Wimax network, Wimax network element, and method of handling QoS requirements therein Background of the invention The present invention relates to a method of handling QoS requirements in a wire¬less communication network, wherein a QoS request message associated with said QoS requirements is sent from a wirelesscore network to a wireless access network responsible for resource allocation to an end terminal requiring a particular service through a wireless access gateway, said method comprising the step of forwarding the QoS request message from a first receiving wireless access gateway to a sec¬ond serving wireless access gateway upon a mobility event of the end terminal. The present invention also relates to a core network for use in a wireless communi¬cation network, to a wireless communication network, and to a computer program product. Providing particular services over a wireless communication network (e.g. Wimax or 3GPP LTE (Long Term Evolution) / SAE (System Architecture Evolution), for in¬stance providing IP Multimedia Subsystem (IMS) related services, requires that cor¬responding Quality-of-Service (QoS) requests be relayed from an element of a wire¬less core network (also referred to a Connectivity Service Network - CSN) generally referred to as Policy Decision Function (PDF) to an Access Network Gateway (ASN-GW, also referred to as Wireless Access Controller- WAG) and further to a particu¬lar base station of the wireless communication network under the coverage of which the end terminal is currently located. Nowadays, end terminals are usually devised in the form of mobile stations or user equipment which are free to move and may therefore change location from base station to base station which may furthermore entail changing the WAC. Conse¬quently, a solution is needed which allows notifying the PDF to which WAC QoS requests for a particular end terminal should be addressed. Prior art solutions rely on specifying a dedicated new interface between WAC and PDF for forwarding dedicated messages which indicate a current terminal location (terminal location update) each time an end terminal changes from one WAC to an¬other. These prior art solutions suffer from the inherent disadvantages of requiring the costly definition of a new interface which furthermore creates a mobility-dependent feature in the CSN/IMS core of the wireless communication network which reduces operational flexibility. Object of the invention It is the object of the present invention to provide a method of handling QoS re¬quirement of the above-defined type which does not rely on defining new interfaces and which will not reduce operational flexibility due to creation of mobility-dependent features in the CSN/IMS core of the wireless communication network. It is also an object of the present invention to provide a network element of the above-defined type as well as a wireless communication network which enable translation of the inventive methods into practise. Summary of the invention According to a first aspect of the present invention the object is achieved by provid¬ing a method of the above-defined type which further comprises the step of allocat¬ing the receiving wireless access gateway at initial authentication of the end termi¬nal. According to a second aspect of the present invention the object is achieved by pro¬viding a core network for use in a wireless communication network, comprising means for receiving an authentication message in connection with an end terminal from an access network of the wireless communication network, said authentication message comprising an identifier of a QoS managing entity in said access network; means for storing said identifier; and means for sending a QoS request message in connection with said end terminal to the entity specified by said identifier.[HN1] According to a third aspect of the present invention, the object is also achieved by providing a wireless communication network, in particular a Wimax network, com¬prising a plurality of access networks, and a core network according to said second aspect of the present invention. Furthermore, according to a fourth aspect of the present invention, the object is achieved by providing a computer program product, comprising program code se¬quences operable to implement in a core network of a wireless communication net¬work: - means for receiving an initial authentication message in connection with an end terminal from an access network of the wireless communication net¬work, said initial authentication message comprising an identifier of a QoS managing entity in said access network; - means for storing said identifier; - means for sending a QoS request message in connection with said end terminal to the entity specified by said identifier. In a further embodiment the computer program product in accordance with the pre¬sent invention comprises program code sequences operable to implement in an ac¬cess network of the wireless communication network: - means for receiving QoS request messages from the core network, said QoS request messages comprising QoS requirements for a service re¬quired by the end terminal; - means for fonwarding said QoS request messages to a corresponding net¬work element of a further access network, if the end terminal is attached to said further access network. In this way and in accordance with a general idea underlying the present invention, a QoS management entity of the WAG, said entity generally being referred to as Service Flow Activation (SFA) entity, is allocated for a given end terminal from a CSN/IMS core point of view at initial authentication (i.e., first terminal network con- nection) of the end terminal in question. That particular WAC is then responsible for forwarding any IMS-QoS requests issued in connection with that end terminal to a WAC which is currently controlling/serving the terminal in question. Therefore, in a further embodiment of the method in accordance with the present invention, the lat¬ter comprises allocating the receiving wireless access gateway at initial authentica¬tion of the end terminal. Corresponding information concerning the allocated wire¬less access gateway (e.g., its address in the network) may be stored at the core network for subsequent use. In another embodiment of the present invention, said service is an IMS-related ser¬vice. In this context, said QoS requests may be identified with IMS-QoS requests comprising measurable QoS parameters, such as required bandwidth, bit rate, or the like. In accordance with the present invention, the WAC to which the PDF sends the IMS-QoS request is fixed during initiation of terminal authentication, i.e. while the end terminal in question is still connected to the network, and thus corresponds to the WAC a given end terminal has initially been attached to. Owing to standard Au¬thentication, Authorization, and Accounting (AAA) features, mobility of said end ter¬minal to another WAC is transparent to an entity of the core network generally re¬ferred to as AAA server. In accordance with the present invention QoS requests will still be sent to the initially allocated WAC ("old WAC") which in turn is responsible for forwarding the received IMS-QoS request messages to the currently serving WAC ("new WAC"). In the context of forwarding said messages, commercially available mechanisms for inter-WAC communication may be employed (e. g., the SuperWAC solution commercialized by the present applicant). Therefore, in a further embodiment of the network in accordance with the present invention the latter further comprises receiving an Initial authentication message from an entity of the serving wireless access gateway at the core network, said ini¬tial authentication message comprising an identifier of the receiving wireless access gateway; storing said identifier in the core network; providing an entity of the core network with said identifier for sending the QoS request message to the receiving wireless access gateway specified by said identifier. In this context, in a further embodiment of the core network in accordance with the present invention said authentication message is an initial authentication message of said end terminal. One particular advantage of the present invention resides in the fact that re¬use/unification of current interfaces for fixed solutions between Wimax access net¬work and Wimax CSN/IMS core networks is possible for mobile IMS/QoS manage¬ment. Further advantages and characteristics of the present invention will become appar¬ent from the following description of exemplary embodiments described with refer¬ence to the enclosed drawings. Features of the present invention described above as well as below can be used either individually or in conjunction. The following de¬scription of embodiments is not meant to be an exhaustive enumeration but rather as an exemplary description in connection with the basic ideas underlying the pre¬sent invention. In particular is should be noted that despite of the Wimax-specific terminology used, the present invention is not limited to Wimax networks and may equally be em¬ployed in connection with other wireless communication networks, e.g. the above-mentioned 3GPP LTE/SAE networks. Brief description of the drawings Fig. 1 is a schematic block diagram of a Wimax network in accordance with the present invention; and Fig. 2 is a flow chart of an embodiment of the method in accordance with the present invention. Detailed description of the drawings The present invention slnail now be described in connection with a wireless commu¬nication networl< in the form of a Wimax network. However, as will be appreciated by a person sl