GB2423216A

GB2423216A - Multimedia traffic handling scheme

Info

Publication number: GB2423216A
Application number: GB0602712A
Authority: GB
Inventors: Marwan Muhammad H Al-Akaidi; Omar Younis Alani
Original assignee: MONFORT UNIVERSITY DE
Current assignee: MONFORT UNIVERSITY DE
Priority date: 2005-02-11
Filing date: 2006-02-10
Publication date: 2006-08-16
Also published as: GB0602712D0; GB0502897D0

Abstract

A method of transmitting multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network comprising the steps of separating a multimedia call to be transmitted over the network into at least two components; and transmitting the components over at least two different layers of the HCS network. In this way different components of the multimedia call can be directed to a layer of the HCS network best suited for carrying it. For example a voice component is supported by a microcell while a video component is supported by a macrocell. A non-real time data component may be supported by a satellite covered layer.

Description

MULTIMEDIA TRAFFIC HANDLING SCHEME

FIELD OF THE INVENTION

The present invention relates to a system for allocating resources for the transmission of multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network, and to the transmission of multimedia traffic in a wireless HCS network.

BACKGROUND

With the increased demand for mobile communication services, there is an increasing problem with allocation of available resources to ensure that the level of service required by users can be provided.

The increased demand for mobile communication services comes both from the increased number of subscribers to such services, and due to the increased number of services available. For example, 3G wireless networks allow users to send and receive data, compressed images, videophone and other multimedia services, in addition to conventional voice services.

In allocating the available resources, it is necessary to ensure that the requirements of the different types of service are met, including the required quality of service (Q0S), bandwidth requirements etc., as well as the availability of the network to initiate and maintain calls.

Typically, mobile networks are arranged as clusters of cells such that data sent to and received from a handset is handled by a base station associated with the cell in which the handset is located. When the handset moves from one cell to another cell, the call is handed over from the base station in the cell where the handset was located to the base station in the new cell, in a procedure known as handoff. When a handset moves from one cell to another, there must be an available channel in the new cell for taking over the call to ensure that the call is not dropped. This is considered particularly undesirable, and indeed more undesirable than not being able to initiate a new call.

Various schemes have been proposed for the allocation of available resources for mobile networks.

Hierarchical Cell Structure (HCS) One scheme used in 3G systems makes use of a Hierarchical Cell Structure.

In such a system, layers of cells of different size are provided. The cells can include picocells that may be the size of an individual office, microcells that cover a few hundred square meters, and macrocells that cover a few square kilometres. Each of the cells will have a base station for receiving and transmitting the call data. Satellites may be used to cover larger areas.

Smaller cells are suited to handling high spot traffic in urban areas, whilst larger cells are suited to providing coverage over a large area where the call density is lower.

Increasing the number of cells and decreasing their size would enable a greater number of calls to be handled. However, increasing the number of cells increases the number of base stations, and this is not preferred. Further, where the cells are smaller, the number of handoffs will be increased, and therefore smaller cells will generally increase the risk that the call is dropped as a handoff cannot be completed. By prioritising call handoffs, this risk may be reduced, but at the expense of reducing the number of new calls that may be initiated.

A number of schemes have been proposed for the assignment of calls to different layers of cells in a hierarchical cell structure.

In an overflow scheme, all calls are initially assigned to a default cell layer which is typically the lower layer. In the event that there are no channels available for the call in the default layer, the call can be handed over to the overflow layer, which is typically the higher layer. The overflow could move in either direction, for example from macrocells to microcells, or from microcells to macrocells.

In a speed-sensitive scheme, a call is determined to be a slow call or a fast call, a slow call being a call involving a handset that is moving at a slow speed, such as one held by a pedestrian, whilst a fast call is one involving a handset moving at a fast speed, for example one in a vehicle. In this case, fast calls are assigned to larger cells, such as macrocells or even satellite cells, with slow calls being assigned to smaller cells such as microcells. In this way, the number of handoffs is reduced.

In a teleservice scheme, delay sensitive data such as video and voice can be allocated to macrocells and microcells, whilst delay insensitive data such as non-real time data can be allocated to a satellite layer.

Despite the above schemes, HCS networks may not guarantee QoS to multimedia traffics as the selected layer might be congested and even if it is not, that layer might not fit the service constraints of the multimedia traffic.

Universal Mobile Telecommunications System (UMTS) HCS systems have been used to increase system capacity and to serve subscribers with different mobilities within UMTS.

UMTS is envisioned as the successor to Global System for Mobile Communications (GSM) and is part of the third generation (3G) of mobile networks. UMTS addresses the growing demand in mobile and internet applications for new capacity in wireless mobile telecommunications. UMTS increases transmission speeds to 2Mbps per mobile user and establishes a global roaming standard. UMTS is also referred to as wideband code division multiple access (W-CDMA) and offers teleservices (like speech and SMS) and bearer services, which provide capability for information transfer between access points. It is possible to negotiate and renegotiate the characteristics of a bearer service at session or connection establishment and during ongoing session or connection. Both connection oriented and connectionless services are offered for Point-ToPoint and Point-To-Multipoint communication.

Bearer services have different QoS parameters for maximum transfer delay, delay variation and bit error rate. Offered data rate targets are: 144 kbits/s satellite and rural outdoor 384 kbits/s urban outdoor 2048 kbits/s indoor and low range outdoor UMTS network services have different QoS classes for four types of traffic: Conversational class (voice, video telephony, video gaming) Streaming class (multimedia, video on demand, webcast) Interactive class (web browsing, network gaming, database access)

Background class (email, SMS, downloading)

Resource Allocation In addition to the use of a hierarchical cell structure, it is known to allocate available resources depending on the class of service - for example voice, multimedia, interactive data, images and video services. Since these different services will have different network requirements, for example different traffic levels, bandwidth requirements, Quality of Service requirements, real-time or non- real time requirements etc., it may be possible to prioritise different services to make the best possible use of the available resources. Different sharing or partitioning schemes have been proposed, including complete sharing in which traffic for all classes of service can access all resources in an unrestricted manner, complete partitioning in which certain resources are reserved exclusively for particular classes of service, and a partial sharing scheme which is a hybrid of the complete sharing and complete partitioning scheme.

For analysis purposes, it can be assumed that for a system that supports i- different classes of service, each call of an i-th service requires m units of capacity over the duration of the call, there being a Poisson call arrival rate with an exponentially distributed channel holding time. This will give an i- dimensional birth-death Markov process, with vector j1j1, 2, ... j} representing the state of the cell, i.e. the number of users from each service that are active at any given time. To determine Quality of Service parameters such as blocking and dropping probability, the steady state probabilities of the system P(j)s can be found by solving the steady state equations of the i-dimensional Markov chain. By way of example, for a complete sharing scheme in a system using two services (i=2), with rates of arrival of A1, A2 and an exponentially distributed holding time p1, I-J2, the steady state balance equations are: k2 u'MHj2 + 18 12) =sj _j2' 12 -1 (i1 M17' +1,12) i2 J21 12 +i) (11,12)EA 8kk =O,(k1,k2)EA where A is the space of acceptable states 1,f(k1,k2) A A product form solution exists for this system, depending upon the individual traffic loads Pi, p2: P(11,j2)= p1JIp2 (P(O))' 1I!/2! where P(O)= /I/2 Ji!j7! and p, = -- for i = 1,2 /1, The blocking probability of the two classes of service traffic are: S C-ni2 /2 J2 Pb = P(O)' n2i2)*12 where C is the total capacity of the channel, S=CIrn, and rn is the number of channels required by the service i.

In the case of complete partitioning, there is a fixed capacity C for each of the i services. In this case, the blocking probability of the traffic of each service will be given by the Erlang-B formula:

- ___

-

In the case of a partial sharing system, part of the capacity C is reserved permanently for the i-th service, whilst the remaining capacity is available for any service, on a first come, first served basis. In this case, the blocking probabilities are given as: /)I = P(j1,j2) C + ( + C1 C -J7fl ( i 12)O!2 I,,, -rnin _L_ rn2 j, tn1 j in1 = P(j,j2) C +C1 I ( C. "2 C -j, m, (/12)IO =/? = I, /2flhIi1J j

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of transmitting multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network comprising the steps of: separating a multimedia call to be transmitted over the network into at least two components; and transmitting the components over at least two different layers of the HCS network.

According to a second aspect of the present invention there is provided a method of allocating resource to multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network, comprising the steps of: separating a multimedia call to be transmitted over the network into at least two components; and allocating the components to channels in at least two different layers of the HCS network.

The HCS network may comprise at least two of the following layers: a picocell layer; a microcell layer; a macrocell layer and a satellite layer. In addition, the network may be a UMTS network. In this case, preferably, the components are carried over channels or bearers in the layers of the HCS network and the channels or bearers are generated using Wideband Code Division Multiple Access by using different spreading factors to generate different spreading codes.

The methods according to the first and second aspects of the present invention may comprise the additional steps of: determining the class of each component of the multimedia call to be transmitted; determining whether resources are available on the layers of the HCS network for the transmission of each determined component; and reserving the required available resources. Each of the components is preferably allocated to the layer of the HCS network that is best suited to the transmission of that component. At least some of the components may require different classes of service for transmission, which classes of service may include data, voice, video, videophone, compressed images and broadband data.

In one embodiment of the methods of the first and second aspects of the present invention, in which a plurality of channels of the network are allocated to the components of the multimedia call for transmitting the call, the channels required for transmission of all components of the data may be reserved concurrently. Alternatively, a channel may be released when the transmission of the respective component has been completed. All the channels may be reserved from the same time or each channel may be reserved from a time when a suitable channel becomes available.

The methods according to the first and second aspects of the present invention may additionally comprise one or more of the following steps: allocating video components of the multimedia call to channels carried by a macrocell layer of the HCS network; allocating voice or high bit rate video components of the multimedia call to channels carried by a microcell layer of the HCS network; allocating non-real time components of the multimedia call to channels carried by a satellite layer of the HCS network; and preferentially allocating at least some components of multimedia calls from fast moving user terminals to a macrocell or satellite layer of the HCS network. The methods may also comprise the additional step of synchronising the components to reform the multimedia call after transmission over the network.

In one embodiment of the method according to the first and second aspects of the present invention components may be allocated to layers of the HCS network based on criteria which take into account at least one of the following: the QoS requirements of the component; the Q0S requirements of the multimedia call as a whole; and the QoS available on the layers of the network. The criteria may be implemented as a computer program at the radio resource control layer of the HCS network.

Also, in the method according to the first and second aspects of the present invention call components may be allocated priorities and the methods may comprises the additional step of dropping a low priority component while continuing to transmit a high priority component of a multimedia call over the network. Additionally or alternatively, the method may comprises the step of dropping a low priority component and reallocating network resource for the low priority component to a high priority component of a multimedia call. The priorities allocated to components of the multimedia call may be dependent on the service class of the components.

According to a third aspect of the present invention there is provided a computer program including code for controlling the transmission of multimedia traffic over a wireless Hierarchical Cell Structure (HCS) network by implementing the method steps as set out above. The code may implement some or all of the steps according to the methods of the first or second aspects of the present invention. According to a fourth aspect of the present invention there is provided a computer program product including the computer program according to the third aspect of the present invention.

According to a fifth aspect of the present invention there is provided a device for facilitating the transmission of multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network comprising: means for separating a multimedia call to be transmitted over the network into at least two components; and means for allocating the components to or transmitting the components over at least two different layers of the HCS network.

The device may additionally comprise: means for determining the class of each component of the multimedia call; means for determining whether resources are available on the layers of the HCS network for each determined component; and means for reserving the required available resources. Also, the device may additionally comprise one or more of the following: means for allocating video components of the multimedia call to channels carried by a macrocell layer of the HCS network; means for allocating voice or high bit rate video components of the multimedia call to channels carried by a microcell layer of the HCS network; means for allocating non-real time components of the multimedia call to channels carried by a satellite layer of the HCS network; and means for preferentially allocating at least some components of multimedia calls from fast moving user terminals to a macrocell or satellite layer of the HCS network. In addition the device may comprise means for synchronising components of a multimedia call so as to reform the multimedia call after transmission over the network.

According to a sixth aspect of the present invention there is provided a Hierarchical Cell Structure (HCS) network comprising at least two of the following layers: a picocell layer supported by at least one base station; a microcell layer supported by at least one base station; a macrocell layer supported by at least one base station and a satellite layer supported by at least one satellite wherein the network additionally comprises at least one device according to the fourth aspect of the present invention. The device may be, for example, an integral part of at least one of the base stations of the network, an integral part of a control station for a satellite of the network or may be an integral part of a network controller for controlling at least a part of the network. Also, the network may be a UMTS network, in which case the components may be carried over channels or bearers in the layers of the HCS network and the channels or bearers may be generated using Wideband Code Division Multiple Access by using different spreading factors to generate different spreading codes.

BRIEF REFERENCE TO THE DRAWINGS

The present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a comparison of the handing of multimedia traffic in the accordance with the present invention and in prior art schemes; Figure 2 shows a state space diagram for a scheme according to the present invention.

Figure 3 shows a state pace diagram for a scheme according to the present invention with concurrent occupation and concurrent release.

Figure 4 shows blocking probability of NRAS, CS, and CF schemes with P1P2 and different number of resources; Figure 5 shows performance of NRAS scheme with equal (c*p) product; Figure 6 shows the division of components of a multimedia call into separate connections; and Figure 7 shows the division of components of a multimedia call into separate layers of a HCS network.

DETAILED DESCRIPTION

A multimedia call will include components in different classes of service, for example data, voice, videophone, compressed images and broadband data.

Unlike conventional systems in which all components of the multimedia call are handled as a unique connection and conveyed in a single wideband channel, according to the present invention a determination is made of the different components in the multimedia call, and a determination is made of the appropriate resources for the transmission of each of the components.

This determination may be made using any of the known schemes for resource allocation in a hierarchical cell system.

Accordingly, the invention is a new model for multimedia traffic. Multimedia calls usually consist of more than one service, e.g. audio, video, text, data, etc., each one classed as a separate component. By transmitting each component independently in the Air Interface, as is shown, for example, in Figure 6 a multimedia call has the flexibility to assign each component the optimum channel as well as to adapt any of its components individually to channel variations. However, all the components logically belong to the same broadband flow and thus need to be jointly handled. By treating the multimedia traffic as individual components that can be transmitted on different layers of a network, optimum Quality of Service for each of the components can be achieved. This leads to call components Quality of Service (Q0S) requirements being fulfilled with resultant improvements in overall system performance. This is different from the classical model where a multimedia call is handled as a unique connection and conveyed on a single wideband channel (single- bearer transport).

Each layer in the HCS network can be considered as a pool serving a certain type of service. Therefore, instead of connecting all the components of a multimedia call to a single layer, the components will be assigned to the optimal layer of the HCS network that fulfils the Q0S requirements of those components. For example, macrocells are able to support video services, while microcells support voice service. Other service types such as non-real time data that has no contingent delay constraint are able to be supported by a satellite covered layer. However, the handoff for components that cannot find channels in the optimal layer could be performed to sub-optimal layers with lower QoS. The interstream that results from multimedia call components flowing to different layers requires synchronisation.

Synchronisation can be used to ensure that the different components travelling in different layers are handled correctly with respect to each other.

A special entity is inserted at the UMTS Terrestrial Radio Access Network (UTRAN) which is responsible for providing synchronisation among the multimedia call components accepted in the network.

In one example of the present invention, the channels are occupied and released concurrently. In this case, once it is determined which resources are required for the transmission of the different classes of data, the availability of the resources is determined, and these are reserved when they become available. The resources are each reserved for a time until all components of the multimedia traffic have been transmitted, and then the resources are concurrently released.

In an alternative embodiment, the required resources are released as soon as the respective component of the multimedia traffic has been transmitted giving a non-concurrent release of resources. In this case, the resources may initially be reserved at the same time, or may be reserved and used as the required resources become available, depending whether all data needs to be transmitted simultaneously. In the case of non-concurrent release the capacity of the system increases; as the release of resources occurs independently, the freed channels could be offered to new users. The non-concurrent release of resources also has a great advantage in the case of congestion. In case of congestion the system can sacrifice the components that are regarded as dispensable in the multimedia call.

Figure 1 shows a comparison of the handling of a multimedia call using different known schemes and the scheme of the present invention. Figure 1A shows the handling of a multimedia call in a complete sharing scheme. In this case, the multimedia traffic (M) must share the available resources with other types of traffic including voice (V) and data (D) traffic. In the case of a complete partitioned system as shown in Figure 1B, the available resources are divided into resources exclusively provided for each of the multimedia, data and voice signals. Therefore, if there are no voice or data signals, these parts of the available resources are not utilised. In Figure 1C, a partial sharing scheme is shown in which there are some dedicated resources for each of the data, voice and multimedia traffic, as well as some shared resources available to any of these types of traffic. However, as shown in Figure ID, with the scheme according to the present invention, a multimedia call is divided into different components, and each of the components are handled in an appropriate resource within the communication system.

With the scheme according to the present invention, it can be assumed that each multimedia call will require resources appropriate to each class of service that can be handled by the system. In the case where there are two classes of service, if a multimedia call is received, and there are no available resources for either or both classes of service, the call will be blocked. Figure 2 shows a state transition diagram for such a system where there are four resources for the first class of service and three resources for the second class of service.

In the case where the number of resources occupied for each class of service, i, is j, the blocking probability for an arriving multimedia call is given by: P8 =P(j,C,)+P(C1,j2) /1=0 For a concurrent occupation and concurrent release scheme, for each multimedia call it is necessary for resources to be available for each class of service. Although each resource needs to be occupied for a negative exponentially distributed amount of time, in the case of a concurrent release scheme, all resources are reserved until the longest of the service periods has ended. This does mean that all but one of the resources are reserved for longer than is necessary. This problem is avoided in a non-concurrent release scheme, in which resources are released at the end of the period required for that class of service. The state space diagram for the non- concurrent release scheme is as shown in Figure 3 for a system including six channels, where each multimedia call requires three channels and each voice call requires a single channel. The blocking probability in this case is given by the equation: - /c1

B

C1 =min{C1,C1,...C}, where n is the maximum number of services in the system.

To obtain equal blocking probabilities for all types of service we need to equate the products of service rate and number of resources 2T, - 1L11. N, for each class / (1 E {l,2 where n is the number of services.

It has been determined that a scheme according to the present invention can provide an improved performance compared to conventional schemes. In the case of a system able to handle two different services, and having four channels available for each of the services, the relative blocking probabilities of the three schemes is shown in Figure 4. It can be seen from this that the blocking probability of the new resource allocation scheme (NRAS) of the present invention is lower than that of the complete partitioning or complete sharing scheme, and is therefore an improvement over these schemes.

Figure 5 shows the relative performance of the schemes taking into consideration the number of resources and the holding time (C*p), considering the cases where C1=4, C2=3 with p=lO, 1J2-4013, and where C1=4, C2=2 with pi=1O, 22O. In the first case, there were C23 (slow) users with P24013, and in the second case there were C2=2 (fast) users with P2=2O.

The present invention is particularly suited for use in a UTMS system. The design of different channels (bearers) for each component in a multimedia call can be obtained in Wide Code Division Multiple Access (WCDMA) by using different spreading factors to generate spreading codes. There is also potential for different channel and speech coding techniques to be applied to individual components to further improve QoS requirements of the system.

Queuing theory can be used in resource allocation to enable bandwidth access problems to be solved.

Applying this new multimedia call model and the corresponding resource allocation scheme to Hierarchical Cell Structure (HCS) networks, as shown in Figure 7, gives shows the allocation of different components in different network layers.

As described above the mobility of the user terminal or handset can decide the selection of the layer that serves a particular call service in a HCS network. In addition, the type of service could decide the serving layer for low mobility and stationary user terminals. For example, a service that requires high power can be served by smaller cells which have lower antenna in order to reduce the interference as well as reducing the path loss and fading impact, thus resulting in higher capacity. However, non-real time data services are best served by the satellite layer as it can tolerate the delay introduced by the long path. Using a multi-component teleservice model will give additional flexibility to Call Admission Control (CAC), as call components will be freely and individually assigned to network layers instead of having all call components served in the same layer. The result of this is that CAC criteria could neglect the mobility factor of the user, relying only on the type of call being handled. However, the resulting potential problem of increased numbers of handoff for fast moving user terminals would then have to be dealt with. Depending on the extent of the handoff problem, mobility could still play an active role in the determination of layer selection in coordination with the type of service. For example, a multimedia call from a fast moving user terminal could be

dealt with by allocating the voice component to the microcell layer, the video component to the macrocell layer and a data component to the satellite layer.

However, if the voice component is deemed to last longer than the video component, which might be a short video clip, then the network may decide to allocate the video component to the microcell and the voice component to the macrocell layer in order to avoid excessive handovers of the voice at the microcell layer. Other CAC criteria could be based entirely on the power budget available at the Base Station as W-CDMA systems are believed to be interference limited systems. In this case high bit rate video component will be assigned to the microcell as they can afford the required power due to their low antennae and short range cell size.

Several criteria can be taken into account by a network controller when allocating resource from different layers of a HCS network to different components of a multimedia call. These criteria might take into account QoS parameters relevant to the components of the calls, QoS parameters relevant to the multimedia call as a whole, as well as the QoS that is available in the different layers of the HCS network. This criteria is implemented as a computer program algorithm at the radio resource control layer, which represents the network layer of the W-CDMA protocol structure.

In one embodiment service class priorities are assigned to the components of a multimedia call and the components are then allocated resource, for example to one or more channels or bearers, on an appropriate layer of the HCS network so that the components can be transmitted across the HCS network. Then if the Q0S of an allocated channel on which a low priority component of the multimedia call is transmitted becomes unacceptable, then that component of the multimedia call can be dropped without adversely effecting the whole multimedia call. For example, a videophone call has two components; a video component and an audio component. However, the call can be continued with only the audio component if the video component is dropped. In this case the video component of a video phone call can be allocated a low priority. Then if the channel carrying the video component has a QoS which becomes unacceptable, for example due to congestion on relevant layer of the HCS network, then based on the low priority of the video component, a network controller is able to drop the video component and the videophone call can continue as an audiophone call, without inconveniencing the customer so much as if the entire multimedia call was dropped. In addition the priorities can be used by a network controller to re-allocate resource allocated to the video component of a videophone call to the audio component of the videophone call in the situation where no resource is available for the audio component in the appropriate layer. In this situation the network controller will drop the video component and allocate the resource previously allocated to the video component to the audio component. Again, the call is able to proceed as an audiophone call, which is a better alternative for the customer than the whole call being blocked.

Claims

1. A method of transmitting multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network comprising the steps of: separating a multimedia call to be transmitted over the network into at least two components; and transmitting the components over at least two different layers of the HCS network.

2. A method according to claim 1 wherein the HCS network comprises at least two of the following layers: a picocell layer; a microcell layer; a macrocell layer and a satellite layer.

3. A method according to any one of the preceding claims wherein the network is a UMTS network.

4. A method according to claim 3 wherein the components are carried over channels or bearers in the layers of the HCS network and wherein the channels or bearers are generated using Wideband Code Division Multiple Access by using different spreading factors to generate different spreading codes.

5. A method according to any one of the preceding claims comprising the additional steps of: determining the class of each component of the multimedia call to be transmitted; determining whether resources are available on the layers of the HCS network for the transmission of each determined component; and reserving the required available resources.

6. The method according to any one of claims 1 to 5, in which each component is transmitted on a layer of the HCS network that is best suited to the transmission of that component.

7. The method according to any one of the preceding claims, in which at least some of the components require different classes of service for transmission, which classes of service include data, voice, video, videophone, compressed images and broadband data.

8. The method according to any one of the preceding claims, in which a plurality of channels of the network are allocated to the components of the multimedia call for transmitting the call wherein the channels required for transmission of all components of the data are reserved concurrently.

9. The method according to any one of claims 1 to 7, in which a plurality of channels of the network are allocated to the components of the multimedia call for transmitting the call, wherein a channel is released when the transmission of the respective component has been completed.

10. The method according to any one of claims 1 to 7, in which a plurality of channels of the network are allocated to the components of the multimedia call for transmitting the call, wherein all the channels are reserved from the same time or each channel is reserved as a suitable channel becomes available.

11. The method according to any one of the preceding claims, additionally comprising one or more of the following steps: allocating video components of the multimedia call to channels carried by a macrocell layer of the HCS network; allocating voice or high bit rate video components of the multimedia call to channels carried by a microcell layer of the HCS network; allocating non-real time components of the multimedia call to channels carried by a satellite layer of the HCS network; and preferentially allocating at least some components of multimedia calls from fast moving user terminals to a macrocell or satellite layer of the HCS network.

12. The method according to any one of the preceding claims comprising the additional step of synchronising the components to reform the multimedia call after transmission over the network.

13. The method according to any one of the preceding claims wherein components are allocated to layers of the HCS network based on criteria which take into account at least one of the following: the QoS requirements of the component; the QoS requirements of the multimedia call as a whole; and the Q0S available on the layers of the network.

14. The method according to claim 13 wherein the criteria are implemented as a computer program at a radio resource control layer of the HCS network.

15. The method according to any one of the preceding claims wherein call components are allocated priorities and the method comprises the additional step of dropping a low priority component while continuing to transmit a high priority component of a multimedia call over the network.

16. The method according to any one of the preceding claims wherein call components are allocated priorities and the method comprises the additional step of dropping a low priority component and re-allocating network resource for the low priority component to a high priority component of a multimedia call.

17. The method according to claim 15 or claim 16 wherein the priorities allocated to components of the multimedia call are dependent on the service class of the components.

18. A method of allocating resource to multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network, comprising the steps of: separating a multimedia call to be transmitted over the network into at least two components; and allocating the components to channels in at least two different layers of the HCS network.

19. A method according to claim 18 wherein the HCS network comprises at least two if the following layers: a picocell layer; a microcell layer; a macrocell layer and a satellite layer.

20. A method according to any one of claims 18 or 19 wherein the network is a UMTS network.

21. A method according to claim 20 in which the components are carried over channels or bearers in the layers of the HCS network and the channels or bearers are generated using Wideband Code Division Multiple Access by using different spreading factors to generate different spreading codes.

22. A method according to any one of claims 18 to 21 comprising the additional steps of: determining the class of each component of the multimedia call; determining whether channels are available on the layers of the HCS network for the transmission of each component; and reserving the required available channels.

23. The method according to claim 18, in which each component is allocated to a layer of the HCS network that is best suited to the transmission of that component.

24. The method according to any one of claims 18 to 23, in which at least some of the components require different classes of service for transmission, which classes of service include data, voice, video, videophone, compressed images and broadband data.

25. The method according to any one of claims 18 to 24, in which a plurality of channels of the network are allocated to the components of the multimedia call wherein the channels required for transmission of all components of the data are reserved concurrently.

26. The method according to any one of claims 18 to 24, in which a plurality of channels of the network are allocated to the components of the multimedia call wherein a channel is released when the transmission of the respective component has been completed.

27. The method according to claim 18 to 24, in which a plurality of channels of the network are allocated to the components of the multimedia call and wherein all the channels are reserved from the same time or each channel is reserved as a suitable channel becomes available.

28. The method according to any one of claims 18 to 27, additionally comprising one or more of the following steps: allocating video components of the multimedia call to channels carried by a macrocell layer of the HCS network; allocating voice or high bit rate video components of the multimedia call to channels carried by a microcell layer of the HCS network; allocating non-real time components of the multimedia call to channels carried by a satellite layer of the HCS network; and preferentially allocating at least some components of multimedia calls from fast moving user terminals to a macrocell or satellite layer of the HCS network.

29. The method according to any one of claims 18 to 28 wherein components are allocated to layers of the HCS network based on criteria which take into account at least one of the following: the Q0S requirements of the component; the QoS requirements of the multimedia call as a whole; and the Q0S available on the layers of the network.

30. The method according to claim 29 wherein the criteria are implemented as a computer program at a radio resource control layer of the HCS network.

31. The method according to any one of claims 18 to 30 wherein call components are allocated priorities and the method comprises the additional step of dropping a low priority component while continuing to allocate resource to a high priority component of a multimedia call over the network.

32. The method according to any one of the claims 18 to 30 wherein call components are allocated priorities and the method comprises the additional step of dropping a low priority component and re-allocating network resource for the low priority component to a high priority component of a multimedia call.

33. The method according to claim 31 or claim 32 wherein the priorities allocated to components of the multimedia call are dependent on the service class of the components.

34. A computer program including code for controlling the transmission of multimedia traffic over a wireless Hierarchical Cell Structure (HCS) network by implementing the method steps according to any one of the preceding claims.

35. A computer program product including a computer program according to claim 34.

36. A device for facilitating the transmission of multimedia traffic in a wireless Hierarchical Cell Structure (HCS) network comprising: means for separating a multimedia call to be transmitted over the network into at least two components; and means for allocating the components to or transmitting the components over at least two different layers of the HCS network.

37. A Hierarchical Cell Structure (HCS) network comprising at least two of the following layers: a picocell layer supported by at least one base station; a microcell layer supported by at least one base station; a macrocell layer supported by at least one base station and a satellite layer supported by at least one satellite wherein the network additionally comprises at least one device according to claim 36.

38. The network according to claim 37 wherein the device is an integral part of at least one of the base stations or is an integral part of a control station for the satellite or is an integral part of a network controller for controlling at least a part of the network.

39. The network according to claim 37 or 38 which is a UMTS network.

40. The network according to any one of claims 37 to 39 wherein the components are carried over channels or bearers in the layers of the HCS network and wherein the channels or bearers are generated using Wideband Code Division Multiple Access by using different spreading factors to generate different spreading codes.

41. The device according to claim 36 additionally comprising: means for determining the class of each component of the multimedia call; means for determining whether resources are available on the layers of the HCS network for each determined component; and means for reserving the required available resources.

42. The device according to claim 36 or claim 41, additionally comprising one or more of the following: means for allocating video components of the multimedia call to channels carried by a macrocell layer of the HCS network; means for allocating voice or high bit rate video components of the multimedia call to channels carried by a microcell layer of the HCS network; means for allocating non-real time components of the multimedia call to channels carried by a satellite layer of the HCS network; and means for preferentially allocating at least some components of multimedia calls from fast moving user terminals to a macrocell or satellite layer of the HCS network.

43. The device according to any one of claims 36, 41 or 42 comprising means for synchronising components of a multimedia call so as to reform the multimedia call after transmission over the network.