Patent Publication Number: US-2007104123-A1

Title: Method to provide centrally coordinated contention-free channel access within a wireless mesh network

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims priority from U.S. Provisional Patent Application No. 60/734,741, filed Nov. 8, 2005, which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION  
      This invention relates to wireless mesh networks, particularly to centrally coordinated contention-free channel access among nodes in such networks.  
     BACKGROUND  
      Hereafter, a station (STA) includes but is not limited to a node, mesh point, wireless transmit/receive unit (WTRU), user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, an access point (AP) includes but is not limited to a base station (BS), Node-B, site controller, node, mesh point, or any other type of interfacing device in a wireless environment.  
      The IEEE 802.11e Quality of Service (QoS) amendment defines new Medium Access Control (MAC) procedures to support QoS requirements in 802.11 WLANs and also introduces the Hybrid Coordination Function (HCF) to provide the QoS support that was lacking in earlier 802.11 legacy systems. Two medium access mechanisms are handled by the HCF: Enhanced Distributed Channel Access (EDCA) and HCF Controlled Channel Access (HCCA). EDCA provides contention-based channel access while HCCA provides controlled channel access. The intent of HCCA is to increase efficiency by reducing the contention on the medium.  
      The HCCA mechanism uses a QoS-aware centralized coordinator called a hybrid coordinator (HC). The HC is collocated with a QoS Access Point (QAP) and uses the HC&#39;s higher priority of access to the wireless medium to initiate frame exchange sequences and to allocate transmission opportunities (TXOPs) to itself and other QoS Stations (QSTAs). This mechanism provides limited duration controlled access for contention-free transfer of data.  
      The HC has privileged access to the medium because it can initiate a transmission after a shorter waiting time than the shortest backoff delay of any station using EDCA. Under control of the HC, a nearly continuous sequence of frame exchanges can be maintained, with short, fixed delays between frames.  
      A non-AP QSTA, based on its requirements, requests the HC for TXOPs—both for its own transmissions as well as for transmissions from the QAP to itself. The HC either accepts or rejects the request based on admission control policy. If the request is accepted, the HC schedules TXOPs for both the QAP and the non-AP QSTA. 
          For transmissions from the non-AP QSTA, the HC polls the non-AP QSTA based on the parameters supplied by the non-AP QSTA at the time of its request. A QSTA may initiate multiple frame exchange sequences during a polled TXOP of sufficient duration.     For transmission to the non-AP QSTA, the QAP directly obtains TXOPs from the collocated HC and delivers the queued frames to the non-AP QSTA, again based on the parameters previously supplied by the non-AP QSTA.        

      The HC traffic delivery and TXOP allocation may be scheduled to meet the QoS requirements of particular traffic streams. TXOP allocations and contention-free transfers of QoS traffic can be based on the HC&#39;s knowledge of the amount of pending traffic within the QAP and its associated QSTAs.  
      In a VLAN mesh network, contention-based channel access can result in collisions, even among members of the same WLAN mesh network. Such collisions can degrade the QoS perceived by end users (STAs) when utilizing the mesh.  
      Current proposals submitted to the IEEE 802.11s task group for standardization of wireless local area network (WLAN) mesh networks only describe distributed solutions for providing contention-free access. Currently, the IEEE 802.11s proposals do not preclude extensions for vendor-specific solutions such as centralized channel access protocols.  
     SUMMARY  
      This invention describes an IEEE 802.11e HCCA-based mechanism adapted for use by mesh points (MPs) wherein a centralized coordination function resides in MPs designated as “master” MPs. In this approach, certain MPs can assume a master role, while other MPs assume a slave role to the master. A collection of a master and its associated slaves is called a cluster. Master-slave roles are negotiated between neighbor MPs and may be based, for example, on MP capabilities, level of connectivity, etc.  
      IEEE 802.11e HCCA provides for coordinated channel access between one access point and multiple client stations in a WLAN. The invention described herein uses an HCCA-based channel access mechanism for coordination between multiple mesh points (MPs) in a WLAN mesh network, thereby providing a centrally coordinated contention-free medium access among members of a WLAN mesh network.  
      For coordinating channel access between clusters, two methods are also described: 
          1. A “hierarchical” structure where one level of clusters is “centrally” controlled by a higher-level master.     2. A “peer-to-peer” structure where all masters are treated as equals and utilize a “distributed” mechanism such as IEEE 802.11e EDCA for channel access between clusters.       

    
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
      A detailed description of the invention will be set forth below in conjunction with the accompanying drawings, wherein like elements are identified by like indicia, and, wherein:  
       FIG. 1  shows a block diagram of the present invention according to a hierarchal mastership.  
       FIG. 2  shows a block diagram of the present invention according to a peer-to-peer mastership. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      The IEEE 802.11e HCCA provides for coordinated channel access between one QAP and multiple non-AP QSTAs. This invention extends the use of an HCCA-based channel access mechanism for coordination between multiple mesh points (MPs) in a WLAN mesh network.  
      The embodiment of the WLAN mesh network described by this invention uses a master-slave architecture in which the master mesh point (MP) coordinates channel access between itself and its associated slave MPs. In this approach, certain MPs can assume a master role, while other MPs assume a slave role to the master. The collection of a master and its associated slaves is referred to herein as a cluster.  
      Channel access is coordinated between the master and its slaves using IEEE 802.11e HCCA-based mechanisms in which the role of the QAP is assumed by the master MP, and the role of the non-AP QSTA is assumed by the slave MPs. The HCCA-based coordination function resides in the master MP. The embodiments herein may be used with any mesh-related standards and specifically IEEE 802.11s.  
      For coordinating channel access between clusters, the following two methods are provided.  
      The first method employs a hierarchical structure where one level of clusters is centrally controlled by a higher-level cluster master. A two-tier example is depicted in  FIG. 1 , where M′ denotes the higher level master, M denotes the subservient masters, and S denotes the slaves.  
       FIG. 1  shows three (3) clusters. The two (2) lower level clusters each have a master M  20 A supporting slaves S  10 C and S  10 D through paths  40 A,  40 B, master M  20 B supporting slaves S  10 E and S  10 F through paths  50 A,  50 B and upper-level master M′  21  supporting slaves  10 A and  10 B through central paths  30 A,  30 B, as well as lower level masters M  20 A and M  20 B through central control paths  30 B,  30 C. It should be understood that the number of slaves served by the upper level master and lower level masters and the number of lower level masters served by the upper level master may be greater in number than is shown in  FIG. 1 , the limited number of masters and slaves shown being chosen for purposes of simplicity. As was described above, a variety of diverse negotiations and criteria employed in such negotiations may be employed to select master and slave MPs.  
      For example, channel access of slave S  10 E in one of the two subservient clusters in  FIG. 1  is conveyed to its (lower level) master M  20 B. Master M′  21 , having been designated as a higher-level cluster master, lower-level cluster master M  20 B conveys the access request from slave S  10 E to higher-level master  21 , which hierarchical structure eliminates contention between and among the three (3) clusters shown in  FIG. 1 . Higher level master M′  21  supports slaves S  10 A and S  10 B, as well as the lower-level masters M  20 A and M  20 B thereby providing the coordination function for the lower-level clusters as well as its own cluster. Although,  FIG. 1 , shows a two-tiered hierarchical structure, it should be understood that a multi-tiered hierarchical structure greater than two (2) may be employed as long as the upper-most level master is directly connected to the subservient masters on the level just below the upper-most master.  
      The second method employs a peer-to-peer structure where all masters are treated as equals and utilize a distributed mechanism such as IEEE 802.11e EDCA for channel access between masters. This is depicted in  FIG. 2 , where M  20 A, M  20 B and M  20 C denote the masters coupled in a distributed manner by distributed control paths  60 A- 60 C, and S  10 A through S  10 F denote the slaves, coupled to respective ones of the masters by centralized control paths  30 A through  50 B.  
      In this peer-to-peer” structure, master/slave/peer roles are negotiated between neighbor MPs and may be based, for example, on MP capabilities or level of connectivity. The method of master/slave/peer role negotiation is not addressed by this invention. Assuming a request for access is conveyed to master M  20 B by slave S  10 E, access is prioritized according to the above-mentioned, as well as, other appropriate criteria. Although,  FIG. 2  shows an example in which three (3) masters of three (3) clusters negotiate, it should be understood that a greater or lesser number of such clusters may be arranged in a peer-to-peer mastership.  
      With respect to this disclosure, coordination applies to devices (client STAs, APs, MPs, MAPs) that are using the services of the same WLAN mesh network.  
      It should be noted that the channel access, as described herein, is per channel specific. For example, in a case of multi-radio/multi-channel nodes, the HCCA mechanism can be used for the access on one channel (for instance to control one channel common to the Master and all its slaves), whereas another access mechanism (e.g., DCA, EDCA) can be used on the other channels.  
      Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.