its own group may be established, it may wait for its assigned time slot  120  and transmit a null transmission with a presumed expanded current position (as in the embodiment of  FIG. 2E ) and the other nodes on the network may synchronize according to the transmitted current position. It will be appreciated that the nodes on the network may synchronize with the most recently transmitted expanded current position, regardless of whether or not it may be the “true” current position as originally defined in schedule  110 . Facilitating synchronization may be of higher priority than determining whether or not the current position accurately reflects a timing diagram as per original schedule  110 . 
     It will also be appreciated that, in the context of the embodiment of  FIG. 3  where explicit null transmissions may not be regularly transmitted, expanded current position information may be occasionally transmitted by synchronized nodes as well. In accordance with a preferred embodiment of the present invention, nominally synchronized nodes that may not require resynchronization may from time to time transmit null transmissions with expanded current position information. Receiving nodes may resynchronize in accordance with the expanded current position without regard to a “true” current position. It will be appreciated that in such manner, the likelihood for synchronization across the network may increase. 
     As discussed hereinabove, ACK and RTS frames may be used in addition to, or instead of null transmissions  220  as an instrument for synchronizing nodes to a current schedule  110 . Accordingly, it will be appreciated that expanded current position information may be added to non null, transmission frames  220  without data payloads, such as, for example, ACK and RTS frames. 
     In any case, it will be appreciated that if no other option may be feasible, a node may resynchronize when a new MAP is transmitted after the end of a transmission cycle  115 . 
     It will be appreciated that the specification of a powerline medium is exemplary. The present invention may be implemented on any data network. It will further be appreciated that the specification of HPNA v3.1 may also 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. 
     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.