Patent Publication Number: US-9413550-B2

Title: Collision avoidance media access method for shared networks

Description:
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) v3.0, which is designed to work over existing telephone lines to create a home/small office network. U.S. Pat. No. 7,408,949, issued Aug. 5, 2008 and assigned to the common assignee of the present invention, describes generally how to extend the HomePNA v3.0 standard to operate over such a hybrid network of telephone and coax lines. 
     HPNA v.3 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. 
     Reference is now made to  FIG. 1 , which depicts a prior art data network  10 , comprising at least two network devices  12  and  14 , connected to computers. Network device  12  comprises a modem  16  which includes, among other items, a carrier sensor  20  and a transceiver  24 . Network device  14  comprises a modem  18  which includes, among other items, a carrier sensor  20 , a scheduler  22  and a transceiver  24 . Scheduler  22  creates and sends to each device on network  10  a media access plan (MAP) at the beginning of each transmission cycle. Transceiver  24  either transmits, or both transmits and receives data transmissions over network  10 . 
     An exemplary timing diagram  40  for an exemplary transmission cycle of network  10  ( FIG. 1 ) is shown in  FIG. 2 , reference to which is now made. Timing diagram  40  shows a detailed schedule of future transmission opportunities (TXOPs) that are made available to specific network devices in the upcoming transmission cycle at specific and non-overlapping times. The start time and length of each scheduled TXOP in the upcoming transmission cycle, such as TXOPs  44 ,  48  and  50  shown in  FIG. 2 , as well as the network device to which each TXOP is assigned, is scheduled by scheduler  22  ( FIG. 1 ) in the MAP for the upcoming transmission cycle. The transmission cycle is then initiated, as shown in  FIG. 2 , with the publication of the MAP by scheduler  22  to the network devices on network  10  ( FIG. 1 ) during MAP publication transmission  30 . For example, as shown in timing diagram  40  of  FIG. 2 , TXOP  44  is shown to be the first TXOP and may be assigned to device  1 , TXOP  48  is shown to be the second TXOP and may be assigned to device  2 , and TXOP  50  is shown to be the third TXOP and may be assigned to device  3 . As shown in timing diagram  40 , the MAP for the represented transmission cycle, also includes a scheduled registration TXOP  54  during which new devices may ask to join network  10 . 
     After publication of the MAP during MAP publication transmission  30 , device transmissions may begin. Each device recognizes a particular TXOP that has been assigned to it according to the MAP, and either utilizes the TXOP or passes on it. 
     In timing diagram  40  shown in  FIG. 2 , it may be seen that device  1  utilizes TXOP  44 , as illustrated by hatched area  56  indicating transmission activity of device  1  during TXOP  44 . However, if, as shown, devices  2  and  3  do not use TXOPs  48  and  50 , these assigned portions of bandwidth are wasted. Furthermore, if no new devices use registration TXOP  54  for registering, the bandwidth of TXOP  54  is also wasted. 
     As can be seen, the prior art MAP wastes significant resources when scheduled TXOPs for transmission and registration are not fully utilized. Due to the predetermined sizes of the TXOPs, which, at a minimum, are required to be sufficiently large to accommodate at least one data frame, the method suffers from inefficient bandwidth utilization and high per device overhead. Predetermined TXOP size per cycle also means that adaptation of bandwidth change is slow and complex. Small TXOPs relative to transmission burst size may cause Head-Of-Line (HOL) Blocking. Scalability suffers as the network capacity drops with increasing network size. For bi-directional protocols such as TCP and TFTP only low data rates may be achieved due to long round-trip time (RTT). 
    
    
     
       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. 1  is a schematic illustration of a prior art data network; 
         FIG. 2  is a timing diagram illustration for an exemplary transmission cycle of the network shown in  FIG. 1 ; 
         FIG. 3  is a schematic illustration of a data network, constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 4  is a timing diagram illustration for an exemplary transmission cycle for an exemplary sub-burst slot media access plan (SBSMAP) constructed in accordance with a preferred embodiment of the present invention and operative in the network shown in  FIG. 3 ; 
         FIG. 5  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring an alternative group scheduling scheme; 
         FIG. 6  is a comparative timing diagram illustration of a repeated type group scheduling scheme and a rotated type group scheduling scheme; 
         FIG. 7  is an illustration of a tabular representation of an exemplary SBSMAP constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 8  is an illustration of an exemplary Group_Type table in which parameters used to define the SBSMAP of  FIG. 7  are catalogued; 
         FIG. 9  is an illustration of a tabular representation of an exemplary alternative SBSMAP having explicit group separators; 
         FIG. 10  is a timing diagram illustration of an exemplary transmission cycle for the SBSMAP of  FIG. 9 ; 
         FIG. 11  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring data packet priority-based media access opportunity allocation; 
         FIG. 12  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring need-responsive data packet priority-based media access opportunity allocation; 
         FIG. 13  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring bandwidth requirement-based media access opportunity allocation; 
         FIG. 14  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring media access starvation prevention; 
         FIG. 15  is a timing diagram illustration for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring bandwidth requirement-based media access opportunity allocation like that provided by the SBSMAP of  FIG. 13 , and starvation prevention like that provided by the SBSMAP of  FIG. 14 ; 
         FIG. 16  is an illustration of a tabular representation of an exemplary SBSMAP in accordance with which media access may be provided to network flows in an advanced quality of service scheme; 
         FIGS. 17A and 17B  are timing diagram illustrations for an exemplary transmission cycle for an exemplary alternative SBSMAP featuring the implementation of Next Map opportunities; 
         FIG. 18  is a timing diagram illustration for exemplary transmission cycles for exemplary SBSMAPs implemented in a multi-tenant unit (MTU) or a multi-dwelling unit (MDU); 
         FIG. 19A  is a timing diagram illustration for an exemplary transmission cycle for a TCP protocol running on the network shown in  FIG. 1 ; and 
         FIG. 19B  is a timing diagram illustration for an exemplary transmission cycle for an exemplary SBSMAP implemented for a TCP protocol. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     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. 
     The present invention may extend media access methods using carrier-sensing and collision avoidance techniques to allow multiple network devices to perform collision-free burst-like media access within shared transmission opportunities (TXOPs). For example, the present invention may be used to extend the media access method of HomePNA (Home Phone Network Alliance) v3.0. It will be appreciated that for HomePNA and other media access methods in which collision detection is feasible, it may be preferable to use collision avoidance due to other benefits which it entails. 
     In order to extend media access methods using carrier-sensing and collision avoidance techniques to allow multiple network devices to perform collision-free burst-like media access within shared TXOPs, the present invention may replace fixed-size, non-overlapping TXOPs with a grid of appreciably smaller time slots, which may each represent an opportunity for the initiation of a data transmission by a network device. The appreciably smaller time slots may be sub-burst slots, i.e., the time allotted to each slot may be shorter than a minimal transmission burst duration. It will be appreciated that the principal advantage provided by minimally sized time slots such as sub-burst slots may be that when a network device does not use its assigned time slot transmission opportunity, minimal time may be wasted before the opportunity to transmit is passed to the next device in the queue. 
     Reference is now made to  FIG. 3 , which depicts a collision avoiding data network  100  constructed and operative in accordance with the present invention. Data network  100  may comprise at least two network devices  102  and  104 , connected to computers. Network device  102  may comprise a modem  106  which may include, among other items, a carrier sensor  110 , which may be similar to prior art carrier sensors, a transceiver  114 , which may be similar to prior art transceivers, and a sub-burst slot (SBS) grid aligner  116 . Network device  104  may comprise a modem  108  which may include, among other items, a carrier sensor  110 , a transceiver  114 , a SBS grid aligner  116  and a sub-burst slot scheduler  112 . Sub-burst slot scheduler  112  may create and send to each device on network  100  a media access plan having sub-burst slots (SBSMAP) at the beginning of each transmission cycle. SBS grid aligner  116  may realign the sub-burst slot grid according to the SBSMAP advertised by scheduler  112 , in accordance with the occurrence of transmissions. 
     It will appreciated that the configuration shown in  FIG. 3  for network  100  is exemplary, and that devices  102  and  104  which are illustrated in  FIG. 3  as stand-alone devices, may operate as stand-alone devices or as integrated components in other devices. For example, in alternative configurations of network  100  constructed and operative in accordance with a preferred embodiment of the present invention, either or both of devices  102  and  104  may be integrated into a personal computer, a set-top box, a television, a PVR or other device. 
     Like the prior art MAP, the SBSMAP provided by the present invention may describe a schedule of transmission opportunities (TXOPs) to each device on network  100 . However, unlike the prior art MAP, the SBSMAP provided by the present invention may not exclusively reserve super-burst sized portions of the TXOP for particular network devices or data flows. Rather, each scheduled sub-burst slot may constitute an opportunity of brief duration for the device or data flow with which it is associated to begin transmitting. Each sub-burst slot may be much shorter in length than the predetermined, super-burst sized TXOPs of the prior art MAP. 
     It will be appreciated that the method for media access provided by the present invention may waste less bandwidth than the prior art due to the brief duration of the sub-burst slots provided by the present invention, which if not used for initiating a transmission, may comprise a much smaller amount of wasted bandwidth than unused prior art TXOPs. 
     Reference is now made to  FIG. 4  which shows an exemplary timing diagram  200  for an exemplary transmission cycle of network  100  ( FIG. 3 ). As shown in  FIG. 4 , the transmission cycle is initiated with the publication of the SBSMAP by SBS scheduler  112  to the network devices on network  100  ( FIG. 3 ) during SBSMAP publication transmission  230 . As shown in  FIG. 4 , a multiplicity of sub-burst slots SBS n  are scheduled in shared TXOP  205  in accordance with the SBSMAP. In accordance with a preferred embodiment of the present invention, each sub-burst slot SBS n  may represent an opportunity for the network device or data flow associated with that sub-burst slot to initiate a transmission. Collectively, the sub-burst slots scheduled in accordance with the SBSMAP may form a grid of transmission opportunity start times. In the example shown in  FIG. 4 , sub-burst slots SBS 1 -SBS 23  form a grid of transmission opportunity start times, initialized at the beginning of shared TXOP  205 . 
     In accordance with a preferred embodiment of the present invention, each sub-burst slot SBS n  in the grid may serve as a placeholder, reserving the opportunity for its associated network device or data flow to transmit at the time which the sub-burst slot occupies in the sequence of sub-burst slots in the grid. For example, as shown in  FIG. 4 , the first opportunity to transmit is reserved for the network device or data flow associated with SBS 1 . The network device or data flow associated with SBS 1  may act on this opportunity to transmit, or pass on it. 
     In order for a network device to act on an opportunity to transmit, it must initiate its transmission at the beginning of its associated sub-burst slot. The duration of sub-burst slots in accordance with a preferred embodiment of the present invention may be 8-32 μsec and the window for acting upon the opportunity to initiate a transmission may be the first 2-4 μsecs of the sub-burst slot. It will be appreciated that the duration of each sub-burst slot in accordance with a preferred embodiment of the present invention may be a variable parameter, which may be varied in a fixed scheme or a dynamic scheme. For example, sub-burst slot duration may be varied according to the characteristics of the devices which may participate in the network. 
     In the event that the transmission opportunity provided by a sub-burst slot SBS n  is not utilized by the network device or data flow associated with it, the network device or data flow associated with the next sub-burst slot SBS (n+1)  in the grid sequence may then be given the opportunity to transmit. 
     Returning now to  FIG. 4 , it may be seen in the illustrated example that no transmissions occur during shared TXOP  205  until the fourth transmission opportunity allotted to network device DEV- 1 , which is associated with the sub-burst slot numbered “0” in the figure. As shown in transmission activity diagram  212  for network device DEV- 1 , network device DEV- 1  passes on the transmission opportunities presented by the sub-burst slots SBS 1 , SBS 5  and SBS 9  numbered “0”, and transmits for the first time during sub-burst slot SBS 13 . During SBS 13  network device DEV- 1  begins transmitting, and occupies the transmission medium until its transmission is complete. It will be appreciated, therefore, as illustrated in  FIG. 4 , that when utilized for a transmission, the sub-burst slots provided by the present invention may be expandable, expanding to allow the completion of the transmission begun by the network device during the sub-burst slot. The lengths to which the sub-burst slots may expand may be unlimited, or they may be limited. The end of a shared TXOP may constitute a limit to the expansion of a sub-burst slot during a transmission. Alternatively, a transmission may extend the length of a shared TXOP. 
     As described hereinabove, at the beginning of each cycle, the network devices on network  100  may receive a schedule of transmission opportunities, i.e. an SBSMAP detailing a lineup of sub-burst slots. Transmissions may then occur, such as the exemplary transmissions shown in  FIG. 4  during sub-burst slots SBS 13 , SBS 17  and SBS 23 . The end of a transmission may be identified by an Inter Frame Gap (IFG)  202 , such as those shown in  FIG. 4  succeeding the transmissions during sub-burst slots SBS 13  and SBS 17 . 
     When transmissions occur, SBS grid aligner  116  ( FIG. 3 ) in each network device on network  100  may recalculate the timing of the sub-burst slot grid so that each network device may know at what time its next transmission opportunity is scheduled. While the order of the sub-burst slots in the grid is known because that information is provided to each network device in the advertised SBSMAP, the timing of the grid is changed due to, and in direct accordance with, the duration of the transmissions which occur. 
     It may further be seen in  FIG. 4  that while the sub-burst slots numbered “0” are associated with network device DEV- 1 , the sub-burst slots numbered “1” are associated with network device DEV- 2 , and the sub-burst slots numbered “3” are associated with the network device DEV- 4 . This may be seen in the transmission activity diagrams  214  and  216 , which show transmission activity for network devices DEV- 2  and DEV- 4  respectively. It is shown that after passing on its first 5 transmission opportunities provided by sub-burst slots SBS 2 , SBS 6 , SBS 10 , SBS 15  and SBS 19  numbered “1”, network device DEV- 2  transmits during SBS 23 . It is also shown that network device DEV- 4  passes on its first three transmission opportunities (SBS 4 , SBS 8  and SBS 12 ) and then transmits during SBS 17 . 
     In the example shown in  FIG. 4  each network device numbered N is associated with the sub-burst slots numbered N−1. It may therefore also be understood from  FIG. 4  that the sub-burst slots numbered “2” are associated with network device DEV- 3  which does not transmit at all during TXOP  205 . However, it will be appreciated that in accordance with a preferred embodiment of the present invention, a particular sub-burst slot may be associated with any network device or data flow, and that the identity of the network device or data flow associated with a particular sub-burst slot may be encoded in the SBSMAP, as will be explained later in further detail with respect to  FIG. 7 . 
     In accordance with an additional preferred embodiment of the present invention, sub-burst slots may be organized in modular groups, and these modular group units may be assembled into group sequences of one or more groups and scheduled in the SBSMAP. A group may consist of a particular sequence of sub-burst slots, each of which is associated with a particular network device or data flow. Each group may also be associated with a particular one of a plurality of group types. In accordance with a preferred embodiment of the present invention, group types may dictate scheduling protocol in various instances. For example, in the event of the interruption of a group sequence by a transmission, SBS scheduler  112  ( FIG. 3 ) may determine which sub-burst slot may be scheduled after the transmission in accordance with the group type of the interrupted group. Scheduling schemes for the repetition, rotation or omission of a group or sequence of groups in a TXOP may also be dictated by group type. 
     Group affiliation, i.e., which sub-burst slots comprise a group of which type, in addition to network device and data flow association, may also be encoded in the SBSMAP, in accordance with a preferred embodiment of the present invention, as will be discussed later in further detail with respect to  FIG. 7 . 
     In the example shown in  FIG. 4 , TXOP  205  is scheduled with successive repetitions of group Fx, a fixed type group comprising the sequence of sub-burst slots “0,1,2,3”, where each sub-burst slot is associated with a network device according to the formula N+1, where N is the number of the sub-burst slot. As shown in  FIG. 4 , the scheduling scheme dictated by a fixed group, in accordance with a preferred embodiment of the present invention, is that a transmission constitutes an interruption of the continuity of the current group sequence, and that after the transmission, the scheduled lineup begins with the first sub-burst slot in the group. This may be seen in  FIG. 4  when the transmission during SBS 13  interrupts the fourth repetition of group Fx, Fx 4 , and the fifth repetition of group Fx, Fx 5 , beginning with sub-burst slot “0”, is scheduled after the transmission. 
     An alternative scheduling scheme, as dictated by a different group type, is shown in  FIG. 5 , reference to which is now made.  FIG. 5  shows timing diagram  300  for an exemplary SBSMAP constructed and operative in accordance with a preferred embodiment of the present invention, similar to timing diagram  200  shown in  FIG. 4 . Similar elements in  FIGS. 4 and 5  are referred to by corresponding reference numerals. 
     In  FIG. 5 , the basic modular group unit R, of which successive repetitions are scheduled in shared TXOP  305 , comprises a sequence of sub-burst slots “0,1,2,3”, which is identical to the sequence of sub-burst slots of group Fx in  FIG. 4 . However, because they are associated with different group types, group Fx and group R dictate different scheduling schemes in shared TXOPs  205  and  305  respectively. 
     In the example shown in  FIG. 5 , three scheduled rounds of group R, R 1 , R 2  and R 3  pass with no transmissions occurring. During sub-burst slot SBS 13  numbered “0” in group R 4 , network device DEV- 1  transmits. As shown in  FIG. 5 , the scheduling protocol associated with group R dictates that after a transmission, the group sequence continues uninterrupted. Thus, the next sub-burst slot scheduled, SBS 14  is shown to be numbered “1”. It may further be seen in  FIG. 5  that the remainder of group R 4 , and similarly, group R 5 , are scheduled fully without regard for ‘interruptions’ caused by transmissions. 
     Reference is now made to  FIG. 6  which illustrates two additional exemplary group types provided in accordance with a preferred embodiment of the present invention, the repeated group type (R 0 ) and the rotated group type (R 1 ). In accordance with a preferred embodiment of the present invention, both the rotated and repeated group types share the post-transmission scheduling characteristics of group R, described hereinabove with respect to  FIG. 5 . That is, after a transmission, the scheduling of the group sequence continues uninterrupted, unlike in the case of fixed groups, described hereinabove with respect to  FIG. 4 . However, the rotated and repeated groups dictate different schemes for the continued scheduling of a group or sequence of groups in a TXOP. 
       FIG. 6  shows two exemplary timing diagrams  600  and  620 . It may be seen in timing diagram  600  that the schedule of groups scheduled by an SBSMAP for TXOP  605  comprises the initial modular sequence  602  of the groups “Fx 0 , R 0 , R 1 ”, where group Fx 0  comprises the sequence of sub-burst slots “0,1,2”, group R 0  comprises the sequence of sub-burst slots “0,1,2,3”, and group R 1  comprises the sequence of sub-burst slots “0,1,2,3,4”. In accordance with a preferred embodiment of the present invention, the type of the last group in initial modular sequence  602  may determine which group or groups may be scheduled for the remainder of the duration of the TXOP. For example, as shown in  FIG. 6 , the last group in initial modular sequence  602  is R 1 , a rotated type group. Accordingly, as per the TXOP remainder scheduling scheme associated with the rotated group type, the sequence of groups in initial modular sequence  602 , i.e. “Fx 0 , R 0 , R 1 ”, is rescheduled in REP 1  in TXOP  605 , as shown in  FIG. 6 . 
     In timing diagram  620 , the constituent groups of initial modular sequence  622  are the groups “Fx 0 , R 1 , R 0 ”, such that the only difference between initial modular sequence  602  and initial modular sequence  622  is the alternative positions of groups R 1  and R 0  in the two sequences. More specifically, the difference between the two sequences is that the last group in initial modular sequence  602  is rotated group R 1 , and the last group in initial modular sequence  622  is repeated group R 0 . It is shown in  FIG. 6  that the TXOP remainder scheduling scheme which may be associated with the repeated type group is the continued repetition of the last group in the initial modular sequence of the SBSMAP. It is accordingly shown in  FIG. 6  that repeated group R 0  is repeated in Rep 1 , Rep 2  and for the remainder of TXOP  625 . 
     It will be appreciated that the group type dependent scheduling schemes and protocols provided by the present invention may not be limited to the exemplary scheduling schemes and protocols described hereinabove with respect to  FIGS. 4, 5 and 6 . For example, an additional post-transmission scheduling protocol which may be incorporated into an SBSMAP provided by the present invention may be an alternative fixed scheme, which may be associated with the alternative fixed group type. The post-transmission scheduling protocol dictated by the alternative fixed group may be similar to the fixed group scheme described with respect to  FIG. 4 . However, after a transmission, the first sub-burst slot to be scheduled may not be the first-sub-burst slot in the group. The alternative fixed group type may dictate that the assignment of the first sub-burst slot to be scheduled after a transmission may rotate through the group sub-burst slots in a round-robin manner. 
     Additional TXOP remainder scheduling schemes may include, for example, alternative repeated schemes in which a selection of the groups in the initial modular sequence, rather than just the final group in the sequence may be repeated for the remainder of the TXOP. For example, the group type “repeated_final_2” may dictate the repetition of the last two groups in the initial modular sequence for the remainder of the TXOP. A group type “initial” may dictate that the group or groups associated with the group type “initial” in the initial modular sequence may only be scheduled once in a TXOP, overriding the scheduling instructions of other scheduling schemes pertaining to that group or groups, which might otherwise dictate that it or they be rescheduled. 
     It will be appreciated that the creation of an SBSMAP using groups as modular building blocks whose constituent participants may be chosen for particular effect, and which may be scheduled in the SBSMAP in schemes designed for particular effect may provide a highly flexible and customizable bandwidth allocation method. A significant measure of control over the allocation of transmission opportunities to particular network devices and data flows may thus be provided by the present invention. 
     Reference is now made to  FIG. 7  which shows how the scheduling schemes and protocols described hereinabove may be encoded into an SBSMAP constructed and operative in accordance with a preferred embodiment of the present invention and represented in tabular form. As shown in table  363  in  FIG. 7 , the parameters which may be used to define an SBSMAP in accordance with a preferred embodiment of the present invention may be Row_Number  350 , Device_ID  352 , Group_Type  354 , Flow_ID  356 , Length  358  and TXOP_Number  360 . Row_Number  350  may be a sequential serial number assigned to each row in the SBSMAP table. Device_ID  352  may be an identification number corresponding to a particular network device. Group_Type  354  may be an identification number associated with a particular group type. Flow_ID  356  may be an identification number associated with a data flow. Length  358  may be the duration allocated to a TXOP, and TXOP_Number  360  may be a sequential serial number assigned to each sequential TXOP. 
     In table  363 , which is an example of an implicit SBSMAP table, each row  365  in the table may correspond to a sub-burst slot in the SBSMAP. This may not be the case in an explicit SBSMAP table which will be explained later with respect to  FIG. 9 . 
     In accordance with a preferred embodiment of the present invention, the association of a sub-burst slot with a particular network device and data flow as well as its group affiliation may be encoded in the SBSMAP table, as shown in  FIG. 7 . The association of a sub-burst slot with a particular network participant may be defined by the parameters Device_ID  352  and Flow_ID  356 . A sub-burst slot may thereby be associated with a particular network device, a particular group of network devices, a particular data flow or group of data flows, or a particular data flow or group of data flows restricted to a particular network device or group of network devices. 
     In accordance with a preferred embodiment of the present invention, certain values of Device_ID  352  and Flow_ID  356  may be reserved for internal network control and assignment definition. For example, a Device_ID value of 0 may be reserved for the group of all the network devices, including unregistered devices, while Device_ID values of 1-62 may be reserved for particular network devices or groups of devices. For any Device_ID 1-62, a Flow_ID of 0 may be reserved for network control flows while the Flow_ID values 1-63 may be reserved for particular flows. The Device_ID value of 63 may be reserved for assignment definition. In conjunction with a Device_ID of 63, particular Flow_ID values may be reserved to trigger particular scheduling schemes, as will be discussed later in further detail with respect to  FIGS. 14-17B . 
     Group affiliation, i.e., which sub-burst slots comprise a group of which type may be encoded in the SBSMAP via particular value assignments in Group_Type parameter  354 . Each group type may be associated with a particular Group_Type value. For example, the Group_Type value “4” may be associated with the group type “fixed”, the Group_Type value “5” may be associated with the group type “rotated” and the Group_Type value “6” may be associated with the group type “repeated”. 
     In the example shown in  FIG. 7 , it is shown that the SBSMAP defined by table  363  has two TXOPs T 0  and T 1 . The first TXOP T 0  has a duration of L 0 , in which one group, group j, which is a fixed type group, is scheduled. It may also be seen that group j comprises one sub-burst slot, and since this sub-burst slot is the sole sub-burst slot in TXOP T 0 , its duration is also L 0 . 
     The second TXOP in the SBSMAP defined by table  363  is T 1 , having a length L 1 . In accordance with a preferred embodiment of the present invention, initial modular sequence  369  is scheduled for the duration L 1  in accordance with the scheduling schemes dictated by the SBSMAP parameters. It is shown in  FIG. 7  that initial modular sequence  369  comprises group k, a rotated type group having two sub-burst slots, followed by group l, a repeated type group having four sub-burst slots. It may therefore be understood that after the first scheduling of initial modular sequence  369  in TXOP T 1 , group l will repeat for the duration of the TXOP length L 1 . 
     The SBSMAP provided by the present invention may also include a registration sub-burst slot during which new devices may request to be added to network  100 . In the example shown in  FIG. 7  it may be seen that the sub-burst slot in group j is a registration sub-burst slot, since its Device_ID is 0 and its Flow_ID is 63. In the example shown in  FIG. 7  the exemplary Device_ID value of “0” indicates “all devices”, including unregistered ones, and the exemplary Flow_ID value of 63 indicates a registration sub-burst slot. 
     As provided by the present invention, access to registration sub-burst slots may be contention-based, and collisions may occur. A higher level protocol may handle retries. 
     It will be appreciated that network bandwidth overhead may be appreciably reduced by the replacement of a long registration TXOP with a shorter registration time-slot, such as a sub-burst slot, which may monopolize a minimum of bandwidth in the event that it is not used. 
       FIG. 8 , reference to which is now made, shows an exemplary Group_Type table  370  in which the group types provided by the present invention may be catalogued. In Group_Type table  370  additional descriptive parameters are listed alongside Group_Type values  354 . The parameters provided in table  370  for each Group_Type value, as shown in table  370 , are TX_Policy  372 , Group_Scheme  374  and Description  376 . 
     It may be seen in  FIG. 8  that the TX policy of group type “0” is “state” while the TX policy of group types “4”, “5” and “6” is “edge”. A “state” TX policy indicates that the transmission opportunity having this TX policy is a prior art TXOP during which transmission of data by the network device associated with that TXOP may occur at any time during the TXOP. In contrast, an “edge” TX policy indicates that the transmission opportunity having this TX policy is a sub-burst slot provided by the present invention, during which transmission of data by the network device associated with the sub-burst slot must occur during the initial few μsecs of the sub-burst slot. The term “collision detection (CD) method” included in the description of group type “0” compared with the term “sub-burst (SB) slot method” included in the description of group types “4”, “5” and “6” further conveys that the transmission opportunities associated with a Group_Type of “0” are prior-art TXOPs, while the transmission opportunities associated with group types “4”, “5” and “6” are sub-burst slots provided by the present invention. 
     It will be appreciated that prior-art group type “0” is not irrelevant to table  370 , as might be expected, due to the association of this group type with the prior-art, and the association of table  370  with the present invention, i.e. SBSMAPS comprising groups of sub-burst slots. It will therefore be appreciated that prior-art group type “0” is included in table  370  because an SBSMAP provided by the present invention may include prior-art TXOPs alongside inventive TXOPs containing sub-burst slots. 
     Reference is now made to  FIG. 9  which shows an explicit SBSMAP table  470 , defining an SBSMAP constructed and operative in accordance with an additional preferred embodiment of the present invention. In exemplary explicit SBSMAP table  470 , each row may not correspond to a scheduled sub-burst slot. An additional row, operating as an explicit group separator, may be included in the table after a row pertaining to the final group member of a group, in order to define a duration allotted to that group which may be shorter than the combined duration of all of the sub-burst slots in the group. This shorter duration may only be sufficient for scheduling sub-burst slots for a portion of the members of the group, rather than for all of them. Alternatively, the duration allotted to a group by an explicit group separator may be longer than the combined duration of all of the sub-burst slots in the group. 
     In the example shown in  FIG. 9 , a Device_ID value of 63 and a Flow_ID value of 63 identify rows  472 ,  473 ,  474  and  475  as explicit group separator rows. Accordingly, as shown in table  470 , group j is explicitly allotted a bandwidth duration of D 0 , rather than the default cumulative duration of all of its group member sub-burst slot durations. Similarly, in TXOP T 1 , the first scheduling of group k, k 1 , is explicitly allotted a bandwidth duration of D 1 , group m is explicitly allotted a bandwidth duration of D 2  and the second scheduling of group k, k 2 , is explicitly allotted a bandwidth duration of D 3 . 
     Reference is now made to  FIG. 10  which shows timing diagram illustration  650  for an exemplary transmission cycle for the SBSMAP shown in  FIG. 9 . Diagrams Dj, Dk and Dm in  FIG. 10  clarify sub-burst slot affiliation with groups j, k and m. 
     Exemplary timing diagram  650  shows two repetitions of each of the two TXOPs T 0  and T 1  scheduled in SBSMAP  470  in  FIG. 9 , such that the four TXOPs shown in timing diagram  650  are T 0   1 , T 1   1 , T 0   2  and T 1   2 . 
     In the two schedulings of group j shown in timing diagram  650 , j-T 0   1  and j-T 0   2 , which are scheduled in TXOPs T 0   1  and T 0   2  respectively, it may be seen that duration D 0  allotted to the scheduling of group j is sufficient to allow the scheduling of three out of the four group members of group j. As shown in timing diagram  650 , the first three sub-burst slots in group j, numbered “0”, “1” and “2” are scheduled accordingly in the first scheduling of group j, j-T 0   1 . In the second scheduling of group j, j-T 0   2 , it may be seen that the scheduling sequence begins at the point where the scheduling in j-T 0   1  ended, with the fourth and final group member, associated with the sub-burst slot numbered “3” in group j. Once a full round of the sequence is complete, after the first sub-burst slot scheduled in j-T 0   2 , the sequence is restarted, as shown by the scheduling of sub-burst slot “0” in the second sub-burst slot position in j-T 0   2 . The orderly, rotational procession through the sequence of group members from first to last, then returning to the first to begin again is illustrated in diagram Dj, where the overall sequence of sub-burst slots scheduled for group j in j-T 0   1  and j-T 0   2  may be seen to be “0,1,2,3,0,1”. 
     With respect to durations D 1  and D 3  assigned to the first and second schedulings respectively of group k in TXOP T 1 , and duration D 2  assigned to the scheduling of group m in TXOP T 1 , as shown in table  470  ( FIG. 9 ), it may be seen in the corresponding timing diagram  650  in  FIG. 10  that durations D 1  and D 3  are each sufficient to allow the scheduling of three out of the five group members of group k, and that duration D 2  is sufficient to allow the scheduling of four out of the six group members of group m. The orderly rotational scheduling of the group k and m members, like that of the group j members, is shown in diagrams Dk and Dm respectively. It may be seen in diagram Dk that the overall sequence of sub-burst slots scheduled for group k in k 1 -T 1   1 , k 2 -T 1   1 , k 1 -T 1   2  and k 2 -T 1   2  is “0,1,2,3,4,0,1,2,3,4,0,1”. In diagram Dm it may be seen that the overall sequence of sub-burst slots scheduled for group m in m-T 1   1  and m-T 1   2  is “0,1,2,3,4,5,0,1”. 
     It will be appreciated that sub-burst slots in different groups identified by the same numeral in timing diagram  650 , i.e. the sub-burst slots numbered “2” in groups j and m, may not be associated with the same network participant. A numeral identifying a sub-burst slot in timing diagram  650  may correspond to the position of the sub-burst slot in its group sequence, while the network participant with which the sub-burst slot is associated is the network participant associated with the Device_ID and Flow_ID values associated with that sub-burst slot, as listed in corresponding SBSMAP table  470  ( FIG. 9 ). For example, the sub-burst slot numbered “1” in group m is the second sub-burst slot in the group sequence, after the sub-burst slot numbered “0” and before the sub-burst slot numbered “2”. In SBSMAP table  470  in  FIG. 9 , the second sub-burst slot in group m is seen to be the sub-burst slot associated with row  13  in table  470 . The network participant associated with the sub-burst slot numbered “1” in group m is therefore the network participant associated with Device_ID “1” and Flow_ID “1” as listed in row  13  of table  470 . 
     Reference is now made to  FIG. 11  in which timing diagram  700  is shown for an exemplary transmission cycle for an SBSMAP constructed and operative in accordance with an additional preferred embodiment of the present invention. In this embodiment media access may be allocated to network devices according to the priority of the data packets which will be transmitted by the devices. Accordingly, high priority transmissions may be provided the first media access opportunities, and lower priority transmissions may only subsequently be provided media access opportunities. Any number of priority levels may be used. For example, the eight IEEE 802.1P priority levels may be mapped to fewer than eight media access priorities, e.g. three levels including high (H), medium (M) and low (L). It will also be appreciated that these eight priority levels, or another group of priority levels, may be mapped to a different group of priority levels according to any suitable scheme. 
     It may be seen in  FIG. 11  that successive repetitions of the group comprising the sequence of sub-burst slots “0, 1, 2, 3” are scheduled in shared TXOP  705 . In accordance with this embodiment of the present invention, and as shown in  FIG. 11 , each group scheduled in TXOP  705  is associated with a priority level, allowing the transmission only of data packets having a priority level equal to or exceeding this level during the sub-burst slot opportunities in the group. For example, as shown in  FIG. 11 , the sub-burst slot opportunities in scheduled groups H 1 , H 2 , H 3  and H 4  are associated with a high priority level H, and may only be used for the transmission of high-priority data packets. The sub-burst slot opportunities in scheduled groups M 1  and M 2  are associated with a medium priority level M, and may be used for the transmission of data packets of medium or higher priorities. The sub-burst slots opportunities in scheduled group L 1  are associated with a low priority level L, and may be used for the transmission of data packets of any priority. 
     It will be appreciated that in accordance with this embodiment of the present invention, the order of priority-affiliated groups scheduled in the SBSMAP descends from higher priority to lower priority. This is shown in  FIG. 11  where the first three groups scheduled in TXOP  705  are the high priority group H 1 , the medium priority group M 1  and the low priority group L 1 , in that order. It may further be seen that after one of each group has been scheduled, i.e. after the scheduling of group L 1 , the scheduling sequence returns to the first group in the sequence of groups, i.e. the high priority group H, and thus group H 2  is seen to be scheduled in the position of the fourth scheduled group. 
     The precedential location of higher priority groups in the sequence of groups in the SBSMAP, ensures quantitatively preferential scheduling status to the higher priority groups due to the post-transmission scheduling scheme provided in this embodiment, which, after a transmission, returns the opportunity to transmit to the first group in the sequence of groups. As shown in  FIG. 11 , after the transmission during sub-burst slot “0” in group H 2 , high-priority group H 3  is scheduled, rather than a medium-priority group. Again after the transmission during sub-burst slot “0” in group H 3 , high-priority group H 4  is scheduled, rather than a medium-priority group. As shown in  FIG. 11 , the result of precedential location of the high-priority groups in the sequence of groups in the SBSMAP results in repeated scheduling of the higher priority groups at the expense of the lower priority groups. 
     In an additional preferred embodiment of the present invention, media access may be allocated according to a more efficient priority-based scheme than that provided in the embodiment described with respect to  FIG. 11 . This embodiment is illustrated with respect to  FIG. 12 , reference to which is now made, in which timing diagram  750  is shown for an exemplary transmission cycle for an SBSMAP constructed and operative in accordance with an additional preferred embodiment of the present invention. 
     In this embodiment, scheduler  112  ( FIG. 3 ) may schedule sub-burst slots associated with a particular priority level to network devices which inform scheduler  112  that they have data packets of that priority level to transmit. Network devices which do not inform scheduler  112  of particular priority needs may be assigned low priority sub-burst slots by scheduler  112  by default. Media access opportunities may thus be more precisely reserved for allocation where they are needed, and not wasted where they are not needed. 
     In order to allocate sub-burst slot transmission opportunities in accordance with data packet priorities to be transmitted by devices, as shown in  FIG. 12 , scheduler  112  ( FIG. 3 ) may allocate sub-burst slot transmission opportunities for each device in all of the groups associated with a priority level equal to or below the priority level requested by each device. In the example shown in  FIG. 12 , it may be seen that the network devices associated with sub-burst slots “0” and “1” informed scheduler  112  that they have high priority data packets to transmit. Accordingly, sub-burst slots numbered “0” and “1” are seen to be scheduled in the groups of every priority, i.e. “H”, “H,M” and “H,M,L”. It may further be seen that the network device associated with sub-burst slot “2” informed scheduler  112  that it has medium priority data packets to transmit. Accordingly, sub-burst slots numbered “2” are seen to be scheduled in the groups having medium and lower priorities, i.e. “H,M” and “H, M, L”. Network devices not informing scheduler  112  that they have medium or high priority data packets to transmit, such as those associated with sub-burst slots “3” and “4”, as may be seen in  FIG. 12 , are scheduled in the low priority group “H,M,L” only. 
     It will be appreciated that in this more efficient priority-based scheme, sub-burst slot opportunities are not wasted on higher priority opportunities for devices that do not have higher priority data packets to transmit. The streamlined result, in which the quantity of members of a group increase with decreasing priority of the group may be seen in  FIG. 12 , where the highest priority group “H” has two members, the next highest priority group “H,M” has three members, and the lowest priority group “H,M,L” has five members. 
     It will also be appreciated that in this embodiment of the present invention, high priority groups may enjoy the same quantitatively preferential scheduling as described previously with respect to  FIG. 11 , owing to the precession of the high priority groups in the sequence of groups in the SBSMAP and the post-transmission scheme of returning to the first group in the sequence of groups after a transmission. As in  FIG. 11 , the initial order of groups in the SBSMAP in  FIG. 12  is shown to descend from the highest priority group to the lowest priority group, as shown in the scheduling of groups H 1 , M 1  and L 1 , in that order, in TXOP  755  in  FIG. 12 . Furthermore, as in  FIG. 11 , the post-transmission return to the scheduling of the first group in the sequence of groups, i.e. the highest priority group, is shown in  FIG. 12 , where after the transmission during sub-burst slot “0” in group H 2 , high-priority group H 3  is scheduled, rather than a medium-priority group. As in  FIG. 11 , the result is repeated scheduling of the higher priority groups at the expense of the lower priority groups. 
     An additional preferred embodiment of the present invention is shown in  FIG. 13 , reference to which is now made. In this embodiment, transmission opportunities may be allotted non-equally to group members, in accordance with the bandwidth requirements of the network participants associated with the members of the group. In this method, sub-burst slot grid scheduler  112  ( FIG. 3 ) may allocate a quantity of sub-burst slot opportunities to each participant which is proportionate to the bandwidth requirements of each participant. 
     An exemplary non-equal allotment of transmission opportunities to network participants in a group is shown in exemplary timing diagram  800  for an exemplary transmission cycle for an SBSMAP constructed and operative in accordance with this embodiment of the present invention. A series of groups g are shown to be scheduled in timing diagram  800 . Each group g comprises the sequence of sub-burst slots “0,1,2,3,4,5”. As shown in rows  810 ,  812  and  819  corresponding to network participants  1  (Dev- 1 ),  2  (Dev- 2 ) and  3  (Dev- 3 ) respectively, it may be seen that for each group g, sub-burst slots “0”, “2” and “4” are associated with network participant  1 , sub-burst slots “1” and “3” are associated with network participant  2 , and sub-burst slot “5” is associated with network participant  3 . 
     As shown in  FIG. 13 , preference for the allotment of transmission opportunities may be conferred upon particular network participants at the expense other network participants, reflecting varying amounts of bandwidth requested by the network participants according to their needs. In the example of group g, as shown in  FIG. 13 , the relative proportions of bandwidth requested by network participants  1 ,  2  and  3  are shown to be 3:2:1 respectively. Network participant  1  is shown to be allotted one half of the transmission opportunities, network device  2  is shown to be allotted two sixths of the transmission opportunities, and network device  3  is shown to be allotted one sixth of the transmission opportunities. 
     An additional preferred embodiment of the present invention is shown in  FIG. 14 , reference to which is now made. In accordance with the embodiment of the present invention illustrated in  FIG. 14 , an SBSMAP constructed and operative in accordance with a preferred embodiment of the present invention may include a mechanism for “starvation prevention”. Network participants may starve for lack of transmission opportunity if, for example, they belong to a group having a low priority for the allotment of transmission opportunities, and other groups having a higher priority for the allotment of transmission opportunities are feasting on those opportunities and generally hogging the communal TXOP. The starvation prevention mechanism provided by the present invention and described with respect to  FIG. 14  may prevent network participant starvation by occasionally throwing a bone, i.e. a transmission opportunity, to the disadvantaged group. This “wild card” transmission opportunity may be rotated through the disadvantaged group so that the participants in the disadvantaged group may receive transmission opportunities on a round robin basis. 
     It may be seen in exemplary timing diagram  830  in  FIG. 14  that groups j and k are scheduled in TXOP  835  according to an SBSMAP associated with timing diagram  830 . It may also be seen that the network participants associated with sub-burst slots “0”, “1”, “2” and “3” in group k are disadvantaged in comparison with the network participants associated with sub-burst slots “0” and “1” in group j, since group j is scheduled five times in TXOP  835  and group k is scheduled only twice. In timing diagram  830  it may be seen that the reason for this inequality is the precession of group j with respect to group k in the SBSMAP, and, as the first group in the sequence of groups “j, k” in the SBSMAP, scheduling restarts with group j after each transmission. This may be seen in timing diagram  830  where after the transmission in group j 2 , group j 3  is scheduled, and after the transmission in group j 3 , group j 4  is scheduled, and after the transmission in group j 4 , group j 5  is scheduled. Consequently, as may be seen in  FIG. 14 , the network participants associated with sub-burst slots “0”, “1”, “2” and “3” in group k are starved for transmission opportunities due to the near monopolization of TXOP  835  by group j. 
     To provide a measure of correction for this type of inequitable distribution of transmission opportunities, Next_Group opportunities may be scheduled by scheduler  112  ( FIG. 3 ) in an SBSMAP constructed and operative in accordance with a preferred embodiment of the present invention. In the example shown in  FIG. 14 , a Next_Group opportunity “N” is shown to be scheduled into group j after the group j sequence “0,1”. As shown in diagram  839  pertaining to group k, the Next_Group opportunity serves as an additional sub-burst slot which is assigned to the next participant in the queue in group k. This is illustrated in diagram  839  in which the group k sub-burst slot transmission opportunities are seen to be numbered in the repeating consecutive sequence “0,1,2,3,0,1,2,3,0,1”, wherein the first, sixth, seventh and eighth sub-burst slot transmission opportunities are Next_Group opportunities contributed to group k by group j. Group k is thereby less starved of transmission opportunities owing to the transmission opportunities bequeathed to it by group j via the Next_Group opportunities scheme. 
     In an additional preferred embodiment of the present invention shown in  FIG. 15 , reference to which is now made, a more deliberate implementation of the Next_Group opportunity may not provide merely a measure of correction for an inequitable distribution of transmission opportunities, but may calculatingly correct the distribution of transmission opportunities among the network participants so that not only the participants in the first group scheduled in the SBSMAP may be guaranteed the bandwidth they require, but also the participants in the succeeding groups may be guaranteed the bandwidth they require. 
     As shown in  FIG. 15 , an amount of bandwidth B may be allotted to the first group scheduled in the SBSMAP. In exemplary timing diagram  850 , an amount of bandwidth B is shown to be allotted to group j 0 , the first scheduled group in the SBSMAP. Because there are four sub-burst slots in group j, bandwidth B may be divided into quarters, with each sub-burst slot allotted one quarter of bandwidth B. Each network flow associated with each of sub-burst slots “0”, “1” and “2” in group j 0  may thereby be assured a minimum bandwidth of B/4. 
     It will be appreciated, as shown in  FIG. 15 , that the fourth sub-burst slot in group j is a Next_Group opportunity. In the example shown in  FIG. 15 , Next_Group opportunity N 0  may bequeath at least one quarter of bandwidth B, but as much as all of the remaining bandwidth not utilized during the duration of the group j 0  sub-burst slots, in the event that one or more of the group j 0  transmission opportunities are not acted upon, to group k 0 . 
     Diagram  860  in  FIG. 15  illustrates the worst case scenario in which only one quarter of bandwidth B is guaranteed by Next_Group opportunity N 0  to group k 0 . The five participants in group k 0 , further to a division of the bandwidth B/4 into five equal parts, are thereby each assured a minimum bandwidth of B/20. Two repetitions Rep 0  and Rep 1  of initial modular sequence  852  of the SBSMAP, are shown to be scheduled in TXOP  855  in accordance with the group type of group k 0 , which is “rotated”. It may further be seen in  FIG. 15  that additional repetitions of initial modular sequence  852  are scheduled for the remaining duration of TXOP  855  in accordance with the TXOP remainder scheduling scheme provided by the present invention for rotated groups. 
     In an additional preferred embodiment of the present invention, shown in  FIG. 16 , reference to which is now made, media access may be provided to network flows in an advanced quality of service scheme. In accordance with the advanced quality of service scheme provided by the present invention, the network services may negotiate with scheduler  112  ( FIG. 3 ) for varying levels of guaranteed bandwidth. As shown in  FIG. 16 , which shows table  905 , the tabular representation of the configuration of an SBSMAP constructed and operative in accordance with an additional preferred embodiment of the present invention, a service, like a network participant, may be associated with a unique pair of values provided by the Device_ID and Flow_ID values  352  and  356  respectively. In accordance with this definition, it may be seen in  FIG. 18  that sub-burst slots “1”, “8” and “12” are associated with a particular service having the Device ID value of 1 and the Flow_ID value of 1. Similarly, it may be seen that sub-burst slots “2”, “4”, “6”, “9” and “13” are associated with a different service having the Device ID value of 2 and the Flow_ID value of 1. Sub-burst slots listed in SBSMAP table  905  as having the same pair of values for the Device_ID and Flow_ID parameters may be similarly associated with a service identified with that pair of values. 
     The groups into which the 17 sub-burst slots shown in SBSMAP table  905  are grouped in the SBSMAP defined by SBSMAP table  905  are also encoded in the table data. As indicated by dashed line  912 , group j comprises the first three sub-burst slots having a Group_Type value of 5, which value may indicate a rotated group. As indicated by dashed line  914 , group k comprises the next four sub-burst slots having a Group_Type value of 6, which value may indicate a repeated group. Similarly, dashed line  916  indicates that group l comprises the next four sub-burst slots which have a Group_Type value of 5, and group m closes out the group sequence assigned to TXOP T 0  with the next six sub-burst slots which have a Group_Type value of 6. 
     It will be appreciated, as explained hereinabove with respect to  FIG. 6 , that in an SBSMAP such as that defined by table  905  in  FIG. 16 , in which only the group type of the last group in a sequence of groups in an SBSMAP may determine the group scheduling protocol for the remainder of the TXOP to which that sequence of groups is assigned, the group types values assigned to each row in the SBSMAP table may serve as separators to delineate the groups in the sequence, and they may not dictate scheduling protocol in the SBSMAP, except in the final group in the sequence. 
     Thus, in the example shown in  FIG. 16 , the alternating group type values of “5” and “6” assigned to the sub-burst slots in groups j, k and l serve to identify the sub-burst slots affiliated with each group, and not group scheduling protocol. I.e., the first three sub-burst slots in the table, which are assigned one group type value, are affiliated with one group, here referred to as group j, the next four sub-burst slots, which are assigned a different group type value, are affiliated with another group, here referred to as group k, etc. In accordance with this embodiment of the present invention, only the group type of the last group, group m, may dictate group scheduling protocol in the shared TXOP. 
     In systems which support advanced quality of service, each network service may conduct negotiations with the scheduler regarding media access parameters such as bandwidth, latency and jitter. For its part, the scheduler may agree to service requests for guaranteed levels of service in accordance with its abilities to meet the service demands. The ability of the scheduler to meet service requests may be dependent upon the demands on the network at the time of the requests. Network services may request that the scheduler provide them with minimum, average, maximum or best effort levels of guaranteed bandwidth. 
     Service requests for a minimum level of guaranteed bandwidth may be the bandwidth requirement requests given the highest priority for scheduling by scheduler  112  ( FIG. 3 ), since a minimum level request by a network service may indicate that the service requires at least this level of bandwidth in order to execute its transmissions. I.e., if the service is not provided this minimum bandwidth, it will not be able to execute its transmissions. Service requests for an average level of guaranteed bandwidth may be the bandwidth requirement given the next highest priority for scheduling by the scheduler, followed by service requests for a maximum level of guaranteed bandwidth, followed by service requests for a best effort level of guaranteed bandwidth. 
     In accordance with the bandwidth allocation hierarchy described hereinabove, the first group in a TXOP scheduled according to the advanced quality of service scheme provided by the present invention may comprise sub-burst slots for the services requesting a minimum level of guaranteed bandwidth, i.e. the highest quality of service. Similarly, the second group in the TXOP may comprise sub-burst slots for the services requesting an average level of guaranteed bandwidth, i.e. the next highest quality of service, the third group in the TXOP may comprise sub-burst slots for the services requesting a maximum level of guaranteed bandwidth, i.e. the third highest quality of service, and the last group in the TXOP may comprise sub-burst slots for the services requesting a best effort level of guaranteed bandwidth, i.e. the lowest quality of service. It may therefore be seen in  FIG. 16  that each of the services associated with each of the sub-burst slots in group j requested and received from the scheduler a transmission opportunity guaranteeing a minimum bandwidth level. It may further be seen in  FIG. 16 , that due to the availability of 60 Mbps to the group of three sub-burst slots in group j, the bandwidth guaranteed to each service in each of the group j sub-burst slots is 20 Mbps. 
     It may further be seen in  FIG. 16  how Next_Group opportunities may be used in the advanced quality of service scheme provided by the present invention to guarantee different levels of bandwidth to network services. In a scheme similar to that described with respect to  FIG. 15  for the guaranteed bandwidth scheme provided by the present invention, Next_Group opportunities may be used in the advanced quality of service scheme provided by an alternative embodiment of the present invention to transfer a guaranteed amount of bandwidth from one group, in which each sub-burst slot represents a guaranteed amount of bandwidth, to the next group. Each sub-burst slot in the beneficiary next group will consequently be guaranteed a fraction of the transferred bandwidth. This fraction may equal the amount of transferred bandwidth divided by the number of sub-burst slots in the next group. 
     For example, as shown in  FIG. 16 , the division of the available 60 Mbps amongst the three sub-burst slots in group j provides the highest priority guaranteed minimum bandwidth of 20 Mbps to each sub-burst slot in group j, one of which is Next_Group opportunity  918 . In the example shown in  FIG. 16 , Next_Group opportunities may be identified by their assigned Device_ID and Flow_ID values of 63 and 0 respectively. They are indicated in table  905  by diagonal hatching and the reference numerals  918 ,  920 ,  922  and  924 . In the example shown in  FIG. 16 . Next 13 Group opportunity  918  transfers at least this 20 Mbps, and at most, all of the bandwidth not utilized in group j, to group k. In the example shown in  FIG. 16 , it may be seen that each of the network services associated with each of the sub-burst slots in group k, the second group in the SBSMAP, requested and received from scheduler  112  a transmission opportunity guaranteeing an average bandwidth level. In the example shown in  FIG. 16 , it may be seen that the guaranteed average bandwidth level is 5 Mbps, which equals the 20 Mbps transferred to group k by group j divided among the four sub-burst slots in group k. 
     It may similarly be seen that the guaranteed maximum bandwidth level in the example shown in  FIG. 16 , for the third group l in the SBSMAP, is 2.5 Mbps, which equals the 10 Mbps transferred to group l by Next_Group opportunities  920  and  922  in group k divided among the four sub-burst slots in group l. 
     It will be appreciated that the advanced quality of service scheme provided by the present invention and illustrated with respect to  FIG. 16 , may provide a method according to which scheduler  112  ( FIG. 3 ) may guarantee different levels of bandwidth to network services in accordance with their requests, subject to the limitations of the network at any given time. Furthermore, through the implementation of Next_Group opportunities as described with respect to  FIG. 16 , highly efficient use of available bandwidth may be achieved through the forwarding of any and all bandwidth remaining unused in one group to the subsequent group. Thus, little or no bandwidth which a network participant may be prepared to use may be wasted. 
     The generally efficient usage of bandwidth in the present invention may be further enhanced by the periodic usage of a minimal MAP. A minimal MAP may be introduced into the transmission cycle during a Next Map opportunity sub-burst slot which may be included in an SBSMAP constructed and operative in accordance with an additional preferred embodiment of the present invention. 
     While a long transmission cycle may provide certain advantages with respect to efficient bandwidth usage, such as minimizing the usage of bandwidth for MAP advertisements over the network, the disadvantage of a long, uninterrupted transmission cycle may be its rigidity. While the network is committed to a particular media access plan, it is unable to react to emergent situations on the network. 
     For example, time and bandwidth may be wasted while network services negotiate with scheduler  112  ( FIG. 3 ) for guaranteed bandwidth, as described hereinabove with respect to  FIG. 16 . Next_MAP opportunity sub-burst slots scheduled in an SBSMAP provided by the present invention may constitute windows of opportunity for the scheduler to quickly interrupt the current MAP and transition to the next MAP. This feature may allow usage of time and therefore bandwidth, which would have otherwise been wasted while the original MAP ran its course. 
     The implementation of a Next_MAP opportunity in accordance with an additional preferred embodiment of the present invention is described with respect to  FIGS. 17A and 17B , reference to which is now made. In exemplary timing diagram  950  for an exemplary SBSMAP shown in  FIG. 17A , it may be seen that groups j and k, having three and four sub-burst slots each respectively, are scheduled through the duration of TXOP  955 . Group j includes a Next_Group opportunity sub-burst slot, indicated by the letter “N” and hatched, and group k includes a Next_MAP opportunity sub-burst slot, indicated by the symbol “M” and circled. It may be seen in  FIG. 17A  that during Next Group opportunity  960 , group j passes the transmission opportunity to the first participant in the queue in the next group, which is the participant associated with sub-burst slot “0” in group k, as indicated by arrow  965 . As can be seen in  FIG. 17A , no transmission occurs during this sub-burst slot. 
     During Next_MAP opportunity sub-burst slot  962 , scheduler  112  may have the opportunity to interrupt the current TXOP  955 , and begin a new TXOP with the first group scheduled in the next MAP. It may be seen in  FIG. 17A  that scheduler  112  passes on Next_MAP opportunity  962  to interrupt the TXOP and transmit the next MAP. Similarly, during Next_MAP opportunity  964 , which occurs as a transmission opportunity is transferred from group j to group k during a Next_Group opportunity as indicated by arrow  966 , scheduler  112  again does not act on the opportunity. 
     Reference is now made to  FIG. 17B  which shows an additional exemplary timing diagram  950 ′ for an exemplary transmission cycle of the same SBSMAP as that for which timing diagram  950  is shown in  FIG. 17A . It may be seen in timing diagram  950 ′ that scheduler  112  again passes on the first Next_MAP opportunity  962  in TXOP T 0 , but does act upon the second Next_MAP opportunity  964 . Consequently, as may be seen in  FIG. 17B , TXOP T 0  is interrupted at this point, as indicated by reference numeral  968 , where scheduler  112  begins the transmission of the next MAP. Accordingly, after the IFG following this transmission, TXOP T 1  is initiated, as indicated by reference numeral  969 . 
     Reference is now made to  FIG. 18  which shows an additional preferred embodiment of the present invention implemented within a multi-dwelling unit (MDU) or a multi-tenant unit (MTU). In a prior art MxU, which may be an MDU or an MTU, the master device is able to communicate with all of the endpoints (EPs) in the units owing to carrier sensing operating between the master device and each of the EPs. In an MxU, each unit may have one or more EPs. In the prior art, each device, i.e., the master and each EP, has a separate TXOP in order to guarantee media access. Media access is thereby guaranteed to the master for advertising media access plans over the network, and to the EPs, for data transmissions. 
     Multiple separate TXOPs in the prior art, required in an MxU for the large number of units if there is only one EP per unit, and for an even larger number of EPs if there is more than one EP per unit, result in an inefficient network having a large overhead and considerable bandwidth wastage. 
     As shown in  FIG. 18 , the present invention may provide a generally more efficient method for utilizing available bandwidth in an MxU. As shown in timing diagram  980 , which is an exemplary timing diagram for an exemplary transmission cycle for an SBSMAP constructed in accordance with a preferred embodiment of the present invention and operative in an MxU in which there is carrier sensing between the master and all of the EPs, all of the network devices may participate in a single shared TXOP T 0 . 
     In an MxU in which there is carrier sensing between the master and all of the EPs, but not between all of the EPs amongst themselves, the EPs between which there is carrier sensing may participate in a shared TXOP in accordance with a preferred embodiment of the present invention. As shown in timing diagram  982 , each of shared TXOPs T 1 , T 2  and T 3  may serve a group of EPs between which there is carrier sensing. 
     Shared TXOPs having sub-burst slots may also be implemented in a network running a TCP protocol, resulting in more efficient bandwidth usage with respect to the prior art, as shown in  FIGS. 19A and 19B , reference to which is now made. The TCP protocol is a bi-directional protocol in which data transmissions (TCP DATA) are followed by transmission acknowledgements (TCP ACK) as shown in timing diagram  990 . Each direction has its own dedicated TXOP, as indicated in diagrams  991  and  992  showing TCP data transmissions by network device DEV- 1  and TCP acknowledgements by network device DEV- 2  respectively. To improve efficiency, there may be a buffer in which multiple data packets are stored until their transmission is acknowledged. The data packets may be stored until acknowledgement is received in case the transmission is not acknowledged and the packets must be sent again. As shown in  FIG. 19A , a network running a TCP protocol may suffer from low effective throughput, due to the limitation of the buffer, as represented by TCP window  991 . As a result of the limited TCP window  991 , bandwidth portions WD 1  and WD 2  may be wasted, resulting in a long round-trip time (RTT). It may further be seen in  FIG. 19A  that the minimal TCP ACK traffic underutilizes the bandwidth allocated for the ACK stream resulting in additional wasted bandwidth portions WA 1  and WA 2 . In view of the amount of wasted bandwidth, as shown in  FIG. 19A , it may be seen that the available bandwidth is inefficiently utilized. 
     As shown in  FIG. 19B , the implementation of shared TXOPs  995  having sub-burst slots in a network running a TCP protocol in an additional preferred embodiment of the present invention may provide a generally more efficient method for utilizing available bandwidth with respect to the prior art. As shown in  FIG. 19B , one device Dev- 1  may be associated with sub-burst slots numbered “1” in the SBSMAP, and may use these sub-burst slots for TCP data transmission. For the acknowledgment traffic, a second device, Dev- 2  may utilize the sub-burst slots numbered “2” in the SBSMAP. As may be seen in  FIG. 19B , through the implementation of sub-burst slot transmission opportunities, the TCP data transmissions may occupy the transmission medium for the exact amounts of time required, as indicated I timing diagram  997  by transmission durations LD 1 , LD 2  and LD 3 , with no wastage of bandwidth. Similarly, as indicated by acknowledgement durations LA 1 , LA 2  and LA 3 , the TCP acknowledgement transmissions may occupy the transmission medium for the exact amounts of time required, with no wastage of bandwidth. It may thus be seen that in accordance with the present invention TCP data and acknowledge transmissions may occupy TXOP opportunities in a shared TXOP as needed in a very efficient manner. The total RTT may also be minimal, and may be the real RTT of the devices and not the shared media. 
     It will be appreciated that SBSMAPs constructed and operative in accordance with a preferred embodiment of the present invention may include any number of TXOPs, any number of which may contain sub-burst slots. The sub-burst slots may be grouped, according to a variety of parameters such as data flow priority, device priority or quality of service (QoS) requirements. The sub-burst slots within a group may be sequenced according to any algorithm, and the groups within a TXOP may be sequenced according to any algorithm. 
     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.