GB2443862A - Real-time transmission of data in the Point Coordination Function Mode of operation of a wireless network - Google Patents

Abstract

An access point (AP) using the point coordination function (PCF) contention free mode of operation sends a beacon frame to all stations located in a basic service set (BSS) within a wireless network (for example a WLAN using the 802.11 standard) indicating the start of the PCF mode of operation. Each station that receives the beacon frame reads the value of a contention free period maximum duration field (CFP-MaxDur). Each station inhibits access to the network for a predetermined period of time in response to the CFP-MaxDur field received in the beacon frame. The access point sends a real-time start frame (RT-Start) indicating the start of a real-time transmission period (RTP) within the CFP to all stations within the BSS and one or more stations then send or receive data within the RTP. This allows the addition of a real-time transmission period which is compatible with existing standards and equipment.

Description

Method and system for real-time transmission of data in the Point

Coordination Function Mode of operation of a wireless network.

Field of the Invention

This invention relates to the development of the 802.11 Medium Access Control (MAC) protocol to provide improved services where real time performance is important, such as Voice over internet protocol (IP).

Background of the Invention

The success of Wired Ethernet led to the development of wireless networks such as Wireless Fidelity (WiFi) networks with the aim of providing similar services to those provided by Wired Ethernet or Local Area Networks (LAN), but without the inconvenience of a wire connecting the networks together.

So that devices from different manufactures could be compatible with the wireless network, a standard for communication across the wireless network was needed This lead to the development of the 802.11 standard for wireless local area network (LAN).

The IEEE 802.11 MAC protocol is a network access technology allowing wireless electronic devices to communicate with one another and connect to networking infrastructures. It was designed to provide asynchronous best-effort data services where data is delivered to a destination as soon as possible but with no guarantee as to the bandwidth of the transmission medium used or the latency (the time it takes for a data to get to its destination) of the network. An example of a best-effort data service is the current implementation of the Internet, where different connections to a network obtain a share of the available bandwidth that depends upon the number of connections that are active The popularity of WiFi and its rapid deployment in hotspots has lead to a study on real time service provision in WiFi networks. This led to combining Global System for Mobile Communication (GSM) I Universal Mobile Telecommunications System (UMTS) and wireless LAN to combine the benefits of both systems; namely 3GPP's wide coverage and high mobility and WiFi's broadband and cost effectiveness. This was proposed by the 3rd Generation Partnership Project (3GPP).

The industry also initiated UMA (Unlicensed Mobile Access), as a solution of FMC (Fixed Mobile Convergence), that is aimed to enable subscribers to

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access mobile core network for 3GPP services via WiFi or Bluetooth. UMA now has been handed over to 3GPP for further standardisation under name Generic Access Network (GAN) These new developments need the IEEE 802.11 MAC protocol to support real time services more effectively. In other words IEEE 802.11 needs to support Quality of Service (Q0S) specific realtime services in order to provide consistent QoS across the different networks. The QoS of a network is usually defined as the speed and reliability of data transmission.

The original 802 11 MAC protocol was designed with two modes of operation: carrier sense multiple access with collision avoidance (CSMNCA) based distributed coordination function (DCF) mode for asynchronous best-effort data services and central polling based point coordination function (PCF) mode for delay-sensitive traffic flows. DCF is mandatory to implement in an AP (Access Point) and stations, whereas PCF is optional.

The current enhancements of the 802.11 MAC protocol are all based around 802.11 DCF mode in order to be compatible and coexist with the existing WiFI devices.

Due to ineffectiveness and lack of scalability in supporting real time services, PCF is, in reality, hardly implemented in the AP and stations. On the other hand, DCF has gained much more acceptance and been very successful.

Considerable research efforts have been made to tackle QoS in WiFi networks, based around DCF. These efforts have led to IEEE proposing the 802.11 enhancements to facilitate QoS over WiFi networks in order to support the differentiation of delay-sensitive applications.

The current enhancements of the 802 11 MAC protocol are all based around 802.11 DCF mode in order to be compatible and coexist with the existing WiFi devices. 802.11 DCF was designed for asynchronous best effort data services, thus it is difficult for the enhancements based around the DCF to produce a breakthrough in real time service provision. A real-time service oriented MAC protocol is expected to produce such a breakthrough.

Current 802.11 MAC protocol

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Figure 1 shows the infrastructure of a wireless network which can use the 802 11 MAC protocol The system comprises a distribution system 101 such as a fixed wired network, for example a local area network (LAN) with one or more AP's 103, 105. Wireless means can also be used as the distribution system 101.

Each AP point provides wireless communications to a number of stations 107, 109, 111, 1 13, and 115 that are located within that particular cell or Infrastructure Basic Service Set (BSS), 117, 119. These AP's perform conversion of frame types used to transmit data to the rest of the network as well as transmitting frames over the wireless medium to stations.

The BSS is simply a group of stations that communicate with each other, either via the AP or directly with one another Stations only communicate directly with one another if they are in an independent Basic Service Set IBSS, when there is no AP.

The propagation characteristics of the wireless medium determine the area within which those communications take place, and this is referred to as the basic service area. These wireless communications are provided by infrared or radio technology.

A problem with wireless networks is they are much less reliable than wired links because of interference of the signal with other devices and because of multipath fading. Therefore this can lead to frames being lost or only partially received by a recipient. In order to overcome this, the 802.11 protocol uses a positive acknowledgement system.

Therefore the 802 11 protocol has a number of services, in addition to data delivery in order to ensure that data is actually received, and also to ensure that two stations do not transmit at the same time. The protocol also has to take into account that stations are usually only equipped with a transmitter and receiver that can only transmit or receive one at a time.

Nine network services have been provided by the 802.11 protocol. Six of these services allow the network to monitor the stations and deliver frames (containing the data) to the appropriate station while the other three are used for actually sending and receiving data, e.g voice data to the appropriate station.

These services are provided using frames. Amongst these services are the Association Request, Re-association Request and MAC Service Data Unit Delivery (MSDU), and these are described in further detail below.

In order for a station to be able to use the wireless medium, a station first needs to associate with an AP. By associating with a particularAP, this allows the distnbution system to know to which AP it should deliver a particular frame, so that it can be transmitted to a station within that AP's BSS.

The Association request is transmitted in an association request frame, Type'=OOOO. This is a type of Management frame (see later) and allows the AP to allocate resources and synchronise with a station. This frame contains information about the station, for example data rates, and the identifier of the network with which it wishes to associate.

A Reassociation request frame is also provided which allows a station to reassociate itself with an AP if the station has roamed away from the currently associated AP and finds another AP with a stronger Beacon signal (see later).

A MSDU allows data to be delivered from one station to another station or other recipient. The data is sent across the wireless medium using MAC frames.

Frames can be broadly divided into three categories; data frames that transport data, for example voice data, between stations; management frames for managing and controlling the wireless link; and control frames that assist in the delivery of data frames between stations.

A schematic representation of a data frame is shown in figure 2 Each frame is divided into a MAC header, a frame body for containing the actual data, and frame check sequence (FCS). The frame check field is provided so that a receiving station can determine if any errors occurred in a frame during transmission.

The MAC header is an important part of any frame, because this determines, amongst other things, where the frame should be sent, as well as where it has come from. The MAC header also determines the type of frame, for example, data, management or control frames The MAC header is subdivided into a number of fields. This is schematically shown in figure 3. The numbers below each field represent the number of bytes of each field. This figure is schematic only because fields with a different number of bytes are represented as the same length. The header contains a frame control field that defines the type of frame, a duration [identifier (ID) field which is used to indicate the time needed to receive the next frame transmission, as well as four address fields. These address fields identify the source and destination address of the frame, as well as indicating the address of the transmitter that transmitted the frame onto the wireless medium, and the address of the next immediate station on the wireless medium to receive the frame. This may not be necessarily the destination address. A BSS identifier is also provided which uniquely identifies the BSS.

Finally, a sequence control frame is also provided which allows the frames to be reassembled in the correct order when they are received at the destination.

The frame control field is further subdivided into 11 fields, and these are schematically shown in figure 4. Once again, this figure is schematic only, and the number of bits of each field is shown below each field. The frame control field comprises Protocol version fields, Type' and Subtype' fields as weD as various other fields which are not be described in detail here.

The Type' field is divided into three types; Management frames (Type' 00), Control frames (Type'= 01) and Data frames (Type'= 10) each with a length of 2 bits.

Management frames are further subdivided into a Subtype' field with a length of 4 bits. These Subtype frames can comprise Association Request or Beacon frames, amongst others. The Subtype' indicates what specific control function the control frame performs.

For example, Type'= 01 and Subtype'l 110 define the CF-End frame, which is used in the PCF mode of operation to indicate the end of that mode of operation.

Further subtypes are Request to Send or Clear to Send fields, and these are used in the DCF mode of operation in order to reduce possibility of two stations transmitting at the same time.

Other Management frames include Beacon frames, Type'=lOOO and a Probe Response frame, Type'=OlOl, and these indicate to all stations how frequently the Beacon frames are broadcast.

Further management frames are also provided which are not described in detail here.

As previously mentioned, two protocol modes for data transmission, DCF and PCF, can be configured by the AP, and operate alternately. The alternation between modes of operation is indicated in the Beacon frames.

In an infrastructure BSS, the AP is the timing master and performs Timing Synchronisation Function. It periodically broadcasts Beacon management frames to maintain all stations in the BSS synchronised. This informs the stations of the presence of the AP and relays information such as the time stamp to those stations. The AP also allows the stations to transmit and receive data to the distribution system and coordinates these transmissions to avoid two stations transmitting on the wireless medium at the same time.

Coordination functions control access to the wireless medium, and there are two modes of operation: CSMA/CA DCF mode where stations contend for access to the wireless medium, and a contention free PCF mode of operation is managed by a point coordinator to provide contention free access to the medium.

DCF Mode of operation This mode of operation allows a number of stations to share access to the wireless medium by using CSMA/CA DCF mode. DCF manages the transmission over a medium by allowing each station to listen to surrounding stations to see if they are transmitting, before transmitting themselves. Two carrier sensing functions are used; physical carrier sensing and virtual carrier sensing using a network allocation vector (NAy) A station, 101, first physically checks the wireless medium to see if it is idle for a period of time known as the DCF interframe space (DIFS).

In order to prevent collisions of transmissions, each station is allocated a Network Allocation Vector (NAy) which indicates when the wireless medium is busy. The NAV is an internal timer which is maintained by each station. The duration field in each frame transmitted to stations is used to reserve the medium for a particular period that indicates how long each transmission will last Each station sets its NAV to a particular valued based on the value of the duration field received in the last frame. Because all stations in the BSS receive frames transmitted from their AP, this allows all stations to update their NAy.

Before transmitting, a station then checks that the NAV is zero. If the NAV is zero, then the station desiring to send information first sends an optional request to send (RTS) frame.

Use of RTS/CTS frames is optional, and depends on the size of MAC Service Data Unit that comes to the MAC layer for transmitting. Thus a station may transmit MAC Service Data Unit straightforward without RTS/CTS handshaking.

The NAV in this frame is then set to a certain value in order to prevent other stations from accessing the wireless medium while the RTS is sent The request to send signal is included in the control frame as Subtype'=lOl 1. If the target station receives the RTS signal it will reply, after waiting for a Short frame interframe space, SIFS, with a Clear to Send (CTS) signal included in the control frame as Subtype=l 100. Once again, this CTS frame will set the NAV to a certain value in order to prevent other stations from attempting to access the wireless medium when it is busy. The sending station will then send the data frame, as a MAC service data unit (MSDU) after it has received the CTS signal from the receiver and waiting for a further SIFS The receiver will then send an acknowledgment frame back to the sender (also setting the NAV to zero allowing other stations to contend for access with the medium) after the sent frame has ended and a further SIFS has elapsed. In this way, the sending station knows that the receiving station safely received the frame.

If, however no acknowledgment is received within the SIFS, the sending station attempts retransmission until the data is sent or until the maximum number of attempts exceeds a predetermined threshold.

Alternatively, if sending station determines that the medium is busy, then the station will wait for the wireless medium to become free. Once it detects for example that the NAV is zero, it will further wait for a period known as the DCF interframe space (DIES). Once this period (DIFS) has expired, stations delay transmission by a randomly generated backoff time. After this backoff time expires stations can either start transmitting if the physical medium is still idle and the NAV is zero.

If however, the medium is not idle anymore, then a station desiring to transmit will hold back and not start transmitting. This period in which stations contend for access to the wireless medium is known as the contention window CW or backoff.

In the PCF mode of operation, contention-free access to the wireless medium is controlled by a point coordinator (PC) which is part of the AP.

When a station enters a BSS for the first time, the station starts an association process with the AP by sending an association request frame to an AP. After receiving the association request, the AP considers associating with the station and if accepted, establishes an association identifier for the station. If the AP accepts the station, then the station is allocated an identifier and is placed on the AP's list of stations which it can poll in order to allow those station to transmit in turn using the PCF mode of operation The PC sends the Beacon frames at regular intervals, by default, every 0.1 second. Between these beacon frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP). In the CP, the DCF mode of operation is used, and all the stations within the BSS contend for the wireless medium using the DCF mode as previously explained.

The Beacon frame announces the start of the CFP using a CF (Contention Free) Parameter set. The set contains fields called the CFP Max Duration, CFP Count, CFP Period, and CFP DurRemaining, and these fields are provided exclusively for use with the CFP, These CFP parameters define the CFP pattern. During the CFP, the AP takes full control of medium access by using NAV and PIFS (PCF lnterFrame Space) mechanism, which ensure that the stations using the DCF mode have no chance to contend for medium access.

The CFP Max Duration field is a field that specifies the length of CFP.

The CFP Count field determines how many Delivery Traffic Indication Map (DTIM) frames are to be transmitted before the next CFP. DTIM frames are those that have been buffered by the AP's and are due to be delivered shortly.

The CFP DurRemaining field indicates the number of time units remaining in the contention free period. This is used by stations to update their NAy.

There are also several other frame types are provided exclusively for use with the CFP, for example CF-Poll, CF-AC K. The CF-Poll frames are sent by access points to a station to give that station the right to transmit a single frame The CF-ACK frame is used by stations to acknowledge the receipt of a frame when no data needs to be transmitted. If data needs to be transmitted, then the acknowledgement is transmitted with that data frame.

Any station receiving the Beacon frame will then set their NAV to its maximum duration to prevent any of these stations attempting to access the medium using the DCF mode of operation.

As an additional mechanism to prevent any station from using DCF in the PCF mode, all transmission during the CFP are separated by either the short interframe space (S IFS) and the PCF interframe space (PIFS) which are shorter than the corresponding DCF interframe space used in the DCF mode, so that no station can access the medium using DCF in the PCF mode.

Once the Beacon frame has been transmitted and after a SIFS has elapsed, the AP polls stations on a polling list by sending a CF-Poll frame (on its own) to one of the pollable stations. The poll to a particular station may also be included in a data frame intended for a different station. If this is the case, a Data + CF-Poll frame is used. This allows more efficient use of the contention free period.

When a station receives a CF-Poll frame intended for it this allows that station to transmit once on the wireless medium during the CFP. The CF-Poll packets are sent in a frame as a subtype of a data frame, Type'lO.

The polled station then transmits the data to the recipient station (via the AP) as a MDSU. The recipient station will acknowledge receipt of the data by sending a CF-Ack frame to the AP.

The AP then sends a CF-Poll to another station in the BSS provided that the CFP-duration is long enough. This CF-Poll to another station can also include an acknowledgment to a station that data was correcuy received by the AP.

The minimum length of the CFP duration is that which allows one maximum size frame to be transmitted and acknowledged. This process continues until all stations have been polled and allowed an opportunity to send data over the wireless network, or until the NAV for the contention free period expires. If however, a station does not respond to a CF-Poll within the SIFS, then the AP will move on to poll the next station on its polling list.

The end of the CFP mode of operation is indicated by the PC transmitting a CF-End control frame. This lets stations know that the end of the period has arrived and they can once again contend for access to the wireless medium using the DCF mode. In this way, a station is only allowed to transmit if it has been centrally polled.

In the 802.11 MAC specification, CF-Pollable and CF-Poll Request part of Capability Information in the Beacon, Probe Response, Association and Reassociation Response frames, tells all stations whether or not an AP supports The CF-Pollable and CF-Poll Request part of Capability Information in the Association and Reassociation Request frames tells the AP whether or not a station that is to join in the BSS is CF-pollable and requesting to be placed on the CF-polling list The enhanced DCF of 802.1 le protocol is based around DCF in order to be compatible and coexist with the existing WiFi devices in a BSS, thus it is not real time service oriented.

Being able to be compatible and coexist with the existing WiFi devices ensures 802.lle to be accepted and leads to a smooth migration forward, but it also leaves little space for a radical MAC protocol to be designed for real time services. A real time service oriented protocol that does not have to follow the design of best-effort is expected to produce a breakthrough in real time service provision, but may turn out not to coexist with the existing WiFi devices.

This invention enables the coexistence of a radically new MAC protocol and the current 802.11 MAC protocol. The technique allows for a new protocol to be designed with a great deal of freedom, and since it makes use of the existing features of 802.11 MAC protocol specifications, it is easy to implement.

Summary of the Invention

The invention is now defined in the independent claims to which reference should now be made. Preferred features are defined in the dependent claims.

Embodiments of the present invention provide a system and method for enabling the coexistence of a new MAC protocol with existing WiFi devices.

Specifically, embodiments of the invention provide a system and method for enabling the coexistence by extending the usage of 802.11 PCF mode. The technique defines one control frame in an AP (Access Point) and one capability information bit for the AP and other stations, using reserved bits of the 802.11 MAC frame format. Therefore it is easy to implement. Importantly, the technique is generic, i.e. it is independent of a specific MAC protocol that will be designed for real time services

Brief description of the Drawings

Figure 1 shows the infrastructure of a wireless network according to an embodiment of the invention;

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Figure 2 shows a schematic representation of a data frame; Figure 3 shows the structure of the MAC header shown in figure 2; Figure 4 shows the structure of the frame control field divided into 11

fields;

Figure 5 shows a timing diagram of an embodiment of the invention; Figure 6 shows the first three fields of the frame control field according to an embodiment of the invention, Figure 7 is a schematic representation of the capability information field; and Figure 8 is a block diagram showing the main functionat components of an Access Point according to an embodiment of the invention as well as Apparatus for transmitting and receiving data according an embodiment of the invention.

Detailed Description of the Preferred Embodiments of the Invention Referring to figures 5 and 8 of the accompanying drawings, figure 5 shows a timing diagram of an embodiment of the invention in which a beacon frame, 151 is transmitted on a wireless medium as soon as possible after the medium has become free and after a target beacon transmission time (TBTT) 153 has elapsed. A block diagram of the main functional components of one of the stations 701 within the BSS as well as an Access Point 711 is shown in figure 8. A PC (Beacon frame generator) 713 in the AP 711 generates the Beacon frame and transmits it using a network transmitter/receiver 715 to all stations within its BSS. This informs all stations within the BSS that PCF mode of operation is to be used.

The Beacon frame comprises a CF parameter set including a CFP Max Duration field, and this is received by all stations, 701 using a transmitter/receiver 705 connected to an antenna 703. A processor 707 connected to the transmitter/receiver determines the value of the CFP Max Duration Field and instructs an inhibitor 709 in each station to set their NAV to this value to prevent stations from attempting to contend for access with the wireless medium using DCF mode of operation.

Then stations are permitted to transmit by the PC sending CF-poll packets to each station, generated by the processor, as previously described. The length of CFP-PCF mode of operation is determined by the AP based on the demands of the stations within a particular BSS. Stations are notified of the length of the CFP by the CFPMaxDuration field. The CFP is shown in figure 5. However, alternatively, depending upon the requirements of stations associated with an AP, the PC could completely close down the CFP-PCF slot, and only allow transmission using the CFP-RT transmission mode according to embodiments of the invention. The AP 711 can then start a real-time slot 155 by generating 717 and transmitting a (newly defined) RT-Start control frame. This RT-Start control frame can be defined as a Subtype' field of the frame control field, as shown in figure 4. It can take one value from the range of 0000-1001 that are currently not in use.

The RT-Start frame can alternatively be defined as a Management frame using one subtype value from the range of 1101-1111, or even as a Data frame using one subtype value from the range of 1000-1111. However, since the RT-Start frame performs a control function, it is preferable to define it as a Control frame.

This is schematically shown in figure 6 which shows the first three fields of the frame control field, namely the protocol field, type field and subtype field. In this example, the Protocol version is Type'=O, whereas the type field is Type'Ol indicating that the frame is a control frame, and the subtype field is Subtype'=OOOO indicating that this frame is a real time start control frame (RT-Start). Alternatively, the subtype field could take the other binary value between "0000" and "1001" (corresponding to decimal 1-9), for example, "0001", "0010", "0011", "0100", "0101", "0110", "0111", "1000", and "1001" The stations using the RT protocol according to embodiments of the invention interpret the RT-Start frame and react accordingly, whereas the stations using PCF or DCF can't recognise the RT-Start frame, and just ignore it.

The stations using the RT protocol know that the CFP-RT duration is to be exclusively used by the RT protocol. The RT protocol that to be used with embodiments of the invention for real time services requires that the CFP-RT duration be repeated periodically to guarantee that every data packet generated from a station can be transferred without delay or dropping. Periodic reservation-based medium access mechanism can be a technique to design the RT protocol.

However, embodiments of the invention are independent of the specific third MAC protocol that will be adopted.

Therefore in the CFP real time protocol according to embodiments of the invention, if the AP does not transmit the CF-End frame, instead it transmits a newly defined control frame, named RT-Start, the duration from reception of the RT-Start frame to the nominal termination time of the CFP 157 can then be defined to be exclusively used by a third MAC protocol that will be specifically designed for real time services. This is referred to as RI (Real Time) protocol.

During this period of time, the stations using PCF or DCF are preventedfrom actively attempting to access medium, since their NAV has been updated with parameter CFPMaxDuration or CFPDurRemaining at the beginning of the current CFP, whereas the stations using the new RT protocol are free to occupy the channel by following certain protocol rules. How stations are allotted slots in which they can send data is dependent on the third MAC protocol that will be adopted.

Once the CFP has ended, the CP starts again, and stations can contend for access to the wireless medium using the DCF mode of operation as previously described. After the CP has ended 159 and provided the medium is not busy 157, a new Beacon frame 151 is transmitted by the AP to all stations in the BSS, and the CFP-RT period starts again. This process of repeating CFP-PCF, followed by CFP-RT, followed by CP is shown in figure 5. The relative lengths of each of these periods can be altered by the AP according to the needs of the stations in the BSS. Furthermore, as shown in figure 5, if the wireless medium is busy at the end of the CP, and the Target Beacon transmission Time (TBTT) has elapsed, the AP delays sending the beacon signal until the wireless medium is free.

AP's and stations supporting the CFP-real time protocol according to embodiments of the invention also need to be identifiable. This is achieved by defining a RT-protocol bit in the Capability Information field. This field is schematically shown in figure 7. The Capability Information is a field located in the body of certain frame types for example AR frames or each Beacon frame.

This field defines the requirements that a station must meet in order to be part of the wireless network.

To indicate that an AP or a station supports the RT protocol as an option, one bit can be taken from the reserved Bit 5-15 of Capability Information field and defined as RT-protocol After the Association or Reassociation procedure that each station has to perform to join in a BSS, the AP will know the number and

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percentage of the three types of station (supporting DCF, PCF or the RT protocol respectively), and can therefore appropriately determine the boundanes between CFP-PCF (portion of CFP used for PCF), CFP-RT (portion of CFP used for RI protocol) and CP duration When a RT protocol-supporting station associates with an AP that does not support the RT protocol, the station shall simply not use the RI protocol.

Because PCF is hardly implemented in the existing WIFI products, theAP can minimise or even close down the CFP-PCF duration Timing relation between CFP and CP, CFP-PCF and CFP-RT is shown in Figure 5. in fact, an AP can reconfigure the CFP-PCF, CFP-RT and CP durations when it thinks necessary to balance the load among the three types of traffic, based on the number of the three types of station and their activities Adding the RT-Start control frame and the RT-protocol capability attribute in the Capability Information field (used in beacon transmissions) to the current 502.11 MAC specification ensures that DCF and PCF are compatible and can coexist with a third MAC protocol specifically designed for real time services and not constrained to following the design of best-effort. Embodiments of the invention are generic, i.e. it is independent of the specific third MAC protocol that is adopted.

Implementation of embodiments of the invention requires calculation of the values for the AP to configure the relevant control parameters to operate as Figure 5 illustrates, based on the characteristics of the real time service to be provided in WiFi networks An embodiment of the invention as applied to voice transfer is described below.

Voice transfer is the dominant real time service being provided in either circuit-switched or packet-switched domain, and VoIP has been widely provided in the public Internet and GPRS/UMTS packet-switched domain. Thus VoIP is a good example of real time service to show how an AP configures the relevant parameters and how many simultaneous voice connections can be expected from a reasonably efficient RT protocol.

In an IP network, voice is typically digitised with the G.71 I coding standard and transported at 64 kbps. The digitised voice is encapsulated into VolP packets with duration of 20 ms, each carrying 1280 ( 64 * 20) bits payload and an IP header, typically 224 bits long. VoIP packets come to the MAC layer as MAC SDUs (Service Data Unit). A real time service oriented MAC protocol that supports VoIP service requires that the CFP-RT be repeated every 20 ms (equivalently, so is the CFP). Such a periodic pattern of the CFP-RT is essential to guarantee that every VoIP packet generated from a station can be transferred without delay or dropping. The CFP-RT duration should be long enough to be able to transfer a number of VoIP packets, one per station. If Beacon Interval (=CFP+CP) is set to 20 ms and the AP controls the CFP-RT to be 10 ms, a theoretical maximum of 36 ((10000*11)/(1280+224)12) VolP packets may be transferred on the 802.1 lb physical layer. A realistic RT protocol may support half the number, but quicker 802.11 a/g physical layer can increase the number several times. The number of packets transferred within the CFP-RT is the number of simultaneous VoIP connections that can be accommodated in a BSS.

The configuration shown in Figure 5 also ensures that foreshorten' caused by Busy Medium at TBTT only happens to the CFP-PCF durations, and will not happen to the CFP-RT durations.

The appended below are the relevant AP parameters set to operate in the way presented when VoIP is the service to provide.

Beacon->Beacon Interval: 19 time units (1024 p s) that is 20 ms Beacon->CF Parameter Set->CFP Count: 0 Beacon->CF Parameter Set->CFP Period: I, which indicates that CFP is repeated every DTIM-marked Beacon Beacon->CF Parameter Set->CFP MaxDuration: 14 time units that approximately equals 15 ms Beacon->CF Parameter Set->CFP DurRemaining: the same as MaxDuration Beacon->TIM->DTIM count: 0 Beacon->TIM->DTIM period: 1, which indicates that all TIMs are DTIMs

Claims (24)

1. A method for sending or receiving data using the Point Coordination Function (PCF) Contention Free mode of operation of a wireless network comprises the steps of: an access point (AP) sending a beacon frame to all stations located in a Basic Service Set (BSS) indicating the start of the PCF mode of operation of the network; each station receiving the beacon frame and reading the value of a Contention Free Period Maximum Duration Field (CFP-MaxDur) indicating the duration of the Contention Free Period (CFP) mode of operation located in the beacon frame; each station inhibiting access to the network for a predetermined period of time in response to the CFP-MaxDur field received in the beacon frame; the AP sending a real time start frame (RI-Start) indicating the start of Real Time transmission Period (RIP) within the CFP to all stations within the BSS; and one or more stations sending or receiving data within the RTP.

2. A method according to claim I in which the CFP comprises CFP-PCF and CFP-RT modes of operation.

3. A method according to claim 1 in which the CFP comprises CFP-RT mode of operation only.

4. A method according to claim 2 in which the AP determines the number of stations supporting CFP-PCF and CFP-RT and sets the length of the CFP-PCF and CFP-RT periods in dependence upon the number of stations supporting

5. A method according to claim 1 in which the beacon frame is sent periodically in time

6 A method according to claim 5 in which the beacon frame is sent every 2Oms.

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7. A method according to claim 1 in which the RI-Start frame is sent periodically in time.

8 A method according to claim 7 in which the RT-Start frame is sent substantially every 2Oms.

9. A method according to claim 1 in which the CFP-MaxDur field is substantially l5ms in length.

10. A method according to claim 1 in which the CFP-RT mode lasts substantially 1 Oms.

11. A method according to claim 1 wherein the RI start frame comprises any one of the subtypes "0001 ", "0010", "0011", "0100", "0101", U 110", "0111", "1000", or "1001" of a control frame

12. A method according to claim 1 wherein the RI start frame comprises any one of the subtypes "1101","lllO", "1111" of a management frame.

13. A method according to claim 1 wherein the RT start frame comprises any one of the subtypes "1000", "1001", "1010", "1011", 1100", "1101", "1110", "1111" of a data frame.

14. A method according to any preceding claim in which each station inhibits access to the network by setting its Network Allocation Vector to the value of the

CFP-MaxDur field received in a frame.

15. A method according to any preceding claim further comprising the step of the AP sending a transmission to all stations within the BSS indicating that theAP supports RTP.

16. A method according to claim 15 wherein the transmission comprises any one of bits 5-15 of the Capability Information Field

17. An access point (AP) for use in the Point Coordination Function (PCF) Contention Free mode of operation of a wireless network comprising: a beacon frame generator for generating and sending a beacon frame to all stations located in a Basic Service Set (BSS) indicating the start of the PCF mode of operation of the network; and a real time start frame (RT-start) generator for generating and sending a real time start frame indicating the start of Real Time transmission Period (RIP) within the CFP to all stations within the BSS.

18. An AP according to claim 17 further comprising a RIP message support generator for generating an sending a transmission to all stations within the BSS indicating that the AP supports RIP.

19. An AP according to claim 18 wherein the transmission comprises any one

of bits 5i5 of the Capability Information Field

20. Apparatus for sending or receiving data using the Point Coordination Function (PCF) Contention Free mode of operation of a wireless network comprising: a transmitter and receiver for receiving a beacon frame indicating the start of the PCF mode of operation of the network and for sending or receiving data; a processor for determining the value of the Contention Free Period Maximum Duration Field (CFP-MaxDur) indicating the duration of the Contention Free Period (CFP) mode of operation located in the beacon frame an inhibitor for inhibiting access to the network for a predetermined period of time in response to the CFP-MaxDur field received in the beacon frame wherein the receiver receives a real time start frame (RT-start) indicating the start of Real lime Transmission Period (RTP) and the transmitter and receiver sends or receives data within the RIP

21. A mobile communication device comprising apparatus according to claim 20.

22. Apparatus for transmitting or receiving data substantially as herein described with reference to the accompanying figures.

I

23. A method for transmitting or receiving data substantaIIy as herein described with reference to the accompanying figures.

24. An Access Point substantialLy as herein describe with reference to the accompanying figures.