Traffic specifications for polling requests of periodic sources

An apparatus and method for informing a coordinator of the particular characteristics of a periodic traffic source are disclosed. A station that generates a periodic traffic stream encodes the temporal period and temporal offset of the traffic stream within a quality-of-service (QoS) traffic specification, and transmits the traffic specification with a poll request. The coordinator, upon receiving a polling request, processes the associated traffic specification and, via appropriate decoding logic, determines whether the requesting station generates periodic traffic, and if so, the temporal period and temporal offset of the traffic stream. The coordinator subsequently can establish, based on the temporal period and temporal offset, a polling schedule that minimizes the delay between (i) the station generating a frame, and (ii) the station transmitting the frame (and thus the destination receiving the frame).

FIELD OF THE INVENTION

The present invention relates to telecommunications in general, and, more particularly, to local area networks.

BACKGROUND OF THE INVENTION

FIG. 1depicts a schematic diagram of wireless local-area network100in the prior art, which comprises: access point101, stations102-1through102-N, wherein N is a positive integer, and hosts103-1through103-N, interconnected as shown. Each station102-i, wherein i is a positive integer in the set {1, . . . N}, enables host103-i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network100via access point101.

Access point101and stations102-1through102-N transmit blocks of data called frames. A frame typically comprises a data portion, referred to as a data payload, and a control portion, referred to as a header. Frames transmitted from a station102-i to access point101are referred to as uplink frames, and frames transmitted from access point101to a station102-i are referred to as downlink frames. A series of frames transmitted from a station102-i to access point101is referred to as an uplink traffic stream, and a series of frames transmitted from access point101to a station102-i is referred to as a downlink traffic stream.

Access point101and stations102-1through102-N transmit frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit one or more frames.

Such protocols can be classified into two types: contention-based protocols, and contention-free protocols. In a contention-based protocol, stations102-1through102-N and access point101compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.

In a contention-free protocol, in contrast, a coordinator (e.g., access point101, etc.) grants access to the shared-communications channel to one station at a time. An analogy for contention-free protocols is a parent (i.e., the coordinator) granting each of several children a limited amount of time on the telephone to talk, one at a time. One technique in which a coordinator can grant access to the shared-communications channel is polling. In protocols that employ polling, stations submit a polling request (also referred to as a reservation request) to the coordinator, and the coordinator grants stations exclusive access to the shared-communications channel sequentially in accordance with a polling schedule. A polling schedule has a temporal period (e.g., 5 seconds, etc.) and continually loops back to the beginning of the schedule after its completion. Since stations transmit only in response to a poll from the coordinator, polling-based protocols can provide contention-free access to the shared-communications channel.

SUMMARY OF THE INVENTION

The present invention enables a station that transmits periodic traffic (e.g., a station that transmits a frame every 25 milliseconds, etc.) to inform a coordinator of the particular characteristics of the periodic traffic. In particular, in the illustrative embodiment a station:(i) encodesa temporal period that specifies the periodicity of the station's traffic stream (e.g., 25 milliseconds, etc.), anda temporal offset that specifies the phase of the periodic traffic stream with respect to a particular reference (e.g., an IEEE 802.11 beacon, etc.)in a traffic specification (e.g., an IEEE 802.11e TSPEC, etc.) that specifies QoS-related information, and(ii) transmits the encoded traffic specification along with its poll request to the coordinator.

When a coordinator receives a polling request, the coordinator processes the associated traffic specification and, via appropriate decoding logic, determines whether the requesting station generates periodic traffic, and if so, the temporal period and temporal offset of the traffic stream. The coordinator then establishes a polling schedule so that the station is polled as soon as possible after generating a frame, thereby minimizing the delay between (i) the station generating a frame, and (ii) the station transmitting the frame (and thus the destination receiving the frame). This is especially advantageous in real-time communications such as voice calls and instant messaging.

The illustrative embodiment comprises: populating a first field of a traffic specification with a function of one of a temporal period and a temporal offset, wherein the temporal period and the temporal offset are for a plurality of expected future transmissions; populating a second field of the traffic specification with the value of the first field; and transmitting a polling request with the traffic specification.

DETAILED DESCRIPTION

FIG. 2depicts a schematic diagram of local-area network200in accordance with the illustrative embodiment of the present invention. Local-area network200comprises access point201, stations202-1through202-N, wherein i is a positive integer in the set {1, . . . N}, and hosts203-1through203-N, interconnected as shown.

As shown inFIG. 2, station202-i enables host203-i to communicate wirelessly with other hosts in local-area network200via access point201.

Host203-i is a device (e.g., a computer, a personal digital assistant, a printer, etc.) that is capable of generating and transmitting data to station202-i. Host203-i is also capable of receiving, processing, and using the data received from station202-i. It will be clear to those skilled in the art how to make and use host203-i.

Station202-i is capable of receiving data from host203-i and of transmitting that data over a shared-communications channel to access point201. Station202-i is also capable of receiving frames from the shared communications channel and of sending that data to host203-i. The salient details of station202-i are described below and with respect toFIGS. 4 and 5.

Access point201receives frames from stations202-1through202-N in accordance with a polling schedule and transmits frames to202-1through202-N in accordance with a transmission schedule. The salient details of access point201are described below and with respect toFIGS. 3 and 6through10.

FIG. 3depicts a block diagram of the salient components of access point201in accordance with the illustrative embodiment of the present invention. Access point201comprises receiver301, processor302, memory303, and transmitter304, interconnected as shown.

Receiver301is a circuit that is capable of receiving frames from shared communications channel203, in well-known fashion, and of forwarding them to processor302. It will be clear to those skilled in the art how to make and use receiver301.

Processor302is a general-purpose processor that is capable of executing instructions stored in memory303, of reading data from and writing data into memory303, and of executing the tasks described below and with respect toFIGS. 6 through 10. In some alternative embodiments of the present invention, processor302is a special-purpose processor (e.g., a network processor, etc.). In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor302.

Memory303is capable of storing programs and data used by processor302, as is well-known in the art, and might be any combination of random-access memory (RAM), flash memory, disk drive, etc. It will be clear to those skilled in the art, after reading this specification, how to make and use memory303.

Transmitter304is a circuit that is capable of receiving frames from processor302, in well-known fashion, and of transmitting them on shared communications channel203. It will be clear to those skilled in the art how to make and use transmitter304.

FIG. 4depicts a block diagram of the salient components of station202-i, in accordance with the illustrative embodiment of the present invention. Station202-i comprises receiver401, processor402, memory403, and transmitter404, interconnected as shown.

Receiver401is a circuit that is capable of receiving frames from shared communications channel203, in well-known fashion, and of forwarding them to processor402. It will be clear to those skilled in the art how to make and use receiver401.

Processor402is a general-purpose processor that is capable of executing instructions stored in memory403, of reading data from and writing data into memory403, and of executing the tasks described below and with respect toFIG. 6. In some alternative embodiments of the present invention, processor402is a special-purpose processor (e.g., a network processor, etc.). In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor402.

Memory403is capable of storing programs and data used by processor402, as is well-known in the art, and might be any combination of random-access memory (RAM), flash memory, disk drive, etc. It will be clear to those skilled in the art, after reading this specification, how to make and use memory403.

Transmitter404is a circuit that is capable of receiving frames from processor402, in well-known fashion, and of transmitting them on shared communications channel203. It will be clear to those skilled in the art how to make and use transmitter404.

In the illustrative embodiment of the present invention, access point201and stations202-1through202-N support at least one IEEE 802.11 protocol. In some other embodiments access point201and stations202-1through202-N might support other protocols in lieu of, or in addition to, one or more IEEE 802.11 protocols. Furthermore, in some embodiments local-area network200might comprise an alternative shared-communications channel (for example, wired instead of wireless). In all such cases, it will be clear to those skilled in the art after reading this specification how to make and use access point201and stations202-1through202-N.

FIG. 5depicts an event loop of the salient tasks performed by a station202-i that transmits periodic traffic, for i=1 to N, in accordance with the illustrative embodiment of the present invention.

At task510, station202-i transmits a polling request that specifies (i) the temporal period of expected future transmissions (e.g., 100 milliseconds, 3 seconds, etc.), and (ii) a temporal offset with respect to a particular reference (e.g., access point201's IEEE 802.11 beacon, etc.) The polling request thus informs access point201of the period of station202-i's future traffic stream, and the phase of the traffic stream with respect to the reference.

As is well-known in the art, in local-area networks that operate in accordance with IEEE 802.11e, a version of 802.11 that supports quality-of-service (QoS), station202-i transmits a polling request to access point201in combination with a traffic specification (TSPEC) that characterizes, via a plurality of fields, traffic generated by station202-i. In some embodiments, an IEEE 802.11e-compliant station202-i might encode one or both of the temporal period and temporal offset in the traffic specification via one or more TSPEC fields. In one such encoding, station202-i populates both the TSPEC Minimum Service Interval and Maximum Service Interval fields with the temporal period. In accordance with this encoding, access point201, upon receipt of a polling request in which the associated traffic specification has a Minimum Service Interval field and a Maximum Service Interval field with the same value, recognizes that station202-i generates periodic traffic with a temporal period equal to this value. (A method by which access point201ascertains the temporal offset when a polling request specifies only the temporal period is disclosed in co-pending U.S. patent application Ser. No. 10/674,178, entitled “Exploratory Polling For Periodic Traffic Sources”)

In another exemplary encoding, station202-i populates the Maximum Service Interval field with the temporal period, and the Minimum Service Interval field with the sum of the temporal period and the temporal offset. In accordance with this encoding, access point201, upon receipt of a polling request in which the associated traffic specification has a Minimum Service Interval field value that is greater than the Maximum Service Interval field value, deduces that station202-i generates periodic traffic with a temporal period equal to the Maximum Service Interval field value, and a temporal offset equal to the difference between the Minimum Service Interval and Maximum Service Interval field values.

As will be appreciated by those skilled in the art, at task510station202-i can encode one or both of the temporal period and temporal offset as arbitrary functions of one or more traffic specification fields, and at task610, described below, access point201can retrieve the temporal period and temporal offset from the traffic specification via the appropriate decoding logic.

At task520, station202-i queues, in well-known fashion, one or more frames in accordance with the temporal period and temporal offset specified at task510.

At task530, station202-i receives a poll from access point201in well-known fashion.

At task540, station202-i transmits the frame(s) queued at task520in accordance with the appropriate protocol (e.g., an IEEE 802.11 protocol, etc.). After task540has been completed, execution continues back at task520.

FIG. 6depicts a flowchart of the salient tasks performed by access point201in establishing a polling schedule, in accordance with the illustrative embodiment of the present invention.

At task610, access point201receives a polling request from station202-i that specifies a temporal offset φ and period π, where i is a positive integer less than or equal to N. As described above, in an IEEE 802.11e network access point201might obtain φ and π from a traffic specification associated with the polling request in some embodiments.

At task620, access point201checks whether a polling schedule P already exists. (P is a schedule for polls to all stations in local-area network200; i.e., P can be thought of as the union of a plurality of polling schedules {P1, P2, . . . , PN}, where Piis a polling schedule for station202-i. If polling schedule P already exists, execution proceeds to task640, otherwise execution proceeds to task630.

At task630, access point201creates a new polling schedule P with one or more polls sent to station202-i in accordance with temporal offset φ and period π. Polling schedule P repeats continually, and thus in some embodiments it is particularly convenient to set the duration of schedule P to a value divisible by period π. After completion of task630, execution of the method ofFIG. 6terminates.

At task640, access point201adds one or more polls of station202-i to polling schedule P in accordance with temporal offset φ and period π. In accordance with the illustrative embodiment, task640comprises, if necessary, adjusting the temporal period of schedule P accordingly. For example, if a poll that occurs every 6 seconds is to be added to a polling schedule that has a temporal period of 4, then the new polling schedule should have a temporal period of 12, comprising (i) three successive instances of the previous polling schedule, and (ii) two instances of the added poll.

Formally, if polling schedule P previously included polls to a set of stations S and previously had a temporal period Q, then the temporal period of the new polling schedule P should be increased, if necessary, to a value Q′ such that for all stations202-n ∈ S, Q′ is divisible by πn, where πnis the temporal period of station202-n. (In other words, Q′ is the least common multiple of the temporal periods of every station in polling schedule P.)

As another example, when:the previous polling schedule P has(i) a temporal period of 6.0 seconds, and(ii) a single poll to station202-j at time 5.0 (i.e., πγ=6.0 and φγ=5.0),wherein j is a positive integer such that j≦N and j≠i;
andstation202-i, which has temporal period πi=4.0 and offset φi=2.0, is added to polling schedule P;
thenthe new polling schedule will have(i) a temporal period of 12.0 seconds,(ii) polls to station202-j at times 5.0 and 11.0, and(iii) polls of station202-i at times 2.0, 6.0, and 10.0.

As will be appreciated by those skilled in the art, the above method of constructing a polling schedule might result in simultaneous polls of two or more stations. As described above, since only one station can be polled at a time, the illustrative embodiment employs polling events in the polling schedule, where each polling event specifies a list of one or more stations to be polled. A description of how the illustrative embodiment processes the lists associated with polling events is disclosed below and with respect toFIG. 7andFIG. 8.

After completion of task640, execution of the method ofFIG. 6terminates.

FIG. 7depicts a flowchart of the salient tasks performed by access point201in establishing a transmission schedule, in accordance with the illustrative embodiment of the present invention. The transmission schedule specifies when access point201transmits buffered downlink traffic to stations202-i in wireless local-area network200, for i=1 to N.

At task710, access point201monitors the arrival times of downlink frames received by access point201and transmitted from access point201to station202-i. The monitoring of task710occurs during a time interval τ that is sufficiently long to serve as an “observation period” for characterizing downlink traffic to station202-i. It will be clear to those skilled in the art how to choose a suitable value for τ.

At task720, access point201determines, in well-known fashion, whether downlink frames for station202-i arrive in accordance with a regular temporal period, based on the observation period of task710. If the determination is affirmative, execution proceeds to task730, otherwise, the method ofFIG. 7terminates.

At task730, access point201determines the temporal period π and temporal offset Φ of downlink frames for station202-i, where offset Φ is relative to the 802.11 beacon transmitted by access point201. It is well-known in the art how to determine the period and offset (i.e., “phase”) of a periodic traffic stream.

At task740, access point201checks whether a transmission schedule T already exists. If none exists, execution proceeds to task750, otherwise execution continues at task760.

At task750, a new transmission schedule T is created with downlink frames for station202-s transmitted in accordance with temporal offset Φ and temporal period π.

At task760, existing transmission schedule T is augmented with the transmission of downlink frames to station202-i in accordance with temporal offset Φ and temporal period π. As will be appreciated by those skilled in the art, it is possible that a downlink transmission to station202-i occurs at the same time as another downlink transmission already in schedule T. Consequently, the illustrative embodiment maintains a list of one or more stations at each “transmission event” that indicates to which station(s) downlink frames should be transmitted. As is the case for polling schedule P, downlink frames are transmitted sequentially to the stations in the list, beginning at the time specified in transmission schedule T. The illustrative embodiment employs a mechanism disclosed below in the description ofFIG. 9for ensuring “fairness” with respect to the order in which downlink frames are transmitted to stations in a list.

At optional task770, any collisions between the new transmission schedule T and polling schedule P (i.e., a transmission in schedule T and a poll in schedule P that occur simultaneously) are overcome by suitably adjusting (i.e., via a slight time shift) the appropriate newly-added transmission of schedule T. In some embodiments, one or more tasks or methods might be performed in lieu of task770for avoiding collisions between schedules P and T. (For example, the illustrative embodiment employs (i) the method ofFIG. 8, disclosed below, for combining polling schedule P and transmission schedule T into a composite schedule, and (ii) the event loop mechanism ofFIG. 9, disclosed below, for rotating between simultaneous polls and transmissions in round-robin fashion, in lieu of task770.

FIG. 8depicts a flowchart of the salient tasks performed by access point201in combining a polling schedule and a transmission schedule into a composite schedule, in accordance with the illustrative embodiment of the present invention.

At task810, index variable i is initialized to 1.

At task820, variable C, which is used to store the composite schedule, is initialized to transmission schedule T.

At task825, for each combination of station and time in composite schedule C, an associated transmission flag is set to true, and an associated poll flag is set to false. In the illustrative embodiment, the transmission and poll flags are stored in composite schedule C; however, it will be clear to those skilled in the art that in some other embodiments these flags might be stored in a separate data structure.

At task830, variable t is set to the time of the ithpoll of polling schedule P, and variable s is set to the station polled in the ithpoll of polling schedule P.

At task840, access point201checks whether composite schedule C has a transmission at time t (obtained from transmission schedule T at task820). If so, execution proceeds to task850, otherwise execution continues at task880.

At task850, access point201checks whether station s is already in the list of stations at time t in composite schedule C. If so, execution proceeds to task870, otherwise, execution proceeds to task860.

At task860, station s is added to the list of stations at time t in composite schedule C. The associated poll flag for station s at time t is set to true, and the associated transmission flag is set to false. After the completion of task860, execution continues at task880.

At task870, the associated poll flag for station s at time t is set to true. After the completion of task870, execution continues at task880.

At task880, access point201checks whether the poll at time t is the last poll in polling schedule P. If so, execution continues at task890, otherwise the method ofFIG. 8terminates.

At task890, index variable i is incremented by 1. After the completion of task890, execution continues back at task830.

FIG. 9depicts an infinite event loop for access point201for processing composite schedule C, in accordance with the illustrative embodiment of the present invention. When the event loop is first started, execution begins at task910.

At task910, variable E is set to the next event in composite schedule C. If the event loop has just been started, the next event is the first event of schedule C.

At task920, variable L is set to the list of stations associated with event E.

At task930, index variable i is initialized to 1.

At task940, access point201checks whether the transmission and poll flags for the ithstation in list L (i.e., L[i]) are both true, indicating that a combined transmission/poll is the appropriate action. If so, execution proceeds to task950, otherwise execution continues at task960.

At task950, access point201transmits a downlink frame with a “piggybacked” poll to station L[i] in well-known fashion. After the completion of task950, execution proceeds to task985.

At task960, access point201checks whether the transmission and poll flags for L[i] are true and false, respectively, indicating that a downlink transmission is the appropriate action. If so, execution proceeds to task970, otherwise execution continues at task980.

At task970, access point201transmits a downlink frame to station L[i] in well-known fashion. After the completion of task970, execution proceeds to task985.

At task980, access point201transmits a poll to station L[i] in well-known fashion. After the completion of task980, execution proceeds to task985.

At task985, access point201checks whether variable i is equal to the size of list L, indicating that all stations in the list have been processed in accordance with tasks940through980. If so, execution proceeds to task990, otherwise execution proceeds to task995.

At task990, list L is rotated one position so that the first station in the list becomes the last, the second station in the list becomes the first, the third station becomes the second, etc. This establishes a new order for list L in preparation for processing event E in the next iteration of schedule C. After the completion of task990, execution continues back at task910for the next invocation of the event loop.

At task995, index variable i is incremented by 1. After the completion of task995, execution continues back at task940for processing the next station in list L.

Although the illustrative embodiment of the present invention is disclosed in the context of IEEE 802.11 local-area networks, it will be clear to those skilled in the art after reading this specification how to make and use embodiments of the present invention for other kinds of networks and network protocols.

It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.