Data transmission system and method
The present invention generally relates to communication over wireless data networks, and particularly, but not exclusively, to maintaining the streaming of audio-visual data over a wireless local area network to an acceptable quality while providing additional quality or service enhancements as channel bandwidth allows.
Certain audiovisual encoding standards, such as MPEG-2 and MPEG-4, provide for scalable encoding of audiovisual information. Scalable encoding provides a tool for the encoding of audiovisual information as multiple data streams. The first of these data streams is considered a primary, or basic stream, which can be received and reconstructed independently of the presence of other data streams. This basic stream is intended to provide a basic level of user experience to the user of a recipient device. The one or more other streams are intended to convey information which, when decoded at the receiving device, enhance the level of user experience provided to the user in conjunction with the decoded basic stream.In these encoding standards, the information conveyed in the one or more other streams is only capable of interpretation by a receiver in combination with the information of the primary stream; this is as a result of the one or more streams being comprised of only additional detail with respect to the primary stream. Due to the normally subtractive process used to derive this additional detail, such data is usually known as 'delta' data..
Two types of scalability can be implemented, namely temporal and spatial scalability. Where temporal scalability is employed, a data stream is encoded with a reduced frame rate base layer stream and one or more enhancement layer streams encoding data to provide interleaved frames in the final decoded data. If a receiver were to receive only a base layer stream of a video sequence, the user experience could possibly be of a jerky video playback, where the rate of frame update could be sufficiently slow that a user could detect frame transitions. The inclusion of the data encoded in the one or more enhancement layer streams will provide intermediate frames to improve the user experience beyond this lowest available frame sequence.
Similarly, using spatial scalability, the data streams are encoded into a reduced resolution base layer and one or more enhancement layers adding detail to increase the resolution of the audiovisual information in the base layer stream. The receiver of solely the base layer spatially scaled stream may offer a user experience which has a blurred or otherwise obscure picture, and inclusion of further spatially scaled enhancement layer streams will provide a decoder with data to improve this user experience, with a higher resolution picture.
If all of the data streams (i.e. the base stream and the one or more enhancement streams) are received at a receiver, the combination of the received data streams will maintain the full frame rate or the full resolution of the original audiovisual information, as the case may be. In the situation where only the base stream is received, or where the base stream and fewer than all of the enhancement streams are received, audiovisual information of an acceptable quality level can still be decoded and displayed. It will be appreciated that in the case that several enhancement layer streams are encoded, these can be associated with progressively greater levels of quality of service (namely spatial or temporal resolution).Alternatively, it is conceivable that, in future Standards and implementations, one or more enhancement layer streams could be associated with other quality of service enhancements, such as additional data (e.g. electronic programme guides or teletext services).
The IEEE802.1 1 e draft Standard concerns the communication of information in a wireless network. This draft Standard addresses the need for an optimal allocation of resources, for different applications' traffic requirements. Access in 802.11 based networks is generally governed by the Distributed Coordination Function (DCF), which employs carrier sense multiple access with collision avoidance (CSMA/CA).
In essence, a station wishing to transmit must wait until the medium over which the transmission is to be made is idle (i.e. free) before initiating transmission. DCF enables the transmitting station to reserve the medium for a period specified in its transmission, to effect a "virtual carrier sense" mechanism so that all other stations are configured not to attempt to access the medium during this time.
Contention arises when two or more nodes in the network each attempt to access a communications resource. To take account of the non-deterministic nature of contention-based DCF access, the IEEE802.11 Standard provides a Point Coordination Function (PCF), wherein access to the medium is controlled and coordinated by the central Point Coordinator (residing normally on the Access Point or a nominated station).
The PC is able to seize control of the medium in order to provide service to and from stations at a pre-agreed time. This is achieved by imposing a "superframe" structure over time on the medium, comprising a broadcast beacon to announce system parameters and mark the start of the superframe, a Contention Free Period (CFP), during which the PC will capture control of the medium, and a Contention Period (CP), in which standard DCF access rules apply. The PC is able to seize the medium by virtue of having to wait for the medium to be idle for less time than other stations have to do. Theoretically the PC could seize the medium at any time, but the PCF rules limit the PC to seizing the medium during the CFP only.
The 802.1 le standard also introduces an improved medium access control mechanism, which is termed the Hybrid Coordination Function (HCF). This is broadly equivalent in function to those techniques identified above. The HCF provides a contention-based channel access method, termed Enhanced Distributed Coordination Access (EDCA), which introduces differentiated levels of service to reflect the different requirements of the traffic being carried. This is achieved by supporting multiple traffic queues, referred to as Access Categories (AC). The designated access categories, in order of decreasing priority are named: voice, video, best effort, and background.Each AC can be assigned (system wide) a different maximum transmission length and may have to wait for the medium to be idle for a different period of time (higher priority ACs being typically expected to wait less time before seizing the medium). The HCF also provides contention-free based channel access, termed Hybrid Central Coordination Access (HCCA), which is broadly analogous to the PCF. Notable changes with HCCA with respect to PCF are the ability for the Hybrid Controller (HC; cf. PC) to seize the medium in both the CFP and CP as needed, and the ability for the HC to constrain the amount of time for which the polled device can transmit, to ensure fairer access for all.
When transmitting delay sensitive video traffic across a wireless network, the standard approach would be to carry the delay-sensitive traffic as polled traffic within the CFP, and then to leave sporadic bursts of browser traffic to fend for themselves in the CP If the bandwidth of the network cannot carry both data streams, then the video traffic will be sent and the internet traffic will be queued, creating large response delays for a user browsing the internet via the access point.
One idea for tackling this problem is to associate each of the multiple streams of scalable-encoded audiovisual data with different 802.1 le access categories. This is proposed in the paper titled "Novel architecture for reliable H.26L video transmission over IEEE 802.11 e," by A. Ksentini, A. Gueroui, and M. Naimi, 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 04), vol. 2.
However, this approach will not solve the problem in the most optimal way. At best, it can only offer a soft, statistical guarantee for the quality of service. This is unacceptable for audio-visual traffic in all but the most trivial loading scenarios. In the case where the network is heavily or fully loaded with audio-visual or speech traffic, any traffic classed as background or best-effort traffic will still be prevented from being transmitted due to the priorities of the 802.1 le access categories. An access point may treat background and best effort traffic as expendable, despite the deleterious effect on the level of service which may be experienced by a user if this traffic fails to arrive or arrives after a significant and notable delay.
It is thus an object of one or more aspects of the invention to overcome the above problems.
It is a further object of one or more aspects of the invention to provide high priority data reliably to nodes over a wireless network, while allowing acceptable distribution, where possible, of low-priority data traffic over the wireless network.
According to one aspect of the invention there is provided a transmitter for transmitting information on a wireless medium in a wireless network, the information comprising a first data stream defining encoded information capable of being independently decoded for presentation to a user, and a second data stream defining encoded information capable of being decoded, in conjunction with at least said first data stream, to provide additional information for presentation to a user in conjunction with the information of the first data stream, the transmitter being operable to determine a first transmission period within which access to said wireless medium is reserved to said transmitter and, depending on capacity of said medium, a second transmission period, said transmitter being further operable to transmit said first data stream within said first transmission period.
Further, with the benefit of the present invention, there is also provided a solution where a higher level of service can be transmitted as high priority traffic over the wireless network where there is available capacity. This is accomplished by transmitting some of the further streams providing further levels of service for the high priority data as high priority traffic over the wireless network, if there is adequate free bandwidth available.
Therefore, preferably, the transmitter is further operable to determine if said second data stream can be transmitted in said first transmission period and, on so determining, being operable to transmit said second data stream in said first transmission period. Preferably, the transmitter is further operable, in the event that the second data stream is not capable of being transmitted in said first transmission period, to determine if said second data stream can be transmitted in said second transmission period and, on so determining, being operable to transmit said second data stream in said second transmission period.
Also, with the benefit of the present invention, there is provided a solution to reduce the instance of transmission of data which is of little or no use to the recipient of such data, because it has encountered significant or notable delay. This is accomplished by assigning expiry information to the packets of the further streams such that, when a packet of further stream data passes the expiry time stored in the expiry information for that packet of data, that packet is identifiable for deletion and transmission is avoided. In the situation where a channel is heavily loaded, and the data packets remain in a queue for transmission for extended periods of time, this prevents the redundant transmission of information that can no longer be used.
Therefore, in a preferred embodiment of the invention, wherein packets in said second data stream comprise expiry information, and for each said packet in said second data stream said expiry information being indicative of whether said packet is suitable for reception given the transmission of a corresponding first data stream, the transmitter is operable to determine on the basis of said expiry information, whether said packet is in an expired condition, and on the basis of said determination, is further operable to transmit said packet only if said packet is not in an expired condition.
Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention and variations thereof, made with reference to the accompanying drawings in which: Figure 1 is a schematic diagram illustrating an infrastructure deployment wireless local area network in accordance with a first embodiment of the invention; Figure 2 is a schematic diagram illustrating an access point of the wireless local area network of the wireless network illustrated in Figure 1; Figure 3 is a flow diagram of a transmission process performed by an access controller of the access point illustrated in Figure 2 when managing the transmission of scalable audio-visual data streams across the wireless network illustrated in Figure 1;Figure 4 is a schematic data diagram illustrating data streams entering and leaving the access controller in a first example of operation of the access point; Figure 5 is a schematic data diagram illustrating a medium access control (MAC) superframe compliant with the draft 802.1 le Standard bearing the data streams output by the access controller in said first example; Figure 6 is a schematic data diagram illustrating data streams entering and leaving the access controller in a second example of operation of the access point; Figure 7 is a schematic data diagram illustrating a MAC superframe compliant with the draft 802.1 1 e Standard bearing the data streams output by the access controller in the second example; and Figure 8 is a schematic diagram illustrating a node of the wireless network illustrated in Figure 1.
With reference to Figure 1, a wireless network 10 comprises an access point 100. The access point 100 is connected to an external network (such as the Internet), in this example by means of a broadband modem. It will be appreciated that other alternative arrangements can be made for the access point to establish connection to an external source of streaming packet-based data.
The access point 100 establishes wireless communication, in accordance with the 802.11 e Standard, with nodes A 102, B 104 & C 106. The access point 100 is thus configured to route data between the external network and the respective nodes 102, 104, 106.
In this embodiment, by way of example only, Node A 102 is a portable laptop computer, Node B 104 is a desktop computer and Node C 106 is a multimedia device (e.g. set-top box) operable in conjunction with a television or hi-fi system. Each of these nodes 102, 104, 106 is equipped with a 802.1 1 g wireless LAN network adaptor and configured to communicate with the access point 100.
Referring to Figure 2, the access point 100 is equipped with a broadband modem 202 as previously described, to establish connection to the Internet. The broadband modem 202 is connected to a general purpose bus 204, which in turn connects to the components of the access point including a random access memory (RAM) 206, a processor 208, a read only memory (ROM) 210, a wireless receive/transmit (RX/TX) controller 214 and a mass storage device 216. The wireless RX/TX controller is, in turn, connected to an antenna 212.
The operation of the access point 100 will now be described. A traffic forwarder, stored in the RAM 206, but not illustrated for reasons of convenience, is executed by the processor 208. The traffic forwarder cooperates with traffic generated at nodes of the network, and consequent download traffic, to effect communication of data to the nodes.
On the basis of execution of an application residing at a network node, data is retrieved from the external network via a connection established through the broadband modem 202. The manner in which the modem establishes connection is not relevant to the present invention, and can be of a conventional nature.
The retrieved data is then stored in the RAM 206. An access controller program 230, stored in the RAM 206 and executed by processor 208 determines, on an ongoing basis, the data to be sent to the respective nodes 102, 104, 106 on the wireless network.
As the presently described embodiment is concerned with the wireless distribution of scalable audio-visual streaming data, such as data describing a video presentation, the example will now proceed on the basis that the data requested by an application residing on one of the nodes is such data. In particular, in the present example, the data is scalable audio-visual data encoded in accordance with the MPEG-2 Standard.
The access controller 230 is thus operable to arrange the various packets of data defining the scalable audio-visual presentation requested for retrieval by the node in question, such that those parts of the data that are required for an acceptable quality of service are placed on the transmission medium in a MAC superframe at a time wherein the access point 100 has control of the medium and does not have to face contention for control of the medium. This corresponds to the Contention Free Period (CFP) of the MAC superframe controlled by the HCCA Scheduler. Other parts of the data, whose receipt by the node requesting retrieval is desirable for a higher than acceptable viewing experience, are arranged for transmission as availability of the medium allows. This may involve use of the Contention Period of the MAC superframe controlled by the EDCA distributed Scheduler.
To do this, the access controller 230 operates in accordance with the process illustrated in figure 3.
The process commences in step S3-2, by analysing the Quality of Service requirements of the various streams intended to be defined in the communications channel. This obviously assumes that the scalable audiovisual data has already been retrieved from the external network (i.e. the Internet) and is for instance stored in the mass storage device 216 until the access controller 230 commences operation.
Then, in step S3-4, the component streams of the scalable audiovisual data, namely the base layer stream and any enhancement layer streams, are determined. These are then split into separate groups of streams.
In step S3-6, the processor 208 determines the available capacity of the wireless network based on factors such as the known capacity of the network, the condition of the channel and the amount of data currently being transmitted and received. Any conventional process of determining the loading and capacity of a network known to the skilled person can be applied at this stage.
Then, in step S3-8, the access controller reserves space for the base stream or streams in the MAC superframe for transmission, in the portion of the MAC superframe reserved as the Contention Free Period. The Contention Free Period is defined, in the present example, as having a minimum acceptable capacity and so the base stream is always expected to be capable of accommodation in the Contention Free Period, as long as the base stream is defined to be smaller than that minimum capacity assigned to the Contention Free Period. Of course, it is also possible that, in good transmission conditions, the Contention Free Period will have substantially higher capacity than the minimum, and so the base stream will only use a small proportion of the total available capacity of the Contention Free Period.
Thereafter, in step S3-10, the access controller 230 considers a first enhancement stream. As noted in step S3-12, for this enhancement stream, and while enhancement streams remain for consideration or remaining capacity of the Contention Free Period of the MAC superframe is exhausted, in step S3-14, the access controller tests, whether the Contention Free Period portion of the MAC superframe will accommodate the enhancement stream under consideration. If this is the case, then in step S3-16, the access controller 230 loads the enhancement stream under consideration into the Contention Free Period portion of the MAC superframe.
Once all enhancement streams have been considered, or the test in step S3-14 has shown that the Contention Free Period has been filled, the process continues. In the latter case, the process continues by considering the first remaining enhancement stream. As initiated in step S3-18, for this enhancement stream, and in step S3-20 while enhancement streams remain for consideration or remaining capacity of the Contention Period of the MAC superframe is exhausted, in step S3-22 the access controller tests, whether the Contention Period portion of the MAC superframe will accommodate the enhancement stream under consideration. If this is the case, then in step S3-24, the access controller 230 loads the enhancement stream under consideration into the Contention Period portion of the MAC superframe.
Thus, as far as possible, enhancement streams are transmitted with the base stream in the Contention Free Period. If certain enhancement streams cannot be accommodated in the Contention Free Period then, again as far as possible, they are transmitted in the Contention Period, using the best effort or background access categories, by means of the EDCA scheduler. Some of the sporadic data which it is also desirable to transport over the network, such as world wide web traffic, will also be in these access categories. Based on the determination of the relative priorities of the data to be transmitted over the network by the EDCA scheduler, and according to the 802.11 e access categories, some of the component stream data will be transmitted in the CP and some component stream data will be queued for transmission in a subsequent MAC superframe.
It will be appreciated that the capacity of the Contention Period need not be treated as an absolute limit on the allocation process. In fact, it may be advantageous to overallocate streams to the Contention Period, on the basis that, once transmission is actually effected, one or more of the streams may be inappropriate for transmission. The streams allocated for transmission can then effectively contend for access to the channel, but in the context of the allocation that has taken place giving rise to much improved prioritisation of the streams to the Contention Free Period.
Examples of operation of the access controller 230 will now be described, with reference to figures 4 to 7. In a first example, set out in figure 4, two scalable audiovisual data streams are being transmitted to one or more nodes on the wireless network. Referring to Figure 1, the audiovisual data streams are streamed from an internet server and are received by the access point 100 from the external network.
As illustrated in figure 4, the left hand portion sets out the data as presented to the access controller 230, exhibiting the base streams and enhancement streams previously described as being characteristic of scalable multi-media data. The right hand portion sets out the data as processed by the access controller, prepared for presentation to the Physical Layer of the communications channel in a MAC superframe.
Then, referring to figure 4, the two audiovisual data streams are split (with reference to step S3-4 in figure 3) into base streams (marked Video 1 Base Stream and Video 2 Base Stream in Figure 4) and enhancement streams (marked Video 1 Enhancement Stream a, Video 1 Enhancement Stream b, Video 1 Enhancement Stream c, Video 2 Enhancement Stream a, Video 2 Enhancement Stream b & Video 2 Enhancement Stream c), by the access controller 230.
The access controller 230 then determines that the loading of the network facilitates both of the base streams and one of the enhancement streams to be selected for transmission by the HCCA scheduler in the contention free period. The remaining five enhancement streams are selected by the EDCA scheduler for possible transmission in the contention period. Dependent on availability of the channel in the contention period, the se component streams are then transmitted over the wireless network in the MAC superframe. Of course, if, during the contention period, another device captures control of the channel, it is conceivable that these queued enhancement streams are not transmitted at this time, and may only be after some considerable delay.
In the first example, the loading of the network allows for all of the component streams to be transmitted in the same MAC superframe, subject to control of the channel for the entire duration of the MAC superframe.
Referring to Figure 5, the MAC superframe (illustrated by the arrow with reference CFPrep in Fig. 5) transmitted by the access point 100 in the first example is illustrated. This figure shows that the MAC superframe is bounded by a pair of beacon frames, defining the beginning and end of the MAC superframe. This figure also shows the "CFP End" frame, which demarcates the Contention Free Period from the Contention Period. Both base streams and an enhancement stream (Video 1 Enhancement Stream a) can be seen loaded in the CFP part of the MAC superframe (illustrated by the arrow with CFP in Figure 5). The remaining enhancement streams can be seen assigned to the CP part of the MAC superframe (illustrated by the arrow with reference CP in Figure 5).
In the event of degradation of the channel, the principle of operation is that the contention free period remains assigned with the same capacity, while the capacity of the contention period is reduced. It will be appreciated that the relative lengths of the contention free period to the contention period in figure 5 is greatly exaggerated, and that in fact, in ideal conditions, the contention free period will normally be only a small proportion of the total. The period of repetition of the contention free period, denoted by CFPrcp, is designed to ensure that the access controller 230 can gain control of the channel with an acceptable degree of regularity, so that streams which are delay sensitive (such as streaming video) are not degraded to an unacceptable level. If channel conditions require, the duration of CFPrep can be reduced to suit system requirements.
In a second example, illustrated in Figure 6, two scalable audiovisual streams have been identified for transmission to one or more nodes on the wireless network, in addition to which one of the nodes on the wireless network is also receiving data from the internet, the form of a browsing stream as indicated in figure 6. This system will, for example, be implemented in the case of Ethernet enabled Internet access, via a domestic cable connection. In such as system, it is desirable to provide a user with a wireless keyboard, and, optionally, a wireless pointing device (e.g. a mouse) for interaction with a set-top box operable with a television receiver for implementation of an Internet browsing service.Such a service can, in a typical implementation, provide sufficient bandwidth to be fairly described as a 'broadband' service, in that it may, in general, offer data download speeds of at least 128kbps and up to and including 1000kbps. In the latter case, this could be considered a premium offering for which a cable service subscriber could be minded to accept a premium charge.
In addition to this, a cable service provider may offer, in addition to Internet access through the combination of a set top box and a television receiver directly connected thereto, a facility for establishment of a wireless network with the set-top box acting as an access point to the Internet. Nodes, such as laptops can then establish wireless communication with the set-top box to enable Ethernet based access to the Internet, via the set-top box.
In this context, it is highly desirable that communication between the laptop and the settop box is sufficiently reliable that a user of the system is not encumbered with communications delays or drop-outs. The user, perhaps having paid a premium subscription for the enhanced service allowing the convenience of a wirelessly routed network established about the set-top box, will reasonably expect the service to be established with the minimum of additional steps or inconvenience. Among the major factors on which a user would determine inconvenience would be interrupted communication, or delays in continuous (i.e. streaming) data.
In this example, the access controller 230 determines that the loading of the network facilitates the base streams and one enhancement stream (Video 1 Enhancement Stream a) being assigned for transmission by the HCCA scheduler in the contention free period. The remaining five enhancement streams and the browsing stream are assigned for possible transmission by the EDCA in the contention period.
In this example, the EDCA scheduler determines that the capacity of the network will not allow all of these streams to be transmitted in the contention period of the current MAC superframe. Therefore, only the browsing stream and two lower order enhancement streams (Video 2 Enhancement Stream a and Video 2 Enhancement Stream c) are selected for transmission in the contention period in the current MAC superframe. The remaining enhancement streams are placed in a queue. The base streams, browsing stream and selected enhancement streams are then transmitted over the wireless network by the access point.
In the second example, there is insufficient capacity to allow all of the data streams to be transmitted in the same MAC superframe.As some enhancement frames are not transmitted, there will be a degradation in quality of the video streams displayed at the nodes, but the video stream will not stop entirely. Also, the internet activity data is transmitted without significant delay.
According to a further embodiment, described below, the streams can have expiry information attached to them. This is particularly appropriate for the scalable video enhancement streams, as their arrival after a delay of one or maybe two MAC superframes will potentially render them too late to be incorporated into the decoding process with the previously received base stream for output of the video presentation to a user. The enhancement frames that are not transmitted and which remain in the queue past their expiry time, as indicated by the respective expiry information, are not transmitted when they reach the head of the queue, irrespective of whether any transmission has been attempted.This is thus different from the approach taken in the draft 802.11 e Standard which describes a transmission process wherein a packet is designated as expired after a number of failed transmission. The present arrangement is not concerned with repeated transmission failure; rather it deals with the fact that information intended for transmission may be rendered useless by delay in transmission for whatever reason and so the transmission of such information can therefore be inhibited after a given time delay. This reduces system loading in an advantageous manner.
The MAC superframe (marked by CFPrep in Fig. 7) output by the access point 100 in the second example is illustrated in Figure 7. Again, this figure shows the two "Beacon" frames and the "CFP End" frame as in Fig. 5. Both base streams and the first enhancement stream of Video 1 can be seen in the contention free period (CFP) of the MAC superframe. The remaining enhancement streams and the browsing stream can be seen in the contention period (CP) of the MAC superframe.
In Figure 8 Node A in the wireless network of Figure 1 is shown in more detail, and will now be discussed. As noted above, Node A is a portable laptop computer and is equipped with a wireless RX/TX controller 820, which is in turn connected to antenna 814, to communicate with the access point 100. The wireless RX/TX controller 820 is connected to a general purpose bus 804, which in turn connects to the various components of Node A including random access memory (RAM) 806, a processor 810, BIOS 812, a display controller 802, an input/output controller 808, a removable media controller 816 and a mass storage device 818.
Data is received from the access point 100 through the antenna 814 into wireless RX/TX controller 820. The data is output to the general purpose bus 804 and stored in the RAM 206. If the data received is video data, specifically component streams of scalable MPEG video data, then a frame extractor program residing in RAM, on an ongoing basis, reproduces the transmitted video data from the received base layer streams and enhanced layer streams. The frame extractor should receive all the base layer stream data, unless there is very severe channel degradation or loss of the connection at some point between the internet server and Node A. Therefore it will be able to reproduce at least the basic video stream.When playing the stream, there will be some video information buffered in memory before playback, allowing any enhancement streams that are transmitted, with the base streams or later, to be received.
Playing the video information carried on the stream in this case will involve reassembly of the information received on the base stream with information carried on any received enhancement streams.
The Frame Extractor program is responsive to the receipt of a beacon indicating the start of a MAC superframe. The Frame Extractor then successively receives a CF-Poll or Data frame, contained in the superframe, determines if this is intended for the present node by means of a destination address and, if the frame is so intended, then delivering the data contained in the frame to the appropriate application residing on the node. Where appropriate, the Frame Extractor program is then operable to respond to receipt and successful extraction with a CF-ACK frame and/or data frames.
This extraction of data continues until the end of the MAC superframe is encountered, which is denoted by the CF-End frame.
This process then repeats for subsequent MAC superframes that are transmitted over the network and received by Node A 102.
This process is described above for Node A 102 of the wireless network shown in Figure 1. It will be clear to the skilled person that it can be similarly applied to the other nodes in the wireless network, and to any other devices on the network.
In a preferred embodiment of the invention, referred to above, all of the frames of the component streams of the scalable MPEG video stream are assigned "transmit by" expiry information. This "transmit by" information will be an expiry time calculated to be roughly the time at which the respective frame of video will cease to be needed, i.e. when that part of the video stream is expected to have been played on the device or devices ultimately receiving the video stream. If, by the time a frame of enhancement stream data (or base stream data) reaches the head of the queue it has exceeded the expiry time as indicated by the "transmit by" information, then it is discarded.
In an alternative embodiment, the invention can be applied to alternative types of network including other types of wireless network, including but not limited to local area networks, personal area networks and wide area networks, which support contention free periods and contention periods and also bandwidth limited wired networks.
In a further alternative embodiment, the access point 100 may be connected to an external network using communication means other than an broadband modem, such as a wireless connection or other wired connection. Additionally, access point 100 may be connected to the internet directly, instead of through an ISP, or not at all.
In another embodiment, nodes on the wireless network shown in Figure 1 can be other devices suitably equipped with wireless networking capabilities such as mobile phones, personal digital assistants, personal video players, personal audio players. Further, the nodes on the network may be connected in a peer-to-peer configuration.
In a further embodiment, the video data received by the access point 100 may be converted from other video formats into scalable MPEG video data.
In another embodiment, the video data can be streamed from any other node, whether on the internet, the external network or on the wireless LAN itself.
A specific embodiment and several potential modifications have been described above. However, it is not intended that the invention be limited to these embodiments. Various modifications will be apparent to those skilled in the art. The features of the above described arrangements may be combined in various ways to provide similar advantages in alternative arrangements.
CLAIMS:
1. A method of processing scalable data for allocation to one or more MAC superframes for transmission in a wireless communications system, the MAC superframe comprising a contention free period, the processing method comprising:
determining at least one base stream and one or more enhancement streams in said scalable data; determining the available capacity of the network for transmitting the streams, including determining the capacity of said contention free period in a given MAC superframe; assigning the or each base stream for transmission in said contention free period of said first stream and, dependent on determined available capacity of said first contention free period, any further contention free periods required; dependent on determined available capacity in said first contention free period, assigning one or more enhancement streams for transmission in said contention free period.
2. A method in accordance with claim 1 wherein said step of assigning dependent on determined available capacity comprises determining a nominal level of overcapacity, relative to said determined available capacity, and assigning up to said nominal level of over-capacity.
3. A method in accordance with claim 1 or claim 2 wherein in the event that an enhancement stream is, dependent on determined available capacity, not transmitted in said contention free period, the method further comprises the step of assigning said enhancement stream for transmission in a corresponding contention period for said first MAC superframe.
4. A method in accordance with claim 3 wherein, in the event that an enhancement stream is, dependent on determined available capacity and contention in said system, not transmitted in said contention period, assigning said enhancement stream for transmission in one or more subsequent MAC superframes as required.
5. A method in accordance with any preceding claim wherein said step of assigning for transmission includes the step of determining the existence of time dependence information associated with said stream and, in the event that time dependence information does exist, discarding said stream for transmission in the event that a time limit for transmission of said stream is exceeded.
6. A transmitter for transmitting scalable data in one or more MAC superframes in a wireless communications system, the MAC superframe comprising a contention free period, the transmitter comprising:
data processing means operable to determine at least one base stream and one or more enhancement streams in said scalable data; channel capacity determining means for determining the available capacity of a transmission channel for transmitting the streams, including for determining the capacity of said contention free period in a given MAC superframe; stream assignment means for assigning the or each base stream for transmission in said contention free period of said first stream and, dependent on determined available capacity of said first contention free period, any further contention free periods required, and, dependent on determined available capacity in said first contention free period, for assigning one or more enhancement streams for transmission in said contention free period.
7. A transmitter in accordance with claim 6 wherein said channel capacity determining means is operable to determine a nominal level of over-capacity, relative to said determined available capacity, and said stream assignment means is operable to assign up to said nominal level of over-capacity.
8. A transmitter in accordance with claim 6 or claim 7 wherein said stream assignment means is operable, in the event that an enhancement stream is, dependent on determined available capacity, not transmitted in said contention free period, to assign said enhancement stream for transmission in a corresponding contention period for said first MAC superframe.
9. A transmitter in accordance with claim 8 wherein said stream assignment means is operable, in the event that an enhancement stream is, dependent on determined available capacity and contention in said system, not transmitted in said contention period, to assign said enhancement stream for transmission in one or more subsequent MAC superframes as required.
10. A transmitter in accordance with any one of claims 6 to 9 wherein said stream assignment means includes time dependency detection means operable to detect the existence of time dependence information associated with a stream to be assigned and, in the event that time dependence information does exist, further operable to discard said stream for transmission in the event that a time limit for transmission of said stream is exceeded.
11. An access point for use in a wireless communications network, comprising a transmitter in accordance with any one of claims 6 to 10.
12. A computer readable carrier medium carrying computer executable instructions which, in use, cause a computer to perform the method of any of claims 1 to 5.
1. A method of processing scalable data for allocation to one or more MAC superframes for transmission in a wireless communications system, each MAC superframe comprising a contention free period, the processing method comprising:
determining at least one base stream and one or more enhancement streams in said scalable data; determining the capacity of said contention free period in a given MAC superframe; assigning the or each base stream for transmission in said contention free period of said given MAC superframe and, dependent on determined available capacity of said contention free period of said given MAC superframe, any further contention free periods required;dependent on determined available capacity in said contention free period, assigning one or more enhancement streams for transmission in said contention free period of said given MAC superframe and, in the event that an enhancement stream is, dependent on determined available capacity, not transmitted in said contention free period of said given MAC superframe, assigning said enhancement stream for transmission in a corresponding contention period for said given MAC superframe.