Abstract:
A method that is useable with a wireless medium and local stations includes communicating a request between one of the local stations and a central authority to reserve a time slot for transmitting from the local station. The central authority is used to selectively reserve the time slot based on at least in part a reservation schedule. If the central authority reserves the time slot, then during the time slot, the central authority prevents the other local stations from transmitting.

Description:
BACKGROUND 
   The invention generally relates to wireless communications, and more specifically, the invention relates to a technique for reserving bandwidth for communications over a wireless system. 
   Wireless systems, such as wireless local area networks (LANs), for example, have been supporting the communication of digital data for the past several years. Most recently, wireless systems are being used to communicate data for real time applications, such as applications that involve the transmission of voice and/or video data. 
   As an example, a wireless LAN may include several local stations, each of which forms a node of the wireless LAN. The wireless LAN may be subdivided into cells, and in each of the cells, the constituent nodes may communicate with an access point (AP). The AP, in turn, typically establishes communication with a wired network, such as an Ethernet network, for example. 
   Due to the wireless communication, one local station may attempt to transmit a frame of data at the same time that another local station or the AP attempts to transmit a frame of data, a condition called a “collision.” For purposes of preventing a collision from occurring, the wireless LAN may use a carrier sense multiple access/collision avoidance (CSMA/CA) contention scheme. In this scheme, before a local station transmits a frame, the local station determines if the wireless transmission medium is free from any carrier waves, and if not, the local station backs off from transmitting the frame for a time that is specified by a collision avoidance algorithm. 
   Because many of the local stations may be simultaneously attempting to communicate frames over the wireless transmission medium, a real time video or voice data stream of frames that is being communicated between one of the local stations and the AP may be continually interrupted due to the above-described contention scheme. Unfortunately, the real time traffic typically requires a relatively high bandwidth and has severe delay constraints, and these interruptions may limit the bandwidth that is available for the real time transmissions. In addition, wireless LANs are plagued with interference problems. Due to the inherent mobility within wireless networks, traditional schemes, such as those used with wired networks, cannot be directly applied. 
   Thus, there is a continuing need for a technique to address one or more of the problems that are stated above. 
   SUMMARY 
   In an embodiment of the invention, a method that is useable with a wireless medium and local stations includes communicating a request between one of the local stations and a central authority over the wireless medium to reserve a time slot for transmitting from the local station. The central authority is used to selectively reserve the time slot based on at least in part a reservation schedule. If the central authority reserves the time slot, then during the time slot, the central authority prevents the other local stations from transmitting. 
   Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic diagram of a wireless system according to an embodiment of the invention. 
       FIG. 2  is a flow diagram depicting a medium reservation technique according to an embodiment of the invention. 
       FIG. 3  is an illustration of a reservation request frame according to an embodiment of the invention. 
       FIG. 4  is an illustration of a reservation vector according to an embodiment of the invention. 
       FIG. 5  is a flow diagram depicting a technique used by a scheduler of a central authority according to an embodiment of the invention. 
       FIG. 6  is a schematic diagram of a central authority according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , an embodiment  10  of a wireless system in accordance with the invention uses a local medium reservation technique to schedule access to a wireless transmission medium. In this manner, the wireless system  10  includes local stations  20  (stations  20   a  and  20   b , as examples) that may each reserve exclusive access to the wireless transmission medium for a scheduled time slot. During this scheduled time slot, the local station  20  that holds the reservation may communicate several frames without being interrupted by another one of the local stations  20 . As a result of this arrangement, the system  10  supports high bandwidth wireless communications that are well suited for real time data, such as voice and/or video traffic. To accomplish these features, the system  10  is designed to maintain information regarding future traffic over the wireless medium, prioritize real time traffic over non-real time data and schedule access to the shared medium, as described below. 
   More particularly, in some embodiments of the invention, the system  10  may include cells  50  that each includes a group of the local stations  20 . Thus, as an example, a cell  50   a  may include the three local stations  20   a  that are depicted in  FIG. 1 , and another cell  50   b  may include the three local stations  20   b  that are also depicted in  FIG. 1 . Each cell  50 , in turn, may include an access point (AP)  30  that establishes communication between the local stations  20  of the cell  50  and a wired network  40 , such as an Ethernet network, for example, or the APs  30  may communicate with each other through the wireless interface. 
   In some embodiments of the invention, the stations  20  and  30  use a contention protocol, such as a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol, for purposes of avoiding collisions for contention access to the wireless transmission medium. In some embodiments of the invention, the IEEE 802.11 wireless local area network (WLAN) standard may generally govern communication across the wireless medium. 
   For purposes of reserving a time slot (i.e., for purposes of reserving bandwidth) to the wireless medium for real time access, the system  10  uses carrier sense information and properties of the real time traffic (voice, streaming video, etc.) in its medium reservation policy, as described below. In this manner, in some embodiments of the invention, each cell  50  includes a central authority  42  (that may be located within the AP  30  of the cell  50 , for example) that interacts with a particular local station  20  to reserve bandwidth using a technique  60  that is depicted in  FIG. 2 . Referring to  FIG. 2 , more particularly, a local station  20  may request (block  61  of  FIG. 2 ) that a particular bandwidth be reserved for a given traffic type over a defined period of time. This reservation request is received and analyzed (block  62 ) by the central authority  42 ; and depending upon the underlying network properties, desired traffic characteristics and the policy applicable to the requesting real time station, the central authority  42  determines (diamond  66 ) whether the central authority  42  will grant or deny the request. Based on this determination, the central authority  42  communicates a frame to the requesting station  20  indicating either denial (block  76 ) or the grant (block  78 ) of the request. 
   If the request is granted, the central authority  42  calculates (block  74 ) the duration and the periodicity of the transmissions for the requesting station  20  to meet the throughput and delay requirements of the real time traffic and indicates the reserved time slot in the frame that is transmitted (block  78 ) to the requesting station. For purposes of tracking the reserved time slots and for purposes of determining which time slots are available, the central authority  42  uses a reservation vector that is described below. 
   In some embodiments of the invention, the system  10  uses the properties of carrier sense functions that determine the state of the wireless transmission medium to ensure that no other local station  20  attempts to transmit during a reserved time slot. For example, in some embodiments of the invention, the system  10  uses the CSMA/CA protocol, a protocol that uses physical and virtual carrier sense functions are used to determine the state of the wireless medium. When either the physical or the virtual carrier sense functions of a local station  20  that needs to transmit indicate that the wireless transmission medium is busy, transmission is deferred. 
   The virtual carrier sense mechanism in CSMA/CA takes the form of a Network Allocation Vector (NAV) that indicates the busy status and the duration of transmission over the wireless medium. The central authority  42  uses this virtual carrier sense mechanism and before each reserved time slot begins (as indicated by the reservation vector), it transmits frames that include information to populate the NAVs (that are maintained locally by all stations  20  within the cell  50 ) with indication that the wireless transmission medium is busy during the upcoming reserved timeslot. This action causes the virtual carrier sense function, provided by the Medium Access Control (MAC) layer in each station  20 , to conclude that the wireless transmission medium is busy during the reserved period. 
   Therefore, in the scheme described herein, each local station  20  makes the medium access decision locally but the medium reservation control is carried out centrally by the central authority  42 . In some embodiments of the invention, the scheme may be used across multiple cells  50  in the following manner. First, the requesting local station  20  transmits the initial reservation request to the local central authority that, in turn, transits the request across all cells  50  where bandwidth reservation is required. For example, the cells  50  in which this occurs may be all cells  50  that are adjacent to and including the cell  50  of the requesting station  20 . Next, the central authorities  42  of these cells  50  reserve the time slots (if possible) and update their reservation vectors. Reserving the bandwidth across adjacent cells  50  provides a solution to prevent interference from adjoining cells. 
     FIG. 3  depicts a Reservation Request Frame (RRF)  100  that is transmitted by a local station  20  to reserve a specified bandwidth over a predefined period of time. One field  102  of the RRF  100  indicates a type of the frame. In this manner, the RRF  100  may be a reservation, cancellation, or refresh frame. A local station  20  transmits a reservation type RRF  100  for purposes of requesting the reservation of bandwidth for a particular time slot. A local station  20  transmits a cancellation type RRF to cancel a previously reserved time slot. A local station  20  transmits a refresh type RRF to renew a reserved time slot. A central authority  42  may also transmit a refresh type RRF after a particular reserved time slot expires to alert the corresponding local station  20  that renewal of the time is required, as further described below. 
   Another field  104  of the RRF  100  indicates a type of the traffic to be transmitted during the reserved time slot. For example, the field  104  may indicate whether the traffic to be transmitted during the reserved time slot is voice or streaming video traffic. A field  106  of the RRF  100  indicates a priority of the traffic. Another field  108  of the RRF  100  indicates the required throughput for the given type of traffic. Use of the field  108 , in some embodiments of the invention, is optional and may be used only if the traffic type is unknown. The RRF  100  may also include a field  110  to indicate the periodicity, the maximum time interval between successive transmissions. In some embodiments, the field  110  is optional and may be used only if the traffic type is unknown. 
   Upon reception of an RRF  100 , the central authority  42  examines the parameters indicated by the RRF  100  and the central authority&#39;s reservation vector to determine if a time slot may be reserved. An exemplary reservation vector  150  is depicted in  FIG. 4 . As shown, the reservation vector  150  indicates a bandwidth reservation window  151  that includes time slots  160  that are reserved for the local stations  20 . For example, a particular local station  20  may have time slots  160  in that are denoted by “STA 1 ”, and another station  20  may have time slots  160  (in the reservation vector  150 ) that are denoted by “STA 2 .” As shown, in some embodiments, any two adjacent reserved time slots  160  are separated from each other by a contention time slot  162 , a time slot in which the local stations  20  that do not have reserved periods may transmit using the CSMA/CA contention protocol. 
   Once the central authority  42  receives an RRF that requests a reservation, the central authority  42  ascertains if sufficient resources are available based on the throughput offered by the underlying network, the latency, carrier sense information, the amount of requested bandwidth, and the policy that is applicable to the requesting local station  20 . If resources are available, then the central authority  42  calculates the duration (called “ι”) of each transmission according to the following formula: 
               t   =       ρ   ·       R   t       A   t         +   λ       ,           Equation   ⁢           ⁢   1             
 
where “R t ” is the required throughput, “A t ” is the throughput offered by the underlying network, “ρ” is the periodicity and “λ” is the network latency. Once the central authority  42  fulfills the request, the central authority  42  transmits an acknowledgment frame to the requesting station  20 , a frame that indicates the time at which the reserved time slot begins.
 
   The central authority  42  includes a scheduler that performs a technique  180  that is depicted in  FIG. 5 . In the technique  180 , the scheduler “wakes” up (block  182 ) before each scheduled transmission period and transmits (block  184 ) a frame that populates the NAVs of the local stations  20  that are not going to transmit during the reserved time slot with the duration of the next reserved transmission. This event causes the virtual carrier sense mechanism that is provided by the MAC layer of each local station  20  to conclude that the medium is busy and defer transmission. Therefore, the medium access decision is taken by the virtual carrier sense mechanism local to each station  20 , and the reservation control information is sent by the central authority  42 . 
   The central authority  42  grants a reserved time slot only for a determined period of time, and the duration of this time may be negotiated with the requesting station  20  when the requesting station  20  transmits a reservation request (in the form of an RRF) for the first time. Basing the duration of the time slot on the available resources (as described above), the central authority  42  responds to the request with an indication of the actual period of time for which the reservation is granted. At the expiration of a reservation period, the central authority  42  sends out a refresh notice (via an RRF frame  100 ) to the appropriate local station  20 . If the local station  20  does not respond with a refresh request (via an RRF frame  100 ), the central authority  42  frees up any resources that are allocated to the local station  20  and removes the associated time slot from its reservation vector. 
   Reservation periods may be cancelled by local stations  20  (via an RRF frame  100 ), and upon reception of a reservation cancellation frame from a particular local station  20 , the central authority frees up the time slot  160  (and thus, the bandwidth) that is allocated to the station  20  and adds the additional time to the contention free time slots  162 . In some embodiments, the central authority  42  may free up the time slots  160  that are allocated to a particular local station  20  if that local station  20  does not transmit during one of its allocated time slots  160 . 
   The scheme described above has the added advantage of working well even when adjacent cells  50  share the same frequency. In this manner, the frame transmitted by the central authority  42  to populate the NAVs would also be received by stations in the adjacent cell  50 , which would lead to them defer transmissions during the reserved period. Thus, interference from neighboring cells  50  is minimized in the above-described scheme. 
   Referring to  FIG. 6 , in some embodiments of the invention, the central authority  42  may include a control unit  202  (a computer, a computer unit, a microprocessor or any other type of processor, as examples) that may execute routines in the form of a program  200  to perform the scheduling and reservation techniques that are described herein. The central authority  42  may include a storage unit  204  (a hard disk drive or a CD-ROM drive, as examples) that may store a copy of the program  200 . Other arrangements are possible. 
   Other embodiments are within the scope of the following claims. For example, in cases where multiple access points are present within the same cell  50 , in some embodiments of the invention, the central authority  42  may dynamically choose to route real time traffic through the access point that has the least existing traffic. As another example, in cases where carrier frequencies overlap in the same cell  50 , in some embodiments of the invention, the central authority  42  may choose to transmit real time traffic using the carrier frequency that best meets a predefined criteria, such as the carrier frequency in which previous communications had the least error rate or the carrier frequency in which previsous communications had the shortest latencies, as examples. 
   While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.