Abstract:
A method and apparatus for implementing path-based traffic stream (TS) admission control in a wireless mesh network having a distributed and/or centralized admission control architecture is disclosed. When the wireless mesh utilizes distributed admission control architecture, a source mesh point (S.MP) transmits a request for TS admission requiring certain resources/quality of service (QoS). The request propagates through the wireless mesh network until the destination mesh point (D.MP) is reached and an admitted path is determined. If an intermediate mesh point (MP) is unable to meet the requested resources/QoS for the TS, the S.MP is notified. When the wireless mesh network utilizes centralized admission control architecture, a S.MP requests a route to the D.MP from a central controller. The central controller maintains a status of MPs in the wireless mesh network, and selects a best route to handle the TS to satisfy the requested resources/QoS.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/660,599, filed on Mar. 11, 2005, which is incorporated herein by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is generally related to wireless communication systems. More particularly, the present invention is related to path-based traffic stream (TS) admission control mechanisms in mesh-type wireless communication networks having centralized or distributed admission control architecture.  
       BACKGROUND  
       [0003]     A wireless local area network (WLAN) mesh network is an IEEE 802.11-based wireless distribution system (WDS) including two or more mesh points (MPs) interconnected via IEEE 802.11 links and communicating via WLAN mesh services.  FIG. 1  shows a typical WLAN mesh network  100  including a plurality of mesh points (MPs)  10 ,  20 , 30 ,  40 ,  50 ,  60  and  70  and gateways A, B and C. Each of the MPs  10 ,  20 ,  30 ,  40 ,  50 ,  60  and  70  of the WLAN mesh network  100  receives and transmits its own traffic, while also acting as a router for other MPs. The gateways A, B, and C provide gateway services to the MPs  10 ,  20 ,  30 ,  40 ,  50 ,  60  and  70 , including, for example, connections to the Internet, the public switched telephone network (PSTN), and other wired and wireless networks. For example, the MP  50  has no direct data connection with any of the gateways A, B, or C. The MP  50  accesses gateway A via the MP  20  and/or the MP  30 .  
         [0004]     Referring still to  FIG. 1 , the WLAN mesh network  100  employs TS admission control to ensure that new incoming TSs can be admitted in the WLAN mesh network  100  without sacrificing current levels of transmission quality. In legacy networks, a new quality of service (QoS) TS requiring admission control will be admitted as long as the access point (AP) can support the resources/QoS requirements of the TS. However, in mesh networks, admission control must ensure that all of the MPs in the network will be able to satisfy the resources/QoS requirements for current and new sessions upon admission of the new TS.  
         [0005]     Therefore, a method and apparatus for performing path-based TS admission control in wireless mesh networks is desired.  
       SUMMARY  
       [0006]     The present invention is related to a method and apparatus for implementing path-based TS admission control in a wireless mesh network having a distributed and/or centralized admission control architecture. When the wireless mesh network utilizes distributed admission control architecture, a source MP (S.MP) transmits a request for TS admission requiring certain resources/QoS. The request propagates through the wireless mesh network until a destination MP (D.MP) is reached and an admitted path is determined. If an intermediate MP is unable to meet the requested resources/QoS for the TS, the S.MP is notified. When the wireless mesh network utilizes centralized admission control architecture, an S.MP requests a route to the D.MP from a central controller. The central controller maintains a status of MPs in the wireless mesh network, and selects a best route to handle the TS to satisfy the requested resources/QoS. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawing wherein:  
         [0008]      FIG. 1  shows a typical WLAN mesh network;  
         [0009]      FIG. 2  is a flow diagram of a process of performing path-based TS admission control in a WLAN mesh network having a distributed admission control architecture in accordance with a preferred embodiment of the present invention;  
         [0010]      FIG. 3  is a signal flow diagram illustrating an example of an S.MP performing path-based TS admission control in a WLAN mesh network having a distributed admission control architecture in which intermediate MPs are able to satisfy the requested/QoS requirements of a new TS generated by an S.MP in accordance with the process of  FIG. 2 ;  
         [0011]      FIG. 4  is a signal flow diagram illustrating an example of an S.MP performing path-based TS admission control in a WLAN mesh network having a distributed admission control architecture in which an intermediate MP is unable to satisfy the requested resources/QoS requirements of a new TS generated by an S.MP in accordance with another embodiment of the present invention;  
         [0012]      FIG. 5  is a flow diagram of a process of performing path-based TS admission control in a WLAN mesh network having central controller in accordance with another embodiment of the present invention;  
         [0013]      FIG. 6  is a signal flow diagram of path-based TS admission control in a WLAN mesh network having centralized admission control architecture in accordance with the process of  FIG. 5 ;  
         [0014]      FIG. 7  is an exemplary block diagram of an MP configured to perform admission control in both a distributed admission controlled wireless mesh network and/or a centralized admission controlled wireless mesh network in accordance with the present invention; and  
         [0015]      FIG. 8  is an exemplary block diagram of a central controller for performing path-based TS admission control in a WLAN mesh network having a centralized admission control architecture in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     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.  
         [0017]     When referred to hereafter, the terminology “MP” includes but is not limited to a WTRU, a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a user station (STA) or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “AP” includes but is not limited to a Node-B, a base station, a site controller or any other type of interfacing device in a wireless environment.  
         [0018]     The present invention is a method and apparatus for path-based TS admission control. In a preferred embodiment of the present invention, a WLAN mesh network has distributed admission control architecture. Accordingly, each MP of the WLAN mesh network shares the responsibility for controlling TS admission, as opposed to this function of the WLAN mesh network being performed at a central location, such as a gateway or an AP.  
         [0019]      FIG. 2  is a flow diagram of a process  200  of performing path-based TS admission control in a WLAN mesh network having a distributed admission control architecture in accordance with a preferred embodiment of the present invention. In this embodiment, an S.MP originates a TS with initial resources/QoS requirements destined to a D.MP. The S.MP transmits an add TS request (ADDTS Req) message to a receiving MP in an admitted path to the D.MP (step  210 ). The receiving MP then determines whether the TS can be admitted or whether to reject the TS (step  220 ). If the TS can be admitted, the receiving MP then determines whether to admit the TS with less resources/QoS than requested, (i.e. a modified, or lower, QoS), (step  230 ).  
         [0020]     An MP may use the following admission criteria parameters to determine whether requested resources/QoS can be satisfied. It should be understood that the following list of parameters is not exhaustive, it is merely exemplary, and multiple parameters may be used in any combination as desired.  
         [0021]     1) Channel occupancy: Individual MPs measure their channel utilization, defined as the percentage of time the physical layer senses that the medium busy, as indicated by either the physical or virtual carrier sense mechanism.  
         [0022]     2) Buffer occupancy: An MP measures the buffer occupancy of its queue of the required access class.  
         [0023]     3) Link conditions: An MP measures the number of frame retransmissions, missing acknowledgement (ACK), or the like.  
         [0024]     In a preferred embodiment, the present invention utilizes channel occupancy (CO) for determining whether a given MP is capable of satisfying a requested resources/QoS. Each MP measures CO constantly, or at dynamic or predetermined intervals. A TS will be admitted by a given MP if the MP CO, after admitting the TS, remains below a threshold CO, CO threshold . This can be shown as follows: 
 
 CO   Current   +ΔCO&lt;CO   threshold    Equation (1) 
 
 where CO current  is the CO before admission of the TS, ΔCO is the change in CO due to the admitted TS, and CO threshold  is a predetermined or dynamic threshold which the given MP cannot exceed and still guarantee the requested resources/QoS. 
 
         [0025]     Alternatively, a CO admission parameter may be applied per TS at a given MP. In this manner, an MP may select a more stringent CO threshold  for a given TS carrying real time services, such as voice over internet protocol (VoIP). For deterministic admission control this may require a quota for each TS as follows: 
 
 CO   current   TS1   +ΔCO   TS1   &lt;CO   threshold   TS1 .   Equation (2) 
 
         [0026]     In both Equations 1 and 2, the calculation of ΔCO must consider both the incoming and outgoing effects of admission of a TS. For an intermediate MP, the admission control considers both the capacity of the MP to process the incoming TS, (i.e., incoming admission control), as well as the capacity of the MP to transmit the outgoing TS, (i.e., outgoing admission control). For an S.MP, only the outgoing admission control need be considered. For the D.MP, only the incoming admission control need be considered.  
         [0027]     Referring still to  FIG. 2 , if the receiving MP determines not to admit, (i.e., rejects), the TS at step  220 , the receiving MP transmits an ADDTS Resp message back to the S.MP indicating the reason for the rejection (step  240 ). The process  200  then returns to step  210 , allowing the S.MP to start the process  200  over with perhaps modified resources/QoS or an alternative admitted path.  
         [0028]     If the receiving MP admits the TS at step  220 , but does not admit the TS with the requested resources/QoS requirements at step  230 , the receiving MP transmits an ADDTS Resp message to the S. MP indicating a modified resources/QoS offer (step  250 ), before returning back to step  210  for further action by the S.MP. If the requested resources/QoS is met by the receiving MP in step  230 , it must then be determined whether the current MP is in fact the D.MP (step  260 ). If not, the receiving MP transmits an ADDTS Req message to the next MP in the admitted path (step  270 ), and the process  200  returns to step  220  for the determination of TS admission by the next MP. Optionally, the current MP sends an ADDTS Resp message to the S.MP indicating success (step  265 ).  
         [0029]     If the receiving MP is in fact the D.MP, then the D.MP sends a path admission (PA) response (PA Resp) message to the S.MP via any route (step  280 ). The S.MP is now ready to begin a session with the -D.MP using the admitted path (step  290 ).  
         [0030]     A PA Resp timeout mechanism is preferably employed at the S.MP, although it is not required. When a timeout mechanism is employed, a timer is initialized and is started when the S.MP transmits the ADDTS Req message. If the PA Resp is not received before a predetermined timeout threshold, the S.MP may abandon the ADDTS Req and optionally transmit another ADDTS Req message along the same or another admitted path. Alternatively, the delay can be measured via time stamping the PA Resp and the TS can be terminated if the delay requirement is not satisfied when the PA Resp is received at the S.MP.  
         [0031]      FIG. 3  is a signal flow diagram illustrating an example of an S.MP performing path-based TS admission control in a WLAN mesh network  300  having a distributed admission control architecture in which intermediate MPs are able to satisfy the requested resources/QoS requirements of a new TS generated by-an S.MP in accordance with the process  200  of  FIG. 2 . The WLAN mesh network  300  includes an S.MP  302 , a plurality of intermediate MPs  304 ,  306 , and a D.MP  308 . Each intermediate MP  304 ,  306 , is capable of satisfying the resources/QoS requested by the S.MP  302 . The S.MP  302  transmits an ADDTS Req message to the MP  304 , specifying a requested resources/QoS (step  310 ). The MP  304  determines that it can meet the requested resources/QoS, and transmits an ADDTS Req message to the MP  306  (step  330 ). Optionally, the MP  304  also transmits an ADDTS Resp message back to the S.MP  302  indicating successful admission of the TS by the MP  304  (step  320 ). The MP  306  determines that it can meet the requested resources/QoS of the TS, and transmits an ADDTS Req message to the D.MP  308  (step  350 ). Optionally, the MP  306  also transmits an ADDTS Resp message back to the S.MP  302  indicating successful admission of the TS by the MP  306  (step  340 ). The D.MP  308  determines that it can meet the requested resources/QoS requirements, and transmits a PA Resp message to the S.MP  302  indicating the admitted path (step  360 ), and a session between the S.MP  302  and the D.MP  308  is now ready to begin (step  370 ).  
         [0032]      FIG. 4  is a signal flow diagram illustrating an example of an S.MP performing path-based TS admission control in a WLAN mesh network  400  having a distributed admission control architecture where an intermediate MP is unable to satisfy the requested resources/QoS requirements of a new TS generated by an S.MP in accordance with the process  200  of  FIG. 2 . The WLAN mesh network  400  includes an S.MP  402 , a plurality of intermediate MPs  404 ,  406 ,  408  and a D.MP  410 . The S.MP  402  transmits an ADDTS Req message to the MP  404 , specifying requested resources/QoS (step  420 ). The MP  404  determines that it can meet the requested resources/QoS, and transmits an ADDTS Req message to MP  406  (step  430 ). Optionally, the MP  404  also transmits an ADDTS Resp message back to the S.MP  402  indicating successful admission of the TS by the MP  404  (step  425 ). The MP  406  receives the ADDTS Req from MP  404 , but determines that it cannot meet the requested resources/QoS of the TS, and transmits an ADDTS Resp message to the S.MP  402  containing the reasons for the rejection (step  435 ), (e.g., the transmission queue is already full, requested resources exceed an established limit, unable to meet requested resources/QoS requirement, or the like). Alternatively, the ADDTS Resp message transmitted by the MP  406  to the S.MP  402  in step  435  may contain an offer for a modified resources/QoS that the MP  406  is able to admit.  
         [0033]     Still referring to  FIG. 4 , upon receiving the ADDTS Resp message from the MP  406  in step  435 , the S.MP  402  determines a new admitted path excluding the MP  406 , either because the MP  406  can not handle the TS at all, or because the S.MP  402  is unwilling to lower its resources/QoS requirements to meet the resources/QoS offered by the MP  406 . The S.MP  402  then transmits another ADDTS Req message to the MP  404 , specifying a requested resources/QoS as well as providing information associated with a new route which bypasses the MP  406  (step  440 ). The MP  404  determines that it can meet the requested resources/QoS, and transmits an ADDTS Req message to the MP  408  (step  450 ). Optionally, the MP  404  also transmits an ADDTS Resp message back to the S.MP  402  indicating successful admission of the TS by the MP  404  (step  445 ). The MP  408  determines that it can meet the requested resources/QoS of the TS, and transmits an ADDTS Req message to the D.MP  410  (step  460 ). Optionally, the MP  408  also transmits an ADDTS Resp message back to the S.MP  402  indicating successful admission of the TS by MP  408  (step  455 ). The D.MP  410  determines that it can meet the requested resources/QoS requirement, and transmits a PA Resp message to the S.MP  402  indicating that the admitted path (step  465 ), and a session between the S.MP  402  and the D.MP  410  is now ready to begin (step  470 ).  
         [0034]     In an alternative embodiment of the invention, centralized TS admission control architecture is implemented in a WLAN mesh network. In this type of WLAN mesh network, a central controller performs the TS admission control functions for the entire WLAN mesh network.  
         [0035]      FIG. 5  is a flow diagram of a process of performing path-based TS admission control in a WLAN mesh network having a central controller in accordance with another embodiment of the present invention. When an S.MP desires to create a TS to a D.MP, the S.MP transmits an ADDTS Req message to the central controller, including resources/QoS and D.MP identification (step  510 ). The ADDTS Req message includes desired resources/QoS and an identification of the D.MP. The central controller selects the best route for satisfying the S.MP&#39;s desired resources/QoS requirements based on control policy and knowledge of available routes in the mesh network (step  520 ). In step  530 , a determination is made as to whether the best route selected by the, central controller does in fact satisfy the desired resources/QoS requested by the S.MP. If the best route selected by the central controller does not satisfy the desired resources/QoS requested by the S.MP, the central controller transmits an add TS modify (ADDTS Mod) message to the S.MP (step  540 ). The ADDTS Mod message contains currently available resources/QoS, of the mesh network. In step  545 , a determination is made as to whether the modified resources/QoS are acceptable to the S.MP. If the determination in step  545  is positive, the process  500  then returns to step  510 , where the S.MP can again send an ADDTS Req message with a modified resources/QoS requirement based on the ADDTS Mod message. If the determination in step  545  is negative, the process  500  ends.  
         [0036]     If it is determined in step  530  that the selected route does meet the S.MP&#39;s resources/QoS requirements, the central controller transmits an add TS commit (ADDTS commit) message to the S.MP and all intermediate MPs comprising the selected path (step  550 ). Each MP receiving the ADDTS Commit message determines whether it can satisfy the requested resources/QoS (step  560 ). If any of the intermediate MPs are unable to support the requested resources/QoS, the MPs unable to satisfy the requested resources/QoS transmit an add TS Reject (ADDTS Rej) message, to the central controller, indicating the MP&#39;s inability to meet the requested resources/QoS (step  570 ). The process  500  central controller then proceeds with steps  540  and  545  as described above. If all intermediate nodes determine they can satisfy the requested resources/QoS in step  560 , a session may now begin for a TS from the S.MP to the D.MP along the selected path (step  580 ).  
         [0037]      FIG. 6  is a signal flow diagram of path-based TS admission control in a WLAN mesh network  600  having centralized admission control architecture in accordance with the process  500  of  FIG. 5 . The mesh network  600  includes an S.MP  602 , an intermediate MP  604 , a D.MP  606  and a centralized controller  608 . The S.MP  602  transmits an ADDTS Req message to the centralized controller  608  requesting a TS with the D.MP  606  (step  610 ). The centralized controller  608  checks an internal database of available resources and determines load on various routes between the S.MP  602  and the D.MP  606 . The centralized controller  608  selects the route with the best resources/QoS metric, and updates its resource database. The centralized controller  608  transmits an ADDTS Commit message containing a TS ID, and a resources/QoS requirement to the S.MP  602  (step  615 ), an ADDTS Commit message to the intermediate MP  604  (step  620 ), and an ADDTS Commit message to the D.MP  606  (step  625 ). Upon receiving the ADDTS Commit from the centralized controller  608 , the MP  604  and the D.MP  606  verify available resources. In the present case, the MP  604  and the D.MP  606  are capable of satisfying the resources/QoS requirement, and a session may now begin between the S.MP  602  and the D.MP  606  via intermediate mesh point MP  604  (step  630 ).  
         [0038]     However, if one or both of the MP  604  and the D.MP  606  is unable to satisfy the requested resources/QoS of the ADDTS Commit messages, the MP  604  and the D.MP  606  transmit ADDTS Rej messages to the centralized controller  608  (step  635 ,  640 ). The MP  604  transmits an ADDTS Rej message to the centralized controller  608  containing resources/QoS that the MP  604  can currently satisfy (step  635 ). Similarly, the D.MP  606  transmits an ADDTS Rej message to the centralized controller  608  containing resources/QoS that the D.MP  606  can currently satisfy (step  640 ). The centralized controller  608  updates its resource database, and transmits an ADDTS Mod message to the S.MP  602  containing the best available resources/QoS for a TS to the D.MP  606  (step  645 ). The S.MP  602  may then transmit an ADDTS Req message containing new resources/QoS requirements (step  650 ). The process then repeats itself, until resources/QoS that can be satisfied is achieved and a session begins.  
         [0039]     Optionally, in order to release resources back to the mesh network  600 , when a session is terminated (step  660 ), the S.MP  602  transmits a delete TS request (DELTS Req) message to the centralized controller  608  (step  665 ). The centralized controller  608  updates its resource database accordingly, and transmits a DELTS Req message to the MP  604  instructing the MP  604  to terminate the TS (step  670 ). Similarly, the centralized controller  608  transmits a DELTS Req message to the D.MP  606  instructing the D.MP  606  to terminate the session (step  675 ). Alternatively, a TS time-out mechanism may be employed for releasing resources back to the mesh network  600 . For example, when an in-session TS is idle for longer than a predetermined period of time, the session is terminated and the resources are released to the mesh network  600 .  
         [0040]      FIG. 7  is an exemplary block diagram of an MP  700  configured to perform admission control in both a distributed admission controlled wireless mesh network and/or a centralized admission controlled wireless mesh network in accordance with the present invention. The MP  700  includes an antenna  705 , a receiver  710 , a processor  715 , a transmitter  720  and an admission control unit  730 . The processor  715  controls the functioning of the receiver  710  and the transmitter  720 , and interfaces with the admission control unit  730 . The admission control unit  730  includes a message processor  740 , a resource (QoS) manager  750  and a PA message processor  760 .  
         [0041]     In a distributed admission control wireless mesh network, the message processor  740  generates ADDTS Req messages when the MP  700  is an S.MP or an intermediate MP. The message processor  740  generates optional ADDTS Resp messages when the MP  700  is acting as an intermediate MP. When a rejection or modified resources/QoS is offered, the message processor  740  informs the resource (QoS) manager  750  of the offered resources/QoS.  
         [0042]     In a wireless mesh network utilizing a centralized admission controller, the message processor  740  generates ADDTS Req messages and ADDTS Rej messages for transmission to the centralized controller. The message processor  740  further processes received ADDTS Commit and ADDTS Modify messages from the centralized controller, and informs the resource (QoS) manger  750  of the required or offered resources.  
         [0043]     The resource (QoS) manager  750  determines available resources of the MP  700 , such as CO, buffer occupancy, or the like. The resource (QoS) manager  750  determines whether resources/QoS requirements of an ADDTS Req message is capable of being satisfied by the MP  700 . When TS specific CO is used to determine whether to admit a TS, the resource (QoS) manger  750  determines CO for each TS, as described above. The resource (QoS) manager  750  further determines the requested resources/QoS for ADDTS Req messages when the MP  700  is the S.MP of a TS. Information provided from the message processor  740  is used to manage resources of the MP  700  and make appropriate resource requests to other MPs in the wireless mesh network.  
         [0044]     In a wireless mesh network utilizing a centralized admission controller, a PA message processor  760  generates DELTS Req messages for transmission to the centralized controller when the MP  700  is the S.MP of a TS to be terminated. The PA message processor  760  processes received DELTS Req messages from the centralized controller when the MP  700  is an intermediate MP. The PA message processor  760  informs the resource (QoS) manager  750  of released resources.  
         [0045]     In a wireless mesh network utilizing distributed admission control architecture, the PA message processor  760  generates PA Resp messages when the MP  700  is the D.MP. The PA message processor  760  also implements any timeout mechanism for receiving a PA Resp message containing the admitted path, such as the one described above, when the MP  700  is the S.MP. When the MP  700  is the S.MP, upon receiving the PA Resp message, the PA message processor  760  informs the resource (QoS) manager  750  of the allocated resources in the admitted path for commencement of the session.  
         [0046]      FIG. 8  is an exemplary block diagram of a central controller for performing path-based TS admission control in a WLAN mesh network having a centralized admission control architecture in accordance with the present invention. The centralized controller  800  is a logical entity which may be implemented in any MP or as a separate entity. The centralized controller  800  includes an antenna  805 , a receiver  810 , a processor  815 , a transmitter  820  and an admission control unit  830 . The processor  815  controls the functioning of the receiver  810  and the transmitter  820 , and interfaces with the admission control unit  830 . The admission control unit  830  includes a resource database  840 , a best route selector  850  and a message processor  860 .  
         [0047]     The resource database  840  stores resource information of all of the MPs in the mesh network and maintains current resources/QoS capabilities of all of the MPs. The resource database  840  may acquire resource information from, for example, queries, synchronization signaling, response to beacons, and by received ADDTS Req messages, ADDTS Commit messages, and ADDTS Rej messages processed by the ADDTS message processor  860 .  
         [0048]     The best route selector  850  calculates traffic loads on the various routes between the S.MP and the D.MP. Based on calculated traffic loads, system operator preferences, and resources/QoS requirements of the TS, the best route selector  850  selects the best route and informs the resource database  840  accordingly.  
         [0049]     The message processor  860  generates ADDTS Commit messages and ADDTS Modify messages in response to ADDTS Req messages and ADDTS Rej messages, respectively. The message processor  860  informs and updates the resource database  840  as available resources are reported in the mesh network. The message processor  860  also generates DELTS Req messages for transmission to intermediate MPs in response to DELTS Req messages received from the S.MP. The message processor  860  informs the resource database  840  of released resources.  
         [0050]     The functionality of the admission control units  730  and  830  described above may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
         [0051]     In another embodiment of the present invention, an admission control mandatory (ACM) field per access category (AC) is included in an ADDTS Req message transmitted by a S.MP. The ACM field indicates to intermediate MPs whether admission control is required for the AC of the TS. If admission control is not required, the TS can be sent directly with no need for resources/QoS guarantees or negotiations. This is particularly useful for low priority data that does not require minimal latency in transmission such as, for example, large file transfers.  
         [0052]     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. Other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.