fig1 shows a schematical representation of a 3gpp umts packet - based wireless communication system architecture 100 . the wireless communication system 100 includes a core network ( cn ) 110 with at least one serving gprs support node ( sgsn ) 111 and at least one core network gateway device , the gateway gprs support node ( ggsn ) 112 — the ggsn 112 comprises an interface 116 being configured for providing interworking of said packet - based wireless communication system with at least one other packet data network 120 , and comprises a cpu 114 and a memory 115 . the cpu 114 performs the process of selecting data packets for dpi based on a critical context table of critical contexts stored in the memory 115 and the process of analysing at least one data packet 121 of at least one traffic flow through deep packet inspection in order to classify the traffic flow . the wireless communication system 100 further comprises a universal terrestrial radio access network ( utran ) 130 which includes one or more radio access networks ( rans ) 131 , radio network controllers ( rncs ) 132 and nodebs 133 . the rncs are configured to transmit congestion information from the ran to the cn by marking a data packet 151 received from a congested radio cell using a gtp - u extension header message 150 of the packet and forwards the marked packet to the core network to thereby transmit the information identifying the congested context . the system 100 also comprises a plurality of wireless user equipment ( ue ) devices 140 . fig2 shows a schematical representation of a 3gpp long term evolution ( lte )/ system architecture evolution ( sae ) packet - based wireless communication system architecture 200 . the following description of the embodiment illustrated in fig2 focuses on the differences to the embodiment of fig1 . for similar and / or identical features , the reader should confer to the above description of the embodiment of fig1 . the wireless communication system 200 includes a core network ( cn ) 210 with at least one core network gateway device , the access gateway ( agw ) 212 . the agw 212 consists of two logical user plane entities , serving gateway 217 and packet data node ( pdn ) gateway 218 , collectively called the sae gw 219 , and one control plane entity , the mobility management entity ( mme ) 216 . these may be implemented in common or separate physical nodes . the wireless communication system 200 further comprises at least one lte radio access network 231 which includes one or more enodebs 232 . the system 200 also comprises a plurality of wireless user equipment devices ( ue ) 240 . the gtp user plane is shown in 253 , the gtp control plane gtp - c is shown in 252 . the ran device , the rnc 132 in fig1 or the enodeb 232 in fig2 is configured for identifying at least one congested context within the radio access network based on a status of bandwidth resources for the congested context . in contrast to prior art , the ran device 132 ; 232 is further configured for transmitting information identifying the congested context within the radio access network to the core network . according to the favourable embodiment of the invention , it is proposed that in the step of identifying the congested context , a radio access network device detects a congested radio cell ; marks at least one packet 151 received from the congested radio cell in a gtp - u extension header message 150 of the packet ; and forwards the marked packet to the core network to thereby transmit the information identifying the congested context . as a consequence , this embodiment does not increase the signalling load between the radio access network and the core network as the information identifying the congested context would be carried with the uplink user data . the cn gateway device , i . e . the ggsn 112 in fig1 . and the agw 212 in fig2 , is configured for providing interworking of said packet - based wireless communication system with at least one other packet data network 120 ; 220 and analysing the incoming traffic flows through deep packet inspection . the cn gateway device is further configured to extract the information on the congested contexts within the ran from the gtp - u extension header messages . dpi is conducted selectively only for those incoming data packets that related to one of the identified critical contexts . fig3 describes the steps for selective dpi in accordance with the embodiment from fig1 based on an illustrative example . in step 1 , a user initiates a p2p download resulting in a traffic flow congesting his radio cell within the radio access network . in step 2 , the rnc notices the shortage of bandwidth resources in that radio cell , e . g . if the traffic volume exceeds a threshold value dependent of the available throughput capacity . as set forth at step 3 , the rnc marks the packets of the traffic flows received from the congested radio cell in a gtp - u extension header message of the packet ; and forwards in step 4 the marked packets to the ggsn via the corresponding sgsn to thereby transmit the information identifying the congested context . the congestion information would thus be carried with the uplink ( ul ) user data . this is also illustrated in fig4 that shows a signalling diagram for transmitting information on congested contexts . the rnc receives the uplink user data 401 and adds the congestion information in the gtp - u extension header 402 before the gtp - u message is transmitted to the ggsn 403 . in step 5 of fig3 , the ggsn is configured to extract the information on the congested contexts within the ran from the gtp - u extension header messages and , as indicated in step 6 , to add the extracted critical context to a table that stores all the information on the critical contexts as indicated . the ggsn is further configured to delete out - dated critical contexts from the table of critical contexts , e . g . by a function that deletes all context entries in the table that have not been indicated as critical contexts for a period of time . as set forth at step 7 , the ggsn selects the data packets for dpi based on the entries of the critical context table . fig5 describes in more detail the steps to determine whether or not a data packet of a traffic flow is selected for deep packet inspection . in step 8 , the ggsn conducts deep packet inspection only for those incoming data packets that relates to one of the critical context as determined in step 7 . according to the findings of the dpi , the data packets of a traffic flow are then classified in step 9 . for example , if the traffic flow contributing to the congested radio cell indicated by the rnc is identified as p2p traffic , then the traffic flow may be classified as p2p traffic and / or as un - wanted traffic . in step 10 , the ggsn then adjusts the bandwidth resources available to this un - wanted traffic according to a function that determines the bandwidth resources based on the identified traffic classification , user profile and status of network resources . a further aspect of the invention relates to a cn gateway device being configured to modify at least one charging parameter and / or service access parameter for the traffic flow according to the classification determined using the deep packet inspection in a modified step 10 in fig3 . such configuration it would allow network operators to differentiate service access control ( allowing certain services only when there is capacity in the network / cell , blocking services if services are constantly misused ) or differentiated charging ( price could vary depending on whether or not a data packets is related to a critical context ) for traffic flows that can only be classified through deep packet inspection . fig5 illustrates a flow diagram of steps involved in determining whether or not a data packet relates to a context out of a set of predetermined critical contexts . the ggsn extracts in step 501 from the data packet header the information to identify the user , area and / or service to which the traffic flow relates . the ggsn then compares this information to a first entry in a critical context table in step 502 . if the data packets relates to a critical context determined by the first entry , then the data packet is selected for dpi in step 503 . otherwise , it is checked whether the table of critical contexts has additional entries in step 504 . if not , then data packet is not selected for dpi in step 505 . if yes , then the procedure jumps to the next entry of the critical context table in step 506 and compares this entry with the information extracted from the data packet header in step 502 . it is understood , that the present disclosure has been made only by way of example , and that numerous changes in the details of implementation of the invention may be made without departing from the spirit and scope of the invention as defined in the independent claims . for example , the ran device may alternatively be configured to transmit the information identifying the congested context by means of signalling , i . e . sending a separate message directly to the core network gateway device by using gtp - u or ranap / gtp - c as a message format . or , as illustrated in 404 of fig4 , congestion could be indicated by means of a network management system that may receive and explicit congestion message from ran or being configured to make a decision based on network load . another alternative to transmit information on congested context may be to use a policy server which could combine congestion and user subscription information . the invention could also be implemented in other 3gpp wireless communication systems ( e . g . in an hspa + system where the inodeb represents the radio access network device that identifies and transmits the information on the congested contexts ) or in non - 3gpp wireless communication systems . moreover , the method could also be employed to perform selective dpi on uplink traffic flows . the skilled person will easily be able to find further combinations and / or sub - combinations of the above described features of the invention in order to adapt the method and the devices to specific circumstances while using the central aspects of the invention as defined in the claims .